NTD Ge Research Articles

Systematic Studies of a Sapphire Bolometer with Phonon Pulses in the Temperature Range of 10–100 mK

Systematic studies with a sapphire bolometer test setup, made with indigenously made NTD Ge sensor, are carried out. A C++ and ROOT-based pulse analysis program implementing Savitzky–Golay filtering technique for analyzing the bolometer signal is discussed. Response of the bolometer subject to heater pulses of energy equivalent of 0.3–5 MeV is presented in the temperature range of 10–100 mK. The energy resolution is found to be 15 ± 3 keV in the temperature range of 15–25 mK. It is observed that the energy resolution of the bolometer is nearly constant $$\sim \,15\,\pm $$ 3 keV over the energy range of 0.3–5 MeV. Performance of the bolometer with the addition of a moderate-size tin sample ($$\sim $$ 0.6 g) to the sapphire substrate is presented. The impact of vibration on the bolometer is also discussed.

Study of the effect of external noise pickups on the performance of a cryogenic bolometer.

This paper reports the detailed noise characterization, investigation of various noise sources, and its mitigation to improve the performance of a cryogenic bolometer detector for the TIN.TIN experiment. The noise spectrum has been measured for a sapphire bolometer test setup with an indigenously developed neutron transmutation doped Ge sensor in the cryogen free dilution refrigerator system at Mumbai. The effect of external noise, arising either from ground loops in the system or from the diagnostic and control electronics of the cryostat, on the performance of the bolometer is assessed. A systematic comparison of the influence of different noise pickups on the bolometer resolution is also presented. The best-achieved resolution (σE) at 15 mK is ∼15 keV for heater pulses and appears to be mainly limited by the noise due to the pulse tube cryocooler.

Study of radioactive impurities in neutron transmutation doped germanium

A program to develop low temperature (mK) sensors with neutron transmutation doped Ge for rare event studies with a cryogenic bolometer has been initiated. For this purpose, semiconductor grade Ge wafers are irradiated with thermal neutron flux from Dhruva reactor at Bhabha Atomic Research Centre (BARC), Mumbai. Spectroscopic studies of irradiated samples have revealed that the environment of the capsule used for irradiating the sample leads to significant levels of 65Zn, 110mAg and 182Ta impurities, which can be reduced by chemical etching of approximately ~50μm thick surface layer. From measurements of the etched samples in the low background counting setup, activity due to trace impurities of 123Sb in bulk Ge is estimated to be ~1Bq/g after irradiation. These estimates indicate that in order to use the NTD Ge sensors for rare event studies, a cooldown period of ~2 years would be necessary to reduce the radioactive background to ≤1mBq/g.

Concept of Metallic Magnetic Calorimeters for Rare Event Search in the LUMINEU Project

The project LUMINEU is mainly aiming at the search for neutrinoless double-beta decay of the candidate nuclide \(^{100}\)Mo using cryogenic ZnMoO\(_4\) detectors with simultaneous heat and scintillation light detection for radioactive background rejection. It also includes some development for dark matter search using cryogenic Ge detectors with simultaneous heat and ionization detection for background rejection. For both cases, metallic magnetic calorimeters (MMCs) are studied among several thermometer types. In double-beta decay search, the intrinsically fast response of MMCs reading out the light detector may allow for a very fast signal rise time and help to reduce the potential background due to pile-up of two-neutrino double beta decay events. In dark matter search, MMCs reading out the heat channel may improve the energy resolution with respect to the standard NTD Ge thermistor readout and hence the sensitivity of the detectors for low-mass WIMPs.

An Improved ZnMoO4 Scintillating Bolometer for the Search for Neutrinoless Double Beta Decay of 100Mo

We present a prototype scintillating bolometer for the search for neutrinoless double β decay of 100Mo, consisting of a single ≈5 g ZnMoO4 crystal operated aboveground in the 20–30 mK temperature range. The scintillation light is read out by two thin Ge bolometers. The phonon signals are collected by NTD Ge thermistors. The ZnMoO4 crystal was grown with an advanced method (low-thermal-gradient Czochralski technique) and after purification of molybdenum. The results are very encouraging: the intrinsic energy resolution of the heat channel is ≈800 eV FWHM, the α/β rejection factor (crucial for background suppression) is better than 99.9% in the region of interest for double β decay (≈3 MeV), and the radiopurity of ZnMoO4 looks substantially improved with respect to previous devices.

Scintillating Bolometers for Double Beta Decay Search

In the field of Double Beta Decay searches the possibility to have high resolution detectors in which a very large part of the natural background can be discriminated with respect to the tiny expected signal, results very appealing. This very interesting possibility can be fulfilled in the case of a scintillating crystal bolometer containing a DBD emitter whose transition energy exceeds the one of the natural 2615 keV gamma line of 208Tl. We present the results achieved in the development of bolometric light detectors for double beta searches. The detectors are 1 mm thick germanium disk coated with a layer of SiO2 in order to increase the light collection. The adopted temperature sensors are NTD Ge thermistors optimized to work at temperatures between 9 and 13 mK. A light detector with a considerable large area (35 cm2) was constructed and run in a test measurement. A 140 g CdWO4 crystal (116Cd has a DBD transition energy of 2802 keV) was operated as bolometer and the scintillation light was read by the light detector. The excellent results combined with extreme easy light detector assembly represent the first tangible proof demonstrating the feasibility of this kind of technique.

Modeling of Planck-high frequency instrument bolometers using non-linear effects in the thermometers

The Planck satellite, which is planned to be launched in 2007, is dedicated to surveying the Cosmic Microwave Background (CMB) to a high precision. Aboard this mission, the High-Frequency Instrument (HFI) will use 52 NTD Ge spiderweb bolometers made by Caltech-JPL and cooled to 100 mK by a dilution cooler. In this paper, we present a model of these detectors that includes non-linear effects seen in NTD Ge thermometers: electron–phonon decoupling and electrical field effect. We show that this model leads to consider only electrical field effect. Furthermore, the optical characterization of the HFI bolometers clearly shows a non-ideal behavior that is explained by non-linear effects in the thermometer. We finally show that these effects have to be taken into account for optimized CMB observations and to fully understand the physics of semi-conducting bolometers.

Electrical characteristics of heavily doped NTD Ge at very low temperatures

We report about the measurement of the electric characteristics of some NTD Ge thermistors at temperatures down to 25mK. The dopant concentration is around 6×1016cm-3, producing a material of characteristics close to the metal-to-insulator transition. Fitting the ρ(T) characteristics with a variable exponent p Mott's law, a p around 0.6 was obtained. This result confirms the hypothesis of a dependence of p on the doping level.

Using a microcalorimeter to measure the Lamb shift in hydrogenic gold and uranium on cooled, decelerated ion beams

A precise determination of the Lamb shift from photons emitted by highly charged, one electron ions represents one of the most sensitive tests of QED in strong electromagnetic fields. Recent progress in the production and cooling of intense beams of fully stripped Au and U in the SIS/ESR synchrotron storage ring at GSI, Darmstadt has made it possible to obtain precision spectroscopy of these ions. A fully stripped beam of either Au79+ or U92+ ions is injected, stored and cooled in the ESR and interacts with an internal gas target. The capture of an electron and the subsequent population of a 2p or 3p state will lead to a decay by either Lyman or Balmer X-ray emission. Although measurements of the 1s Lamb shift in U with Ge ionization detectors accurate to ∼3% have provided a test of QED for the high Z domain, the experimental errors (±13eV) are about one order of magnitude larger than the accuracy theoreticians presently claim (±1eV). We present the results from initial broad band experiments using NTD Ge microcalorimeters to measure the 2s Lamb Shift in Au and U at the ESR. The broad band coverage of the microcalorimeter makes it possible to reduce the systematic uncertainties in the Doppler corrections while the high-energy resolution reduces the statistical error in the absolute energy calibration.

Modelling and Optimizing of High Sensitivity Semiconducting Thermistors at Low Temperature

We have modelled low temperature semiconducting thermometers by using a semi-analytical approach of Anderson insulators, taking into account both the electrical field and the electron/phonon decoupling effects. We first used this simulation to fit measured V(I) curves to validate this method and to obtain physical parameters of NTD Ge and NbSi thin film samples around 100 mK. We show that considering the two effects separately leads to an overestimate of each one. Once validated, our algorithm was used to optimise the design of high sensitivity thermometers for the High Frequency Instrument (HFI) of the Planck satellite.

Low noise cryogenic electronics: preamplifier configurations with feedback on the bolometer

We have developed preamplifier configurations used to readout resistive bolometers such as those based on Neutron Transmuted Doped Germanium thermometers (so called NTD Ge thermometers)[1] or NbSi thin film thermometers [2, 3]. We introduce the impedance regulating preamplifier configuration. This configuration, is compared to previously proposed readout configurations with feedback on the bolometer [4]. It is shown that the impedance regulating preamplifier achieves Extreme Electrothermal Feedback without using transition edge thermometer and SQUID readout. Thus the detector and readout electronics are straightforwardly calibrated and can handle much larger event rates. Looped configurations simplify the design and improve performance of detection such as bandwidth. Finally, we analyse the impedance regulating amplifier noise contributions, with JFETs as front end device, relative to other configurations.

Evaluation of prototype 100 mK bolometric detector for Planck Surveyor

The High-Frequency Instrument (HFI) for the Planck Surveyor mission will measure anisotropies of the Cosmic Microwave Background (CMB) down to scales of 6 arcmin and to an accuracy of Δ T/ T=2×10 −6. Channels ranging in frequency from 100 to 857 GHz will use 100 mK spider web bolometer detectors with NTD Ge thermistors. The detectors must be photon noise limited and fast enough to preserve signal information at the 1 r.p.m. scan rate of the satellite. The prime low-frequency CMB channels at 143 and 217 GHz are the most technically demanding owing to the lower background limited NEPs. For the 143 GHz channel the requirements are that the time constant τ<5.7 ms and the NEP bol <1.53×10 −17 WHz −1/2 including contribution from amplifier noise. We present here thermal, electrical and optical data on a prototype detector which, although optimised for the 100 GHz channel, satisfies most of the requirements of the more demanding 143 GHz channel. The measurements are consistent with ideal thermal behaviour of the detector over the appropriate bias and temperature ranges for optimum performance. From optically blanked electrical measurements we determined the dependence of resistance and thermal conductance on temperature over a wide range, 70–200 mK. The optical responsivity and NEP were measured under photon background conditions similar to those expected in flight. Measurements of speed of response as a function of bias at different temperatures allowed us to determine the variation of total heat capacity with temperature. Extrapolation of these data show that in principal performance for all the Planck HFI channels can be met.

Milano Group Development of Bolometric Detectors: a 6.8Kg TeO 2 bolometer array for ββ decay and high energy resolution μ-bolometers for nuclear and x-ray physics

In this paper we want to discuss the results obtained on bolometer detectors in the last year by the Milano Group. At first we will show the results on double decay of 130 Te obtained running an array of twenty cryogenic detectors for about 1450h (0.31Kg/y of 130 Te under test). The set-up is made with crystals of TeO 2 of 340 grams each. It was run in the Gran Sasso Underground Laboratory at a depth of about 3500 m.w.e. By recording the pulses of each detector, in anticoincidence with the others, a 90% c.l. lower limit of 0.87×10 23 years has been obtained on neutrinoless ββ decay of 130 Te. Then we will show the exceptional energy resolution of two bolometers composed of tin absorbers and NTD Ge thermistors. They have been fabricated in preparation of experiments in nuclear and subnuclear physics. Both detectors fully resolve the two K α1 and K α2 lines of 55 Mn. The deconvolved FWHM resolution in this energy region ranges from 4.5 to 5.7 eV, according to different intrinsic measured value of those lines.

Hot Electrons Effect in a #23 NTD Ge Sample

We have measured thermal properties of Neutron Transmutation Doped Ge (type #23 from E. E. Haller) at very low temperatures (down to 20 mK). Both R versus T characteristics and thermal decoupling were considered. We show that, for our sample, the thermal decoupling can be well described with the hot electron model. Our results are consistent with those obtained for other doping concentrations NTD Ge. Finally, we compare the two decoupling mechanisms (electron-phonon and phonon-bath) and show that they are comparable in magnitude for standard running conditions of bolometers.

Esr of heavily neutron-transmutation-doped germanium

Integrated electron spin resonance is used to monitor the magnetic properties of the electron spin in a sample of NTD Ge using temperatures down to 40 mK and uniaxial stresses up to 0.36 GPa. The sample density is just below the critical density at ambient stress for the metal-non-metal transition. The electron spin susceptibility under stress shows no temperature variation; a small broadening as T is lowered is matched by a similar decrease of intensity. Further, uniaxial stress enhances the intensity of the esr line. The contrast with Si∶P is discussed. A large ‘stress-tuning’ effect is inferred.

A study of incomplete charge collection in cryogenic detectors using a segmented 60 gram germanium phonon and ionization detector

We have tested a new 60 gram germanium phonon and ionization detector to study incomplete charge collection effects in cryogenic detectors. An understanding of such effects is necessary to further improve the nuclear vs. electronic recoil discrimination capability of cryogenic detectors. The detector is operated at ≈ 20 mK. It has three concentric ionization channels, and four electric field shaping structures on its sides. Phonons are sensed using NTD Ge thermistors. First results obtained using this device and moveable radioactive sources of241Am and57Co will be presented.

Nb-Si thin films as thermometers for low temperature bolometers

Thin alloy films with compositions leading to the Anderson metal-to-insulator transition can potentially lead to high detection sensitivities and adjustable input impedances as well as to excellent coupling to the heat absorber. We demonstrate some of these advantages in the case of Nb-Si films, whose bias power is shown to be at least 50 times that of NTD Ge detectors at about 30 mK.

Electrical and thermal properties of neutron-transmutation-doped Ge at 20 mK.

We report on hot-electron effects in neutron-transmutation-doped Ge (NTD Ge) near 20 mK. Both static and dynamic electrical properties were measured and compared with a model including both variable-range-hopping conduction and hot-electron effects.