Analysis of long-lived isotopes in the presence of short-lived isotopes using zero dead time correction

摘要

High Purity Germanium (HPGe) detector systems are routinely used in counting laboratories in many types of nuclear facilities such as nuclear power plants and fuel production sites. These systems generally consist of a lead-shielded HPGe detector, Multi Channel Analyzer (MCA), and analytical software. These systems are used to analyze a wide variety of sample types for many different isotopes. Analysis of certain sample types, such as those from the reactor coolant or the off-gas extraction system, in nuclear power plant radiochemistry laboratories is complicated by the presence of short-lived isotopes. With these isotopes present, the sample count rate begins at a higher value than the ending count rate with a rapid change in count rate often observed. This decaying of the sample count rate causes the true count rate of the peaks to be unknown for those MCAs that use the traditional Live Time Clock extension methods. The current method of compensating for these short-lived isotopes is simply to delay starting the acquisition until these isotopes decay (typically 45-60 minutes). This has the effect of reducing the throughput capacity of the laboratory meaning fewer samples can be counted in any given period. The use of "loss free counting" methods in radiochemistry laboratories has been unacceptable because these methods do not provide the uncertainty in the measurement which must be reported with the activity calculation from the counting laboratory. An innovative MCA with a zero dead time (ZDT/spl trade/) correction method will be presented which (1) compensates for the decaying count rate caused by the short-lived isotopes, thus eliminating the need for delaying the start time of the acquisition; and (2) calculates the uncertainty in the activity determination, thus satisfying the reporting requirements of the counting laboratory. Data from the analysis, including the uncertainty, of long-lived isotopes in reactor coolant samples both in the presence and absence of short-lived isotopes will be presented.