The very large n2EDM magnetically shielded room with an exceptional performance for fundamental physics measurements

Abstract

We present the magnetically shielded room (MSR) for the n2EDM experiment at the Paul Scherrer Institute, which features an interior cubic volume with each side of length 2.92 m, thus providing an accessible space of 25 m3. The MSR has 87 openings of diameter up to 220 mm for operating the experimental apparatus inside and an intermediate space between the layers for housing sensitive signal processing electronics. The characterization measurements show a remanent magnetic field in the central 1 m3 below 100 pT and a field below 600 pT in the entire inner volume, up to 4 cm to the walls. The quasi-static shielding factor at 0.01 Hz measured with a sinusoidal 2 μT peak-to-peak signal is about 100 000 in all three spatial directions and increases rapidly with frequency to reach 108 above 1 Hz.

Achieving ultra-low and -uniform residual magnetic fields in a very large magnetically shielded room for fundamental physics experiments

“…A successful degaussing of the MSR, with low field values of below for the entire inner volume of the MSR ( ), has already been demonstrated in Ref. [ 6 ]. While the residual field found there is sufficient enough to allow for a sensitive n2EDM measurement, improvements were sought due to the long duration of the degaussing procedure (3.5 h) and the long thermal relaxation time after a degaussing (12 h).…”

“…This degaussing procedure already allowed us to achieve extremely low residual field values over the inner volume of the MSR, as published in Ref. [ 6 ].

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“…This was not used in the work of Ref. [ 6 ]. Furthermore, here, the full magnetic coil system and vacuum vessel were both installed, neither of which were in place during the work of Ref.…”

“…The active magnetic shielding compensates external magnetic-field drifts, maintaining a constant field environment on the outside of the MSR of [ 7 ]. In the MSR, passive shielding materials are used to reduce the magnetic flux to the innermost volume [ 6 ]. Passive material often used for shielding have a high magnetic permeability, such as MUMETALL ® , a soft ferromagnetic alloy of nickel and iron used in the construction of this MSR.…”

“…The outside dimensions of the MSR is horizontally and vertically. To achieve a quasi-static shielding factor of at 0.01 , the MSR is composed of seven shielding layers: one aluminium layer (serving as an eddy current shield) and six soft magnetic layers (five layers of MUMETALL ® and one layer, “layer 6”, of the alloy ULTRAVAC 816 ® ) [ 6 ].…”