Pulse Shape Analysis with Scintillating Bolometers

Among the detectors used for rare event searches, such as neutrinoless Double Beta Decay (0νDBD) and Dark Matter experiments, bolometers are very promising because of their favorable properties (excellent energy resolution, high detector efficiency, a wide choice of different materials used as absorber, …). However, up to now, the actual interesting possibility to identify the interacting particle, and thus to greatly reduce the background, can be fulfilled only with a double read-out (i.e. the simultaneous and independent read out of heat and scintillation light or heat and ionization). This double read-out could greatly complicate the assembly of a huge, multi-detector array, such as CUORE and EURECA. The possibility to recognize the interacting particle through the shape of the thermal pulse is then clearly a very interesting opportunity. While detailed analyses of the signal time development in purely thermal detectors have not produced so far interesting results, similar analyses on macro-bolometers (～10–500 g) built with scintillating crystals showed that it is possible to distinguish between an electron or γ-ray and an α particle interaction (i.e. the main source of background for 0νDBD experiments based on the bolometric technique). Results on pulse shape analysis of a CaMoO₄ crystal operated as bolometer are reported as an example. An explanation of this behavior, based on the energy partition in the heat and scintillation channels, is also presented.     

The CUORE Experiment: Bolometric Detectors in 1-Ton Scale Projects

CUORE is the first of a new generation 1-ton scale cryogenic detectors for rare events physics. CUORE, a detector to search neutrinoless Double Beta Decay of ¹³⁰Te, is an array of 988 TeO₂ bolometers that will work at 10 mK. Latest developments on detectors performances are reported together with improvements reached in background reduction. Results of CUORICINO, a single CUORE tower running since 2003, that widely showed the feasibility of the project are also reported and discussed. P. Gorla on behalf of the CUORE collaboration.     

An active noise cancellation technique for the CUORE Pulse Tube cryocoolers

The Cryogenic Underground Observatory for Rare Events (CUORE) experiment at Gran Sasso National Laboratory of INFN searches for neutrinoless double beta decay using TeO₂ crystals as cryogenic bolometers. The sensitivity of the measurement heavily depends on the energy resolution of the detector, therefore the success of the experiment stands on the capability to provide an extremely low noise environment. One of the most relevant sources of noise are the mechanical vibrations induced by the five Pulse Tube cryocoolers used on the cryogenic system which houses the detectors. To address this problem, we developed a system to control the relative phases of the pulse tube pressure oscillations, in order to achieve coherent superposition of the mechanical vibrations transmitted to the detectors. In the following, we describe this method and report on the results in applying it to the CUORE system.     

The Design of the Input Stage for the Very Front-End of the CUORE Experiment

We describe the criteria for the selection of the input transistor of the very front-end for the detector, an array of 988 macro bolometers. Each of such macro bolometer is composed of a crystal of TeO₂, having a mass of 750 g, to which a thermistor is glued. Due to the very large mass of the crystals, the detector response time is very slow and limited to a few Hz. The quoted characteristic for the CUORE (Cryogenic Underground Observatory on Rare Events) detectors make it attractive a solution based on a complete room temperature operated front-end. The very important requirements of such a solution are a very small parallel and low frequency series noise. We worked to the selection of a Si JFET having a gate area and pinch-off voltage that fulfils the CUORE requirements. The selected JFET has an input gate current of less than 60 fA at 40°C (the expected operating temperature at regime). The noise at 1 Hz is about 3 nV/ at the operating point of I _DS of 0.5 mA and V _DS of 0.5 V, selected for having small power dissipation. Thanks to the quoted results the limit in the energy resolution given by the preamplifier is about 10 times better than that presently achievable by the crystals tested and candidate for CUORE. The wafer from which the semi-custom JFET has been cut was previously probed. All the production consisting of 1200 JFETs pairs is already available.      

First results from the Cuoricino experiment

At the end of 2001 the Milano Double Beta Decay (MI-DBD) experiment on double beta decay of ¹³⁰Te has been completed. The project Cryogenic Underground Observatory for Rare Events (CUORE), proposed as a natural extension of MI-DBD, will be a tightly packed array of 1000 TeO₂ bolometers, each being a cube 5 cm on a side with a mass of 790 g. The array will consists of 25 vertical towers, arranged in a square of 5 towers by 5 towers, each containing 10 layers of 4 crystals. A single CUORE tower has been constructed as a smaller scale experiment called CUORICINO. The technical feasibility of CUORE is now being tested in CUORICINO, running since few weeks. The CUORICINO experiment consists of 44 TeO₂ detectors 5×5×5 cm³ and 18 TeO₂ detectors 3×3×6 cm³ for a total mass of approximately 41 kg. An analysis of the detector performances is presented together with the new limit obtained on neutrinoless double beta decay of ¹³⁰Te.     

Measurement of Low Temperature Specific Heat of Crystalline TeO2 for the Optimization of Bolometric Detectors

The optimization of bolometric detectors, like those that will be developed for the rare event experiment CUORE, requires a complete knowledge of the detector's thermal parameters. Since the CUORE detecting elements will consist of TeO₂ crystals, we have measured the specific heat of this material down to 60 mK with the thermal relaxation method. Previous available data were taken at temperatures higher than 0.6 K. Our results are clearly consistent with a lattice dominated specific heat. The Debye temperature, evaluated to be (232±7) K, is in excellent agreement with the elastic constant values measured by other authors. The knowledge of the Debye temperature allows a simple prediction of the pulse amplitude of presently working bolometers.     

Scintillating Bolometers for Rare Events Searches: The LUCIFER Experiment

The main goal of LUCIFER is the study of the neutrino-less double beta decay, a rare process that, if detected, could demonstrate the Majorana nature of neutrino and set the absolute mass of this particle. Dealing with rare decays, one of the most critical issues of the experiment is the background reduction. This requirement will be satisfied by LUCIFER thanks to the use of Zn \(^{82}\) Se scintillating bolometers: the simultaneous read-out of heat and light emitted by the interactions in the detector will allow to reject most of the spurious events, providing a background of 10 \(^{-3}\)  counts/keV/kg/year at the transition energy of \(^{82}\) Se (2,997 keV). The detector will be made by tens of \(\sim \) 0.5 kg ZnSe crystals and Ge light detectors operated as bolometers at 10 mK. We present the results obtained with a single detector module in terms of energy resolution, radio-purity and background rejection capability. In addition, we discuss the feasibility of dark matter searches in the framework of the LUCIFER experiment.     

Silicon nitride Micromesh Bolometer Array for Submillimeter Astrophysics

We present the design and performance of a feedhorn-coupled bolometer array intended for a sensitive 350-mum photometer camera. Silicon nitride micromesh absorbers minimize the suspended mass and heat capacity of the bolometers. The temperature transducers, neutron-transmutation-doped Ge thermistors, are attached to the absorber with In bump bonds. Vapor-deposited electrical leads address the thermistors and determine the thermal conductance of the bolometers. The bolometer array demonstrates a dark noise-equivalent power of 2.9 x 10(-17) W/ radicalHz and a mean heat capacity of 1.3 pJ/K at 390 mK. We measure the optical efficiency of the bolometer and feedhorn to be 0.45-0.65 by comparing the response to blackbody calibration sources. The bolometer array demonstrates theoretical noise performance arising from the photon and the phonon and Johnson noise, with photon noise dominant under the design background conditions. We measure the ratio of total noise to photon noise to be 1.21 under an absorbed optical power of 2.4 pW. Excess noise is negligible for audio frequencies as low as 30 mHz. We summarize the trade-offs between bare and feedhorn-coupled detectors and discuss the estimated performance limits of micromesh bolometers. The bolometer array demonstrates the sensitivity required for photon noise-limited performance from a spaceborne, passively cooled telescope.     

Status of the Cryogen-Free Cryogenic System for the CUORE Experiment

The CUORE detector will be made of 988 TeO₂ crystals and will need a base temperature lower than 10 mK in order to meet the performance specifications. To cool the CUORE detector a large cryogen-free cryostat with five pulse tubes and one specially designed high-power dilution refrigerator has been designed. The detector assembly has a total mass of about 1.5 ton and uses a vibration decoupling suspension system. Because of the stringent requirements regarding radioactivity, about 12 tons of lead shielding need to be cooled to 4?K and below, and only a limited number of construction materials are acceptable. The eight retractable radioactive sources for detector calibration and about 2600 signal wires add further complexity to the system. The many stringent and contrasting requirements together with the overall large size made the design of the CUORE cryogenic system a real mechanical and cryogenic engineering challenge. The cryogenic system is expected to be fully operational in the Gran Sasso Laboratory in July 2013. We report here about the current status of the cryogenic system construction, which has started about one year.     

Cryogenic Wide-Area Light Detectors for Neutrino and Dark Matter Searches

Large-mass arrays of bolometers proved to be good detectors for neutrinoless double beta decay (0 \(\nu \) DBD) and dark matter searches. CUORE and LUCIFER are bolometric 0 \(\nu \) DBD experiments that will start to take data in 2015 at Laboratori Nazionali del Gran Sasso in Italy. The sensitivity of CUORE could be increased by removing the background due to \(\alpha \) particles, by detecting the small amount of ?erenkov light ( \(\sim \) 100 eV) emitted by the \(\beta \) s’ signal and not by \(\alpha \) s. LUCIFER could be extended to detect also dark matter, provided that the background from \(\beta \) / \(\gamma \) particles ( \(\sim \) 100 eV of scintillation light) is discriminated from nuclear recoils of about 10 keV energy (no light). We have recently started to develop light detectors for CUORE, LUCIFER and similar bolometric experiments. The aim is to obtain detectors with an active area of \(5\times 5~\mathrm{cm}^2\) (the face of bolometric crystals), operating at 10 mK, and with an energy resolution at the baseline below 20 eV RMS. We have chosen to develop phonon-mediated detectors with KID sensors. We are currently testing the first prototypes.     

New material for bolometer design

In an attempt to build up a bi-dimensional array of bolometers we investigate the thermometer properties of a thin film. The small size of the thermal detector and the requirement of a fast response time imply an excellent electron-phonon coupling. This has been achieved by the DC sputtering of the metastable hexagonal phase of the gold germanium alloy on a sapphire substrate. The chemical composition of the film is adjusted to 0.1% thanks to a high resolution deposition rate monitor. The polarization power density decreases very rapidly with the decreasing temperature. So, for the small elementary detector the best sensitivity is not achieved at the lowest temperature. An optimum for the polarization power density, 20kW/m³ is observed at 0.4K.     

Optimizing Feedhorn-Coupled TES Polarimeters for Balloon and Space-Based CMB Observations

Maximizing the sensitivity of balloon-based and space-based observations of the cosmic microwave background (CMB) requires detectors with substantially lower saturation power and background noise than ground-based observations, because of reduced atmospheric loading and lower photon noise. We have fabricated and tested prototype transition-edge sensor (TES) bolometers that have architecture identical to that used in feedhorn-coupled TES polarimeter arrays developed for ground-based CMB observations, but have saturation power appropriate for balloon-based or space-based observations (0.5?pW–7?pW). The operating resistance of these bolometers (～3?mΩ) is appropriate for readout with time-division or gigahertz frequency-division SQUID multiplexers. Dark bolometer measurements show that the noise levels are near the expected thermal-fluctuation-noise background (<10^?17?W/Hz^1/2), that the thermal response times are faster than the observation requirements, and that low-frequency 1/f noise can be strongly suppressed to <10?mHz by pair differencing. We report on the performance of the prototype devices and progress towards optimizing them for balloon-based and spaced-based observations.     

Science and technology of large-mass bolometer arrays

After almost 20 years of R&D, cryogenic detectors are now officially inserted in the list of detectors for fundamental and applied physics. One of the main research fields where bolometers are conveniently exploitable is in the search of rare events, like Double Beta Decay, Dark Matter or Axions. The first requirement that had to be fulfilled for this application was to progress from the mg range of the first absorbers towards the kg range of nowadays detectors, together with the development of the array technique, that climbed from the first four detector style up to the present 60 and, possibly, the future 1000 channel array. The increase in mass and in number of channels must go on together with crucial scientific and technological achievements, some of which are discussed in this paper.     

Design of the Cryogen-Free Cryogenic System for the CUORE Experiment

We present here the final design of the cryogenic system where the CUORE detector will be installed in 2010. It is a large cryogen-free cryostat cooled by pulse tubes and by a high-power dilution refrigerator. To avoid radioactive background, about 15000 kg of lead will be cooled to below 1 K and only few construction materials are acceptable. The detector assembly will have a total mass of about 1500 kg and must be cooled to less than 10 mK in a vibration-free environment. We discuss the adopted technical solutions, the results of the preliminary thermal analysis of the system, and its expected performance.      

Large Bolometer Arrays with Superconducting NbSi Sensors for Future Space Experiments

New techniques in microelectronics allow to build large arrays of bolometers filling the focal plane of submillimeter and millimeter telescopes. The expected sensitivity increase is the key for the next generation of space experiments in this wavelength range. Superconducting bolometers offer currently the best prospects in terms of sensitivity and multiplexed readout. We present here the developments led in France based on NbSi alloy thermometers. The manufacturing process of a 23 pixel array and the test setup are described.      

Characterization of NbSi TES Bolometers: Preliminary Results

We present preliminary results on characterisation of voltage biased superconducting bolometers with an NbSi alloy sensor. I(V) curves as well as complex impedance and noise measurements are carried out in a dilution fridge with a pulse tube as a first stage cooler. SQUIDs polarization and addressing are controlled by a cold ASIC in which a SiGe amplifier has been implemented. First characterisations were done on full membrane bolometers and show a noise level of about $10^{-16}\ \mathrm{W}/\sqrt{\mathrm{Hz}}$ . Those detectors are developed for the QUBIC experiment, a B-modes telescope that will be deployed in Antactica in 2013.     

Superconducting Transition-Edge Sensor Bolometers with Integrated Electron-tunneling Refrigerators

We discuss the development of an array of superconducting Transition-Edge Sensor (TES) bolometers intended for use at millimeter wavelengths. Each bolometer in the array uses a proximity-effect TES sensing element and each has integrated Normal-Insulator-Superconductor (NIS) refrigerators to cool the bolometer below the reference bath temperature. The NIS refrigerators and bolometers are fabricated on a single Silicon substrate. Four short, thin and narrow legs suspend the millimeter wavelength radiation-absorbing element. The leg geometry is used to control and optimize the thermal conductance of the bolometer. Using the technology developed at NIST, we fabricated NIS refrigerators at the base of each of the suspension legs. These NIS refrigerators remove hot electrons by quantum-mechanical tunneling and should be capable of reducing the temperature of the biased (∼10 pW) bolometers by more than 100 mK when operating from a local thermal reservoir provided by a ³He system at ∼300 mK. This lower temperature at the bolometer will allow the detectors to approach background-limited performance despite the simple cryogenic system. In this paper we describe the progress toward the development of these detectors designed for use in observations at the 100 m Green Bank Telescope at 3 mm wavelength and the results of laboratory tests of a prototype detector.      

AC Bias Characterization of Low Noise Bolometers for SAFARI Using an Open-Loop Frequency Domain SQUID-based Multiplexer Operating Between 1 and 5 MHz

SRON is developing the Frequency Domain Multiplexing (FDM) read-out and the ultra low NEP TES bolometers array for the infra-red spectrometer SAFARI on board of the Japanese space mission SPICA. The FDM prototype of the instrument requires critical and complex optimizations. For single pixel characterization under AC bias we are developing a simple FDM system working in the frequency range from 1 to 5 MHz, based on the open loop read-out of a linearized two-stage SQUID amplifier and high Q lithographic LC resonators. We describe the details of the experimental set-up required to achieve low power loading (<1?fW) and low noise ( $\mathrm{NEP}\sim 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ ) in the TES bolometers. We conclude the paper by comparing the performance of a $4\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ TES bolometer measured under DC and AC bias.     

Electron-Nuclear Recoil Discrimination by Pulse Shape Analysis

In the framework of the “ULTIMA” project, we use ultra cold superfluid ³He bolometers for the direct detection of single particle events, aimed for a future use as a dark matter detector. One parameter of the pulse shape observed after such an event is the thermalization time constant τ _b . Until now it was believed that this parameter only depends on geometrical factors and superfluid ³He properties, and that it is independent of the nature of the incident particles. In this report we show new results which demonstrate that a difference for muon- and neutron events, as well as events simulated by heater pulses exist. The possibility to use this difference for event discrimination in a future dark matter detector will be discussed.