Semitransparent CsI Photocathode Research Articles

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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex> </formula> , 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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> . The studies were carried out with a GEM-MIGAS coupled to a semitransparent CsI-photocathode operated in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm Ar}/5\%{\rm CH}_{4}$</tex></formula> gas mixture at atmospheric pressure. The increase of the induction gap thickness from 50 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> to 300 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> leads to an increase of the maximum achievable charge gain by a factor of 100, from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim 2\times 10^{3}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim 2\times 10^{5}$</tex></formula> . 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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex> </formula> 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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim$</tex></formula> 1% and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim$</tex></formula> 4%, respectively, was obtained for the corresponding charge gains of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim 5\times 10^{4}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim 2\times 10^{5}$</tex></formula> , respectively.

Sealed gaseous photomultiplier with CsI photocathode

A sealed gaseous photomultiplier tube (PMT) with a CsI photocathode has been developed using a micropattern gas detector (MPGD) made of Pyrex glass. A microblasting technique (MB) was employed for the production of a new hole-type MPGD. We investigated gaseous PMTs with a semi-transparent CsI photocathode for two different gas mixtures of Ne(90%)+i-C 4H 10 (10%) and Ne (90%)+CF 4 (10%) at a gas pressure of 700 torr. Gains of up to 10 5 were attained with the microblasted glass plate (MB-GP) for both neon gas mixtures. Quantum efficiencies of up to 0.5% were obtained for UV light at the wavelength of 170 nm. The results of the aging test and the characteristics in a high magnetic-field environment are described for the gaseous PMT filled with neon gas mixture.

A new imaging device based on UV scintillators and a large area gas photomultiplier

A new type high spatial resolution radiation detector based on a UV scintillator+a UV imaging gas photomultiplier is presented. The prototype UV detector consists of a 10 cm×10 cm μPIC, 2 GEMs and a semitransparent CsI photocathode deposited on a MgF 2 window. The effective photo-sensitive area is ϕ34 mm and the readouts are 400 μm pitch strips. A newly developed 20 mm size LaF 3(Nd) crystal which emits 172 nm photons is coupled to the detector for the first step. The detector was tested in pulse mode operation with 5.5 MeV α particles from 241Am. The single photoelectrons were successfully detected and the images of the crystal shape were clearly obtained.

Developments of a large area VUV sensitive gas PMT with GEM/μPIC

A new large area UV photon detector with micro pattern gaseous detectors isdeveloped and evaluated.A semitransparent CsI photocathode deposited on a MgF2 window wascombined with 10cm × 10cm GEM and μPIC.Using Ar+C2H6 (10%) gas, we achieved the gas gain of more than 105 which is enough to detect single photoelectron.We, then, irradiated vacuum UV photons (VUV, around 172nm) from the newly developed LaF3(Nd)scintillator to the detector and the single photoelectrons were successfully detected. We also demonstrated the imaging capability of the detector with μPIC readout systems.

Photosensitive gaseous detectors for cryogenic temperature applications

There are several proposals and projects today for building LXe time projection chambers (TPCs) for dark matter search. Important elements of these TPCs are the photomultipliers operating either inside LXe or in vapours above the liquid. We have recently demonstrated that photosensitive gaseous detectors (wire type and hole type) can operate perfectly well, until temperatures of LN 2. In this paper, results of systematic studies of operation of the photosensitive version of these detectors (combined with reflective or semi-transparent CsI photocathodes) in the temperature interval of 300–150 K are presented. In particular, it was demonstrated that both sealed and flushed by a gas detectors could operate at a quite stable fashion in a year/time scale. Obtained results strongly indicate that they can be cheap and simple alternatives to photomultipliers or avalanche solid-state detectors in LXe TPC applications.

Thick GEM-like multipliers—a simple solution for large area UV-RICH detectors

We report on the properties of thick GEM-like (THGEM) electron multipliers made of 0.4 mm thick double-sided Cu-clad G-10 plates, perforated with a dense hexagonal array of 0.3 mm diameter drilled holes. Photon detectors comprising THGEMs coupled to semi-transparent CsI photocathodes or reflective ones deposited on the THGEM surface were studied with Ar / CO 2 (70:30), Ar / CH 4 (95:5), CH 4 and CF 4 . Gains of ∼ 10 5 or exceeding 10 6 were reached with single- or double-THGEM, respectively; the signals have 5–10 ns rise times. The electric field configurations at the THGEM electrodes result in an efficient extraction of photoelectrons and their focusing into the holes; this occurs already at rather low gains, below 100. These detectors, with single-photon sensitivity and with expected sub-millimeter localization, can operate at MHz / mm 2 rates. We discuss their prospects for large-area UV-photon imaging for RICH.

GEM photomultiplier operation in CF 4

The properties of a 3-GEM (Gas Electron Multiplier) element photomultiplier, with a semitransparent CsI photocathode and CF 4 gas filling, are presented. Compared to other gas mixtures, such as CH 4, Ar/CH 4, Ar/N 2 and He/Ar/N 2, CF 4 has superior performance: the highest gain, approaching 10 7, the fastest, 8 ns wide signal and the lowest photoelectron backscattering; the latter allows to reach photocathode quantum efficiency values approaching that in vacuum. The time resolution of the multi-GEM photomultiplier for single photoelectrons was measured to be 2 ns. These properties are of high relevance for applications in Cherenkov detectors and in tracking devices.

Sealed GEM photomultiplier with a CsI photocathode: ion feedback and ageing

We present the performance of a sealed gaseous photomultiplier consisting of a cascade of 3 or 4 Gas Electron Multiplier (GEM) elements coupled to a semitransparent CsI photocathode, in Ar/CH 4 (95/5). A few-month stability study of the photocathode in a sealed mode is presented. Increasing the number of GEMs in cascade substantially reduces the ageing of the detector under strong irradiation. The ion feedback to the photocathode has probably a minor effect on the ageing rate.

Sealed gas UV-photon detector with a multi-GEM multiplier

A sealed gas photon detector with a gas electron multiplier (GEM) cascade coupled to a semitransparent CsI photocathode has been studied. High gains, reaching 10/sup 6/, and fast anode signals, of a width of 15 ns, were obtained with this GEM-photomultiplier in Ar-CH/sub 4/ (95/5), in a photon counting mode. The lifetime of this first prototype detector under high photon flux and large anode currents is provided.

A UV light photo-detector based on a MicroGap Chamber with single electron response

Abstract A MicroGap Chamber (MGC) used as a fast UV light detector with single electron sensitivity is presented. The detector has a semitransparent or reflective CsI photocathode, 100–200 μm anode pitch and 2D capability. It works with gas mixtures based on DME and light noble gases at atmospheric pressure. A gas gain >106 has been achieved with a two-stage gas amplification (G > 102 in the drift region and G ≈ 104 in the strip region). The use of light noble gases, like neon, has brought to a sensible reduction of VUV light output (hence photon feedback) during electron multiplication in gas. Due to the very thin gap (5 μm) a very large fraction of the avalanche charge is quickly delivered to the fast amplifier (20 ns peaking time) thus improving the single electron detection capability of the MGC. Because of its higher gain the MGC with a semitransparent CsI photocathode provides a better single electron detection efficiency (>90%) than the MGC with a reflective photocathode. This latter, vice versa, has a higher quantum efficiency and a faster response. Experimental results on gas gain, drift properties, positional sensitivity and single electron detection efficiency are reported.

Properties of reflective and semitransparent CsI photocathodes

Various aspects related to the quantum efficiency of the reflective and semitransparent CsI photocathodes have been investigated experimentally. The investigation explored the dependence of the quantum efficiency on the following: the thickness of the photocathode film, the gas used in the parallel-plate avalanche chamber, exposure to water vapor, and the density of the CsI film. With reflective CsI photocathodes quantum efficiencies of more than 25% at 180 nm have achieved in three different gases: CH 4, C 2H 6, and C 4H 10. Our study reveals that high quantum efficiency is also achievable for the semitransparent CsI photocathodes. For a 95-Å semitransparent photocathode the quantum efficiency at 185 nm can be as high as 20%. A simple model to explain the dependence of quantum efficiency on the thickness of CsI photocathodes is suggested, which may be a useful guide for further improvement of the performance of CsI photocathodes. Also reported are UV-transmittance measurements for various materials used in our studies, such as methane, ethane, isobutane, CF 4, CO 2, helium, neon, air, TMAE vapor, thin CsI films, and quartz windows, all in the wavelength range 115 nm to 230 nm.

Some studies of the applications of CsI photocathodes in gaseous detectors

Cesium-iodide photocathodes have been investigated in combination with gaseous detectors. The quantum efficiency of a semi-transparent CsI photocathode has been measured in the UV range and found to be 10% at 140 nm. We report experimental results obtained with such photocathodes coupled to BaF 2 and KMgF 3 scintillators and read out by parallel-plate gaseous detectors.

Mesh-based semitransparent photocathodes

Semitransparent photocathodes consisting of vacuum-deposited layers of CsI on electroformed nickel meshes with 40 and 60 lines/mm spacing have been investigated for applications in vacuum-ultraviolet electronic imaging devices. These photocathodes are found to have about half the quantum yield of an opaque CsI photocathode, but a factor of 2 to 3 greater quantum yield than a conventional semitransparent CsI photocathode. The limitations of window materials on short-wavelength spectral response, and their disadvantages in an energetic charged-particle environment, are eliminated. Unlike conventional opaque photocathodes, the mesh photocathodes can be used interchangeably with conventional semitransparent photocathodes in a wide variety of electronic imaging devices. Tests of mesh photocathodes in laboratory electrographic cameras indicate that best results for most applications are obtained if the output electron image has a somewhat poorer resolution than that corresponding to the mesh spacing.