Cascaded Electron Multipliers Research Articles

Development of high-gain gaseous photomultipliers for the visible spectral range

We summarize the development of visible-sensitive gaseous photomultipliers, combining a semitransparent bi-alkali photocathode with a state-of-the-art cascaded electron multiplier. The latter has high photoelectron collection efficiency and a record ion blocking capability. We describe in details the system and methods of photocathode production and characterization, their coupling with the electron multiplier and the gaseous-photomultiplier operation and characterization in a continuous mode. We present results on the properties of laboratory-produced K2CsSb, Cs3Sb and Na2KSb photocathodes and report on their stability and QE in gas; K2CsSb photocathodes yielded QE values in Ar/CH4(95/5) above 30% at wavelengths of 360–400 nm. The novel gaseous photomultiplier yielded stable operation at gains of 105, in continuous operation mode, in 700 Torr of this gas; its sensitivity to single photons was demonstrated. Other properties are described. The successful detection of visible light with this gas-photomultiplier pave ways towards further development of large-area sealed imaging detectors, of flat geometry, insensitive to magnetic fields, which might have significant impact on light detection in numerous fields.

High Pressure Operation of the Photon-Assisted Cascaded Electron Multiplier

We present the operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in xenon at high pressure. The PACEM is a multi step electron multiplier where the VUV scintillation produced in the electron avalanches is used for signal propagation: the VUV scintillation produced in the first element of the cascade induces the emission of photoelectrons from a CsI photocathode placed on the top-surface of the second element. These photoelectrons are further multiplied, via charge avalanche. A metallic mesh electrode placed between the first and the second elements of the cascade completely blocks the charge transfer between them. Optical gains of 103 were achieved in xenon at atmospheric pressure, dropping to 25 at 3.3 bar, for applied voltages of 700 and 1100 V, respectively. Taking into account the subsequent charge multiplication, total gains are higher than those obtained with a triple GEM and double THGEM.

The Photon-Assisted Cascaded Electron Multiplier Operation in CF<formula formulatype="inline"><tex>$_{4}$</tex></formula> for Ion Backflow Suppression

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in CF<formula formulatype="inline"><tex>$_{4}$</tex></formula> is investigated. The PACEM uses the VUV scintillation produced in the electron avalanches of the first multiplier of the cascade to transfer the signal to the subsequent ones. The VUV scintillation induces the emission of a large number of photoelectrons from a CsI photocathode placed on the top-surface of the second multiplier. The photoelectrons are further multiplied in the subsequent stages of the cascade, resulting in efficient signal amplification. A mesh, set at a fixed voltage, is placed between the first and the second multipliers to block the charge transfer between them, thus suppressing all the ion backflow (IBF) heading to the first cascade element. The PACEM electrically isolates the first multiplier of the cascade and only the ions produced in the electron avalanches of the first element may flow back into the drift region. Operating in CF<formula formulatype="inline"><tex>$_{4}$</tex></formula>, absolute IBFs as low as <formula formulatype="inline"><tex>${\sim} 1$</tex> </formula> ion per primary electron are achieved for drift fields of 0.1 kV/cm, while for fields of 0.5 kV/cm the absolute IBF is <formula formulatype="inline"> <tex>${\sim} 10$</tex></formula> ions/primary electron. This corresponds to an IBF of 10<formula formulatype="inline"><tex>$^{-4}$</tex></formula> at gains of 10<formula formulatype="inline"><tex>$^{4}$</tex></formula>, for Time Projection Chambers (TPC) operating conditions, and IBFs of <formula formulatype="inline"><tex>${\sim} 10^{-5}$</tex></formula> at gains of 10<formula formulatype="inline"><tex>$^{6}$</tex></formula> for Gaseous Photomultipliers (GPM) operating conditions. </para>

PACEM: a new concept for high avalanche-ion blocking

We present the Photon-Assisted Cascaded Electron Multiplier (PACEM) as a potential alternative for ion back-flow suppression in gaseous cascade electron multipliers. Using a Micro Hole and Strip Plate–Gas Electron Multiplier (MHSP-GEM) configuration, the number of ions flowing back to the scintillation region is about 1.5 ions per primary electron at an optical gain of 6.5 and a drift field of 0.1 kV/cm, and about 10 ions per primary electron at an optical gain of 10 and a drift field of 0.5 kV/cm. These allow reaching ion back-flow values close to 10 −4 and 10 −5 at typical operation conditions of TPCs and GPMs, respectively.

The Photon-Assisted Cascaded Electron Multiplier: a concept for potential avalanche-ion blocking

We present a Photon-Assisted Cascaded Electron Multipliers (PACEM) which has a potential for ion back-flow blocking in gaseous radiation detectors: the avalanche from a first multiplication stage propagates to the successive one via its photons, which in turn induce photoelectron emission from a photocathode deposited on the second multiplier stage; the multiplication process may further continue via electron-avalanche propagation. The photonmediated stage allows, by a proper choice of geometry and fields, complete blocking of the ion back-flow into the first element; thus, only ions from the latter will flow back to the drift region. The PACEM concept was validated in a double-MHSP (Micro-Hole & Strip Plate) cascaded multiplier operated in xenon, where the intermediate scintillation stage provided optical gain of ∼60. The double-MHSP detector had a total gain above 104 and energy resolution of 18% FWHM for 5.9 keV x-rays.