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4 Radiation Detection Technology
4.1 Types of Radiation Surveys
There are three primary reasons for conducting radiation surveys:
1. To measure radiation dose rates. Radiation dose rate surveys are recorded in
units of mGy/hr, µGy/hr (or their American counterparts) and they are used to:
• Determine health and safety requirements,
• Demonstrate compliance with regulatory requirements,
• Identify the presence of unexpected radioactive materials.
Note: Radiation dose is a measure of energy deposition; it requires the ability
to measure the energy of the radiation detected by the radiation detector.
Detectors that can register only count rates are not normally effective at
accurately measuring radiation dose rate.
2. To measure levels of radioactive contamination. Contamination surveys are
recorded in units of counts per minute (cpm) or counts per second (cps) and
they are used to determine if protective anti-contamination clothing is required,
if contamination levels are too high to permit an object, area, or person to be
released from a contaminated area, or if decontamination is required.
3. To identify radionuclides and categorize them as natural, medical, industrial,
and so forth
Note: some radionuclides can have multiple uses, and categorizing them simplistically can be misleading. For example, Cs-137 can be used for industrial radiography, temporarily implanted as a small source into tumors, or stolen for use in a
terrorist attack; Ra-226 can be naturally occurring (present in granite or in sludge and
pipe scales in the petroleum and natural gas industry), used in older compasses and
watches, present as a contaminant in areas formerly used for the production of selfluminous products, or can be accidentally disposed of into the trash. In addition, the
186 keV gamma from Ra-226 is almost impossible to distinguish from the 185 keV
gamma from U-235, the isotope of uranium used to make nuclear weapons. Although
a radionuclide identifier might identify and categorize a radionuclide, those finding
it must be able to understand the context in which it was found to understand the
threat (if any) it poses (Fig. 4.1).
4.2 Gas-Filled Detectors
When ionizing radiation interacts with matter it will interact and strip an electron
from an otherwise electrically neutral atom; this creates two ions, one positive and
one negative. If this ion pair is created in a gas and the gas is embedded in an electrical
field then the electrical field will cause the positive and negative charges to separate.
If the voltage applied is high enough (a few to several hundred volts) the ions will be
accelerated to high velocities. As they accelerate, they will interact with other atoms
4 Radiation Detection Technology
4.1 Types of Radiation Surveys
There are three primary reasons for conducting radiation surveys:
1. To measure radiation dose rates. Radiation dose rate surveys are recorded in
units of mGy/hr, µGy/hr (or their American counterparts) and they are used to:
• Determine health and safety requirements,
• Demonstrate compliance with regulatory requirements,
• Identify the presence of unexpected radioactive materials.
Note: Radiation dose is a measure of energy deposition; it requires the ability
to measure the energy of the radiation detected by the radiation detector.
Detectors that can register only count rates are not normally effective at
accurately measuring radiation dose rate.
2. To measure levels of radioactive contamination. Contamination surveys are
recorded in units of counts per minute (cpm) or counts per second (cps) and
they are used to determine if protective anti-contamination clothing is required,
if contamination levels are too high to permit an object, area, or person to be
released from a contaminated area, or if decontamination is required.
3. To identify radionuclides and categorize them as natural, medical, industrial,
and so forth
Note: some radionuclides can have multiple uses, and categorizing them simplistically can be misleading. For example, Cs-137 can be used for industrial radiography, temporarily implanted as a small source into tumors, or stolen for use in a
terrorist attack; Ra-226 can be naturally occurring (present in granite or in sludge and
pipe scales in the petroleum and natural gas industry), used in older compasses and
watches, present as a contaminant in areas formerly used for the production of selfluminous products, or can be accidentally disposed of into the trash. In addition, the
186 keV gamma from Ra-226 is almost impossible to distinguish from the 185 keV
gamma from U-235, the isotope of uranium used to make nuclear weapons. Although
a radionuclide identifier might identify and categorize a radionuclide, those finding
it must be able to understand the context in which it was found to understand the
threat (if any) it poses (Fig. 4.1).
4.2 Gas-Filled Detectors
When ionizing radiation interacts with matter it will interact and strip an electron
from an otherwise electrically neutral atom; this creates two ions, one positive and
one negative. If this ion pair is created in a gas and the gas is embedded in an electrical
field then the electrical field will cause the positive and negative charges to separate.
If the voltage applied is high enough (a few to several hundred volts) the ions will be
accelerated to high velocities. As they accelerate, they will interact with other atoms