Gaseous Detector Laboratory

120° High Precision Neutron Imager

A new, large area spectrometer, based on the gas proportional detector, has been constructed for an advanced protein crystallography beam-line at the LANSCE spallation source at Los Alamos Laboratory. The detector has been operational at the beam line since 2001. Some key specifications are a position resolution of about 1.3 mm FWHM, a count rate capability in excess of one million per second, and timing resolution of about one microsecond.

The basis of the detector is neutron absorption in ³He followed by low gain multiplication in a 2D wire proportional chamber. Eight independent segments of wire chambers, read out in parallel, are housed in a pressure vessel, with a radius of curvature of 70cm. Each of the segments has a cathode wire plane, an anode wire plane and a strip cathode plane, all bonded onto a curved, rigid, aluminum support plate. Position readout of each segment is performed along both cathodes, with resistive charge division between a number of nodes. This readout provides an extremely stable response, and low noise position information, thus allowing operation at low gas gain, which greatly extends the life of the detector. The charge signal from each cathode node is processed by charge sensing preamps, followed by newly developed gated baseline restorer (GBLR) circuits. The GBLR outputs are digitized by ADCs and the ionization centroid is calculated using an optimized algorithm with a DSP and a FPGA dedicated to each segment. An overlapping readout scheme ensures that events at the boundaries of the segments are correctly processed, effectively forming a single seamless detector 150cm×20cm in area. Additionally, within each segment, the anode and cathode wires are constructed with an arrangement that significantly reduces anode wire position modulation across the anode wires. The detector operates at a pressure of approximately 3 atm. of propane and 6 atm of ³He, with a circulating gas purification system.

The figure below shows the time profile (right to left) of measured neutron intensity from coenzyme of vitamin B12 at LANSCE’s protein crystallography station. Time represents duration of one beam pulse (~ 30ms) from the spallation source, highest energy neutrons being recorded to the right, lowest energy neutrons to the left. Diffraction peaks sit on top of a scattered background of neutrons.

The 120° Neutron Detector Team

References:
J. Fried, J.A. Harder, G.J. Mahler, D.S. Makowiecki, J.A. Mead, V. Radeka, N.A. Schaknowski, G.C. Smith, and B. Yu,  "A Large, High Performance, Curved 2D Position-Sensitive Neutron Detector," Nucl. Instrum. & Meth. A478 (2002) 415-419.

B. Yu, Z. Zojceski, J.A.Harder, A. Hrisoho, V. Radeka, and G.C. Smith, "Front-End Electronics for High Rate, Position Sensitive Neutron Detectors," Nucl. Instrum. & Meth. A485 (2002) 645-652.


G.J. Mahler, V. Radeka, N. A. Schaknowski, G. C. Smith, B. Yu, and Z. Zojceski, "A New Thermal Neutron Detector for Protein Crystallography," IEEE Trans. Nucl. Sci. NS-46 (1999) 1916-1919.