A new imaging proportional counter using a Penning gas improves energy resolution

Abstract

In recent years Siegmund et al.^1,2have attempted to combine in one detector the position resolution of the multi-wire proportional counter (MWPC)^3–5or the parallel-plate imaging proportional counter (PPIPC)^6,7with the energy resolution of the gas-scintillation proportional counter (GSPC)^8–11, using light from avalanches in a PPIPC. By counting the light flashes from individual primary electrons, Fano factor limited energy resolution was obtained but only for energies below 2 keV. The position resolution was ≈350 µm over an active diameter of 25mm. Sipilä and coworkers^12,13used Penning gas^14,15mixtures to improve the energy resolution to 11% at 6 keV. However, this was achieved at gas gains of <50, and in a single wire proportional counter (PC) without imaging capability. We have achieved similar resolution with gains up to 550 by using a Penning gas in a uniform field. Although satisfactory for energy measurement down to 0.1 keV this gain is not high enough for good position resolution in a PPIPC. We therefore use another stage of uniform field avalanch-ing to boost the overall gain to values >10⁵. The concept of two-stage avalanching was first reported by Charpak et al.¹⁶and used by Breskin et al.¹⁷. We have built and tested a laboratory version of such a detector (Fig. 1), which we call the Penning gas imager (PGI). A Penning gas and a two-stage parallel grid avalanche geometry are combined with a wedge and strip anode (WSA) position read-out system¹⁸. Preliminary experimental energy resolution is 12% FWHM at 6 keV (24% FWHM at 1.5 keV) and the position resolution is 100 µ m FWHM at 1.5 keV. This compares with ≈ 10% and ≈ 1 mm for the GSPC and ≈18% and a few hundred micrometres for the MWPC. The particle background can be reduced by an estimated 99% using an anticoincidence technique.