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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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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
Representative terms from entire chapter:
beta particles
