The Use of ²⁵²Cf for Calibrating Safeguards Monitors

ABSTRACT

Spontaneous fission ²⁵²Cf neutron sources are commonly used to calibrate and test the operating performance of safeguards instruments and portal monitors. ²⁵²Cf is preferred over (á,n) sources for many operational and safety reasons, but most importantly can be designed to emit an energy-dependent spectrum that closely matches that of ²⁴⁰Pu. ²⁴⁰Pu is the pre-dominant neutron emitter of plutonium metal and oxides. Because portal monitors and other border monitoring devices are designed to detect low neutron fluence rates at distances of 1-3 metres from the detector, several characteristics in the manufacture and decay of the source need to be understood and accounted for in order to make an accurate measurement of the performance: isotopic composition and ²⁵⁰Cf to ²⁵²Cf evolution, age, traceability, angular dependence of the neutron flux, and design and manufacture of the moderating cover are examples. This paper describes these issues and presents the range of possible error terms in the utilization of these physically small, relatively weak, (10 4 n.s -1) neutron sources for calibration of portal monitors.

INTRODUCTION

For calibration of neutron coincidence counters and multiplicity counters, ²⁵²Cf is used as a surrogate for ²⁴⁰Pu, the principle isotope of interest in either low or high burnup plutonium fuels. In 1993 Croft [1] presented the results of Harwell Laboratory’s work with ²⁵²Cf as a calibration source for passive neutron counters. His work presents a comprehensive review of the issues and effects observed, in time, with the neutron multiplicity distribution, mean neutron energy, the Reals to Totals ratio, and the overall effect of source isotopic composition, particularly as it influences the neutron output over time. In passive neutron counters it is imperative to understand and account for these effects so that an accurate representation of the counter’s performance relative to a mixture of spontaneously fissioning isotopes found in safeguarded and waste materials of plutonium ( ²³⁸Pu/ ²⁴⁰Pu/ ²⁴²Pu).

In the last few years, a significant effort is underway for a large-scale development and deployment of homeland defense and border monitoring equipment. Two types of neutron detection systems have been deployed: large, fixed-geometry portal monitors and hand-held radioisotope identifiers (RIDs). Most of these systems use moderated ³He proportional counters to detect fission neutrons. Various algorithms are used to analyze the signal neutron pulse train and alarm when the rate is determined to be greater than the background rate. ²⁵²Cf sources are used to calibrate the systems for efficiency and to test overall system performance. Performance-based testing and calibration with ²⁵²Cf provides assurances to the instrument users that IAEA Significant Quantities (SQs)¹ can be effectively detected and interdicted, as necessary.

This paper discusses the calibration issues of ²⁵²Cf relative to the performance testing of these devices for international safeguards and interdiction, and presents an additional framework to the original efforts of McGarry and Boswell. [2]