Download New Developments of Scintillating Crystal

“ New Developments of Scintillating Crystal-Based Hybrid Single Photon Detectors (X-HPDs) for
Charged Particle and Neutrino Detection Applications ”
Al Samarai et al
Communicating Author: G. Hallewell Centre de Physique des Particules de Marseille
Under a scientific-industrial partnership bwteeen the IN2P3 (Institut National de Physique
Nucléaire et de Physique des Particules) division of the CNRS and Photonis. S.A.S. Brive la
Gaillarde, a European manufacturer of photomultiplier tubes and night vision devices, a new
series of scintillating crystal-based hybrid single photon light detectors (X-HPDs) is being
developed.
Such devices allow the detection of single photons with sub-nanosecond timing
precision and offer additional improvements in performance over standard large photomultiplier
tubes. Improved overall detection efficiency is possible through the more efficient electrostatic
collection of photoelectrons emitted from the photocathode, which also makes X-HPDs
insensitive to the Earth’s magnetic field allowing identical efficiency in any orientation. The use
of a scintillating crystal allows better energy resolution through the capability of separating
single, double and multiple photon signals.
As in conventional photomultiplier tubes, X-HPD devices contain a photocathode
deposited on the inside of a vacuum envelope. The window/photocathode combination may be
chosen to be sensitive to visible light or to other wavelengths. Photoelectrons emitted from the
cathode are accelerated, however, in a much stronger electric field (exceeding 1kV/cm) than in a
photomultiplier (typically < 100 V/cm), and impact a central crystal anode rather than a standard
secondary-emission first dynode structure. High energy photoelectrons (typically 25kV) can
penetrate a thin aluminium coating on a non-organic scintillating crystal to produce detectable
scintillation light. The use of modern scintillators including YSO, LSO ad LYSO result in a gain
of around thirty scintillation photons for each photoelectron emitted by the photocathode.
X-HPD detectors have been demonstrated as viable single photon detectors since 1996 in the
Lake Baikal neutrino telescope. Prior to this, the Philips XP2600 'SMART' X-HPD had been
developed under the DUMAND program, while more recently, developments at CERN, Geneva
have demonstrated the advantages of a true concentric geometry with a scintillator at the
geometric centre of a spherical photocathode, giving almost 100% electrostatic collection
efficiency over a ~ 3*pi solid angle coverage.
Under a collaboration centred on the Groupement d'Intêret Scientifique cooperation signed
between Photonis S.A. and the IN2P3, a series of quasi-spherical X-HPDs will be developed with
sizes ranging between 8" and the maximum that can be constructed for fitting in a standard 17"
optical pressure sphere for use in a deep sea neutrino telescope. The thrust of this R&D will be to
investigate the industrialisation of the X-HPD to the point where it represents a significant cost
reduction per cubic kilometre of instrumented volume compared to conventional PMTs, thereby
allowing for extremely large telescope target volumes.
Such gains will arise from industrialisation of an all-glass envelope construction, the internal
deposition of enhanced efficiency bi-alkali photocathodes, and either from cost reductions in the
central scintillating crystal or the use of a deposited phosphor on a truncated glass sphere viewed
by a small PMT.
Remeasurements of the quantum efficiency of recuperated PHILIPS XP2600 tubes at various
polar angles illustrate the possibility of quantum efficiency enhancement by the double cathode
phenomenon demonstrated to greater effect in the CERN true spherical X-HPD.
Details of the development program will be presented