Genie2000 Cascade Summing Correction Validation Testing

To validate the accuracy and precision of the Genie2000 Cascade Summing Correction method, over 800 archived measurements were made on calibrated sources (filter paper, 20cc liquid scintillation vial, 400 ml beaker and Marinelli beaker) containing cascading (Y and Co) and non-cascading isotopes. These measurements used 133 different high purity germanium detectors that were characterized for use by the ISOCS/LabSOCS mathematical efficiency software. These detectors ranged in size from 10cc to 450cc (100% relative efficiency), and covered all types of Ge detectors. MCNP was used to calculate required peak-to-total efficiency calibrations. This study compared the Genie2000 cascade summing corrected results for the cascading isotope activities to the known activities. The analysis shows that the method is effective and accurate. The method appears to be valid for all sizes of detectors tested. Evaluation of the accuracy as a function of the amount of correction reveals a small systematic error for which a variable precision adjustment is recommended. Introduction A common but under-recognized error in gamma spectroscopy is from cascade summing. This can happen when calibrating with or when measuring certain nuclides in high efficiency geometries. These nuclides include Co-60, Y-88, Ba-133, Eu-152, and Cs-134, among many others. Cascade summing can happen with a nuclide that has a decay scheme with more than one gamma ray, and when two or more of these gammas from the same decaying atom interact with the detector. This process can sometimes increase the full energy peak count rate [called summing-in] but normally results in the loss of peak count rate [summing-out]. When this process happens to the nuclide being measured, the results are normally underreported. When this happens to the nuclide being used to generate the efficiency calibration, then the entire calibration curve near to the affected energies will be [normally] low, and therefore the samples measured using this calibration curve will be reported high. The magnitude of these errors is commonly in the 5-30% range, but can be greater than 50% for certain nuclide-detector-geometry combinations. For those nuclides emitting cascade gammas, the amount of error is directly proportional to the efficiency. High efficiency samples have higher errors. This high efficiency can be because the geometry is high [small samples close to the detector] or because the detector is large, or both. It should be noted that this error is totally independent from the activity of the sample; it applies to low activity samples as well as high activity samples. And, if the calibration efficiency is in error, it also applies to the MDA values, even if there was no activity detected. The fact that Y-88 and Co-60 are extremely common calibration nuclides, and that they form the only high energy datapoints in the calibration curve, and that all 4 of the high energy calibration lines have significant cascade summing errors makes source-based efficiency calibrations for close-in geometries have this error. And, it is difficult to see this problem by inspection of the calibration curve, since all 4 of the high energy datapoints are in error by a similar amount. In general, keeping a source more than 10 cm from a high purity germanium (HPGe) detector will reduce potential cascade summing effects to less than 2%. However, the count time would have to be increased by more than an order of magnitude to achieve the same precision or detection limits, which is normally not desirable.