Last Update: October 2000

MIBETA (Milano e gruppo collegato di Trento)

Laboratory and beam: LASA (Mi) for cryogenic measurements and Milano-Bicocca for Radioactivity Measurements, IRST of Trento for the preparation of Silicon implanted thermistors)

1. Goal of the experiment

This experiment is based on the use of microbolometers for high resolution energy independent X - ray spectroscopy and measurements of the decay of 187Re in view of the determination of the mass of the electron antineutrino. In thermal detectors microbolometers due to their low heat capacity can reach energy resolutions (see later) superior by more than an order of magnitude then any other detector. The measurement of the spectrum of 187Re is very promising because this b active nucleus has the lowest transition energy in Nature. The basic idea of these experiments, carried out surprisingly in the world only in the Sezioni of Genoa and Milan, is to perform a "calimetric" experiment to determine , unlike in electron spectrometers, not only the electron energy , but the entire energy released in the detector. In this sense these experiments are to be considered as complementary to those carried out with electron spectrometers. In addition the lifetime of the 187Re decay (~43 Gyear) is of the same order of the life of the Universe and therefore very important in Cosmochronology.

The Genoa and Milan groups , in collaboration, have decided to diversify their experimental calorimetric approach. The Genoa group is using sources of metallic Rhenium and for the moment Neutron Transition Doped (NTD). The Milan group uses dielectric compounds of Rhenium , which could provide better energy resolution, and Silicon (Si:P) implanted thermistors constructed in collaboration with the IRST institute in Trento.

2. Physics achievements during 2000

As far as X-spectroscopy is concerned the Milan group has realized microdetectors where a resolution of 5 eV FWHM has been reached for the 55Mn lines. This resolution is twenty times better than the one obtained so far with any other type of X-ray detectors.

The Rhenium spectrum has been measured first with four crystals of Rhenium perrhenate and presently with an array of 8 crystals of the same material with an average mass of 300 micrograms each. The lifetime and transition energy have been measured with very high precision and the results already pubblished. The experiment with the array of 8 microbolometers read independently is continuously running since August 2000 and the results are being analyzed. A very careful and conservative analysis of all parameters is in fact needed before claiming a meaningful results on the value or limit of the neutrino mass.

3. INFN contribution to the experiment in terms of manpower and financial support

- Manpower: 12 physicists and 4 technicians are involved almost equally in the MIBETA and CUORE experiments
- - The financial support for MIBETA in 2000 was of 506 ML

4. Number of publications in refereed journals: 4

5. Number of talks to conferences: 3

6. Number of undergraduate and doctoral thesis on the experiment:

- n.1 Ph.D.;
- n.2 Laura diplomas

7. Leadership of the experiment

- the spokesman of the experiment and the leader of the contacts with the Berkeley and Trento groups is a member of the Milano group
- the coordinating group of the European Network for Cryogenic detectors is the Milano group

8. Innovative instruments:

- The results on X-ray spectroscopy are the best in the literature
- The Genoa and Milano groups are the only ones to perform, with different cryogenic techniques , a calorimetric measurement on the neutrino mass
- Application of the technique of microbolometers in Material Science and Astroparticle Physics is being considered

9. Competing experiments

- There are only four groups so far that can compete with Milano in X-ray spectroscopy, and only one uses a similar technique
- For the Rhenium experiment the Milano group is proud to compete with an highly scientific and technically qualified group like the Genoa one

10. International committee which has reviewed the experiment

- The experiment has been reported to three meeting of the CEE TMR and successfully reviewed by the European panel