A large area neutron detector array for the LET instrument at ISIS

LET is a direct geometry cold neutron spectrometer at the ISIS spallation neutron source, working in the range between 0.5 and 30 meV (1.6 to 12.8 A). The development and installation of a large area neutron detector array for LET has recently been completed. The detector array consists of 384 3 He filled resistive wire tubes, 4 m long and 25.4 mm in diameter. The 3 He pressure is 10 bars, to guarantee an efficiency above 90% for 1.6 A neutrons, and 3 bars of argon are added as a stopping gas. The tubes are organized in twelve panels installed inside the LET vacuum tank that is operated at cryogenic vacuum. Here we will report about the rigorous assembly and test program of the detectors before their installation on the instrument and their performance on LET. The two main tasks in the assembly of the detectors were: aligning the support of the tubes with a precision better than 0.5 mm along the 4 m length of the tubes and guaranteeing the vacuum tightness for all the ∼2000 vacuum feeds-through. We will outline the methods used to achieve these goals and the tests performed to check these requirements before the installation of the detectors on the instrument. Regarding the operation of these detectors on LET, we will describe the digitization and processing of the signals from the tubes. In particular we will focus the attention on how the signal processing is optimized to operate these detectors at high neutron rates whilst preserving other performances like gamma rejection and position resolution.