Figure 1. From M.W. Trow et al, Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 232-236. (doc, ps)
1 April 93 - 31 March 95
M.W. Trow
Although the development of alternative technologies now means that gaseous devices such as the proportional counter are in decreasing use in space applications, they are still competitive where large area detectors are needed for x-rays and soft gamma rays. The development of the microstrip gas proportional counter is important for the field. Expertise in position sensitive gas detectors will remain important for MSSL.
The most recent deployment of an MSSL gas detector in a flight project was the position sensitive proportional counter used in the Yohkoh BCS. Much of the work in this area has been driven by the requirements of the solar physicists at MSSL and elsewhere who use that instrument. This includes the study of rate dependent image distortions which is focused on the Yohkoh BCS detector.
In many situations, for example when constructing models of phenomena such as image distortions, it is necessary to know the absolute value of detector gain. Work on a new technique to obtain this measurement continues.
Numerical modelling is an increasingly important part of the detector development process. We have developed a Monte Carlo model of a counter proposed for the Integral XRM. The main purpose of the model was to test the operation of a new position sensing technique. Another output was the optimum value of the electron diffusion coefficient, which would be controlled by choosing a suitable gas mixture.
These three programmes will all be pursued in the future, although preparations for the flight of proportional counters on the Koronas-F/ ReSIK instrument will take priority.
M.W. Trow, A.C. Bento, A. Smith
The effects of counting rate on proportional counters, as described in the last report, are well known. In particular, the position linearity of imaging proportional counters is affected by the intensity distribution of the incident radiation.
When a detector is illuminated by a narrow beam or point, a sheet of ions will be produced in the drift regions. This charge generates an electric field which has a component perpendicular to the plane of the sheet. Primary electrons are attracted to this region of ion density as they drift toward the anode.
The amount of lateral drift will depend on the density of ions in the sheet, the location of the primary ionisation, and the drift velocity of the electrons. At any point in the electron's path, the direction of drift is that of the gradient of the total electric field, and the drift velocity is a complex function of the field strength. This function is usually highly dependant on the proportions of the gas mixture.
Figure 1. From M.W. Trow et al,
Nucl. Instr. and Meth. in Phys.
Res. A 348 (1994) 232-236.
(doc,
ps)
The magnitude of perturbing field of the ion sheet decreases with distance. Therefore, photons absorbed close to the ion sheet will drift more than those absorbed further away. In addition, the depth of the initial ionisation sites varies randomly, so a distribution of lateral drifts will be observed.
To determine the effect of counting rate on a test image, we illuminated a position sensitive detector through a mask mounted against its window. We obtained a series of images at a range of anode voltages and count rates. The qualitative features of the mechanism were confirmed.
Two classes of numerical model were developed. The first, which used an exponential shift function was found to be inadequate. A second model, which follows the trajectories of the drifting electrons, is proving more successful. Data from this model, which has of order of ten parameters, most of which are adjustable in this case, must be fitted to the observed images, so that extrapolations can be made. This will require the development of a suitable gradient minimisation procedure (Powell's method is a candidate), since the model's partial derivatives are unavailable. Measurement of some of the physical parameters (absolute gain, ion mobility) reduces the number of free model parameters.
I. Douglas, M.W. Trow, A. Smith
The work on rate dependent image distortions is an example of a need for the absolute value of detector gain (i.e. the charge produced by a known input). However, in most charge measurement systems, some charge is lost due to the properties (ballistic deficit) of charge integrators.
A new method for measuring this quantity was proposed shortly before the publication of the last report. The method has now been extensively tested in the laboratory and we have demonstrated the utility of the technique, which can be applied to other photon counting detectors including MCPs, MSGCs and semiconductor detectors. Our tests showed a loss of 20% of the signal to this effect. The fraction lost in any given case depends on the shape of the detector pulse and the time constants of the preamplifier.
M.W. Trow, J.S. Lapington
This work was done in preparation for a proposal to participate in the Integral XRM consortium. Although that opportunity is no longer available, the modelling work will be useful elsewhere.
The model simulated the detection of soft gamma rays (<100 keV) by a high pressure (5 atm) xenon-filled chamber with a microstrip device in its base acting as the multiplication and readout element. The model was needed to verify the proposed readout technique, in which signals from the cathodes of the microstrip pattern were physically combined in cyclic groups. A requirement for the Integral application was that fluorescence photons which were reabsorbed in the gas were to be detected and located.
Because this technique depended on the charge distribution over the cathodes, it was necessary include in the model all the features which affected this, namely the penetration of photons into the gas, primary ionisation (including Fano factor, photoelectron production, Auger electron ejection and fluorescence), drift and diffusion of the primary electrons and multiplication at the microstrip (including avalanche factor and spreading onto the neighbouring cathodes). Collection of charge from the cathodes was subject to electronic noise and the grouping arrangement under test.
The modelled collected charges were then fed into a series of position decoding algorithms, and statistics gathered for each. Of particular interest was the efficiency of recognition of the fluorescence photons.
We found that our strip grouping arrangement, which used less than 28 channels per axis could equal the performance of a system comprising 224 independent channels, whilst maintaining the required fluorescence detection efficiency, under realistic detector operating conditions.
The underlying physical model will be refined for use in other gaseous detector projects, whilst the readout concept can be used in any detector which creates a charge distribution.
M.W. Trow
The position sensitive proportional counters developed for the Yohkoh will be utilised in the ReSIK project. The ReSIK instrument, which is a bent crystal spectrometer similar to the BCS has been designed with theYohkoh detectors in mind. The flight spare detectors (including the front end electronics package) will be used in the ReSIK flight model, and other parts from the Yohkoh project will be prepared to flight standard so that two spare detectors will be available at the time of ReSIK's calibration.
This activity involves careful monitoring of the existing detectors and a series of quality-control tests during the preparation of the flight spares.
Prepared by M.W. Trow 16-May-95