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Mapping of the magnetic field to correct systematic effects in a neutron electric dipole moment experiment

C. Abel et al.
Phys. Rev. A 106, 032808 – Published 16 September 2022
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Abstract
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Article Text
  • INTRODUCTION
  • SYSTEMATIC EFFECTS RELATED TO FIELD…
  • THE COIL SYSTEM
  • THE MAGNETIC-FIELD MAPPING
  • ANALYSIS OF A SINGLE FULL MAP
  • GLOBAL ANALYSIS
  • COMPARISON WITH SIMULATIONS
  • DISCUSSION
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    Experiments dedicated to the measurement of the electric dipole moment of the neutron require outstanding control of the magnetic-field uniformity. The neutron electric dipole moment (nEDM) experiment at the Paul Scherrer Institute uses a Hg199 co-magnetometer to precisely monitor temporal magnetic-field variations. This co-magnetometer, in the presence of field nonuniformity, is, however, responsible for the largest systematic effect of this measurement. To evaluate and correct that effect, offline measurements of the field nonuniformity were performed during mapping campaigns in 2013, 2014, and 2017. We present the results of these campaigns, and the improvement the correction of this effect brings to the neutron electric dipole moment measurement.

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    • Received 3 May 2022
    • Accepted 29 June 2022

    DOI:https://doi.org/10.1103/PhysRevA.106.032808

    ©2022 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Atomic & molecular processes in external fieldsSpin dynamics
    1. Physical Systems
    Neutrons
    1. Properties
    CP violationElectric momentT-symmetry
    1. Techniques
    Precision measurements
    Atomic, Molecular & OpticalNuclear PhysicsParticles & Fields

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    Vol. 106, Iss. 3 — September 2022

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    Images

    • Figure 1
      Figure 1

      Side view of the B0 coil (red cables) and trimcoils (green, yellow, and white cables) wound on the surface of the vacuum tank.

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    • Figure 2
      Figure 2

      Simulation of the field generated by the B0 coil and a four-layer mu-metal shield. The represented geometry is a quarter of the complete volume. The external dimensions of the fourth (outermost) layer of the shield were RMS4simu=0.98 m and HMS4simu=2.79 m. The coil's windings are represented in red. The central volume, the area of the heat map, is a cylinder of diameter 80 cm and height 50 cm, larger than the mapping volume.

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    • Figure 3
      Figure 3

      Magnetic-field mapper installed in the empty vacuum vessel. The fluxgate is inside the tube on the left, on which the helical groove used for the calibration motion can be seen. The inset illustrates the relative position of the three individual fluxgate sensor axes, which are offset from each other by 20mm in the radial direction.

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    • Figure 4
      Figure 4

      Recording of the field measured by the fluxgate every ten seconds at the center of the coil to see the drifts of the three offsets.

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    • Figure 5
      Figure 5

      Bz field for a full map of the B0 coil. The axes are defined as in Fig. 2.

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    • Figure 6
      Figure 6

      Fit of the Bz field with a Fourier series up to order m=6 for a B0 up map. In panel (a) is the fit for the ring ρ=22cm, z=6 cm. In panel (b) are the square root of mean squared residuals of all the rings. Each square corresponds to the rms residual after fitting the ring at the position (ρ,z).

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    • Figure 7
      Figure 7

      Fit of gradients Gl,0 to the Fourier coefficients a0,z for a B0 up map. The index m=0 denotes the field components without ϕ dependence, which are responsible for the “phantom” fields contributing to Ĝ. The colors represent the different values of the ring's height z, for the same fit. Each point represents the fitted a0,z of a ring. Error bars are too small to be visible.

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    • Figure 8
      Figure 8

      Principle of the global analysis of all maps.

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    • Figure 9
      Figure 9

      Histogram of the values of Ĝ and its global averages for all the B0 up (red) and down (blue) maps and the remnant field (green) maps. The reproducibility and repeatability were only calculated with the B0 maps.

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    • Figure 10
      Figure 10

      Comparison of the measured and predicted values for the maps of the nEDM sequence configurations. The green line is the first bisector y=x. The rms written in the top left corner of each plot is the mean square difference square root. (a) Comparison for the gradient Ĝ. The large dots are the average values of the gradient extracted from the analysis of the B0 maps, see Fig. 9. (b) Comparison for the transverse inhomogeneity 〈BT2〉. The 〈BT2〉∼15nT2 point in the upper right corner corresponds to the magnetic configuration of one of the first nEDM data sequences, when the uniformity optimization method [10] was not used yet.

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    • Figure 11
      Figure 11

      Predicted values of Ĝ and the corresponding corrections of dn for the 99 nEDM measurement sequences.

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