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Preliminary results on aluminium and zinc superconducting granules at very low temperature
Nuclear Physics B (Proc. Suppl.) 16 (1990) 510 North-Holland
510
ELIMINA Y RESULTS ®N ALUMINIUM AND ZINC SU E C N UCTING GRANULES AT VERY LOW TEMPERATURE NIFACEa, . FREUNDE, J. GE AUER', L. G®NZALEZ-MESTRESa E ET-GALLIXa
a L.A .P.P (CNRS/IN2P3) Annecy-le-Vieux, France
6 Max-Planck-Institut fuer Physik und Astrophysik, Muenchen FRG Recent results on the behaviour of aluminium and zinc superconducting grain colloids at very low temperature are reported, exhibiting possible evidence for avalanche phenomena . Superheated superconducting granule (SSG) detectors are foreseen for solar neutrino and dark matter searches. They are made of a colloid of type I superconducting microspheres, embedded into a dielectric material (e.g. varnish GE 7031). The behaviour of the SSG colloid as a collective medium remains to be explored. Evidence for thermal interactions between grains due to latent heat exchanges through the dielectric material, was claimed by the Garching group 1 from tests with Cd granules at T < 300 mK. When the applied magnetic field Ho was raised to a value well below the maximum field compatible with superheating, most of the superconducting grains suddenly changed state . This was very :unusual, as previously a continuous andbroad distribution of grain flips per unit increase in o had always been obtained. We present here further possible evidence for collective phenomena using Al and Zn granules down to T > 40 mI{ . Zinc powder (diameter ^= 4 pm) was embedded in GE varnish 7031 , at filling factor between 4 and 30% in volume. For all samples prepared, new phenomena were observed already at temperatures above 350 mI{ . Although the used electronics was not sensitive enough to read individually such small grains, large pulses with different heights and different risetimes were seen when raising the magnetic field. Risetimes were often in the range 10 - 50 ps , whereas for single grains signal risetimes compatible with that of the pre-amplifiers (in the 200 ns range) were expected . The "avalanche" signals lie in a well defined region of Ho . They form distribution similar to the ordinary differential superheating curves previously observed in tests with other samples, where single grain flips had been detected and counted . At lower temperatures (T ^_" 40 mK), in a dilution refrigerator, "avalanche" signals appeared also under irradiation when, with a 3601 source mixed with the colloid, the magnetic field was set fixed for a few minutes at a value inside the "avalanche" region . 0920-5632/90/$3 .50 © Elsevier Science Publishers B.V. North-Holland
Aluminium grains of size 12 pm < 0 < 20 pm exhibited a similar behaviour at dilution temperatures, with somewhat shorter signals (,. 1 ps risetime) produced by bunches . More details can be found in [2] . 280
Voltage
lac 92 -2 -9r, -190 k
31
93 62 microseconds
Fig . 1 - (a) A digitized pulse obtained from a Zn colloid at 40 mK , when raising the applied magnetic field Ho . (b) An "avalanche" differential superheating curve (number of counts per unit increase in Ho) obtained by adding 20 sweeps in Ho . The pulses were shaped at 5 ps risetime to improve signal/noise ratio . References 1 . F. von Feilitzsch et al. in "Low Temp. Detectors for Neutrinos and Dark Matter", Springer-Verlag 1987 . 2. P. Freund et al. in "Low Temperature Detectors for Neutrinos and Dark Matter - III", Frontieres 1989.
510
ELIMINA Y RESULTS ®N ALUMINIUM AND ZINC SU E C N UCTING GRANULES AT VERY LOW TEMPERATURE NIFACEa, . FREUNDE, J. GE AUER', L. G®NZALEZ-MESTRESa E ET-GALLIXa
a L.A .P.P (CNRS/IN2P3) Annecy-le-Vieux, France
6 Max-Planck-Institut fuer Physik und Astrophysik, Muenchen FRG Recent results on the behaviour of aluminium and zinc superconducting grain colloids at very low temperature are reported, exhibiting possible evidence for avalanche phenomena . Superheated superconducting granule (SSG) detectors are foreseen for solar neutrino and dark matter searches. They are made of a colloid of type I superconducting microspheres, embedded into a dielectric material (e.g. varnish GE 7031). The behaviour of the SSG colloid as a collective medium remains to be explored. Evidence for thermal interactions between grains due to latent heat exchanges through the dielectric material, was claimed by the Garching group 1 from tests with Cd granules at T < 300 mK. When the applied magnetic field Ho was raised to a value well below the maximum field compatible with superheating, most of the superconducting grains suddenly changed state . This was very :unusual, as previously a continuous andbroad distribution of grain flips per unit increase in o had always been obtained. We present here further possible evidence for collective phenomena using Al and Zn granules down to T > 40 mI{ . Zinc powder (diameter ^= 4 pm) was embedded in GE varnish 7031 , at filling factor between 4 and 30% in volume. For all samples prepared, new phenomena were observed already at temperatures above 350 mI{ . Although the used electronics was not sensitive enough to read individually such small grains, large pulses with different heights and different risetimes were seen when raising the magnetic field. Risetimes were often in the range 10 - 50 ps , whereas for single grains signal risetimes compatible with that of the pre-amplifiers (in the 200 ns range) were expected . The "avalanche" signals lie in a well defined region of Ho . They form distribution similar to the ordinary differential superheating curves previously observed in tests with other samples, where single grain flips had been detected and counted . At lower temperatures (T ^_" 40 mK), in a dilution refrigerator, "avalanche" signals appeared also under irradiation when, with a 3601 source mixed with the colloid, the magnetic field was set fixed for a few minutes at a value inside the "avalanche" region . 0920-5632/90/$3 .50 © Elsevier Science Publishers B.V. North-Holland
Aluminium grains of size 12 pm < 0 < 20 pm exhibited a similar behaviour at dilution temperatures, with somewhat shorter signals (,. 1 ps risetime) produced by bunches . More details can be found in [2] . 280
Voltage
lac 92 -2 -9r, -190 k
31
93 62 microseconds
Fig . 1 - (a) A digitized pulse obtained from a Zn colloid at 40 mK , when raising the applied magnetic field Ho . (b) An "avalanche" differential superheating curve (number of counts per unit increase in Ho) obtained by adding 20 sweeps in Ho . The pulses were shaped at 5 ps risetime to improve signal/noise ratio . References 1 . F. von Feilitzsch et al. in "Low Temp. Detectors for Neutrinos and Dark Matter", Springer-Verlag 1987 . 2. P. Freund et al. in "Low Temperature Detectors for Neutrinos and Dark Matter - III", Frontieres 1989.