High Magnetic Field Environment Research Articles

High sensitivity digital SQUID magnetometers

A single-chip digital SQUID magnetometer integrates a sensitive analog SQUID sensor with a comparator gate and feedback circuitry on the same chip. The comparator gate is an asymmetric SQUID gate driving two DC-to-SFQ converters in series with the feedback coil. In an optimized digital magnetometer chip, sensitivity and noise level are determined by the input analog SQUID. The dynamic range of such digital chips is extremely wide enabling them to be operated in a relatively high magnetic field environment without extensive magnetic shielding. Their slew rate is determined by the frequency of an external two-phase clock. This chip simplifies room temperature electronics and, due to its digital output, can be easily multiplexed on-chip. In this paper, we describe this digital SQUID magnetometer chip and summarize our experimental results demonstrating a digital SQUID chip with sensitivity of 20 /spl mu//spl Phi//sub 0///spl radic/(Hz) and a system slew rate of 5/spl times/10/sup 6/ /spl Phi//sub 0//s at the pick up coil.

Performance and testing of Russian mesh photomultiplier tubes for high magnetic field environments

Future experiments on RHIC and LHC require the performance of photodetectors with stable operation in strong magnetic fields, 0.5 T for the STAR (RHIC) Time of Flight system (TOF) and more than 3 T for the CMS (LHC) electromagnetic calorimeter. Such photodetectors have to possess a good energy and time resolution, for TOF measurements less than 100 ps [1] simultaneously being radiation hard up to some Mrad. Fine mesh dynode proximity photomultiplier tubes FMPMT developed and produced in recent years by Hamamatsu are the most promising detectors for the above specified goals. Different kinds of PMT were tested by some experimental groups in axial magnetic field up to 1.5 T [2–4]. The obtained PMT time resolution in beam test together with Bicron scintillator was better than 100 ps [1].

Design and development of high-sensitive vacuum photodetectors in the National Research Institute “Electron”

The last years' developments of Photomultiplier Tubes (PMTs) in the “Electron” National Research Institute are presented. There were a number of PMTs with superior characteristics developed during the last decade. Among these are such devices as PMTs with low radiation background, microchannel plate PMTs (MCP-PMT), PMTs with GaAs photocathodes. The latest developments of the new kind and innovative range of photodetectors, and such novel devices as Fine-Mesh PMTs for high magnetic field environment and proximity focused Hybrid PMTs (HPMT) with an electron bombarded p-i-n diode are also discussed.

Performance of a prototype aerogel counter readout by fine mesh photo-tubes

The BaBar experiment, in order to achieve its physics program, requires pion/kaon identification capability up to 4.3 GeV/ c. The recent development of new processes has lead to the fabrication of low density silica aerogel with high optical quality. An aerogel threshold counter using the combination of two refractive indices (1.055 and 1.007) can be used to complete the angular coverage of the particle identification system in the forward region of the BaBar experiment. We present final test-beam results on a two-layer aerogel prototype, readout by Hamamatsu fine mesh photo-tubes, as required by the high magnetic field environment of the BaBar experiment. Several configurations have been tested, with different aerogels, photo-tubes and reflective materials. A comparison between data and a detailed Monte Carlo simulation is also presented. The result of the test shows that such a detector can achieve the desired performances.

An integrated digital SQUID magnetometer with high sensitivity input

A single chip SQUID magnetometer is described that integrates a SQUID-based pre-amplifier with a high sensitivity comparator gate and feedback circuitries on the same chip. The comparator gate is an asymmetric SQUID gate driving two SQUID quantizers in series with the feedback coil. The chip's sensitivity and noise level are primarily determined by the pre-amplifier SQUID. The pick up coil is in series with the feedback transformer. Since the current in the feedback coil is maintained close to zero, the dynamic range of the chip can be extremely wide and is independent of the SQUID pre-amplifier or comparator architectures. The chip's slew rate is determined by the bipolar clock biasing the comparator gate. Clocks running in the tens of MHz result in a magnetometer system with slew rate exceeding 10/sup 5/ /spl Phi//sub 0//s (/spl Phi//sub 0/=2.07/spl times/10/sup -7/ Gauss-cm/sup 2/). This chip simplifies room temperature electronics and, due to its digital output, can be easily multiplexed on-chip. A system based on this chip can be operated in a relatively high magnetic field environment without extensive magnetic shielding. The details of the chip as well as preliminary measurement results for the pre-amplifier as well as the digital circuit will be presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A wide dynamic range single-chip SQUID magnetometer

A single-chip SQUID magnetometer is described and demonstrated that integrates a SQUID sensor with feedback circuitry on the same chip. This chip has a very large dynamic range, determined by the sensitivity of the input SQUID and by the current-carrying capacity of the input superconducting lines. This chip can eliminate the need for the sophisticated room temperature circuitry currently used with conventional analog SQUID magnetometers and replace these electronics with a simple bi-directional counter. Furthermore, on-chip multiplexing can be easily implemented for use in multi-channel systems where arrays of more than 100 sensors are required for magnetic imaging. In addition, due to its extremely wide dynamic range and high slew rate, a system based on this chip can be operated in a relatively high magnetic field environment without extensive magnetic shielding.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Left-right resolution in high magnetic fields for the CHAOS inner drift chamber

Design considerations for a pion spectrometer being built at TRIUMF led to the development of a thin, low mass, single plane cylindrical drift chamber with a simple cell structure consisting of alternating anode and cathode wires. To date no solution to the left-right ambiguity in chambers of this type has been found which works in high magnetic field environments and for arbitrary incident track angles. We hypothesized that the left-right ambiguity could be resolved in a chamber of this type by comparing the induced charge on cathode strips which are parallel to the sense wire and located in diagonally opposed corners of the cell. Measurements with prototype chambers in magnetic fields up to 1.6 T show that the left-right ambiguity is indeed resolved well for tracks with incident angles between −45° to +45°. The measured intrinsic chamber resolution for tracks at normal incidence at B = 1 T is 150 μm ( σ).

A proposed diagnostic for time-resolved 14 MeV neutron measurements on TFTR

A novel method for time-resolved measurements of the 14 MeV neutron flux in an intense 2.5 MeV neutron and γ-ray background has been developed. Discrimination against the background 2.5 MeV neutron and γ-ray flux is achieved by the use of polyethylene and lead shielding. A high detection efficiency of DT neutrons is obtained by the use of large volume plastic scintillators and photomultiplier tube designed for operating in high magnetic field environments. Design computations for such a detector system on TFTR show that an absolute detection efficiency of ∼10−8 counts per DT neutron may be obtained. A source strength of 1013 DT n/s may readily be detected by this method using both count mode and current mode operation with a resolution of ∼10 ms within a statistical accuracy of ∼5%.

The Inchworm ® as a precision translator in a high magnetic field and uhv environment

A new set-up has been designed and tested for on-line, high-precision mass measurements of short-lived radioactive isotopes via a determination of the ion cyclotron resonance. Ions delivered by the on-line isotope separator ISOLDE at CERN/Geneva are stored in a Penning trap installed in a superconducting solenoid. Due to severe space limitations in the bore of the solenoid, it is impossible to use conventional mechanical feedthroughs for the necessary manipulations inside the uhv chamber. Instead, a number of Inchworms, a high-precision positioning device based on the piezo-electric effect are employed. This publication reports on the first application of this device in a uhv environment at a magnetic field of nearly 6 T.

Wide range temperature compensated cryogenic scanning tunneling microscope

We describe a compact scanning tunneling microscope using piezoelectric bimorph elements to achieve a large scanning area of 20 μm2 at 4.2 K. This instrument provides inherent temperature compensation, compatibility with a high magnetic field environment, and a novel means of rough sample–tip (z) adjustment.

アモルファス・シリコン温度計

The carbon glass resistance thermometers (CGRT) shows an unstable drift by heat cycles. Since we were looking for a more stable element of thermometer for cryogenic and high magnetic field environments, we selected amorphous silicon as a substitute for CGRT. The resistance of many amorphous samples were measured at 4K, at 77K, and at 300K. We eventually found an amorphous silicon (Si-H) alloy whose the sensitivity below 77K was comparable to that of the germanium resistance thermometer with little magnetic field influence.

Development of cryogenic electrical components for the demonstration poloidal coils

Several cryogenic electrical components have been developed for the Demonstration Poloidal Coils (DPC) in Japan Atomic Energy Research Institute (JAERI). The test coils of the DPC were designed for high-voltage and forced-cooled conductors. The rated voltage of each coil is AC 6.9 kV. Maximum operating pressure is 20 atm. JAERI is developing and evaluating (1) insulation coupling, (2) instrumentation cable with new insulator, (3) a feedthrough for high voltage use, (4) a high-voltage isolation amplifier and (5) an amorphous silicon thermometer for use in high magnetic fields. The insulation coupling and feedthrough are designed on the specification of testing voltage AC 16 kV, testing pressure 30 atm and 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> Acc/s helium leakage. The insulation coupling consists of glass-cloth-epoxy and stainless steel pipes. The feedthrough is made with ceramics. We usually use Teflon or polyimid (KAPTON) tape insulator for instrumentation cables at cryogenic temperatures. However, Teflon is not strong against irradiation and polyimid tape does not have a sufficient high voltage performance because of a withstanding corona. Therefore, poly-ether-imid (PEI) is selected because of its irradiation and corona withstanding. Amorphous silicon thin film has a good sensitivity in the range of 4 K - 30 K and exhibits few magnetic field influences. The stability of amorphous silicon may be high because amorphous silicon has good mechanical and chemical properties. This element can be applied to thermometers in a high magnetic field environment.

Tetrode Photomultiplier Base for High Magnetic Field Environments

A photomultiplier base for scintillation counters has been developed to be used with a four stage multiplier, the Hamamatsu R2107. The design described incorporates a high gain, low noise amplifier, to compensate for the low charge gain, on the order of 600, of the multiplier; the base also contains a discriminator ECL output stage to dtrive a twisted pair cable. The tests of scintillation counters using this base have been carried out in a magnetic field of a few hundred gauss. With only one layer of ferromagnetic shielding for this photomultiplier, efficiencies of > 95% were obtained for signals from minimum ionizing particles. Eighty counters of this type have been fabricated.

A highly segmented NaI(Tl) detector with vacuum photodiode readout for measuring electromagnetic showers at the CERN ISR

In this report we describe construction and performance of two identical units of electromagnetic shower counters which were installed in the axial-field spectrometer at the CERN-ISR in 1982 to provide improved detection of photons and electrons over a 1.3 sr solid angle of the AFS calorimeter. Thallium doped sodium-iodide in the form of small blocks served as an active shower material. Vacuum photodiodes and low-noise charge sensitive electronics were used for the deposited energy measurement and signal amplification. The stable performance of the detectors over a period of more than 18 months until the closure of the ISR has proven that vacuum photodiodes can reliably be utilized in highly modularized large scale detectors operating in a high magnetic field environment.

New Phototubes and Photomultipliers Resistive against High Magnetic Fields

The energy of particles in recent high energy physics experiments has been increasing, so that detector apparatus is inevitably placed in high magnetic field environment. Therefore new photodetectors are required which, unlike conventional photomultipliers, are inherently insensitive to high magnetic fields. We have developed four types of new photodetector for the use in high magnetic fields: a one-atm. xenon filled phototube, a rectangular vacuum photodiode, a one-stage photomultiplier and a photomultiplier with fine-mesh dynodes. Each of them is specially designed on the basis of the analysis of the behavior of electrons in electromagnetic fields and is capable of stable operation in magnetic fields as high as several kilogauss or more.

The Secondary Scintillation of Rare Gases under the Influence of Magnetic Fields

The influence of magnetic fields on the secondary scintillation of xenon and argon is studied. A small gas cell is used with excitation by alpha-particles and magnetic fields perpendicular to the direction of the electric fields. For reduced electric fields normally used in gas proportional scintillation counters the intensity of the secondary scintillation remains constant, within an accuracy of about ± 1 % for xenon and ± 2% for argon, when the magnetic field intensity varied from 0 to 0.4 Tesla. A discussion and interpretation of the experimental data obtained is presented. It is concluded that gas proportional scintillation counters can be used in high magnetic field environments.

Low Temperature Thermometry in High Magnetic Fields. II. Germanium and Platinum Resistors

The effect of static magnetic fields up to 210 kG on the characteristics of commercial germanium and platinum resistance thermometers was measured at various temperatures in the range 3.5–78 K. Measurements of the transverse (sample current I normal to magnetic field H) and longitudinal (I∥H) magnetoresistance were carried out on both types of thermometers. A description is given of the dc resistance measuring technique used in these experiments. This method is a semiautomatic potentiometric scheme which increases the speed of measurement while maintaining high resolution and reasonable accuracy. The magnetoresistance of the germanium sensors was found to be large and orientation dependent over the temperature range investigated. These effects preclude the use of germanium elements, as presently fabricated, for temperature sensors in a high magnetic field environment. In the case of platinum resistance thermometers, the magnetoresistance at temperatures above 30 K is small enough so that it becomes practical to correct for it. In fact, these thermometers have been recently used in studies of the magnetic phase transitions in antiferromagnets in fields up to 200 kG.