Experiment – ORNL

The Experiment

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.

Detailed Description

First, we must create a clean magnetic field environment: a horizontal field of 30 mG with gradients of a few ppm/cm and fluctuations < 1 picoTesla. This is achieved by a combination of careful design of the experimental building, earth-field compensation coils, a double-layer magnetically shielded enclosure and the magnet module, an intricate multi-layer combination of magnetic coils, shields and boundary-condition-defining layers. A cryogenic magnetic field monitor allow us to do a high-level optimization of the fields.
Next, we must introduce a sample of spin-polarized neutrons that can be observed for a long time. The Fundamental Neutron Physics Beamline at ORNL’s Spallation Neutron Source provides the world’s highest flux of pulsed cold neutrons (E ~ few meV) for fundamental physics research. Neutrons interact optically with surfaces. As a result, those below a critical angle (i.e., having transverse energy less than the critical energy (few hundred neV)) can be reflected. This property can be used to polarize neutrons and efficiently guide them over long distances. Magnetic fields around the neutron guide preserve the neutron polarization. Neutrons with total energy less than the critical energy can be reflected at any angle and can therefore be stored in a material box, or “measurement cell” for hundreds of seconds – in principle, limited only by the free neutron lifetime. Such neutrons are known as Ultracold Neutrons, or UCNs. To get a useful UCN density we use a “superthermal” process in which neutrons with a “magic” energy (1 meV, corresponding to a wavelength of 8.9 Å) can scatter off phonons in liquid helium and lose essentially all their energy. Two high-powered dilution refrigerators, one with very strict non-magnetic requirements, are required to keep the apparatus cold.
We need a way to measure the neutron precession frequency and the magnetic field the neutrons see. By introducing a small concentration of spin-polarized Helium-3 we can do both. The n+Helium-3 absorption cross section is strongly spin-dependent. So, when the spins of the spins two species are pointed in opposite directions there will be a relatively high rate of capture events; when the spins are aligned there will be very few. So, by measuring the rate of n+Helium-3 capture events we can determine the difference between the neutron and Helium-3 precession frequencies (ω_n3 = ω_n – ω₃). Helium-3 atoms have a magnetic dipole moment and we can use a density high enough that theprecession frequency (ω₃) can be measured directly, using an array of SQUID magnetometers. Helium-3 has a negligibly small electric dipole moment and has useful temperature-dependent properties that make it an ideal co-magnetometer. We obtain the neutron precession frequency by adding these two frequencies: ω_n = ω₃ + ω_n3.
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.