M.J.L. Turner

Lifetime of proportional counters filled with xenon-methane and argon-methane

Abstract Methane is a useful quench gas for proportional counters, but is known to polymerize on the anode wire causing a loss of performance after long exposure to ionizing radiations. Results are presented on the degradation of both argon and xenon filled proportional counters using methane quenching. The effects observed vary, and depend on whether the irradiation is isotropic or anisotropic, and on its energy. However, the onset of degradation appears to depend only on the total charge accumulated on the anode, irrespective of the gas gain. An allowable total charge before degradation may be expected is derived from the results, and means of applying this to detectors of different geometry are provided.

Advanced deconvolution techniques for coded aperture imaging

The standard deconvolution technique for coded aperture imaging is cross correlation with the relevant mask pattern. In practice such an imaging system is imperfect, giving incomplete coding, and direct cross correlation with the mask introduces “ghosting” and enhanced noise fluctuations. This paper describes the application of image processing techniques such as the Wiener Filter and the Maximum Entropy Method to deconvolving the recorded shadow pattern and illustrates the advantage of such methods using a computer simulation of a wide field camera for X-ray astronomy.

Avalanche fluctuations in a uniform field proportional counter filled with the penning mixture neon-argon

Abstract The energy resolution of a proportional counter depends on fluctuations in the primary ionization and subsequent electron avalanche. In general avalanche fluctuations dominate and an improvement in energy resolution can be expected by raising the efficiency of the ionization mechanisms involved. This can be achieved using Penning mixtures. Previous theoretical investigations of avalanche fluctuations predict such an improvement but detailed comparison between theory and experiment requires the use of an uniform field proportional counter. Avalanche fluctuations in the Penning mixture neon-argon have been investigated under uniform field conditions. Comparison between the theory and the experimental results suggests that factors other than ionization efficiency play an important role and limit the achievable energy resolution.

A High Resolution Sealied Xenon Filled Position Sensitive Proportional Counter

Xenon filled imaging proportional counters enable X-ray Astronomy images to be obtained at higher energies than with Argon filled counters. Such a counter filled with Xenon/Methane (90/10) at 2 atmospheres pressure has been built. It has a sensitive area 9 cm × 9 cm and a FWHM position resolution of 360 microns. A very low noise readout system using pulse amplitude ratio, and a Graded Density Cathode, which has no resistive element, enables intrinsic counter resolution to be measured. Resolution appears to be limited by secondary avalanche effects rather than by diffusion, and is improved by use of faster pulse shaping if signal to noise in the electronic readout allows.

A Sealed High Pressure Xenon Filled Imaging Proportional Counter with a Sensitive Area 30cm Square

The performance of an imaging proportional counter (IPC) 30cm square is evaluated in this paper. The scaling effects involved in the construction of such a device were investigated using an electrode array alone and a xenon-methane filled IPC. In the IPC, a spatial resolution of 1.2mm FWHM was achieved; and as expected this limitation is shown not to be due to the electronic signal to noise ratio, but to the widely dispersed secondary avalanches initiated by ultra violet photons emitted by excited xenon atoms.

Gas limits to MWPC position resolution and linearity in xenon-methane at 2 atm pressure☆

Abstract A sealed position sensitive proportional counter was built in order to examine the limits to the performance of xenon-methane gas mixtures. It appears that the position resolution is limited by effects intrinsic to the nature of the gas filling rather than any electronically imposed limitation. It is proposed that this limitation in resolution is due to secondary avalanching and a simple model is used to describe this effect. The linearity obtained with this gas filling is also measured and possible explanations to the observed trends are discussed.

A Sealed High Pressure Xenon-Carbon Dioxide Filled Position Sensitive Proportional Counter

A sealed position sensitive proportional counter, used in previously reported work with xenon-methane and intended for applications in X-ray astronomy, has been operated with xenon-carbon dioxide at 2 atmospheres pressure. As expected, the improved UV absorption of carbon dioxide limits the effect of secondary avalanches so that a position resolution of 230 microns FWHM is obtained compared with 360 microns in xenon-methane. The modulation of the position signal at the anode wire pitch, observed in xenon-methane is greater in xenon-carbon dioxide for the same amplifier shaping time constant. However, it does diminish for longer time constants and the position resolution improves, contrary to its behaviour in xenon-methane. Thus considerably improved position resolution with marginally degraded linearity are achieved with xenon-carbon dioxide.

Charge Cloud Diffusion in a Sealed Position Sensitive Proportional Counter Filled with Xenon-Methane and Xenon-Carbon Dioxide

The influence of diffusion in a xenon filled imaging proportional counter has been studied in two gas mixtures, xenon-methane and xenon-carbon dioxide. As expected in xenon-methane, by increasing the charge cloud diameter from 280 to 1130 microns, interpolation between the 2mm pitch anode wires is greatly improved. The use of a long diffusion region also delays secondary avalanches and consequently improves the resolution. In xenon-carbon dioxide the charge clouds are smaller, and the improvement in interpolation is less. The resolution in this case is not limited by secondary avalanches and diffusion causes a very small degradation.

Improved Background Rejection in Proportional Counters a Front Guard

Details are presented of a new technique for providing a front guard in multi-wire array proportional counters. It involves the use of a grid plane 2 mm below the entrance window. An electron drift region is set up between the window and the grid and rise-time discrimination is used to veto electron tracks emanating from the window. A Xe/CH4 MWA counter fitted with 3 sided anti-coincidence anQ RTD shows a reduction in residual background by a factor of 2 when the front guard is applied.

A large area, high pressure, sealed, xenon-filled imaging proportional counter for applications in X-ray astronomy

There are now several applications in X-ray Astronomy which require sealed, imaging proportional counters of relatively large area. These include missions such as AXAF and XMM. These applications also require xenon at high pressure as the majority gas constituent to give adequate stopping power to prevent image blurring. An imaging proportional counter of this type is described with a sensitive area 30 cm square. The position resolution, linearity and energy resolution are described, and the factors limiting these parameters in large area imaging proportional counters are examined.