Characteristics of scintillators for well logging to 225°C

Application-oriented performance characteristics for scintillators [NaI(Tl), CsI(Tl), CsI(Na), BGO, and BC-438 plastic] were evaluated as a function of temperature from 25°C to 225°C, as appropriate. Relative pulse height and pulse height resolution are reported. It is shown that the scintillator most appropriate for measurement above 100°C is NaI(Tl). At 200°C, NaI(Tl) still has more than 50% of its light output and its pulse height resolution has increased by less than a factor of 1.5. Because the decay constants of scintillators tend to shorten, shorter integration time constants can be used at higher temperatures to collect all the charge generated by the photomultiplier tube. Differences in the data due to geometry suggest that volume absorption effects are present and that these effects have a stronger temperature dependence than the scintillator itself