Two-stage Adiabatic Demagnetization Refrigerator Research Articles

Design and performance of dual-polarization lumped-element kinetic inductance detectors for millimeter-wave polarimetry

Aims. Lumped-element kinetic inductance detectors (LEKIDs) are an attractive technology for millimeter-wave observations that require large arrays of extremely low-noise detectors. We designed, fabricated and characterized 64-element (128 LEKID) arrays of horn-coupled, dual-polarization LEKIDs optimized for ground-based CMB polarimetry. Our devices are sensitive to two orthogonal polarizations in a single spectral band centered on 150 GHz with Δν∕ν = 0.2. The 65 × 65 mm square arrays are designed to be tiled into the focal plane of an optical system. We demonstrate the viability of these dual-polarization LEKIDs with laboratory measurements. Methods. The LEKID modules are tested with an FPGA-based readout system in a sub-kelvin cryostat that uses a two-stage adiabatic demagnetization refrigerator. The devices are characterized using a blackbody and a millimeter-wave source. The polarization properties are measured with a cryogenic stepped half-wave plate. We measure the resonator parameters and the detector sensitivity, noise spectrum, dynamic range, and polarization response. Results. The resonators have internal quality factors approaching 1 × 106. The detectors have uniform response between orthogonal polarizations and a large dynamic range. The detectors are photon-noise limited above 1 pW of absorbed power. The noise-equivalent temperatures under a 3.4 K blackbody load are <100 μK √s. The polarization fractions of detectors sensitive to orthogonal polarizations are >80%. The entire array is multiplexed on a single readout line, demonstrating a multiplexing factor of 128. The array and readout meet the requirements for 4 arrays to be read out simultaneously for a multiplexing factor of 512. Conclusions. This laboratory study demonstrates the first dual-polarization LEKID array optimized specifically for CMB polarimetry and shows the readiness of the detectors for on-sky observations.

ADR design for the Soft X-ray Spectrometer instrument on the Astro-H mission

In its instrument suite, the Japanese Astro-H mission will include the Soft X-ray Spectrometer (SXS), whose 36-pixel detector array of ultra-sensitive X-ray microcalorimeters will be cooled to 50 mK. This will be accomplished using a two-stage adiabatic demagnetization refrigerator (ADR). A complicating factor for its design is that the ADR will be integrated into a superfluid helium dewar at 1.3 K that will be coupled to a 1.8 K Joule–Thomson (JT) stage through a heat switch. When liquid helium is present, the coupling will be weak, and the JT stage will act primarily as a shield to reduce parasitic heat loads. When the liquid is depleted, the heat switch will couple more strongly so that the ADR can continue to operate using the JT stage as its heat sink. A two-stage ADR is the most mass efficient option and it has the operational flexibility to work well with a stored cryogen and a cryocooler. The ADR’s design and operating modes are discussed, with emphasis on how they reflect the capabilities and limitations of the hybrid cryogenic system.

Compositional Analysis by a Superconductor-Based Energy Dispersive Spectrometer

Energy dispersive spectrometers (EDS) are widely used in material science to provide elemental identification by analyzing the energy of characteristics fluorescence X-rays of an excited specimen. We present compositional analysis of materials of interest in various contexts (from electronics to cultural heritage). They were obtained by a superconductor/based - energy dispersive spectrometers (S-EDS) mounted on a scanning electron microscope. The superconducting detector is a Au/Mo/Pd transition edge sensor (TES) with a Au absorber and a SQUID read-out. The achieved energy resolution is 10 eV at an energy of 1.486 keV (one order of magnitude better than semiconductor based EDS). The working temperature is T = 110 mK and is obtained with a cryostat which uses a combination of a pulse tube cooler and a two-stage adiabatic demagnetization refrigerator. In this way there is no need of liquid coolant during the cool down operation. The complete system allows high performances morphological and compositional analysis representing a successful example of an electronics application of superconductivity.

Nuclear diagnostics with cryogenic spectrometers

Cryogenic spectrometers offer an energy resolution an order of magnitude higher than conventional high-purity Germanium detectors, since low-temperature operation reduces thermal fluctuations and the associated noise. They have been developed over the last several decades, initially driven by particle and astrophysics applications, more recently also for material science, biophysics and nuclear science applications. The Advanced Detector Group at LLNL is developing cryogenic Gamma-ray and fast-neutron spectrometers for nuclear diagnostics in fundamental science and national security. They are based on measuring the increase in temperature upon photon or particle absorption with a sensor operated at the transition between its superconducting and its normal state. The approach can be adapted for different types of radiation with the appropriate choice of absorber material. We describe our Gamma-ray spectrometers consisting of a bulk Sn absorber attached to a superconducting Mo/Cu multilayer sensor. The detectors are operated at a temperature of ∼0.1 K the end of a cold finger in a two-stage adiabatic demagnetization refrigerator, and have, depending on design, achieved an energy resolution between 50 and 150 eV FWHM for energies below 100 keV. Here we give an overview about cryogenic detectors for nuclear diagnostics, and discuss the application of this detector technology to the measurement of uranium enrichment.

Design and performance of a 50 mK rapid turnaround detector characterization system

Upcoming major NASA missions such as the Einstein Inflation Probe and the Single Aperture Far-Infrared Observatory will require arrays of detectors with thousands of background-limited elements sensitive to wavelengths from ∼ 50 μ m to ∼ 3 mm . Because of the low power levels present in space with cryogenic telescopes, these detectors will likely operate at temperatures below 100 mK. In order to make rapid progress on detector development, the cryogenic testing cycle must be made convenient and quick. We have developed a cryogenic detector characterization system capable of testing superconducting detector arrays in formats up to 8 × 32 , read out by SQUID multiplexers. The system relies on the cooling of a two-stage adiabatic demagnetization refrigerator immersed in a liquid helium bath. This approach permits a detector to be cooled from 300 K to 50 mK in under 6 h, so that a test cycle begun in the morning will be over by the end of the day. The system is modular, with two identical immersible units, so that while one unit is cooling, the second can be reconfigured for the next battery of tests.

A multichannel superconducting tunnel junction detector for high-resolution X-ray spectroscopy of magnesium diboride films

We have built a high-resolution soft X-ray spectrometer using a 3 /spl times/ 3 array of superconducting Nb-Al-AlO/sub x/-Al-Nb tunnel junctions. The detector is cooled to /spl sim/0.1 K by a two-stage adiabatic demagnetization refrigerator while being held at the end of a 40-cm-long detector snout that can be inserted into an experimental chamber. The 0.6 mm /spl times/ 0.6 mm array has an energy resolution of 10-15 eV FWHM for X-ray energies below 1 keV and total count rate capabilities above 100 000 counts/s. We demonstrate the capabilities of the instrument in a study of anisotropy in c-axis oriented MgB/sub 2/ films by near edge X-ray absorption spectroscopy. The absorption fine structure at the B K-edge differs for excitation with X-rays polarized along the c-axis and in the ab-plane of the MgB/sub 2/ film. We discuss the implications of the anisotropy for the density of states in MgB/sub 2/ at the Fermi surface.

A multichannel superconducting soft x-ray spectrometer for high-resolution spectroscopy of dilute samples

We have built a high-resolution high-efficiency superconducting soft x-ray spectrometer for synchrotron-based fluorescence-detected absorption spectroscopy. The sensor is a 3×3 array of 200×200 μm2 superconducting Nb–Al–Al2O3–Al–Nb tunnel junctions with an energy resolution around 15 eV below 1 keV and a total count rate capability of ≈100 000 counts/s. This sensor array is cooled to ≈0.1 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into an UHV sample chamber for x-ray fluorescence measurements. We present L-edge absorption spectra of dilute transition metals (≈few 100 ppm) and discuss spectrometer performance with respect to the analysis of metalloproteins.

Characterization of superconducting tunnel junction X-ray detectors by means of monochromatized undulator radiation

A superconducting tunnel junction X-ray detector system was designed to match current energy resolution, count rate and UHV requirements in soft X-ray spectroscopy. It employs a two-stage adiabatic demagnetization refrigerator with a 23 cm long cold finger to operate the detector inside a sample chamber for X-ray fluorescence measurements. This cryogenic detector system was characterized in the energy range from 100 eV to 900 eV with respect to its energy resolution ranging from 12 eV to 25 eV FWHM, count rate capability up to 80 kcps and absolute detection efficiency using undulator radiation monochromatized by means of a plane grating monochromator beamline. Collimating the beam to the detector area improves the energy resolution by about 10%. The potential of this novel detector for beamline characterization was investigated.

A superconducting detector endstation for high-resolution energy-dispersive SR-XRF

We have built a two-stage adiabatic demagnetization refrigerator (ADR) to operate cryogenic high-resolution X-ray detectors in synchrotron-based fluorescence applications. The detector is held at the end of a 40 cm cold finger that extends into a UHV sample chamber. The ADR attains a base temperature below 100 mK with about 20 h hold time below 400 mK, and does not require pumping on the liquid He bath. We will discuss cryostat design and performance.

Two-stage magnetic refrigerator for astronomical applications with reservoir temperatures above 4 K

We propose a novel adiabatic demagnetization refrigerator (ADR) to produce temperatures as low as 100 mK starting from a high temperature reservoir between 4 and 8 K. The high temperature reservoir for the ADR can be provided by a mechanical cooler or an unpumped liquid helium bath. This refrigerator can be used to cool bolometric infrared detectors for low background astronomy from mountain tops, balloons or satellites, as well as to cool cryogenic X-ray detectors. The two-stage ADR consists of a single magnet with a paramagnetic chromic caesium alum (CCA) salt pill to produce the low temperature and paramagnetic gadolinium gallium garnet (GGG) as the first stage to intercept heat from the high temperature reservoir. Thermal contact between the paramagnets and the reservoir during magnetization is made with a mechanical heat switch. The ADR is suspended with Kevlar cords under tension for high mechanical stiffness and low parasitic heat leak. In a single cycle, the ADR maintains a temperature of 100 mK for 10–100 h. This time depends strongly on the magnetic field and reservoir temperature but not on the volume of the paramagnetic material as long as the heat leak is dominated by the suspension.