Early Fundamental Neutron Experiments at ORNL – ORNL

Early Fundamental Neutron Experiments at ORNL

Following the end of World War Two, ORNL had a monopoly on powerful neutron sources. As a result, the first measurement of the neutron’s decay lifetime and the first measurement of the neutron’s electric dipole moment (EDM) were both made at ORNL under the aegis of the Physics Division.
In 1948 Arthur Snell and Leonard Miller made the first observation of neutron decays into a proton and electron at the Graphite Reactor. In 1950 Snell, along with Physics Division colleagues Frances Pleasonton and Rube McCord made the first measurement of the neutron lifetime.
In 1950 the Graphite Reactor also enabled the first measurement of the neutron EDM by James Smith, Edward Purcell and Norman Ramsey of Harvard University. Smith was a graduate student at the time, in residence at ORNL and working with Seymour Bernstein’s group in the Physics Division. This first measurement, which determined that the neutron’s electric dipole moment was less than 5×10^-20 e·cm, went unpublished for seven years due to its consistency with the then-accepted view of parity conservation.
The Harvard/ORNL collaboration held the title for the most precise measurement of the neutron EDM for another two decades as Ramsey, along with Bill Dress, Phil Miller and Jim Baird of the Physics Division, steadily improved the apparatus, identifying and eliminating systematic errors along the way. These measurements utilized beams of neutrons traversing a region with a strong transverse electric field parallel/anti-parallel to a static magnetic field. Ramsey’s separated oscillating field method was used to look for a shift in the neutron resonant frequency that depended on the strength and orientation of the electric field. This team moved their apparatus to the Institute Laue-Langevin and carried out the last measurement of this type in in the mid-1970’s, having improved the precision by over four orders of magnitude (d_n < 3×10^-24 e·cm). Measurements in the intervening decades have utilized trapped ultracold neutrons whose lower velocity was key to reducing the most severe systematic error, the Exv effect. The current precision record for the neutron EDM is held by an experiment running at the Paul Sherrer Institute, (d_n < 1.8×10^-26 e·cm).
In 1994 Golub and Lamoreaux proposed an experimental approach to measuring the neutron EDM that takes advantage of a serendipitous set of liquid helium properties (high electric field breakdown strength; phonon/roton spectrum that allows high-density in situ neutron production; compatible with polarized Helium-3, which serves as both a co-magnetometer and spin analyzer; UV scintillation following ionizing energy deposition, allowing signal detection). This experiment is the flagship effort of the Fundamental Neutron Physics Beamline at ORNL’s Spallation Neutron Source and is currently under construction with a goal sensitivity of d_n < 3×10^-28 e·cm.