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E Radiation Measurement This appendix provides tutorial information about radioactivity, radiation, and their detection. It is important to understand the basic concepts of ionizing radiation, its interaction with matter, and its detection to be able to address many issues associated with the release of slightly radioactive solid material (SRSM) from regulatory control. Note that the levels of radioactive material concentration under consideration for release are very low relative to most licensed sources. In fact, these levels are close to those of the natural background radiation. As the concentration or amount of radioactive material decreases, detection and identifi- cation of the source or sources become more difficult. First, consider some elementary but important aspects of matter. Atoms are composed of electrons that orbit around a nucleus. It is the number of electrons surrounding the nucleus that determines the chemical properties of the atom, and in an atom, the number of orbital electrons is equal to the number of protons in the nucleus, since protons are positively charged and electrons are negatively charged. Atoms gain electrons (to become anions), lose electrons (to become cations), or share electrons to form molecules. Neutrally charged particles- neutrons also exist in the nucleus. The relative numbers of protons and neutrons play a key role in determining the stability of an atom's nucleus. Nuclei with the same number of protons but different numbers of neutrons are called isotopes. Unstable nuclides radionuclides radiate particles and electromagnetic ra- diation when they transform to a more stable configuration. All isotopes of an element will behave the same chemically. For example, radioactive 60Co will act just like stable 59Co when steel is melted. Radioactive material can be either naturally occurring or created by man. Radioactive decay is a random process. The half-life of a radionuclide is the 212

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APPENDIX E 213 average time it takes for a sample of that radionuclide to reduce in quantity by one-half. The activity of a collection of radionuclides is a measure of the number of nuclear transformations per unit time occurring in a sample in units of becquerels (Bq) and curies (Ci). One becquerel is defined as one disintegrating nucleus per second. The curie is a customary unit that is equal to 3.7 x 10~ Bq. In any radiation measurement, there is a small statistical uncertainty resulting from the radioactive decay process. It is the emitted radiation and its subsequent interaction with matter that can be detected. The type, energy, half-life, and frequency of detected radiation can be used to determine the amount of each radionuclide present in a sample. By comparing the quantity of each radionuclide present in a sample with the activity limits established from a dose standard, a determination can be made of whether the sample meets release criteria. THE MEASUREMENT PROCESS The method used to detect the radiation emitted from radioactive material plays an important role in determining the presence and quantity of a specific radionuclide or collection of radionuclides that are present. Two general ap- proaches can be applied, each giving different levels of information. One method is to attempt to survey 100 percent of the material entering or leaving a facility. An example of this is the use of portal detectors to survey scrap metal entering a steel production site. The truck with a load of scrap pulls between two large detectors and slows or stops briefly while the load is "counted"; then, based on the number of counts obtained during the counting period, an essentially immedi- ate determination is made of whether the load contains radioactive material. No attempt is made to identify or quantify the specific radionuclides that are present. An alternative method is to survey each piece of scrap metal individually, using a more sensitive detector capable of determining the identity and quantity of each of the radionuclides in the material by determining radiation type, energy, and activity. The first method has the clear advantage of being capable of a large throughput. Its major disadvantage is the inability to detect small quantities of radioactive material and its insensitivity to radiation that is easily stopped in matter. The second approach gives a very accurate and complete assessment of the radionuclide inventory (i.e., identity and quantity), but the process is tedious, leading to high personnel costs (more skilled personnel required) and low through- put. Thus, the measurement process selected will vary depending on the goal. RADIATION TYPES AND INTERACTIONS There are unique types and combinations of radiation emitted by individual radionuclides as they decay. This uniqueness permits identification of the radio-

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214 APPENDIX E nuclide that decayed from its detected radiations. The most common types of radiation are alpha particles, beta particles, and gamma rays (or photons). An alpha particle is a helium-4 nucleus with two protons, two neutrons, and a +2 charge. Alpha particles travel only a short distance before coming to a stop, having transferred all their kinetic energy to the target material. An alpha particle can usually be stopped by 2 to 3 cm of air or one sheet of paper. After the alpha particle stops, it simply picks up two free electrons and becomes a helium atom. Alpha particles are easy to shield and, thus, are of little hazard to humans when outside the body. Conversely, when alpha particles are emitted from radionu- clides within the body, all of their kinetic energy is deposited in a small amount of tissue, resulting in a large, highly localized absorbed dose. Beta particles originate in the nucleus when a neutron transforms to a proton. Beta particles are electrons that have been given this special name to differentiate them from the atomic orbital electrons. Like alpha particles, beta particles take energy away from the nucleus. Beta particles travel a longer distance through matter than alpha particles. A typical range of a beta particle is 1 to 3 meters in air or 0.1 to 1 cm in solids and liquids. Radionuclides emit a third type of radiation, gamma rays, which are zero- mass, zero-charge photons. Usually, gamma photons are emitted in conjunction with particle decay to rid the nucleus of the remaining excess energy. Gamma photons also interact with a target material' s orbital electrons, but with very low frequency compared to the interaction frequency of charged particles. This means that gamma photons are the most penetrating of the common types of radiation. The attenuation of photon radiation is described by an exponential relationship. The interaction of radiation with matter is extremely important in the overall assessment of the radioactive material content of an unknown sample. To suc- cessfully measure the radioactive material in a sample, radiation emitted from the decaying nuclei must be able to penetrate everything between its point of emis- sion and the detector. The radiation must then interact within the active volume of the detector. Some radionuclides are difficult to measure because the radiation is not very penetrating. Radionuclides emitting only alpha or beta particles fall within this category. Special procedures must be used to quantify the radioactive material content of solid materials containing alpha- and beta-particle emitters. The diffi- culty in assaying materials contaminated with radionuclides that emit only par- ticle radiation is getting the radiation to the detector. Many radionuclides that decay by emission of alpha or beta radiation also simultaneously emit one or more gamma photons. Gamma photons are very penetrating relative to particles, with the exception of low-energy photons. For radioactive materials emitting gamma photons, different detectors (from those used for alpha and beta particles) are employed depending on the purpose of the measurement.

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APPENDIX E 215 If it were necessary to determine only whether radiation is present, a detector that responds to alpha, beta, and gamma radiation would be preferred. An ex- ample of such a detector is the Geiger-Muller (GM) detector. A GM detector is a gas-filled chamber that is coupled to an electronic circuit to detect the pulses generated by a radiation interaction within the detector's active volume. These devices are portable and inexpensive. GM instruments are often used for initial surveys, since they register detected radiation events as "counts." By knowing the details of how the measurement was made and the sample characteristics, the radioactive material concentration in the sample can be estimated. There are many other types of radiation detectors, including ion chambers, scintillation detectors, and solid-state detectors. Ionization chambers are air-filled detectors operated in the current mode. Ion chambers are insensitive at radiation intensities associated with the proposed clearance levels. Scintillation detectors are based on detection of the small light flashes produced by radiation interac- tions within a scintillation material. Scintillators can be manufactured in liquid, crystal, or plastic form. Because scintillators are usually designed to respond to one type of radiation, it is possible to eliminate some radionuclides from consid- eration when assaying an unknown sample. Additionally, the intensity of the flash is proportional to the energy; thus, scintillation detectors can be used to gain some information on the radiation's energy. Solid-state detectors utilizing silicon or germanium are preferred for radia- tion spectroscopy because of the high-energy resolution possible from these de- vices. Solid-state detectors are available for particle and photon measurement. When coupled with a computer and spectral analysis software, these detectors provide a powerful tool for quantifying both the activity level and the radionu- clide inventory in a sample. It is perhaps easier to illustrate radiation detection and measurement proce- dures using two examples. The first example is the decision process made on scrap steel entering a steel plant. The objective of the measurement is to deter- mine whether or not the shipment contains radioactive materials. A truckload of scrap is pulled between two detectors. If activity is detected, the shipment is rejected. Usually no attempt is made to sort the scrap or investigate the cause of the radiation alarm. Since the material is scrap metal contained in a truck, any particle radiation would be shielded from the detectors by the truck wall, the other scrap metal, and the air between the truck and the detectors. If sufficient quantities of radioactive materials that emit gamma rays are present, the detectors will respond accordingly. This example illustrates several important points. The goal in many cases is to determine the presence or absence of radioactivity in a large amount of mate- rial. In order to maximize the probability of detection of the radiation from any radioactive materials present, the measurement system must be optimized, usu- ally by the use of large gamma scintillation detectors. The go/no go type of

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216 APPENDIX E system gives no information about the radionuclide inventory in the shipment, since the detectors used are not capable of providing sufficient data for radionu- clide identification and the parameters necessary to convert from counts per unit time to activity are unknown. A second hypothetical example is a U.S. Nuclear Regulatory Commission (USNRC) licensee who has a quantity of concrete for disposal that is probably not radioactive. However, the licensee is aware of the possibility that the concrete may have been irradiated with neutrons that would have created some radionu- clides. External measurements with a survey instrument indicate that the activity, if present at all, is about at the background level. Thus, the problem is to deter- mine whether the concrete contains neutron-produced radionuclides or only natu- rally occurring radionuclides. Since it would be reasonable to assume that neu- trons could penetrate deeply into the concrete, it would follow that radionuclides could have been produced within the concrete, not just on its surface. An addi- tional assumption would be that a wide variety of radionuclides could have been produced. A solution would be to perform a measurement of the concrete in a labora- tory. This requires collection of a statistically representative group of samples from the batch of concrete. Each sample would be analyzed carefully using standard methods to determine the radionuclides present and their respective activities. One method would be to crush the concrete to a fine powder and then count small volumes of the powder to eliminate source self-shielding, making it possible to determine if alpha or beta radiation is present. Spectroscopy could then be utilized to gather the data to determine the energies and intensities of each radiation type. Analysis of the data would yield a complete radionuclide inven- tory and determine whether any of the detected radionuclides were produced by neutron activation or whether they were naturally occurring. This second example illustrates the difficulty with a quantitative assay of volumetrically contaminated or irradiated materials. Although exact activity in- ventory determinations are possible (and routinely performed), they utilize spe- cialized, nonportable instrumentation in a laboratory environment. Such an analy- sis may take several weeks to complete at a fairly high cost (relative to simple scanning of materials). Thus, it is not realistic to anticipate that this type of analysis would be performed in most high-volume, high-throughput manufactur- ing processes. BACKGROUND RADIATION Background radiation is present in every counting situation. It results from several different sources, including naturally occurring radioactive materials, cos- mic radiation, and man-made radionuclides from weapons tests. Some naturally occurring radionuclides have long half-lives, often more than a billion years. These are residual isotopes that were once present in much larger abundances but

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APPENDIX E 217 have slowly decayed with time. Examples of these include 40K, 147Sm, and 235U. Other naturally occurring radionuclides are produced by activation by cosmic-ray bombardment of stable isotopes. An example of this is the production of radioac- tive 14C from stable 14N. Table E-1 gives some specific examples of background and man-made source activities. Since the distribution of radionuclides varies around the world depending on the geology of the area, some of these activities represent typical numbers. All detection systems must account for and subtract background levels to obtain true sample radioactive material concentrations. TABLE E- 1 Radiation Sources and Their Activities Radiation Source Radioactivity (Bq) 70 kg adult human (male) 40Ka 1 kg of fresh vegetablesa 1 kg of super phosphate fertilizerb Air inside 2000 ft2 home (radon) (593 m3)a Household smoke detectorb Radionuclide for medical diagnosisC Radionuclide source for medical therapy 1 kg natural uraniuma 1 kg low-level radioactive waste (Class A, 137cs)e 1 kg of coal fly ashb 1 kg of granite (U. Th, K)b ~5,000 10 5,000 36,000 3,700- 1 10,000 11-740 x 106 3.7 x 1014 24 x 106 4x 107 150-410 72 aNational Council on Radiation Protection and Measurements (NCRP) Report No. 94 (NCRP, 1987b). bNCRP Report No. 95 (NCRP, 1987d). CNCRP Report No. 100 (NCRP, 1989a). dNCRP Report No. 105 (NCRP, 1989b). elo CFR Part 61.55