Robin Cantor

Superconducting Transition-Edge Sensor Microcalorimeters for Ultra-High Resolution Alpha-Particle Spectrometry

Alpha-particle spectrometry is a powerful analytical tool for nuclear forensics and environmental monitoring. Superconducting transition-edge sensor microcalorimeters have been shown to yield unsurpassed energy resolution for alpha spectrometry. With nearly an order of magnitude better energy resolution (1.06 keV FWHM at 5.3 MeV) than the current state-of-the-art silicon detectors (8-10 keV at 5.3 MeV), it is possible to measure samples containing multiple radioisotopes that would require expensive and time-consuming radiochemical separation prior to measurement with a silicon detector. This paper presents recent results from the Los Alamos four-channel microcalorimeter alpha spectrometer. We have prepared a source from weapons-grade plutonium and demonstrated the ability of microcalorimeter alpha spectrometry to simultaneously resolve alpha energies from 239Pu, 240Pu, 238Pu, and 241Am. The low-energy performance of the spectrometer system has been improved to allow measurement of energies as low as 5 keV, which gives a dynamic range of 1000. We have demonstrated this capability by simultaneously measuring the alpha particles and low-energy x-rays and internal conversion electrons emitted by an electroplated 240Pu source.

Development of Ta-Based Superconducting Tunnel Junction X-Ray Detector Arrays

We are developing new Ta-based superconducting tunnel junction (STJ) X-ray detectors for high-resolution soft X-ray spectroscopy at synchrotrons. STJ detectors combine the high-energy resolution of cryogenic detectors with the high count rate capabilities of athermal devices and the high efficiencies of solid state detectors, which increases the sensitivity for material analysis by fluorescence-detected X-ray absorption spectroscopy. Our STJ detectors are fabricated using thick, high-Z Ta absorber films that enhance quantum efficiency and spectral purity, and extend operational range to several keV compared with earlier Nb-based STJs. They offer an energy resolution of ~ 5 to 10 eV FWHM for soft X-rays up to ~1 keV, and count rates of several 1000 counts/s per detector pixel. For increased solid angle coverage, we have fabricated 36- and 112-pixel Ta-based STJ detector arrays with total areas of 1.4 and 4.5 mm2, respectively. The 208 × 208 μm2 pixels have an energy resolution between 6.8 and 7.6 eV FWHM at 525 eV with a low-energy shoulder, and their responsivity is uniform to within 2% across the array. Here we discuss the performance of the array in the context of synchrotron science.

First-Order Planar Superconducting Quantum Interference Device Gradiometers With Long Baseline

We have developed several types of low-Tc first-order planar gradiometers with long baselines ranging from 3.6 cm to 4 cm. Thin-film planar gradiometers are very attractive for measurements of the off-diagonal components of the magnetic field gradient tensor in noisy environments because of the high intrinsic balance that can be achieved with the precision photolithographic techniques used to fabricate these devices. The gradiometer pickup loops are series configured and connected to the input circuit of a dual-washer gradiometric dc SQUID. Series and parallel SQUID washer configurations have been investigated. For a given chip size, model calculations using a magnetic dipole point source and a distributed current dipole source show that gradiometer performance can be improved by increasing the size of the pickup loops at the expense of a small reduction of the baseline length. Based on these results, an improved planar gradiometer has been developed with 1.175 cm times 1.175 cm square pickup loops integrated with a series-configured dual washer SQUID on a 1.2 cm x 4.8 cm chip. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 muPhi0/Hz1/2 (rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/Phi0. This results in a magnetic field gradient noise of 0.42 fT/cm - Hz1/2. The gradiometers are operable unshielded in typical laboratory environments without losing lock.

Tantalum Passive Persistence Shunts for On-Chip Current Trapping in Metallic Magnetic Calorimetry

Ultrahigh resolution photon detectors based on metallic magnetic calorimeters (MMCs) employ a weakly magnetized paramagnetic sensor to measure the energy of the absorbed particles. MMCs can require large on-chip magnetizing currents of order ~100 mA to achieve optimal performance. To minimize noise injected from room-temperature current supplies, it is useful to trap these currents in on-chip persistent superconducting loops. These loops have so far used electrically heated persistent current switches. However, wire count can be reduced and design flexibility increased by using a passive superconducting persistent current switch with a Tc intermediate between Tc of the Nb loop and the operating temperature of the MMC. In addition, it is desirable for the Tc of the switch to be above the regeneration temperature on singleshot adiabatic demagnetization refrigerators (ADRs). We present passive persistent current switch measurements obtained with Ta film grown on a 100 Å Nb base layer. We have demonstrated trapping of up to 150 mA with no evidence of flux creep over 20 h, and persistence of 100 mA trapped current through several regeneration cycles of our ADR with a regeneration temperature of ~2 K.

Thin-Film Planar Gradiometer with Long Baseline

Gradiometers are attractive for magnetic field measurements in noisy environments. Thin-film planar gradiometers are in particular attractive for measurements of the off-diagonal components of the magnetic field gradient tensor, and they can be fabricated with high intrinsic balance owing to the precision photolithographic techniques used to fabricate these devices. We have developed a low-Tc first-order planar gradiometer with a long baseline of 4 cm. The pickup loops are series configured and connected to the input circuit of a dual washer gradiometric dc SQUID. The white rms flux noise measured in a shielded environment is 1.6 µΦ0/Hz½. The magnetic field sensitivity referred to one pickup loop is 2 nT/Φ0, resulting in an rms magnetic field noise referred to one pickup loop of 3.2 fT/Hz½ and an rms gradient noise of 0.8 fT/cm-Hz½. Based on the lithographic methods used to fabricate the gradiometers, the expected balance level is 1 part in 24, 000 or 0.004%. The gradiometer is operable without shielding in typical laboratory environments without losing lock, with an rms white noise at 10 kHz of 3.5 µΦ0/Hz½.

A High-Resolution, High-Speed, Superconducting Tunnel Junction X-ray Spectrometer for Synchrotron Science

X-ray absorption spectroscopy (XAS) is widely used at every synchrotron to characterize the electronic structure of materials. For XAS on dilute samples, the partial fluorescence yield (PFY) from the element of interest is typically recorded as a measure of absorption. This requires an X-ray detector with sufficient solid angle coverage to capture as much of the weak fluorescence signal as possible, with sufficient energy resolution to separate this fluorescence from the X-ray background due to other elements in the sample, and with sufficient speed to handle the total X-ray fluorescence at modern synchrotron beam lines.

Cryogenic Microcalorimeter System for Ultra‐High Resolution Alpha‐Particle Spectrometry

Microcalorimeters have been shown to yield unsurpassed energy resolution for alpha spectrometry, up to 1.06 keV FWHM at 5.3 MeV. These detectors use a superconducting transition‐edge sensor (TES) to measure the temperature change in an absorber from energy deposited by an interacting alpha particle. Our system has four independent detectors mounted inside a liquid nitrogen/liquid helium cryostat. An adiabatic demagnetization refrigerator (ADR) cools the detector stage to its operating temperature of 80 mK. Temperature regulation with ∼15‐μK peak‐to‐peak variation is achieved by PID control of the ADR. The detectors are voltage‐biased, and the current signal is amplified by a commercial SQUID readout system and digitized for further analysis. This paper will discuss design and operation of our microcalorimeter alpha‐particle spectrometer, and will show recent results.

Oxidation State Determination from Chemical Shift Measurements using a Cryogen-Free Microcalorimeter X-Ray Spectrometer on an SEM

Cryogenic microcalorimeter X-ray detectors based on superconducting transition edge sensors (TES) offer up to a roughly 60-fold improvement in energy resolution as compared with conventional detectors for energy-dispersive spectrometry. The best energy resolutions demonstrated to date are 2.0 eV full width at half maximum (FWHM) at 1.5 keV (Al-K) and 2.4 eV FWHM at 5.9 keV (Mn-K). The energy resolution of state-of-the-art microcalorimeter detectors rivals the resolution of spectrometers for wavelength-dispersive spectrometry (WDS), yet microcalorimeters offer all the advantages of EDS detectors ease of use, long-term stability, and the ability to quickly provide qualitative as well as quantitative chemical analysis.

Carbon Bonding Determination with XES Using a TES Microcalorimeter Detector

TES microcalorimeter detectors are capable of high-resolution X-ray emission spectroscopy (XES) which rivals XANES spectroscopic probes found only within the confines of synchrotrons. Commercial microcalorimeters offer spectral resolution around 5-7 eV which rivals that of wavelength dispersive XRF instruments yet provide full spectra of the material of interest not merely a single element, thereby surpassing WDXRF systems capabilities.

Fabrication of Gamma Detectors Based on Magnetic Ag:Er Microcalorimeters

This report discusses the photolithographic fabrication of ultra-high resolution gamma-ray detectors based on magnetic microcalorimeters (MMCs). The MMC uses a novel Er-doped silver sensor (Ag:Er) that is expected to have higher sensitivity than the Er-doped gold (Au:Er) sensors currently in use. The MMC also integrates the first-stage SQUID preamplifier on the same chip as the MMC gamma detector to increase its signal-to-noise ratio. In addition, the MMC uses a passive Ta-Nb heat switch to replace one of the common long-term failure points in earlier detectors. This report discusses the fabrication process we have developed to implement the proposed improvements.