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| Fig. 1: Geiger-Müller radiation detector.(Source: Wikimedia Commons) |
The detection of radiation and the measurement of its properties are required in all aspects of the nuclear field - in scientific research, in the operation of nuclear-power plants, in medical applications, and even in counterterrorism. [1] Radiation detectors enable humans to determine the radioactivity of nuclear waste, the severity of nuclear fallout, and the amount of radionuclides in the air, water, soil, and food etc. Since radiation has detrimental impact on human health and ecosystems, it is crucial to constantly monitor the radiation level around people, to set dose limits for personnel working in high radioactivity environments, and to strictly regulate the amount of radionuclides encountered in everyday life. In the United States, RadNet, managed by EPA, is the nationwide system that continuously monitors ambient environmental radiation levels and those resulting from major nuclear accidents. Nonetheless, many individuals and private non-profit groups volunteer for radiation monitoring, thanks to the ready availability of detector technologies. [2] As radiation detection is a huge subject, this paper will limit its focus to detectors used for measuring radionuclide levels in air and water.
Before jumping into the discussion of detectors, it is beneficial to go over some radiation basics relevant to detection: [3]
Activity refers to the amount of ionizing radiation released by a radioactive substance. Ionizing radiation can be α particles, β particles, γ rays, X-ray, neutrons, or any combinations of these. Activity (A) is defined as
where λ represents the decay constant of the radioisotope (or 0.693 over the decay half-life) and N is the number of radioisotope atoms. The unit of measure in SI system for activity is becquerel (Bq). 1 Becquerel is defined as 1 disintegration per second. Another unit for activity is Curie (Ci), which is traditionally defined as the activity of one gram Radium. 1 Ci equals 3.7 × 1010 Bq. In other words, 1 gram of radium atoms disintegrates 37-billion times per second. More often, activity is presented as specific activity, which is the activity per mass or per volume. For example, the activity for meat is usually in Bq/kg, and the activity of the air is in Bq/m3.
Exposure describes air molecule ionization caused by radiation. The units for exposure are the Roentgen (R). 1 R means the amount of X-ray or γ ray that results in 2.58 × 10-4 coulomb per kg of ions generated in air.
Absorbed dose describes the amount of radiation absorbed by a person, quantitatively represented by the amount of energy deposited on the target. The units for absorbed dose are the radiation absorbed dose (rad) and the gray (Gy). 1 Gy equals to 1 joule of absorbed energy per kilogram of exposed biological substances. An older unit of absorbed dose is rad. 1 Gy equals 100 rad. To calculate the absorbed dose from an exposure of 1 R depends on the energy of the radiation and the composition of the irradiated material. For example, if soft tissue is exposed to γ-radiation of 1 R, the absorbed dose will be approximately 9.3 milligray (mGy).
Dose equivalent is used when considering medical effects resulted from radiation absorption. It is a common measurement for the regulatory agencies because it directly relates to health concerns. Dose equivalent is defined as
where D is the absorbed dose and QF is the quality factor. The quality factor depends on the type of radiation and relates to the biological effect of certain radiation. For example, the QF of X-ray and γ-ray is 1, while for the α-particle emitted from Radon it is 20. This means that α-particle leads to a larger biological damage for same absorbed dose of X-ray and α-particle. Units for dose equivalent are the roentgen equivalent man (rem) and sievert (Sv). If the D is expressed in Gy, the dose equivalent will be in Sv; if the D is in rad, the dose equivalent will be in rem. Therefore, 1 Sv is equal to 100 rems. 1 Sv represents the equivalent biological effect of a joule of radiation energy absorbed in a kilogram of tissue.
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| Fig. 2: Pictured is a researcher seated in front of a scintillation counter. This device measures radioactivity incorporated into a cell culture. (Source: Wikimedia Commons) |
Background radiation comes from both natural and man-made sources. Some examples of background radiation include cosmic rays, terrestrial radiation, and radiation in food. Human may encounter one, or more of those in their normal activities. Passengers are subject to small doses of cosmic radiation that comes from outer space on their airplane flights. Terrestrial radiation comes from many naturally occurred radioactive elements, for example Radon, in the water and air. Food that contains isotopes such as C-14 and K-40 could lead to small radiation doses. In addition, plants and animals that uptake radioactive elements from soil or food could accumulate radioactive materials. The averaged annual natural radiation a U.S. citizen encounters is about 3.1mSv, mainly contributed from Radon gas (2mSv) in the air. It is important to note that the actual background radiation dose varies from one location to one location with at least a factor of 10. Because of that, background radiation can become very critical to one's radiation detection if one simply considers the averaged value, resulting in great artifacts or measurement errors.
In general, there are two types of ionizing radiation detectors. One type is counting equipment that is used to determine radioactivity, and the other type is dosimeter that is used to determine radiation dose. The qualities of radiation that are of interests include radioactivity, type, energy, and dose of the radiation. For instance, a nuclear waste treatment company may want to determine how radioactive a nuclear waste is when a radiotherapist is more interested in the dose of radiation to the workers. Therefore, various detectors have been designed for different purposes although nearly all of them follow the same principle - radiation causes changes in a compound, which can be converted into measurable signals. Some examples of detecting instruments are film, thermo luminescence dosimetry, ionization tube, scintillation counters, solid-state semiconducting detectors etc.
In the U.S., RadNet has more than 100 air monitors nationwide that measure β particle level and γ radiation in the air, and it collects drinking water samples for γ composite analysis from 78 sites across the country. [4] According to NAREL's radiological methodologies report, various detectors are employed to analyze the radioactivity and dose level of the air and water samples. [5] A few of them are illustrated in the following:
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| Fig. 3: Setup for a γ spectroscopy experiment: germanium detector connected to a cooling dewar, scintillation detector and sample mounting. (Source: Wikimedia Commons) |
Gas counter has a gas-filled chamber where external voltage is applied. When a charged particle or γ ray from the radioactive decays of air sample, for example, enters into the chamber, it ionizes the gas molecules and current pulses (signals) will be generated when the gas ions arrive the cathode. There are three operating modes of gas counter, depends on the applied voltage. At low applied voltage, number of current pulse is a measure of the number of incident particles that have entered the chamber. This is called ionization chamber mode. At moderate applied voltage, cascade of ionization events occurs. Now the current signal is proportional to the original number of incident particles; however, it can be used to distinguish α and β particles for their distinct ionization abilities. This mode is therefore called proportional counter. The final mode at high applied voltage is called Geiger-Müller counter. Due to the high voltage, the current signal generated in this mode no longer depends on the number of original particles. Even though the Geiger-Müller counter is not able to measure the number and types of the original decay emission, it measures radiation exposure in R/hr or rem/hr.
LSC detects photons generated by interaction of charged particles from decays with scintillators (can be either organic or inorganic materials). The generated photons are then converted into signals by a photomultiplier tube (PMT). The energy of signals is proportional to the energy deposited on the scintillators. Although α and β particles within certain energy range are detectable, LSC is good for measuring energy of γ radiation and possibly detecting γ-radiating isotopes. Modified scintillation counter can be used as dosimetry to measure energy of radiation absorbed in Gy.
Cherenkov counter involves photon radiation (signals) occurred when charged particles travel in a dielectric medium at paces greater than speed of light. PMT is normally used to amplify the signals. Cherenkov counter is good for measuring high-energy β-emitting radionuclides. This counter has good selectivity to high-energy β emission and is easy for construction.
Besides the radioactivity, type, energy, does of radiation, one may want an analyzer that identifies the radionuclides in the sample. GS-HPGe is a powerful technique that provides information of multiple nuclides in one sample, made possible by the high energy- resolution of HPGe and the available γ-ray data libraries. The signals come from various emission of γ rays from the radionuclides in the sample, which is then absorbed by the HPGe detector by photoelectric effect. The GS-HPGe spectrum does not only yield identification of the radionuclides in the sample, but also offer clues on the relative concentration/radioactivity of these elements.
Radiation detection is an important subject. This paper first briefly reviewed some important units and qualities of radiation. Subsequently, a few types of radiation detectors that are employed by RadNet were discussed. Background radiation exists almost everywhere with great variation, so detector users must be aware of that when they take and analyze their measurements.
© Hoi Ng. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. L. Murray, Nuclear Energy: an Introduction to the Concepts, Systems, and Applications of Nuclear Processes, 6th Ed (Butterworth-Heinemann, 2008), pp. 125-140.
[2] H. Kazem, "U.S. Residents Monitor Fukushima Radiation," Al Jazeera, 19 Jan 14.
[3] T. Henriksen and D.H. Maillie, Radiation and Health (CRC Press, 2002).
[4] "Historical Uses of RadNet Data," U. S. Environmental Protection Agency, EPA-402-R-08-007, November 2008.
[5] "Inventory of Radiological Methodologies," U. S. Environmental Protection Agency, EPA 402-R-06-007, October 2006.
