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 100 μm×100 μm–2 μm 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 6 keV X-rays in the film causes heating on the order of 1 mK with a decay time of 1 ms 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.     

Development of cryogenic alpha spectrometers using metallic magnetic calorimeters

Cryogenic particle detectors have recently been adopted in radiation detection and measurement because of their high energy resolution. Many of these detectors have demonstrated energy resolutions better than the theoretical limit of semiconductor detectors. We report the development of a micro-fabricated magnetic calorimeter coupled to a large-area particle absorber. It is based on a planar, 1 mm² large paramagnetic temperature sensor made of sputtered Au:Er, which covers a superconducting meander-shaped pickup coil coupled to a low-noise dc-SQUID to monitor the magnetization of the sensor. A piece of gold foil of 2.5×2.5×0.07 mm³ was glued to the Au:Er film to serve as an absorber for incident alpha particles. The detector performance was investigated with an ²⁴¹Am source. The signal size comparison for alpha and gamma peaks with a large difference in energy demonstrated that the detector had good linear behavior. An energy resolution of 2.83±0.05 keV in FWHM was obtained for 5.5 MeV alpha particles.     

Microstructured Magnetic Calorimeter with Meander-Shaped Pickup Coil

In the last years metallic magnetic calorimeters (MMC) showed an energy resolution of a few eV for x-rays up to 10 keV. This makes MMCs a promising and powerful tool for many applications where photons or energetic massive particles have to be detected—like absolute activity measurements of radioactive isotopes, high resolution x-ray spectroscopy and x-ray fluorescence material analysis. However, in order to fulfill all requirements of these applications and to allow to reach the maximum resolving power a consequent micro-fabrication of the MMC detectors is needed. The micro-fabrication of metallic magnetic calorimeters requires reliable deposition and patterning processes for niobium structures with high critical currents and for paramagnetic sensors. As one result of our advances in microstructuring a fully microfabricated MMC which consists of a meander shaped niobium thin film pickup coil and a 3 μm thick sputter deposited paramagnetic Au:Er temperature sensor will be presented. Deposition of energy in the paramagnetic sensor causes a rise in temperature and results in a change of magnetization, which is measured by a low noise high bandwidth dc-SQUID. The sputter deposited Au:Er films we report on are working well and show thermodynamic properties close to the ones known from bulk material down to temperatures of 45 mK.      

Detection of single X-ray quanta with a magnetic calorimeter

We give a status report on the development of a particular low temperature calorimeter with new experimental results. On absorption of an X-ray photon the increase of temperature changes the magnetization of a diluted magnetic sample, and this quantity is measured with a SQUID-magnetometer. It is a special feature of this experimental method that the magnetic sample has a very high heat capacity and an additional absorber for a compound detector does not change the sensitivity essentially. Besides a short summary on earlier measurements we present new results with metallic magnetic samples, which give shorter signal rise times (below 100 μs). On a compound detector with 0.1 g of LaB₆:Er and an absorber of 12 g sapphire, the energy resolution for 5.9 keV and 60 keV X-ray sis 1.6 keV and 2.6 keV (FWHM), respectively. With a silicon absorber an energy resolution of 1.4 keV at 5.9 keV has been found. The energy resolution is in any case limited by two effects. On the one hand the signal height is strongly reduced due to an additional heat capacity of the magnetic sample and on the other had we have an additional noise from the conduction electrons of the metallic sample. Possible improvements with respect to both effects are discussed.     

maXs: Microcalorimeter Arrays for High-Resolution X-Ray Spectroscopy at GSI/FAIR

Highly-charged heavy ions like U⁹¹⁺ provide unique conditions for the investigation of relativistic and quantum electrodynamical effects in strong electromagnetic fields. We present two X-ray detectors developed for high-resolution spectroscopy on highly-charged heavy ions. Both detectors consist of metallic magnetic calorimeters (MMCs) forming linear eight-pixel arrays. The first detector, maXs-20, is developed for the detection of X-rays up to 20?keV with an energy resolution below 3?eV. The second device, maXs-200, is designed for X-ray energies up to 200?keV with an energy resolution of 40?eV. The results of characterization measurements of single detectors of both arrays will be shown and discussed. In both cases, the performance of the detectors agrees well with their design values. Furthermore, we present a prototype MMC for soft X-rays with improved magnetic flux coupling. In first characterization measurements the energy resolution of this device was 2.0?eV (FWHM) for X-rays up to 6?keV.     

Metallic magnetic calorimeters (MMC): detectors for high-resolution X-ray spectroscopy

X-ray detectors based on the concept of magnetic calorimetry are well suited for high-resolution spectroscopy. Metallic magnetic calorimeters (MMC) make use of a metallic paramagnetic temperature sensor, which is in tight thermal contact with a metallic X-ray absorber. The paramagnetic sensor is placed in a small magnetic field. Its magnetization is used to monitor the temperature, which in turn is related to the internal energy of the calorimeter. 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 ΔE_FWHM=3.4 eV for X-ray energies up to 6.5 keV has been achieved. We discuss general design considerations, the thermodynamic properties of such calorimeters, the energy resolution, and the various sources of noise, which are observed in MMCs.     

Development of MMC Gamma Detectors for Nuclear Analysis

Non-destructive assay (NDA) of nuclear materials would benefit from gamma detectors with improved energy resolution in cases where line overlap in current Ge detectors limits NDA accuracy. We are developing metallic magnetic calorimeter gamma-detectors for this purpose by electroplating \(\sim \) 150  \(\upmu \) m thick Au absorbers into microfabricated molds on top of Au:Er sensors. Initial tests under non-optimized conditions show an energy resolution of \(\sim \) 200 eV FWHM at 60 keV. Monte Carlo simulations illustrate that this resolution is starting to be sufficient for direct detection of \(^{242}\) Pu in plutonium separated from spent nuclear fuel.     

Beta Spectrometry with Magnetic Calorimeters

Absolute activity measurements of alpha, beta and gamma emitting radioactive sources are important in numerous fields such as therapeutic radiology and the characterization of nuclear waste. Conventional ionization and liquid scintillation detectors, which are commonly used for these applications, have an energy dependent quantum efficiency and severe limitations in energy resolution. As a novel alternative we have developed a detector based on a metallic magnetic calorimeter (MMC) with a gold absorber that covers the full solid angle of 4π around the radioactive source. Deposition of energy in the absorber causes a temperature rise and results in a change of magnetization of a parametric Au:Er sensor, which can be measured by a low-noise high-bandwidth dc-SQUID. The detector has equal sensitivity for beta and gamma radiation. In this paper we describe a detector which has a deviation from linear behavior for energies up to 700 keV of smaller than 0.5% and an overall quantum efficiency for beta particles in this energy range close to unity. We show the data of our experiments measuring the decay of ³⁶Cl and compare the results to the theoretically expected spectrum for this second order forbidden non-unique β-decay. We discuss the observed contributions to noise, the quantum efficiency and the achieved energy resolution.     

Fabrication of Metallic Magnetic Calorimeter for Radionuclide Analysis

We present a detailed report on the fabrication process of a metallic magnetic calorimeter (MMC). The MMC is configured in a planar geometry with a meander-shaped pickup coil covered with a Au:Er temperature sensor layer. The meander coil is used to apply a magnetic field to magnetize the erbium ions and to measure the magnetization change of the spin system. The MMC is designed to have a large area (1 mm \(^2\) ) and 3  \(\upmu \) m thickness Au:Er layer, which is suited for large metal absorbers with a few nJ/K heat capacity in radionuclide analysis applications. The completed devices are used in alpha and Q spectrometries.     

A Metallic Magnetic Calorimeter for Hard X-Ray and Gamma Ray Spectrometry

The CEA/LNHB is responsible for the determination and publication of atomic and nuclear data such as X-ray and gamma ray emission probabilities. In order to reduce uncertainties on the determination of these data, a high energy resolution associated with a good intrinsic detection efficiency is required. Hence taking into account these two aspects, we are developing cryogenic detectors, especially metallic magnetic calorimeters (MMCs) for photon spectrometry from few keV up to 200 keV. A MMC using a meander pick-up coil made of niobium thin films has been optimized. The gold absorber (diameter: 1.1 mm, thickness: 335 μm) has an intrinsic detection efficiency larger than 70% for photons from few keV up to 100 keV. From an energy spectrum obtained with a ¹³³Ba multi-gamma source, we have characterized this first detector. The energy resolution is 320 eV and 560 eV respectively at 30 keV and 357 keV. Possible improvements of the performance of the detector are discussed.     

Comparative Study of TiAu-Based TES Microcalorimeters with Different Geometries

Seven microcalorimeters with different geometries have been tested and their performance is compared. The study, for TiAu TESs with a Cu absorber, indicates the presence of so-called constant voltage noise and internal thermal fluctuation noise. The constant voltage noise is not changed by a normal metal pattern on the TES, or by a magnetic field. The energy resolution of the detectors, having different heat capacities, is 2.5 and 5.0 eV (at 5.9 keV).      

Optimization of magnetic calorimeters

The properties and performance of magnetic calorimeters based on Au containing a few hundred ppm of Er can be fully described by equilibrium thermodynamics. As a consequence, the magnitude of the change of magnetization of the sensor, resulting from the absorption of a particle, can be calculated with confidence. The magnetization change depends upon a number of parameters such as the external magnetic field, the temperature and the concentration of the Er ions. This theoretical understanding of the calorimeter also allows us to calculate the flux signal detected by a SQUID and how that signal depends on the detector geometry and other relevant parameters. To date we have measured only cylindrically shaped sensors, which are located directly in a circular SQUID loop. However, a sensor having the shape of a thin strip, possibly in form of a meander pattern, enclosed by a loop of the same geometry, has the potential of providing enhanced flux coupling to the SQUID. We discuss the relation of sensor geometry to other parameters such as the dimensions and heat capacity of an attached particle absorber. The values of the adjustable parameters that optimize the performance of a magnetic calorimeter are investigated under a number of different experimental constraints.     

Development of Low Temperature SQUID Gradiometer Array for Metallic Magnetic Microcalorimeters

We report on the performance of two types of SQUID gradiometers developed for the readout of magnetic calorimeters. Our previously developed low dissipation SQUID gradiometer optimized for low temperature operation has demonstrated the flux noise level of under a magnetic field of 2.5 mT and 150 mK. With a cylindrical Au:Er paramagnetic sensor mounted inside the octagonal pickup washer of the SQUID gradiometer, we succeeded in detecting X-ray signals. However, our achieved energy resolution was 47.2±2.1 eV at 5.9 keV limited by the high operating temperature of 150 mK and by a magnetic field, small for that temperature, due to the limited critical current of the field coils. Based on these results, we fabricated new arrays of SQUID gradiometer by tuning the line width and the number of turns of the field coils and shunt resistance to realize a lower noise level and a larger magnetic field. Furthermore, arrays of SQUID gradiometer with meander patterned pickup washer was fabricated which provides a stronger coupling between the paramagnetic sensor and the pickup washer, and a larger magnetic field at the sensor.      

High resolution detection of x-rays with a Nb-based STJ detector

A high energy-resolution of 88 eV has been achieved for 5.9-keV x-rays with a large area (178×178 µm²) Nb/Al-AlOx/Al/Nb superconducting tunnel junction (STJ) detector, which is stable at room temperature and robust to thermal cycles. The energy resolution is higher than those of semiconductor detectors. The resolution and the short shaping-time-constant, 2 µs, of the main amplifier used to obtain the energy resolution indicate that STJ detectors can be developed as nuclear radiation detectors with high energy resolution even for high rate radiations. Besides, a theoretical limit of energy resolution due to the statistical fluctuation of signal charges is discussed.     

High-resolution superconducting x-ray detectors with two aluminum trapping layers

Superconducting tunnel junctions coupled to superconducting absorbers may be used as high-resolution, high-efficiency x-ray spectrometers. We have tested three detectors with niobium x-ray absorbing layers coupled to aluminum layers that serve as quasiparticle traps. Two detectors differed only in barrier thickness. A third detector includes an extra absorbing layer. Here we present a comparison of detector performance. The best energy resolution measured was 36 eV full width at half maximum at 6 keV.     

Toward a 256-Pixel Array of Gamma-Ray Microcalorimeters for Nuclear-Materials Analysis

We are developing a gamma-ray spectrometer for the analysis of nuclear materials based on an array of superconducting transition-edge-sensor microcalorimeters. The instrument will include eight columns of time-division-SQUID multiplexing circuitry capable of reading out 256 sensors. Our most recent sensors are bulk (1.5 mm square×0.25 mm thick) superconducting Sn absorbers glued to Mo/Cu bilayer thermometers. When fully populated, the active area of the spectrometer will be 5.76 cm², and the maximum count rate of the array will approach 20 kHz. Thus, our spectrometer will be comparable to the state-of-the-art 100 keV high-purity-Ge detector in count rate and collecting area, but with an order of magnitude better energy resolution. Half the detectors will be optimized for operation up to 100 keV, and the other half for operation up to 200 keV. A version of the spectrometer with a partially populated detector array was delivered to Los Alamos National Laboratory in June, 2007. We describe the present status of that instrument. In addition, we review results from a prototype array of 14 detectors that achieved 47 eV average energy resolution (full width at half maximum at 103 keV) and 25 eV resolution in the best detector. An important application of this technology is determining the total Pu content in spent reactor fuel without detailed knowledge of the reactor’s operating history. This work was supported in part by DOE-NNSA, NIST-EEEL Director’s Reserve, and the Intelligence Community Postdoctoral Fellowship Program.     

New Developments in Superconducting Tunnel Junction X-Ray Spectrometers for Synchrotron Science

We are developing Ta-based superconducting tunnel junction (STJ) X-ray detectors for high-resolution soft X-ray spectroscopy at the synchrotron. For scaling to large detector arrays, we have also built a compact, low-cost, remote-controllable preamplifier with <3?eV electronic noise. Current Ta-STJs attain an energy resolution between 6.5 and 9?eV FWHM for energies up to ～2?keV, and can be operated at rates up to ～5,000 counts/s as long as the signals decay with a single exponential time constant.     

Characterization and Performance of Magnetic Calorimeters for Applications in X-ray Spectroscopy

We have developed prototype arrays of metallic magnetic calorimeters for applications in X-ray astronomy. Each pixel consists of an all-gold X-ray absorber in good thermal contact to a gold-erbium paramagnetic thin film thermometer that is operated in the temperature range of 30–100 mK. The para-magnetic response is coupled to a SQUID amplifier. We have characterized pixels in an array and observed the expected temperature dependence of the magnetization and heat capacity. We have demonstrated a full width at half maximum energy resolution of 1.7  \(\pm \) 0.1 eV at 6 keV and have also read out these devices using time-division multiplexing.     

Development of Metallic Magnetic Calorimeters for High Precision Measurements of Calorimetric 187Re and 163Ho Spectra

The measurement of calorimetric spectra following atomic weak decays, beta?(β) and electron capture (EC), of nuclides having a very low Q-value, can provide an impressively high sensitivity to a non-vanishing neutrino mass. The achievable sensitivity in this kind of experiments is directly connected to the performance of the used detectors. In particular an energy resolution of a few eV and a pulse formation time well below 1?μs are required. Low temperature Metallic Magnetic Calorimeters (MMCs) for soft X-rays have already shown an energy resolution of 2.0?eV FWHM and a pulse rise-time of about 90?ns for fully micro-fabricated detectors. We present the use of MMCs for high precision measurements of calorimetric spectra following the β-decay of ¹⁸⁷Re and the EC of ¹⁶³Ho. We show results obtained with detectors optimized for ¹⁸⁷Re and for ¹⁶³Ho experiments respectively. While the detectors equipped with superconducting Re absorbers have not yet reached the aimed performance, a first detector prototype with a Au absorber having implanted ¹⁶³Ho ions already shows excellent results. An energy resolution of 12?eV FWHM and a rise time of 90?ns were measured.