The sensitivity of an overlapped tube cryogenic current comparator (CCC), coupled to a SQUID by means of a flux transformer, is calculated and compared with measurements. Conditions for optimal coupling between the CCC and the SQUID are derived. Based on these, an optimal CCC for precision measurements of very small currents in the nanoampere range has been designed. The paper describes the construction and testing of some parts of the system. An essential element is a home-made dc SQUID with low current noise and low inductance input coil equal to that of the CCC overlapped shield, which we use as sensing coil.
AbslmcL A 19-channel DZSQUID based neummagnerometer is under mnstructian at the Univenity of 'kente PT). Except for the myasrat all elements of the system are developed at the UT It comprises 19 wire-wound fint-order gradiometen in a hexagonal mnfiguration. ?he gradiometen are connected to planar ocsoulos fabricated with a NbIAI, AIO,/Nb technology. For this mnnection we developed a method to bond a Nb wire to a Nb thin-film. The SQUIB are placed in mmpanmenlalised Nb modules.Further, aternal feedback is incorporated in order to eliminate crou mlk ktween the gradiometen. The electronics basically mnsist of a phase-locked Imp operating with a modulation frequenq of LOO kHz. Between SQUID and preamplifier a small Iransformer is used to limit the noise mntribulion of the preamplifier. In the paper the werall system is desmibed, and special attention is paid to the SQUID module @ending, mmpanments, alernal-feedback setup, output Iransformer). IntmductionDuring the last few years biomagnetic instrumentation has changed from singlechannel SQUID systems to multichannel magnetometer units. In this way the magnetic field distribution around a subjects's body can be measured much faster and more reliably. Furthermore, spontaneous activity in the body can now be studied. Multichannel systems of about 7.0 to 30 channels aredeveloped by or Are under construction in university groups, for instance in Helsinki and Rome, at the PTB in Berlin and in industries like BTi, Siemens and Philips.At the University of Wente a 19-channel DC SQUID magnetometer for brain research is under construction. In this paper several aspects of the system are described. First, the sensing-coil unit will be considered, that has been optimised with respect to the signal-to-noise ratio of the overall system. Then, attention is paid to the SQUID module in which other topics such as cross talk elimination by means of external feedback and bonding of a niobium wire to a niobium thin-film pad are involved. Finally, the electronics are shortly described. L. Sensing-coil unitFirst-order gradiometers are used as the sensing coils. Second-order gradiometers are not required, bccause our biomagnetim laboratory is on a low-noise location and t On leave from:
Abstract. A method for bonding a niobium wire to a niobium thin film i s described The bonds are to be used as superconducting connections between wire-wound gradiometers and thin-film coupling coils on oc SOUIOS. The method is characterized by two steps. Firstly. the hardness of the niobium wire is reduced by a heat treatment. Secondly, the niobium film is covered with a thin layer of palladium to prevent it from oxidizing. Superconducting bonds were realized using an ultrasonic bonding technique. We tested the bonds and measured superconductivity (to a sensitivity level of 6 x IO-'' n) with currents up to 80 PA, which is equivalent l o 10 times the dynamic range of the oc soulo systems. Even at 80pA, limited by the measuring set-up, the critical current of the bonds is not reached.Present SQUID magnetometers are most often equipped with thin-film DC SQUIDS to which external signals can be coupled via spiral-shaped thin-film input coils [I]. Usually a signal is transferred to such a SQUID by means of superconducting wire. Therefore a proper connection between wire and thin-film input coil has to be realized. This superconducting contact can he made with lead [2] or a lead alloy [3]. In this communication an alternative approach is described. We present a method to bond a niobium wire to a niobium thin film. The annealing of the wire is first considered and after that the bonding and a test experiment.In order to obtain a highly ductile niobium bonding wire, it is first processed in z vacuum annealing step. The niobium bonding wire has a diameter of 50 pm after its insulation layer is removed by sulphuric acid. About 90cm of the wire is hung in a vacuum chamber in a vertical V-shape with an additional weight of 0.2 g in the centre, The chamber is pumped to a pressure lower than Pa (typically 3 x IO-& Pa). Then a DC current of 0.45 to 0.50A is fed through the wire. We turn on the current in about 20 s. The temperature of the wire follows the current according to the brightness. A stable temperature is thus quickly reached and results from a dissipation in the wire of 90 to 100 W for 1 m length. The wire is heated for typically 5 min. If the heating exceeds 6 min the niobium may recrystallize. This effect was clearly observed after 10 min of heating. We tried to measure the temperature of the wire by means of a pyrometer. However, because the wire is very thin and also located at a relatively large distance from the chamber windnw, we did not succeed in an accurate measurement. Nevertheless, the wire is bright white and an estimate of its temperature is 2200 "C. After annealing the current is turned off in about 2 s, and the wire is allowed to cool down via its heat exchange with the environment. After this procedure, the ductility of the wire can be inspected directly by simply bending it. Measurements showed that the Vickers hardness of the niobium is reduced from 180 to about 80kgf1nm-~ (1 kgfmm-2=107pa).The annealed niobium wire is bonded to the film using an ultrasonic wedge bonder of Mech-EL Industries Inc (...
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