Neutron multiplicity analysis has been a valuable technique for safeguards measurements of plutonium oxide and mixed oxides.
Historically, most of these measurements have been performed using shift register based electronics. The shift register data
acquisition lacks certain flexibility because the basic coincidence parameters (e.g., pre-delay, gate width, and long delay)
must be fixed prior to the start of the measurement and the values may potentially, therefore, be sub optimal. List mode or
time stamped data acquisition records the arrival time of each pulse thereby preserving the history of the pulse stream and
allowing analysis and reanalysis using software analogs to the shift register circuit with adjustable parameters. Until recently,
the data rates encountered in the assay of modest amounts of plutonium using efficient multiplicity counters were beyond the
capacity of readily available personal computers. The calibration of the large epithermal neutron multiplicity counter (LEMC)
for assay of plutonium scrap materials is used as a vehicle to compare the performance of the multiplicity shift register
and a commercially available list mode acquisition module.
In order to estimate by calculation the magnitude of the true coincidence summing losses that may be affecting the observed gamma-ray spectrum of a given nuclide, measured using a spectrometer, knowledge of the total detection efficiencies at the gamma-ray energies within the cascades is essential. The total efficiency can be determined from the full energy peak efficiency, provided the peak-to-total ratio is known. For a given high purity germanium (HPGe) detector, one can establish an intrinsic peak-to-total (P/T) efficiency curve using a set of measurements performed with “single” (ideally monoenergetic) gamma-emitting nuclides (e.g., 241Am, 109Cd, 57Co, 113Sn, 137Cs, 65Zn). Some of these nuclides are short lived and so have to be replaced periodically. Moreover, the presence of low energy gamma-rays and X-rays in most of the decay schemes complicate the empirical determination of the P/T ratios. This problem is especially severe if measurements are made using HPGe detectors that have a very thin dead layer. The problems posed by low energy gamma-rays and X-rays can be avoided by using absorbers, but then one has to be careful not to perturb the intrinsic value of the P/T ratio being sought. This paper addresses these problems. Measurement related limitations are avoided if one can use a computational technique instead. In the work presented here, the feasibility of using a Monte-Carlo based technique to determine the P/T ratios at a wide range of energies (60 keV to 2000 keV) is explored. The Monte-Carlo code MCNP (version 4B) is used to simulate gamma-ray spectra from various nuclides. Measured P/T ratios are compared to calculated ratios for several HPGe detectors to demonstrate the generality of the approach. Reasons for observed disagreement between the two are discussed.
Authors:S. Croft, R. McElroy, S. Philips, R. Venkataramen, and D. Curtis
The application of quantitative high-resolution gamma-ray spectrometry for the non-destructive assay of plutonium bearing
items, such as waste drums, is complicated by self-attenuation if the plutonium is present as lumps. By definition, lumps
are small compared to the bulk matrix and so are not accounted for in the gross matrix correction yet can exert a significant
influence on the assay result. Compared to a calibration using dilute standards, self-attenuation results in an under-reporting
of the mass of plutonium present. The availability of representative standards is unrealistic for diverse waste streams and
so a means to detect and compensate for the presence of lumps is needed. An experimental approach that can in principle generate
an item specific correction factor is to exploit the differential attenuation between a set of gamma-lines of known relative
emission intensity. In the case of routine measurements of drummed Pu wastes the choice of lines is often limited, the most
appropriate often being those at 129 keV and 414 keV from 239Pu. This paper discusses the problems and potential of exploiting this pairing in a simple dual energy approach to the long
standing and challenging problem of self-attenuation.
Authors:A. Bosko, S. Croft, S. Philips, and R. Gunnink
Nondestructive measurements of γ-ray and X-ray emissions are often made to characterize special nuclear materials. Various
computer codes are available to determine the relative isotopic composition of uranium or plutonium (along with certain other
associated nuclides) from analysis of the spectra resulting from such measurements. MGA (Gunnink, Proceedings of the 9th ESARDA
symposium on safeguards and nuclear management 167, 1987) and MGAU (Gunnink et al., Proceedings of the IAEA symposium on international
safeguards 541, 1994) are among the major isotopic codes. The purpose of this study was to investigate MGA and MGAU performance
versus energy resolution of the counting system.
Authors:H. Zhu, R. Venkataraman, N. Menaa, W. Mueller, S. Croft, and A. Berlizov
When radionuclides decay by cascading photons, the accuracy of the measured nuclide activity may be affected by true coincidence
summing effects. The effects can be quantified by Monte Carlo simulations that can handle correlated γ-and X-ray emissions
from a radionuclide. Analysis techniques are also available commercially to correct for the effects due to cascading γ-rays.
The MCNP-CP code was used to compute the effects in high purity germanium detectors for several commonly used nuclides and
geometries and the results were compared to measurements and an analysis technique. Excellent agreement in true coincidence
summing corrections predicted by MCNP-CP and the analysis technique was obtained. In addition, the X-ray true coincidence
summing effects were evaluated.