G. Czapek

Detection of nuclear recoils in prototype dark matter detectors, made from Al, Sn and Zn superheated superconducting granules

Abstract This work is part of an ongoing project to develop a superheated superconducting granule (SSG) detector for cold dark matter and neutrinos. The response of SSG devices to nuclear recoils has been explored irradiating SSG detectors with a 70 MeV neutron beam. The aim of the experiment was to test the sensitivity of Sn, Al and Zn SSG detectors to nuclear recoil energies down to a few keV. The detector consisted of a hollow teflon cylinder (0.1 cm 3 inner volume) filled with tiny superconducting metastable granules embedded in a dielectric medium. The nuclear recoil energies deposited in the SSG were determined measuring the neutron scattering angles with a neutron hodoscope. Coincidences in time between the SSG and the hodoscope signals have been clearly established. In this paper the results of the neutron irradiation experiments at different SSG intrinsic thresholds are discussed and compared to Monte Carlo simulations. The results show that SSG are sensitive to recoil energies down to ∼ 1 keV. The limited angular resolution of the neutron hodoscope prevented us from measuring the SSG sensitivity to even lower recoil energies.

The ORPHEUS dark matter experiment

Abstract A progress report of the ORPHEUS dark matter experiment in the Bern Underground Laboratory is presented. A description of the ORPHEUS detector and its sensitivity to WIMPs is given. The detector will consist of 1 to 2 kg Sn granules operating in a magnetic field of approximately 320 G and at a temperature of 50 mK. In the first phase, the detector will be read out by conventional pickup coils, followed by a second phase with SQUID loops. Preliminary results on background and radioactivity measurements are shown.

Performance of a BGO calorimeter with photodiode readout and with photomultiplier readout at energies up to 10 GeV

Abstract Bismuth germanate (BGO) calorimeter arrays, consisting of up to 12 elements of 30 × 30 × 200 mm 3 have been tested at the CERN PS with pions and electrons of up to 10 GeV/ c momentum, and at SIN with pions, electrons and protons up to 450 MeV/ c . Both photomultiplier (PM) and photodiode (PD) readouts were used. Accurate calibration in the 100 MeV energy range was achieved with stopping protons, stopping pions and minimum ionizing pions. With 212 MeV electrons and PM readout, a time resolution of the BGO signal of 640 ps fwhm has been measured. The energy resolution for electrons above 1 GeV (PD readout) was found to be roughly constant at σ / E ∼ 1%. This is consistent with a negligible intrinsic resolution for BGO at these energies, after taking into account shower leakage and PD noise. For electrons of 92 and 200 MeV, we obtained (PM readout) energy resolutions close to the theoretical limit given by photon statistics and shower leakage. The electron/hadron separation was better than 1:500 over the energy range of 0.5 to 10 GeV, and improved to better than 1:1000 after a simple pattern cut. The energy deposition of the e.m. showers, both laterally and longitudinally (rear leakage), was found to be in agreement at the 0.1% level with Monte Carlo calculations using the SLAC-EGS program.

Superheated superconducting granule device: detection of minimum ionizing particles

We present the first efficiency measurement of a superheated superconducting granule (SSG) detector irradiated with minimum ionizing particles (mips). The SSG device is a solenoid readout coil filled with tin granules (22 μm average diameter) embedded (16% volume filling factor) in a good thermal conductor varnish cooled down to T = 490 mK. n nIt is shown that a 2 mm thick detector is close to 100% efficient (1σ over the electronic noise) to mips. The probability of a mip to hit at least one granule, sensitive to an energy deposition of 16 keV or more over the detector length, is approximately 100%. Only 10% of the granules in the detector were sensitive to mips because of the finite granule size distribution and the phase transition smearing observed with this sample.

Feasibility study of a Superheated Superconducting Granule detector for cold dark matter search

The presented results are part of a feasibility study of a Superheated Superconducting Granule (SSG) device for weakly interacting massive particles (WIMPs) detection. The sensitivity of SSG to nuclear recoils has been explored irradiating SSG detectors with a 70MeV neutron beam proving that energy thresholds of ∼1keV can be reached in 30µm Zn and 17µm Sn granules. The successful irradiation experiments with neutrons encouraged us to plan a prototype SSG dark matter detector. The status of the project will be presented and the expected counting rate for spin-independent WIMP interactions in SSG detectors will be discussed.

First results with the ORPHEUS dark matter detector

Abstract The ORPHEUS dark matter detector is operating at our underground laboratory in Bern (70 m.w.e.). The detector relies on measuring the magnetic flux variation produced by weakly interacting massive particles (WIMPs) as they heat micron-sized superheated superconducting tin granules (SSG) and induce superconducting-to-normal phase transitions. In an initial phase, 0.45xa0kg of tin granules in a segmented detector volume have been used. In this paper a general description of the experimental setup, overall performance of the detector, and first results are presented.

Phase transition study of superheated planar arrays of tin cylinders

Abstract We have investigated the superheating and the supercooling fields of a planar array of tin cylinders with diameters ranging from 40 to 80 μm and thickness 8.3 μm evaporated onto a glass substrate and onto a kapton foil. Experimental values of the Ginzburg–Landau parameter have been obtained and compared with results from tin spheres. The spread of the superheating curves is below 5%.

Improvement of the phase transition homogeneity of superheated superconducting tin granules

Abstract A considerably improved phase transition homogeneity was observed with superheated superconducting Sn spheres as a result of laser melting and fast cooling. These spheres exhibited a very homogeneous phase transition behaviour independent of the orientation of the spheres with respect to the direction of the external magnetic field. Compared to previously untreated Sn spheres the spread of the phase transition boundaries was reduced by almost an order of magnitude. In addition, we studied mass production of Sn structures using vacuum evaporation and deposition. First encouraging results are reported.

Superheated superconducting granule detector tested with nuclear recoil measurements

Abstract The presented results are part of a superheated superconducting granule (SSG) detector development for neutrino and dark matter. The aim of the experiment was to measure the sensitivity of the detector to nuclear recoil energies when exposed to a 70 MeV neutron beam. The detector consists of a small readout coil (diameter 5 mm, length 10 mm) filled with aluminum granules of average diameter 23 μm embedded in an Al 2 O 3 granulate with a 6% volume filling factor. The neutron scattering angles were determined using a scintillator hodoscope. Coincidences between the SSG and the hodoscope signals have been clearly established. Data were taken at an operating temperature of 120 mK for different SSG intrinsic thresholds. The results prove the sensitivity of the detector to nuclear recoils around 10 keV.

Nuclear recoil measurements in Superheated Superconducting Granule detectors

The response of Superheated Superconducting Granule (SSG) devices to nuclear recoils has been explored by irradiating SSG detectors with a 70MeV neutron beam. In the past we have tested Al SSG and more recently, measurements have been performed with Sn and Zn detectors. The aim of the experiments was to test the sensitivity of SSG detectors to recoil energies down to a few keV. In this paper, the preliminary results of the neutron irradiation of a SSG detector made of Sn granules 15–20µm in diameter will be discussed. For the first time, recoil energy thresholds of ∼1keV have been measured.