An Indirect Measurement of then−nInteraction

The gamma-ray spectrum from the process ${\pi }^{-}+d\to 2n+\gamma$ has been remeasured using a high resolution 110-channel pair spectrometer. The negative pions produced in an internal target by 335-Mev protons of the Berkeley 184-inch cyclotron, are captured at rest in a high-pressure deuterium vessel. Comparison of the spectrum with the gamma-ray spectrum from the process ${\pi }^{-}+p\to n+\gamma$ measured with the same apparatus shows the effect of the $n-n$ interaction as a broadening of approximately three times the resolving width. In spite of the low counting rates and resulting statistical inaccuracy, it is possible to establish limits for the scattering length "$a$" for $S$-state neutron-neutron scattering. The experimental data are analyzed by evaluating a weighted first moment of the spectrum. Comparison of the weighted first moments of the theoretical spectra with that of the data leads to a value of $a=-15.9\mathrm{}\times {10}^{-13\mathrm{}}$ cm with limits, based on the probable error, of $a=-8.5\mathrm{}\times {10}^{-13\mathrm{}}$ cm and $a=-\infty$ cm. The value $a=-15.9\mathrm{}\times {10}^{-13\mathrm{}}$ cm corresponds to an unbound state (∼160 kev positive) for the hypothetical dineutron. The bound dineutron of more than 50 kev is less than 0.1 percent probable. This result is consistent with the $n-p$ and $p-p$ scattering data and is in agreement with the theory of charge independence of nuclear forces.