Superheated Superconducting Granule Detector Research Articles

The Bern Cryogenic Detector System for dark matter search

In this paper we report on the design, construction, and operation of a cryogenic system designed to cool a Superheated Superconducting Granule (SSG) detector for dark matter search. The volume accommodating SSG detector is 20 cm in diameter and 67 cm long. The system provides a lowest operating temperature of 115 mK for long-lasting (several months) experiments in the Bern Underground Laboratory.

New fabrication techniques for PASS

The Planar Array of Superheated Superconductors was introduced by the UBC group in 1990. It is a regular array of micron-sized spheres of indium or tin that acts as a detector of radiation or particles in a similar fashion to the Superheated Superconducting Granule Detector. Over the years, it was demonstrated that this device exhibited good energy sensitivity, a narrow spread of individual transition temperatures, position sensitivity, and, using line geometry, partial avalanche effects. The great difficulty, however, was to produce a detector of sufficient size. We discuss three possible approaches to this problem. The first method is to fabricate the array photolithographically in much the same way as before, but on a much larger scale. The second way is to emboss squares in a metal film with an ultrafine cutting tool before melting. The third technique involves depositing the metal on to a structured surface then melting. Some experimental tests are described.

Simulations of transitions in superheated superconducting granules

We perform simulations of both colloidal suspensions and planar arrays systems of superheated superconducting granules. Transitions induced by an increasing external field appear in both situations as an ordering mechanism. Concentrated planar arrays present an interval of external field for which no transitions are produced. These effects can be interesting in applications for superheated superconducting granule detectors.

Dynamical ordering induced by preferential transitions in planar arrays of superheated superconducting granules

We perform simulations of planar arrays of superheated superconducting granules (PASS) under an external magnetic field, analyzing transitions undergone by the system when the external field is slowly increased from zero. We observe, for high concentrations, the existence of an interval of external fields for which no transitions are induced. This effect is analogous to a ‘hot border zone’ identified in the response of superheated superconducting granule detectors. We explain such behaviour as produced by a geometrical ordering dynamically induced in the system by transitions in preferential sites due to diamagnetic interactions.

Surface field in an ensemble of superconducting spheres under external magnetic field

We perform calculations of the magnetic field on the surface of an ensemble of superconducting spheres when placed into an external magnetic field, which is the configuration employed in superheated superconducting granule detectors. The Laplace equation is numerically solved with appropriate boundary conditions by means of an iterative procedure and a multipole expansion.

Transitions in disordered suspensions of superconducting granules under external magnetic field

We perform simulations of transitions in three-dimensional suspensions of superconducting granules, placed completely at random in a sample, when an external magnetic field is slowly increased. The results show that these transitions induce order in the system. This effect appears as advantageous in applications for Superheated Superconducting Granule detectors.

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

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.

Energy transport via quasiparticle and phonon diffusion in superheated superconducting granule detectors after nuclear recoils.

The energy transport via thermal diffusion inside superheated superconducting granules (SSG) after nuclear recoils is discussed. The decay towards equilibrium of the initial disturbance is described by a set of coupled heat-flow equations for the effective quasiparticle and phonon temperatures. The solution is carried out analytically for a point source located anywhere inside the superconducting granule with the initial energy distributed in both quasiparticle and phonon systems. The calculated sensitivity to nuclear recoils and the decay in time between interaction and phase transition are compared with irradiation measurements performed on Sn and Zn granules. The derived expressions for the quasiparticle and phonon temperatures are useful to predict the sensitivity of SSG detectors to nuclear recoils produced in dark matter particle or neutrino interactions.

Superheated superconducting granule detectors for dark matter

The presented results are part of a feasibility study of a Superheated Superconducting Granule (SSG) detector for weakly interacting massive particles (WIMPs). Using a 70 MeV neutron beam, the response of aluminum SSG to nuclear recoil energies of a few keV has been studied. The proved sensitivity of the detector to elastic scatterings will be discussed comparing the SSG performance at different detector thresholds with the theoretical expectations. The irradiation results allow to calibrate the detector energy threshold versus the magnetic threshold ΔH H and are in good agreement with the theoretical values. We are planning to start in the near future a dark matter search with a prototype SSG detector. The status of the project will be presented and the expected counting rate for spin-independent WIMP interactions will be discussed.

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.

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.

The position sensitivity of a planar array of superheated superconductors

A new technique for manufacturing the superheated superconducting granule detector is reported. The new sensor element has the potential for position sensitive detection of particles and radiation with a resolution of at least 100 μm. It consists of a planar array of spherical granules for which the spread in transition temperatures is an order of magnitude smaller than that of the colloidal device. Positional sensitivity (in one dimension) is achieved by analyzing the amplitudes of the signals from two SQUID pick-up coils and therefore only a few channels of electronics are required. The narrow spread in the size and positional ordering of the granules reduces the distribution of signal amplitudes originating from individual grains thereby allowing the dependence of signal amplitude on the distance from the coils to be utilized as signature of position.

Planar array of superheated superconductors: An improved superheated superconducting granule detector

We have invented a new superheated superconducting granule detector which is a prime candidate for sensing dark matter and which could prove very useful for other applications. The sensor element is a planar array of spherical granules for which the spread in transition temperatures is nearly an order of magnitude smaller than that for a colloidal device. The detecting efficiency for low-energy deposition is thereby significantly increased. Preliminary results are described.