D. Renker

Design, commissioning and performance of the PIBETA detector at PSI

Abstract We describe the design, construction and performance of the PIBETA detector built for the precise measurement of the branching ratio of pion beta decay, π + →π 0 e + ν e , at the Paul Scherrer Institute. The central part of the detector is a 240-module spherical pure CsI calorimeter covering ∼3 π sr solid angle. The calorimeter is supplemented with an active collimator/beam degrader system, an active segmented plastic target, a pair of low-mass cylindrical wire chambers and a 20-element cylindrical plastic scintillator hodoscope. The whole detector system is housed inside a temperature-controlled lead brick enclosure, which in turn is lined with cosmic muon plastic veto counters. Commissioning and calibration data were taken during two 3-month beam periods in 1999/2000 with π + stopping rates between 1.3·10 3 π + / s and 1.3·10 6 π + / s . We examine the timing, energy and angular detector resolution for photons, positrons and protons in the energy range of 5– 150 MeV , as well as the response of the detector to cosmic muons. We illustrate the detector signatures for the assorted rare pion and muon decays and their associated backgrounds.

Beam tests of lead tungstate crystal matrices and a silicon strip preshower detector for the CMS electromagnetic calorimeter

Tests of lead tungstate crystal matrices carried out in high-energy electron beams in 1996, using new crystals, new APDs and an improved test set-up, confirm that an energy resolution of better than 0.6% at 100 GeV can be obtained when the longitudinal uniformity of the struck crystal is adequate. Light loss measurements under low dose irradiation are reported. It is shown that there is no loss of energy resolution after irradiation and that the calibration change due to light loss can be tracked with a precision monitoring system. Finally, successful tests with a preshower device, equipped with silicon strip detector readout, are described.

Investigation of the avalanche photodiodes for the CMS electromagnetic calorimeter operated at high gain

Abstract Avalanche Photodiodes (APD) with improved characteristics were developed by Hamamatsu Photonics for the Electromagnetic Calorimeter of the CMS experiment. This report presents measurements of the latest generation of APDs, which are capable to operate at high gains (∼2000).

Comparison between high-energy proton and charged pion induced damage in PbWO4 calorimeter crystals

Abstract A PbWO4 crystal produced for the electromagnetic calorimeter of the CMS experiment at the LHC was cut into three equal-length sections. The central one was irradiated with 290 MeV / c positive pions up to a fluence of ( 5.67 ± 0.46 ) × 10 13 cm - 2 , while the other two were exposed to a 24 GeV / c proton fluence of ( 1.17 ± 0.11 ) × 10 13 cm - 2 . The damage recovery in these crystals, stored in the dark at room temperature, has been followed over two years. The comparison of the radiation-induced changes in light transmission for these crystals shows that damage is proportional to the star densities produced by the irradiation.

Wavelength dependence of avalanche photodiode (APD) parameters

New types of Hamamatsu avalanche photodiodes (APD) have been investigated for the readout of PbWO4 crystals. The results presented cover quantum efficiency measurements of APD prototype A-E, which is passivated by a SiO2 layer or a Si3N4 layer, before and after exposure to a 5.5 Mrad dose of60Co photons. The measurements of the gain and of the excess noise factor as a function of the wavelength are also presented and compared to analytic calculations based on a simple model for the internal APD structure. This work was performed within the frame of the CMS ECAL group.

Cosmic muon tomography of pure cesium iodide calorimeter crystals

Abstract Scintillation properties of pure CsI crystals used in the shower calorimeter being built for precise determination of the π + →π 0 e + ν e decay rate are reported. Seventy-four individual crystals, polished and wrapped in Teflon foil, were examined in a multiwire drift chamber system specially designed for transmission cosmic muon tomography. Critical elements of the apparatus and reconstruction algorithms enabling measurement of spatial detector optical nonuniformities are described. Results are compared with a Monte Carlo simulation of the light response of an ideal detector. The deduced optical nonuniformity contributions to the FWHM energy resolution of the PIBETA CsI calorimeter for the π + → e + ν 69.8 MeV positrons and the monoenergetic 70.8 MeV photons were 2.7% and 3.7%, respectively. The upper limit of optical nonuniformity correction to the 69.8 MeV positron low-energy tail between 5 and 55 MeV was +0.2%, as opposed to the +0.3% tail contribution for the photon of the equivalent total energy. Imposing the 5 MeV calorimeter veto cut to suppress the electromagnetic losses, GEANT -evaluated positron and photon lineshape tail fractions summed over all above-threshold ADCs were found to be 2.36±0.05 (stat) ±0.20 (sys)% and 4.68±0.07 (stat)±0.20 (sys)%, respectively.

Studies of Lead Tungstate Crystal Matrices in High-Energy Beams for the CMS Electromagnetic Calorimeter at the LHC

Using matrices of lead tungstate crystals, energy resolutions better than 0.6% at 100 GeV have been achieved in the test beam in 1995. It has been demonstrated that a lead tungstate electromagnetic calorimeter read out by avalanche photodiodes can consistently achieve the excellent energy resolutions necessary to justify its construction in the CMS detector. The performance achieved has been understood in terms of the properties of the crystals and photodetectors.

Properties of the most recent Hamamatsu avalanche photodiode

Abstract In a development program Hamamatsu Photonics improved their avalanche photo-diodes significantly. The first milestones, reduction of the so-called nuclear counter effect, the effective thickness contributing to signals from charged particles, without increasing the capacitance, the dark current and the excess noise factor has been achieved. In parallel the radiation hardness against neutron and gamma irradiation has been improved. Results on the most recent prototype will be presented.

The AX-PET concept: New developments and tomographic imaging

The Axial PET (AX-PET) concept proposes a novel detection geometry for PET, based on layers of long scintillating crystals axially aligned with the bore axis. Arrays of wavelength shifting (WLS) strips are placed orthogonally and underneath the crystal layers; both crystals and strips are individually readout by G-APDs. The axial coordinate is obtained from the WLS signals by means of a Center-of-Gravity method combined with a cluster algorithm. This design allows spatial resolution and sensitivity to be decoupled and thus simultaneously optimized. In this work we present the latest results obtained with the 2-module AX-PET scanner prototype, which consists of 6 radial layers of 8 LYSO crystals each (crystal size: 3 × 3 × 100 mm3). The WLS arrays comprise 26 strips (3-mm wide) per layer. The estimated energy resolution from point-like measurements is 11.8% (FWHM at 511 keV). The intrinsic spatial resolution was measured for the two modules in coincidence at two different configurations using point-like sources, showing very little degradation when the modules were placed oblique to each other. The axial spatial resolution was 1.5 mm (FWHM) in all the studied cases. Tomographic data of extended phantoms filled with fluorine-18 have been acquired. Imaging a larger transaxial Field-of-View (when compared to the previous measurement campaign) was possible thanks to implementing secondary motion of one of the modules. We have also developed various reconstruction approaches which take into account the particular nature of AX-PET data, as well as a count rate model which allowed us to develop an acquisition protocol able to compensate for count losses. The reconstructed phantom images confirm the imaging capabilities of AX-PET, and the recent advancements in the DAQ let us expect significant improvements for future campaigns.

Detection of Cherenkov light from air showers with Geigermode-APDs

Abstract We have detected Cherenkov light from air showers with Geigermode-APDs (G-APDs). G-APDs are novel semiconductor photon detectors, which offer several advantages compared to conventional photomultiplier tubes in the field of ground-based very high energy (VHE) γ -ray astronomy. In a field test with the MAGIC telescope we have tested the efficiency of a G-APD/light catcher setup to detect coinciding Cherenkov light from air showers. From our test we estimate a detection efficiency of our G-APD/light catcher setup to be ≥ 60 % higher than the efficiency of a MAGIC camera pixel. The dark count rates of the tested G-APD at ambient temperature are 10 times lower than the rates of detected photons from the night sky light background (NSB). This relaxes the need for cooling the G-APD to the level where the noise rate is sufficiently low. According to this recent test, G-APDs promise a major progress in ground-based VHE γ-ray astronomy.