Final Report Summary - ARBRES (Active Rejection of Background in Rare Event Searches)
Project aims and objectives
ARBRES (active rejection of background in rare event searches) is a proposal for a 24-month Marie Curie fellowship dealing with a key topic in astroparticle physics: the tagging, control and rejection of the radioactive background of surface origin in rare-event searches. The scientific objective of the proposal is to significantly extend the discovery potential of searches for neutrinoless double beta decay and for weakly interacting massive particles (WIMPs), hypothetical particles composing dark matter.
In particular, the control of the radioactive background thanks to its discrimination is a crucial aspect in the search for rare events, such as neutrinoless double beta decay (0nBB). When the detection technique involves bolometers operated in the so-called source=detector technique, alpha particles released by internal contamination are not harmful, since they give origin to sharp peaks typically above 4 MeV, whereas 0nBB signals are expected in the 2-3 MeV region where the Q-value of the more interesting candidates lies. On the contrary, near-surface events constitute a serious problem, due to the absence of a dead layer at the detector surface. Radioactive contamination in materials facing the detectors, consisting of 210Po (polonium 210), 210Pb (lead 210), 226Ra (radium 226) or directly the natural radioactive chain progenitors 238U (uranium 238) and 232Th (thorium 232), give origin to alpha particles that may reach the detector after losing a significant part of their energy and they may enhance the background in the region close to the Q-value. The 0nBB community that operates with bolometers is making big efforts to fight against this problem with different approaches.
This proposal has provided an innovative technique for discrimination of the surface background, which combines the use of thin layers of superconducting aluminium (Al) deposited on a crystal operated as a macro-bolometer and equipped with a fast thermistor, such as a niobium silicide (NbSi) thin film acting as out-of-equilibrium phonon sensor. The rationale of this approach is based on the fact that particles that transfer energy into the Al film create quasi-particles that release heat to the crystal with a considerable delay, which may be in the millisecond range. It is thus clear that pulses acquired from a fast thermistor in the case of surface and bulk events should have different shapes, depending on the place of the initial energy release. We have obtained good preliminary results on a small tellurium dioxide (TeO2) crystal equipped with NbSi films and an Al film with a thickness of 10 um. We have proved to be able to identify alpha surface events by pulse shape analysis with a good separation between bulk and surface events. The possibility to extend this rejection technique to bolometers equipped with slow temperature sensors as neutron transmutation doped germanium (Ge) thermistors is under study.
We know that future tests are mandatory to optimise the rejection capability and underground tests could be very useful but we can conclude that the presented results are a good starting point to identify surface background in 0?ßß bolometric searches.
A second topic of ARBRES has been the study of NbSi films with the co-evaporation method, both in the form of Anderson insulators and superconductors. Different tests on low temperature characterisations of the produced films in terms of transport properties and specific heat have been carried out. The obtained results have shown that these devices are usable as thermal sensors for more general bolometric applications. In particular, we have realised a sample consisting of a 204-pixel array of NbxSi1-x transition edge sensors with a meander structure fabricated on a 2-inch silicon wafer using electron-beam co-evaporation and a clean room lithography process. The performed test has proved that these devices can be used also for cosmic microwave background (CMB) applications as a very promising by-product. In fact the new bolometer structure, able to increase pixel sensitivities and simplify the fabrication procedure, can replace delicate membrane-based structures and eliminate the mediation of phonons in the detection. To validate the application of this device to CMB measurements, we have performed an optical calibration of our sample in the focal plane of a dilution cryostat test bench. We have demonstrated a light absorption close to 20 % and an optical noise equivalent power of about 7x10-16 W/sqrt(Hz), which is highly encouraging given the scope for improvement in this type of detectors.
Relevant contact details:
Fellow researcher: Claudia Nones - claudia.nones@cea.fr
Project coordinator: Stefanos Marnieros - stefanos.marnieros@csnsm.in2p3.fr.
ARBRES (active rejection of background in rare event searches) is a proposal for a 24-month Marie Curie fellowship dealing with a key topic in astroparticle physics: the tagging, control and rejection of the radioactive background of surface origin in rare-event searches. The scientific objective of the proposal is to significantly extend the discovery potential of searches for neutrinoless double beta decay and for weakly interacting massive particles (WIMPs), hypothetical particles composing dark matter.
In particular, the control of the radioactive background thanks to its discrimination is a crucial aspect in the search for rare events, such as neutrinoless double beta decay (0nBB). When the detection technique involves bolometers operated in the so-called source=detector technique, alpha particles released by internal contamination are not harmful, since they give origin to sharp peaks typically above 4 MeV, whereas 0nBB signals are expected in the 2-3 MeV region where the Q-value of the more interesting candidates lies. On the contrary, near-surface events constitute a serious problem, due to the absence of a dead layer at the detector surface. Radioactive contamination in materials facing the detectors, consisting of 210Po (polonium 210), 210Pb (lead 210), 226Ra (radium 226) or directly the natural radioactive chain progenitors 238U (uranium 238) and 232Th (thorium 232), give origin to alpha particles that may reach the detector after losing a significant part of their energy and they may enhance the background in the region close to the Q-value. The 0nBB community that operates with bolometers is making big efforts to fight against this problem with different approaches.
This proposal has provided an innovative technique for discrimination of the surface background, which combines the use of thin layers of superconducting aluminium (Al) deposited on a crystal operated as a macro-bolometer and equipped with a fast thermistor, such as a niobium silicide (NbSi) thin film acting as out-of-equilibrium phonon sensor. The rationale of this approach is based on the fact that particles that transfer energy into the Al film create quasi-particles that release heat to the crystal with a considerable delay, which may be in the millisecond range. It is thus clear that pulses acquired from a fast thermistor in the case of surface and bulk events should have different shapes, depending on the place of the initial energy release. We have obtained good preliminary results on a small tellurium dioxide (TeO2) crystal equipped with NbSi films and an Al film with a thickness of 10 um. We have proved to be able to identify alpha surface events by pulse shape analysis with a good separation between bulk and surface events. The possibility to extend this rejection technique to bolometers equipped with slow temperature sensors as neutron transmutation doped germanium (Ge) thermistors is under study.
We know that future tests are mandatory to optimise the rejection capability and underground tests could be very useful but we can conclude that the presented results are a good starting point to identify surface background in 0?ßß bolometric searches.
A second topic of ARBRES has been the study of NbSi films with the co-evaporation method, both in the form of Anderson insulators and superconductors. Different tests on low temperature characterisations of the produced films in terms of transport properties and specific heat have been carried out. The obtained results have shown that these devices are usable as thermal sensors for more general bolometric applications. In particular, we have realised a sample consisting of a 204-pixel array of NbxSi1-x transition edge sensors with a meander structure fabricated on a 2-inch silicon wafer using electron-beam co-evaporation and a clean room lithography process. The performed test has proved that these devices can be used also for cosmic microwave background (CMB) applications as a very promising by-product. In fact the new bolometer structure, able to increase pixel sensitivities and simplify the fabrication procedure, can replace delicate membrane-based structures and eliminate the mediation of phonons in the detection. To validate the application of this device to CMB measurements, we have performed an optical calibration of our sample in the focal plane of a dilution cryostat test bench. We have demonstrated a light absorption close to 20 % and an optical noise equivalent power of about 7x10-16 W/sqrt(Hz), which is highly encouraging given the scope for improvement in this type of detectors.
Relevant contact details:
Fellow researcher: Claudia Nones - claudia.nones@cea.fr
Project coordinator: Stefanos Marnieros - stefanos.marnieros@csnsm.in2p3.fr.