Answer to Question #8948 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

I am currently doing some experiments using the portable gamma detector named HDS-100GN, which is made by MGP Instruments. I want to use it to detect the gamma radiation of an area. This portable gamma detector is made by CsI (Tl), and it can give the gamma counts per second and ambient dose equivalent of gamma field. But I do not know how it works to get the dose rate of the gamma field. How is the scintillator usually made to get the dose rate of the gamma field? I have seen some other scintillation dosimeters such as NaI(Tl), so there must be some general rule for making gamma dosimeters from inorganic scintillators. Could you tell me some information about this issue?

A

According to its specifications, the instrument you describe has a low-dose-rate range that extends from 1 µSv h-1 to 100 µSv h-1 and an extended range from 0.1 mSv h-1 to 10 mSv h-1. I am not sure whether the extended range is available on all instruments or whether it is an option that may be elected at the time of purchase. At any rate, it appears that the CsI(Tl) detector is used for the lower-dose range. The higher-dose range is covered by a silicon diode detector.

In order to interpret ambient dose equivalent rate from the output of the CsI(Tl) detector, the instrument must include, among other things, an algorithm that accounts for variation in detection efficiency with photon energy. Since the instrument has gamma spectral accumulation ability, the energy definition task is readily achievable, and efficiency would have been generated independently and incorporated into firmware in the instrument. Given the interpreted interaction rate of photons of a given energy, the algorithm must go on to calculate ambient dose equivalent rate. At least indirectly, this involves a determination of the fluence rate on the detector and an assumption as to the directionality of the field. I assume that the detector is located at an effective depth of 10 mm tissue equivalence since this is the depth used to specify ambient dose equivalent normally used to simulate effective dose equivalent. There have been conversion factors developed.

There is no general rule by which inorganic scintillation detectors may be applied to the measurement of dose (rate). If only a single photon energy is of interest, the problem becomes simple since the detector may be calibrated at the photon energy of interest so that a known dose rate produces a specific count rate. If the detector is calibrated at one energy, however, and it is then used to measure a photon field of a different energy or of mixed energies, the original calibration would have no validity. This is because the inorganic scintillators are all very different in atomic composition from soft tissue, the medium of usual interest in dosimetry. The scintillators all have effective atomic numbers considerably higher than that of tissue. As a result, low-energy photons tend to produce a markedly greater response than do mid- to higher-energy photons (up to about 1.5 MeV) because of enhanced photoelectric effect. At still higher energies of several MeV, the detector response will also be excessive because of enhanced pair production interactions in the high-atomic-number material compared to tissue.

Thus, for the instrument with the inorganic scintillator detector to be able to interpret a tissue dose quantity, it must have information about the energies of the photons and the detection efficiencies as a function of energy. It must also have specified the particular dose quantity of interest, such as the ambient dose equivalent, so that the proper conversion factors can be applied through the stored algorithms. The great advances in miniaturization of sophisticated electronics have allowed today's instruments to incorporate sophisticated microprocessors and firmware (and software) to perform complicated tasks that were impossible not many years ago.

I hope this information is helpful.

George Chabot, PhD, CHP

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