Microcalorimeter design for fast-neutron spectroscopy
Introduction
High-energy resolution neutron spectroscopy has widespread applications in nuclear science, material characterization and non-proliferation. Precise spectroscopy of fast-neutrons with an energy of and above is challenging, and current instrumentation has either low resolution or is voluminous [1]. For example, a time-of-flight detector with an energy resolution of less than 0.1% for fast-neutrons will need to be tens of meters long. Calorimeter-based neutron detectors offer a compact alternative, with a theoretical energy resolution below 0.1% at for neutrons and an efficiency of 1% or more. The expected theoretical resolution of is especially advantageous for fast neutrons with a kinetic energy of or more.
Neutrons are usually detected indirectly by inducing a nuclear reaction resulting in the emission of prompt energetic particles. Most interesting for our purpose are reactions that release heavy charged particles, since reaction products have a range of only a few tens of micrometers and can be completely stopped inside the absorber. The energy liberated following neutron capture is the sum of the Q-value of the reaction and the kinetic energy of the neutron, the Q-value of commonly used materials being usually positive and on the order of a few MeV. It is thus possible to discriminate between gamma rays and neutrons since the neutron response will be shifted towards higher energy by the amount of the Q-value. This is of particular interest since neutron sources also emit gamma rays, usually in much higher proportion.
Previous work on microcalorimeters for neutron spectroscopy has focused on LiF absorbers coupled to a thermistor [2], [3]. We discuss here the design of a transition edge sensor (TES) based microcalorimeter with a large absorber using and compounds. First results using a small TiB2 absorber are presented.
Section snippets
Detector design
The two most popular isotopes for conversion of neutrons into detectable particles are and due to the high cross-section of the (n,α) reaction. The Q-value for the (n,α) reaction in is 2.31 and (two decay channels) and in . The (n,α) cross-section for thermal neutrons is for and for , and is rapidly decreasing with neutron energy with a 1/v dependence, where v is the neutron velocity. We are using different lithium and boron compounds such as LiF
Demonstration experiments
In order to perform neutron spectroscopy with a microcalorimeter, we made a small volume test detector. The microcalorimeter was composed of a TiB2 absorber coupled with stycast to a Mo/Cu multilayer TES. TiB2 was chosen because of its large heat capacity per unit mass. The TiB2 sample weighed only a few mg and could be supported on a thick Si–N membrane window on which the TES was deposited. The absorber heat capacity of at was sufficient to measure neutron capture events
Acknowledgements
This work was performed under the auspices of the US Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48 and BWXT Y-12, L.L.C., under Contract DE-AC05-00OR-22800.
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