A precise value for the ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ mass excess has been obtained by comparison of the $Q$ values for the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(d,{}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^6{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}(d,{}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^{18}{}_{}{}^{}\mathrm{O}_{}^{*}{}_{}{}^{}$ reactions, the latter populating the first excited state of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. The measurements were performed at 0° in a magnetic spectrograph. The result for the ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ mass excess is (17593.7 ± 1.1) keV, about 3 keV less than the previous value, and 3 times more accurate. The effect on the $A=8\mathrm{}$ isobaric quintet and on the $\beta$ decay of ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ is discussed.

[NUCLEAR REACTIONS ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(d,{}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^6{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}(d,{}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. $E_d=20.8\mathrm{}$ MeV, $\theta =0^{\circ }$, magnetic spectrograph. ${}_{}{}^6{}_{}{}^{}\mathrm{He}$, measured mass excess.]

• Received 4 August 1977

DOI:https://doi.org/10.1103/PhysRevC.17.4