General structure of gamma rays spectrum usually has the following principle features. Workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. Radiation Dosimetry
Analysis of gamma spectra is very interesting, since it has a structure and workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.
Cobalt-60 is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.
As can be seen from the figure, there are two gamma ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
Figure: Caption: Comparison of NaI(Tl) and HPGe spectra for cobalt-60. Source: Radioisotopes and Radiation Methodology I, II. Soo Hyun Byun, Lecture Notes. McMaster University, Canada.
General structure of gamma rays spectrum usually has the following principle features:
Photopeaks are full energy peaks, that are created, when γ-ray transfers its full energy (hω −Ebinding) into the sensitive material (scintillation crystal or germanium crystal). Than a single delta function should be observed in the spectrum.The cobalt-60 spectrum contains two photopeaks, one at 1173.2 keV and another at 1332.5 keV.
Compton Continuum. In the crystal, a gamma ray undergoes a number of interactions, but for intermediate energies compton scattering dominates. In compton scattering, the incident gamma ray photon is deflected through an angle Θ with respect to its original direction. The photon transfers a portion of its energy to the recoil electron. The energy transferred to the recoil electron can vary from zero to a large fraction (maximum E) of the incident gamma ray energy, because all angles of scattering are possible. The size of the scintillation crystal changes the ratio between photopeak and Compton continuum. For an infinitely large spherical detector centered around a source no photons would be able to escape and only a photopeak would be seen on the spectrum. For very small detectors the chance for a photon to leave after Compton scattering is high and the Compton continuum would be large compared to the photopeak.
Compton Edge. The Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. This feature is due to photons that undergo Compton scattering with a scattering angle of 180° and then escape the detector. When a gamma ray scatters off the detector and escapes, only a fraction of its initial energy can be deposited in the sensitive layer of the detector. It depends on the scattering angle of the photon, how much energy will be deposited in the detector. This leads to a spectrum of energies. The Compton edge energy corresponds to full backscattered photon. The counts between Compton edge and the photopeaks are caused by multiple Compton scattering events, where scattered gamma photon exits the sensitive material.
Sometimes the structure of gamma rays spectrum has the following secondary features:
Compton edge of 60Co on gamma spectrometer Na(Tl).
Backscatter Peak. A Compton backscatter peak is found when γ-rays enter the material around the detector and are scattered back into the detector. Gamma photon can interact by Compton mechanism in shield or surrounding materials and is backscattered out of a material into sensitive volume. Maximum photon energy in this interaction is ~200 keV regardless of initial photon energy. This results in a secondary peak at -200 keV.
X-ray Peaks. When the gamma rays undergoes photoelectric effect in surrounding materials (for example lead shield) the outgoing X-ray can be captured again by the detector. This gives an characteristic X-ray peak with an energy depending on the material it came from. In case of lead the characteristic X-ray energies is in the 72-84 keV range. Photoelectric absorption by K-shell electron in lead of shielding, resulting in a K-shell vacancy. The K -> L transition for lead = 72 keV. If this characteristic x-ray is absorbed in the crystal then a secondary peak at 72 keV is observed.
Coincidence Peak. Simultaneous absorption of two gamma photons in crystal, resulting in a peak higher in energy than the photopeak. The gamma photons may result from serial isomeric transitions (e.g. 2 keV plus 1332.5 keV in case of cobalt-60) or simultaneous decays in separate radionuclides (e.g. 2 x 1173.2 keV in case of cobalt-60).
Annihilation Peak. For high energy gamma rays (greater than 1.02 MeV), electron-positron pair production is a possible interaction. But a positron may then annihilate in the detector or in the surrounding material. If both 0.51 MeV annihilation photons are absorbed in the crystal, a secondary peak 1.02 MeV below the photopeak can be found. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Gamma Spectroscopy
We hope, this article, Gamma Ray Spectrum – Structure – Features, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
A gamma ray spectrometer (GRS), or spectroscope, is a sophisticated device for measuring the energy distribution of gamma radiation. Germanium-based semiconductor detectors are most commonly used for gamma spectroscopy. Radiation Dosimetry
Gamma Spectroscopy
Source: wikipedia.org License: Public Domain
In general, gamma spectroscopy is the study of the energy spectra of gamma ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Spectroscopes, or spectrometers, are sophisticated devices designed to measure the spectral power distribution of a source. The incident radiation generates a signal that allows to determine the energy of the incident particle.
Most radioactive sources produce gamma rays, which are of various energies and intensities. Gamma rays frequently accompany the emission of alpha and beta radiation. When these emissions are detected and analyzed with a spectroscopy system, a gamma-ray energy spectrum can be produced. Gamma rays from radioactive decay are in the energy range from a few keV to ~8 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. As was written, they are produced by the decay of nuclei as they transition from a high energy state to a lower state. A detailed analysis of this spectrum is typically used to determine the identity and quantity of gamma emitters present in a sample, and is a vital tool in radiometric assay. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source.
Gamma Ray Spectrometer – Gamma Ray Spectroscope
As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. In the previous articles, we described the gamma spectroscopy using scintillation detector, which consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The advantages of a scintillation counter are its efficiency (large size and high density) and the high precision and counting rates that are possible. Due to the high atomic number of iodine, a large number of all interactions will result in complete absorption of gamma-ray energy, so the photo fraction will be high.
HPGe detector with LN2 cryostat Source: canberra.com
But if a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
Gamma Spectrum Structure – The Cobalt-60 Spectrum
Analysis of gamma spectra is very interesting, since it has a structure and workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.
Cobalt-60 is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.
As can be seen from the figure, there are two gamma ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
Figure: Caption: Comparison of NaI(Tl) and HPGe spectra for cobalt-60. Source: Radioisotopes and Radiation Methodology I, II. Soo Hyun Byun, Lecture Notes. McMaster University, Canada.
General structure of gamma rays spectrum usually has the following principle features:
Photopeaks are full energy peaks, that are created, when γ-ray transfers its full energy (hω −Ebinding) into the sensitive material (scintillation crystal or germanium crystal). Than a single delta function should be observed in the spectrum.The cobalt-60 spectrum contains two photopeaks, one at 1173.2 keV and another at 1332.5 keV.
Compton Continuum. In the crystal, a gamma ray undergoes a number of interactions, but for intermediate energies compton scattering dominates. In compton scattering, the incident gamma ray photon is deflected through an angle Θ with respect to its original direction. The photon transfers a portion of its energy to the recoil electron. The energy transferred to the recoil electron can vary from zero to a large fraction (maximum E) of the incident gamma ray energy, because all angles of scattering are possible. The size of the scintillation crystal changes the ratio between photopeak and Compton continuum. For an infinitely large spherical detector centered around a source no photons would be able to escape and only a photopeak would be seen on the spectrum. For very small detectors the chance for a photon to leave after Compton scattering is high and the Compton continuum would be large compared to the photopeak.
Compton Edge. The Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. This feature is due to photons that undergo Compton scattering with a scattering angle of 180° and then escape the detector. When a gamma ray scatters off the detector and escapes, only a fraction of its initial energy can be deposited in the sensitive layer of the detector. It depends on the scattering angle of the photon, how much energy will be deposited in the detector. This leads to a spectrum of energies. The Compton edge energy corresponds to full backscattered photon. The counts between Compton edge and the photopeaks are caused by multiple Compton scattering events, where scattered gamma photon exits the sensitive material.
Sometimes the structure of gamma rays spectrum has the following secondary features:
Compton edge of 60Co on gamma spectrometer Na(Tl).
Backscatter Peak. A Compton backscatter peak is found when γ-rays enter the material around the detector and are scattered back into the detector. Gamma photon can interact by Compton mechanism in shield or surrounding materials and is backscattered out of a material into sensitive volume. Maximum photon energy in this interaction is ~200 keV regardless of initial photon energy. This results in a secondary peak at -200 keV.
X-ray Peaks. When the gamma rays undergoes photoelectric effect in surrounding materials (for example lead shield) the outgoing X-ray can be captured again by the detector. This gives an characteristic X-ray peak with an energy depending on the material it came from. In case of lead the characteristic X-ray energies is in the 72-84 keV range. Photoelectric absorption by K-shell electron in lead of shielding, resulting in a K-shell vacancy. The K -> L transition for lead = 72 keV. If this characteristic x-ray is absorbed in the crystal then a secondary peak at 72 keV is observed.
Coincidence Peak. Simultaneous absorption of two gamma photons in crystal, resulting in a peak higher in energy than the photopeak. The gamma photons may result from serial isomeric transitions (e.g. 2 keV plus 1332.5 keV in case of cobalt-60) or simultaneous decays in separate radionuclides (e.g. 2 x 1173.2 keV in case of cobalt-60).
Annihilation Peak. For high energy gamma rays (greater than 1.02 MeV), electron-positron pair production is a possible interaction. But a positron may then annihilate in the detector or in the surrounding material. If both 0.51 MeV annihilation photons are absorbed in the crystal, a secondary peak 1.02 MeV below the photopeak can be found. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Gamma Spectroscopy
We hope, this article, Gamma Ray Spectrometer – Gamma Ray Spectroscope, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
In general, spectroscopy is the science of studying the interaction between matter and radiated energy while spectrometry is the method used to acquire a quantitative measurement of the spectrum. Radiation Dosimetry
In general, spectroscopy is the science of studying the interaction between matter and radiated energy while spectrometry is the method used to acquire a quantitative measurement of the spectrum. Spectroscopy (scopy means observation) does not generate any results. It is the theoretical approach of science. Spectrometry (metry means measurement) is the practical application where the results are generated. It is the measurement of the intensity of the radiation using an electronic device. Often these terms are used interchangeably, but every spectrometry is not spectroscopy (e.g. mass spectrometry vs. mass spectroscopy).
Distinction between X-rays and gamma rays
The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. Yes, X-rays are being said to have lower energies, but this is not the rule and the main difference. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
Gamma Spectroscopy
Source: wikipedia.org License: Public Domain
In general, gamma spectroscopy is the study of the energy spectra of gamma ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Spectroscopes, or spectrometers, are sophisticated devices designed to measure the spectral power distribution of a source. The incident radiation generates a signal that allows to determine the energy of the incident particle.
Most radioactive sources produce gamma rays, which are of various energies and intensities. Gamma rays frequently accompany the emission of alpha and beta radiation. When these emissions are detected and analyzed with a spectroscopy system, a gamma-ray energy spectrum can be produced. Gamma rays from radioactive decay are in the energy range from a few keV to ~8 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. As was written, they are produced by the decay of nuclei as they transition from a high energy state to a lower state. A detailed analysis of this spectrum is typically used to determine the identity and quantity of gamma emitters present in a sample, and is a vital tool in radiometric assay. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source.
X-Ray Spectroscopy
X-ray spectroscopy is a general term for several spectroscopic techniques for characterization of materials by using x-ray excitation. When an electron from the inner shell of an atom is excited by the energy of a photon, it moves to a higher energy level. Since the process leaves a vacancy in the electron energy level from which the electron came, the outer electrons of the atom cascade down to fill the lower atomic levels, and one or more characteristic X-rays are usually emitted. As a result, sharp intensity peaks appear in the spectrum at wavelengths that are a characteristic of the material from which the anode target is made. The frequencies of the characteristic X-rays can be predicted from the Bohr model. Analysis of the X-ray emission spectrum produces qualitative results about the elemental composition of the specimen.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Gamma Spectroscopy
We hope, this article, Spectroscopy – Spectrometry – Difference – Distinction, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus. Gamma Rays – X-Rays – Difference – Distinction
The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. Both are high-energy photons (electromagnetic radiation) with very short wavelengths and thus very high frequency. Yes, X-rays are being said to have lower energies, but this is not the rule and the main difference. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
Source: Tour of the Electromagnetic Spectrum www.nasa.gov
Characteristics of X-rays
Key features of X-rays are summarized in following few points:
X-rays are high-energy photons (about 100 – 1 000 times as much energy as the visible photons), the same photons as the photons forming the visible range of the electromagnetic spectrum – light.
X-rays are usually described by their maximum energy, which is determined by the voltage between the electrodes. It may range from about 20 kV up to 300 kV. Radiation with low voltage is called “soft” – and radiation with high voltage is called “hard”.
Photons have no mass and no electrical charge, therefore they cannot directly ionize matter, neither X-rays.
X-rays ionize matter via indirect ionization.
Although a large number of possible interactions are known, there are three key interaction mechanisms with matter.
X-rays travel at the speed of light and they can travel hundreds of meters in air before spending their energy.
Since the hard X-rays are very penetrating matter, it must be shielded by very dense materials, such as lead or uranium.
The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
For X-rays generated by X-ray tube, there are two different types of X-rays spectra:
Key features of gamma rays are summarized in following few points:
Gamma rays are high-energy photons (about 10 000 times as much energy as the visible photons), the same photons as the photons forming the visible range of the electromagnetic spectrum – light.
Photons have no mass and no electrical charge, therefore they cannot directly ionize matter, neither gamma rays.
Despite this fact, gamma rays ionize matter via indirect ionization.
Although a large number of possible interactions are known, there are three key interaction mechanisms with matter.
Gamma rays travel at the speed of light and they can travel thousands of meters in air before spending their energy.
Since the gamma radiation is very penetrating matter, it must be shielded by very dense materials, such as lead or uranium.
The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Gamma Rays
We hope, this article, Gamma Ray – X-Ray – Difference – Distinction, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
X-ray spectroscopy is a general term for several spectroscopic techniques for characterization of materials by using x-ray excitation.
In general, spectroscopy is the science of studying the interaction between matter and radiated energy while spectrometry is the method used to acquire a quantitative measurement of the spectrum. Spectroscopy (scopy means observation) does not generate any results. It is the theoretical approach of science. Spectrometry (metry means measurement) is the practical application where the results are generated. It is the measurement of the intensity of the radiation using an electronic device. Often these terms are used interchangeably, but every spectrometry is not spectroscopy (e.g. mass spectrometry vs. mass spectroscopy).
Distinction between X-rays and gamma rays
The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. Yes, X-rays are being said to have lower energies, but this is not the rule and the main difference. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
X-Ray Spectroscopy
X-ray spectroscopy is a general term for several spectroscopic techniques for characterization of materials by using x-ray excitation. When an electron from the inner shell of an atom is excited by the energy of a photon, it moves to a higher energy level. Since the process leaves a vacancy in the electron energy level from which the electron came, the outer electrons of the atom cascade down to fill the lower atomic levels, and one or more characteristic X-rays are usually emitted. As a result, sharp intensity peaks appear in the spectrum at wavelengths that are a characteristic of the material from which the anode target is made. The frequencies of the characteristic X-rays can be predicted from the Bohr model. Analysis of the X-ray emission spectrum produces qualitative results about the elemental composition of the specimen.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Spectroscopy
We hope, this article, X-Ray Spectroscopy, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
The scintillation based spectrometers have the advantage of availability in large size and high density which can result in high interaction probabilities for gamma rays. Radiation Dosimetry
Source: wikipedia.org License: Public Domain
In general, gamma spectroscopy is the study of the energy spectra of gamma ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Spectroscopes, or spectrometers, are sophisticated devices designed to measure the spectral power distribution of a source. The incident radiation generates a signal that allows to determine the energy of the incident particle.
Most radioactive sources produce gamma rays, which are of various energies and intensities. Gamma rays frequently accompany the emission of alpha and beta radiation. When these emissions are detected and analyzed with a spectroscopy system, a gamma-ray energy spectrum can be produced. Gamma rays from radioactive decay are in the energy range from a few keV to ~8 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. As was written, they are produced by the decay of nuclei as they transition from a high energy state to a lower state. A detailed analysis of this spectrum is typically used to determine the identity and quantity of gamma emitters present in a sample, and is a vital tool in radiometric assay. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source.
Gamma Ray Spectrometer – Gamma Ray Spectroscope
The gamma spectroscope consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The advantages of a scintillation counter are its efficiency and the high precision and counting rates that are possible. These latter attributes are a consequence of the extremely short duration of the light flashes, from about 10-9 (organic scintillators) to 10-6 (inorganic scintillators) seconds. The intensity of the flashes and the amplitude of the output voltage pulse are proportional to the energy of the radiation. The pulses are counted and sorted by their height, producing a x-y plot of scintillator flash brightness vs number of the flashes, which approximates the energy spectrum of the incident radiation, with some additional artifacts. A monoenergetic gamma ray produces a photopeak at its energy. The scintillation detector also shows response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
The scintillation based spectrometers have the advantage of availability in large size and high density which can result in high interaction probabilities for gamma rays. On the other hand, their energy resolution scintillators is poor, when compared to high purity germanium detectors. Germanium detectors are better choice for separation of closely spaced gamma energies. The typical energy resolution of a good germanium system is few tenths of a percent compared to 5-10 % for sodium iodide.
Gamma Spectrum Structure – The Cobalt-60 Spectrum
Analysis of gamma spectra is very interesting, since it has a structure and workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.
Cobalt-60 is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.
As can be seen from the figure, there are two gamma ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
Figure: Caption: Comparison of NaI(Tl) and HPGe spectra for cobalt-60. Source: Radioisotopes and Radiation Methodology I, II. Soo Hyun Byun, Lecture Notes. McMaster University, Canada.
General structure of gamma rays spectrum usually has the following principle features:
Photopeaks are full energy peaks, that are created, when γ-ray transfers its full energy (hω −Ebinding) into the sensitive material (scintillation crystal or germanium crystal). Than a single delta function should be observed in the spectrum.The cobalt-60 spectrum contains two photopeaks, one at 1173.2 keV and another at 1332.5 keV.
Compton Continuum. In the crystal, a gamma ray undergoes a number of interactions, but for intermediate energies compton scattering dominates. In compton scattering, the incident gamma ray photon is deflected through an angle Θ with respect to its original direction. The photon transfers a portion of its energy to the recoil electron. The energy transferred to the recoil electron can vary from zero to a large fraction (maximum E) of the incident gamma ray energy, because all angles of scattering are possible. The size of the scintillation crystal changes the ratio between photopeak and Compton continuum. For an infinitely large spherical detector centered around a source no photons would be able to escape and only a photopeak would be seen on the spectrum. For very small detectors the chance for a photon to leave after Compton scattering is high and the Compton continuum would be large compared to the photopeak.
Compton Edge. The Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. This feature is due to photons that undergo Compton scattering with a scattering angle of 180° and then escape the detector. When a gamma ray scatters off the detector and escapes, only a fraction of its initial energy can be deposited in the sensitive layer of the detector. It depends on the scattering angle of the photon, how much energy will be deposited in the detector. This leads to a spectrum of energies. The Compton edge energy corresponds to full backscattered photon. The counts between Compton edge and the photopeaks are caused by multiple Compton scattering events, where scattered gamma photon exits the sensitive material.
Sometimes the structure of gamma rays spectrum has the following secondary features:
Compton edge of 60Co on gamma spectrometer Na(Tl).
Backscatter Peak. A Compton backscatter peak is found when γ-rays enter the material around the detector and are scattered back into the detector. Gamma photon can interact by Compton mechanism in shield or surrounding materials and is backscattered out of a material into sensitive volume. Maximum photon energy in this interaction is ~200 keV regardless of initial photon energy. This results in a secondary peak at -200 keV.
X-ray Peaks. When the gamma rays undergoes photoelectric effect in surrounding materials (for example lead shield) the outgoing X-ray can be captured again by the detector. This gives an characteristic X-ray peak with an energy depending on the material it came from. In case of lead the characteristic X-ray energies is in the 72-84 keV range. Photoelectric absorption by K-shell electron in lead of shielding, resulting in a K-shell vacancy. The K -> L transition for lead = 72 keV. If this characteristic x-ray is absorbed in the crystal then a secondary peak at 72 keV is observed.
Coincidence Peak. Simultaneous absorption of two gamma photons in crystal, resulting in a peak higher in energy than the photopeak. The gamma photons may result from serial isomeric transitions (e.g. 2 keV plus 1332.5 keV in case of cobalt-60) or simultaneous decays in separate radionuclides (e.g. 2 x 1173.2 keV in case of cobalt-60).
Annihilation Peak. For high energy gamma rays (greater than 1.02 MeV), electron-positron pair production is a possible interaction. But a positron may then annihilate in the detector or in the surrounding material. If both 0.51 MeV annihilation photons are absorbed in the crystal, a secondary peak 1.02 MeV below the photopeak can be found. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Gamma Spectroscopy
We hope, this article, Spectroscopy using Scintillation Counter, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
HPGe detector and germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. Radiation Dosimetry
As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. In the previous articles, we described the gamma spectroscopy using scintillation detector, which consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The advantages of a scintillation counter are its efficiency (large size and high density) and the high precision and counting rates that are possible. Due to the high atomic number of iodine, a large number of all interactions will result in complete absorption of gamma-ray energy, so the photo fraction will be high.
HPGe detector with LN2 cryostat Source: canberra.com
But if a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
Gamma Spectrum Structure – The Cobalt-60 Spectrum
Analysis of gamma spectra is very interesting, since it has a structure and workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.
Cobalt-60 is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.
As can be seen from the figure, there are two gamma ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
Figure: Caption: Comparison of NaI(Tl) and HPGe spectra for cobalt-60. Source: Radioisotopes and Radiation Methodology I, II. Soo Hyun Byun, Lecture Notes. McMaster University, Canada.
General structure of gamma rays spectrum usually has the following principle features:
Photopeaks are full energy peaks, that are created, when γ-ray transfers its full energy (hω −Ebinding) into the sensitive material (scintillation crystal or germanium crystal). Than a single delta function should be observed in the spectrum.The cobalt-60 spectrum contains two photopeaks, one at 1173.2 keV and another at 1332.5 keV.
Compton Continuum. In the crystal, a gamma ray undergoes a number of interactions, but for intermediate energies compton scattering dominates. In compton scattering, the incident gamma ray photon is deflected through an angle Θ with respect to its original direction. The photon transfers a portion of its energy to the recoil electron. The energy transferred to the recoil electron can vary from zero to a large fraction (maximum E) of the incident gamma ray energy, because all angles of scattering are possible. The size of the scintillation crystal changes the ratio between photopeak and Compton continuum. For an infinitely large spherical detector centered around a source no photons would be able to escape and only a photopeak would be seen on the spectrum. For very small detectors the chance for a photon to leave after Compton scattering is high and the Compton continuum would be large compared to the photopeak.
Compton Edge. The Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. This feature is due to photons that undergo Compton scattering with a scattering angle of 180° and then escape the detector. When a gamma ray scatters off the detector and escapes, only a fraction of its initial energy can be deposited in the sensitive layer of the detector. It depends on the scattering angle of the photon, how much energy will be deposited in the detector. This leads to a spectrum of energies. The Compton edge energy corresponds to full backscattered photon. The counts between Compton edge and the photopeaks are caused by multiple Compton scattering events, where scattered gamma photon exits the sensitive material.
Sometimes the structure of gamma rays spectrum has the following secondary features:
Compton edge of 60Co on gamma spectrometer Na(Tl).
Backscatter Peak. A Compton backscatter peak is found when γ-rays enter the material around the detector and are scattered back into the detector. Gamma photon can interact by Compton mechanism in shield or surrounding materials and is backscattered out of a material into sensitive volume. Maximum photon energy in this interaction is ~200 keV regardless of initial photon energy. This results in a secondary peak at -200 keV.
X-ray Peaks. When the gamma rays undergoes photoelectric effect in surrounding materials (for example lead shield) the outgoing X-ray can be captured again by the detector. This gives an characteristic X-ray peak with an energy depending on the material it came from. In case of lead the characteristic X-ray energies is in the 72-84 keV range. Photoelectric absorption by K-shell electron in lead of shielding, resulting in a K-shell vacancy. The K -> L transition for lead = 72 keV. If this characteristic x-ray is absorbed in the crystal then a secondary peak at 72 keV is observed.
Coincidence Peak. Simultaneous absorption of two gamma photons in crystal, resulting in a peak higher in energy than the photopeak. The gamma photons may result from serial isomeric transitions (e.g. 2 keV plus 1332.5 keV in case of cobalt-60) or simultaneous decays in separate radionuclides (e.g. 2 x 1173.2 keV in case of cobalt-60).
Annihilation Peak. For high energy gamma rays (greater than 1.02 MeV), electron-positron pair production is a possible interaction. But a positron may then annihilate in the detector or in the surrounding material. If both 0.51 MeV annihilation photons are absorbed in the crystal, a secondary peak 1.02 MeV below the photopeak can be found. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Gamma Spectroscopy
We hope, this article, Spectroscopy using Germanium Semiconductor – HPGe, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
A semiconductor detector is a radiation detector which is based on a semiconductor, such as silicon or germanium to measure the effect of incident charged particles or photons. Radiation Dosimetry
A semiconductor detector is a radiation detector which is based on a semiconductor, such as silicon or germanium to measure the effect of incident charged particles or photons. In general, semiconductors are materials, inorganic or organic, which have the ability to control their conduction depending on chemical structure, temperature, illumination, and presence of dopants. The name semiconductor comes from the fact that these materials have an electrical conductivity between that of a metal, like copper, gold, etc. and an insulator, such as glass. They have an energy gap less than 4eV (about 1eV). In solid-state physics, this energy gap or band gap is an energy range between valence band and conduction band where electron states are forbidden. In contrast to conductors, electrons in a semiconductor must obtain energy (e.g. from ionizing radiation) to cross the band gap and to reach the conduction band.
Semiconductor detectors are very similar in operation as photovoltaic panels that generate electric current. In a similar way a current can be induced by ionizing radiation. As ionizing radiation enters the semiconductor, it interacts with the semiconductor material. It may excite an electron out of its energy level and consequently leave a hole. This process is known as electron–hole pair generation. In semiconductor detectors, the fundamental information carriers are these electron-hole pairs, which are produced along the path taken by the charged particle (primary or secondary) through the detector. By collecting electron-hole pairs, the detection signal is formed and recorded.
Semiconductor detectors are widely used in radiation protection, assay of radioactive materials and physics research because they have some unique features, can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation. These detectors are employed to measure the energy of the radiation and for identification of particles. Of the available semiconductor materials, silicon is mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors while germanium is widely used for gamma ray spectroscopy. A large, clean and almost perfect semiconductor is ideal as a counter for radioactivity. However, it is difficult to make large crystals with sufficient purity. The semiconductor detectors have, therefore, low efficiency, but they do give a very precise measure of the energy. Semiconductor detectors, especially germanium based detectors, are most commonly used where a very good energy resolution is required. In order to achieve maximum efficiency the detectors must operate at the very low temperatures of liquid nitrogen (-196°C). Therefore, the drawback is that semiconductor detectors are much more expensive than other detectors and require sophisticated cooling to reduce leakage currents (noise).
Types of Semiconductors
Types of Semiconductors
There are many semiconductors in nature and others synthesized in laboratories; however, the best known are silicon (Si) and germanium (Ge). Types of semiconductors:
Silicon. Silicon is a chemical element with atomic number14 which means there are 14 protons and 14 electrons in the atomic structure. The chemical symbol for Silicon is Si. Silicon is a hard and brittle crystalline solid with a blue-grey metallic lustre, it is a tetravalent metalloid and semiconductor. Silicon is mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors. The large band-gap energy (Egap= 1.12 eV) allows us to operate the detector at room temperature, but cooling is prefered to reduce noise. Silicon based detectors are very important in high-energy physics. Since silicon-based detectors are very good for tracking charged particles, they constitute a substantial part of detection system at the LHC in CERN.
Germanium. Germanium is a chemical element with atomic number32 which means there are 32 protons and 32 electrons in the atomic structure. The chemical symbol for Germanium is Ge. Germanium is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. Pure germanium is a semiconductor with an appearance similar to elemental silicon. Germanium is widely used for gamma ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Germanium is more used than silicon for radiation detection because the average energy necessary to create an electron-hole pair is 3.6 eV for silicon and 2.9 eV for germanium, which provides the latter a better resolution in energy. On the other hand, germanium has a small band gap energy (Egap = 0.67 eV), which requires to operate the detector at cryogenic temperatures.
Diamond. Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamonds are also very good electrical insulators which strangely is both useful and problematic for electrical devices. Diamond is a wide-band gap semiconductor (Egap= 5.47 eV) with high potential as an electronic device material in many devices. Diamond detectors have many similarities with silicon detectors, but are expected to offer significant advantages, in particular a high radiation hardness and very low drift currents.
CdTe and CdZnTe. Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and gamma ray detection. The high atomic number and the high density of these materials mean they can effectively attenuate X-rays and gamma rays with energies of greater than 20 keV that traditional silicon-based sensors are unable to detect. This significantly increases their quantum efficiency in comparison with silicon-based . The large band-gap energy (Egap= 1.44 eV) allows us to operate the detector at room temperature. On the other hand, a considerable amount of charge loss in these detectors produces a reduced energy resolution.
Principle of Operation of Semiconductor Detectors
The operation of semiconductor detectors is summarized in the following points:
Ionizing radiation enters the sensitive volume of the detector and interacts with the semiconductor material.
Particle passing through the detector ionizes the atoms of semiconductor, producing the electron-hole pairs. The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band.
Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit,
This pulse carries information about the energy of the original incident radiation. The number of such pulses per unit time also gives information about the intensity of the radiation.
The energy required to produce electron-hole-pairs is very low compared to the energy required to produce paired ions in a gaseous ionization detector. In semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.
Silicon-based Semiconductor Detectors
Silicon-based semiconductor detectors are mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors while germanium is widely used for gamma ray spectroscopy. A large, clean and almost perfect semiconductor is ideal as a counter for radioactivity. However, it is difficult to make large crystals with sufficient purity. The semiconductor detectors have, therefore, low efficiency, but they do give a very precise measure of the energy. Detectors based on silicon have sufficiently low noise even by room temperature. This is caused by the large band gap of silicon (Egap= 1.12 eV), which allows us to operate the detector at room temperature, but cooling is prefered to reduce noise. The drawback is that silicon detectors are much more expensive than cloud chambers or wire chambers and require sophisticated cooling to reduce leakage currents (noise). They also suffer degradation over time from radiation, however this can be greatly reduced thanks to the Lazarus effect.
Application of Silicon Detectors
Since silicon-based detectors are very good for tracking charged particles, they constitute a substantial part of detection system at the LHC in CERN. Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to turn them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents that can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can yield an accurate picture of what paths particles take. For example, the Inner Tracking System (ITS) of a Large Ion Collider Experiment (ALICE) contains three layers of silicon-based detectors:
Silicon Pixel Detector (SPD)
Silicon Drift Detector (SDD)
Silicon Strip Detector (SSD)
Silicon Strip Detectors
Silicon-based detectors are very good for tracking charged particles. A silicon strip detector is an arrangement of strip like shaped implants acting as charge collecting electrodes.
Silicon strip detectors 5 x 5 cm2 in area are quite common and are used in series (just like planes of MWPCs) to determine charged-particle trajectories to position-accuracies of the order of several μm in the transverse direction. Placed on a low doped fully depleted silicon wafer these implants form a one-dimensional array of diodes. By connecting each of the metalized strips to a charge sensitive amplifier a position sensitive detector is built. Two dimensional position measurements can be achieved by applying an additional strip like doping on the wafer backside by use of a double sided technology. Such devices can be used to measure small impact parameters and thereby determine whether some charged particle originated from a primary collision or was the decay product of a primary particle that traveled a small distance from the original interaction, and then decayed.
Silicon strip detectors constitute a substantial part of detection system at the LHC in CERN. Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to turn them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents that can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can yield an accurate picture of what paths particles take.
For example, the Inner Tracking System (ITS) of a Large Ion Collider Experiment (ALICE) contains three layers of silicon-based detectors:
Silicon Pixel Detector (SPD)
Silicon Drift Detector (SDD)
Silicon Strip Detector (SSD)
Delta E – E Detector – Telescope
Example of histogram from ΔE-E detector. Each hyperbola-like curve represents different particle in the beam.
In experimental physics, ΔE-E detectors, known as telescopes, are powerful devices for charged particles identification. In order to provide charged-particle identification, telescopes consisting of pairs of thin and thick surface-barrier detectors can be used. These detectors must be positioned in series. The velocity is deduced from the stopping power measured in the thin detectors (ΔE detectors). There is a strong correlation between the energy deposited in each detector. This correlation depends on the mass (A), the charge (Z) and the kinetic energy (E) of each particle. The mass is deduced from the range or from the total kinetic energy loss in the thicker detector (E detector).
Telescopes can be composed of several detectors (ionization chambers, silicon detectors and scintillators for instance) stacked in order to slow down incident charged particles, the first detector being the thinnest and the last one the thickest. CsI scintillation counters can be, for example used as final E counters. As an example of telescope, an assembly based on two front ΔE silicon detectors (10 or 30 µm) and an E silicon counter 1500 µm thick may be used for detection of high-energy charged particles.
Germanium-based Semiconductor Detectors
Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. A large, clean and almost perfect germanium semiconductor is ideal as a counter for radioactivity. However, it is difficult and expensive to make large crystals with sufficient purity. While silicon-based detectors cannot be thicker than a few millimeters, germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV.
On the other hand, in order to achieve maximum efficiency the detectors must operate at the very low temperatures of liquid nitrogen (-196°C), because at room temperatures the noise caused by thermal excitation is very high.
Since germanium detectors produce the highest resolution commonly available today, they are used to measure radiation in a variety of applications including personnel and environmental monitoring for radioactive contamination, medical applications, radiometric assay, nuclear security and nuclear plant safety.
Application of Germanium Detectors – Gamma Spectroscopy
As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. In the previous articles, we described the gamma spectroscopy using scintillation detector, which consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The advantages of a scintillation counter are its efficiency (large size and high density) and the high precision and counting rates that are possible. Due to the high atomic number of iodine, a large number of all interactions will result in complete absorption of gamma-ray energy, so the photo fraction will be high.
HPGe detector with LN2 cryostat Source: canberra.com
But if a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Detection
We hope, this article, Semiconductor Detector, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
A photodiode is a semiconductor device that converts light into an electrical current. This is a semiconductor device which consists of a thin layer of silicon in which the light is absorbed after which free charge carriers (electrons and holes) are created. Radiation Dosimetry
A scintillation detector or scintillation counter is obtained when a scintillator is coupled to an electronic light sensor such as:
a photomultiplier tube (PMT),
a charge-coupled device (CCD) camera,
photodiode
All these devices may be used in scintillation counters and all convert the light to an electrical signal and contain electronics to process this signal. A photodiode is a semiconductor device that converts light into an electrical current. This is a semiconductor device which consists of a thin layer of silicon in which the light is absorbed after which free charge carriers (electrons and holes) are created. A conventional photodiode most often refers to a PIN diode. PIN means that the p and the n doped sides are separated by a depleted i-region. Electrons and holes are collected at the anode and cathode of the diode. This results in a photocurrent that is the output of the diode. The charge is however not amplified making the output signal amplitude small. This makes the photodiode sensitive to electronic noise. On the other hand, the quantum efficiency for the photodiode is high (60-80%) compared to the PMT (20-30%) which gives a higher energy resolution.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Scintillation Counters
We hope, this article, Photodiode – Scintillation Counter, helps you. If so, give us a like in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about radiation and dosimeters.
Gamma spectroscopy is the study of the energy spectra of gamma ray sources, such as in the nuclear industry and geochemical investigation. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source. Radiation Dosimetry
In general, spectroscopy is the science of studying the interaction between matter and radiated energy while spectrometry is the method used to acquire a quantitative measurement of the spectrum. Spectroscopy (scopy means observation) does not generate any results. It is the theoretical approach of science. Spectrometry (metry means measurement) is the practical application where the results are generated. It is the measurement of the intensity of the radiation using an electronic device. Often these terms are used interchangeably, but every spectrometry is not spectroscopy (e.g. mass spectrometry vs. mass spectroscopy).
Distinction between X-rays and gamma rays
The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. Yes, X-rays are being said to have lower energies, but this is not the rule and the main difference. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
Gamma Spectroscopy
Source: wikipedia.org License: Public Domain
In general, gamma spectroscopy is the study of the energy spectra of gamma ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Spectroscopes, or spectrometers, are sophisticated devices designed to measure the spectral power distribution of a source. The incident radiation generates a signal that allows to determine the energy of the incident particle.
Most radioactive sources produce gamma rays, which are of various energies and intensities. Gamma rays frequently accompany the emission of alpha and beta radiation. When these emissions are detected and analyzed with a spectroscopy system, a gamma-ray energy spectrum can be produced. Gamma rays from radioactive decay are in the energy range from a few keV to ~8 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. As was written, they are produced by the decay of nuclei as they transition from a high energy state to a lower state. A detailed analysis of this spectrum is typically used to determine the identity and quantity of gamma emitters present in a sample, and is a vital tool in radiometric assay. The gamma spectrum is characteristic of the gamma-emitting nuclides contained in the source.
X-Ray Spectroscopy
X-ray spectroscopy is a general term for several spectroscopic techniques for characterization of materials by using x-ray excitation. When an electron from the inner shell of an atom is excited by the energy of a photon, it moves to a higher energy level. Since the process leaves a vacancy in the electron energy level from which the electron came, the outer electrons of the atom cascade down to fill the lower atomic levels, and one or more characteristic X-rays are usually emitted. As a result, sharp intensity peaks appear in the spectrum at wavelengths that are a characteristic of the material from which the anode target is made. The frequencies of the characteristic X-rays can be predicted from the Bohr model. Analysis of the X-ray emission spectrum produces qualitative results about the elemental composition of the specimen.
Gamma Ray Spectrometer – Gamma Ray Spectroscope
As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. In the previous articles, we described the gamma spectroscopy using scintillation detector, which consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The advantages of a scintillation counter are its efficiency (large size and high density) and the high precision and counting rates that are possible. Due to the high atomic number of iodine, a large number of all interactions will result in complete absorption of gamma-ray energy, so the photo fraction will be high.
HPGe detector with LN2 cryostat Source: canberra.com
But if a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
Gamma Spectrum Structure – The Cobalt-60 Spectrum
Analysis of gamma spectra is very interesting, since it has a structure and workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.
Cobalt-60 is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.
As can be seen from the figure, there are two gamma ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
Figure: Caption: Comparison of NaI(Tl) and HPGe spectra for cobalt-60. Source: Radioisotopes and Radiation Methodology I, II. Soo Hyun Byun, Lecture Notes. McMaster University, Canada.
General structure of gamma rays spectrum usually has the following principle features:
Photopeaks are full energy peaks, that are created, when γ-ray transfers its full energy (hω −Ebinding) into the sensitive material (scintillation crystal or germanium crystal). Than a single delta function should be observed in the spectrum.The cobalt-60 spectrum contains two photopeaks, one at 1173.2 keV and another at 1332.5 keV.
Compton Continuum. In the crystal, a gamma ray undergoes a number of interactions, but for intermediate energies compton scattering dominates. In compton scattering, the incident gamma ray photon is deflected through an angle Θ with respect to its original direction. The photon transfers a portion of its energy to the recoil electron. The energy transferred to the recoil electron can vary from zero to a large fraction (maximum E) of the incident gamma ray energy, because all angles of scattering are possible. The size of the scintillation crystal changes the ratio between photopeak and Compton continuum. For an infinitely large spherical detector centered around a source no photons would be able to escape and only a photopeak would be seen on the spectrum. For very small detectors the chance for a photon to leave after Compton scattering is high and the Compton continuum would be large compared to the photopeak.
Compton Edge. The Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. This feature is due to photons that undergo Compton scattering with a scattering angle of 180° and then escape the detector. When a gamma ray scatters off the detector and escapes, only a fraction of its initial energy can be deposited in the sensitive layer of the detector. It depends on the scattering angle of the photon, how much energy will be deposited in the detector. This leads to a spectrum of energies. The Compton edge energy corresponds to full backscattered photon. The counts between Compton edge and the photopeaks are caused by multiple Compton scattering events, where scattered gamma photon exits the sensitive material.
Sometimes the structure of gamma rays spectrum has the following secondary features:
Compton edge of 60Co on gamma spectrometer Na(Tl).
Backscatter Peak. A Compton backscatter peak is found when γ-rays enter the material around the detector and are scattered back into the detector. Gamma photon can interact by Compton mechanism in shield or surrounding materials and is backscattered out of a material into sensitive volume. Maximum photon energy in this interaction is ~200 keV regardless of initial photon energy. This results in a secondary peak at -200 keV.
X-ray Peaks. When the gamma rays undergoes photoelectric effect in surrounding materials (for example lead shield) the outgoing X-ray can be captured again by the detector. This gives an characteristic X-ray peak with an energy depending on the material it came from. In case of lead the characteristic X-ray energies is in the 72-84 keV range. Photoelectric absorption by K-shell electron in lead of shielding, resulting in a K-shell vacancy. The K -> L transition for lead = 72 keV. If this characteristic x-ray is absorbed in the crystal then a secondary peak at 72 keV is observed.
Coincidence Peak. Simultaneous absorption of two gamma photons in crystal, resulting in a peak higher in energy than the photopeak. The gamma photons may result from serial isomeric transitions (e.g. 2 keV plus 1332.5 keV in case of cobalt-60) or simultaneous decays in separate radionuclides (e.g. 2 x 1173.2 keV in case of cobalt-60).
Annihilation Peak. For high energy gamma rays (greater than 1.02 MeV), electron-positron pair production is a possible interaction. But a positron may then annihilate in the detector or in the surrounding material. If both 0.51 MeV annihilation photons are absorbed in the crystal, a secondary peak 1.02 MeV below the photopeak can be found. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.
See also:
Radiation Detection
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