4.2 Gas-Filled Detectors
33
GM detectors are useful for performing contamination surveys (particularly a
“pancake-type GM) and can be useful for measuring radiation dose rates if used as
noted above. Some GM tubes have beta windows that can be exposed to measure
beta radiation dose rate, but the dose rate readings suffer from the same sources of
inaccuracy as noted for measuring dose from multiple energies of gamma radiation.
Geiger-Mueller detectors are relatively simple, inexpensive, and relatively rugged.
However, the process of ionization, amplification, counting, and recombination
requires a handful of microseconds and, during that time, additional counts will not be
registered—this is called “dead time.” Thus, GM detectors can saturate in high doserate radiation fields. If this happens with an older meter the needle might drop to zero
because the detector is no longer registering individual counts; on newer (electronic)
detectors the meter will typically indicate “overload” or something similar.
4.3 Scintillation Detectors (Handheld and Mobile Systems)
Gas-filled detectors are one major family of radiation detectors, scintillation detectors
are the other. In a scintillation detector, ionizing radiation interacts with a material
(usually a crystal or liquid) of some sort and causes it to emit photons of visible light;
these photons can be collected and analyzed.
Unlike gas-filled detectors, which will react to any ionizing radiation that enters
the detector, scintillation media are typically only used to detect and analyze one type
of radiation; alpha, beta, or gamma. Sodium iodide and cesium iodide detectors, for
example, are used to measure gamma radiation; zinc sulfide is used to measure
alpha radiation; and various organic materials are used to measure beta radiation.
Some of these compounds are sensitive to more than one type of radiation, but most
scintillation media lack the generality of gas-filled detectors.
The properties of scintillation media are well-understood, including the number
of scintillation photons produced for each MeV deposited in the detector. The energy
of the scintillation photons is also well-known, as is the quantum efficiency of the
photodiode and the amount of amplification of the signal as it progresses through
the dynodes. Thus, it is possible to determine the amount of energy in the incident
radiation by measuring the number of electrons collected from the instrument; this,
in turn, can be used to identify the radionuclide that emitted the radiation.
The process of generating, amplifying, and processing each “count” takes a up to
a microsecond [1] and any additional radiation that strikes the detector in that time
will produce a pulse that overlaps with that of the earlier count—this is called “dead
time.” This can make it difficult to analyze each of these pulses and, if the radiation
field is sufficiently high, it can become impossible to differentiate between counts;
the detector can saturate, no longer providing useful information. Many detectors
will identify when dead time becomes an issue making accurate nuclide ID and/or
dose rate measurements difficult or impossible; other instruments will simply note
“Overload” or the equivalent.
33
GM detectors are useful for performing contamination surveys (particularly a
“pancake-type GM) and can be useful for measuring radiation dose rates if used as
noted above. Some GM tubes have beta windows that can be exposed to measure
beta radiation dose rate, but the dose rate readings suffer from the same sources of
inaccuracy as noted for measuring dose from multiple energies of gamma radiation.
Geiger-Mueller detectors are relatively simple, inexpensive, and relatively rugged.
However, the process of ionization, amplification, counting, and recombination
requires a handful of microseconds and, during that time, additional counts will not be
registered—this is called “dead time.” Thus, GM detectors can saturate in high doserate radiation fields. If this happens with an older meter the needle might drop to zero
because the detector is no longer registering individual counts; on newer (electronic)
detectors the meter will typically indicate “overload” or something similar.
4.3 Scintillation Detectors (Handheld and Mobile Systems)
Gas-filled detectors are one major family of radiation detectors, scintillation detectors
are the other. In a scintillation detector, ionizing radiation interacts with a material
(usually a crystal or liquid) of some sort and causes it to emit photons of visible light;
these photons can be collected and analyzed.
Unlike gas-filled detectors, which will react to any ionizing radiation that enters
the detector, scintillation media are typically only used to detect and analyze one type
of radiation; alpha, beta, or gamma. Sodium iodide and cesium iodide detectors, for
example, are used to measure gamma radiation; zinc sulfide is used to measure
alpha radiation; and various organic materials are used to measure beta radiation.
Some of these compounds are sensitive to more than one type of radiation, but most
scintillation media lack the generality of gas-filled detectors.
The properties of scintillation media are well-understood, including the number
of scintillation photons produced for each MeV deposited in the detector. The energy
of the scintillation photons is also well-known, as is the quantum efficiency of the
photodiode and the amount of amplification of the signal as it progresses through
the dynodes. Thus, it is possible to determine the amount of energy in the incident
radiation by measuring the number of electrons collected from the instrument; this,
in turn, can be used to identify the radionuclide that emitted the radiation.
The process of generating, amplifying, and processing each “count” takes a up to
a microsecond [1] and any additional radiation that strikes the detector in that time
will produce a pulse that overlaps with that of the earlier count—this is called “dead
time.” This can make it difficult to analyze each of these pulses and, if the radiation
field is sufficiently high, it can become impossible to differentiate between counts;
the detector can saturate, no longer providing useful information. Many detectors
will identify when dead time becomes an issue making accurate nuclide ID and/or
dose rate measurements difficult or impossible; other instruments will simply note
“Overload” or the equivalent.