F.A.F. 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.

Quality control of GEM detectors using scintillation techniques

Abstract Non-destructive quality control of microstructures at the manufacturing stage is an important issue in the foreseen use of huge numbers of such gaseous detectors in the future high luminosity colliders. In this work we report on the use of the scintillation light emitted by the avalanches in GEM channels for checking defects in the foils. The test system is described and data on the relative efficiency of several gaseous mixtures are presented. The foil images obtained with a low-noise CCD system are analysed and compared with the optical images obtained with an industrial inspection system of high magnification. The validity of this test method is established and possible extensions of its use are discussed.

Dose imaging in radiotherapy with an Ar-CF4 filled scintillating GEM

A gaseous scintillation detector consisting of a Ar+CF4 filled GEM and a CCD camera has been investigated for the application as a position sensitive dosimeter in proton-beam radiation therapy. The light yield has been measured as a function of the CF4 concentration, operating voltage, beam intensity and beam energy. In conditions where the gas gain decreases due to a lower electric field, an increase of the amount of light per secondary electron is observed. The mixture of Ar+5% CF4 has the largest light output in absolute sense as well as per secondary electron. The scintillating Gas Electron Multiplier emits >1.5 times more light than needed and the signal-quenching in the Bragg peak which occurs in solid scintillators is reduced by a factor 4.

Imaging detectors based on the gas electron multiplier scintillation light

We have developed a novel type of two-dimensional, large area, low granularity gaseous position sensitive detector using a gas electron multiplier foil (GEM) and a standard CCD camera with glass optics. First results obtained with a 10/spl times/10 cm/sup 2/ GEM foil demonstrate that the position resolution with 7 keV X-rays using a Xe-CO/sub 2/ mixture is better than 100 /spl mu/m (RMS). The complete system is described and experimental results on position resolution, efficiency and linearity are shown. Data on light efficiency for mixtures of interest for detection of other types of radiation, such as neutrons, are also presented. Possible applications of this type of detector are considered.

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.