Suppression of the isospin-½, three-body photodisintegration ofHe3

The suppression of the isospin-½, three-body photodisintegration of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is shown to result from two facts: (1) For the $S=\frac{1}{2}$, $I=\frac{1}{2}$ states of three nucleons, the spin-isospin symmetries lead to integral equations dominated by a single effective interaction $V^{+}$, which is the average of the $\mathrm{NN}$ $s$-wave spin-singlet and spin-triplet interactions and (2) This $V^{+}$ supports a bound state due to the nature of the $\mathrm{NN}$ effective-range parameters. In the approximation that the (½,½) states are generated from $V^{+}$ alone, it is clear that a fraction of the isospin-½ bremsstrahlung-weighted sum rule must come from two-body photodisintegration with a corresponding reduction of the three-body channel contribution, since the total isospin-½ sum rule is fixed. The substantial reduction of the actual physical isospin-½, three-body photodisintegration is directly related to the strength of $V^{+}$.