Polarized neutrons of energies 2 meV -15 meV are incident on a liquid parahydrogen target. The neutrons are captured, forming Deuterium and releasing a gamma ray with an energy of 2.2 MeV (the binding energy of Deuterium), which is subsequently detected. The detector array consists of 4 rings of 12 detectors each, where each ring is concentric around the neutron beam. The polarization of the incoming neutron beam is then reversed and the resulting gamma-ray is emitted in a different direction.
The measured quantity is the difference in the number of gamma rays emitted between the two neutron spin states. This difference (divided by the sum) forms a parity-violating asymmetry, A. By studying the correlation between the spin of the incoming neutron (its polarization) and the direction of the emitted gamma ray, one primarily probes f, the long-range coupling constant used to describe the hadronic weak interaction.
The first phase of the NPDGamma experiment was carried out at Los Alamos National Lab in 2006, but did not have the statistical sensitivity to test the theoretical prediction (A~-5x10-8).