Evaluation of Dead-Time Effect in HPGe Detector

Measurements and counting of gamma emitting radionuclides are usually performed by high-purity germanium spectrometers. Nearly in all detector systems there is a minimum amount of time ("dead-time") between two sequential pulses. This resolving time depends on the capacity of the supporting electronics to record a large number of events during short period of time. Due to the random nature of radioactive decay, there is always some probability that a true event will be lost because it occurs too quickly following a preceding event. These losses can become rather important when high counting rates are encountered. Thus, accurate counting measurements made under these conditions must include proper correction to account for the lost events due to the dead time effect. In modern counting systems the dead time is inherently evaluated using dedicated software and hardware and presented to the user.
As part of the verification of the proper operation of a spectrometry system, we directly measured the dead time effect. The tested system was a mobile HPGe detector (model Falcon5000, Canberra). Radioactive sources of varying activity levels were used to evaluate the response of the detector, thereby achieving dead times of up to 90%. We found the measurements to be in the normal range. We intend to present the methods used and results of the tests.