(formerly Matthew W. Trow)
13 August 2002
We describe the gas proportional counter used for X-ray detection on the Solar-A Bragg Crystal Spectrometer instrument. The sealed detector utilises a multi-anode geometry together with a wedge and wedge (or backgammon) cathode pattern to provide one-dimensional imaging along the position axis. We discuss the development programme which has led up [to] the design of the Prototype Detector. Finally we present results of the imaging performance, energy resolution and count-rate capability.
The concept for the 2D position-readout device for the SPAN photon-counting detector is presented with attention to the count rates, spatial resolution, and charge-measurement precision. The electrodes which are deposited on the planar substrate result from charge division induced by a charge cloud, the centroid position of which is encoded by the ratio of charge magnitudes. The SPAN electrode design is analyzed and theorized to permit 1000 x 1000-pixel resolution at 1 MHz. The SPAN spiral-anode six-electrode design is compared to the Vernier-anode twelve-electrode structure for encoding 2D position, and digital precision is analyzed at count rates up to 1 MHz. The SPAN readout affords resolution levels of up to 1/1000 across the entire active area at 8-bit digitization.
A one-dimensional position-sensitive proportional counter has been developed. It incorporates a wedge-and-wedge cathode as the position sensitive element, which operates by measuring the charge induced from a nearby anode wire. The detector is sealed and contains a Xe-Ar-CO2 gas mixture. Tests of a number of detectors of this type are described. Position resolutions of ~ 350 mm are demonstrated, as well as good energy resolution and gain characteristics. These detectors will form part of a soft X-ray spectrometer on board Solar-A, a solar studies satellite to be launched in 1991.
We describe a novel, charge centroiding, position readout, the spiral anode (SPAN) which combines excellent spatial resolution with very high count rate performance. SPAN is a planar structure of six electrically isolated electrodes. The ratio of the charges collected by the electrodes determines a two-dimensional position. The novel design allows the spatial resolution to be an order of magnitude better than the charge measurement accuracy. We present results from a prototype detector consisting of a microchannel plate stack in conjunction with the SPAN readout and discuss opportunities for the use of the SPAN readout with other photon counting detectors.
The Bragg crystal spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employers four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like Fe XXXVI and He-like Fe XXV, Ca XIX and S XV in four narrow wavelength ranges with a resolving power (l/Dl) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarises the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.
The position sensitive proportional counter used in the Bragg Crystal Spectrometer (BCS) on the Yohkoh spacecraft (formerly known as Solar-A) is described. The BCS provides time-resolved spectra of solar flares in soft X-rays (2.4 keV to 7.0 keV) with improved sensitivity over previous studies. This sealed detector, filled with a Xe-Ar-CO2 gas mixture, uses thin (15 mm) anodes, and a backgammon cathode board to provide one-dimensional imaging along the dispersion axis of the spectrometer. The performance and calibration procedures for the detectors are discussed, and we illustrate how the calibrations can be used to interpret BCS data.
Proportional counters are relatively sensitive to contamination through outgassing and the range of electrical insulators suitable for use in their manufacture is quite limited. Although small amounts of plastics such as have been used as feedthroughs, ceramics are most commonly used when sealed counters with long lives are required. Ceramics have poor and widely scattered mechanical properties of the use of a more robust material is often highly desirable. Of particular interest is the use of polymers and this work examines polycarbonate in particular. To investigate its suitability in terms of outgassing a simple cylindrical, single anode proportional counter containing a large sample of polycarbonate was baked out at approximately 100 °C and filled with a CO2/Ar/Xe mixture (5 : 47.5 : 47.5 by pressure, respectively). Subsequent measurements of the counter indicated an increase in gain, which, after a second similar filling, was identified to be associated with a preferential loss of CO2 to the polycarbonate. The consequences of this result and the circumstances under which polycarbonate could be used on a large scale and the construction of proportional counters are discussed.
The positional linearity of imaging proportional counters is affected by the intensity distribution of the incident radiation. A mechanism for this effect is described, in which drifting positive ions in the gas produce a distorting electric field which perturbs the trajectories of the primary electrons. In certain cases, the phenomenon causes an apparent improvement of the position resolution.
We demonstrate the effect in a detector filled with a Xenon-Argon-CO2 mixture. The images obtained are compared with the results of a simulation. If quantitative predictions for a particular detector are required, accurate values of the absolute detector gain, ion mobility and electron drift velocity are needed.
The microsphere plate (MSP) is a new type of electron multiplier device operating along similar lines to the well known microchannel plate (MCP). The MSP is manufactured by El- Mul Technologies Ltd., using glass beads 20 to 60 micrometer diameter, sintered together to form a wafer less than 1 mm thick. Conductive coatings are applied to the upper and lower surfaces, and a high voltage is applied between these two electrodes, allowing secondary electron multiplication to take place. The device uses the surfaces of the randomly arranged interstices of the sintered glass beads as dynodes, whereas in the MCP, dynodes are constituted by the inner surfaces of the longitudinal pores. The homogeneous composition of the MSP causes charge to spread laterally during multiplication, resulting in a spatial resolution of about 2 linepairs/mm when proximity focused to a phosphor. Charge division readouts benefit from this charge spreading, such as the wedge and strip anode which requires a charge footprint of order 1 - 2 mm diameter. We present results of experiments on the imaging performance of detectors using MSPs with readouts such as the wedge and strip anode. We discuss and quantify the potential advantages to be gained from MSPs, such as the higher gain achievable per stage, reduced susceptibility to paralysis owing to their isotropic conductivity, etc. Potential MSP disadvantages, such as image nonlinearities, quantum efficiency variability, and pulse height saturation are analyzed.
We describe a new position readout scheme, applicable to proportional counters, which provides the attributes required for large format, high energy x-ray detectors, such as that proposed for the Eixon x-ray monitor instrument on the esa integral mission. Large format detectors for coded mask imaging require a position resolution of typically < 1 mm, in order to over-sample the projected mask pixel. Background rejection at higher energies can be improved by using fluorescence gating. However, this technique requires the position readout to be capable of detecting the simultaneous double event signature. The scheme we propose combines both excellent position resolution with the ability to resolve simultaneous events.
The readout scheme consists of an array of charge measurement electronic channels connected to groups of cathode strips. The particular cathode grouping arrangement allows a large reduction (~1/6) in the number of channels required compared to the fully parallel scheme, with one channel per electrode. However, the new design still retains the charge centroiding and parallel processing capabilities of the fully parallel scheme, enabling it to provide high spatial resolution and resolve multiple simultaneous events.
We present results of a Monte Carlo simulation of the detector and readout. The simulation models the physics involved in each x-ray interaction and predicts the primary ionisation distribution. Simulation of electron diffusion and gas multiplication are used to predict the charge induced on each cathode strip. Electronic noise and other signal degradation factors are included for a realistic assessment of readout performance. Thus far, the position readout is modelled in one axis only. The success of the new scheme is assessed by comparison with the fully parallel readout.
The microsphere plate is a new type of electron multiplier similar to the microchannel plate in dimensions and mode of operation, but manufactured from microscopic glass beads sintered to form a wafer of user definable thickness.
We present further measurements taken using a detector utilising one or more microsphere plates with a charge division anode for imaging purposes. Charge division anode systems are well suited to the microsphere plate as they benefit from the relatively large charge footprint which occurs during multiplication unlike other read-out systems, such as phosphors, whose performance is degraded by a large charge footprint. A comparison between the imaging performance of single thickness and double thickness MSPs is given and we investigate the mechanism causing variability of quantum efficiency with position using the bare microsphere plate illuminated with ultraviolet light.
The potential advantages in the immediate performance of a hybrid detector using microchannel and microsphere plates in tandem are discussed and imaging performance measurements with such a combination are presented.
The MUPUS experiment on the Rosetta Lander will measure thermal and mechanical properties as well as the bulk density of the cometary material at and just below the surface of the nucleus of comet 46P/Wirtanen. A profile of bulk density vs. depth will be obtained by measuring the attenuation of 662 keV gamma rays emitted by a 137Cs source. Compton scattering is the dominant interaction process at this energy, the attenuation depending directly on the total number of electrons along the source-detector path. This in turn is approximately proportional to the column density. We report here on the design of the bulk density instrument and the results of related Monte Carlo simulations, laboratory tests and calculations of the instrument's performance. The 137Cs radioisotope source is mounted in the tip of the MUPUS thermal probe- a 10 mm diameter rod, to be hammered into the surface of the nucleus to a depth of ~370 mm. Two cadmium zinc telluride (CZT) detectors mounted at the top of the probe will monitor the count rate of 662 keV photons. Due to the statistics of photon counting, the integration time required to measure column density to a particular accuracy varies with depth as well as with bulk density. The required integration time is minimised for a material thickness equal to twice the exponential attenuation length. At shallower depths the required time rises due to the smaller fractional change in count rate with varying depth, while at greater depths the reduced count rate demands longer integration times. The former effect and the fact that the first 45 mm of the source-detector path passes not through the comet but through the material of the probe, mean that the first density measurement cannot be made until the source has reached a depth of perhaps 100 mm. The laboratory experiments indicate that at this depth an integration time no less than 348 s (falling to 93.9 s at full penetration) would be required to measure a bulk density of 1000 kgm-3 to 5% accuracy, assuming a source activity of 1.48 mCi (decayed from an initial 2 mCi). Although solutions involving feedback of the measured bulk density into a time-budgeting algorithm are conceivable, a simple approach where equal time is spent per unit depth may be best, providing an accuracy in bulk density of around 5-20%, for 25 mm slices and the expected range of parameters.
We present examples of early, unreduced results obtained from the instrument package dedicated for spectroscopic studies of coronal flaring plasma composition. The package consists of two Bragg spectrometers, ReSIK and DIOGENESS. These instruments have been launched aboard the Russian Koronas-F Solar Observatory on 31st July, 2001. The data being received from these two spectrometers allow for determining the absolute abundances of several important elements including that of low and high first ionisation potential (FIP). Based on data which have been (and hopefully will be) obtained, we will investigate fast temporal variations (~ 1 min) of the plasma composition in flares and hotter active regions. The analysis of spectra obtained will certainly allow for spectroscopic studies of several, so-called “triplet” lines forming in He-like, highly ionised plasma since the spectra received are apparently of unprecedented quality. The Coronas-F mission is expected to be supported for at least two years.