Answer to Question #1013 Submitted to "Ask the Experts"

Q

What are the effects of temperature on the NaI(Tl) scintillator detectors? Precisely, I want to know what kind of transition occurs during de-excitation (direct or indirect).

A

As you know, NaI(Tl) has been the standard against which other scintillators have been compared, especially with regard to light yield, for many years. As is common for most scintillators, the light yield per unit energy deposition decreases with increasing temperature in NaI(Tl). For a typical three-inch diameter by three-inch deep crystal, one might expect an approximately linear drop in light output of about 0.15 to 0.2% per degree centigrade in the interval from 20 degrees centigrade to 180 degrees centigrade. The decrease in light yield with increasing temperature may vary with the physical dimensions of the NaI(Tl) crystal. Knoll (Radiation Detection and Measurement, 3rd ed., John Wiley & Sons, Inc, New York, 2000) suggests that this may be due to differences in surface reflectivity among crystals. More than 90% of the light output from NaI(Tl) is from short-lived fluorescence with a decay time of less than 250 ns, which is what I assume you are referring to as the direct component. The remainder is associated with longer-lived phosphorescence (indirect component) with a much longer lifetime of about 150 ms. The decrease in light yield is accompanied by a simultaneous decrease in resolution and a decrease in the fluorescence decay time. I hope this information is useful to you.

George Chabot, CHP, PhD