Transition Edge Thermometer Research Articles

Calorimetric low temperature detectors for low-energetic heavy ions and their application in accelerator mass spectrometry

The energy-sensitive detection of heavy ions with calorimetric low temperature detectors was investigated in the energy range of E=0.1-1 MeV/amu, commonly used for accelerator mass spectrometry (AMS). The detectors used consist of sapphire absorbers and superconducting aluminum transition edge thermometers operated at T approximately 1.5 K. They were irradiated with various ion beams (13C, 197Au, 238U) provided by the VERA tandem accelerator in Vienna, Austria. The relative energy resolution obtained was DeltaE/E=(5-9) x 10(-3), even for the heaviest ions such as 238U. In addition, no evidence for a pulse height defect was observed. This performance allowed for the first time to apply a calorimetric low temperature detector in an AMS experiment. The aim was to precisely determine the isotope ratio of 236U/238U for several samples of natural uranium, 236U being known as a sensitive monitor for neutron fluxes. Replacing a conventionally used detection system at VERA by the calorimetric detector enabled to substantially reduce background from neighboring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of 236U/238U=6.1 x 10(-12) could be obtained, representing the smallest 236U/238U ratio measured at the time. In addition, we contributed to establishing an improved material standard of 236U/238U, which can be used as a reference for future AMS measurements.

Al/Au Bilayer Thermometer for Third Sound Detection

We present results from an experiment to develop bilayer metallic films of Al and Au as superconducting transition edge thermometers for the detection of third sound waves in thin superfluid 4He films. We compare transition edge data for such an Al/Au thermometer to that for a pure Zn thermometer and document its utility for third sound detection.

Steepness, noise and instabilities of Ti/Au transition edge thermometers

We report on several properties of Ti/Au thermometer layers that limit the experimentally obtained energy resolution of microcalorimeters, based on this thermometer. We have observed excess noise, instabilities in the setpoint and a process dependent steepness of the superconducting transition. We report on attempts to clarify on these imperfections.

Development of an array of transition edge sensors for application in X-ray astronomy

We report on the development activities towards a cryogenic array of micro-calorimeters, based on voltage-biased Ti/Au transition edge thermometers. Fabrication issues are discussed along the lines of two fabrication routes. One route utilizes bulk micromachining in [110] Si wafers, the other route surface micromachining with a sacrificial layer. Prototype 5×5 arrays have been fabricated and we present the first performance data: Two arrays were irradiated with 5.9keV X-ray irradiation and an energy resolution of 6–7eV FWHM was obtained. The arrays have been designed and their performance is analyzed with the aid of finite element simulation of the electrothermal behavior of a single pixel and thermal conductivity in the supporting structure.

First application of calorimetric low-temperature detectors in accelerator mass spectrometry

For the first time, calorimetric low-temperature detectors were applied in accelerator mass spectrometry, a well-known method for determination of very small isotope ratios with high sensitivity. The aim of the experiment was to determine with high accuracy the isotope ratio of 236U/238U for several samples of natural uranium, 236U being known as a sensitive monitor for neutron flux. Measurements were performed at the VERA tandem accelerator at Vienna, Austria. The detectors consist of sapphire absorbers and superconducting transition edge thermometers operated at T≈ 1.5K. The relative energy resolution obtained for 17.39MeV 238U is ΔE/E=4–9×10−3, depending on the experimental conditions. This performance enabled to substantially reduce background from neighbouring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of 236U/238U=6.5×10−12 could be obtained, representing the smallest 236U/238U ratio measured until now.

Structural and electrical characterisation of Mo films for transition-edge sensors

Detectors based on transition-edge thermometers are under investigation for material analysis and space observations thanks to their high-energy resolution. However, fabrication of stable structures using superconducting/normal proximity bilayer is not trivial. For example, the characteristics of thermometers made of Al/Ag are quickly degraded by interdiffusion. In this work, we report details on fabrication and characterisation of Mo films and Mo/Ag bilayers that being immiscible and non-reactive with each other should have a long time stability. First results show that bilayers with transition widths of 1 mK are achievable, adequate to realise high sensitivity thermometers.

A Compound Thin Film Thermometer

We describe an arrangement of thin film superconducting transition edge thermometers, useful for third sound measurements, that provides an extended range of sensitivity beyond that of a typical single thermometer. We connect thin film strips of zinc and aluminum in series but position them physically in close proximity on a glass substrate where they operate over the range 0.1K≤T≤1.7K. A description of the construction and a full characterization of a typical such thermometer arrangement is provided.

Development of an AC-DC thermal converter at millivolt level operating at cryogenic temperature

A device for the measurement of AC-DC transfer difference at millivolt signal level is under development at the IEN. In the fabrication of this device, a chromium heater, niobium leads, and a niobium transition edge thermometer have been deposited by evaporation on a silicon nitride (SiN) membrane. The sensitivity, the temperature rise of the heater, the temperature control of the support, and other possible effects influencing the AC-DC transfer difference have been investigated. This analysis and first experimental tests show the possibility of high accuracy AC-DC transfer measurements at the level of 1 mV.

Thermal fluctuation noise in a voltage biased superconducting transition edge thermometer

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition.

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.

Fabrication and characterization of infrared and sub-mm spiderweb bolometers with low-T/sub c/ superconducting transition edge thermometers

Spiderweb bolometers with a superconducting transition edge thermometer combine a high sensitivity and absorption efficiency for far infrared and sub-mm radiation with a low susceptibility to high energetic particles. They are among the most promising detectors today for future astronomy applications. In this paper we present our fabrication process for a fully integrated bolometer consisting of absorber, thermometer and suspension structure, based on micromachining of a silicon nitride membrane and various thin film deposition and etching steps. The optimization of Ti/Au transition edge thermometers is described, as well as electrical measurements on a fully integrated bolometer, indicating an electrical noise equivalent power of 2/spl times/10/sup -17/ W//spl radic/Hz at an operating temperature of 270 mK.

Fabrication of superconducting bilayer transition edge thermometers and their application for spaceborne X-ray microcalorimetry

The transition between normal conduction and superconductivity in superconducting materials can be exploited as a highly sensitive thermometer. Transition temperatures can be tailored through the selection of materials, their component cases of more than one material. Two bilayer configurations, Ag/Al and Au/Mo, are examined, including details of preparation, testing, and encountered difficulties. Proposed designs for spaceflight detector applications are discussed.

The Next Generation of EDS: Microcalorimeter Eds With 3 eV Energy Resolution

Abstract Semiconductor energy dispersive spectrometers (EDS), the most commonly used detectors for x-ray microanalysis, have matured to the point that significant improvement in energy resolution is not expected in the future. We believe a revolutionary advance in x-ray microanalysis will occur in the next few years due to the development of new x-ray spectrometers based on microcalorimeters. Energy resolution comparable with wavelength dispersive spectrometers, 3 eV to 10 eV, has already been achieved; future detectors may reach a fundamental limit as low as 0.5 eV to 1 eV. In a microcalorimeter, the energy of an x-ray is converted into heat, and a measurement of the temperature rise of the detector gives the deposited photon energy. Our microcalorimeter detector consists of a superconducting transition edge thermometer cooled to an operating temperature of 100 mK by a compact adiabatic demagnetization refrigerator, a read-out SQUID (Superconducting Quantum Interference Device) preamplifier followed by pulse-shaping amplifier and pile-up rejection circuitry, and a multi-channel analyzer with real-time computer interface.

A monolithic superconducting micro-calorimeter for X-ray detection

A cryogenic micro-calorimeter detector using a small bulk iridium single crystal as both absorber and as transition edge thermometer has been prepared. The detector is voltage biased, and the current through it is measured with a DC-SQUID. In this mode of operation an improvement of the detector performance by the electrothermal feedback is expected. In fact, we observed a stabilization of the working temperature due to this effect. By irradiating the micro-calorimeter with an 241Am X-ray and γ-ray source an RMS energy resolution of σ=150 eV for 14 keV X-rays and σ=250 eV for 60 keV γ-rays was observed.

Application of low temperature calorimeters for the detection of energetic heavy ions

The energy sensitive detection of energetic heavy ions with calorimetric low temperature detectors is investigated. The temperature readout was done with an aluminum transition edge thermometer operated at T ≈ 1.5 K. For 20 Ne-ions with an energy of E = 100 MeV/u from the SIS accelerator at GSI Darmstadt the best energy resolution obtained was ΔE E = 1.9 × 10 −3 . This value corresponds to the energy spread of the ion beam of the SIS. In a first application of such detectors the excitation of the giant resonance in lead nuclei via the reaction nat Pb( 20 Ne, 20 Ne′) nat Pb ∗ was investigated by separating inelastically from elastically scattered 20 Ne-ions in the energy spectrum. At a scattering angle Θ Lab = 3° the excitation energy and the strength of the giant resonance were found to be in good agreement with theoretical predictions. In a first test with an extracted cooled 238 U-beam with an energy of E = 360 MeV/u from the storage ring ESR with an intrinsic beam energy spread of ΔE E ≤ 2 × 10 −4 an energy resolution of ΔE = 97 MeV ( ΔE E = 1.1 × 10 −3 ) was measured. The baseline noise was ΔE = 17 MeV. Further improvement of the energy resolution seems possible.

Physics, fabrication, and characterization of thin films

Superconducting tunnel junctions as well as transition edge thermometers could benefit from high quality thin films. We point out the important features of various cryo detectors and show where high purity, or even epitaxially grown films are advantageous for detector performance. Some fabrication methods are discussed and methods of film characterization introduced. Photolithography has proven necessary to fabricate the complex structure of the present cryo detectors.

Sensitive bolometers using high-T c superconducting thermometers for wavelengths 20–300 μm

Two types of sensitive high-Tc bolometer have been fabricated on 20-μm-thick sapphire substrates with YBa2Cu3O7−δ thin film transition edge thermometers. The sensitivity of both devices is superior to commercially available, room temperature pyroelectric detectors. A 1×1 mm bolometer with a radiation absorber of gold black smoke has a noise equivalent power NEP=2.4×10−11 W/Hz1/2 at 10 Hz and has useful sensitivity over wavelengths 20–100 μm. A 3×3 mm bolometer with a Bi film radiation absorber has NEP=3.6×10−10 W/Hz1/2 at 10 Hz and has useful sensitivity over wavelengths 100–300 μm. Water vapor transmittance spectra were measured to demonstrate the sensitivity of both devices.

Calorimetric particle detectors with superconducting transition edge thermometers

The author discusses the development of calorimetric detectors utilizing large absorber crystals coupled to superconducting transition edge thermometers. At low temperature, high energy resolution results from the low detector heat capacity and the strong temperature dependence of the resistance of the thermometer at the normal/superconducting phase transition. The transition edge thermometer is a thin film at very low T/sub c/ superconductor deposited directly on the absorber. Results from two detectors yielding excellent energy resolution are presented. One of these detectors was fabricated with an Ir strip thermometer, and the other with an Ir/Au proximity effect bilayer. Detailed studies have resulted in a model which will be used to optimize future detectors. >

New detector developments for nuclear radiation

New detectors operated at temperatures in the millidegree range offer the possibility for detecting radiation of nuclear origin with improved energy resolution and energy threshold. Basic features of two types of such detectors are described: (1) calorimetric detectors, measuring phonons via transition edge thermometers. The resistance matching problems of such thermometers can be solved with SQUID readout techniques; (2) detectors based on superconducting tunnel diodes, measuring excitations of quasielectrons. Diffusion loss and ambiguity in signal response occurring with such diodes can be coped with by employing proper geometries and quasielectron trapping techniques.

Design analysis of a novel low temperature bolometer

The authors propose a novel antenna-coupled superconducting bolometer which makes use of the thermal boundary resistance available at low temperatures. The radiation is collected by a planar self-complementary antenna and thermalized in a small thin-film resistor. The resulting temperature rise is detected by a transition edge thermometer which can be (but need not be) a separate film. All components are deposited directly on substrate so that arrays can be conveniently produced by conventional lithographic techniques. The active area of the bolometer is thermally decoupled by its small size and by the thermal resistance of the boundaries with the substrate and the antenna terminals. Design calculations based on a 2- mu m*2- mu m film of a superconductor with T/sub c/ approximately=0.1 K give an NEP approximately=10/sup -18/ WHz/sup -1/2/, a time constant approximately=10/sup -6/ s, and responsivities up to approximately=10/sup 9/ V/W. These specifications meet the requirements for NASA's Space Infrared Telescope Facility and Sub-Millimeter Moderate Mission. Useful applications also exist at /sup 3/He and /sup 4/He temperatures. The calculated NEP scales as T/sup 5/2/. Materials, architectures, and readout schemes are discussed.