Detection of gamma radiation is very specific, because gamma rays interact differently with matter. Gamma rays can travel thousands of feet in air and can easily pass through various materials. Moreover, gamma rays can ionize atoms indirectly and directly (despite they are electrically neutral) through the photoelectric effect and the Compton effect. But secondary (indirect) ionization is much more significant.
In order to describe principles of detection of gamma radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interactions of gamma rays (radiation as a flow of these rays) separately.
Interaction of Gamma Radiation with Matter
Gamma rays consist of high energy photons. These photons are particles/waves (Wave-Particle Duality) without rest mass or electrical charge. They can travel 10 meters or more in air. This is a long distance compared to alpha or beta particles. However, gamma rays deposit less energy along their paths. Lead, water, and concrete stop gamma radiation. Photons (gamma rays and X-rays) can ionize atoms directly through the Photoelectric effect and the Compton effect, where the relatively energetic electron is produced. The secondary electron will go on to produce multiple ionization events, therefore the secondary (indirect) ionization is much more significant.
Although a large number of possible interactions are known, there are three key interaction mechanisms with matter.
The photon is completely absorbed in photoelectric effect and pair production, while only partial energy is deposited in any given Compton scattering. The probability of photoelectric absorption (dominates at lower gamma rays energies) per unit mass is approximately proportional to:
τ(photoelectric) = constant x ZN/E3.5
where Z is the atomic number, the exponent n varies between 4 and 5. E is the energy of the incident photon. The probability of Compton scattering per one interaction with an atom increases linearly with atomic number Z, because it depends on the number of electrons, which are available for scattering in the target atom. The probability of pair production (dominates at higher gamma rays energies), characterized by cross section, is a very complicated function based on quantum mechanics. In general the cross section increases approximately with the square of atomic number (σp ~ Z2) and increases with photon energy, but this dependence is much more complex.
As a result, effective sensitive material for gamma radiation detection is in most cases based on use of materials with two following material properties:
- high-density of material.
- high atomic number of material (high Z materials)
Detectors of Gamma Radiation
Detectors may be also categorized according to sensitive materials and methods that can be utilized to make a measurement:
Detection of Gamma Radiation using Ionization Chamber
Gamma rays have very little trouble in penetrating the metal walls of the chamber. Therefore, ionization chambers may be used to detect gamma radiation and X-rays collectively known as photons, and for this the windowless tube is used. Ionization chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. Some problems are caused by the fact, that alpha particles are more ionising than beta particles and than gamma rays, so more current is produced in the ionization chamber region by alpha than beta and gamma. Gamma rays deposit significantly lower amount of energy to the detector than other particles.
The efficiency of the chamber can be further increased by the use of a high pressure gas. Typically a pressure of 8-10 atmospheres can be used, and various noble gases are employed. For example, high-pressure xenon (HPXe) ionization chambers are ideal for use in uncontrolled environments, as a detector’s response has been shown to be uniform over large temperature ranges (20–170°C). The higher pressure results in a greater gas density and thereby a greater chance of collision with the fill gas and ion-pair creation by incident gamma radiation. Because of the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring.
Detection of Gamma Radiation using Geiger Counter
Geiger counter can detect ionizing radiation such as alpha and beta particles, neutrons, and gamma rays using the ionization effect produced in a Geiger–Müller tube, which gives its name to the instrument. The voltage of detector is adjusted so that the conditions correspond to the Geiger-Mueller region.
The high amplification factor of the Geiger counter is the major advantage over the ionization chamber. Geiger counter is therefore a much more sensitive device than other chambers. It is often used in the detection of low-level gamma rays and beta particles for this reason.
Detection of Gamma Radiation using Scintillation Counter
Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.
Scintillation counters can be used to detect alpha, beta, gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used.
- Gamma Rays. High-Z materials are best suited as scintillators for the detection of gamma rays. The most widely used scintillation material is NaI(Tl) (thallium-doped sodium iodide). The iodine provides most of the stopping power in sodium iodide (since it has a high Z = 53). These crystalline scintillators are characterized by high density, high atomic number, and pulse decay times of approximately 1 microsecond (~ 10-6 sec). Scintillation in inorganic crystals is typically slower than in organic ones. They exhibit high efficiency for detection of gamma rays and are capable of handling high count rates. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. This feature is widely used in medical imaging to detect X-rays or gamma rays. Inorganic scintillators are better at detecting gamma rays and X-rays. This is due to their high density and atomic number which gives a high electron density.
Detection of Gamma Radiation using Semiconductors – HPGe Detectors
High-purity germanium detectors (HPGe detectors) are the best solution for precise gamma and x-ray 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. 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.
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