S.R. Elliott, R.G.H. Robertson, T.D. Steiger, D.I. Will and J.F. Wilkerson
The emiT experiment is a search for a violation of time-reversal (T) invariance in the beta decay of free neutrons. The experiment will utilize a beam of cold (<10 meV), polarized neutrons from the Cold Neutron Research Facility at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. A sizable team of scientists has been assembled to perform this experiment from Los Alamos National Laboratory, NIST, the University of California at Berkeley/Lawrence Berkeley National Laboratory, the University of Michigan, the University of Notre Dame, and the University of Washington's Nuclear Physics Laboratory (NPL).
emiT will probe the T-odd triple correlation (between the neutron spin and the momenta of the neutrino and electron decay products) in the neutron beta-decay distribution.1 The coefficient of this correlation, D, will be measured by detecting decay electrons in coincidence with recoil protons while controlling the neutron polarization. Technological advances in neutron polarization and proton detection---in addition to an improved detector geometry---will allow emiT to attain a sensitivity to D of 3 ×10-4. This level of sensitivity represents a factor of five improvement over previous neutron T tests, and may permit restrictions to be placed on several extensions to the Standard Model that allow values of D near 10-3.
emiT will be the first neutron T test to make use of a 'supermirror' neutron polarizer. Thus, emiT will achieve polarizations of > 95%, as opposed to the 65--85% polarizations typical in previous experiments. The emiT detector consists of four plastic scintillator paddles for electron detection and four arrays of large-area PIN diodes to detect the protons. The PIN diodes have been extensively tested and shown to be efficient and economical detectors of low-energy (~ 30 keV) protons.2 The eight detector segments are arranged in an alternating octagonal array about the neutron beam so that each segment of one type lies at an angle of 135° relative to two segments of the other type. This geometry takes advantage of the fact that the electron--proton angular distribution is strongly peaked due to the disparate masses of the decay products. When compared to the 90° geometry used in previous experiments, this octagonal geometry results in an increase in signal rate which is the equivalent of a factor of seven increase in neutron beam flux.
During 1995 the emiT experiment was in an intensive building mode. Most of the major systems were constructed, assembled, and tested. The primary responsibilities of the NPL team include delivery of required cryogenics and vacuum systems as well as the production of the proton detector segments---including data acquisition electronics. The cryogenics and vacuum systems are complete and the PIN diodes are in hand. The electronics to run these diodes consist of custom-made precision on-board preamplifiers and a custom VME-based data acquisition board (see Section 1.10). These electronics have been designed and built and are currently undergoing final testing. It is expected that data collection will commence in the fall of 1996.