Magnetic Microcalorimeters Research Articles

Measurement of the ^{229}Th Isomer Energy with a Magnetic Microcalorimeter.

We present a measurement of the low-energy (0-60keV) γ-ray spectrum produced in the α decay of ^{233}U using a dedicated cryogenic magnetic microcalorimeter. The energy resolution of ∼10 eV, together with exceptional gain linearity, allows us to determine the energy of the low-lying isomeric state in ^{229}Th using four complementary evaluation schemes. The most precise scheme determines the ^{229}Th isomer energy to be 8.10(17)eV, corresponding to 153.1(32)nm, superseding in precision previous values based on γ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be b_{29}=9.3(6)% and b_{42}<0.7%.

An impedance-modulated code-division microwave SQUID multiplexer

Large arrays of cryogenic detectors, including transition-edge sensors (TESs) or magnetic micro-calorimeters (MMCs), are needed for future experiments across a wide range of applications. Complexities in integration and cryogenic wiring have driven efforts to develop cryogenic readout technologies with large multiplexing factors while maintaining minimal readout noise. One such example is the microwave SQUID multiplexer (μmux), which couples an incoming TES or magnetic calorimeter signal to a unique GHz-frequency resonance that is modulated in frequency. Here, we present a hybrid scheme combining the microwave SQUID multiplexer with code division multiplexing: the impedance-modulated code-division multiplexer (Z-CDM), which may enable an order of magnitude increase in multiplexing factor particularly for low-bandwidth signal applications.

Integrated SQUID/Sensor Metallic Magnetic Microcalorimeter for Gamma-Ray Spectroscopy

Metallic magnetic microcalorimeters (MMCs) achieve energy resolution comparable to transition-edge sensors (TESs) but rely on different measurement physics that may allow MMCs to surpass TESs in some future applications. We have recently completed fabrication of new MMC γ-ray detector arrays using several exploratory sensor designs. All designs integrate the SQUID and sensor on the same chip and use a superconducting cap layer on the paramagnet, but explore different combinations of combined/separate sensing and magnetization coils and direct/flux transformer coupling to the input SQUIDs. This report describes the design and initial testing of one of these devices, which has so far demonstrated an energy resolution of 38 eV at 60 keV near 10 mK using natural-abundance silver–erbium paramagnet.

Simulation and optimization of the implantation of holmium atoms into metallic magnetic microcalorimeters for neutrino mass determination experiments

Several novel experiments designed to investigate the electron neutrino mass in the sub-eV region are based on the calorimetric measurement of the 163Ho electron capture spectrum. For this the 163Ho source, with a required activity of the order of 1 to 100Bq, needs to be enclosed in the detector, having a volume smaller than 10−3mm3. Ion implantation is presently considered to be the most reliable method to enclose this source in the detector homogeneously distributed in a well defined volume.We have investigated the distribution of implanted holmium ions in different target materials and for different implantation energies by means of Monte Carlo simulations based on the SRIM software package. We show that, for a given implantation energy, a given target material and implantation area, the number of holmium ions that can be implanted in a single implantation run is limited. We discuss possible methods to overcome this saturation limit in order to fabricate detectors with an enclosed 163Ho source of the activity required by the experiments.

Development of Magnetic Microcalorimeters for Gamma-Ray Spectroscopy

Integrating the SQUIDs and sensing coils of magnetic microcalorimeters onto the same die is a promising approach for maximizing flux coupling and signal/noise. However, new challenges in microfabrication must be overcome, because the underlying SQUID devices are sensitive to chemical attack and elevated processing temperatures. In this report, we describe development and details of a microfabrication process for integrated SQUID/sensor gamma-ray magnetic microcalorimeters with electroformed gold absorbers, starting from a modified version of the STAR Cryoelectronics “Delta 1000” Josephson Junction process.

Cryogenic micro-calorimeters for mass spectrometric identification of neutral molecules and molecular fragments

We have systematically investigated the energy resolution of a magnetic micro-calorimeter (MMC) for atomic and molecular projectiles at impact energies ranging from E≈13 to 150 keV. For atoms we obtained absolute energy resolutions down to ΔE≈120 eV and relative energy resolutions down to ΔE/E≈10−3. We also studied in detail the MMC energy-response function to molecular projectiles of up to mass 56 u. We have demonstrated the capability of identifying neutral fragmentation products of these molecules by calorimetric mass spectrometry. We have modeled the MMC energy-response function for molecular projectiles and concluded that backscattering is the dominant source of the energy spread at the impact energies investigated. We have successfully demonstrated the use of a detector absorber coating to suppress such spreads. We briefly outline the use of MMC detectors in experiments on gas-phase collision reactions with neutral products. Our findings are of general interest for mass spectrometric techniques, particularly for those desiring to make neutral-particle mass measurements.

Magnetic calorimeter arrays for x-ray astronomy

Ever since the early 1980s, microcalorimeters were thought to have the potential to determine the energy of an x-ray photon with an accuracy of 1eV, a goal that promises to greatly enhance our understanding of astrophysics.1 Amicrocalorimeter detector at the focus of a high-angular-resolution x-ray telescope is a unique instrument because of its ability to simultaneously determine the energy of each x-ray photon extremely precisely while also producing x-ray images of various astronomical sources with very high detection efficiency. By studying the precisely measured distribution of x-ray photon energies emitted by various astronomical objects, we learn important information about them such as their composition, temperature, density, and dynamics. We can study a broad spectrum of astrophysical sources, such as the matter-accretion process near black holes, supernovae remnants, and the growth and evolution of galaxies and galaxy clusters. Since the invention of microcalorimeters, scientists around the world have developed ever more sensitive and innovative techniques to detect photons.2 At present, microcalorimeters with superconducting transition-edge sensors (TESs) hold the record for the best energy resolution (1.8eV) for detecting a 6keV x-ray.3 Metallic magnetic microcalorimeters (MMCs) are now on the verge of demonstrating the first ever ‘sub-eV’ energy-resolution x-ray detector. MMCs use magnetism to produce a high-precision temperature sensor. The MMCs that have been developed use the paramagnetic susceptibility of gold doped with a low concentration of erbium ions (Au:Er) when placed in a dc magnetic field.4 In a paramagnet, the magnetization is inversely proportional to temperature, making it very sensitive to small changes at low temperatures. The paramagnet is attached to an x-ray absorber as depicted in Figure 1. When an incoming x-ray hits the microcalorimeter’s absorber, its energy is converted into heat, which a thermometer then measures. The temperature rise is Figure 1. Schematic representation of a magnetic calorimeter. SQUID: Superconducting quantum interference device.

Fabrication of Metallic Magnetic Calorimeter X-ray Detector Arrays

Microcalorimeters with metallic magnetic sensors show great promise for use in astronomical X-ray spectroscopy. We describe the design and fabrication of a lithographically patterned magnetic microcalorimeter. A paramagnetic AuEr film is sputter-deposited as the sensor, which is coupled to a low noise SQUID via a meander superconducting pickup loop used as an inductor. This inductor also provides the magnetic field bias to the sensor. The AuEr film is deposited over this meander such that the field created by a large current flowing in the loop magnetizes the sensor material. The use of thin film techniques in the fabrication of these magnetic sensors not only allows strong magnetic coupling between the sensor and the inductor, it also is scalable for array fabrication.

Erbium-doped gold sensor films for magnetic microcalorimeter x-ray detectors

We briefly review the principles of a magnetic microcalorimeter x-ray detector and the requirements for fabricating an array of such sensors, which include a paramagnetic sensor fabricated with thin-film techniques. We discuss two methods for depositing an Er-doped Au sensor film and present magnetization measurements from 4to300K in 5T and near the detector’s operating temperature and field. The properties of the deposited films that we have studied to date deviate from the expected behavior of bulk materials.

Development of a low temperature SQUID gradiometer for magnetic microcalorimeters

We will report on the performance of a first order SQUID gradiometer developed for the readout of the magnetic microcalorimeters. The SQUID is designed to operate at temperatures below 0.1 K with flux noise level lower than 1 μ Φ 0 / Hz in a magnetic field of 3.5 mT at its washers. It has a baseline of 500 μ m and washer size of 50 μ m in diameter. To produce an ambient magnetic field in a persistent current mode, field coils with a superconducting short equipped with a heater as a switch are integrated on the same chip. Care was taken to decrease the heat input generated by shunt resistors and cooling fins were attached to them to minimize the over-heating of the electrons in the shunts. In order not to degrade the performance by the readout circuit, the SQUID gradiometer is connected to and read out through a low noise series SQUID amplifier.

SQUID-Gradiometers for Arrays of Integrated Low Temperature Magnetic Micro-Calorimeters

We designed first-order integrated planar SQUID gradiometers especially for readout of signals from magnetic micro-calorimeters used for the detection of single x-ray quanta. These SQUIDs must operate in magnetic fields up to 5 mT and at temperatures down to 10 mK. Various new designs of eight-pixel gradiometer SQUID arrays on one chip were developed. The dimensions of the gradiometers are adapted to the magnetic thermometer of the micro-calorimeter. The intrinsic noise of the SQUID gradiometers measured at 4.2 K is lower than 2.5/spl mu//spl Phi//sub 0//Hz/sup 1/2/. They withstand external magnetic fields up to 7 mT without degradation of their performance. A new concept for thermal contact of the detector to the environment based on normal-conducting metallic wiring on the chip is introduced. Field coils with a persistent current switch were integrated on chip to produce the necessary magnetizing field. Appropriate changes in niobium technology compared to our previous work were introduced to achieve necessary critical current of the coil. Thermal coupling between the switch heater and the superconducting short was optimized. A persistent current of up to 80 mA can be injected in the field coil using a heater power of 4 mW. This current corresponds to magnetizing field of 6 mT at the paramagnetic sensor. During the heat pulse the SQUID remains superconducting.

Metallic magnetic microcalorimeters: Energy dispersive X‐ray detectors with high spectral resolving power

Metallic magnetic calorimeters (MMC) are energy dispersive detectors for photons and energetic particles. Recently, MMCs have been shown to be well suited for high resolution X-ray spectroscopy. These devices have potential applications in nano-scale chemical analysis and in focal-plane arrays in X-ray telescopes. In an MMC the magnetization of localized paramagnetic ions in a small magnetic field is used to monitor the temperature. A dilute concentration of ions is embedded in a metallic host to achieve fast response. This sensor is attached to a metallic X-ray absorber with a short thermal time constant. High-energy resolution can be obtained by using a low-noise, high-bandwidth DC SQUID to measure the small change in magnetization upon the absorption of an X-ray. With recent prototype detectors an energy resolution of ΔEFWHM = 3.4 eV for X-ray energies up to 6 keV has been achieved. We discuss the thermodynamic properties of MMCs, the energy resolution, various sources of noise and general design considerations, which are important for a fully integrated micro-fabrication of MMCs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lithographically patterned magnetic calorimeter X-ray detectors with integrated SQUID readout

We describe the design, fabrication and performance of a fully lithographically patterned magnetic microcalorimeter X-ray detector. The detector is fabricated on the same chip as a low-noise SQUID that measures the change in the magnetic sensor film's magnetization as the film is heated by absorbed X-rays. Our proof-of-principle detectors use a 100μm×100μm–2μm paramagnetic Au:Er film coupled to a low-noise on-chip SQUID via a meandering superconducting pickup loop that also provides the magnetic field bias to the film. Absorption of 6keV X-rays in the film causes heating on the order of 1mK with a decay time of 1ms or less, the fastest reported using a magnetic calorimeter. However, the resolution is currently poor due to poor Au:Er film properties and non-optimized coupling to the SQUID. We describe the design and fabrication of this device and present measurements of the heat capacity, decay time constant and effective thermal conductance of the microcalorimeter as a function of temperature. Because the SQUID and calorimeter are lithographically patterned on the same substrate, this technology can be readily applied to the fabrication of arrays of multiplexed magnetic microcalorimeter detectors.

Feasibility study of absolute activity measurement with metallic magnetic microcalorimeters

We report on a feasibility study on precise determination of mass-specific activity of low-energy emitting radioisotopes. Conventional methods of activity measurement suffer from source self-absorption and a strong decrease in detection efficiency for low-energy electrons and photons. We propose a new method based on metallic magnetic microcalorimeters with the source embedded in the detector target in a 4π geometry. First results with a 55Fe source show that electrons and photons are detected with a detection efficiency close to unity and with little loss of energy for electrons. The aim of this study is to provide standards of activity with very low uncertainties in the framework of radiation metrology.

SQUID gradiometer for ultra-low temperature magnetic micro-calorimeter

First-order integrated planar SQUID gradiometers with pick-up loops of 40 µm in diameter and a baseline of 200 µm were designed specially for the readout of signals from magnetic micro-calorimeters. The SQUID has to operate in magnetic fields up to 5 mT and at temperatures down to 7 mK. The design is a first step to develop a multi-pixel x-ray detector, which can be used in high-resolution x-ray fluorescence analysis. Chip coils with a heater switch produce a magnetic field up to a few millitesla in persistent current mode. A SQUID-array amplifier was used to readout the signal of the SQUID gradiometer. The intrinsic noise of the gradiometer at 4.2 K is lower than 1 μΦ0 Hz−1/2. The SQUID can withstand external magnetic fields up to 7 mT without losing its performance. The results of a test with the SQUID gradiometer at ultra low temperatures are reported.

Particle detection using cryogenic magnetic calorimeters

Magnetic microcalorimeters have been developed to detect X-rays with energies up to 100 keV. The magnetization of localized paramagnetic ions embedded in a metal is measured with a SQUID and is used to determine very small temperature changes resulting from the absorption of energy. The response of such detectors can be calculated from equilibrium statistical thermodynamics. Our present best result of 90 eV resolution at 6 keV has been obtained with a calorimeter of 600 ppm Er 3+ in Au at 55 mK, at which temperature the detector had a heat capacity of 3×10 −12 J/K. Upon optimization of the parameters of the calorimeter a resolution of less than 2 eV is predicted to be achievable at 50 mK and less than 1 eV at lower temperatures. The coupling of the conduction electrons to the ions leads to very fast thermalization times even at low temperatures.