High Quality Content by WIKIPEDIA articles! NPDGamma is an ongoing effort to measure the parity-violating asymmetry in polarized cold neutron capture on parahydrogen. 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.

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