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This content was downloaded from IP address 137.138.152.64 on 08/08/2018 at 09:03 2008 JINST 3 P11003 SISSA July 10, 2008 : October 16, 2008 October 29, 2008 : : November 14, 2008 : ECEIVED R EVISED b http://www.iop.org/EJ/jinst/ UBLISHING AND R CCEPTED A Analysis and statistical UBLISHED f the cryoEDM experiment P IOP P elve hours; and after correcting t in one loop is detected by the he published values from nearby the crosstalk between an array of ectromagnetic environment inside a SQUID system for this purpose, spectrum of our system and studied ucting state has on the other, parallel f our prototype system in the very low SBB), on Building, and G. Waysand UBLISHED BY a P V.B. Mikhailik a M. Malek, a H. Kraus, : Control and monitor systems online; Cryogenic detectors; ∗ : High precision magnetometry is an essential requirement o a [email protected] Corresponding author. ∗ Université de Nice Sophia-Antipolis, La Grande Combe, 84400 Rustrel, France E-mail: University of Oxford, Department of Physics,Keble Denys Road, Wilkins Oxford, OX1 3RH, U.K. Laboratoire Souterrain à Bas Bruit de Rustrel-Pays d’Apt (L 2008 IOP Publishing Ltd and SISSA EYWORDS BSTRACT a b c K A SQUID magnetometry for the cryoEDM experiment — Tests at LSBB S. Henry,

at the Institut Laue-Langevin, Grenoble.however tests done We in have Oxford developed our have been laboratory, therefore limited we by have the testednoise noisy a environment el smaller at version LSBB, o Rustrel,parallel France. pick-up We loops have — studied others. where We the monitored field the generated magneticthese field by data in a for the curren SQUID LSBB forgeomagnetic resets, observatories. over and tw We crosstalk, have also wethe measured compare the effect it that noise to heating one t loops. of the pick-up loops into its cond methods. 2008 JINST 3 P11003 1 1 6 8 3 4 3 × 10 23 12 16 9 . 2 < | n d | s one of the most sig- persymmetric theories. nge in the magnetic field during frequency. l do this using a new UCN source eriment did this very successfully ment (EDM) of the neutron based eutrons (UCN), and measuring any e have developed a suitable SQUID it on this parameter of ntly improve the sensitivity it will be oEDM aims to improve this limit to oid this we need to monitor the mag- s this operates at cryogenic tempera- electric field is reversed. If the neutron – 1 – cm, which will cover the range predicted by the majority of su · e cm set by the previous phase of the experiment (nEDM) [2, 3], i · e 28 As the neutron has a significant magnetic dipole moment, a cha 1.1 CryoEDM 1.2 Tests of our prototype system at LSBB 3.1 SQUID resets 3.2 Crosstalk 3.3 Calibration 4.1 The difference between signals in two pick-up loops 13 − 26 − 10 The experiment works by storing achange large in number their of ultra precession cold frequency n whenhas the a external non-zero EDM, we will measure a change in thea precession measurement cycle could give anetic false field positive to signal. a To very av highusing resolution. atomic The mercury nEDM spectroscopy phase [4]. ofnecessary However the to to exp significa greatly increase the numberwhich of exploits UCN. CryoEDM the wil properties oftures, superfluid the helium mercury [5]; cannot but be a used. Therefore w nificant results in determining the∼ scale of T violation. Cry 10 1. Introduction 1.1 CryoEDM The cryoEDM experiment is aat search for the the Institut electric Laue-Langevin dipole (ILL) mo in Grenoble [1]. The lim Contents 1. Introduction 2. SetupatLSBB 3. Data corrections 4. Magnetometry measurements 6. Heating pick-up loops to destroy superconductivity7. Conclusions 20 5. Noise spectra and resolution 2008 JINST 3 P11003 (1.2) (1.1) 17 fT. To is given by B cm in an electric · He atoms in a neutron e 3 28 − ure the neutron EDM to our ny drift in the magnetic field ged magnetic field; SQUIDs ent at this sensitivity. However sh to monitor the magnetic field s are stored and the precession and magnetic field er the duration of each measure- e located a safe distance from the E ment of 10 T. From this we conclude it is fea- e precise bandwidth required for a f SQUID magnetometry to directly higher frequency to ensure any drift cy bounds than those given above), ed to generate the electric field. As k-up loops. We intend to record the 2 13 that we understand the crosstalk be- rent applied electric fields. Each pre- use a total of twelve SQUID readout etic field in the region of interest. This . − lop a SQUID magnetometer to monitor 2 1 / 10 1 he precession frequency of B × − above 10 Hz. At low frequencies the noise with fewer longer magnetic field measurements and a µ 2 2 / 130 s — limited by the lifetime of the 1 THz ± − ∼  ssion frequency measurements). the magnetic dipole moment. From this we can Hz dE – 2 – · 14 in an electric field 2 µ − f fT υ = 10 ∼ √ υ × h 2 70,000 measurements (several months of data taking) and a T for each measurement.  ∼ 13 − 10 ∼ is the same as that caused by a change in the magnetic field of 0. 1 − cm . A typical performance reported in [7] is · f / 50 kV is the electric dipole moment and ± d There is still a great deal of work to be done before we can meas Integrating this from 1/130 s (the time for which the neutron During a neutron measurement we will use the SQUIDs to track a Whereas the mercury magnetometer measured the volume avera SQUIDs can achieve noise levels of The neutron spin precession frequency This is one possibility. We may achieve the same sensitivity Reference [6] describes a proposal to use SQUIDs to measure t 2 1 targeted precision. In this paper we report work done to deve sible to use SQUIDs as magnetometerswe in stress a that neutron this EDM is experim cryoEDM measurement only cannot an be order defined at of thisover magnitude stage. much estimate. We may longer wi periods, Th butment we (in will effect take operating the withalthough lower average it upper field will be and ov necessary lower to frequen monitoris the within SQUID acceptable output limits. at a frequency measured) to 10 Hz gives a resolution of 1.3 between measurements of the precessioncession frequency frequency for measurement diffe is averaged over where in the storage volume. are only sensitive to relativesignals changes from in multiple pick-up the loops flux tois through extrapolate complicated the pic magn by the presenceSQUIDs of are easily high damaged voltage by electrodeselectrodes. static, us the To pick-up achieve loops the mustchannels. necessary b extrapolation It we will is importanttween these. for magnetic field reconstruction increases as 1 EDM experiment. higher resolution (by taking the average over several prece measure the magnetic field in a neutron EDM experiment. magnetometer system to replace it. This will be the first use o achieve this sensitivity will require magnetometer resolution of field of show that the frequency shift caused by an electric dipole mo 2008 JINST 3 P11003 5 [SQUID]2 n by TekData. ries of pick-up loops to elled #5) was connected to a ingle turn of 0.127 mm diam- t LSBB; we then outline the ingle pick-up loop; and some ur own software, to control the ips to LSBB, one in March and will be to run the magnetometry eille, a former control centre for mited only by fluctuations of the tored using the 3 axes oEDM system at the Laboratoire red by a set of STS2 seismometers heart of this laboratory, there is a y not a suitable site to test such a e magnetic field with three SQUIDs signal processing and data analysis vatories and discuss the significance tus) to measure the neutron EDM. a heater to allow us to heat the loop ns are stored. In these tests, we have ee SQUIDs; two of these (labelled #3 alk, and calibrate the system; then we of the loops into its conducting state. th their planes parallel, and used these d around the loops. ng on shock absorbers, surrounded by ied out further tests using a five channel nt pick-up loops. In the final sections we with readout electronics from Star Cryo- 3 -1412, U.S.A. on-Trent, ST6 4HY, U.K. – 3 – Woven cables to read out the SQUIDs have been made to our desig 4 6 The data discussed in this paper were taken during two field tr In the following section we describe the setup we installed a In the cryoEDM experiment, we need to use the signals from a se Our laboratory, located in central Oxford, is unfortunatel TekData, Westport House, Federation Road, Burslem, Stoke- Shielding QUalified for Ionosphere Detection with SQUID. Star Cryoelectronics, 25A Bisbee Court, Santa Fe, NM-87508 Supracon AG, Wildenbruchstr. 15, D-07745 Jena, Germany 5 6 4 3 system [9]. extrapolate the magnetic field in themeasured volume the where signals the in neutro several loops,data mounted to coaxially study wi the crosstalk between loops. the other in September 2006. Inconnected the March to trip parallel we monitored pick-up th loops;above one its loop superconducting was transition. fitted In September with system, we carr where two ofmeasurements the were SQUIDs carried were out with connected an in additional series lead shiel to a s data analysis process to correct forcompare this SQUID to resets measurements and from crosst nearby geomagneticof the obser difference between the signals measuredgive with the differe noise spectra and describe the effect of heating one 2. Setup at LSBB In March 2006 we carried out someand preliminary #4) tests using were thr connected to twoeter 24 NbTi mm diameter wire, loops wound made on from the a s same former. The other SQUID (lab We have designed and made oursystem, own and for pick-up data loops, acquisition and and written analysis. o system. Therefore we haveSouterrain tested à Bas a Bruit smaller (LSBB) versionthe in of French Rustrel, the 100 ground-based km cry nuclear northroom missile of (called Mars arsenal the [8]. capsule) shielded500 by m At 1cm of the thick rock. steel, In resti magnetic this field environment of we the expect Earth. our The system(5 environment to at underground, LSBB be one is li on monito the top), and the magnetic field is moni electronics. system in a suitably shieldedtechniques; and cryostat to at use ILL; it (with to the develop rest the of the1.2 cryoEDM appara Tests of our prototype system at LSBB Our system uses SQUID sensors supplied by Supracon the magnetic field in the cryoEDM experiment. The next stages 2008 JINST 3 P11003 0.25 0.25 0.25 0.25 0.25 (mm) 0.127 0.127 Wire diameter No No No No No Yes Yes located inside the LSBB Heater his had four pick-up loops inder open at one end) to mm below the other loops. to the inputs of two SQUIDs 10 V) was recorded using a 12 ick-up loops. A photograph of ere twisted together. A heater re. Two loops were wound on omplete and the cables running ss the noise level (discussed in eld over two periods of around s 1 and 2 were not used during the llow us to heat the loop into its ± ertical component of the magnetic UID sensors. This shield reduced calibration coils to be distorted. tside of the capsule) by 15 m long e mounted next to the cryostat, con- cs ( ils of the loops and shows which one h only DC power lines to control the s and magnetometry measurements as ucting. 46 46 56 59 73 59 73 coil (mm) – 4 – Distance from calibration Connected to the same loop as SQUID 4 24 24 34 30 30 30 30 (mm) Loop diameter #3 #4 #5 #1 #2 #3 #4 #5 SQUID Details of the pick-up loops attached to the SQUIDs. Channel Setup The September setup was designed to allow a lead shield (a cyl In September 2006 we installed a five channel system at LSBB. T In both setups, a calibration coil was mounted away from the p The SQUIDs and pick-up loops were lowered into a helium dewar The capsule is not a true Faraday cage as the shielding was inc March September March measurements. bit Aurora 14 transient recorder. fit over the pick-upthe loops, magnetic calibration field coil by and asection part factor 5). of of the The around SQ shield 30,the was and compact not was dimensions of installed used the for to shield the asse caused calibration the field from the 3. Data corrections In section 4 we will present our measurement of the magnetic fi 34 mm diameter loop made from 0.25 mm NbTi wire, positioned 10 All loops were mounted parallelfield. to the ground The to wires measure between thewas the v attached loops to and the SQUID twistedconducting input wire state, terminals while pair the w connected other to loops SQUID remained supercond 3 to a Table 1. consisting of 30 mm diametereach loops of of two 0.25 mm formers diameter separated(connected NbTi by in wi 14 series), mm. giving five One channels loop in was total. connected the September setup is shown in figureis 1. connected to Table 1 each lists SQUID. the deta capsule. The SQUID preamplifier andnected control electronics to wer a datascreened acquisition cables, system and and a fibre-optic computer link. (located ou inside were unfiltered. All mainsequipment power was by kept the outside, cryostat. wit The output of the SQUID electroni 2008 JINST 3 P11003 e (sampling interval) 84 samples (referred to pplied to the calibration ops and calibration coil used for how these data were acquired ops. The setup here is pictured horizontally, meter). A heater was installed on one twisted rs, made to accommodate two loop diameters hown are in mm. The SQUIDs are connected he bottom and lowered vertically into a dewar – 5 – ) using a transient recorder every 2 or 4 seconds. The timebas s, giving a total sample of 0.82 or 1.63 s. A step function was a Photograph, drawing and schematic of the SQUIDs, pick-up lo µ event The measurements were made by recording a frame of 8192 or 163 was 100 by twisted wire pairs to pick-up(in loops the wound September on setup plastic all loops forme wire were pair, wound and on a the calibration larger dia coil was mounted away from the lo the measurements carried out in September. The dimensions s but it was mounted on a dipstick with the calibration coil at t and analysed. as an of liquid helium. twelve hours (for March and September). But first, we explain Figure 1. 2008 JINST 3 P11003 However , and correct ) stalk between 1 − 0 Φ 1V ∼ 10 V), the SQUID resets, ± he calibration throughout the t signal is the voltage on the he SQUIDs are only sensitive to range ( d value whenever the baseline stor in the feedback line. In the et by the same number of flux set as an example. SQUIDs are 10]. These jumps can be seen on range for all the tests discussed in ied to the feedback coil is adjusted ange ( d by taking the average over 95% of ined in turn. nected to SQUIDs 3 and 4, and 0.21nT olution of the data acquisition system. data to improve clarity. efore the absolute scale shown on plots medium ract a value for the magnetic field strength medium bitrary. The step is due to the calibration signal, – 6 – only one SQUID reset was recorded over a 14 hour measurement. ) 1 − 0 value, we need to first correct the data for SQUID resets, cros Φ 0.1V ∼ baseline Three consecutive events recorded in the March dataset. As t The SQUIDs can be run in different ranges by changing the resi The software corrects the data by adding or subtracting a fixe The data plotted in the graphs in this section were calculate range ( in this range the drift overTherefore a short it time was was preferable limited by to the operate res the SQUIDs in the the SQUID resets using software. The SQUIDs were in jumping an integer number of fluxthe quanta uncorrected to data a in value figure close 3. to 0 V [ low this paper unless otherwise stated. changes abruptly. For this method to work, the SQUID must res channels, and then convert it to nT. These steps are now expla 3.1 SQUID resets The SQUIDs are run in a flux-lockedto loop where keep the the current appl fluxfeedback through line. the SQUID Whenever loop the constant, output and reaches the the outpu limit of its r relative changes in the magnetic fluxwhich the produces absolute scale an is average ar fieldthrough the of loop 0.13nT connected to through SQUID 5. the loop con coil 1/4 of themeasurement. way through Figure 2 each shows sampleonly three sensitive to events to allow from relative changes us the inof March to the our data magnetic monitor data flux, is t ther arbitrary and we frequently offset the plotted the pretrigger region (before the calibration step). To ext from this Figure 2. 2008 JINST 3 P11003 of values is re 5) gave a smaller range the data corrected for resets. The x quantum (measured as 1.91, of the resets for the September ing the correction program as the tude of abrupt changes in the baseline ed correctly. The corrected data are e drift of the baseline over the measurement ptember dataset). Each SQUID resets by an ets as the same magnitude and we correct for offset for clarity. – 7 – 06 V for SQUIDs 1, 2 and 3), this seems to be the case. The spread . 0 ± 34 . The uncorrected SQUID output (ranging from -10 to +10V), and A histogram showing the distribution of values for the magni A study of events containing a reset (such as that shown in figu due to the drift in the signal between adjacent events. of values. These events werepresence cut of out a of reset the oftenshown sample meant in before figure the 3. runn baseline was not calculat Figure 4. Figure 3. data. As the spread1.66 of and values 2 for each SQUID is less than one flu quanta each time. Figure 4 shows a histogram of the magnitude integer number of flux quanta. The spread of values is duethis to by th adding or subtracting a fixed value at each reset. between two events, attributed to SQUID resets, (from theperiod; Se as this is less than one flux quantum, we assume all res absolute value is arbitrary and the corrected curve has been 2008 JINST 3 P11003 , lk n (3.1) (3.2) , so it simply i I flowing around i will fall (or rise) ∝ I i f I I l from the calibration coil. At ick-up loop. The last term change in flux through pick- dback current. This contributes simple model, the change in the nm be ignored as rises (or falls), can be eliminated by applying the M is the total inductance of the input re will be a small crosstalk between ext m is the mutual inductance between the ID). In this paper we explain how to T fi ) is the current flowing through loop i fi L I In this case the current M n ( ∆Φ f ) M i 7 I . I m s because the current induced in a loop by ∑ m + s for just the flux through the pick-up loop instead of + T n), ( L T i and I 6= L n n + ) i – 8 – I ext + ( ∆Φ ext = 0V, and the change in the feedback current produces a crossta ∆Φ ∼ ∆Φ = n 10 to ∼ ∆Φ through a SQUID input circuit is given by 0. ∆Φ = is the current applied to the feedback coil, and is given by The output of all three SQUIDs. The jump at t=0 is due to a signa ∆Φ is the supercurrent circulating in the input loop, is the mutual inductance between loops f n i I I nm The first two terms of equation (3.1) give the flux through the p When we have two or more pick-up loops in close proximity, the M The feedback current term is not present as this expression i 7 Figure 5. the whole input circuit. t=0.11s SQUID 3 resets from signal in channels 4 and 5. 3.2 Crosstalk When two or more pick-up loopsthem are [10]. located close together In the referencefeedback [11] to ter the Brake input et circuit al.correct (instead crosstalk explain of effects how using directly this software. toa the Crosstalk change arise SQU in the magneticmagnetic flux flux, will itself generate a field. In a where feedback and input coils. When the external applied flux such that where the sum is over all other pick-up loops (m gives the extra flux throughonly the a few SQUID percent input and coil in due a to single channel the magnetometer fee it can circuit, increases the effective inductance of the input loop. up loop and 2008 JINST 3 P11003 A; to 0 µ (3.3) (3.4) f R has had 100 / = V is a matrix 7.4 nH, but re the total f for this loop I = i X I ∆ noresets fi V M ; and 10 V). ∼ f each SQUID channel to k-up loop of itive reset (-10 to 0V) in another 100 nH. 0.42 0.40 =0 for this loop, e this. By changing the current ∼ SQUID 5 will increase. This keeps the total ignal on the other, we determined all change in the flux through the h reset ( ∆Φ ate. From the values in table 2 we uring the September tests we used i r loop, and as we do not know the I nm o keep the flux through pick-up loop he other SQUIDs are listed in table 2 e 5. trated by looking at the signal on all M =520 nA. Given this and the crosstalk i resets . In practice this is complicated as we The data sheet gives is loop must change, giving a crosstalk I =0, y V ∆ 8 · ce is 350 nH, and we calculate pick-up loop induc- ∆Φ 0.47 0.39 . X fi SQUID 4 − T M f 435 nH. L I = ∆ – 9 – noresets T induced by the SQUID reset = L V i 0.58 0.30 V I = ∆ ∆ SQUID 3 giving a change in the feedback current of Ω 10 V to 0 V, the feedback current changes from 10 corrected ∼ V term for the next loop. To keep 100 k 3 4 5 = nm f M R 435 nH. m = ) i Reset SQUID T I ( L ∑ It will be even lower for loops attached to a single SQUID, whe . parameter is a vector containing the voltages for all SQUIDs ··· V 4.3 nH is the resistor on the feedback line). To keep f = The signal induced in each SQUID in the March setup due to a pos R fi M A SQUID reset will cause a change in the current though the pic In the medium range When SQUID 3 resets from Substituting these numbers into equation (3.3) gives Using the data in table 2 we can rescale the measured voltage o Taken from the Supracon data sheet, the SQUID input inductan 8 SQUID. flux through the SQUID inputpick-up circuit loop, constant, which is but picked causes up4 a by constant the sm neighbouring (being loops. parallel T to 3),signal. the The voltage input steps current in around each SQUIDfor th induced the by March resets setup. in t (where we estimate the input circuit inductance as will change, so we havechannels a when crosstalk one signal. SQUID resets, This which is is illustrated best in illus figur one loop affects the Table 2. tance as 85 nH [12] giving this is reduced when athe pick-up setup loop with is two attached SQUIDs toapplied connected the to to the input. the feedback same D loop loop ofthat to one measur SQUID, and measuring the s inductance of the input circuit is less. currents, in theory we canmust calculate consider the mutual the inductances crosstalk betweeninductances each with loop any and accuracy, we every canestimate othe only the mutual give inductances a between rough loops estim are typicall correct for this crosstalk by using equation (3.4) where each containing the data in table 2 divided by the magnitude of eac 2008 JINST 3 P11003 A µ (3.6) (3.5)    resets resets resets 5 4 3 contains the output with V V V tached to the input circuit    · entical, but the twisted wire resets ing a current step of 62.5    d 5 in figure 6. As this curve V f the coil and the measurements each other, a negligible distance els in the September data using a rosstalk correction matrix was 041 037 of the calibration of one or both riably triggered a flux jump in the in table 3. e signal is correspondingly higher. 3), . . sted wire pair was coiled; this also ctance of the loop and explains why 00 turns of copper wire on a 1 mm SQUIDs will show the same signal, h SQUIDs (shown in figure 7), the s was larger than the size of the coil above) two SQUIDs were connected , when taking the magnetometry data ed by numerical modelling of the coil. e difference between the signals from up, we can write this as However we can see by comparing the    hich shows a larger signal than the other loops. A) of 0.13 nT for SQUIDs 3 and 4, and 10%. By measuring the average voltage ised by abrupt changes whenever one of rrection gives a noticeable improvement. 045 0 065 0 µ . . ∼ 009 004 . . 62.5 = etail in section 6. 0 0 ghtly around the heater, so we would not expect a significant 041 0 045 0 0 . . c i 0 0 040 0 024 0 . . – 10 –    −    0 0 033 0 009 0 0 The loop attached to SQUID 5 had approximately twice the . . 0 0 9    noresets noresets noresets 3 4 5 V V V    =    corrected corrected corrected 5 4 3 V V V    We could in principle correct the crosstalk for the five chann In the March setup, the loops attached to SQUIDs 3 and 4 were id 5 matrix. In practice this is difficult because (as explained A heater was also attached to SQUID 2 in the September setup, w 9 × The top plot (before thethe crosstalk other correction) SQUIDs is resets. character 3.3 Calibration The first jump shown on the event in figure 5 was produced by send In order to assess thetwo accuracy SQUIDs. of this As correction, the we pick-upapart, plot loops we th were would expect all (to mounted aand parallel first to the approximation) difference that would the be two is zero. roughly proportional This is tomain shown the for reason magnetometry SQUIDs signal for 3 from the an magnetometers bot difference (discussed further is in due sections to 3.3plot and before the 4.1). and limited after accuracy the crosstalk correction, that this co to the same pick-up loop. Insecond this and case the a two reset effects in couldshown one not in SQUID be section inva separated. 4, we Therefore ran only three SQUIDs. In this case the c 5 to the calibration coil.former. The As calibration the coil distance between waswe the made treat coil from it and as 2 the a magnetic pick-up dipole. loop Thus, This we assumption calculate was support a magnetic field signal (for 0.21 nT for SQUID 5 (inof the its March dimensions setup). are not As perfect the we construction expect o an error of pair connected to SQUID 3 was longer.SQUID This 3 increased the showed indu a lowerof signal SQUID than 3, this SQUID consisted 4.increased of the inductance A a of resistor, heater the around circuit. was which also the at twi jump on each SQUID due to this signal, we calculate the values area of the loops attached to SQUIDs 3 and 4 (see table 1), so th the SQUID reset jumps removed (the “corrected” line in figure increase in the inductance. The heater is described in more d However in this case the twisted wire pair was not coiled so ti resets (the “uncorrected” line in figure 3). For the March set 2008 JINST 3 P11003 ections and calibrations. The op) and after (bottom) correcting agnetic field measurement shown in figure 7 so – 11 – The magnetic field measured by all three SQUIDs after the corr The difference between signals from SQUIDs 3 and 5, before (t Figure 7. Figure 6. the crosstalk. This follows roughly theit same appears pattern the as largest the error m is in the calibration. traces have been offset for clarity. 2008 JINST 3 P11003 (4.1) ies record he same diameter, therefore ta. As the geomagnetic field ed by many factors and these 2 rcent. The different magnitudes 3 5 6 . ] il. The errors show the RMS spread ements to Rustrel, assuming the 9 . . . eld measured by SQUID 5. The . 1 e variation than that recorded by been offset to show all lines at a 0 QUIDs. As the loop had a total c field in the laboratory, we com- rtical component of the magnetic red only by 1.1%, the magnitude 0 0 0 0 − espondingly lower compared to the ± ± ± ± the Earth’s magnetic field to 0.1 nT ± n loops. As SQUIDs 4 and 5 were h’s magnetic field shows fluctuations n France), Ebro (Spain), and Fürsten- 1 factors including the loop size, length hour March measurement is shown in 1 2 2 . 9 nd converting the voltage to nanotesla, . . . nything better. The vertical component . nts every minute. 8 5 5 11 There is no single geomagnetic observa- 12 longitude − − − − × September setup c + 5 . 4 4 0 . . – 12 – 0 0 ± 0 ± ± . latitude 5 8 Signal response [VnT . . 21 7 9 × − March setup b + a = 1 2 3 4 5 B SQUID to fit the geomagnetic data at each data point. The observator c and b , a The response of each SQUID to a signal from the calibration co In the September setup all the loops were wound on formers of t The result, as figure 8 shows, is an approximate match to our da We carried out a simple linear extrapolation of these measur 10 nT each day due to currents in the ionosphere. It is influenc of values. we would not expect the loopof area calibration to signal are differ by likely to moreof be than the due a to few twisted-wire-pairs a pe and combinationconnected of the to mutual the inductance same betwee expected loop, due the signals to in theinductance these variation of SQUIDs of 785 nH, diffe the instead of inputloops 435 connected coupling nH, to the between only magnitude one S is SQUID. corr 4. Magnetometry measurements After removing the SQUID resets, correctingthe the resulting crosstalk a magnetic field strength recordedfigure during 7. the To 12 be surepared this our really results is with a published measurement∼ geomagnetic of data. the The magneti Eart the absolute field strength, butscale which the shows data the shown fluctuations. in figure 8 have shows significant regional variations, we would notof expect a the magnetic field recorded by Ebro shows significantly mor tory close to Rustrel. Therefore we compare our data to the ve fluctuations can be quite different at different locations. field measured by three observatories at:feldbruck Chambon la (Germany) Forêt [13]. (i Thisgeomagnetic is shown observatories in monitor figure all 8,precision three with using components fluxgate the of magnetometers, fi recording data poi geomagnetic field is given by Table 3. and calculating 2008 JINST 3 P11003 omagnetic data we are t measured by three geomagnetic Rustrel, but they do share some n la Forêt and Fürstenfeldbruck; 10 seismometers did not record any al measured by SQUIDs 3 and 5. re appropriate comparison we first er SQUID, this appears to be due to outh. ach dataset to present the traces on the same l solar eclipse, which produced a time and posi- itive to currents induced in the Mediter- moved across the ionosphere [9], although this effect atterns as fluctuations in the geomagnetic field on r extrapolation of these data to Rustrel to compare – 13 – Fluctuations in the magnetic field measured at LSBB, with tha As the signal we measured is close to what we expect from the ge We also note that on that day (29 March 2006) there was a partia 10 Figure 8. tion dependent magnetic field signalwas as small. the shadow of the moon observatories (to 0.1nT precision). Thesethis show scale different vary p between different locations.with We plot our a measurement. linea A fixed value has been subtracted from e scale, and the lines have been offset for clarity. other observatories as due to its coastal location it is sens ranean Sea [14]. Thesefeatures are with not the present Ebro data inthese which our are were likely measurements not to at recorded be caused by by Chambo ionospheric currents to the S confident that it is duesignificant to activity the over Earth’s this magnetic period. field. The LSBB 4.1 The difference between signals in two pick-upFigure loops 6 shows the differenceAs between this the curve is magnetometry roughly sign the proportional imperfect to calibration the of the signal magnetometers. from eith To make a mo 2008 JINST 3 P11003 (4.2) are cal- β calculated he reference α and α 80 pT, but this is ∼ 10% and our comparison h-South component of the stalk) and ∼ r, we conclude the magnetic ic errors on our measurement ith the calculated areas. From aset. Doing the same analysis of the horizontal pick up from s of the magnetic field in the irections (vertical, north-south, 3 and 5, and is a similar magni- d a resistor. It is likely the twist- t field parallel to the loop, to that ion is up of the horizontal component of tic field, and some rough estimates field signal from SQUID 4, rescaled ch data. ts shown in figure 12 and figure 13. ic observatory data). The two lines extrapolated geomagnetic data. This tly horizontal, therefore we can only aligned. The twisted wire pair from ifference between the signals on each s estimate neglects any pick-up in the tory data, we recorded the signals from tra magnetic flux. with the value of the geographic north 3 B β − ) + 3 4 pT. Correcting the SQUID resets will produce 4 ( 5 ∼ B B α – 14 – (sum over all data points). The values of = 4.2%. The loop is unlikely to be folded such that the ) 2 10 nT, while the difference between our measurements 3 ( 4  is the error on our measurement of the voltage difference ≤ ∼ 0 3 B Φ B V − δ ) 3 4%. The largest crosstalk in this setup was ( 4 ∼ B  where ∑ N √ 0 Φ are 0.93, 1.16, 1.25. The difference between the scaled and t V ) 0.4 nT, the expected order of magnitude. is given by 4 δ ( 5 ) ∼ B 3 ∝ ( 4 B 5% accuracy reducing this effect to and ) ∼ 3 ( 5 is the unscaled field (but corrected for SQUID resets and cros varied by B 4 3 , ) B B 3 ( 4 To test this hypothesis, figure 10 compares Figure 9 shows the correlation is best between pick-up loops The likely cause is pick-up of other (horizontal) component From figure 10 we can see that during our measurement, the Nort The data discussed so far in this section is from the March dat We conclude this section with a short summary of the systemat − B ) 3 ( 5 a cumulative error culated to give the minimum where to fit SQUID 3, rescale one of the SQUID signals to fit the other. The magnetic corrected to twisted wire pairs, or ifSQUID 3 the passed pick-up through loops the are heater,ing where is not it less perfectly than was perfect wrapped at aroun this point, causing it to pick up ex SQUID output is shown in figure 9 for all three SQUIDs. tude between 4 and 5.field gradient As between these loops 3/4 loops and areSQUID 5 wound is is mainly on negligible, and due the the to same d other forme factors. field component at Rustrel (extrapolatedmatch from very the well. geomagnet We canfigure make 11, a which rough shows estimate a ofthis side the and diagram magnitude end we view expect of thedue the ratio pick to of up the the loops, field signal w perpendicular due to to it the to magne be twisted wire pair, and assumes thetreat plane this as of a the rough loop estimate. is perfec geomagnetic field varied over a range of horizontal area is the maximum size calculated here, but thi for B for a long measurement in theThis time, September in dataset addition gives to the looking plo atthe the [SQUID]2 geomagnetic observa magnetometer.and This east-west) monitors and the gave a field moreanalysis in accurate gives the three calibration same than d conclusion the as that drawn from the Mar of the relative change inof the their vertical component magnitude. of the We magne estimate the accuracy of the calibrat of the signals inthe two field loops due suggest to this misalignment is is about right. Pick- 2008 JINST 3 P11003 , middle 5 r 12 hour B − ) 5 ( 3 f, then we would expect √ as 1/ puts (in nT), top B th shorted input, and if the noise o drift over a long time period. If of the SQUID magnetometer. The – 15 – is the number of resets. This gives an effective drift over ou N . 5 B and , 0 − ) 5 Φ 20 pT. ( 4 ∼ The difference between the rescaled and reference SQUID out , bottom B 4 B In the following section we discuss in detail the resolution − ) 4 ( 3 the drift over 12 hours to be 32 fT. B Figure 9. measurement of corresponding to 1 intrinsic 1/f noise of thethis SQUID contribution will is cause the the same SQUIDspectrum as output shown that t in measured figure with 14 a for SQUID a wi bare SQUID continues to scale 2008 JINST 3 P11003 (inverted to give 3 B − ) 3 ( 5 herefore we predict a ratio of s to 100 ms. An 8-pole But- nent of the magnetic field. In this µ lotted using data taken with and ) component of the magnetic field at m 1 ber run. These were produced from and used to block any noise above the it seems the dominant error is pick-up of the eldbruck, and Ebro geomagnetic observatories; ence SQUID baseline B . The maximum possible horizontal area for the pick-up 2 – 16 – 30/707 = 4.2%. ≤ , (the actual area will depend on exactly how the wires lie). T 2 The pick-up loops are designed to measure the vertical compo Top trace: a linear extrapolation of the X (geographic north 80 recorded baseline samples with 6 different timebases fro 5. Noise spectra and resolution Figure 14 shows the noise spectra∼ recorded during the Septem horizontal pick-up to vertical pickup of terworth filter was installed beforeNyquist the frequency transient in recorder thesewithout measurements. the lead Two shield spectra in place. were p loop is 30mm horizontal field. the same polarity). As the two traces follow the same pattern plane a 30mm diameterloop has an areaof 707mm Figure 11. Figure 10. Rustrel from the data measured at Chambonbottom la trace: Forêt, Fürstenf the difference between the rescaled and refer 2008 JINST 3 P11003 that measured by the three 06. The top two lines show the ations visible), the bottom line shows the ), and that measured by the permanent LSBB d line shows the scaled difference between the ese data to Rustrel as was shown in figure 8. – 17 – The magnetic field signal measured at LSBB in September, with 12 hour magnetic field measurement taken during September 20 geomagnetic observatories and a linear extrapolation of th Figure 13. Figure 12. field measured by one of ourSQUID SQUIDs magnetometer (after (measuring all the the vertical corrections field).signals The on thir two of ourLSBB SQUIDs magnetometer (multiplied for by the 5 North-South to direction. make the fluctu 2008 JINST 3 P11003 Ω This (5.1) 11 30 by the 100 Hz the ∼ 1 Hz; below ∼ ∼ magnetic field f is the bandwidth / max above f , although the lowest below this. Note that 2 2 / / 2 1 1 / 1 − − , by integrating the noise rtz), − t LSBB with and without the B ower frequencies the noise Hz Hz δ · rnal field fluctuations. · Hz t). · n the magnetic field noise and fT t LSBB [15]). There is a small factor). This shows the intrinsic ord with an identical sensor, but bandwidth. Below fT ) f  f e in the laboratory (this is the same / oise, reduced by a factor ng wire, shorting the input terminals 9 . √ f power spectrum (or 1 4 / ( d 2 2 UID with the pick-up loop replaced by a 0 . 2 n f 3 ns why the lines in figure 14 do not overlap perfectly fT using the same calibration factor). The data B / t circuit is different, but as the pick-up loop inductance max f 1 T Z . s – 18 – f / 35 . 2 = above 1 Hz and B 2 δ / 1 − Hz · . f / is the noise level at frequency f (in units of tesla per root he ) The noise spectra for our SQUID magnetometer (#1) recorded a f ( n 20% of the total inductance this effect is small. This explai B is the measurement time (which sets the lower frequency limi ∼ We can calculate the FWHM resolution of the magnetometer, At high frequencies the intrinsic SQUID noise is greater tha The shielded spectrum shows a flat white noise level of 4.0 fT T There will be a small difference as the inductance of the inpu 11 gives a white noise of 2.6 fT frequency points are even higher. This is the magnetic field n shield. Figure 14 alsowith shows the a pick-up noise loop spectrum replacedon measured by the a in very SQUID Oxf short chip superconducti (convertednoise to of fT the using SQUID, the which same is calibration what we would measure with no exte where while the geomagnetic field shows an approximately 1 spectrum), the SQUID power only rises as 1 and spectrum; as shown in equation (5.1), the spectra are flat, until the noise drops off above the SQUID points have been smoothed. superconducting shield. The spectrum measuredshort for across a the bare SQ input terminals is also shown (converted to Figure 14. is only at high frequency. this it increases as the frequency falls, approximately as unshielded spectrum shows the spectrumlevel of as the that magnetic nois measured bypeak the at permanent 50 SQUID Hz magnetometer dueincreases a as to 1 the field generated by the power cables. At l 2008 JINST 3 P11003 f). In practice we √ cted to the same loop as 8 49 fT 26 fT 2.7 pT r target of 0.1 pT, but this is eceives the noise produced by trated at Rustrel is 0.57 pT for d by these noise spectra).If the e spectra in figure 14 for a cut-off shown that these low frequency ed the signal (so when converted is environment. It is limited by al in both SQUIDs, so the scaled rom the spectra in figure 14. With e scaling as 1/ . o note that this spectrum does not vironment at LSBB we still detect her analysis of the noise measured took the frequency spectrum of the e resolution we could achieve with 80 wo SQUIDs. In practice, at some ed in section 4.1 we calculated the e source, as the frequency spectrum are SQUID. This gives 27 fT for an hielded environment (unless limited by uencies, then we estimate we would ved shielding, as there will always be 27 fT lief that this feature, which limits the etup for a cut-off frequency of 10 Hz, 14 pT However as our target of 100 fT is 3-4 570 fT field. on assumes that the geomagnetic field n figure 16 for two events. the performance. The resolution calculated ion of the transient recorder, so the line ithout the shield) were limited by fluctuations – 19 – 0.1 pT for a 130 s measurement with 10 Hz cutoff. However ≤ Measurement time [s] Resolution (no shield) Resolution (with shield) Resolution from bare SQUID noise The resolution of the magnetometer calculated from the nois Table 4 shows the resolutions calculated using equation 11 f This is higher than our target of Figure 15 shows the noise spectrum for SQUID 4, (the one conne To conclude this section, the best resolution we have demons In theory the magnetic field noise will give an identical sign the shield in place, we deriveand a resolution a of measurement 0.57 fT time forgeomagnetic of our spectrum s 80 continues s to (the scalemeasure longest as a time 1/f resolution period at of 0.73 lower covere pT freq for a measurement time of 130 s we stress that thisfluctuations is in merely the the Earth’s magnetic bestperfect field. we magnetic can shielding We can from demonstrate estimate the80 in s th noise th measurement, spectrum which for would the increase to b 28 fT for 130 s (nois Table 4. a measurement time oflimited 80 s by the and magnetic 10 Hz environment.fluctuations cutoff. Even in in the the This Earth’s low is magnetic noisefluctuations less field en are below than due 10 to ou Hz. the magnetic We field have and not another nois are unlikely to achieve such asome low resolution thermal magnetic even with noise impro due totimes the shielding this materials. value, we believenoise it we is measured achievable. is not Thisby masking conclusi the another two noise SQUIDs source. connected A to furt the same loop supports this. SQUID 5). This isSQUID 5; significantly and higher the higher than inductance SQUID ofto the 1 fT two as SQUID the loop it reduc noise alsoscaled level r difference is between higher). the two Using SQUIDsresult, the which for is all procedure also events, describ shown and in figure 15. This is illustrated i difference spectrum should showfrequencies, the this intrinsic difference was noise limited of by the the bit t resolut increase so dramatically at low frequency,resolution confirming we our demonstrated, be is due to the external magnetic in figure 15 is only an upper limit. However it is encouraging t from the bare SQUID noise is what we could achieve in a better s frequency of 10Hz. The magnetometerresolutionsin (with the and w geomagnetic field, so these figures are an upperother limit factors). on 2008 JINST 3 P11003 with SQUID 5 to the pick-up e can achieve this. e aim to achieve a resolution of ke the calibration coil, which is d to the heater, the input circuit al conducting state. This setup . SQUID 4 mirrors SQUID 3 to Ds when one loop changed from lot of work to do to achieve this, re the magnitude of this signal as ick-up loops to be heated into their ution of 27 fT (same measurement d the resistor, which was held in a fferent each time the heater current esigned to work when submerged in e noise level equal to that measured nnected to the same pick-up loop did nt is illustrated in figure 17. troyed. In our March tests at LSBB we fference between the two samples (calculated er gain. This gives rise to the apparent jump gher frequency datasets, which were recorded he lowest spectrum, the quantization noise of the he data points are not smoothed so the boundaries resistor, on the twisted wire pair between SQUID – 20 – Ω The noise spectrum for SQUID 4 (which was connected in series These results were obtained with a small prototype system. W When the heater is turned on it creates a magnetic field just li using the procedure described in sectionbetween 4.1). different In datasets this are plot visible. t Attransient the recorder upper dominated. end of t This is not the caseat for 12Hz. the hi of the scaled difference between the signals in two SQUIDs co loop); together with the frequency spectrum of the scaled di with the filter preceding the transient recorder set to a high not rise so rapidly atwith low a frequencies. SQUID with If shorted we input,time could then and we achieve cutoff). could th achieve a resol better than 0.1 pT inbut the the final results cryoEDM we system. have obtained There at is LSBB still mean we a are confident6. w Heating pick-up loops to destroy The cryoEDM SQUID system has heaters installedconducting to allow state the so p the circulatinginstalled supercurrent a can be similar des heater, consisting of a 390 Figure 15. 3 and its pick-up loop.plastic guide The of twisted 5 wire mm diameter, pair filledliquid was with or coiled Stycast. superfluid aroun This helium. was d Whenof a this current SQUID of was 10 mAallowed driven us was from to applie its study superconducting thesuperconducting change to to in its conducting. the The norm signals result on of this the experime other SQUI picked up by all threeit SQUIDs. exceeds Unfortunately the we slew rate cannot of measu thewas SQUIDs changed, (the so voltage jump it was appears di the SQUIDs were losing flux quanta) 2008 JINST 3 P11003 ion for s dominated by hand column shows the signals ting of the wires around the conducting due to the crosstalk ant the magnetic field generated ghtly, but only wrapped only once t remains superconducting. SQUID ective, and a current of over 50 mA quired a voltage of 50 V, and as the this point (discussed in section 4.1). led difference between these two signals. tra experiments with the heater were – 21 – s timebase, the bottom row for 10ms timebase. The bit resolut µ 0.01V, so the Fourier spectrum of the bottom right hand plot i ∼ Two of the events used to plot the spectra in figure 15. The left One problem with the heater design is that the imperfect twis In the setup installed in the cryoEDM experiment, we did not w an extent as the two pick-up loops are on the same former, but i quantization noise at the higher frequencies. 5 also shows a small shiftbetween at channels. the point at which SQUID 3 becomes heater, leads to the unwanted pick-upIn of the the September magnetic setup field at thearound twisted the wire resistor. pair However was thermalwas coupling not needed was coiled to not ti heat so the eff high loop voltage into power its supply conducting generated state.done additional during noise, This the no September re run. ex the transient record was Figure 16. measuredby SQUIDs 4 and5. TheThe right top handcolumnshows row is the for sca an event with a 1 2008 JINST 3 P11003 is shows the 50 Hz signal ssfully shielded the wires so we he status of the heater and calibration ter. This is shown in figure 18. heater was fixed to the twisted wire m this ran through a superconducting ned on, and then off. The jump at t=0 is e this point, showing that the lead shield er on the twisted wire pair for one SQUID mes conducting. The other SQUIDs record n, and 0.45s later the input to SQUID 3 became y the heater current. – 22 – The performance of the heater on the cryoEDM SQUID system. Th The output signal of three SQUIDs (offset for clarity), and t measured by five SQUIDs. At 0.086swas increased. the current At supply 0.098s tothis a the heat change input of circuit state, ofaround this but the SQUID there heater beco is successfully shields no the signal field generated on b any SQUID befor by the heater to interferepair with inside measurements. a small Therefore lead cylinder, the shield and made the from wire solder pair wire todid and with not fro pick the up flux any removed. signal when This a succe current was applied to the hea Figure 18. Figure 17. currents as the heater ondue the to the loop calibration connected coil. to Atnon-superconducting. t=1.9s SQUID 3 the was heater was tur turned o 2008 JINST 3 P11003 (6.1) ibration coil, as the heater on ration signal for SQUID 4, UID 3 becomes conducting, an write the change in the flux e prototype cryoEDM SQUID then off. When SQUID 3 is su- . consisted of several parallel pick- onitored the geomagnetic field. ter we have constructed and tested er correcting the effects of SQUID teraction between pick-up loops. in loop 4 must increase so the total 34 ation coil was installed and one loop into its normal conducting state. We the crosstalk between the two loops. 4 M I . We have achieved this objective; and ed for clarity. 3 techniques to extract a useful magnetic I signal, we compare our measured signal + T L 4 I + ext – 23 – ∆Φ = 0 disappears, the current = 3 4 I ∆Φ The voltage step in SQUID 4 due to the 266pT signal from the cal Figure 19 shows the height of the voltage step due to the calib The primary purpose of these tests was to demonstrate that th SQUID 3 was turned on, then off. The data points have been binn plotted against time, as theperconducting, heater SQUID for 4 shows SQUID an 3this average was increases jump turned to of 3.2 on, V. 2.5 This V.Loops When can 3 SQ be and explained 4 by are wound considering through on loop the 4 same as former; from equation 4, we c When the supercurrent in loop 3, Figure 19. system would function as intended inin a suitable addition, environment the project provided anfield opportunity signal to from develop the SQUID output voltage; and to study the in with data from geomagnetic observatories to confirm we have m flux through the superconducting loop remains constant. 7. Conclusions In this paper we have described aduring multi-loop two SQUID field magnetome trips toloops LSBB connected in to Provence, the France. SQUIDs by This twistedhad setup wire a pairs. heater A installed calibr on thehave wire monitored the pair SQUID to output allow for it upresets to to and be twelve crosstalk heated hours between and the aft loops, and calibrating the 2008 JINST 3 P11003 , , , eutron f the Neutron n EDM experiment uccessfully tested at the software algorithm. The isted-wire-pair between the .57 pT over an 80 s measure- o thank Jim Grozier for numer- gest we can achieve the desired mal conducting state (even when ic observatories, so we conclude g one signal to match another and n that the frequency spectrum of through PPARC and the EC IHP gnetic noise. better shielding. spitality; and allowing us to record ted pair in lead. ank the geomagnetic observatories Experiments Report te have been limited by the electro- than the 0.1 pT resolution we aim to sed the signal in a parallel loop by ked-up by other loops; and we have nduced on other channels when one UIDs connected to the same pick-up cts. A suitably small calibration coil caled difference correlates well with our signals to give a final value in nT. rnal geomagnetic field. Our measure- er study the coupling between pick-up d component. rtment, – 24 – (1998) 381. A 404 (2006) 131801. (1999) 904. Improved Experimental Limit on the Electric Dipole Moment o New Experimental Limit on the Electric Dipole Moment of the N 97 82 Performance of an atomic mercury magnetometer in the neutro , pg. 1–5, RAL-TR-2003-006. Nucl. Instrum. Meth. Phys. Rev. Lett. Phys. Rev. Lett. 2002 A SQUID magnetometer for cryoEDM has now been installed and s Finally we have shown how a suitably designed heater on the tw We have measured a noise level equivalent to a resolution of 0 We have examined the variation between channels by rescalin We have shown how SQUID resets can be corrected using a simple [1] Rutherford Appleton Laboratory, Particle Physics Depa [2] P.G. Harris et al., [4] K. Green et al., [3] C.A. Baker et al., ILL in Grenoble, however as expected,magnetic all environment. measurements We to are da working on improving this with Acknowledgments We would like to thank everyone atthe LSBB signal for from their excellent their ho magnetometermentioned together and with INTERMAGNET for ours. publishing their Weical th data. modelling We of als thenetwork calibration HPRN-CT-2002-00322 on coil. Applied Cryodetectors. This work was funded References ment time and 10 Hz cut offachieve frequency. in Although the this cryoEDM is apparatus, higher thisments is of limited by the the noise exte ofperformance a SQUID with with improved no magnetic pick-up shielding.the loop scaled attached We difference sug between have the show signalsloop measured by does two not SQ increase so much at low frequencies as theSQUID geoma and pick-up loop can beused used in to liquid heat or superfluid the helium). wireloops into This and allowed its us nor show to that furth heating28%. one The superconducting heater current loop itself increa shown generated how a this field can which be was avoided pic by shielding the heater and twis then taking the difference betweenthe these North-South two component signals. of the Thisthe field s difference measured is mainly by due geomagnet to pick-up of the horizontal fiel crosstalk between channels can beSQUID measured resets, from and the these signal figures i cancan be be used modelled to as a correct magnetic its dipole effe allowing us to calibrate 2008 JINST 3 P11003 , , , He 4 . SQUIDS as First oise Environment (1986) 667. Elimination of 26 in superfluid , (2008) 31. Cryogenics , 61 eters c dipole moment oper and S. Lamoreaux, noise underground laboratory (LSBB) of ndu_pt_2.pdf. labdjian and Ph. Le Thiec, Power Control and Intelligent Motion Ulfrnan and J. Flokstra, , Wiley-VCH, Berlin (2004) Phys. Today , in , (2000) 336. nch). – 25 – European Physical Journal of Applied Physics A 444 Integrated dc SQUID magnetometer with a high slew rate (1999) 3696. 9 The SQUID Handbook , to be published in (1984) 952. Nucl. Instrum. Meth. Seismo-Ionosphere Detection by Underground SQUID in Low-N , 55 (2003) 67. Experimental measurement of ultracold neutron production Inductance calculation techniques A 308 Magnetic monitoring of earth and space IEEE Trans. Appl. Supercond. Rev. Sci. Instrum. Phys. Lett. Rustrel-Pays d’Apt characterization of the ultra-shielded chamber in the low- [ISBN: 3527404082]. flux-transformer crosstalk in multichannel SQUID magnetom December 1999, http://www.classictesla.com/download/i in LSBB-Rustrel, France detectors in a new experiment to measure the neutron electri [9] G. Waysand et al., [5] C.A. Baker et al., [8] G. Waysand, D. Bloyet, J.P. Bongiraud, J.I. Collar, C. Do [7] F. Wellstood, C. Heiden and J. Clarke, [6] M.A. Espy, A. Matlashov, R.H.Jr. Kraus, P. Ruminer, M. Co [15] http://lsbb.unice.fr/caracteristiques.htm (in Fre [11] H.J.M. ter Brake, H.J.M. ter Brakea, F.H. Fleuren, J.A. [10] J. Clarke and A.I. Braginski, [13] INTERMAGNET, http://www.intermagnet.org. [14] J.J. Love, [12] M.T. Thompson,