Dieter Renker

Advances in solid state photon detectors

Semiconductor photodiodes were developed in the early `Forties approximately at the time when the photomultiplier tube became a commercial product (RCA 1939). Only in recent years, with the invention of the Geiger-mode avalanche photodiodes, have the semiconductor photo detectors reached sensitivity comparable to that of photomultiplier tubes. The evolution started in the `Sixties with the p-i-n (PIN) photodiode, a very successful device, which is still used in many detectors for high energy physics and a large number of other applications like radiation detection and medical imaging. The next step was the development of the avalanche photodiode (APD) leading to a substantial reduction of noise but not yet achieving single photon response. The weakest light flashes that can be detected by the PIN diode need to contain several hundreds of photons. An improvement of the sensitivity by 2 orders of magnitude was achieved by the development of the avalanche photodiode, a device with internal gain. At the end of the millennium, the semiconductor detectors evolved with the Geiger-mode avalanche photodiode into highly sensitive devices, which have an internal gain comparable to the gain of photomultiplier tubes and a response to single photons. A review of the semiconductor photo detector design and development, the properties and problems, some applications and a speculative outlook on the future evolution will be presented.

Design and Operation of FACT -- The First G-APD Cherenkov Telescope

The First G-APD Cherenkov Telescope (FACT) is designed to detect cosmic gamma-rays with energies from several hundred GeV up to about 10 TeV using the Imaging Atmospheric Cherenkov Technique. In contrast to former or existing telescopes, the camera of the FACT telescope is comprised of solid-state Geiger-mode Avalanche Photodiodes (G-APD) instead of photomultiplier tubes for photo detection. It is the first full-scale device of its kind employing this new technology. The telescope is operated at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain) since fall 2011. This paper describes in detail the design, construction and operation of the system, including hardware and software aspects. Technical experiences gained after one year of operation are discussed and conclusions with regard to future projects are drawn.

Evaluation of Geiger-mode APDs for PET block detector designs

This paper presents an evaluation of two types of Geiger-mode avalanche photodiodes (G-APDs) for their potential to be used in a positron emission tomography (PET) detector. While the MPPC G-APD had only 3600 cells, the solid state photomultiplier (SSPM)-type G-APD had 8100 cells. In a single-channel G-APD/LSO setup, the energy resolution (DeltaEpsilon/Epsilon) of the SSPM at 511 keV was 25%, while the (DeltaEpsilon/Epsilon) of the MPPC was 13.5% (FWHM). No influences were observed while the detectors were inside a 7 T magnetic resonance (MR) scanner. A time resolution of 2.7 ns (FWHM) was measured for the LSO/SSPM and 0.9 ns for the LSO/MPPC detector setup. Although the linearity was superior for the SSPM in the single detector readout, the inferior energy and time resolution excluded them to be used for the block detector readout. All 12 x 12 LSO crystals of the block could be resolved in a crystal map using a 3 x 3 MPPC G-APD array. The time resolution of the block detector was 950 ps. While the energy spectra for the MPPC-based single-channel setup were nonlinear, they reached linearity better than 5% in the block detector. A high number of G-APD cells provide a linear signal in a single-channel detector setup, but not necessarily a good timing or (DeltaEpsilon/Epsilon) due to a larger inactive surface resulting in lower photon detection efficiency. G-APDs with a low number of cells provide a good timing and (DeltaEpsilon/Epsilon) and linear signals in block detector designs, where the scintillation light is shared over many G-APDs.

Fast readout of plastic and crystal scintillators by avalanche photodiodes

Abstract Avalanche photodiodes (APDs) of improved quality and a sensitive area up to cm2 area are becoming commercially available now. In certain applications they can replace photomultipliers for fast readout of scintillation detectors. Expected performances will be discussed and some test measurements presented. Examples of possible applications will be given.

The first telescope of the HEGRA air Cherenkov imaging telescope array

Abstract In search of VHE γ ray emission from cosmic point sources a system of imaging Cherenkov telescopes is constructed at present on the Canarian island of La Palma; the first telescope has been operational since 1992. The Cherenkov light from air shower particles is collected by a 5 m2 reflector. The camera at the focus contains 37 photomultipliers which sample the images of the Cherenkov flashes. The subsequent image analysis allows the discrimination of γ ray induced events from the much more abundant charged cosmic ray induced showers. The telescope has an effective energy threshold for γ showers of about 1.5 TeV. During the first year of operation a signal from the Crab nebula was detected.

A small animal PET prototype based on LSO crystals read out by avalanche photodiodes

Abstract Feasibility studies of modern avalanche photodiodes (APDs), coupled to LSO scintillation crystals, have given encouraging results for new detector designs in positron emission tomography (PET). We constructed an experimental set-up for measuring the performance of new arrays of 2 × 8 detector channels of APD/LSO. This gantry allows us to simulate a complete ring of these array detectors and will ultimately serve as a simple and flexible small animal PET prototype.

Search for isotropic γ radiation in the cosmological window between 65 and 200 TeV

Abstract Electromagnetic energy injected into the universe above a few hundred TeV is expected to pile up as γ radiation in a relatively narrow energy interval below 100 TeV due to its interaction with the 2.7°K background radiation. We present an upper limit (90% C.L.) on the ratio of primary γ to charged cosmic rays in the energy interval 65–160 TeV (80–200 TeV) of 10.3 · 10 −3 (7.8 · 10 −3 ). Data from the HEGRA cosmic-ray detector complex consisting of a wide angle Cerenkov array (AIROBICC) measuring the lateral distribution of air Cerenkov light and a scintillator array, were used with a novel method to discriminate γ-ray and hadron induced air showers. If the presently unmeasured universal far infrared background radiation is not too intense, the result rules out a topological-defect origin of ultrahigh energy cosmic rays for masses of the X particle released by the defects equal to or larger than about 10 16 GeV.

The μSR facilities at PSI

TheμSR Facility Instruments presently available at PSI and the envisaged medium- and long-term developments are presented. The plans focus on further upgrades of the existing instruments and the development of new techniques using the very high fluxes becoming available at PSI, in particular the setup of a beamline with a fast kicker for ‘muons on request’ (MORE) and the development of very low energy muon beams.

A novel camera type for very high energy gamma-ray astronomy based on Geiger-mode avalanche photodiodes

Geiger-mode avalanche photodiodes (G-APD) are promising new sensors for light detection in atmospheric Cherenkov telescopes. In this paper, the design and commissioning of a 36-pixel G-APD prototype camera is presented. The data acquisition is based on the Domino Ring Sampling (DRS2) chip. A sub-nanosecond time resolution has been achieved. Cosmic-ray induced air showers have been recorded using an imaging mirror setup, in a self-triggered mode. This is the first time that such measurements have been carried out with a complete G-APD camera.

A Monte-Carlo model of a SiPM coupled to a scintillating crystal

The silicon photomultiplier (SiPM) is a promising photodetector for a number of applications. In this paper, we present a Monte-Carlo based statistical model, which includes the fundamental device characteristics of the SiPM, such as fill factor, quantum efficiency, Geiger discharge probability, afterpulse probability, crosstalk probability, cell size, non-linearity and dark count rate. The impact of these parameters on the performance of a SiPM coupled to a scintilla-ting crystal were studied. In this work special focus was put on the influence of different parameters on the energy resolution of a SiPM coupled to a LYSO, which provides important guidelines for further optimization of SiPMs for positron emission tomography. The developed model shows that afterpulses and the crosstalk have little influence on the detector performance in terms of energy resolution. The energy resolution of the SiPM with 400 cells/mm2 is strongly affected by the recovery time of the detector. It is due to the combination of the high PDE and short recovery time that a SiPM with 400 cells/mm2 shows better performance in term of energy resolution when coupled with LYSO compared to a SiPM with 1600 cells/mm2.