Last Update: October 2000

MANU2 (Genova)

Researchers: F.Gatti, S.Vitale, F.Fontanelli, G.Gallinaro, D.Pergolesi, M.Razeti, M.Zondlowsky.

1. Goal of the experiment.

The MANU2 experiment aims to measure the beta spectrum near the end-point of 187-Re by means of cryogenic microcalorimeters with an energy resolution similar to the one of magnetic spectrometers. The physics goals are the high sensitivity kinematic measurement of the antineutrino mass and the observation of the spike anomaly at the end-point detected by the Russian of Troitsk experiment on tritium beta decay. The cryogenic detectors allow performing a total energy dispersive beta spectroscopy, avoiding complex and model dependent corrections of the impusle dispersive spectra of tritium experiments. MANU2 physics backgrounds have been studied in the previous experiment, MANU, i.e.:(i) feasibility of cryogenic detector working at about 100 mK made by superconducting metal, which contains 63% of 187-Re, (ii) first measurements of 187-Re beta spectrum with energy resolution down to 30 eV fwhm and statistics up to about 107counts, (iii) discovery of fine structure of the beta decay BEFS.

The first objective is the feasibility check of a cryogenic rhenium detector with energy resolution similar to the Mainz and Troitsk apparatus (~5eV). In order to improve the actual performances we have planned to develop new detectors, which are based on the use of different sensors, i.e., the superconducting transition edge sensor (TES). Therefore in year 2000, first year of the experiment, the following milestones have been foreseen:(1) production of TES of Ir or Al-Ag, (2) microlithography of TES sensors on silicon or Re single crystal, (3) functionality test of Rhenium-TES microcalorimeter, (4) electronics for SQUID-Array, (5) building of a working prototypes of detector. In parallel we planned to continue the study of the BEFS effect.

2. Achievements.

In the first stage of the experiment we have fixed the problems in the production of 3-layers Ag-Al-Ag films. Now we are able to obtain good 3-layers films, which show transition temperature in the range of 50-100 mK and thermometric responsivity [ 1/R(dR/dT) ] between 1500 and 3000, more than one order of magnitude of the sensors of the microcalorimeters of the previous experiment (milestones 1). We have produced sensors with different patterns and with dimensions lower than 200 x 200 um, by means of microlithographic processes on silicon and directly on Rhenium single crystals (milestones-2). We have tested one of these microcalorimeters to study the dynamical response to low energy x-ray. We have observed very fast thermal response respect to the old microcalorimeters (20-50 ms), being the actual thermal time constant about 3 ms, about 1 order of magnitude lower. This improvement is in agreement with the model prediction in the regime of strong electrothermal feedback (mileston-3). The development of the new SQUID electronics and of the working prototype of TES-microcalorimeter are under course.

On the line of study of the BEFS effect, we have planned and executed a run of measurements of EXAFS on Rhenium samples at the European Radiation Synchrotron Facility (ESRF) in Grenoble, within the Italian Beamline Facility supported by INFN. We have obtained a good K-edge EXAFS spectrum that will be used for parameter extraction for BEFS models.

3. Manpower and financial support.

Manpower: 7 researcher (2.85 FTE), 2 technician and support from the mechanics and the electronics workshops of INFN-Genova.

Financial support: 122 Mlit.

4. Number of publication in referred juornals: 3

5. Number of talks to conference: 3

6. Number of undergraduate and doctoral thesis: 2 undergraduates, 1 Ph.D.

7. Leadership role in the experiment.

Scientific management and coordination of the detector development is done by F.Gatti. Data analysis is coordinated by S.Vitale.

8. Innovative instruments

The experiment developed unique thermal detectors with superconducting absorber with high Z (=75) and high density (=21) capable to reach energy resolution (best results ~ 14 eV FWHM at 5.9 keV) about 1 order of magnitude better than commonly available solid-state detectors. Better performances are predicted in the near future. Applications to X-ray spectroscopy with same spectroscopic resolution of dispersive Bragg crystal are foreseen. We have developed very low noise J-FET cryogenic electronics. We developed high performances TES sensors: we obtained one of the best thermometric responsivity among the groups working on cryogenic detectors. It is also of interest the development of unique detector for nuclear spectroscopy by detection of small energy release: 7-Be E.C. in ground state of 7-Li, Ho-163 E.C. We have successfully examined the perspective of possible application of the BEFS effect for the study of microcrystalline structure in situations where EXAFS is not usable.

9. Competing experiments

No other group developed superconducting rhenium detector. No other group has achieved the sensitivity to observe the beta environmental fine structure of beta decay BEFS. Our group first proposed the use of Rhenium for neutrino mass measurements, and observed for the first time the beta spectrum with cryogenic detectors. The competing experiment is Mi-Beta that started the data taking after MANU was completed. Mi-Beta makes use of absorber made of dielectric crystal of silver perrhenate. The resolution of Mi-Beta is slightly better than our old detectors, with equivalent or lower mass of rhenium. MANU-2 is developing detectors with better performances.