The 2004 NPDGamma Commissioning Run-Measurement of Parity-Violating Gamma-Ray Asymmetries in Neutron Capture on Al, Cu, Cl, In, and B

摘要

The NPDGamma experiment will measure with a high precision, 5×10–9, the small parity-violating gamma-ray asymmetry, A, in polarized cold neutron capture in a para-hydrogen target to determine unambiguously the weak pion-nucleon coupling constant H. For the experiment the collaboration has built a new high-flux pulsed cold neutron beam line at LANSCE. In 2004, we first commissioned the beam line and then the apparatus with exception of the hydrogen target. The sensitivity of the apparatus was tested by measuring A on Al, B, Cl, Cu, and In. The Cl has a well-known large parity-violating gamma-ray asymmetry that was used to verify the performance of the apparatus. The other nuclei that were studied during the commissioning run are present in materials used for construction of the experiment and are, therefore, possible sources of the false asymmetries since backgrounds are expected to be about 10% of the signal from the neutron capture on hydrogen. We measured A0 for these nuclei except for Cl. We report the status of the experiment and preliminary results of the 2004 commissioning run.