The next generation of cosmology space missions will be sensitive to parasitic signals arising from cosmic rays. Using a composite bolometer, we have investigated pulses produced by α particles in order to understand the movement of energy produced by ionising radiation. Using a series of measurements at 100 mK, we have compared the typical fitting algorithm (a mathematical model) with a second method of pulse interpretation by convolving the detector’s thermal response function with a starting profile of thermalised athermal phonons, taking into account the effects of heat propagation. Using this new fitting method, we have eliminated the need for a non-physical quadratic nonlinearity factor produced using more common methods, and we find a pulse form in good agreement with known aspects of thermal physics. This work is carried forward in the effort to produce a physical model for energy deposition in this detector. The modelling is motivated by the reproduction of statistical features in the experimental dataset, and the new interpretation of α pulse shapes represents an improvement in the current understanding of the energy propagation mechanisms in this detector.
Cosmic rays affect the performance of any detector in space through the creation of spurious signal and/or slow build-up of radiation damage. To mitigate the effects of these high-energy particles on the observations of next-generation space missions, the interaction between state-of-the-art detectors will have to be understood through simulation and experimental verification. We present the first measurement results of a new cryogenic system designed to become a common-user test facility to evaluate the effects of high-energy particles on arrays of these high-sensitivity detectors. The system is based on pulse-tube precooled dilution refrigerator with a large experimental volume (ø = 29 cm, H = 30 cm). At 100 mK the system provides 650 µW of cooling power and an out-of-the box thermal stability of 76 µK rms. A first experiment with a semiconducting bolometer from the DIABOLO experiment shows a responsivity and noise level consistent with previous measurement in different cryogenic systems. However, the pulse-tube induced vibrations show as clear features in the noise. To irradiate the detectors a particle beam, such as the 25 MeV proton beam of the nearby ALTO facility, can be coupled to one of four ports. Simulations show that the aluminum-coated Mylar windows do not significantly affect the 25 MeV proton beam of TANDEM. First experiments at the ALTO facility for system verification are expected early 2019. Until that time, the thermal stability, vibration damping and EMI shielding will be improved and a flexible wiring will be developed, to accommodate multiple detector types.
We present the design, performance, and tests of a new generation of cooled large-area (5 cm2) optical composite bolometers with a pure germanium absorber, permitting a good efficiency from near-IR to x-rays. With a sensitivity often better than photomultipliers or semiconductor diodes, they allow fluorescence measurements of cold targets with no window, no infrared background, good optical couplings, and a flat response on a large spectral band. Performance obtained at 25 mK is very promising: noise equivalent power as low as 4×10–17 W/ in the continuous mode, energy threshold about 50 eV in the pulse detection mode, and time constant ~3 ms. These detectors of low mass (0.25 g) have been successfully used for detecting the fluorescence emitted by much more massive bolometers, having, for example, a BGO (92 g) or a CaWO4 (54 g) target. The simultaneous detection of heat and light in these double bolometers permits the identification of each event in the massive target ( decay, , or cosmic ray interaction, neutron recoil). Thanks to the consecutive excellent subtraction of the radioactive and cosmic ray background, it is a powerful tool developed by several groups for fundamental research: study of very rare decays of atoms, measurement of internal very low radioactivity content in single crystals, direct detection of dark matter recoils in massive fluorescence targets, and detection of solar neutrino fluorescence events in liquid 4He. Recently obtained results are reviewed: the first detection of the rare alpha decay of 209Bi, and new scintillation data on Al2O3 (sapphire), LiF, or TeO2 at 20 mK. By cooling at 10 mK, sensitivity can yet be increased by more than one order of magnitude.
At very low temperature large area bolometers may show a better sensitivity than photomultipliers or semiconductor diodes, while allowing fluorescence measurements of cool targets with no window, no infrared background, good optical couplings and a flat response on a large absorption bandpass. The optical absorber of these composite bolometers can be matched to the desired bandpass. Here we present the design, the performances and calibration tests of a new generation of large area (5 cm<sup>2</sup>) optical bolometers with a pure germanium disk absorbing on a wide spectral band from near-IR to X-rays. Performances obtained at 25 mK are very promising : Noise Equivalent Power as low as 4x10<sup>-17</sup> W/√Hz in the photometry mode, energy threshold about 50 eV in the single photon detection mode, and time constant τ~3 ms. These detectors of low mass (0.25 g) have been recently successfully used for detecting the fluorescence emitted by much more massive bolometers, having for example a BGO (92 g), or a CaWO<sub>4</sub> (54 g) target. The simultaneous detection of heat and light in these <<double bolometers>> permits the identification of each event in the massive target (α decay, or γ cosmic ray interaction, neutron recoil...). Thanks to the consecutive excellent subtraction of the radioactive and cosmic rays background, it is a powerful tool developed by several groups for fundamental research : study of very rare decays of atoms, measurement of internal very low radioactivity content in single crystals, direct detection of dark matter recoils in massive fluorescence targets, detection of solar neutrino fluorescence events in liquid <sup>4</sup>He...Recently obtained results which support this new promising field are reminded: the first detection of the rare alpha decay of <sup>209</sup>Bi, and new scintillation data on Al<sub>2</sub>O<sub>3</sub> (sapphire), LiF or TeO<sub>2</sub> at 20mK. We discuss the ultimate performances at 12 mK of the optical bolometers as a function of their area, as well as the optimisation of their absorbing part to the desired bandpass, and finally, we estimate achievable improvements of our current technology.