Operation of scintillators and SiPMs at high temperatures and their application for borehole logging

Gamma-ray detection is extensively used in borehole logging - a technique widely employed in oil and gas, and mineral exploration. The workhorse of this detection application for many years has been traditional NaI(Tl) scintillators coupled to photo-multiplier tubes (PMTs) which can provide the performance and energy resolution required in this application. PMTs are a well proven technology which can operate in the high temperature conditions (typically of order 100 degrees C) and pressures (10 MPa) encountered during logging activities. PMTs are, however, fragile in that they incorporate an evacuated tube. They also have a large form factor and require ancillary electronics such as a high-voltage supply meaning they occupy significant space within the borehole probe. It would be advantageous to have a compact replacement to allow additional instruments to be included within the borehole probe. Silicon photo-multipliers (SiPMs) are an attractive PMT replacement since they are robust, compact and operate at low voltage. However, SiPMs suffer from dark current which increases rapidly with temperature leading to increased noise and degraded energy resolution. We have evaluated CsI(Tl) scintillators coupled to standard 6 x 6 mm(2) SiPMs from Hamamatsu and SensL as a function of temperature. We have shown that these prototypes operate effectively up to a temperature of 80 degrees C which could satisfy the requirements of some applications of borehole logging where the maximum temperature encountered is 75 degrees C.