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Register a new userA new pulse shape description for $alpha$ particle pulses in a highly-sensitive sub-Kelvin bolometer
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Added by
Samantha Lynn Stever
Publication date
2018
fields
Physics
and research's language is
English
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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 $alpha$ 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 detectors 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 $alpha$ pulse shapes represents an improvement in the current understanding of the energy propagation mechanisms in this detector.
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We developed a cryogenic system on a rotating table that achieves sub-Kelvin conditions. The cryogenic system consists of a helium sorption cooler and a pulse tube cooler in a cryostat mounted on a rotating table. Two rotary-joint connectors for electricity and helium gas circulation enable the coolers to be operated and maintained with ease. We performed cool-down tests under a condition of continuous rotation at 20 rpm. We obtained a temperature of 0.23 K with a holding time of more than 24 hours, thus complying with catalog specifications. We monitored the systems performance for four weeks; two weeks with and without rotation. A few-percent difference in conditions was observed between these two states. Most applications can tolerate such a slight difference. The technology developed is useful for various scientific applications requiring sub-Kelvin conditions on rotating platforms.
In the field of astrophysics, the faint signal from distant galaxies and other dim cosmological sources at millimeter and submillimeter wavelengths require the use of high-sensitivity experiments. Cryogenics and the use of low-temperature detectors are essential to the accomplishment of the scientific objectives, allowing lower detector noise levels and improved instrument stability. Bolometric detectors are usually cooled to temperatures below 1K, and the constraints on the instrument are stringent, whether the experiment is a space-based platform or a ground-based telescope. The latter are usually deployed in remote and harsh environments such as the South Pole, where maintenance needs to be kept minimal. CEA-SBT has acquired a strong heritage in the development of vibration-free multistage helium-sorption coolers, which can provide cooling down to 200 mK when mounted on a cold stage at temperatures <5K. In this paper, we focus on the development of a three-stage cooler dedicated to the BICEP Array project led by Caltech/JPL, which aims to study the birth of the Universe and specifically the unique B-mode pattern imprinted by primordial gravitational waves on the polarization of the Cosmic Microwave Background. Several cryogenic receivers are being developed, each featuring one such helium-sorption cooler operated from a 4K stage cooled by a Cryomech pulse-tube with heat lifts of >1.35W at 4.2K and >36W at 45K. The major challenge of this project is the large masses to be cooled to sub-kelvin temperatures (26 kg at 250mK) and the resulting long cool-down time, which in this novel cooler design is kept to a minimum with the implementation of passive and active thermal links between different temperature stages. A first unit has been sized to provide 230, 70 and 2{mu}W of net heat lifts at the maximum temperatures of 2.8K, 340 and 250mK, respectively, for a minimum duration of 48 hours.
The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne telescope mission to search for inflationary gravitational waves from the early universe. PIPER employs two 32x40 arrays of superconducting transition-edge sensors, which operate at 100 mK. An open bucket dewar of liquid helium maintains the receiver and telescope optics at 1.7 K. We describe the thermal design of the receiver and sub-kelvin cooling with a continuous adiabatic demagnetization refrigerator (CADR). The CADR operates between 70-130 mK and provides ~10 uW cooling power at 100 mK, nearly five times the loading of the two detector assemblies. We describe electronics and software to robustly control the CADR, overall CADR performance in flight-like integrated receiver testing, and practical considerations for implementation in the balloon float environment.
We present three Monte Carlo models for the propagation of athermal phonons in the diamond absorber of a composite semiconducting bolometer `Bolo 184. Previous measurements of the response of this bolometer to impacts by $alpha$ particles show a strong dependence on the location of particle incidence, and the shape of the response function is determined by the propagation and thermalisation of athermal phonons. The specific mechanisms of athermal phonon propagation at this time were undetermined, and hence we have developed three models for probing this behaviour by attempting to reproduce the statistical features seen in the experimental data. The first two models assume a phonon thermalisation length determined by a mean free path $lambda$, where the first model assumes that phonons thermalise at the borders of the disc (with a small $lambda$) and the second assumes that they reflect (with a $lambda$ larger than the size of the disc). The third model allows athermal photons to propagate along their geometrical line of sight (similar to ray optics), gradually losing energy. We find that both the reflective model and the geometrical model reproduce the features seen in experimental data, whilst the model assuming phonon thermalisation at the disc border produces unrealistic results. There is no significant dependence on directionality of energy absorption in the geometrical model, and in the schema of this thin crystalline diamond, a reflective absorber law and a geometrical law both produce consistent results.
The next generation of far infrared space observatories will require extremely sensitive detectors that can be realized only by combining extremely low intrinsic noise with high optical efficiency. We have measured the broad-band optical response of ultra-sensitive TES bolometers (NEP$approx2rm aW/sqrt Hz$) in the 30--60-$murm m$ band where radiation is coupled to the detectors with a few-moded conical feedhorn and a hemispherical backshort. We show that these detectors have an optical efficiency of 60% (the ratio of the power detected by the TES bolometer to the total power propagating through the feedhorn). We find that the measured optical efficiency can be understood in terms of the modes propagating through the feedhorn with the aid of a spatial mode-filtering technique.
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