Performance characteristics of the new detector array for the SANS2d instrument on the ISIS spallation neutron source

• Published in: Journal of instrumentation Vol. 9; no. 12; p. C12051
• Main Authors: Duxbury, D., Heenan, R., McPhail, D., Raspino, D., Rhodes, N., Rogers, S., Schooneveld, E., Spill, E., Terry, A.
• Format: Journal Article
• Language: English
• Published: 23.12.2014
• Subjects: Arrays; Counting; Detectors; Neutron sources; Position sensitive; Spallation; Tanks; Tubes
• ISSN: 1748-0221; 1748-0221
• DOI: 10.1088/1748-0221/9/12/C12051

The performance of the new position sensitive neutron detector arrays of the Small Angle Neutron Scattering (SANS) instrument SANS2d is described. The SANS2d instrument is one of the seven instruments currently available for users on the second target station (TS2) of the ISIS spallation neutron source. Since the instrument became operational in 2009 it has used two one metre square multi-wire proportional detectors (MWPC). However, these detectors suffer from a low count rate capability, are easily damaged by excess beam and are then expensive to repair. The new detector arrays each consist of 120 individual position sensitive detector tubes, filled with 15 bar of super(3) He. Each of the tubes is one metre long and has a diameter of 8mm giving a detector array with an overall area of one square metre. Two such arrays have been built and installed in the SANS2d vacuum tank where they are currently taking user data. For SANS measurements operation of the detector within a vacuum is essential in order to reduce air scattering. A novel, fully engineered approach has been utilised to ensure that the high voltage connections and preamps are located inside the SANS2d vacuum tank at atmospheric pressure, within air tubes and air boxes respectively. The signal processing electronics and data acquisition system are located remotely in a counting house outside of the blockhouse. This allows easy access for maintenance purposes, without the need to remove the detectors from the vacuum tank. The design will be described in detail. A position resolution of 8mm FWHM or less has been measured along the length of the tubes. The initial measurements taken from a standard sample indicate that whilst the detector arrays themselves only represent a moderate improvement in overall detection efficiency (~ 20%), compared to the previous detector, the count rate capability is increased by a factor of 100. A significant advantage of the new array is the ability to change a single tube in situ within approximately one day with a relatively small staff effort. The results obtained from the first user trials are reported.