Design of an active magnetic field compensation system for MiniCLEAN

Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Technical Notes

Design of an active magnetic field compensation system for MiniCLEAN M. Bodmer a, F. Giuliani a,n, M. Gold a, A. Christou b, M. Batygov b a b

Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM, USA SNOLab, Sudbury, ON, Canada

a r t i c l e i n f o

abstract

Article history: Received 5 January 2012 Received in revised form 2 September 2012 Accepted 17 September 2012 Available online 26 September 2012

MiniCLEANis a single-phase noble liquid scintillator experiment designed to detect nuclear recoils due to weakly interacting massive particles hypothesized to constitute the dark matter. The principle of the detector is to monitor scintillation light resulting from ionizing radiation using 92 photomultiplier tubes surrounding a spherical target. Photomultiplier tube response is known to be affected by subGauss magnetic fields, so that the Earth’s magnetic field has a non-negligible effect on the photomultiplier tube efficiency. In this experiment, the crucial nuclear recoil energy threshold depends on the ability to detect very small amounts of scintillation light; high photomultiplier tube efficiency is critical. Therefore, the MiniCLEAN collaboration has designed active compensation coils to mitigate the Earth’s local magnetic field. Two features of the experimental environment make this situation unique: first, the underground laboratory (SNOLAB) is located in a nickel mine, so that direct measurement of the potentially distorted geomagnetic field is mandatory. Second, the close proximity of another experiment based on photomultiplier tubes (DEAP-3600) makes the compensating field outside our detector a concern. An additional complication is that MiniCLEANis surrounded by a steel water tank needed for shielding and a muon veto composed of four strings of 12 photomultipliers suspended in the water. We describe our design based on these considerations, survey data, field calculations and simulations of the photomultiplier tube response. & 2012 Elsevier B.V. All rights reserved.

Keywords: Photomultiplier tube Geomagnetic field measurement Magnetic compensation Dark matter

1. Introduction MiniCLEAN [1] is a single-phase noble liquid scintillator experiment designed to search for dark matter. The experiment employs 92 photomultiplier tubes (PMTs) to monitor the 500 kg liquid argon or neon central target. The layout is illustrated in Fig. 1. A complete description of the detector structure and status can be found in Ref. [2]. The detector will be submerged in a water shield contained inside a steel tank which acts both as passive shielding of the environmental radiation (primarily neutrons), and as an active veto. This shield is instrumented with 48 additional PMTs to detect the Cherenkov radiation from cosmic-ray muons energetic enough to penetrate the 6 km of water equivalent overburden of SNOLAB. The expected number of detected events depends exponentially on the detector threshold. The scintillation light yield (LY) measured as detected photoelectrons (PE) in MiniCLEAN with liquid argon is expected to be  6 PE=keV with a desirable nuclear recoil threshold being a few tens of keV. Thus, minimizing its energy threshold and maximizing light yield are crucial for the success of the experiment. The response of the target PMTs1 is known to be affected by magnetic

n

Corresponding author. Tel.: þ1 505 277 3604; fax: þ1 505 277 1520. E-mail addresses: [email protected], [email protected] (F. Giuliani). 1 Hamamatsu R5912-02MOD. The veto PMTs are Hamamatsu R1408.

0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.09.030

fields at the sub-gauss level (see Fig. 2). The most economical ! choice to mitigate the effect of the local geomagnetic field ( B Earth ) in MiniCLEAN is an active magnetic field compensation system.2 As SNOLAB is located at 461280 N latitude, 811110 W longitude, close to the Earth’s magnetic pole, the field is primarily vertical, so we have chosen a minimal design, consisting of three pairs of horizontal Helmholtz coils, three coils above and three below the center of the detector, attached to the outside of the shield tank’s wall. We also considered employing an additional pair of Helmholtz coils to compensate the horizontal component of the field. These additional coils are made problematic by the small clearance (  5 in:) between MiniCLEAN’s and the neighboring DEAP3600’s [3] tank. Our measurements (see below) show that the horizontal component is not nearly as uniform as the vertical. Fortunately, simulations suggest that compensating for the horizontal component is not necessary. Our conclusions are based on ! two surveys of B Earth before and after installation of the steel tank ð18 in: |  25 in: WESTEEL MODEL 1907) which will contain the water shielding, calculations of the expected field using SUPERFISH [4], and simulations of the effect on MiniCLEAN’s PMTs using the detailed MiniCLEAN simulation package RAT [5].

2 Experiments with different geometries have used passive compensation. See for example, Ref. [12].

100

M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106 y B

l’ = 23’ β

l

l = 10’

A

a

α

P

l

b

x

Fig. 1. Conceptual drawing (left) of MiniCLEAN’s layout, with the central sphere of PMTs inside the cylindrically symmetric cryostat, in turn immersed in the cylindrical water tank. Photo of the SNOLAB Cube Hall infrastructure (right). View is facing west. Water tanks for DEAP-3600 (on left) and MiniCLEAN (on right) are visible.

b P α

N

l β

a

A

B

Fig. 3. Schema of the infrastructure footprint in the Cube Hall. The open circles mark the positions measured in 2010.

than that of a nuclear recoil in the active volume. Indeed, detailed calculations (described in the simulation section below) have confirmed that the performance of the veto is not seriously degraded by the compensating field.

2. Surveys in the cube hall In both surveys, we used a hand-held, directional hall probe to measure the field components. 2.1. 2010 survey

Fig. 2. Effect of a magnetic field on the Hamamatsu R5912 PMTs of MiniCLEAN, for ! B parallel to the width (blue line with open circles) and to the height (red line with triangles) of the dynodes, for a bias voltage of 1500 V. Data kindly provided by Hamamatsu photonics. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

The goal of the surveys was to accurately measure a number of benchmark points in the key regions of interest: the inner PMT shell and the walls, where the veto PMTs will be placed. The first survey, performed in 2010, spanned the whole volume below the deck structure in the Cube Hall of SNOLAB (the  14 m side cubic cavern whose inside is shown in the righthand side of Fig. 1), including the region where the neighboring DEAP-3600 experiment will be located. As the magnetic permeability of the tank’s steel is not known, the difference between the field measured in 2011 and in 2010 was used to scale the magnetic permeability in the SUPERFISH simulations. SUPERFISH is a natively two dimensional simulation which is sufficient for three dimensions for problems such as ours with azimuthal symmetry. In the second survey (conducted in July 2011), a preliminary check showed a strong dependence of the vertical field component Bz on height at the tank’s walls near the ground, reaching a maximum of  850 mG at a height of 144 cm, while at  79 cm height Bz  350 mG. This is consistent with the fact that the tank is magnetically open at the bottom (the water being contained in a NSF-61 [6] compliant liner), making the bottom wall a magnetic pole, and shows that without compensation the veto PMTs may suffer from the effect of up to twice the field acting on the inner PMTs. Based on Fig. 2, we expect that the effect on the veto PMT efficiency may be significant. However, the signal (Cherenkov light) due to a minimum ionizing muon is considerably larger

The instruments employed in the 2010 survey were: Hall probe magnetometer3; level (for horizontal alignment); laser pointer; plumb line (for vertical alignment); Distro D2 laser range finder; cage elevator. Angles were not directly measured; rather, they were obtained a posteriori by triangulation. In each side of the hall (MiniCLEAN and DEAP-3600), two of the columns supporting the deck were chosen as reference directions, pointing the laser to the corners shown in the scheme of Fig. 3. The distance between reference columns was measured to be 2l ¼ 609:6 cm. The columns are 30.48 cm wide. Then, for every position on the MiniCLEAN side (the north side), the distances a and b from the two columns on the right of Fig. 3 were measured. For the DEAP-3600 side, the scheme was rotated by 1801 with the ‘‘A’’ column in the SE corner instead of the NW. 2.2. 2011 survey For this survey, a more precise measuring apparatus (see Fig. 4) was developed. From the first survey we learned that the alignment of the hand-held hall probe limited the precision of our results. We constructed a 17.78  12.7  5.08 cm aluminum box, which was used to mount the measuring equipment, and which was then secured on a Bogen professional tripod. The tripod allowed for both vertical and horizontal rotational degrees of freedom, and had a built-in level on the base. The rangefinder had a usable range of 0.05–60 m with an accuracy of 71.5 mm. The magnetometer is designed to measure the magnetic field along its length, with less than 4% error. The head was oriented with the aid of the two circular bubble levels shown in Fig. 4. The accuracy of the orientation is crucial due to the location of SNOLAB, near the magnetic north pole, which makes Bz dominant. 3 Integrity Design Geomagnetometer IDR-321. The offset was minimized with a zero gauss chamber.

M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

y

101

Y

~W β b

P

! Fig. 4. Tripod head employed to survey the B Earth in MiniCLEAN’s tank.

a

x

α

N

Table 1 Statistical error measurements. Each position was measured five times. Dhor and Dz are the distances to a horizontal reference point and to the floor, respectively.

Mean

s Mean

s

Bhor (mG)

Dz (m)

Bz (mG)

5.131 0.007 7.423 0.004 3.432 0.011

176.0 4.5 213.2 5.4 256.2 1.9

0.801 0.002 0.801 0.001 0.803 0.001

 291.0 5.0  310.8 3.6  333.0 2.1

Hence, a small error in vertical alignment can induce a significant error in the measured horizontal field. The vertical component of the position has a small systematic error due to the change of the position of the range finder when the head is oriented vertically, so that Bz is actually measured at a different height than the horizontal components. This systematic error was measured to be 10 cm by comparing the height of the head as measured with a metric tape to the reading of the range finder when the head is pointing towards the ground. Finally, the statistical error intrinsic to the measurement apparatus was determined off-site, in New Mexico (351 latitude), where B is not predominantly vertical, as clear from the values in Table 1, which lists the results of measurements of BEarth at three different distances from the reference point. While the fields in the three benchmark points are clearly different, the largest standard deviation, which is the error of a single measurement, was 5.4 mG.

3. Raw data and conversion to a cartesian reference frame As described in the previous section, the horizontal position was determined by triangulation, using, in the 2010 survey, the pillars of the deck. Whereas in the second measurement these pillars were unreachable to the laser range finder, this time we used the bolts of the outer tank’s stiffener rod (N) closest to the geographic north and the bolts of a stiffener rod at  901 from the N as reference points. The distance between these reference points was measured with a metric tape to be 389 cm. As shown in Fig. 5, we first determine by triangulation the coordinates of the point P in the (x,y) frame with origin in N, x-axis orthogonal to the N  W segment and pointing outwards, and y axis the direction of N  W . So, naming a and b the segments from the survey point to N and W, and a and b the angles adjacent to N  W , and to a and b respectively, the cosines of a and b can be obtained from the cosine theorem. Then, in the (x,y) reference frame, the horizontal cartesian coordinates are ( x ¼ a sin a ð1Þ y ¼ a cos a ! while the B hor components are 8 Ba cosb þ Bb cosa > > > < Bx ¼  sinða þ bÞ : B sin bBb sina > a > > : By ¼ sinða þ bÞ

ð2Þ

O

X

Fig. 5. Schema of the triangulation employed in 2011 and the natural cartesian coordinate system. The range finder measured the distances a and b, and the ! magnetometer measured the components of B along those same sides, Ba,b .

800 700 600 z (cm)

Mean

s

Dhor (m)

500 400 300 200 100 200 150 100

50 0 y (c -50 m) -100 -150 -200 -250

-300

-200

-100

0

100

200

x (cm)

Fig. 6. Positions of the points measured in the 2011 survey.

To then convert to the XY reference frame of the 2010 survey, with X from the center of MiniCLEAN to the geographic north, we used two of the four anchor points that will hold MiniCLEAN’s vacuum cryostat (which in water is buoyant) in place, using the support structure depicted in the left hand of Fig. 1. MiniCLEAN’s anchors form a square with medians oriented parallel to X and Y, and centered in the origin. The distances from the two anchors closest to N to the reference points were measured ((164.6,244) and (250.4,413.6) cm respectively), allowing to determine the coordinates of the origin of the XY system in the xy reference (  201.5, 195.5) cm, as well as the angle between the x axis and the X axis ð611560 900 Þ. The distance between the two anchors as determined with these calculations is 207.4 cm, in good agreement with the metric tape measurement of 208 cm. The resulting coordinates of the measured points are illustrated in Fig. 6. The distribution is intentionally not uniform, since the interest is focused on the regions where PMTs will be located. Since, in order to allow access into the tank, a steel sheet was removed, we also measured its position: the distances to N and  W where measured to be 500.1 and 539.9 cm respectively, yielding the coordinates ( 479.7,141.3) cm in the xy frame and ( 83.0, !  271.0) cm in the XY. The final values of the B hor components ! are calculated by rotating the B hor of Eq. (2).

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M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

4. Survey results The mean and root-mean-square (RMS) dispersion parameters of the spatial distributions of the field components measured in both surveys in MiniCLEAN’s tank region are shown in Table 2. Although ! the local B Earth in a nickel mine can be strongly distorted, it is interesting to compare our measurements with expectations from geomagnetic models, like those made available online by NOAA [7]. In our coordinate system (X,Y,Z), with X and Y defined in Section 3, and Z upwards, the International Geomagnetic Reference Field (IGRF) model ! estimates B ¼ ð165:79,29:629,533:375Þ mG in July 2011, versus ð165:094,29:505,535:379Þ mG in January 2010. As reported in ! Table 2, we measured B ¼ ð183:5 7 11:5,26:17 10:1,473:5 7 8:2Þ mG, showing a clear distortion relative to the IGRF field, while back in 2010 our measurement was ð181:6 76:6,42:0 7 7:42, 520:57 3:4Þ mG. Although the local field distortion due to the ferromagnetic ores of the mine could be significant, it should also be quasi constant, so the increase in distortion is primarily due to the tanks. The estimate of the magnetic declination based on the average field is d  81E,  181 different from that obtained from Ref. [7]. Since the PMTs which are expected to detect dark matter interactions are contained in the sphere centered 487.68 cm above ground with radius o 150 cm (the purpose of the veto being, by definition, to tag background events from relativistic charged particles like muons), the region where the magnetic compensation is most critical is precisely this sphere. The mean and RMS dispersion of the distribution for all field components in this region are reported in the last column of Table 2, showing an apparent good uniformity, at least in Bz. Nevertheless, even conservatively assuming the largest standard deviation of ! Table 1 (5.4 mG) as the typical error of a B field measurement in the SNOLAB, it is clear that the spread of the survey data is not accounted for by the statistical error. Hence, the field is not

Table 2 Summary of the two surveys’ results. The first two columns display the measured mean (with its statistical error) and statistical RMS dispersion of the field components in the two surveys, while the third column reports the same values for the 2011 survey restricted to the region of interest (inner PMTs).

/Bx S (mG) sx (mG) /By S (mG) sy (mG) /Bz S (mG) sz (mG)

2010

2011

2011 inner

181.6 7 6.6 23.82  42.0 7 7.4 26.75  520.5 7 3.4 12.16

183.5 7 11.5 101.6  26.1 7 10.1 88.9  473.5 7 8.2 72.58

203.0 75.0 29.49  19.5 74.5 26.43  464.9 71.6 9.56

uniform, even in the inner region, where the RMS dispersion of Bz is almost twice the error measured in New Mexico, but those of Bx and By 5–6 times larger than the measurement error. The dispersions in the second data column of Table 2, however, are over three times larger than those of the inner region, confirming the expectation that near the walls and the outer lid (at the time of the measurement, only the outer section of the tank’s lid was mounted) the field can become quite non-uniform. This circum! stance will limit the B compensation for the veto PMTs. As noted earlier, a magnetic compensation for the veto is not critical for MiniCLEAN. The strong non-uniformity near the tank’s wall is displayed in a more detailed way by Fig. 7, and agrees with the quantitative RMS dispersions of Table 2. In fact, the RMS disper! sions of the distributions of each B component in the inner region, where the distortion due to the tank’s steel is minimum, agree with the 2010 measurements without tank, so the increased dispersions of the second column measure the distortions at the walls and edges. This is visualized by the direct comparison of the left and right panels of Fig. 7, which show that near the walls and the outer lid the deviations from average can reach 40–60%, vsversus 4–5% in the inner PMT region. This requires a careful investigation of the effect on the veto PMTs, both with and without compensation. ! Based on the relatively small non-uniformity of B Earth in the region of the inner PMTs, the effect on the compensating field calculated before the 2011 survey should be small. Assuming the ! B with no tank was essentially uniform, and neglecting the small non-uniformity of Bz in the sphere centered at 487.68 cm height and 150 cm in radius we estimate the central Bz field was only reduced by a factor of ð464:9 7 1:6Þ=ð520:5 7 3:4Þ ¼ 0:893 70:007.

5. SUPERFISH calculations The survey data were employed, in combination with the measurements without the tanks performed in 2010, to check and refine SUPERFISH simulations of the magnetic field and estimate the magnetic permeability m of the tank’s steel. As the ! main component of B in the Cube Hall is vertical, and the main effect on the PMTs due to Bz, these simulations neglected the horizontal component of  200 mG, which, based on Fig. 2, reduces the PMT performance by less than 10%, while the intank Bz exceeds the scale of Fig. 2, producing a 20% effect or higher. The Earth’s Bz was simulated by using a current-carrying solenoid on the edges of the problem region (500 rz r 1500 cm and 0 rr r 725 cm, with r measured from the tank’s axis and z ¼0 corresponding to the floor). This solenoid’s current

rsph < 150 cm 0.05 0.4

0.04 0.03 Bz/ - 1

Bz/ - 1

0.2 0 -0.2

0.02 0.01 0 -0.01 -0.02

-0.4

-0.03 -0.04

-0.6 100

200

300

400 z (cm)

500

600

700

800

400

420

440

460

480

500

520

540

560

z (cm)

Fig. 7. Relative residual of Bz versus z for all points of the 2011 survey (left) and for the points in the inner PMT region (right). Note the order-of-magnitude better uniformity in the inner region.

M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

was tuned to produce Bz ¼  520.0270.01 mG without tank, and the RMS dispersion of the resulting field was, for r o 720 cm and the full z range, 0.71 mG, a mere 0.14% of the average field. For r o 300 cm, containing the region of MiniCLEAN’s tank, this dispersion reduces to 7.7 parts per million. The data measured in 2011 were then interpolated with a Biharmonic Spline method [8] and averaged over the spherical shell centered in the MiniCLEANdetector’s center, r ¼0 cm and z ¼487.68 cm, with radii 70 and 100 cm, which is our region of interest, yielding an average field /Bz S ¼ 464:35 7 14:93 mG. Fig. 8 shows both the survey data, the SUPERFISH data and their biharmonic fit. On the top the field of the central region is plotted versus the polar angle y, whereas on the bottom (near the tank wall) the field is plotted versus Z. We note that the measured errors are considerably larger near the tank wall. SUPERFISH simulations were then carried on with the tank in the geometry, and the value of the magnetic permeability of the tank’s steel was varied to match the simulated /Bz S to the one measured in the region of interest. The best match was /Bz S ¼ 465:62 7 1:21 mG, corresponding to m ¼ 320. Although during each of these simulations m was held constant, control simulations (described below) show that at the low fields of interest the nonlinearity of the steel should not have a significant effect. Fig. 9 displays the difference between the measured (Bz_exp ) and the simulated (Bz_sim ) data within the region of interest, whose average is /Bz_exp Bz_sim S ¼ 1:27 7 14:97 mG. The next set of simulations was carried out using m ¼320 and the full

103

Fig. 9. Residual Bz estimated as difference between the survey data Bz_exp and the SUPERFISH calculation Bz_sim in the region of interest plotted versus the polar angle y. In this comparison the survey data have been interpolated using a biharmonic fit.

geometry with six coils outside the tank to try to balance out the natural field. The coil current was set to various values, finding the best compensation for 66 A-turns, which produces the calculated residual field shown in Fig. 10. The average compensated field in the region of interest is /Bz_res S ¼ 1:14 75:13 mG, while the field in the region of the veto PMTs is shown by the lower panel of Fig. 10. Finally, the default mðBÞ table in SUPERFISH was scaled to match the above found m ¼ 320 for B  466 mG, and further runs were performed to confirm that the results with constant m do not appreciably differ from those with m varying as a function of B.

6. Simulation results

Fig. 8. Bz in the spherical shell centered in the MiniCLEAN detector’s center, with radii 70 and 100 cm (top) and in the region near the tank wall (bottom) defined by cylindrical radius r 4240 cm. Along with the survey data points (1) are shown SUPERFISH calculated points (x) and biharmonic fits [8] to the SUPERFISH points in gray. On the top the center field is plotted versus the polar angle y, whereas on the bottom (near the tank wall) the field is plotted versus Z. We note that the measured errors are considerably larger near the tank wall (see Table 2).

The effect on MiniCLEAN’s light yield (the total number of photoelectrons from all 92 PMTs per keV of event energy) has been then simulated with the MiniCLEANRAT simulation, which is a highly detailed simulation based on the GEANT4 [9] and ROOT [10] frameworks and GLG4sim [11]. Four simulations of 1000 b events, each of 40 keV and generated at the center of MiniCLEAN’s ! active volume, were executed. While the / B S with and without the tank are listed in Table 2 (for the case with tank, the last column was employed), the compensated field was the sum of the output of the SUPERFISH simulation and the average horizontal field with the tank in the inner region. The results are shown in Fig. 11. Although the distributions are wide (  0:4 PE/keV RMS dispersion), there is a clear 13% loss in LY ! due to B Earth . The fact that this is less than the 20% of Fig. 2 can be understood by the spherical geometry: the field component along the PMT axis does not affect the response, and the direction of such axis takes effectively all possible values. So, for PMTs close to ! the poles, Bz has a small effect, and likewise B hor has little influence on the PMTs close to the equator. We note that the LY in the absence of the compensating field is nearly identical with and without the tank as the tank’s steel provides essentially no shielding. Also, as the LY with Bz compensation is essentially the same as with no field at all, the simulations suggest that ! compensating for the horizontal components of B is not necessary. For completeness, Fig. 12 shows how Bz outside the tank is altered by the compensation coils. As expected, the influence on outside equipment, like the DEAP-3600’s detector, decreases with increasing distance from the axis of MiniCLEAN’s tank, tending to the bare Earth field. Within  5:4 m from MiniCLEANtank’s axis, where the nearest inner PMTs of DEAP-3600 will be located, the calculated coil contribution to 9Bz 9 does not exceed  80 mG.

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M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

Fig. 10. (Top) compensated Bz versus spherical polar angle in the region of interest, as simulated with SUPERFISH after including six coils carrying 66 A-turns each (black dots) and fitted with the method of Ref. [8] (grey). (Bottom) compensated Bz versus z near the tank’s wall.

B=0 no tank (2010) in tank (2011) 66 A Bz comp.

60 5.08 PE/keV 50

5.20 PE/keV

5.87 PE/keV 40

30 5.89 PE/keV

20

10

0

4

4.5

5

5.5

6

6.5

7

! average yields reported in Table 3: for B ¼ 0 we simulate 503 PEs, ! versus 445 PEs with uncompensated B , i.e. the veto retains 88% of the ideal LY. Adding the compensation, the loss is contained to 27% of the ideal yield with no field, or 6% of the yield with the natural in-tank field. Thus, the veto performance is not expected to be significantly degraded by the addition of the compensation coils, while the LY of the main detector will improve. The primary reason that the effect of the coils on the veto is not dramatic is seen in Fig. 13: SUPERFISH calculates that Bz drops below 600 mG within 10 cm from the tank wall, while the PMTs are going to be located at 30 cm from the wall itself. The RAT simulation of the veto light yield used as compensated field the sum of the SUPERFISH field and the average horizontal field measured within 40 cm from the wall in the 2011 survey.

7.5

LY (PE/keV)

! ! Fig. 11. RAT simulations of MiniCLEAN’s LY for (a) B ¼ 0, (b) the average B Earth ! measured in 2010 (no tank), (c) the average B Earth measured in 2011 (in tank) and (d) the SUPERFISH-simulated compensated field of Fig. 10.

We have also calculated the effect of the compensating field on the veto system inside the water tank close to the tank wall. We simulated 4 GeV minimum ionizing muons and calculated the

7. Conclusions ! Based on the measured B field in the Cube Hall of SNOLAB before and after installation of the steel tanks needed for the water shielding of both MiniCLEAN and DEAP-3600, it was possible to design the needed compensation system of 6 Helmholtz coils that will reduce the Bz field on MiniCLEAN’s inner PMTs to within 15 mG. We used SUPERFISH to extrapolate our measurements into the full detector volume and to estimate the effect

M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

105

Fig. 12. Simulation of the compensated Bz (top), and the coil Bz (bottom) versus the distance form MiniCLEAN’s tank axis outside the tank. The inner PMTs of DEAP-3600 nearest to MiniCLEAN’s coils will be located at 540 cm from MiniCLEAN tanks axis.

Table 3 Veto yield for m s reaching the water tank with 4 GeV.

0.2

PEs

! No B

5037 12

! Measured in-tank B

4457 11

! 66 A-turn compensated new B

418 710

0

Bz (G)

Condition

0.4

-0.2 -0.4 -0.6 -0.8 -1

of the steel water tank. We then performed detailed simulations of the detector performance using RAT and our calculated residual field. These simulations show that our compensating coils will recover 13% of the LY. This improvement in the LY lowers the energy threshold and increases the sensitivity of the experiment. The horizontal field is approximately 200 mG, but our simulations predict that we do not need to compensate this component of the field. Furthermore, our simulations show that the performance of the veto system, despite the location of veto PMTs near the edge of the water tank and close to the coils, is not significantly degraded.

240

250

260

270

280

290

R (cm) Fig. 13. Compensated Bz near MiniCLEAN’s tank wall, as calculated by SUPERFISH. Note that the y-axis is cut off and that the field extends up to 0.6 G at the tank wall.

Acknowledgments This work was supported by the U.S. Department of Energy (DOE) under award number DE-FG03-92ER40732. F. Giuliani was

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M. Bodmer et al. / Nuclear Instruments and Methods in Physics Research A 697 (2013) 99–106

partially supported by the LDRD program of Los Alamos National Laboratory.

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[6] National Sanitation Foundation’s Drinking Water System Components Standard 61 /http://www.nsf.org/business/water_distribution/standard61_over view.asp?program=WaterDistributionSysS. [7] International Geomagnetic Reference Field and World Magnetic Model /http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jspS; C.C. Finlay, et al., Geophysical Journal International 183 (2010) 1216. [8] David T. Sandwell, Geophysical Research Letters 14 (2) (1987) 139. [9] S. Agostinelli, J. Allison, K. Amako, et al., Nuclear Instruments and Methods in Physics Research Section A 506 (2003) 250; J. Allison, K. Amako, J. Apostolakis, et al., IEEE Transactions on Nuclear Science NS-53 (2006) 270. [10] For Documentation and Software, see /http://root.cern.ch/drupal/S. [11] A Generic Liquid-Scintillator Anti-Neutrino Detector Geant4 Simulation Derived from the Most General Parts of KGL4sim, a Monte Carlo Simulation Developed for KamLAND. For Documentation, see /http://neutrino.phys.ksu. edu/  GLG4sim/S. [12] F.P. An, et al., (Daya Bay) Nuclear Instruments and Methods in Physics Research Section A 685 (2012) 78.