elements Up: Double
beta decay Previous: Double
There are three ways to look for double beta decay:
1) direct searches, in which one looks with a detector for the two
electrons emitted by a source. The energy of the electrons is measured,
so that the three decay modes can be distinguished.
2) geochemical experiments, in which one searches, in an ore containing
double beta decay candidates, for an abnormal isotopic abundance of the
daughter nuclei. The three modes cannot be distinguished.
3) radiochemical experiments, similar to the geochemical ones, but
in which the daughter nuclei are unstable, and can be identified by their
decay. We shall here focus on direct searches. Experiments have become
more and more sensitive over the years, with larger target masses. Much
progress has been achieved in selecting radiochemically clean components
for the construction of detectors, leading to a substantial reduction of
the background from natural activities. To minimise the background from
direct cosmic hits, or from cosmogenic activations, detectors are usually
operated in underground laboratories. Calorimeters with superior energy
resolution are particularly well suited for the search of the
decay peak. Tracking devices are also used. In these good event candidates
can be selected from their topology, which leads to a further background
decay has been observed in several nuclei (48Ca, 76Ge,
82Se, 100Mo, 128Te, 130Te,
150Nd). At the same time the limits on the other modes keep
getting more constraining. We shall here mention only the most recent developments.
76Ge. This nucleus has been studied for several years,
since Ge detectors with large mass can be operated as calorimeters with
excellent energy resolution. Presently the best results are those obtained
by the Heidelberg-Moscow collaboration, which operates an array of 5 crystals
made from Ge enriched to 86-88 % in 76Ge, with a total mass
of 11 kg, in the Gran Sasso underground lab. Data corresponding to 15 kgyr
have been accumulated so far.
was clearly observed, with a half life
yr. An upper limit of
yr for the half-life of the Majoron mode was derived. The energy resolution
in the region of the transition energy E0=2038.6 keV
is of order 3 keV FWHM. There is no evidence of a peak there, and the limit
yr (90% CL) was derived. A pulse shape discrimination system is being implemented
ed. It allows to identify and reject multi-site events, for instance multi
Compton scattering events. This reduces the background by a factor 5. A
similar experiment, IGEX (South Carolina, Pacific Northwest, Zaragoza,
ITEP, INR collaboration), has now also started producing data. It uses
several Ge detectors enriched to 87.4 % in 76Ge in Homestake,
Canfranc and Baksan. The reported half-life for
decay is in rough agreement with the Heidelberg-Moscow value. The limit
yr (90% CL) was obtained. Pulse shape discrimination is also being implemented
130Te. Another calorimeter, of a rather innovative
type, a cryogenic bolometer, has been built by the Milano group to study
130Te. The central component is a 334 g TeO2 crystals
mounted in a low background cryostat, and operated at 10 mK. Natural tellurium
with 34.5 % 130Te is used. Excellent energy resolution, around
10-15 keV FWHM, is achieved, close to that of Ge detectors. Data have been
accumulated in the Gran Sasso laboratory during 10500 hours. No evidence
decay has been found, and the limit
yr (90% CL) was derived. Now a 20 crystal detector is being built. Efforts
are also made to grow crystals made from enriched 130Te.
136Xe. This nucleus is presently being investigated
by the Caltech-Neuchâtel-PSI collaboration in the Gotthard underground
lab. The detector is a tracking device, to be specific a gas time projection
chamber filled with Xe enriched to 62.5 % in 136Xe. The source
mass is 3.3 kg of 136Xe.
Table: Measured lower limits for the half-life of various
transitions. Deduced upper limits on the neutrino mass
using various matrix elements.
(90 % CL)
||> 2.6 - 7.7
Data were taken during 10'000 hours. The limits yr
(90% CL) and yr
have been reported.
100Mo. Another tracking device, NEMO II, is being
used in the Fréjus lab. The source is a thin foil, stretched in
the middle of the fiducial volume filled with He gas and instrumented with
Geiger cells. Several nuclei can be investigated. The energy of the electrons
is measured in two planes of scintillators. Using a 172 g source of Mo,
enriched to 98.4 % in 100Mo,
decay was observed with the half-life yr.
From the same data, a limit yr
was derived for decay. The sensitivity
decay however is limited because of the small mass of the source. More
recently, the NEMO group has also observed
decay in 116Cd, as have the Osaka and Kiev groups.
elements Up: Double
beta decay Previous: Double