To measure the neutron electric dipole moment we must place a sample of neutrons in a uniform magnetic field and a strong electric field. The neutron spins will precess due their finite magnetic dipole moment The electric dipole moment is determined by measuring a tiny difference in the precession frequency that is linearly dependent on the electric field strength.
The statistical sensitivity of an EDM experiment depends on the electric field (E), the number of neutrons (N) and the neutron storage time (τ): σ ∝ (E√Nτ)-1. The nEDM@SNS experimental approach, conceived by Golub and Lamoreaux, takes advantage of an amazing confluence of superfluid helium properties to substantially increase E, N and τ:
A large density of ultracold neutrons (UCNs) can be produced through a superthermal process (8.9Å neutrons can be effectively stopped by scattering off excitations in superfluid Helium).
UCNs can be stored in a material bottle for times longer than their decay lifetime.
Superfluid Helium can support strong electric fields.
Furthermore, spin-polarized Helium-3 can be used as a co-magnetometer (to minimize and control systematic errors associated with non-zero magnetic field gradients), and as a spin analyzer, producing scintillation light (signal) in response to the spin-dependent n-Helium-3 capture reaction.
The change in the neutron precession frequency can also be measured using an atomic physics technique known as spin dressing. By applying an RF magnetic field perpendicular to the primary field we can force the neutrons and the Helium-3 to precess with the same frequency in the absence of an electric dipole moment – a non-zero EDM is revealed by a slow drift in the relative spin orientation, as determined by the n+Helium-3 capture rate. This method has certain statistical and systematic advantages. Having both techniques available provides a vital experimental check on the final results.
Neutron+Helium-3 capture events produce a proton and a triton with a total kinetic energy of 764 keV. This energy is deposited in the Helium bath filling the measurement cell, producing scintillation light. The measurement cell walls are coated with a deuterated compound that converts the UV light to visible wavelengths allowing its detection. A system of optical fibers transports the light to an array of silicon photomultipliers which measure the total light output.
The sensitivity of the measurement is linearly dependent on the applied electric field. To generate the required high fields we use an electrostatic induction machine known as a Cavallo high-voltage amplifier. To satisfy non-metallic requirements in the measurement cell region we have developed electrodes that are fabricated from an acrylic substrate and ion-implanted with a thin (~200 nm) conducting layer.
It takes ~ 1,000 seconds to fill the measurement cell with UCN and Helium-3, after which a measurement cycle can begin. A measurement cycle lasts ~ 1,000 seconds – both times governed by the lifetime of the neutron. After a measurement cycle the Helium-3 atoms will be partially depolarized, so they must be removed, after which the sequence is started again.
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