Implementation and Testing of the New Collimator Algorithm in ISOCS (In situ Object Calibration Software)


R. Venkataraman, F. Bronson


About 2 years ago, Canberra introduced the In Situ Object Calibration Software (ISOCS) in-situ Ge detector measurement system, which included collimated Ge detectors, and mathematical efficiency calibration software. Each user also received a Validation Testing document which contained the results of a series of >100 independent evaluations of the accuracy mathematical calibration routines; about 30 of the tests were to evaluate the collimator accuracy. That document showed adequate accuracy for most collimated in-situ measurements (~30% for normal moderately collimated situations), but cautioned the users that highly collimated detectors used to measure large source could have uncertainties approaching a factor of 2. While this is not too bad for these geometries, and certainly better than the inherent (and usually unrecognized) errors in small-quantity sampling for laboratory analysis, improvements were desirable. It was also desired to make the collimator algorithm suitable for both rectangular and cylindrical collimators. A series of new algorithms was designed and tested for accuracy improvements, and minimal computational speed degradation. The elements of the new algorithm that was chosen will be described. Although many improvements are made internally, the user interface is the same and quite simple. The algorithm is now very generic, and equally applicable to rectangular side shields and end collimators with trapezoidal holes, as well as cylindrical side shields and end collimators with conical holes. Extensive testing has been done which now indicates that there is no measurable degradation in accuracy of the overall ISOCS efficiency calibration by the use of collimated detectors. Accuracies of both collimated and uncollimated detector efficiencies are about 4-5% (1s) at photon energies >150 keV, and 8-10% at energies <150 keV. The computation time is somewhat longer than before, but today's faster computers have just about made up the difference.


This abstract was presented at the 33rd Annual Midyear Meeting, "Instrumentation, Measurements, and Electronic Dosimetry", Laboratory Use Session, 1/30/2000 - 2/2/2000, held in Virginia Beach, VA.

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