J.A. Mir

Application of the microhole and strip plate detector for neutron detection

We introduce the microhole and strip plate (MHSP) detector as a micropattern detector for the detection of thermal and epithermal neutrons. Detection sensitivity is obtained by filling these detectors with /sup 3/He at high pressures. We propose the use of argon-xenon penning mixtures as the stopping gas as opposed to the usual carbon based stopping gases. These argon-xenon mixtures provide suitable gas gains for the high pressure/high resolution neutron detector applications. With these mixtures it is possible to obtain a sealed detector with only rare-gas filling which is simple to purify and not subject to ageing. An MHSP gas detector filled with a 3-bar argon/50-mbar xenon/6-bar helium mixture can achieve gains above 2/spl times/10/sup 3/. This mixture allows neutron detection efficiencies of about 70% at 1 /spl Aring/ for a 2.5-cm thick absorption region and intrinsic position resolution (full-width at half-maximum) of about 1.8 mm. The sensitivity to /spl gamma/-rays of the present mixture will be the same when compared to that of 2.6-bar CF/sub 4/.

Single photon counting x-ray imaging system using a micro hole and strip plate

A single photon counting x-ray imaging system based on a MicroHole and Strip Plate (MHSP) using a resistive charge division method was implemented. The MHSP is a hybrid microstructure that combines within the same device the features of the Gas Electron Multiplier (GEM) and the Micro-strip Gas Chamber (MSGC). The MHSP presents two multiplication stages thus allowing reaching high gains. Two thin orthogonal resistive lines of about 100 Omega between each strip (top side) and between each anode strip (bottom side) allows us to obtain the actual position in both x and y directions. The readout electronics use only two charge preamplifiers in each dimension and a TNT module with four independent ADCs controlled through FPGAs that allow converting and registering the four signals through the USB port of a computer. Position resolution in both dimensions as a function of the detector parameters are presented for a 30 keV copper-target x-ray tube. Detector position resolution of sigmax=130 mum and sigmay=250 mum were achieved for 8keV, making it suitable for many applications in single photon X-ray imaging. The first x-ray images produced by the system are also presented together with a discussion of the image quality and the future prospects.

Studies of the gain properties of gas microstrip detectors relevant to their application as X-ray and electron detectors

The microstrip gas counter (MSGC) makes an excellent planar (position-sensitive) amplifier of incident electron clouds because both the anodic and cathodic gain-defining elements are produced lithographically on the same rigid substrate. We have studied the dependence of the gas gain and pulse-height resolution of the plate as a function of various geometric and gas parameters. The results show that an MSGC can be made very insensitive to. the shape of the drift electrode, allowing it to be used in a wide variety of applications. An example of an electron-yield XAFS study is given. The aim of the work reported in this paper is to produce a well-defined technology platform from which to build detectors that meet the requirements of high-flux synchrotron radiation and neutron facilities, both of which are key CLRC facilities.

Single-Electron Response Using a GEM-MIGAS Electron Multiplier

A gas electron multiplier with a micro-induction gap amplifying structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically set at 50 mum. Experimental studies were carried out to investigate the single-electron response of a GEM-MIGAS, using a He/iso-C4H10 (85/15%) gas mixture operated in a flow mode at atmospheric pressure. The additional charge multiplication in the induction gap results in a gain increase up to one order of magnitude when compared to the GEM mode operation alone. A series of measurements were undertaken to examine the pulse height distributions induced by single-electrons under a wide range of bias voltages applied across the GEM holes, 100 to 550 V, and for a wide range of electric fields in the induction gap, 0.6 to 100 kV/cm. It was possible to sustain effective charge gains in excess of 3times105 B and multiplication relative variances around 0.4 over a large range of GEM voltages, enabling us to demonstrate single-electron detection efficiencies above 98%.

Optimising the design of gas microstrip detectors for soft X-ray detection

This report describes development work in which systematic changes in the electrode pattern of a gas microstrip detector are explored in the search for higher avalanche gains and enhanced stability. It is found that the width of the cathode structure is the main determinant of the detector stability. With the correct cathode width, gas gains of >50 000 are comfortably attainable with low detector noise so that X-rays can potentially be detected down to the limit of a single X-ray produced photoelectron.

First results with THGEM followed by submillimetric multiplying gap

This work presents the first results dealing with THGEMs coupled to submillimetric multiplication gaps, operated in an atmospheric pressure of Ne/CF4. The experimental studies done so far with a THGEM coupled to submillimetric multiplication gaps achieved charge-gains of 4 × 104 and 1 × 105 in Ne/CF4 (95:5), for 0.4 mm and 0.8 mm gaps, respectively, values that are one order of magnitude higher than those obtained in single-THGEM configuration and approximately half from those obtained for a cascaded-THGEM configuration. The present studies evaluate the performance operation in terms of the charge-gain characteristics and X-ray energy resolution.

GEM-MIGAS electron multiplier operation in Argon-Methane mixtures

A Gas Electron Multiplier with a Micro-Induction Gap Amplifying Structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically 50 μm wide. A three-dimensional electric field simulation in a GEM-MIGAS detector was performed in order to evaluate the electric field profile in the different regions of the structure. Different electrostatic conditions were undertaken to forecast the detector performance in gas medium. An experimental study of GEM-MIGAS operating in an Ar/CH4 (90/10%) gas mixture and in pure CH4 was also carried out. Performance characteristics, including charge gain and energy resolution for 5.9 keV X-rays, are presented.

Operational characteristics of a GEM-MSGC system for X-ray detection

This report outlines a recent study undertaken at CCLRC Rutherford Appleton Laboratory to evaluate the performance of a gas electron multiplier (GEM) coupled with a microstrip gas counter (MSGC). The parameters investigated during this study were effective gain, effective gain stability and energy resolution at 8.05 keV using Cu-K X-rays. These parameters were studied as a function of drift field, induction field, potential differences across the GEM holes and that across the MSGC anodes and cathodes. This report demonstrates that a single stage GEM can sustain effective gains up to 6000 whilst retaining adequate X-ray energy resolution. By utilising the MSGC as well as the GEM amplification these gains easily exceed 100,000 and allow the MSGC operation at much lower voltages. This report also demonstrates that the introduction of the GEM preamplification to the MSGC enables the operation of the latter at much higher effective gains (30,000) before any degradation in the X-ray energy resolution.

Ion Back-Flow Suppression in GEM-MIGAS

The Gas Electron Multiplier with a MIcromegas Gap Amplifying Structure (GEM-MIGAS) is obtained by the coupling of a GEM to a short induction gap, typically 50 μm , where additional charge multiplication occurs. In this work, the GEM-MIGAS gain and ion back-flow are investigated, for induction regions in the range of 50-300 μm. The studies were carried out with a GEM-MIGAS coupled to a semitransparent CsI-photocathode operated in Ar/5%CH4 gas mixture at atmospheric pressure. The increase of the induction gap thickness from 50 μm to 300 μm leads to an increase of the maximum achievable charge gain by a factor of 100, from ~ 2 × 103 to ~ 2 × 105. Moreover, the high field ratio between amplification and conversion regions, which prevents ions to drift towards the conversion region, allows a strong reduction, by a factor ~ 20, of the ion back-flow ratio to the drift region, when compared with the operation in GEM-mode (i.e. at low induction field). For typical drift fields of 0.1 and 0.5 kV/cm, an ion back-flow fraction ~ 1% and ~ 4%, respectively, was obtained for the corresponding charge gains of ~ 5 × 104 and ~ 2 × 105, respectively.

GEM-MIGAS Electron Multiplier Operation in Argon-Methane Mixtures

A Gas Electron Multiplier with a Micro-Induction Gap Amplifying Structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically 50 mum. A three-dimensional electric field simulation of a GEM-MIGAS detector was performed in order to evaluate the electric field profile in the different regions of the structure. Different electrostatic conditions were undertaken to forecast the detector performance in gas medium. An experimental study of GEM-MIGAS operating in an Ar/CH4 (90/10%) gas mixture, in pure CH4 and in pure Ar was carried out. Performance characteristics including charge gain and energy resolution for 5.9 keV X-rays are presented.