S. Biagi

Monte Carlo simulation of electron drift and diffusion in counting gases under the influence of electric and magnetic fields

Abstract A fast and accurate computer simulation program for electron drift and diffusion in gases under the influence of electric and magnetic fields is described and some calculated results are compared to precise experimental results in carbon tetraflouride and methane mixtures. The calculated Lorentz angles are shown to be typically within 1° of the measured experimental values. The program allows the electric and magnetic fields to be at any angle to each other.

High rate behavior and discharge limits in micro-pattern detectors

Abstract We present and discuss a set of systematic measurements, carried out with gaseous proportional micro-pattern detectors, in order to assess their maximum gain when irradiated with high-rate soft X-rays and heavily ionizing alpha particles. The inventory of detectors tested includes: micro-strips, micromegas, micro-dot, gas electron multiplier, CAT (compteur a trous), trench (or groove), micro-CAT (or WELL) detectors, as well as systems with two elements of gaseous amplification in cascade. We confirm the general trend of all single-stage detectors to follow Raethers criterion, i.e. a spontaneous transition from avalanche to streamer, followed by a discharge, when the avalanche size reaches a value of a few 10 7 ; a noticeable exception is the micro-dot counter holding more than 10 8 . In multiple structures, where the gain is shared between two devices in cascade, the maximum overall gain under irradiation is increased by at least one order of magnitude; we speculate this to be a consequence of a voltage dependence of Raethers limit, larger for low operating potentials. Our conclusion is that only multiple devices can guarantee a sufficient margin of reliability for operation in harsh LHC running conditions.

A multiterm Boltzmann analysis of drift velocity, diffusion, gain and magnetic-field effects in argon-methane-water-vapour mixtures

The effect of adding water vapour to argon-methane mixtures used in TPC-type detectors is investigated using the exact multiterm Boltzmann expansion. The transverse and longitudinal diffusion coefficients and the drift velocity are calculated with an accuracy of ± 3, ± 2 and ± 1% respectively. The calculations are extended into the avalanche region and compared to avalanche gain measurements. The effect of misalignment of the electric and magnetic fields on the drift direction is also investigated.

The microdot gas avalanche chamber: an investigation of new geometries

Abstract The gains of the microstrip and microgap gas avalanche chambers are limited to below gains of 5 × 10 3 by geometrical and gas properties. In order to increase the maximum gain to greater than 2 × 10 4 a new detector geometry, the microdot, is proposed that can be produced at a standard silicon MOS microfabrication facility at similar cost to present microstrip gas detectors. In addition to charged particle tracking, the new geometry is also well suited to imaging applications. The operating characteristics of the new device have been investigated using a numerical simulation and a comparison with conventional strip geometries is presented. These results show that the microdot geometry reaches higher gas gains than the strip-like geometries, for the same set of electrode potentials, with a reduced cathode electric field enhancing operational stability.

GEM OPERATION IN PURE NOBLE GASES AND THE AVALANCHE CONFINEMENT

Abstract We study the operation of the Gas Electron Multiplier (GEM) in pure Ar and almost pure Xe. Rather high gas gains obtained in pure Ar, of the order of 1000, are explained by the effect of the avalanche confinement to a GEM micro-hole. Applications to the development of non-ageing sealed photon detector filled with pure noble gases are discussed. In particular, it is shown that the photoelectron collection efficiency deteriorated in pure Ar due to electron backscattering, can be recovered by operation at a higher electric field.

Accurate solution of the Boltzmann transport equation

Abstract We have written a computer code in Fortran which solves the Boltzmann transport equation in the Lorentz approximation for any angle between the electric and magnetic fields. The solution is obtained by backward prolongation and Gauss-Seidel iteration. All electron scattering, excitation, ionization and attachment are treated to all orders of solution. An example of an accurate solution for an argon- isobutane-methylal mixture up to fields of 50 kV/cm is given.

First experimental results from a microdot gas avalanche detector integrated onto a silicon wafer

First results on the voltage dependence of the gas gain of a microdot detector are presented. No variation of the gas gain with rate is observed up to a measured maximum rate of 4 x lo3 X-rays/mm*/s. Some possible applications of this detector are discussed.

A simulation toolkit for electroluminescence assessment in rare event experiments

Abstract A good understanding of electroluminescence is a prerequisite when optimising double-phase noble gas detectors for Dark Matter searches and high-pressure xenon TPCs for neutrinoless double beta decay detection. A simulation toolkit for calculating the emission of light through electron impact on neon, argon, krypton and xenon has been developed using the Magboltz and Garfield programs. Calculated excitation and electroluminescence efficiencies, electroluminescence yield and associated statistical fluctuations are presented as a function of electric field. Good agreement with experiment and with Monte Carlo simulations has been obtained.

Large-area low-pressure microstrip gas chambers for thermal neutron imaging

Abstract Thermal neutron imaging gas detectors using counting gas mixtures with gaseous neutron convertors (e.g. 3 He) suffer from inherent limitations in position and time-of-flight resolutions resulting from the long ranges of the released secondary ions in the counting gas and from parallax errors due to the necessary depth of the detector gas volume of >1xa0cm. In order to overcome these limitations a novel detector generation is presently being constructed utilizing composite neutron convertor foils ( 157 Gd or 6 Li overcoated with CsI) in combination with arrays of large-area microstrip and microdot gas chambers (MSGCs and MDOTs) which are operated in low-pressure, two-stage amplification mode and delivering unsurpassed gains. In this paper the optimization of MSGCs for low-pressure operation with 157 Gd convertors is discussed. The MSGCs are of technologically advanced, robust design, using 3xa0mm thick synthetic quartz substrates and two closely spaced electrode planes for true two-dimensional position readout. These metal planes are separated by a ∼3xa0μm thick SiO 2 /DLC double-layer deposited by means of plasma-enhanced CVD. The same fabrication technology is presently prepared for production of large-area MDOTs. The work shall be conducted in collaboration with nine teams from research institutes and industry.

Experimental results from a microdot detector overcoated with a semiconducting layer

Abstract A Microdot (MDOT) detector has been overcoated with a boron-doped amorphous silicon carbide semiconductive layer. The stable operation of the device in mixtures of argon, neon and helium with di-methyl ether (DME) at high gains and at high counting rates is shown. Radiation damage tests give a lifetime of over 120xa0mC/cm using a gas system with plastic gas pipes.