Proc. Neutrino Astrophysics - MPP Theory Group

Neutrino Experiments with Cryogenic Detectors
P. Meunier 1,2
1 Dipartimento di Fisica Universita’ di Genova, INFN Sezione di Genova, Genoa, Italy
2 Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
In general an ideal cryogenic thermal detector consists of an absorber for radiation or particles
where the energy released has to be rapidly converted into thermal energy, changing the
detector temperature. The absorber is coupled to a thermometer which responds to a change
of temperature by a change of resistance. In the case of current biasing of the thermistor,
the temperature change is measured by observing the voltage drop across it. In order to have
an increase of the detector temperature which is not negligible (in the range of µK) for a
very small amount of energy release (100 eV), the detector heat capacity has to be extremely
small. Since heat capacities decrease at low temperature, the sensitivity is enhanced by
running the detector at very low temperatures (≪ 1 K). In the case of cryogenic thermal
detectors characterized by a small mass and a heat capacity of the order of 10 −12 J/K at
0.1 K, it is theoretically possible to obtain an energy threshold and an energy resolution
of a few eV. The intrinsic slowness of the developed low temperature detectors (total pulse
duration is about 100 ms) limits their application only to low activity measurements.
A possible application of this kind of detector is the measurement of the β calorimetric
spectrum with a high energy resolution. The investigation of the β decay spectrum of particular
isotopes, such as 187 Re and 167 Ho can provide more stringent kinematics limits on
anti-neutrino and neutrino mass respectively. The isotope 187 Re is a β − emitter; it has the
lowest value of the end-point Q among the known β − isotopes. The study of the β − spectrum
of 187 Re can be an alternative to the tritium experiment to set a limit on the anti-neutrino
mass. The effect of a finite anti-neutrino mass should manifest itself as a count deficit, in
first approximation proportional to m 2 ν , near the spectrum end-point. The isotope 187 Re is
naturally occurring at the 62% level and the element Re is a superconductor at the detector
working temperature of 0.08 K. Therefore it is possible to use a crystal of natural Rhenium as
a detector absorber to perform the 187 Re spectrum calorimetric measurement. As a thermistor
an heavily doped germanium thermistor has been used.
In the preliminary measurement [1] of the 187 Re beta spectrum performed in Genoa, an
energy resolution of 30 eV FWHM has been obtained. The end-point energy, obtained from
the linear fit of the Kurie Plot in the interval 112–2360 eV, is Eend−point = (2482 ± 12)eV.
The detector energy calibration has been made using a multi-lines fluorescence x-ray source;
the Kα and Kβ x-ray lines of Cl, Ca, Va, Mn have been detected, in the energy range 100–
7000 eV (see Fig. 1). Using this external source it has been possible to investigate the detector
linearity and the energy dependence of the detector energy resolution. A fit of the measured
energy lines versus the expected values with the function y = ax 2 + bx + c has given the
following result: a = (−0.7 ± 0.7)E −4 eV −1 , b = (1.000 ± 0.005), c = (2.2 ± 1.5)eV. The
rms value of the energy resolution has been plotted against the energy lines, and a fit of
this plot with the function y = Ax + B has given the following result: A = (0.9 ± 2.5)E −4 ,
B = (15.3 ± 1.4)eV. It should be possible to set a kinematics limit to the anti-neutrino
mass by means of this calorimetric method lower than 20 eV/c 2 in 50 days of data taking. A
high-statistics measurement of the 187 Re spectrum is under development.
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