Ronald F. Fleming

High accuracy determination of the shape of the efficiency curve of the HPGe detector in the energy range 900 to 1300 keV

Abstract A method for high accuracy characterization of the shape of a High Purity Germanium (HPGe) detector efficiency curve has been developed. Radionuclides with decay schemes having at least two γ lines, for which the yield ratio can be calculated accurately, are used to establish the shape of the detector efficiency curve. Measurements using 60Co and 46Sc sources show that the behavior of the efficiency curve of the detector on a log-log scale is indistinguishable from that of a straight line with a slope of −0.828 ± 0.004. Measurements using a 24Na source show that above 1368 keV this behavior no longer holds due to pair production and escape effects. The measured value of the slope in that case is −0.951 ± 0.002. The proposed method is to be used in measurements requiring the minimum calibration bias in the determination of the reaction rate ratios, such as reactor dosimetry applications, integral cross section measurements and k0 determination for neutron activation analysis.

The half-life of 76As

In the course of making high-accuracy measurements of arsenic, we found that the most recently published and compiled half-life of 76As did not agree with our data as well as the earlier accepted value. To redetermine this parameter, 76As sources were measured on four Ge gamma detector systems, and an exponential function was fitted to the decay data by two different nonlinear least-squares methods. We obtained T1/2 = 1.09379 days with a standard uncertainty of 0.00045 days. This result is 1.5% higher than the most recent value, but is in agreement with the older, less precise, consensus value.

High-accuracy determination of the relative full energy peak efficiency curve of a coaxial HPGe detector in the energy range 700–1300 keV

A method for the high-accuracy determination of the relative full energy peak efficiency is established. Radionuclides that emit at least two gamma-ray lines for which the relative intensity can be found (from the decay scheme) to much better than ±0.1% were used as calibration standards. Specifically, the 889 and 1120 keV lines of 46Sc, the 983 and 1312 keV lines of 48Sc, the 1173 and 1332 keV lines of 60Co, and the 702 and 871 keV lines of 94Nb were implemented. The high-accuracy calibration was taken to extend from the lowest line of 94Nb at 702 keV to the highest line of 60Co at 1332 keV. An analytical expression, based on linear least-squares fitting, was developed to describe the behavior of the relative efficiency curve in that energy range. As a result, the ability of predicting relative full energy peak efficiencies to within ±0.1% (over most of the energy range) was demonstrated. The presented method is applicable in any measurement that requires the minimum calibration bias in the determination of reaction rate ratios. Applications in Neutron Activation Analysis (NAA) and in nuclear reactor dosimetry represent examples of such situations.

Digital signal processing and zero-dead-time counting

One of the most recent developments in equipment for high and/or varying count rates, as encountered in the gamma-ray spectrometry needed for INAA via short-lived radionuclides, is the advent of EG&G Ortecs DSPECplus™. This all-in-one apparatus provides digital signal processing and zero-dead-time counting. One such DSPEC unit was tested at IRI with respect to performance as a function of count rate. Optimum parameter settings for different applications were established. It is concluded that this spectrometer allows for measurements with count rates up to at least 105 cps with an accuracy of a few percent (depending on the calibration approach), a peak position stability of 5.10-3%. and a peak width stability of 5%. It is also concluded that the normal spectrum provides a good estimate for the uncertainties in the zero-dead-time spectrum acquired with this spectrometer.

Direct measurement of ko for monitor neutron activation analysis

This paper introduces the thermal column or the cold neutron guide beam of the 20 MW NBSR at the National Institute of Standards and Technology as a directko measurement facility. Measurement ofko at this facility not only produces accurate values, but avoids the additional correction factors needed in other measurement methods. Theko of Sb, Ag and Cr with respect to Sc as monitor have been measured and their values are comparable to values based on tabulated nuclear constants and to those measured by other researchers.

Statistical properties of gamma-ray spectra obtained with loss-free or zero-dead-time counting, and ORTEC'S variance spectrum

Abstract In the “dual spectrum” approach to non-Poisson counting statistics of gamma-ray spectra obtained with loss-free or zero-dead-time (ZDT) counting, an uncorrected spectrum acquired in parallel is employed to obtain the uncertainties in the dead-time-corrected spectrum. In this paper, it is mathematically proved that this approach can only yield correct results under very special conditions. It is also proved that a variance spectrum acquired by adding the squares of the increments added to the dead-time-corrected spectrum do provide the correct variances in a loss-free or ZDT spectrum.

Detector systems for imaging neutron activation analysis

This paper compares the performance of two imaging detector systems for the new technique of imaging neutron activation analysis (imaging NAA). The first system is based on secondary electron imaging, and the second employs a position sensitive charged particle detector for direct localization of beta particles. The secondary electron imaging system has demonstrated a position resolution of 20 /spl mu/m. The position sensitive beta detector has the potential for higher efficiencies with resolution being a trade off. Results presented show the feasibility of the two imaging methods for different applications of imaging NAA. >

A position sensitive β-γ coincidence technique for multiplexed gamma spectrometry of many small samples

Abstract A technique based on β-γ coincidence has been developed to perform multiplexed gamma-ray spectrometry of small samples using a single gamma-ray detector and a position sensitive beta detector. A system is described that uses a position sensitive photomultiplier tube coupled to a thin plastic scintillator as the beta imaging detector. Multiplexed gamma-ray spectrometry is demonstrated by results obtained with this system for a 4 × 4 array of Au, Co and Ag samples. The advantages of this technique over gamma spectrometry on individual samples are the substantial reduction in total counting time and the reduction in background, which are especially significant in neutron activation analysis of particles.

Uncertainty of centroid and full-width at half maximum determinations and the effect of binning data

The uncertainties in determining the centroid and FWHM of Gaussian-shaped peaks by moments calculations and least squares fitting have been studied. The theoretical uncertainty for the moment calculation method has been derived and found to be the limiting uncertainty for both the moment calculation and least squares fitting. The results of the uncertainty studies have been used to assess the legitimacy of assuming that histogrammed data can be approximated by the continuous Gaussian shape. Increasing the number of bins in the peak was observed to improve the approximation of a continuous distribution. For example, FWHM>or=11 and FWHM>or=15 bins were necessary for the experimental standard deviation of the FWHM to be <or=5% and <or=1%, respectively, of the expected standard deviation for a peak with 10000 counts. Likewise, the uncertainty in determining the FWHM will be minimized under this criteria. The centroid uncertainty is not minimized under these conditions; however, this uncertainty is some fraction of a single bin and thus is relatively small.<<ETX>>

The Materials Dosimetry Reference Facility

A new, high-intensity reference neutron field for reactor dosimetry is in the early stages of operation at the Ford Nuclear Reactor (FNR) at the University of Michigan. Designed and constructed by the National Institute of Standards and Technology (NIST), the facility hosts calibration and validation experiments in support of materials neutron dosimetry for the nuclear power industry and for the metallurgical community engaged in estimating radiation damage in steel. This benchmark is a natural extension of a long-term NIST program to develop standard and reference neutron fields for measurement assurance applications and for testing new detectors and techniques. Field characterization and user operation of the facility is a joint effort by NIST and the Phoenix Memorial Laboratory of the University of Michigan. The materials dosimetry reference facility (NDRF) complements the [sup 235]U cavity fission source at NIST by providing a tenfold increase in fast-neutron fluence, a much larger irradiation volume with modest flux gradients and a neutron spectrum rich in intermediate-energy neutrons. Two spectrum options are available to investigate detector response characteristics and to validate the interpretation of dosimetry measurements.