π−−He4Inelastic and Capture Reactions Leading to Excited and Multineutron Final States

A beam of (140±0.5)-MeV ${\pi }^{-}$ was produced at the Berkely 184-in. cyclotron and used to study the finalstate interactions of three and four neutrons, and to look for excited levels of the $\alpha$ particle through the reactions $\{{\pi }^{-}+{}_{}{}^4{}_{}{}^{}\mathrm{He},\to {\pi }^{-}+{}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{*}{}_{}{}^{},\}\{\to p+3n,\}\{\to d+2n,\}\{\to {\pi }^{+}+4n.\mathrm{}\}$ Only one such level is found, with an excitation energy of 32 MeV and an intrinsic width smaller than our 1-MeV resolution. We find that our data on the four-neutron final state, although not inconsistent with phase space, agree more closely with the assumption that there is a ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ final-state interaction between two of the neutrons, the other two not interacting. We find, too, that deuteron production is down by a factor of $\approx {10}^3$ from proton production, and that the proton spectrum indicates a stronger-than-expected interaction between the three neutrons in the final state. Lower limits for the production of a tri- or tetraneutron are set.