Measurement of the γ-anisotropy in n→+p→→d+γ

The study of the radiative neutron capture by protons, n+p→d+γ, provides valuable information about the nucleon–nucleon interaction. So far, no experimental value has existed for the γ-anisotropy which may appear if neutrons and protons both are polarised. A non-vanishing γ-anisotropy η is a clear-cut signal for the existence of transitions 3S1→ 3d1 from the triplet initial state to the ground state of the deuteron. We report the first measurement of this observable. The result is η=(1.0±2.5)×10−4 at 50.5% polarisation of neutrons and protons.