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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 6.2, p. 136
Section 6.2.1.4.1. Multiwire proportional counters
a
Life Sciences Division M888, University of California, Los Alamos National Laboratory, Los Alamos, NM 8745, USA, and bSmall Angle Scattering Facility, Australian Nuclear Science & Technology Organisation, Physics Division, PMB 1 Menai NSW 2234, Australia |
The principles of a multiwire proportional counter (MWPC) are well established (Sauli, 1977![[link]](/graphics/greenarr.gif)
) and have wide application. For thermal neutron detection (Radeka et al., 1996), the reaction of choice is ![[^{3}\hbox{He} + \hbox{n} \rightarrow {}^{3}\hbox{H} + \hbox{p} + 764\hbox{ keV}.]](/teximages/fach6o2/fach6o2fd7.svg)
The 191 keV triton and the 573 keV proton are emitted in opposite directions and create a charge cloud whose dimensions are determined primarily by the pressure of a stopping gas. Depending on the work function of the gas mixture, approximately ![[3\times 10^{4}]](/teximages/fach6o2/fach6o2fi28.svg)
electron–ion pairs are created. Low-noise gas amplification of this charge cloud occurs in an intense electric field created in the vicinity of the small diameter (20–30 µm) anode wires (Radeka, 1988). Typical gas gains of ~10–50 lead to a total charge on the anode of ~50–100 fC. The efficiency of the detector is determined by the pressure of 3He, and the spatial resolution and count-rate capability are determined by the detector geometry and readout system. The event decoding is selected from the time difference (Borkowski & Kopp, 1975), charge division (Alberi et al., 1975), centroid-finding filter (Radeka & Boie, 1980), or wire-by-wire techniques (Jacobé et al., 1983; Knott et al., 1997). Present MWPC technology offers opportunities and challenges to design a detector system that is totally integrated into the instrument design and optimizes data collection rate and accuracy (Schoenborn et al., 1985, 1986; Schoenborn, 1992b).
A concept related to the MWPC is the micro-strip gas chamber (MSGC). With the MSGC, the general principles of gas detection and amplification apply; however, the anode is deposited on a suitable substrate (Oed, 1988, 1995; Vellettaz et al., 1997). The MSGC can potentially improve the performance of the MWPC in some applications, particularly with respect to spatial resolution and count-rate capability.
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