Neutron-Hydrogen Mass Difference from theT3(p,n)He3Reaction Threshold

The threshold for the ${\mathrm{T}}^3\mathrm{}(p,n)\mathrm{}{\mathrm{He}}^3$ reaction has been carefully measured using protons from the Los Alamos electrostatic generator and a thick target of tritium absorbed in a zirconium disk. The neutrons were observed at 0°, and the proton energy was measured relative to the accurately measured ${\mathrm{Al}}^{27}(p,\gamma ){\mathrm{Si}}^{28}$ resonance at 993.3 kev. The threshold for neutrons is found to be at a proton energy of 1019±1 kev, giving for this reaction $Q=-764\mathrm{}\pm 1$ kev. Assuming the rest mass of the neutrino to be zero, and the maximum energy of the $\beta$-particle from the decay of tritium to be 18.5 kev, 782±2 kev is obtained for the neutron-hydrogen mass difference.