Absolute Measurement of theT3(p, n)He3Reaction Threshold Energy and the Neutron-Hydrogen Mass Difference

The threshold energy of the ${\mathrm{T}}^3(p, n){\mathrm{He}}^3$ reaction has been precisely measured with a 2-meter radius electrostatic analyzer, calibrated by absolute methods. A clean tritiated zirconium target, protected from vacuum-system contaminants, was bombarded by protons having energy inhomogeneities as low as 0.02%. The result, obtained by an extrapolation of ${\left(\mathrm{net}\mathrm{neutron}\mathrm{counts}\right)}^{\frac{2}{3}}$, gives a threshold value of 1019.7±0.5 kev, a reaction $Q$ value of -764±0.4 kev, and a neutron-hydrogen mass difference of 782.9±0.4 kev.