During the last decades, low temperature detectors have undergone a considerable growth and are now widely
acknowledged as useful instruments in many fundamental physics experiments. In this field, the phonon mediated
particle detectors known as bolometers are remarkable and are successfully used in various branches of physics
research for their good sensitivity, energy resolution and flexibility in the choice of the constituting materials.
Bolometers have proved to be powerful devices for radiation detection; in particular, they are able to detect
Gamma-rays with resolutions comparable to those obtained with the best Ge diodes. They are also suited for
applications in the area of nuclear and particle physics, like the study of rare events or dark matter. Although
an effective technique, the use of bolometers in the specific field of the search for neutrinoless double beta
decay is affected by the lack of spatial resolution. This results in the expected signal of this rare decay hidden
under an indistinguishable background due to possible surface radioactive contaminations in the materials facing
the detectors. An approach to this problem is to make bolometers surface sensitive by applying ultra-pure
crystalline foils on the main detector through direct thermal contact and by operating them as active shields.
In this contribution we present for the first time surface sensitivity achieved with large mass TeO<sub>2</sub> bolometers
(~800 g) operated underground at ~10 mK, dedicated to the detection of neutrinoless double beta decay of
<sup>130</sup>Te. Our encouraging measurements suggest that this could be a viable method for the discrimination of
In this contribution, we describe two innovations of the structure of large mass bolometers, proposed by the cryogenic group of the Insubria University (Como) and developed in collaboration with the Firenze group. First, up to now, low temperature calorimeters do not have any sort of spatial resolution. This means that it is not possible to reject events coming from the material that faces the detectors (holder, refrigerators shields, ...). In order to cope this problem, we developed a new kind of composite bolometers able to discriminate, by means of active ultra-pure semiconductor shields, external surface events from those coming from the absorber bulk.
A second innovation that we discuss here concerns the temperature sensors. Presently, neutron transmutation doped Ge thermistors are the most common kind of phonon sensors. Unfortunately, this kind of readout dissipates power on the detector because of the thermistor biasing and also introduces a Johnson noise term. To improve energy resolution we studied and test the application of capacitive sensors that in principle could allow us to achieve a better signal-to-noise ratio. Modeling, simulations and first encouraging measurements on surface sensitive bolometers will be discussed along with preliminary results on capacitive sensors.