A new Readout of large area Smart Photomultipliers by Geiger-mode APDs

A new Readout of large area Smart Photomultipliers by Geiger-mode APDs

E. Lorenz

, D. Ferenc

a)

Max Planck Inst. for Physics, Foehringer Ring 6, D 80805 Munich, Germany

b)

Physics Dep. UC-DAVIS, One Shield Av, Davis, CA 95616-8677 USA

Abstract

Future Neutrino detectors require large, transparent detection volumes (water, ice, scintillator) and in turn a large number of large photomultipliers (PMT). We present a possible photon detector design based on the light amplification concept, e.g., a large, nearly spherical vacuum photon detector where the photoelectrons are accelerated onto a small scintillator. As secondary readout element Geiger- mode APDs are used. Advantages and drawbacks of this readout will be discussed. Some results from initial studies will be presented.

Keywords: Photon detectors, Neutrino experiments, Geigermode Avalanche photodiodes PACS: 95.55 Vj, 95.85 Ry

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1

Corresponding author. Tel.: +49-89-32354241; fax:+49-89-3226704; e-mail: e.lorenz@mac.com.

1. Introduction

An experimental challenge in high energy astroparticle physics is given by the low flux of cosmic particles. Very large detection volumes and large angular acceptances are required. The only suitable materials available today are natural ones of high optical transmission, such as water, ice (bubble-free) and the atmosphere to detect Cherenkov light (in case of the atmosphere also air fluorescence light) emitted by relativistic secondary particles generated by the cosmic messenger particles interacting in these media. The detection of this light requires large photon sensors with high sensitivity for single photons. For some neutrino detector projects the costs of these photon detectors will approach half of the entire project costs. Therefore low cost large area PMTs are required. Here we present a progress report of an earlier study (ref. 1) for a variant of the so- called smart PMT. The smart PMT was originally proposed by the company Philips (now Photonis). Later, an improved design -dubbed Quasar- has been built in the former Soviet Union. This PMT has been in use for quite a few years in the Lake Baikal neutrino experiment. The classical smart PMT is basically a light amplifier with a small secondary PMT for the readout of the intensified light signals. In detail, photoelectrons (PE) generated in a large spherical photo-cell are accelerated by high voltage (typ. 25 kV) and focussed onto a small scintillator. The scintillator is coated with aluminium acting as a light barrier towards the cathode. The scintillator is coupled to a short light guide (part of the vacuum wall), which transmits the scintillation light out of the PMT where it can be detected by another sensor. The light conversion for a PE impinging onto a fast YAP scintillator is ≈ 20 photons per KeV energy. Not all of

this light will emerge though the light guide and can be re- converted into an electrical signal. Using a second PMT one can detect typically 10-20 secondary PEs for one initial 25 KeV PE, i.e., such a detector has a single photoelectron response. The main advantages of the concept are:

• Simpler and cheaper production because of the absence of simultaneous processing of the photocathode and the dynodes.

• Perfect circular symmetry in PE collection efficiency.

• Practically 100% PE collection efficiency (the PE collection in large classical PMTs is much lower).

• Only low power HT required as no bleeder current is needed.

• Negligible sensitivity to the earth magnetic field

• Very robust

Disadvantages are the long de-excitation time of common high light yield scintillators (typically 40-100 nsec) and the need for a secondary PMT with a separate power supply, both being a major cost factor. Our study focuses exactly on this issue. We carried out an experiment to prove that it is possible to replace the secondary PMT by another class of photon detectors, the so-called Geigermode-APDs (G- APD), often also called Silicon Photomultipliers or Avalanche Microchannel Photodiodes. We have reported in the past, ref.[1], about a first study using a small G-APD of only 1x1 mm

area, to detect first signals from a Quasar PMT. Here we present a progress report using a more advanced 3x3 mm

G-APD allowing us to carry out some timing and pulse height studies and such to demonstrate the feasibility of this novel readout. Nevertheless, the G-APD used was still too small and had a photon detection efficiency (PDE) too low to detect the signals from a single PE in the Quasar. The G-APD was obtained from Z.

Sadygov/Micron Company, Russia. The essential

parameters were an area of 3x3 mm

, 10

cells/mm

, a bias

Submitted to Elsevier Science 2

voltage of 132 V and a gain ≤ 5.10

. The PDE was 12.5%

at 450 nm with a further reduction at shorter wavelength.

2. Some measurements with the Quasar Photo- multiplier and Conclusions

For the study we used a standard Quasar of below average performance. The tube was biased at 22 kV. The cathode was illuminated by a weak, pulsed, blue LED light source, adjustable between < 1 photon per pulse to a few 100 photons per pulse. The scintillator was viewed by the G- APD at a distance of ≈ 1mm to the exit window (due to the G-APG package rim). The signal was amplified by a chain of 2 preamps, MAN-1LN. and analysed with a LeCroy qVt analyser triggered by the light pulser. Fig 1 shows an oscilloscope screen shot from a two photoelectron signal displaying the timing structure of the emitted scintillation photons. Fig. 2 shows the corresponding pulse height distributions. One can clearly see that the single electron response is still insufficient. Fig. 3 shows the results of the timing resolution analysis for a mean of two photoelectrons. The FWHM is 3.0 nsec.

Fig.1 Screen shot for a 2 PE signal

Fig 2. Pulse height distribution for <2> PE.

This study is basically the proof of the concept. With the G- APD used it was not possible to demonstrate single PD sensitivity. The PE sensitivity can be largely improved by:

• Using an G-APD of at least 10x10 mm

area (or a larger size matrix to correlate sections of the cathode to single elements, respectively using the Anger camera readout principle)

• Improving the PDE to ≥ 30% (this would limit the dynamic range to < 1000 pe)

• Use of optical coupling, a thinner light guide or a fiber optic faceplate.

We estimate a possible improvement by a factor > 50, but further studies are needed.

Fig 3. Timing distribution for <2> PE. One dot = 400 psec.

Fig. 4 shows a sketch of a possible future arrangement with a photocathode extending over 270 °. A ZnO, or LaBr crystal would result in either a very fast or a large signal.

Fig. 4. Sketch of a possible configuration with a spherical scintillator for perfect spherical symmetry

3. Acknowledgements

Herewith we would thank Z. Sadygov for the provision of the G-APDs and I. Britwich for the measurement of the PDE. The project was in part supported by the US NNSA.

References:

[1] D. Ferenc et al. : The Novel Light Amplifier Concept:

Procs. Beaune Photon Detector Conf. 2005, in print Nuc.

Inst. Meth.