Answer to Question #1221 Submitted to "Ask the Experts"

Q

I have a 10 mm×10 mm×5 mm-thick CsI:Tl scintillator attached to the photodiode. If I expose it to 1 µR h^{-1 137}Cs radiation dose then how many counts should I expect from the detector?

A

This is a relatively small detector, and I could only find efficiency data for a 3"×3" CsI(Tl) in Glenn Knoll's text Radiation Detection and Measurement, available from Wiley. For that size detector one figure notes a 60% efficiency at the ¹³⁷Cs photon energy, which I assume was with a photomultiplier tube.

Not knowing your photodiode efficiency for light photons from CsI(Tl), the light output from such a small size of the detector, and the electronic window you're setting (that is, to pick up some of the lower-energy compton scatter), the count rate is not easily calculated. My recommendation is an experimental measurement. If you can get the CsI(Tl) light output wavelength and photodiode detection efficiency from the manufacturer(s), you might calculate expected counts from first principles.

That is, I found in the 1970 Radiological Health Handbook, the gamma fluence rate for a 662 keV photon to produce a 1 R h^-1 field is 7.8E5 photons cm^-2s^-1. Thus, for 1 µR h^-1 with ¹³⁷Cs, one would have about 0.8 photons cm^-2s^-1. From the detector dimensions and published mass attenuation coefficient data, the number of interactions per unit time could be determined and, assuming little self attenuation of the light in the thin detector, coupled with the light detection efficiency of the photodiode and assumed wide open-pulse counting window, an upper limit of the count rate could be estimated.

David J. Allard, CHP