NPDGamma Collaboration

The NPDGamma (n + p --> d + gamma) collaboration is relatively large, with 28 members at 13 institutions. Unlike my other nuclear collaborations, this experiment will not test the Standard Model. Instead, it will test the weak interaction between two nucleons (such as between a neutron and a proton). Nucleons interact primarily through the strong interaction and the weak part of the coupling is masked by the strong part. A parameterization of this interaction was proposed thirty years ago, but the coupling constants are incredibly small and difficult to measure. The neutron is a very simple nuclear system so the results of this measurement will be theoretically clean. This experiment is designed to measure fp, the most important weak nucleon-nucleon coupling constant.

The experiment will detect the small (Ag = 10-8!) asymmetry in gamma rays produced by neutron capture on protons. We send neutrons into a liquid hydrogen target, and measure the direction of the gamma rays coming out. The collaboration received funding in 1999, commissioned the experiment at Los Alamos in the spring of 2003, and will have its first data run in the fall of 2004. Unfortunately, several years of data at Los Alamos may not provide enough statistics for the experiment. The experiment may need to be moved to a source with a higher neutron flux such as HFIR at Oak Ridge National Lab in Tennessee or possibly the Spallation Neutron Source (SNS) coming on line in 2009 at Oak Ridge.

NPDGamma will be the first long timescale experiment to make use of a 3He based neutron polarizer. Since arriving at Hamilton, my primary responsibility has been NMR diagnostics for the 3He-based neutron polarizer. In the summer of 2001, Andrew Magyar, ‘03, developed the basic NMR system for the NPDGamma experiment. The DOE grant to Los Alamos for NPDGamma included money for the NMR systems, and I received a subcontract from Los Alamos for the NMR in 2002. A second, portable NMR system built by Professor Brian Collett in our department may be useful in tuning the RF spin flipper for the experiment this fall. In addition, Andrew Yue, ’04, spent the summer of 2002 at Los Alamos working on this experiment under Scott Wilburn. Andrew was inspired by his summer at a major nuclear lab and will attend graduate school at the University of Tennessee near Oak Ridge National Lab next year.