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

Abstract

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 photon-mediated 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 10 4 and energy resolution of 18% FWHM for 5.9 keV x-rays.

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

“…The main difference between these two structures is the strip pattern present on the bottom face of , and in the region around the anode strips, which is established by the voltage difference between cathode and anode strips, . Compared to the GEM, the MHSP allows for an extra multiplication stage, resulting in higher scintillation and charge gains [1], [18]. In addition, depending on the electric field between the MHSP and the wire mesh, only % of the anode-avalanche ions may drift back through the MHSP holes [19].…”

“…As the current measured at the reflective photocathode, , includes the contribution from both photoelectrons induced by the gas scintillation and photoelectrons induced by the direct exposure of the reflective photocathode to the UV photons emitted by the Hg lamp, , the optical gain is given by (1) Both and are measured for null electric fields in the MHSP, i.e., V, and in the GEM, i.e., V, and extraction field of 1.0 kV/cm in the region between the reflective photocathode and the wire mesh . This value was found to ensure good extraction efficiency in the gas, above 90% [7].…”

“…A different approach to the IBF blocking was introduced with the Photon Assisted Cascaded Electron Multiplier (PACEM) detector [1], [3]. In this detector, only a fraction of the ions produced in the first cascade element will flow back to the drift volume.…”

“…T HE Photon-Assisted Cascaded Electron Multiplier (PACEM) [1] was developed as an alternative for blocking avalanche-induced ions in cascaded gaseous electron multipliers. The avalanche originated in the first element propagates to the subsequent ones solely via its photons which, in turn, induce photoelectrons in the next multiplier of the cascade, while the charge transfer between the first and second cascade elements is blocked by a mesh electrode placed between them.…”

“…This optical coupling is only effective in highly scintillating gases such as the heavy noble gases [2], [3], excluding applications in mixtures with quenching gases [2]. The PACEM principle and operational characteristics were demonstrated in xenon [1], [3]; by a proper choice of the gas, geometry and electric fields, the photon-assisted propagation can be made very efficient, while the charge transport between the first and the second element is completely blocked [3]. Manuscript Amongst the molecular gases, CF is known to have a high scintillation output, either under bombardment by ionizing radiation [4] or produced in electron avalanches [5], having a significant component in the VUV region (peak around 170 nm) [4].…”