Precision Neutron Flux Measurement and Advancement Using the Alpha Gamma Technique

Abstract

Absolute neutron flux detection is important for many scientifi c applications and critical for the measurement of the free neutron lifetime in a beam of cold neutrons [1]. The neutron lifetime is a critical input for Big Bangnucleosynthesis and tests of unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) matrix [2, 3] as well as the measurement of standard neutron cross sections and branching ratios of neutron interactions for a variety of isotopes. This thesis discusses the results of a new method to measure neutron flux called the Alpha-Gamma (AG) technique. A transfer calibration technique allows the detection efficiency of a pair of HPGe gamma detectors to be coupled to the measured absolute decay rate of a 239Pu source. A thick 10B target is then used to measure the absolute flux of neutrons in the beam. Using this technique we show that it is possible to determine the detection efficiency of the Flux Monitor (FM) used in the in-beam neutron lifetime experiment to an absolute accuracy of better than 0.1%. We present the results and systematic error analysis for the calibration of the neutron monitors now in use in the new neutron lifetime measurement at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) [4].