A high-sensitivity paramagnetic salt second-sound detector

We describe a second sound detector based on the observation of temperature-dependent magnetization changes in paramagnetic materials. The sensitive element consists of a small pill of powdered paramagnetic salt placed in a region of high magnetic field and surrounded by a superconducting pickup loop connected to a SQUID magnetometer. At 10 Hz, the ac sensitivity was estimated to be 3×10^–9 K/Hz, increasing to 1×10^–10 K/Hz for dc signals. The device is particularly useful for experiments near the lambda point, which can be approached much more closely than with superleak transducers.     

Performance of a 2-hole Y-Ba-Cu-O rf SQUID

A two-hole rf SQUID has been fabricated out of bulk YBCO by drilling two holes and careful erosion of the wall between the holes. Commercial SQUID electronics is inductively coupled to the SQUID through a copper coil glued in one of the holes. Periodic oscillations in V-B characteristic of the SQUID are observed at 77 K. The spectral density of the flux noise in the white noise region is 5·5 × 10⁻⁴Φ₀ / √Hz. The flux noise is frequency-dependent below 200 Hz.     

Observation of NMR Signals from a Thin 3He Slab

NMR free precession signals from a slab of normal ³ He of thickness comparable to the superfluid coherence length have been observed using a SQUID NMR spectrometer at a frequency of 880 kHz. This spectrometer is based on a SQUID with Additional Positive Feedback, directly coupled to a low noise room temperature amplifier, and operated in flux locked loop mode using the direct offset integration technique, with a bandwidth of several MHz. The sensitivity is such that the signal from a 100 nm slab on a 1 cm ² surface, corresponding to 2×10 ¹⁷ spins, is clearly resolved.     

The Design of a Frequency Multiplexed Ir-Au TES Array

The IrAu TESs which have been developed in Genoa in past years, will be assembled in large numbers in a detector to reach the required accuracy in neutrino mass measurement (by β decay of ¹⁸⁷Re). When the number of bolometers increases a multiplexed readout is needed to reduce the thermal load on the low temperature refrigerator and to keep low the cost of SQUID electronics. We present the scheme of the circuit that distributes the AC supply to the TESs in the array and collects the signal from a row of devices, decoding the address by the signal frequency. The TESs are polarized by voltage sources of different frequencies. The TES working at different frequencies can be read by a single SQUID, which reads the current induced in a Superconducting transformer, and feeds in the transformer itself a feedback flux. The signal from the TES in the idle state will be compensated. The circuit will be built with planar film technology and will act as heat sink for each TES. The requirements for the SQUID will be discussed. The construction of a small prototype of the circuit is under way.      

Low-frequency magnetic interference in a SQUID gradiometer caused by a Cryotiger gas-mixture cooler

A Cryotiger^® gas-mixture cooler was applied for cooling of three high-T_c SQUID magnetometers. These SQUID magnetometers were mounted on an alumina holder in an axial gradiometer configuration. From 20 Hz upward, the system noise was about 0.1 pT/√Hz. Below this frequency, the noise gradually increased to a level of 10 pT/√Hz at 1 Hz. This low-frequency excess noise appeared to be due to remnant magnetization of the Cryotiger cold head. Movement of magnetic cold-head parts with respect to the SQUIDs are induced by pressure fluctuations in the heat exchanger lines. By using one SQUID as a reference for the cooler noise, a first-order gradiometer can be formed in which the cooler noise is eliminated. To establish a proper second-order gradiometer either a fourth SQUID has to be added, or the spatial separation between cold head and SQUIDs has to be increased significantly.     

The superheated superconducting colloid: A new particle detector

The use of micron sized superconducting grains in the search for “dark matter” is discussed. Cosmions and massive neutrinos could produce count rates up to 10⁶ per kg of active detector per day, and the signal could be differentiated from the background by the modulation produced by the Earth's motion around the Sun.We report a radiation test of a small superheated superconducting colloid in which 90 KeV γ-rays were detected using an rf SQUID. Individual “flips” of 5 μm radius grains were observed with a signal/noise ratio of 10.     

Current comparators with superconducting shields

In this paper it is shown that the iron cores of conventionál current comparators may be replaced by superconducting shields. Their function is to transmit the magnetic field of the exciting ratio windings regardless of their position and shape to a detector winding. A toroidal shield system with two modifications - a nested one and a helical one - has been combined with a SQUID to build up current comparators for dc and ac applications. The smallest dc error was found to be 5 × 10^?10. Finally a practical example is given of the application of high precision cryogenic current comparators.     

Cryogenic GaAs-FET amplifiers for SQUIDS

Gallium arsenide field effect transistor amplifiers for use with SQUIDs and operating at 4.2 K in the frequency range 50–500 MHz have been developed. The system sensitivity is limited by intrinsic SQUID noise and is 5 × 10^–30 J/Hz at 500 MHz. Further improvement can be obtained by cooling the SQUID. The methods of obtaining proper impedance matching to the transistor are discussed both theoretically and in connection with practical measurements.This work has been supported financially by the Academy of Finland and the Technical Research Centre of Finland.     

Cryogenic detection of particles with a SQUID-assisted thermocouple Au:Fe-Nb and silicon absorber

The feasibility of thermoelectric detection in cryogenic detectors was demonstrate at elevated temperature of 1 K with a 33 mm³ volume silicon absorber placed in electrical field. Au:Fe-Nb thermocouple was used as a thermometer, installed directly on the heat absorber. The resolution, provided by Au:Fe-Nb thermocouple with SQUID read-out was 2.10^–7 K/Hz^1/2, which is one order of magnitude less than expected, and was limited by a noise of SQUID electronics.     

Development of integrated ΔE-E silicon detector telescope using silicon planar technology

An integrated ΔE-E silicon detector telescope using silicon planar technology has been developed. The technology developed is based on standard integrated circuit technology and involves double sided wafer processing. The ΔE and E detectors have been realized in a PIN configuration with a common buried N⁺ layer. Detectors with ΔE thicknesses of 10, 15 and 25 μm, and E detector with thickness of 300 μm have been fabricated and tested with alpha particles using ²³⁸Pu-²³⁹Pu dual alpha source. The performance of the detector with ΔE detector of thickness 10 μm and E detector of thickness 300 μm has been studied for identification of charged particles using 12 MeV ⁷Li⁺ ion beam on carbon target. The results of these tests demonstrate that the integrated detector telescope clearly separates the charged particles, such as alpha particles, protons and ⁷Li. Due to good energy resolution of the E detector, discrete alpha groups corresponding to well known states of ¹⁵N populated during the reaction could be clearly identified.     

The Superfluid 4He Analog of the RF SQUID

We describe the theory, design, fabrication, and performance of a super fluid ⁴ He device which is the analog of the superconducting RF SQUID. This device is a sensitive rotation detector and is used to sense the rotation of the Earth. We also describe the experimental developments and observations which lead to the construction of this successful device.     

Development of an inconel self powered neutron detector for in-core reactor monitoring

The paper describes the development and testing of an Inconel600 (2 mm diameter×21 cm long) self-powered neutron detector for in-core neutron monitoring. The detector has 3.5 mm overall diameter and 22 cm length and is integrally coupled to a 12 m long mineral insulated cable. The performance of the detector was compared with cobalt and platinum detectors of similar dimensions. Gamma sensitivity measurements performed at the ⁶⁰Co irradiation facility in 14 MR/h gamma field showed values of −4.4×10⁻¹⁸ A/R/h/cm (−9.3×10⁻²⁴ A/γ/cm²-s/cm), −5.2×10⁻¹⁸ A/R/h/cm (−1.133×10⁻²³ A/γ/cm²-s/cm) and 34×10⁻¹⁸ A/R/h/cm (7.14×10⁻²³ A/γ/cm²-s/cm) for the Inconel, Co and Pt detectors, respectively. The detectors together with a miniature gamma ion chamber and fission chamber were tested in the in-core Apsara Swimming Pool type reactor. The ion chambers were used to estimate the neutron and gamma fields. With an effective neutron cross-section of 4b, the Inconel detector has a total sensitivity of 6×10⁻²³ A/nv/cm while the corresponding sensitivities for the platinum and cobalt detectors were 1.69×10⁻²² and 2.64×10⁻²² A/nv/cm. The linearity of the detector responses at power levels ranging from 100 to 200 kW was within ±5%. The response of the detectors to reactor scram showed that the prompt response of the Inconel detector was 0.95 while it was 0.7 and 0.95 for the platinum and cobalt self-powered detectors, respectively. The detector was also installed in the horizontal flux unit of 540 MW Pressurised Heavy Water Reactor (PHWR). The neutron flux at the detector location was calculated by Triveni code. The detector response was measured from 0.02% to 0.07% of full power and showed good correlation between power level and detector signals. Long-term tests and the dynamic response of the detector to shut down in PHWR are in progress.     

High-performance dc SQUIDs with submicrometer niobium Josephson junctions

We report on the fabrication and performance of low-noise, all-niobium, thin-film planar dc SQUIDs with submicrometer Josephson junctions. The junctions are evaporated obliquely through a metal shadow evaporation mask, which is made using optical lithography with 0.5 µm tolerance. The Josephson junction barrier is formed by evaporating a thin silicon film and with a subsequent oxidation in a glow discharge. The junction parameters can be reproduced within a factor of two. Typical critical currents of the SQUIDs are about 3 µA and the resistances are about 100 Ω. With SQUIDs having an inductance of 1 nH the voltage modulation is at least 60 µV. An intrinsic energy resolution of 4×10^?32 J/Hz has been reached. The SQUIDs are coupled to wire-wound input coils or with thin-film input coils. The thin-film input coil consists of a niobium spiral of 20 turns on a separate substrate. In both cases the coil is glued onto a 2-nH SQUID with a coupling efficiency of at least 0.5. Referred to the thin-film input coil, the best coupled energy resolution achieved is 1.2×10^?30 J/Hz measured in a flux-locked loop at frequencies above 10 Hz. As far as we know, this is the best figure achieved with an all-refractory-metal thin-film SQUID. The fabrication technique used is suited for making circuits with SQUID and pickup coil on the same substrate. We describe a compact, planar, first-order gradiometer integrated with a SQUID on a single substrate. The gradient noise of this device is 3×10^?12 T m^?1. The gradiometer has a size of 12 mm×17 mm, is simple to fabricate, and is suitable for biomedical applications.     

Multiplexed Readout of MMC Detector Arrays Using Non-hysteretic rf-SQUIDs

Metallic magnetic calorimeters (MMCs) are widely used for various experiments in fields ranging from atomic and nuclear physics to X-ray spectroscopy, laboratory astrophysics or material science. Whereas in previous experiments single pixel detectors or small arrays have been used, for future applications large arrays are needed. Therefore, suitable multiplexing techniques for MMC arrays are currently under development. A promising approach for the readout of large arrays is the microwave SQUID multiplexer that employs non-hysteretic rf-SQUIDs to create a frequency shift of high \(Q\) resonators that is in accordance with the detector signal and that can be monitored by using standard microwave measurement techniques. In this paper we discuss the design of a recently developed and fabricated 64 pixel detector array with integrated microwave SQUID multiplexer that was produced to test the suitability of this readout technique. The characterization of dc-SQUIDs with virtually identical washer design compared to the rf-SQUIDs of the SQUID multiplexer revealed that the crucial SQUID parameters such as the critical current of the Josephson junctions or the washer inductance are close to the design values and anticipates a successful operation of the SQUID multiplexer.     

NMR on Systems of Low Spin Density Using DC Squids

We discuss the use of SQUID NMR for the study of systems of low spin density. The sample is located inside a coil which forms part of a series tuned resonant circuit attached to the input coil of a SQUID. Such a scheme was first discussed by Freeman et al. ¹ We have studied the pulsed NMR response at 1 MHz of a ³ He film adsorbed on the surface of closely packed Mylar sheets. In this case a monolayer corresponds to a spin density a factor of order 104 smaller than that of bulk liquid and a factor of order 10 ² smaller than with a Grafoil substrate. For our particular SQUID and input coil the calculated noise temperature is 60 mK, significantly better than that so far achieved with a cooled semiconductor preamplifier. We evaluate the present and potential performance of the spectrometer, some of the practicalities involved in its implementation, and discuss the minimum number of detectable spins.     

Properties of Superconducting Rhenium as an Absorber for Magnetic Calorimeters

A still puzzling problem in the development of low temperature micro-calorimeters for the measurement of the ¹⁸⁷rhenium β-spectrum is the understanding of the thermalization of energetic electrons in the superconducting rhenium absorber. We studied metallic magnetic calorimeters (MMC) with single crystal rhenium absorbers and paramagnetic Au:Er temperature sensors. The energy released into the detector leads to a change of magnetization of the paramagnetic sensor located in a weak magnetic field. A SQUID with meander shaped inductance is used to read out this change. This setup allows the study of several properties of the superconducting absorber. The transition to the superconducting state is studied by measuring the magnetic flux expelled by the rhenium sample. The resistivity of rhenium above T _c can be estimated from the measurement of the spectral power density of the Johnson noise. Furthermore the quasiparticle lifetime can be investigated through the analysis of the shape of detector signals caused by intrinsic β-decays and the absorption of X-rays. We present the data obtained in these experiments and discuss the physical quantities which can be derived from these.      

Effects on DC SQUID characteristics of damping of input coil resonances

The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally. A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID. Ten differentRC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances. It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise. At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID. The noise level is a function of the shunt resistanceR _s only, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency. At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies asR _s ^/–0.7 . The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account.     

SQUID behavior at liquid nitrogen temperature in high-T c superconductors of the type Y-Ba-Cu-O

Quantum interference at liquid nitrogen temperature in a bulk sample of high-T _c superconductor YBa₂Cu₃O_9–y is described. The SQUID quantization loop is formed at random inside the sintered sample as an internal percolation loop with a Josephson weak link. The estimated values of this multiply connected quantization loop are: the inductance of the loopL _SQ4×10^–10 H, the critical currentI _c1×10^–6 A, the area of the loopA1×10^–8 m², and the hysteresis parameter _L 1. In the flux-locked loop regime the field sensitivity of this liquid nitrogen SQUID with a random loop is 10^–9 T/Hz^1/2 and the resolution is 5×10^–3 ₀/Hz^1/2.     

Characterization of the Input Noise Sources of a dc SQUID

From the theory of the intrinsic noise in a dc SQUID by Tesche and Clarke, we derive the expressions of the current and voltage input noise spectral densities in a dc SQUID current amplifier operated in a flux locked mode. The expected current and voltage noises are compared, at audio frequencies, with the experimental results obtained with a low noise dc SQUID in which the input load (resonant and not) and the operating temperature (1–4 K) are changed. In order to evaluate the input voltage noise, which is directly related to the current noise around the SQUID loop and is usually neglected, we have used as the input circuit a LC resonator with a very high quality factor (10⁶). Both the voltage and current input noises exceed the expected values by the same factor of about 8. This means that the modulus the optimum source impedance of the SQUID amplifier is still in agreement with the value expected from the theory, which is approximately given by the product of the input coil inductance and the angular frequency. To explain the excess noise results, we propose a model in which the voltage and current input noises are due to a thermal magnetic noise source which is present near the SQUID.