Nick Connor

Radiation Frisker is a handheld, wand-shaped radiation detector, featuring digital display and an internal pancake, halogen-quenched, Geiger-Mueller detector. Geiger counters are mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation. Although the major use of Geiger counters is probably in individual particle detection, they are also found in gamma survey meters. They are able to detect almost all types of radiation, but there are slight differences in the Geiger-Mueller tube. For alpha and beta particles to be detected by Geiger counters, they must be provided with a thin window. This “end-window” must be thin enough for the alpha and beta particles to penetrate. However, a window of almost any thickness will prevent an alpha particle from entering the chamber. The window is usually made of mica with a density of about 1.5 – 2.0 mg/cm² to allow low-energy beta particles (e.g. from carbon-14) to enter the detector. Effective window diameter in a Frisker is about 50 mm.

The Frisker detects alpha, beta, gamma, and X-ray radiation. Typical Frisker detects:

• gamma rays and X-rays down to 10 keV typical through the window, 40 keV minimum through the case.
• alpha particles down to 2 MeV.
• beta particles down to 0.16 MeV

Efficiencies vary, depending on energy and isotope. The Frisker is ideal for general purpose survey in laboratory use, nuclear power plant frisking, emergency response, border patrol, airport security, and other applications.

See also:

Dosimetry in NPPs

Portal monitors, are instruments for external contamination measurement. They are used for monitoring of personnel, which exit a nuclear facility. Portal monitors are usually last line of defence against contaminated people and equipment leaving the station. These detectors are able to measure simultaneously beta and gamma contamination. The total body surface including hands, head and feet is measured by many beta gas flow proportional counters and several large additional gamma scintillation detectors, which are positioned in the breast area (left and right side). Large scintillation detectors (plastic or liquid) are installed in order to be able to detect the passage of low levels of gamma activity. As they are only sensitive to gamma radiation, they are able to detect incorporated gamma activity if present. In this way, if some internal contamination exists, the worker is immediately sent for whole-body counting and the dose assessment is much more accurate.

See also:

Dosimetry in NPPs

Full-Body Monitors, or Whole-Body Monitors, are instruments for surface contamination measurement. They are used for personnel exit monitoring, which is the term used in radiation protection for checking for external contamination (or surface contamination) of a whole body of a person leaving a radioactive contamination controlled area. The main purpose is to prevent the spread of contamination outside the controlled area. Generally, surface contamination means that radioactive material has been deposited on surfaces. It may be loosely deposited, much like ordinary dust, or it may be quite firmly fixed by chemical reaction. This distinction is important, and we classify surface contamination on the basis of how easily it can be removed.

These monitors may utilize gas-flow proportional counters with a large area, and workers must use it every time they leave the controlled area. These monitors usually require a two-step measurement. First the measured person face the detectors. When they’ve done their thing, the person turn around to monitor his/her back. Proximity sensors check that the person is standing in the right position for the measurement. If not, the monitor actually talks to the person and tells what to do. Full-body monitors are able to detect beta and gamma contamination on the body. In order to detect beta radiation, these large area detectors have thin Mylar windows to allow low-energy beta particles to enter the detector. When instruments are operated in the proportional region, the voltage must be kept constant. If a voltage remains constant the gas amplification factor also does not change. Proportional counter detection instruments are very sensitive to low levels of radiation. By proper functional arrangements, modifications, and biasing, the proportional counter can be used to detect alpha, beta, gamma radiation. The electronics sorts the alpha, the beta-gamma pulses. This feature may be used in alpha, beta-gamma Hand & Shoe Monitors, Full-Body Monitors, some alpha Contamination Monitors. Although such detectors are very sensitive, their drawback is that the windows are punctured quite easily by misuse. The detector is then dead until its window is repaired.

See also:

Dosimetry in NPPs

Hand & Shoe Monitors are instruments for surface contamination measurement. In nuclear facilities, Hand & Shoe Monitors are installed usually at the exit from the controlled areas and workers are usually required to measure, whether their hands and shoes are contaminated or not. Generally, surface contamination means that radioactive material has been deposited on surfaces. It may be loosely deposited, much like ordinary dust, or it may be quite firmly fixed by chemical reaction. This distinction is important, and we classify surface contamination on the basis of how easily it can be removed.

These monitors may utilize gas-flow proportional counters with a large area, they are used in various locations in the station to detect beta gamma contamination on hands, shoes and clothing. In order to detect beta radiation, these large area detectors have thin Mylar windows to allow low-energy beta particles to enter the detector. For example, they can easily detect the low energy beta particles emitted by carbon-14 (E_max = 156 keV). When instruments are operated in the proportional region, the voltage must be kept constant. If a voltage remains constant the gas amplification factor also does not change. Proportional counter detection instruments are very sensitive to low levels of radiation. By proper functional arrangements, modifications, and biasing, the proportional counter can be used to detect alpha, beta, gamma radiation. The electronics sorts the alpha, the beta-gamma pulses. This feature may be used in alpha, beta-gamma Hand & Shoe Monitors, Full-Body Monitors, some alpha Contamination Monitors. Although such detectors are very sensitive, their drawback is that the windows are punctured quite easily by misuse. The detector is then dead until its window is repaired.

See also:

Dosimetry in NPPs

At nuclear facilities, remote radiation monitoring systems (RMS) are installed to monitor radiation levels at selected plant locations. The radiation monitoring system with pre-set alarm levels (e.g. for dose, dose rate or airborne activity) provide a reliable means of real time monitoring of the radiological conditions to which a worker is exposed. If these levels are exceeded, alarms are activated and in some cases automatic protective functions initiated. Thus the system serves to:

• Warn of any radiation health hazard
• Give an early warning of a plant malfunction
• Initiate automatic protective functions.

All data are collected in a radiation protection control room. The radiation monitoring system may gather all information on the radiological conditions at various working areas, as well as voice and visual feedback, with a minimum presence of RP technicians in radiation areas, therefore reducing dose to such personnel. Several types of radiation monitors are used in the RMS, depending on the source and strength of the radiation source.

• Airborne Contamination Monitors
• Area Monitors
• Iodine Monitors
• In-vent Gas Monitors

See also:

Dosimetry in NPPs

Portable survey meters are radiation detectors used by radiological technicians to measure ambient dose rate. These portable instruments usually have rate meters. In nuclear facilities, these portable survey meters are typically used by radiation protection technicians, which are responsible for following operations in the field to help assure that radiation protection policies are carried out and that jobs are implemented in accordance with the ALARA principle. Their responsibilities include:

• Providing assistance and advice to workers to motivate them to adopt an ALARA behaviour.
• Following jobs to ensure the respect of safety and radiation protection procedures.
• In some plants, stopping work in case of serious deviation from dosimetric objectives, or when there is a significantly increasing radiological risk for workers.

The typical radiation survey meter is, for example, the RDS-31, which is a multi-purpose radiation survey meter that uses a G-M detector. It has optional alpha, beta, and gamma external probes. It measures 3.9 x 2.6 x 1.3 inches and can be handheld, or worn by pocket, belt clip, or pouch. It has a five-digit, backlit, LCD display. Geiger counters operate at such a high voltage that the size of the output pulse is always the same, regardless of how many ion pairs were created in the detector. Geiger counters are mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation. Although the major use of Geiger counters is probably in individual particle detection, they are also found in gamma survey meters. They are able to detect almost all types of radiation, but there are slight differences in the Geiger-Mueller tube.

The operational quantities for area and individual monitoring of external exposures are defined by the ICRP. The operational quantities for area monitoring are:

• Ambient dose equivalent, H*(10). The ambient dose equivalent is an operational quantity for area monitoring of strongly penetrating radiation.
• Directional dose equivalent, H’ (d,Ω). The directional dose equivalent is an operational quantity for area monitoring of weakly penetrating radiation.

See also: Work Management to Optimise Occupational Radiological Protection at Nuclear Power Plants. NUCLEAR ENERGY AGENCY, OECD 2009. ISBN 978-92-64-99089-0.

See also: The Radiation Dosimeters for Response and Recovery Market Survey Report. National Urban Security Technology Laboratory. SAVER-T-MSR-4. <available from: https://www.dhs.gov/sites/default/files/publications/Radiation-Dosimeters-Response-Recovery-MSR_0616-508_0.pdf>.

High-pressure Ionization Chamber – Gamma Survey Meter

Ion chambers may be also used as the Emergency Gamma Survey Meter, the Beta Survey Meter, and the Tritium-in-Air Monitor. They are very useful for high levels of gamma radiation. But in this case, there are some difficulties.

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.

See also:

Dosimetry in NPPs

TLD – Thermoluminescent Dosimeter

A thermoluminescent dosimeter, abbreviated as TLD,  is a passive radiation dosimeter, that measures ionizing radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal in the detector when the crystal is heated. The intensity of light emitted is measure by TLD reader and it is dependent upon the radiation exposure. Thermoluminescent dosimeters was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison. TLD dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In dosimetry, both the quartz fiber and film badge types are being superseded by TLDs and EPDs (Electronic Personal Dosimeter).

Advantages and Disadvantages of TLDs

Advantages of TLDs

• TLDs are able to measure a greater range of doses in comparison with film badges.
• Doses from TLDs may be easily obtained.
• TLDs can be read on site instead of being sent away for developing.
• TLDs are easily reusable.

Disadvantages of TLDs

• Each dose cannot be read out more than once.
• The readout process effectively “zeroes” the TLD.

Film Badge Dosimeter

Film badges, film badge dosimeters, are small portable devices for monitoring cumulative radiation dose due to ionizing radiation. Principle of operation is similar as for X-ray pictures. The badge consists of two parts: photographic film, and a holder. The film is contained inside a badge. The piece of photographic film that is the sensitive material and it must be removed monthly and developed. The more radiation exposure, the more blackening of the film. The blackening of the film is linear to the dose, and doses up to about 10 Gy can be measured.

Advantages and Disadvantages of Film Dosimeters

Advantages of Film Dosimeters

• A film badge as a personnel monitoring device are very simple and therefore they are not expensive.
• A film badge provides a permanent record.
• Film badge dosimeters are very reliable.
• A film badge is used to measure and record radiation exposure due to gamma rays, X-rays and beta particles.

Disadvantages of Film Dosimeters

• Film dosimeters usually cannot be read on site instead of they have to be sent away for developing.
• Film dosimeters are for one-time use only, they cannot be reused.
• Exposures of less than 0.2 mSv (20 millirem) of gamma radiation cannot be accurately measured.

See also:

TLD

EPD – Electronic Personal Dosimeter

An electronic personal dosimeter is modern dosimeter, which can give a continuous readout of cumulative dose and current dose rate, and can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded. EPDs are especially useful in high dose areas where residence time of the wearer is limited due to dose constraints.

Characteristics of EPDs

The electronic personal dosimeter, EPD, is able to display a direct reading of the detected dose or dose rate in real time. Electronic dosimeters may be used as a supplemental dosimeter as well a primary dosimeter. The passive dosimeters and the electronic personal dosimeters are often used together to complement each other. To estimate effective doses, dosimeters must be worn on a position of the body representative of its exposure, typically between the waist and the neck, on the front of the torso, facing the radioactive source. Dosimeters are usually worn on the outside of clothing, around the chest or torso to represent dose to the “whole body”. Dosimeters may also be worn on the extremities or near the eye to measure equivalent dose to these tissues.

The dosimeter can be reset, usually after taking a reading for record purposes, and thereby re-used multiple times. The EPDs have a top mounted display to make them easy to read when they are clipped to your breast pocket. The digital display gives both dose and dose rate information usually in mSv and mSv/h. The EPD has a dose rate alarm, and a dose alarm. These alarms are programmable. Different alarms can be set for different activities.

For example:

• dose rate alarm at 100 μSv/h,
• dose alarm: 100 μSv.

Advantages and Disadvantages of Electronic Personal Dosimeters

Advantages of Electronic Personal Dosimeters

• EPDs are able to display a direct reading of the detected dose and dose rate in real time.
• EPDs have a dose rate alarm, and a dose alarm, which can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded.
• The dosimeter can be reset, usually after taking a reading for record purposes, and thereby re-used multiple times.
• EPDs are capable of measuring a wide radiation dose range from routine (μSv) levels to emergency levels (hundreds mSv or units of Sieverts) with high precision

Disadvantages of Electronic Personal Dosimeters

• EPDs are generally the most expensive dosimeters.
• EPDs are generally large in size.
• EPDs are used to measure and record radiation exposure due to gamma rays, X-rays, sometimes beta particles. For neutrons, TLDs are more capable.

DIS Dosimeter

Direct-ion storage dosimeter, DIS, is an electronic dosimeter, from which the dose information for both Hp(10) and Hp(0.07) can be obtained instantly at the workplace by using an electronic reader unit. The DIS dosimeter is based on the combination of an ion chamber and a non-volatile electronic charge storage element.

DIS dosimeter use an analog memory cell inside a small, gas-filled, ionization chamber. Incident radiation causes ionizations in the chamber wall and in the gas, and the charge is stored for subsequent readout. The DIS dosimeter is read at the user’s site through connection to an electronic reader unit. The DIS dosimeter is designed to clip to a breast pocket. The DIS series of personal electronic radiation dosimeters can operate at high dose rates and inside pulsed fields. It’s lightweight, but rugged. The DIS dosimeter represents a potential alternative for replacing the existing film and thermoluminescence dosimeters (TLDs) used in occupational monitoring due to its ease of use and low operating costs.

See also:

EPD

TLD – Thermoluminescent Dosimeter

A thermoluminescent dosimeter, abbreviated as TLD,  is a passive radiation dosimeter, that measures ionizing radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal in the detector when the crystal is heated. The intensity of light emitted is measure by TLD reader and it is dependent upon the radiation exposure. Thermoluminescent dosimeters was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison. TLD dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In dosimetry, both the quartz fiber and film badge types are being superseded by TLDs and EPDs (Electronic Personal Dosimeter).

Advantages and Disadvantages of TLDs

Advantages of TLDs

• TLDs are able to measure a greater range of doses in comparison with film badges.
• Doses from TLDs may be easily obtained.
• TLDs can be read on site instead of being sent away for developing.
• TLDs are easily reusable.

Disadvantages of TLDs

• Each dose cannot be read out more than once.
• The readout process effectively “zeroes” the TLD.

OSL Dosimeter

The OSL dosimetry (Optically Stimulated Luminescence) is a method that has established itself in the whole-body dosimetry. As can be deduced, this method is based on optically stimulated luminescence. The OSL dosimeter provides a very high degree of sensitivity by giving an accurate reading as low as 1 mrem for x-ray and gamma ray photons with energies ranging from 5 keV to greater than 40 MeV. OSL dosimeters are designed to provide X, gamma, beta and neutron radiation monitoring using OSL technology. OSL dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In diagnostic imaging the increased sensitivity of the OSL dosimeter makes it ideal for monitoring employees working in low-radiation environments and for pregnant workers. OSL dosimeters offer advantages that include the ability to be re-read and a high sensitivity (low minimum measurable dose), and they have become popular because of these favourable properties.

OSL materials (e.g. beryllium oxide ceramic) contain defects in their crystal structure that trap electrons released by exposure to radiation. In TLDs, the trapped electrons are subsequently freed by stimulation with heat, while OSL uses stimulation with light. After stimulation by light, the detector releases the stored energy in the form of light, i.e., it is stimulated to emit light. The light output measured with photomultipliers is a measure unit for the dose. In comparison with TLDs, their major difference is that luminescence is produced by a light beam, rather than by heat.

See also:

TLD

EPD – Electronic Personal Dosimeter

An electronic personal dosimeter is modern dosimeter, which can give a continuous readout of cumulative dose and current dose rate, and can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded. EPDs are especially useful in high dose areas where residence time of the wearer is limited due to dose constraints.

Characteristics of EPDs

The electronic personal dosimeter, EPD, is able to display a direct reading of the detected dose or dose rate in real time. Electronic dosimeters may be used as a supplemental dosimeter as well a primary dosimeter. The passive dosimeters and the electronic personal dosimeters are often used together to complement each other. To estimate effective doses, dosimeters must be worn on a position of the body representative of its exposure, typically between the waist and the neck, on the front of the torso, facing the radioactive source. Dosimeters are usually worn on the outside of clothing, around the chest or torso to represent dose to the “whole body”. Dosimeters may also be worn on the extremities or near the eye to measure equivalent dose to these tissues.

The dosimeter can be reset, usually after taking a reading for record purposes, and thereby re-used multiple times. The EPDs have a top mounted display to make them easy to read when they are clipped to your breast pocket. The digital display gives both dose and dose rate information usually in mSv and mSv/h. The EPD has a dose rate alarm, and a dose alarm. These alarms are programmable. Different alarms can be set for different activities.

For example:

• dose rate alarm at 100 μSv/h,
• dose alarm: 100 μSv.

Advantages and Disadvantages of Electronic Personal Dosimeters

Advantages of Electronic Personal Dosimeters

• EPDs are able to display a direct reading of the detected dose and dose rate in real time.
• EPDs have a dose rate alarm, and a dose alarm, which can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded.
• The dosimeter can be reset, usually after taking a reading for record purposes, and thereby re-used multiple times.
• EPDs are capable of measuring a wide radiation dose range from routine (μSv) levels to emergency levels (hundreds mSv or units of Sieverts) with high precision

Disadvantages of Electronic Personal Dosimeters

• EPDs are generally the most expensive dosimeters.
• EPDs are generally large in size.
• EPDs are used to measure and record radiation exposure due to gamma rays, X-rays, sometimes beta particles. For neutrons, TLDs are more capable.

TLD – Thermoluminescent Dosimeter

A thermoluminescent dosimeter, abbreviated as TLD,  is a passive radiation dosimeter, that measures ionizing radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal in the detector when the crystal is heated. The intensity of light emitted is measure by TLD reader and it is dependent upon the radiation exposure. Thermoluminescent dosimeters was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison. TLD dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In dosimetry, both the quartz fiber and film badge types are being superseded by TLDs and EPDs (Electronic Personal Dosimeter).

Advantages and Disadvantages of TLDs

Advantages of TLDs

• TLDs are able to measure a greater range of doses in comparison with film badges.
• Doses from TLDs may be easily obtained.
• TLDs can be read on site instead of being sent away for developing.
• TLDs are easily reusable.

Disadvantages of TLDs

• Each dose cannot be read out more than once.
• The readout process effectively “zeroes” the TLD.

See also:

EPD