Realizing Thick-Film Boron Carbide Direct-Conversion Neutron Detectors
Abstract
Developing neutron detectors for the detection of special nuclear materials is an important goal in the counter weapons of mass destruction mission. Of the various neutron detector configurations, solid-state semiconductor designs offer the promise of smaller/lighter, cheaper, and lower-power detectors. In a semiconductor-based detector, neutrons are transduced (converted), via charged particle reaction products, to electron–hole pairs that can be swept out and detected by the semiconductor device. There are two main types of solid-state configurations: conversion-layer devices, where a neutron-sensitive layer capable of capturing neutrons and generating charged particle products is coupled to a traditional semiconductor charge detection device within which electron–hole pairs are generated, separated, and collected (detected), and direct-conversion devices, in which the entire neutron transduction process—from neutron absorption to electron–hole pair detection—takes place within the bulk semiconductor device. Conversion-layer devices have been successfully developed and marketed, but they are ultimately limited by geometry. Charged particle products only travel so far, and if the conversion layer is thick enough to absorb all of the incoming neutrons, not all of the charged particle will make it to the semiconductor layer, but if the conversion layer is made too thin, not all of the incoming neutrons are absorbed. Planar devices made with an optimum conversion layer thickness only reach theoretical thermal neutron detection efficiencies on the order of 5–10%. Complex conversion-layer structures based on trenches and pillars have been designed to alleviate part of this problem, but still cannot achieve 100% theoretical efficiency and are complicated and expensive to produce. Direct-conversion detectors are, in principle, an ideal solution, as all neutrons can be absorbed and converted to detectable charge, with the potential for 100% theoretical detection efficiency in a simple planar device, but these require very specialized semiconductor materials capable of absorbing neutrons, which are technologically immature and generally perform poorly from a charge detection standpoint. Of the few isotopes with adequately high neutron sensitivity (3He, 6Li, 10B, 113Cd, 157Gd, 235U/238U), 10B is one of the most promising for neutron detection as it has a very high neutron absorption cross section and low sensitivity to interfering gamma rays. Amorphous boron carbide (a-BC:H) has shown potential for detector applications due to low electrical leakage current (necessary for detecting neutrons over electrical noise) and demonstrated tunability of thin-film properties. Although prototype detectors based on thin (~1 ?m) films have been demonstrated, scale up to the thicknesses required to achieve maximal thermal neutron detection efficiencies (i.e., tens to hundreds of microns) is limited by the poor charge transport properties in these materials and challenges in fabricating thick films. The objective of this project is to solve both of these limitations for a specific a-BC:H variant, grown by plasma-enhanced chemical vapor deposition (PECVD). The effort will focus on determining and strategically optimizing critical charge transport metrics that define charge collection efficiency/neutron detection efficiency in a-BC:H films by tuning film growth conditions within a PECVD regime conducive to thick-film growth. Charge transport measurement and analysis techniques will be adapted to extract key properties (e.g., electron/hole mobility and lifetime) in this non-traditional amorphous semiconductor, providing guidance toward this optimization. In parallel, studies on film stress, adhesion, and environmental stability will be completed, such that any limitations to ultimately scaling up film thicknesses once charge transport metrics are met can be identified and mitigated.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 26, 2016
- Source ID
- HDTRA11510020
Entities
People
- Michelle Paquette
Organizations
- Defense Threat Reduction Agency
- University of Missouri–Kansas City