Straw detector for high rate, high resolution neutron imaging

Instrument requirements in soon to be commissioned and planned neutron scattering facilities call for large area detectors that can sustain rates up to 10/sup 8/ cps and millimeter-level spatial resolution. Although /sup 3/He pressurized area detectors can provide good spatial resolution, sensitivity and gamma ray discrimination, this technology cannot achieve the required rates without further development. Moreover, achieving large detection areas with pressurized /sup 3/He technology is expensive because of the complexity of the pressure containing vessels required. We propose a detector technology, based on thin-walled straws, lined with a 1 /spl mu/m thick sputter coating of enriched boron carbide (/sup 10/B/sub 4/C). Neutrons converted in /sup 10/B generate charged particles that subsequently ionize the gas contained within each straw. Because the /sup 10/B/sub 4/C coating is very thin, efficient escape of the reaction products can be achieved. A panel detector consisting of several thousand close-packed individual straws, which are read out independently, can easily support high event rates. We present performance testing of two 50-straw prototypes (non-enriched B/sub 4/C), including detection efficiency, spatial resolution, and two-dimensional imaging. Each straw has a diameter of 4 mm and a length of 1 m. Additional tests of a single 50 cm long straw, 2 mm in diameter are also presented.

I. Introduction

THE Spallation Neutron Source (SNS) facility, due for completion at Oak Ridge in 2006, will push available thermal neutron flux at least an order of magnitude above that achievable at any other neutron science facility [1]. The increased flux imposes extreme rate requirements on the neutron detectors in many of the proposed instruments. Moreover, many of the SNS experiment stations require detector areas of several square meters. Detector requirements for the Small Angle Neutron Scattering (SANS) instrument, as an example, are listed in Table 1.