M.M.F.R. Fraga

CCD readout of GEM-based neutron detectors

We report on the optical readout of the gas electron multiplier (GEM) operated with a gaseous mixture suitable for the detection of thermal neutrons: 3 He–CF4. A CCD system operating in the 400–1000 nm band was used to collect the light. Spectroscopic data on the visible and NIR scintillation of He–CF4 are presented. Images of the tracks of the proton and triton recorded with a triple GEM detector are also shown. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Gx

The GEM scintillation in He–CF4, Ar–CF4, Ar–TEA and Xe–TEA mixtures

Light emitted during the development of electron avalanches in a gas electron multiplier (GEM) can be efficiently used for optical readout of the detector. We report on measurements of total light yields in Ar/CF 4 , He/CF 4 , Ar/TEA and Xe/TEA mixtures. Information on the energy resolution obtained with low-energy X-ray photons is also presented. The spectral distribution of the light produced in the GEM is analysed and the mechanisms associated with the process of light production are considered.

Optical readout of GEMs

Abstract We present an overview of results from our recent studies on the use of the visible and NIR scintillation emitted by the gas electron multiplier (GEM) and on the possibility of using detectors operated with cascaded GEMs to build tracking chambers.

Observation of electron multiplication in liquid xenon with a microstrip plate

Abstract We report here on the observation of electron multiplication in liquid xenon in a microstrip chamber with an amplification factor of the order of 10. The measurements were carried out at a temperature between 208 and 215 K (liquid density of about 2.7 g/cm 3 ).

Study of scintillation light from microstructure based detectors

In a previous work it has been pointed out that scintillation light, extending up to the infrared, emitted in microstructure based gaseous detectors (microstrips, microgaps, GEMs, etc.) can be used for non destructive testing of these detectors when they are associated to a CCD readout system. The choice of the gas mixture is an important issue, in so far as its emission spectrum should overlap efficiently the sensitivity region of the CCD (400-1100 nm). In the present work we report on a systematic study for several gas mixtures which includes measurements of the total light yields as a function of the electric field and of the spectrometric distribution of the light emitted, in the wavelength region between 250 and 930 nm. Results are presented for pure argon and argon and xenon based gas mixtures. A comparison is made between the results obtained with the CCD coupled to a GEM detector and with a gaseous scintillation proportional counter.

Secondary scintillation in CF4: emission spectra and photon yields for MSGC and GEM

Secondary scintillation (defined here as photon emission originating from electron avalanches) was studied for two gaseous micropattern detectors: MSGC (MicroStrip Gas Chamber) and GEM (Gas Electron Multiplier) operated in pure CF4. For MSGC, the study was performed in the pressure range from 1 to 5 bar; for GEM all experiments were carried out at a fixed pressure of 1 bar. Charge gains from ~ 10 to ~ 150 were used in both cases. The primary ionization of the gas was produced by alpha particles from an Am-241 source. Emission spectra of the secondary scintillation were recorded in the wavelength range from 200 to 800 nm and corrected for the response of the detection system. Photon yields (number of photons generated per electron collected at MSGC or GEM) were measured for the integrated UV (200–500 nm) and visible (500–800 nm) emission bands. The obtained emission spectra and photon-per-electron ratios were compared to the corresponding data for the primary scintillation.

Fragments and radicals in gaseous detectors

Abstract The need for detectors able to provide fast and reliable information, particularly in view of applications for high counting rates, requires a better knowledge of the physics of the processes involved, in order to master the after-effects resulting from the emission of photons. In turn, the identification of radicals and fragments formed in the avalanche is important to understand and eventually overcome the problems related with the ageing of the present gaseous detectors and those arising in the substrata of the new-coming microstrip gas chambers. Emission spectra, in the region from 120 to 450 nm, of the mixtures of argon with quenching gases (methane, isobutane and carbon dioxide) in the proportional and self-quenching streamer modes are reported. Especial attention is given to the carbon lines (7.94, 7.48 and 6.42 eV). For each mixture several fragments and radicals are identified.

Modelling of an IR scintillation counter

A systematic study of the excitation and de-excitation mechanisms in ternary gas mixtures Ar+CO2+N2 is presented regarding the possibility of developing a proportional scintillation counter based on the detection of the infrared molecular emissions associated with the lowest vibrational states of molecules. The use of visible or near-infrared photons (λ<1 μm) for applications like imaging and quality control of microstructure detectors has been reported. In view of these applications we analyse the processes leading to near-infrared emissions in pure argon and give an estimation of the number of photons emitted per electron, at several pressures, as a function of the charge gain.

Rate effects in a proportional counter with resistive cathode

In this work, the rate e⁄ects associated with the gain of detectors with resistive electrodes are considered. To study these e⁄ects it was decided to use the classical proportional counter geometry, with a cylindrical glass tube as the cathode, rather than parallel plate devices. Indeed, the generality of the information obtained is not a⁄ected and the cylindrical proportional counter geometry has several advantages to gather detailed experimental data and to compare it with simple electric field calculations. A self-consistent set of data is presented for the e⁄ects of counting rate from a few tens of Hz up to several hundreds of Hz, using several gas mixtures, di⁄erent anode wires, cathode radii and thickness of the resistive electrodes. Although a few sources of systematic errors can be identified, all the observed experimental data can be quantitatively explained. Indeed, the measured gas amplification, from a few tens up to &5]104 and for the range of counting rates quoted above, depends only on the local electric field as it should be expected. These results are directly relevant to approach the problem of counting eƒciency in resistive plate chambers. ( 1998 Elsevier Science B.V. All rights reserved.

Transient behaviour and rate effects in resistive detectors

Abstract The dependence of the detection efficiency (or charge gain) of a resistive detector on the counting rate has been considered by several authors. The influence of the resistivity of the electrodes, the counter geometry and the gas mixture composition have also been studied. Most of those measurements assume a stationary regime. In the present work we report on studies of the transient behaviour of detectors with a resistive cathode for various counting rates, charge gains and detector geometries. We show that in some cases the time-decay curves can be fitted by a single exponential plus a constant term, while in others (higher charge gains or higher counting rates) a sum of two or even three exponentials plus the constant term is needed to fit the experimental data. A study of the electric properties of the dielectrics used is also presented and a comparison is made between these results and the data obtained under irradiation conditions.