Initial Performance of the TUCAN Magnetically Shielded Room

Abstract

The TRIUMF Ultracold Advanced Neutron (TUCAN) collaboration has commissioned a large magnetically shielded room to be used for measuring the neutron electric dipole moment (nEDM) to a precision of $10^{-27}e\mathrm{cm}$. The room is composed of five layers of MuMetal and one layer of copper and sits within the $\lesssim 370\hat{I}¼\mathrm{T}$ ambient field produced by the TRIUMF cyclotron. Within this environment, the quasi-static shielding factor was measured to be $3.25(2)\times 10^4$ at $0.01\mathrm{Hz}$ with an external peak-to-peak perturbation of $2\hat{I}¼\mathrm{T}$. Without the large ambient cyclotron field, the shielding factor improves to $3.75(4)\times 10^4$ at the same perturbation amplitude and frequency. After idealization in the cyclotron field, the residual field at the room center was $B=1.8(2)\mathrm{nT}$ and the vertical first-order gradient across the central 1~m${}^3$ ($d{B}_{\mathrm{z}}/dz$) was $-279(64)pT/m$. With additional improvement to the idealization, and with active compensation, we expect the room to be adequate for a $10^{-27}e\mathrm{cm}$ nEDM search.