Comparison of Gamma and Passive Neutron Non-Destructive Assay Total Measurement Uncertainty Using the High Efficiency Neutron Counter

ABSTRACT

Since 1997, the High Efficiency Neutron Counter (HENC) has been employed by the Los Alamos National Laboratory (LANL) to assay and ship to the Waste Isolation Pilot Plant (WIPP) plutonium waste using passive neutron methods. During 2004 and early 2005, the HENC has undergone a modification and recertification to add a Canberra broad energy germanium detector (BEGe), DSA-1000 digital spectrum analyzer, and JSR-14 neutron analysis shift register so as to conduct simultaneous gamma and passive neutron measurements. The BEGe detector provides both isotopic and quantitative measurement data to complement the neutron detectors. The assay is under the control of Canberra’s NDA-2000 software suite that provides acquisition control and analysis.

As part of the recertification effort, the total measurement uncertainty (TMU) is being evaluated. A comparison of the gamma and the passive neutron TMU will be provided and the NDA 2000 settings used to achieve the results will be summarized and reported.

INTRODUCTION

The High Efficiency Neutron Counter (HENC) has been employed by the Los Alamos National Laboratory (LANL) to assay plutonium waste using passive neutron methods since 1997. During 2004 and early 2005, the HENC has undergone a modification and recertification to add a Canberra broad energy germanium detector (BEGe), DSA-1000 digital spectrum analyzer, and JSR-14 neutron analysis shift register so as to conduct simultaneous gamma and passive neutron measurements. The BEGe detector provides both isotopic and quantitative measurement data to complement the neutron detectors. The assay is under the control of Canberra’s NDA-2000 software suite that provides acquisition control and analysis.

As part of the recertification effort, the total measurement uncertainty (TMU) is being re-evaluated. Unfortunately, the measurements to support a full TMU determination could not be performed so, in lieu, data from a similar instrument, the MCS HENC, was used to evaluate the effect of the changes in operating methodology¹ LANL intends to use.

The focus of this study is to evaluate how the gamma and neutron measurements, and the associated TMU, should be used for the LANL HENC in light of the operational experience of the MCS HENC. There are significant differences in how the neutron data is to be used, utilizing only the add-a-source corrected doubles coincidences and neutron Ones rates. Further, while the MCS HENC utilizes a second order calibration curve, the LANL HENC uses a linear calibration constant. These are the changes with which the MCS data are evaluated, and care should be exercised in extending these results to MCS Henc operations.