Carlos A. N. Conde

Incidence angle dependence of the output response uniformity of photomultipliers

A simple system is described that allows the study of the photoelectron collection and quantum efficiency uniformity, i.e. the output response of a photomultiplier as a function of the angle of incidence of the light for a variety of wavelengths. Results are presented for the EMI 9956QQB and are discussed for both this model and the 9956QB photomultipliers, tubes frequently used in gas proportional scintillation counters. It is shown that the response uniformity is better for the shorter wavelengths and for incidence angles which are not too large. >

Monte Carlo simulation study of the characteristics of Xe-Ne gas mixtures as detection media in gas proportional ionization counters

A Monte Carlo simulation has been developed in order to assess the use of Xe-Ne gas mixtures as detection media in gas proportional ionization counters, examining in particular the parameter most relevant for the performance of a detection medium: the intrinsic energy resolution R/sub int/. This parameter, which establishes a theoretical limit to the detection capability of a detector, has been calculated as a function of the mixtures composition. The Monte Carlo simulation code developed fully reproduces the growth of single-electron-initiated avalanches in cylindrical geometry and results for R/sub int/ are discussed in terms of the parameter f, the statistical fluctuations parameter characterizing single-electron-avalanches. This parameter is calculated for each mixture as a function of the reduced anode voltage K=V/ln(c/a), where V is the voltage applied to the anode-wire and c/a is the cathode-to-anode radius ratio. Results for the gas multiplication factor M, characterizing each mixture composition at a given applied electric field, were also obtained.

Monte Carlo Simulation Study of the Characteristics of Xe-Ne Gas Mixtures as Detection Media in Gas Proportional Ionization Counters

A Monte Carlo simulation has been developed to investigate the performance of Xe-Ne filled proportional ionization counters, with a special interest on the intrinsic energy resolution Rint as a function of the Xe-Ne mixtures composition. The Monte Carlo simulation reproduces the growth of single-electron-initiated avalanches in cylindrical geometry, and the results for Rint are discussed in terms of the influence of the Penning ionization on the statistical parameter f characterizing the fluctuations of single-electron avalanches gain. The results for Rint, and f, as well as for the multiplication factor M, are calculated for each mixture as a function of the reduced anode voltage K=V/ln(c/a), where V is the voltage applied to the anode-wire and c/a is the cathode-to-anode radius ratio

A New Technique for Gaseous Radiation Detectors: The Multigrid High-Pressure Xenon Gas Proportional Scintillation Counter

A new technique is presented for high-pressure gaseous radiation detectors leading to pulse amplitudes at least one order of magnitude larger than in ionization-chamber-based gamma-ray detectors. The technique uses room-temperature pure Xe or Xe-based gaseous mixtures at pressures of about 5-20 bar (or higher) to detect ionizing radiation in a multigrid high-pressure gas proportional scintillation counter (MGHP-GPSC) with a CsI deposit as the photocathode in direct contact with the gas, with no optical windows. The detector relies on secondary scintillation as the amplification stage followed by photoelectron production in a CsI photocathode layer. Experimental results are presented for a small prototype filled with pure Xe up to 5 bar, showing that the principle of operation of the detector works. The maximum detector gain obtained so far is about 10.

Slotted-electron-sieve integrated photosensor for gas-proportional scintillation counters

The performance of a new slotted electron sieve design in an integrated photo-sensor for use in xenon gas proportional scintillation detectors is described. The new design exhibits an enhanced photoelectron collection when compared to the earlier circular holes design, used for the first time in such application. The present design uses an electron sieve composed of a 50-micrometers pitch. The front surface is made photosensitive with a 150-thick CsI film. When an appropriate voltage is applied between the copper electrodes, the resulting electric field directs photoelectrons produced on the front surface through the holes in the sieve and onto a wire chamber where charge amplification occurs. Positive feedback is essentially eliminated since the charge amplification stage is optically decoupled from the photo-cathode. The electron sieve also provides a small amount of charge gain up to 2.8. The measured effective quantum efficiency, namely the number of photoelectrons traversing the electron sieve holes per incident 170-nm scintillation photon, as measured under present conditions, is about 8.3 percent. A discussion of the results is presented.

Light yield and Fano factor for x rays in xenon gas proportional scintillation counters

The light yield, or electroluminescence yield, of pure xenon for reduced electric fields below the ionization threshold (about 6 Vcm-1torr-1) was measured for pressures of 825, 570 and 276 torr using a uniform field gas proportional scintillation counter excited with 5.9 keV x rays. A linear behavior was exhibited from the 1 Vcm-1torr-1 electroluminescence threshold, but the measured yield (for lambda greater than 165 nm) diminished with the gas pressure. The best energy resolution obtained was 7.6% for collimated 5.9 keV x rays with xenon at 825 torr. A Fano factor of 0.26 was measured for the three xenon pressures.

W value in argon-xenon mixtures for x-rays

Experimental measurements were made for the w-value for 5.9 keV x rays in pure argon, argon with 4% xenon, argon with 21% xenon, and pure xenon at pressures just above the atmospheric. The values obtained are 26.2 eV +/- 0.7 eV, 22.0 eV +/- 0.9 eV, 21.4 +/- 0.9 eV, and 21.7 eV +/- 0.5 eV, respectively. The importance of the 96% argon - 4% xenon mixture in gas proportional scintillation counters for the detection of soft x rays with reduced spectral distortion is discussed.

High-efficiency hard x-ray xenon gas proportional scintillation counter

A large-volume (3 liters), xenon-filled, gas proportional scintillation counter with a curved grid and focusing rings was developed and tested with hard X-rays. The efficiency at 60 keV was calculated to be 64%. The experimentally measured energy resolution was observed to decrease from 6 to 5% when the X-ray energy was increased from 22.1 to 59.6 keV. Spectra are presented for the 241Am gamma rays and lanthanum K(alpha ) and B(beta ) X-rays. Spectra for the same K lines of erbium, praseodymium and tin are also presented for the detector with and without the focusing rings. These spectra show that the focusing rings increase the detection efficiency and improve the energy resolution. This effect is attributed to the increased electron collection in the scintillation region, to faster pulse rise times and to reduced electron attachment to impurities.

Tertiary Scintillation Gas Proportional Scintillation Counter (TS-GPSC): First Experimental Results

A new concept for a gaseous radiation detector is presented: the Tertiary Scintillation Gas Proportional Scintillation Counter (TS-GPSC). In this detector the electric field induced secondary scintillation is first detected by a CsI-coated GEM-like structure where it releases photoelectrons which are transferred through the GEM holes, with no charge multiplication, to another region where further field induced scintillation (tertiary) is produced and then again detected on a planar CsI-coated photocathode at the backplane of the detector. The electrons released therein are collected at a grid and constitute the detectors signal. Since there is no avalanche charge multiplication in the detector, the gain will be quite stable and, moreover, as the field induced scintillation yield is very high, and in spite of the low photocathode quantum efficiency, the gain will be high and with low fluctuations, so an improvement in the energy resolution as compared to other types of Gas Proportional Scintillation Counters is expected. The first prototype of the tertiary scintillation detector was tested in a xenon atmosphere for hard X-rays and experimental results are presented. Energy resolutions of 8.2% were achieved for 22.1 keV X-rays.

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A new technique is presented for high pressure gaseous radiation detectors leading to pulse amplitudes about one order of magnitude larger than ionization chamber based gamma ray detectors. The technique uses room temperature Xe based gaseous mixtures at pressures of about 5 to 20 atm (or higher) to detect ionizing radiation in a multigrid high pressure gas proportional scintillation counter (MGHP-GPSC) with a CsI deposit as the photocathode and no optical windows. The detector relies on secondary scintillation for the charge amplification stage rather than on charge avalanche multiplication as in proportional counters. Experimental results are presented for a small prototype filled with pure Xe up to 5 atm showing that the principle of operation of the detector works. The detector maximum gain obtained in the first results is about 10.