Microcalorimeter design for fast-neutron spectroscopy

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Abstract

We are developing microcalorimeters for fast-neutron spectroscopy. The goal is to develop a detector with an energy resolution of 0.1% for 1–20MeV neutrons with an efficiency of 1%. We discuss the design of such a detector and present the first results of a transition edge sensor based microcalorimeter with a small TiB2 absorber. The best energy resolution obtained was 5.5keV FWHM for a total energy deposition of 2.792MeV by thermal neutrons.

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 1MeV 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 0.1K for 1MeV neutrons and an efficiency of 1% or more. The expected theoretical resolution of 1keV is especially advantageous for fast neutrons with a kinetic energy of 1MeV 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 6Li and 10B 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 10B and 6Li due to the high cross-section of the (n,α) reaction. The Q-value for the (n,α) reaction in 10B is 2.31 and 2.79MeV (two decay channels) and 4.78MeV in 6Li. The (n,α) cross-section for thermal neutrons is 3840b for 10B and 940b for 6Li, 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 1mm3 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 1μm thick Si–N membrane window on which the TES was deposited. The absorber heat capacity of 10nJ/K at 130mK 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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