Larger Volume Detector Efficiency Parameterization with the MGA Code

ABSTRACT

The Multiple Group Analysis (MGA) code is widely used for the non-destructive determination of the relative isotopic composition of plutonium items. The MGA method is based solely on the analysis of the gamma-ray spectrum recorded using a high resolution gamma spectrometer. The form and function of the code has evolved greatly since its origins in the 1980’s. Over the lifetime of the code there have been tremendous developments in detector technology. The small volume planar Low Energy Germanium (LEGe) detectors available for safeguards work in the early years have given rise to the use of large volume Broad Energy Germanium (BEGe) detectors today in many applications - including the challenging arena of nuclear waste assay. Certain algorithms within MGA have been hardened to meet this challenge and the code is now more robust to poor counting statistics and poorer pulse height resolution as a consequence. The universe of applications to which MGA is being applied is constantly growing. These applications frequently step outside the initial expectations of the code and/or of the technical boundaries of the day. In particular we address here the functional form used within MGA to describe the energy dependence of the relative full energy peak efficiency curve of the Ge detector. The relative efficiency curve is represented by the product of three terms. The filter attenuation factor and self-attenuation factors together modify the basic response function of the Ge crystal. This in turn is parameterized in terms of the active volume of the crystal. At the time MGA was conceived it was envisioned as a safeguards tool to be used exclusively with LEGe detectors. The parameterization was therefore based on this assumption and the maximum detector volume that could be entered was 20 cm ³. In waste assay applications it is now routine to use large BEGe detectors for reasons of higher sensitivity. BEGe detectors have volumes up to about 150 cm 3. We have reviewed the database of BEGe detectors characterized at our facility to derive an appropriate parametric form for this class of large volume detector. The impact on MGA performance is considered.

INTRODUCTION

Plutonium is a special nuclear material that must be accurately measured and accounted for at all times and at all places within the nuclear fuel cycle. Non-invasive gamma-ray measurements of the isotopic composition are important in this regard because such measurements determine the fissile content of items without altering the items in any way. The relative abundance vector forms a unique attribute that can subsequently be used to identify it. Furthermore the results of isotopic analyses can be used to interpret other non-destructive data coming from, for example, neutron coincidence counters and calorimeters.

The Multiple Group Analysis code MGA [1-8] was developed to perform plutonium isotopic analysis on high-resolution gamma ray spectra gathered in-situ. Because of this the following defining characteristics are key to MGA:

• The code was designed to require no calibration apart from that of the energy scale. That is, the only data required by the code are fundamental constants (e.g. gamma-ray energies and branching intensities, half lives of the isotopes, and mass absorption coefficients that are written into the body of the code), the spectrum data and a file containing “setup” parameters.
• It was designed to attain the highest possible precision and accuracy in the shortest measurement and analysis time. This specification implies that the most intense peaks of the spectrum be used. By using the most intense, but also the most complex, regions in the spectrum in certain safeguards applications it requires only a few minutes of measurement time and can attain accuracies of better than 1%.
• It was developed to analyze a wide variety of samples usually without regard to: 1) physical form, size, shape, or container, 2) chemical form or elemental distribution, 3) actinide isotopic distribution (plus small amounts of other activities), and 4) the sample age. That is, both freshly processed and aged samples can be analyzed. The current version of MGA is capable of measuring the following actinides: all of the important isotopes of plutonium (except ²⁴²Pu which is derived using a correlation technique), ²⁴¹Am, ²³⁵U, ²³⁸U, and ²³⁷Np- ²³³Pa. Under certain circumstances, it can also measure the U/Pu ratio, detect the presence of ²⁴³Am- ²³⁹Np and whether the ²⁴¹Am content is homogeneous, and measure (or determine the upper limits of) several long–lived fission products, when using the two-detector mode.
• The code was designed to operate with little or no user interaction. That is, all decisions regarding the treatment of the data must be made internally in the code based on the values of the spectrum data. MGA analyzes the data quickly and runs on conventional PC computers. It can be imbedded in, or linked to, other application codes. Within this minimal set-up environment MGA measures almost any size and type of plutonium sample. It can be applied to U/Pu ratio in MOX samples and can be used to measure the Pu concentration in solutions.

The evolutionary development of MGA began the early 1980’s. Many capabilities have been added since that time as new needs, new measurement situations or new methods for analysis have become apparent. The dramatic increase in power and capabilities of computers has also greatly affected its development.

There is considerable interest in using MGA in association with the neutron coincidence method to measurements of plutonium in waste containers [2]. Since high accuracy (i.e. 1%) measurements are not usually required for this application, the isotopic values do not need be as good as for accountability type measurements. The principal gamma ray spectrometry problem that arises here is the very low number of counts obtained in spectra taken of low plutonium-content samples. Significant efforts have been devoted to “hardening” the code to analyzed statistically poor data, or exit properly so that unattended measurements may continue. Our experience indicates that a plutonium mass of 10 mg can be detected in a measurement time of 10-30 minutes in a 208 liter waste drum filled with a low atomic number matrix (e.g. combustibles) of medium density (i.e. 0.3g.cm ^-3).

Pu has a fairly soft gamma ray emission spectrum. The performance (detection limits – assay times) for the measurement of contact handleable waste items is therefore considerable improved if a large area detector is used so that the geometrical efficiency (solid angle) is increased. However when fission and activation products that emit high energy lines are also of interest it is also an advantage to have a thick detector to achieve a high stopping power. A similar need arises when the matrix attenuation is severe or when a high mass of Pu is present in a form that is highly self attenuating. In these cases the weak but more highly penetrating Pu lines may be used in the assay as a way of reducing the overall uncertainty. Differential peak analysis (plotting the apparent mass vs energy) forms the basis of a ‘lump’ correction strategy. Clearly therefore a wide energy range is a useful feature. A single detector covering both the low and high energy range effectively has a cost and complexity advantage since it requires only a single collimator housing and nucleonics chain. The challenge is to achieve the large volume detector and sought after aspect ratio with the energy resolution needed for analysis of the complex spectral regions used by MGA.

The Broad Energy Germanium (BEGe) detector configuration comprises a near right circular piece of high purity Ge typically with a large area flat face and a short length. This enhances the low energy efficiency and reduces high energy background in typical counting geometries. These detectors have thin (20-30 mm) stable ion implanted front contacts so that the nominal energy range extends down to 3keV with appropriate low atomic number end-cap. This means that the efficiency is relatively easy to calculate using transport codes. Usually we choose to control the low energy response through the use of external filters as will become apparent later. The filters have the important role of limiting the dead-time rate loss particularly in those situations where the 59keV radiation from ²⁴¹Am is dominant. The BEGe is designed with an electrode structure that enhances low energy resolution and is fabricated from select Ge having an impurity profile that improves charge collection and thus resolution and charge collection at medium and high energies also. Consequently the resolution at low energies is equivalent to that of traditional Low Energy Ge (LEGe) planar detectors (widely used below 150keV) while at high energies it is comparable to that of good quality coaxial detectors. Additionally because of their larger volume compared to LEGe detectors they have high efficiency at medium to high energies (200-3000keV say). These are the features that make the BEGe geometry attractive for the measurement of the complex spectra from special nuclear materials and other gamma emitting radionuclides found in waste.