The technology base of the scintillating bolometers used by ROSEBUD is the bolometer. In its simplest model, it consists of a crystal (absorber), thermally coupled to a thermal bath, and a phonon sensor glued on it. The bolometer measures as heat the energy ΔE deposited by the interaction of a particle with the nuclei or atoms of the target. This heat energy is measure as a temperature increase ΔT proportional to that deposited energy in the crystal. In order to be sensitive and able to quantify very small temperature increases, we have to use crystal absorbers with very small heat capacity C(T) at the working temperature T0. This condition is satisfied with the use of dielectric and diamagnetic crystals which have at very low temperature a heat capacity which depends only on the cube of the working temperature, i.e. C ∝ T03.
The main characteristics of the bolometer for rare event searches are:
- High avaliability of a wide range of different absorbers.
- High efficiency in heat energy conversion of the energy deposited by nuclear recoils.
- Very low energy threshold for energy detection.
- High capability for background rejection in hybrid measurements: heat-light and heat-ionization signals.
The very low working temperature (≈20 mK or lower) is attained using the special properties of a mixture of 3He and 4He in a dilution refrigerator.
The dilution refrigerator is a cryogenic device which works in its first phase with a thermal bath of liquid Nitrogen (LN2) at 77 K and then with a thermal bath of liquid He (LHe) at 4.2 K. To reach the base temperature a mixture of 3He-4He circulates into a closed-cycle circuit from the mixing chamber up to the vacuum pumps, among other stages, in the dilution refrigerator. Below 867 mK, a phase separation in the mixture between 3He and 4He is produced. At this point we have a diluted phase of 3He in 4He and a concentrated phase of 3He, having at a temperature of ≈0 K a minimum concentration of 6.6% of 3He in the diluted phase. This phase separation allows to reach very low temperature by evaporating 3He atoms in the still, 3He atoms which come from the mixing chamber. In this case, we have a continuous passing of 3He from the concentrated to the diluted phase which allows to reach the base temperature of the system in the mixing chamber. With a constant rate of evaporation and constant fraction of 3He-4He concentration in the mixing chamber we obtain a the dilution refrigerator working at a steady-state temperature.
Particle detection with scintillating bolometers
The scintillating bolometer consists of a scintillating crystal and faced to it a second bolometer-a Ge disk. They are mounted inside a copper frame with internal reflecting cavity coated with Ag and thermally coupled to the mixing chamber of a dilution refrigerator working at 20 mK.
The energy deposited into the crystal by a particle interaction is converted into heat and light signals, where the heat energy is measured as a temperature increase of the crystal and the scintillating photons that escape from the crystal and are absorbed by the Optical detector are again converted into heat and measured as a temperature increase. These temperature increases are measure with NTD (neutron transmutation doped) Ge thermistors glued on the crystal and the optical bolometer.
The 20 mK working temperature allows us to be sensitive to very low energy events (≈keV).
The simultaneous measurement of heat and light signals allows us to discriminate among the different sort of particles interacting with the crystal, depending their light response in the density of the energy deposited and the type of particle interacting. This is clearly seen in the pulses produced by a photon and nuclear recoil depositing the same energy of 569.7 keV in the BGO crystal. Photons produce more scintillating photons than nuclear recoils but with the heat response is the same for them, so the light response is use to discriminate particles whereas the heat response is used to measure the energy deposited by particles.
The combined depiction of these magnitudes in a heat-light discrimination diagram allows us clearly to identify two well separated bands: an upper band originated by photon and electron interactions (β/γ events), an intermediate band produce by α-particle events and a lower band with feeble light response produced by nuclear recoils (NR). Considering its application in direct dark matter searches, the nuclear recoils band is the region with main interest for WIMPs detection.
ROSEBUD at IAS and the LSC
Design, development and preliminary characterization of scintillating bolometers were carried out at the Institut d'Astrophysique Spatiale (IAS, France), a ground laboratory, where radiopurity conditions and ultralow background environment are not needed. Here the experimental set-up consists of a Faraday cage, 20 cm Pb shielding, a cryostat (INSU) and a gas handling and pumping system.
The low background characterization measurements of the scintillating bolometers were carried out at the Canfranc Underground Laboratory, scientific facility located below the Tobazo Peak in the Spanish Pirenees and completely covered by a rock overburden of 850 m or 2450 m.w.e. Here, the μ flux is reduced by a factor ≈105 in regard to sea level facilities. This feature allows to study and perform rare event searches (low rate events).
ROSEBUD was initially located since 1999 up to August 2011 in the old Lab 3 of the LSC, where different bolometers (Al2O3, Ge) and scintillating bolometers (CaWO4, Al2O3, BGO, LiF) were characterized for dark matter experiments or, in the case of LiF, to monitor the neutron flux inside the experimental set-up.
Nowadays this Collaboration is finished but ROSEBUD was located in the Hall B of the LSC since September 2011 and til the end of the year 2012.
The experimental set-up of ROSEBUD
The experimental set-up at LSC is similar to that used at Orsay. The set-up at the LSC consisted of a Dilution Refrigerator, a low background cryostat, a Faraday Cage, the Pb shielding and the Gas handling and Vacuum pumping system.
Scintillating bolometers are mounted inside a copper tray thermally coupled to the mixing chamber of a dilution refrigerator of 3He-4He working at a temperature of 20 mK. This dilution refrigerator is mounted inside a cryostat with appropriate modifications on radiopure materials: low background Pb and Cu pieces were added to the cryostat (CTL2) to reduce the background seen by the detectors. Also, lateral holes were made to introduce sources for energy calibration.
The dilution refrigerator is mounted inside a Pb shielding of 25 cm in thickness (laterally and in the base), polyethylene foils (40 cm on two sides and 20 cm at the bottom), 1 mm in thickness μ-metal foil and a PVC box sealed where a flux of LN2 vapour is introduced in order to remove the airborne radon contamination present in the lab. To avoid electromagnetic interferences the whole assembly was located inside a Faraday cage, isolated acoustically and vibrationally decoupled.
Research and Applications
- WIMP searches.
- Light-heat discrimination of nuclear recoils.
- Multi-target strategy (distinctive WIMP signal).
- Rare alpha decays (W isotopes) and other nuclear processes.
- REF (quenching factor).
- Absolute light yield and energy partition.
- Neutron detection (LiF).
For details of the experimental set-up and results, background levels and detectors capabilities see publications and talks.