M. Furlan

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.

Optimization of the readout electronics for a superheated superconducting granule particle detector

Abstract The sensitivity of Superheated Superconducting Granule (SSG) detectors is limited by the detectability of phase transition of single granules. The corresponding voltage pulse induced by the magnetic flux change in a pick-up coil was measured with an amplifier at room temperature. The readout circuit was optimized on the basis of a simple theoretical noise model, and a signal-to-noise ratio close to the expected theoretical limit was achieved. The minimal granule size required for a detectable signal has been determined as a function of the pick-up coil volume.

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.

Phase transition time measurements of superheated superconducting granules

Abstract We investigated the phase transition time of single Sn, In and Al granules from 20 μm up to 50 μm in diameter. The transitions from the superconducting to the normal state were induced by ramping the external magnetic field above the superheating field or by irradiating the granules with X-ray or β sources at a constant magnetic field. The readout electronics was optimized to allow real-time measurements with a signal resolution of 11 ns. Within the experimental errors, the irradiation measurements revealed no difference between magnetically and thermally induced phase transitions. The dependence of the transition time on the temperature, the granule size and the orientation of the sample with respect to the magnetic field is discussed. The presented results are part of an ongoing project to develop a Superheated Superconducting Granule (SSG) detector for cold dark matter search and neutrino detection.

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.

A long-lasting experiment with a 13 g SSG detector

Abstract A SSG detector has been tested in a first long-lasting experiment. It consisted of 13 g of tin granules with diameters of 29±0.5 μm embedded in Teflon powder with a volume filling factor of 11%. The detector has been exposed to cosmic rays and radioactive sources and has been kept for seven weeks at a constant temperature of 70 mK followed by shorter periods at 545 mK and 1.04 K. The detector performance as well as results of rate measurements are presented. These tests were made in preparation of the ORPHEUS dark matter experiment.

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.

BACKGROUND CONSIDERATIONS FOR LOW RATE SSG DETECTOR EXPERIMENTS

Abstract The low energy thresholds achieved with detectors made of superheated superconducting granules (SSG) encourage to use these devices for low rate measurements like WIMP search or coherent neutrino scattering experiments. Such applications need powerful rejection criteria for background events from electronic noise, cosmic and gamma radiation. In order to develop background rejection strategies, the origin of background sources and their interactions with SSG detectors need to be known. We present a discussion about electronic noise as well as background radiation together with their effects on the SSG detector counting rate.

Properties of a colloidal Superheated Superconducting Granule particle detector

Abstract Superheated Superconducting Granule detectors, consisting of tiny superconducting granules acting as microcalorimeters and being randomly distributed in space, deserve a careful investigation of their intrinsic properties, i.e. characteristic superconductivity effects, the target efficiency and the feasibility as an energy-resolving system. Experimental results and simulations, mainly focusing on the superheating curve and connected problems arising from magnetostatic interactions between the granules, are discussed in order to establish a profound frame for the development of or improvements on such detectors.