J. Gebauer

A scintillator-lead photon calorimeter using optical fiber readout systems

The construction and performance of a tower structured scintillator-lead photon calorimeter using a novel fiber optics readout system is described. An energy resolution of σE = 0.10√E was obtained with incident electrons in the range of 0.5–5.0 GeV/c. The uniformity of response across the front face of the tower was measured. Results obtained with a silicon photodiode are compared to those obtained with a photomultiplier.

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.

Studies of single superconducting grains for a neutrino and dark matter detector

Abstract We have conducted various tests of the properties of single superconductinggrains, of the type that has been suggested for use in detectors for dark matter and coherent neutrino scattering. These tests included study of the variation of the critical fields of a single grain when it is rotated relative to the applied field, observations concerning the existence of the intermediate state, and the effects of irradiation by alpha and gamma particles. The materials studied were Sn, Al and Zn at 4 He and 3 He temperatures. The variation of the critical (superheating) field under rotation suggests that the uncertainty of the phase boundary observed in the past for colloids of such grains in large measure already present in the different orientations of a single grain. The irradiation experiments indicate that for certain materials a simple “global heating” picture of the grain flip process applies in that the grain shows a well defined, sharp flip threshold in an applied field, one which furthermore is in accordance with the superconducting specific heat of the grain.

A tower structured scintillator-lead photon calorimeter using a novel fiber optics readout system

Described is the construction and the performance of a tower-structured scintillator-lead photon calorimeter using a novel fiber optics readout system. The calorimeter is divided into 9 individual towers. Each tower has a cross section of 5 × 5 cm2 and consists of 60 layers of 2 mm lead plus 5 mm thick scintillator. The four sides of each tower are covered by thin acrylic sheets (1.5 mm thick) doped with a wavelength shifting material. The light produced in each scintillator plate is first converted in these sheets, then converted a second time in a set of polystyrene optical fibers (diameter 2 mm) which run longitudinally through the calorimeter along the corners of each tower. A small diameter photomultiplier was attached to the fibers at the back end of the calorimeter. The obtained energy resolution with incident electrons in the range of 0.25−5.0 GeVc is σE = 0.10√E. The uniformity of response across the front face of each tower was measured.

TEST OF THE NEW HYBRID INTEVAC INTENSIFIED PHOTOCELL FOR THE USE IN AIR CHERENKOV TELESCOPES

Abstract For a new large diameter (17 m ∅ mirror collector) air Cherenkov telescope project a “camera” of photosensors with high, red extended quantum efficiency is needed. A candidate is a variant of the INTEVAC intensified photocell with a GaAsP photocathode (≈45% QE). A version with an avalanche diode as secondary amplifying element with fast (ns) single electron response is under consideration. The motivation for the development and first test results are discussed.

An optimized method for the reconstruction of the direction of air showers for scintillator arrays

Abstract An optimized method is presented for the reconstruction of air shower directions for scintillator arrays. The method takes into account that both the expectation value and the spread of the measured arrival times not only depend on the distance of a counter from the shower axis, but also on the number of particles registered in that counter. It also takes into account that the distributions of the measured arrival times are not Gaussian. For showers recorded with the HEGRA scintillator array above the threshold energy of E thres = 20 TeV the mean angular resolution obtained with this method is 〈 σ 63% θ 〉 = 1.0°, and above the threshold E thres = 50 TeV it is 〈 σ 63% θ 〉 = 0.6°. Comparing the new procedure with the HEGRA standard procedure the angular resolution has improved on average by a factor of 1.33. The mis-pointing has been determined with an accuracy of 0.15°. The method is developed using experimental data.

Superconducting grains as micro-calorimeters

Abstract Under irradiation with monoenergetic α particles certain single selected superconducting grains show a relatively sharp and reproducible threshold in the applied magnetic field for the transition to the normal state. This threshold corresponds to a well-defined heating of the grain, indicating that the grain acts like little calorimeter. Both the absolute value of the threshold and its temperature variation are in good agreement with the superconducting specific heat.

Investigation of Superconducting Tin Granules for a Low-Energy Neutrino or Dark Matter Detector

The properties of single superconducting tin granules with a diameter between 20 and 112 µm were studied between 3.26 and 1.4 Kelvin in a magnetic field. The granules were rotated around an axis perpendicular to the magnetic field axis and the superheating and supercooling fields were determined. We observed that each granule exhibits its own characteristic superheating and supercooling field which strongly depends on the rotational angle. For granules with effective superheating fields just above the critical thermodynamical field, a phase transition was observed which took place over only a part of the granule (intermediate state). Single tingranules were also irradiated with α-particles of 5.5 MeV energy. Phase transitions were clearly observed. The results are consistent with local heating.