MARE-1 in Milan: Status and Perspectives

The international project MARE (Microcalorimeter Array for a Rhenium Experiment) aims at the direct and calorimetric measurement of the electron neutrino mass with sub-eV sensitivity. Although the baseline of the MARE project consists in a large array of rhenium based thermal detectors, a different option for the isotope is also being considered. The different option is ¹⁶³Ho. The potential of using ¹⁸⁷Re for a calorimetric neutrino mass experiment has been already demonstrated. On the contrary, no calorimetric spectrum of ¹⁶³Ho has been so far measured with the precision required to set a useful limit on the neutrino mass. The first phase of the project (MARE-1) is a collection of activities with the aim of sorting out both the best isotope and the most suited detector technology to be used for the final experiment. One of the MARE-1 activities is carried out in Milan by the group of Milano–Bicocca in collaboration with NASA/GSFC and Wisconsin groups. The Milan MARE-1 arrays are based on semiconductor thermistors, provided by the NASA/GSFC group, with dielectric silver perrhenate absorbers, AgReO₄. The experiment, which is presently being assembled, is designed to host up to 8 arrays. With 288 detectors, a sensitivity of 3 eV at 90% CL on the neutrino mass can be reached within 3 years. This contribution gives an outlook for the MARE activities for the active isotope selection. In this contribution the status and the perspectives of the MARE-1 in Milan are also reported.     

Developments of Microresonators Detectors for Neutrino Physics in Milan

Superconducting microwave microresonators are low temperature detectors which are compatible with large-scale multiplexed frequency domain readout. We aim to adapt and further advance the technology of microresonator detectors to develop new devices applied to the problem of measuring the neutrino mass. More specifically, we aim to develop detector arrays for calorimetric measurement of the energy spectra of ¹⁶³Ho EC decay (Q～2–3 keV) for a direct measurement of the neutrino mass. In order to achieve these goal, we need to find the best design and materials for the detectors. A recent advance in microwave microresonator technology was the discovery that some metal nitrides, such as TiN, possess properties consistent with very high detector sensitivity. We plan to investigate nitrides of higher-Z materials, for example TaN and HfN, that are appropriate for containing the energy of keV decay events, exploring the properties relevant to our detectors, such as quality factor, penetration depth and recombination time.     

Cryogenic composite detectors for the dark matter experiments CRESST and EURECA

Weakly interacting massive particles (WIMPs) are candidates for non-baryonic dark matter. WIMPs are supposed to interact with baryonic matter via scattering off nuclei producing a nuclear recoil with energies up to a few 10 keV with a very low interaction rate of 10⁻⁶ events per kg of target material and day in the energy region of interest. The dark matter experiment cryogenic rare event search with superconducting thermometers (CRESST) and the European underground rare event calorimeter array (EURECA) project are aimed at the direct detection of WIMPs with the help of very sensitive modularised cryogenic detectors that basically consist of a transition edge sensor (TES) in combination with a massive absorber crystal. In the CRESST experiment the search for coherent WIMP-nucleon scattering events is validated by the detection of two processes. In the scintillating absorber single crystal, CaWO₄, heat (phonons) and scintillation light are produced and detected with two independent cryogenic detectors: a phonon channel and a separate light channel.The development of such cryogenic detectors and the potential ton-scale production are investigated in this paper. To decouple the TES production from the choice of the target material in order to avoid heating cycles of the absorber crystal and to allow pretesting of the TESs, a composite detector design (CDD) for the detector production has been developed and studied. An existing thermal detector model has been extended to the CDD, in order to investigate, understand, and optimize the performance of composite detectors. This extended model, which has been worked out in detail, can be expected to provide a considerable help when tailoring composite detectors to the requirements of various experiments.     

Distributed Antenna-Coupled TES for FIR Detector Arrays

We report progress toward large arrays of sensitive TES bolometers for submillimeter and far-infrared wavelengths with noise equivalent power (NEP) suitable for either imaging from a cooled space telescope or ground based spectroscopy. The arrays are based on a pixel design that makes use of a distributed transition edge sensor (TES) coupled to a slot antenna array. We have electrically characterized prototype detectors consisting of 256 TiN hot-electron TES microbolometers biased in parallel with T _c =50 mK. The measured electron-phonon thermal conductance of prototype devices is as low as 1.1 pW/K at 50 mK corresponding to an electrical NEP of 4×10⁻¹⁹ W/Hz^1/2. The time constant of two detectors with different geometries and transition widths was measured under a range of bias conditions. We have measured time constants ≳10⁻³ seconds, which is long enough for straightforward multiplexing with existing multiplexer technology.      

Microfabrication of Transition-Edge Sensor Arrays of Microcalorimeters with <Superscript>163</Superscript>Ho for Direct Neutrino Mass Measurements with HOLMES

HOLMES is aiming at a direct measurement of neutrino mass by performing a calorimetric measurement of the energy released in the decay of ¹⁶³Ho. In such approach, the ¹⁶³Ho source, with the required activity, needs to be embedded in the detector. HOLMES will deploy a large array of transition-edge sensor microcalorimeters with implanted ¹⁶³Ho ions. While good progress has been made in optimizing single pixel design and fabrication to achieve the target resolution, a major challenge is the fabrication of arrays of such microcalorimeters with the required amount of ¹⁶³Ho ions embedded in the detectors absorber. We describe the multi-step microfabrication process implemented to produce the detector arrays for HOLMES. One crucial part of such process is the ability to perform co-deposition of gold during the ¹⁶³Ho implantation process on the detectors absorber. We describe the UHV target chamber, with integrated gold deposition system, we have built to achieve this goal.     

The cryogenic multiplexer and shift register for submillimeter-wave digital camera

We have been developing cryogenic readout integrate circuits using SONY GaAs JFETs for large format arrays of high impedance type detectors especially for submillimeter/terahertz astronomy. The GaAs JFETs manufactured by SONY CO. Ltd have excellent static properties at less than 1 K. Besides, these JFETs have good performance for electrical switches; they have very low gate capacitance (<50 fF), low on resistance (∼10 kΩ), and high off resistance (>100 TΩ). To realize a cryogenic readout system for submillimeter-wave/terahertz camera, we designed multiplexers with sample-and-holds and shift registers. We report the first test results of each circuit and show prospect of a cryogenic multiplex system for a submillimeter-wave/terahertz digital camera.     

Das Karlsruhe Tritium Neutrino Experiment

The Karlsruhe Tritium Neutrino Experiment — The Word's Most Accurate Scale Scale The mass of the neutrino is one of the big open questions in particle physics, astrophysics and cosmology. The Karlsruhe TRItium Neutrino (KATRIN) experiment aims to measure the neutrino mass with a sensitivity of 200 meV/c². This corresponds to an improvement by a factor of 10 compared to predecessor experiments. In order to achieve this improvement, the KATRIN experiment needs to solve technical challenges from various technical fields such as high voltage, cryogenics or vacuum technology. A considerable challenge is the operation of a large spectrometer with a volume of 1240 m³ in a pressure regime of 10^?11 mbar.     

Determination of lead concentration in Allchar lorandite

The lead/thallium ratio in a sample of Allchar lorandite (Tl·As·S₂) has been measured to be (5.2 ± 1.3) × 10⁻⁶ by means of an alpha activation analysis. The mineral of this mine is expected to be suitable for the geochemical ²⁰⁵Tl solar neutrino experiment.     

Cryogenic SiGe ASICs for readout and multiplexing of superconducting detector arrays

This paper presents an ultra low noise instrumentation based on cryogenic electronic integrated circuits (ASICs: Application Specific Integrated Circuits). We have designed successively two ASICs in standard BiCMOS SiGe 0.35 μm technology that have proved to be operating at cryogenic temperatures. The main functions of these circuits are the readout and the multiplexing of TES/SQUID arrays. We report the cryogenic operation of a first ASIC version dedicated to the readout of a 2 × 4 pixel demonstrator array. We particularly emphasize on the development and the test phases of an ultra low white noise (0.2 nV/sqrtHz) cryogenic amplifier designed with two multiplexed inputs. The cryogenic SiGe amplifier coupled to a SQUID in a FLL operating at 4.2 K is also presented. We finally report on the development of a second version of this circuit to readout a 3 × 8 detectors array with improved noise performances and upgraded functionalities.     

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.     

Development of a Cryogenic Detector for Coherent Neutrino Nucleus Scattering

In Coherent Neutrino Nucleus Scattering (CNNS) the neutrinos interact coherently with all nucleons leading to a cross section which is much larger than for all other neutrino interactions. Because of the small momentum transfer as well as the small recoil energy in CNNS, and the relatively low count rate, a low energy threshold and a large target mass (several hundred grams) are required to observe CNNS. Our aim is to build a cryodetector for that purpose. Such a cryodetector, installed in the vicinity of a nuclear power plant, could probe new physics like non-standard neutral current interactions or a neutrino magnetic moment. We describe the results of three detectors, with Ge absorbers of 0.8 and 3.2 g and with a CaWO₄ absorber of 10 g. For Ge (0.8 g) an energy threshold of 0.43 keV and an energy resolution of 0.27 keV at ∼6 keV could be reached. We demonstrate that surface roughness effects deteriorate both threshold and resolution. For the 10 g CaWO₄ absorber we obtained 0.27 keV and 0.35 keV for threshold and energy resolution at ∼6 keV respectively.      

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.     

Characterization and enhanced field emission properties of carbon nanotube bundle arrays coated with N-doped nanocrystalline anatase TiO2

Anatase TiO₂ nanocrystals (NCs) were deposited onto patterned carbon nanotube (CNT) bundle arrays to form a TiO₂/CNT composite using metal organic chemical vapor deposition (MOCVD) using titanium-tetraisopropoxide (Ti(OC₃H₇)₄) as a source reagent. The N-doped TiO₂/CNT composite was then fabricated using nitrogen plasma treatment. The structural and spectroscopic properties of TiO₂/CNT composites were characterized by field-emission scanning electron microscopy, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. The combined geometrical structure and low electron affinity effects of N-doped TiO₂ led to a low turn-on field of 1.0 V μm⁻¹ at a current density of 10 μA cm⁻², a low threshold field of 1.9 V μm⁻¹ at a current density of 1 mA cm⁻², a high field enhancement factor of 3.0 × 10³, and long-term stability for the N-doped TiO₂/CNT composite. The results revealed that the N-doped TiO₂/CNT composite can be a potential candidate for field emission devices.     

Evaluation of 400 low background 10-in. photo-multiplier tubes for the Double Chooz experiment

The Double Chooz is a reactor neutrino experiment which measures the last unknown neutrino mixing angle θ₁₃. The Double Chooz experiment uses two identical detectors placed at sites far and near from Chooz reactor cores. The detector uses 390 low-background and high performance 10-in. Photo-Multiplier Tubes (PMTs) to detect scintillation light from gadolinium loaded liquid scintillator. In order to test and characterize the PMTs and to tune operation parameter, we developed two types of PMT test system and evaluated 400 PMTs before installation. Those PMTs fulfilled our requirements and half of them were installed in the far detector in 2009 and physics data have been successfully taken since 2011.     

Robust Fabrication of Hybrid Lead-Free Perovskite Pellets for Stable X-ray Detectors with Low Detection Limit

X-ray detectors are widely utilized in medical diagnostics and nondestructive product inspection. Halide perovskites are recently demonstrated as excellent candidates for direct X-ray detection. However, it is still challenging to obtain high quality perovskites with millimeter-thick over a large area for high performance, stable X-ray detectors. Here, methylammonium bismuth iodide (MA₃Bi₂I₉) polycrystalline pellets (PPs) are developed by a robust, cost effective, and scalable cold isostatic-pressing for fabricating X-ray detectors with low limit of detection (LoD) and superior operational stability. The MA₃Bi₂I₉-PPs possess a high resistivity of 2.28 × 10¹¹ Ω cm and low dark carrier concentration of ≈10⁷ cm⁻³, and balanced mobility of ≈2 cm² V⁻¹ s⁻¹ for electrons and holes. These merits enable a sensitivity of 563 μC Gy_air⁻¹ cm⁻², a detection efficiency of 28.8%, and an LoD of 9.3 nGy_air s⁻¹ for MA₃Bi₂I₉-PPs detectors, and the LoD is much lower than the dose rate required for X-ray diagnostics used currently (5.5 μGy_air s⁻¹). In addition, the MA₃Bi₂I₉-PPs detectors work stably under high working bias field up to 2000 V cm⁻¹ after sensing an integrated dose >320 Gy_air with continuous X-ray radiation, demonstrating its competitive advantage in practical application. These findings provide an approach to explore a new generation of low LoD, stable and green X-ray detectors based on MA₃Bi₂I₉-PPs.     

Cryogenic design and operation of liquid helium in an electron bubble chamber towards low energy solar neutrino detectors

We are developing a new cryogenic neutrino detector: electron bubble chamber, using liquid helium as the detecting medium, for the detection of low energy p-p reaction neutrinos (<420 keV), from the Sun. The program focuses in particular on the interactions of neutrinos scattering off atomic electrons in the detecting medium of liquid helium, resulting in recoil electrons which can be measured. We designed and constructed a small test chamber with 1.5 L active volume to start the detector R&D, and performed experimental proofs of the operation principle. The test chamber is a stainless steel cylinder equipped with five optical windows and ten high voltage cables. To shield the liquid helium chamber against the external heat loads, the chamber is made of double-walled jacket cooled by a pumped helium bath and is built into a LN₂/LHe cryostat, equipped with 80 K and 4 K radiation shields. A needle valve for vapor helium cooling was used to provide a 1.7-4.5 K low temperature environments. The cryogenic test chamber has been successfully operated to test the performance of Gas Electron Multipliers (GEMs) in He and He + H₂ at temperatures in the range of 3-293 K. This paper will give an introduction on the cryogenic solar neutrino detector using electron bubbles in liquid helium, then present the cryogenic design and operation of liquid helium in the small test chamber. The general principles of a full-scale electron bubble detector for the detection of low energy solar neutrinos are also proposed.     

Data acquisition system for dark matter detectors

Progress in Ge detector technology has resulted in ultralow backgrounds of less than 0.3 counts keV^–1 kg^–1 d^–1 at energies between 5 and 12 keV, and less than 1.0 counts kev^–1 kg^–1 d^–1 for energies between 3 and 5 keV. Coupled with good energy resolution, 0.4 keV FWHM at 10 keV, this allows searches for DM particles with m 8 GeV/c².Electromagnetic interference and acoustical pick-up are the main sources of background in the best Ge detectors. These problems are even more important in cryogenic WIMP detectors under development. A PC-based on line pulse shape analysis system is presented which permits rejection of about 95% percent of the EMI/ acoustical background. The hardware uses a low cost, commercially available digital storage oscilloscope. The software consists of about 20,000 lines of code in Pascal and assembly language. We tested this system using a low radioactive background Ge-system on the Earth's surface. For low energy events (27 keV photons) this system permits improvement in the background from 0.1 cpm to 2 cpd.     

Review of ββ decay and dark matter searches with ββ apparatus

Physics beyond the standard model is being sought in Ge experiments in two quite different ways: looking for neutrinoless double beta decay in ⁷⁶Ge and for dark matter collisions with Ge atoms. Lifetime limits on the former, now (5–9) × 10²³ yr, provide severe constraints on (1) lepton number nonconservation; (2) the effective mass of a light Majorana electron neutrino; (3) right-handed currents; (4) the mass of a heavy Majorana neutrino; (5) the ũ squark mass as a function of gaugino mass in supersymmetric theories with R-parity violation; and (6) for a different decay mode, the coupling of a Majoron to the electron neutrino. The dark matter search has eliminated a class of shadow matter and is setting constraints on masses of various kinds of weakly interacting massive particles. In both pursuits new techniques are starting to be used which promise even better results soon.     

Performance of a Low-Noise Test Facility for the SAFARI TES Bolometer Arrays

We have constructed a test facility for characterizing the focal plane arrays of SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays are populated with extremely sensitive ( $\mathit{NEP}\sim 2\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ ) transition edge sensors with a transition temperature close to 100?mK. The extreme sensitivity and low saturation power (～4?fW) of SAFARI’s detectors present challenges to characterizing them. In optimizing the SAFARI Detector System Test Facility we have paid careful attention to stray-light exclusion as well as electrical, magnetic, and mechanical isolation. We present measurements verifying the facility’s performance and analyze them in terms of a two-fluid model of the TES current on the transition to investigate the background power level. We have measured a detector NEP of (5.1±0.4)×10^?19?W?Hz^?1/2, showing that the facility is ready to test the SAFARI prototype arrays and is approaching the performance needed for testing the flight arrays.