How does this CsI detector measure dose?

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