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The reaction D (?i,2n)p has been measured at i£ inc =14.1 MeV, 9 n^ =0 n^ = 30°,

150 MeV, 1 " 3 ' 20 but it fails at E inc <100 MeV. 4 " 10 Various simple modifications have been proposed: (i) off-the-energy-shell effects, but they do not seem to be large enough to account for the discrepancy between the data and the theory 10 ' 20 * 21 ; (ii) final-state interaction (FSI) effects, which improve the fit to the data at E inc £150 MeV, 2 ' 3 * 20 but do not explain the 46-MeV data 5 ; (iii) modified simple impulse approximation (MSIA), 22 which explains the energy variation of the absolute cross section and the shape of the spectra. 6 These simple models are inadequate at low energies, 23 ' 24 and indeed they do not explain, e.g., the observed ratio D(p,pn)/D(p, 2£). 6 ' 9 ' 13 ' 14 ' 5 Neutron-neutron interaction remains a problem of considerable interest. 26 The latest analysis of the reaction D(ti~,nn)y gives a value of a nn = ~16.6 ±1.3 fm. 27 A comparison procedure 28 applied to the reactions D(p,n)2p, D(n,p)2n, 3 Re(d,t)2p, and 3 R(d, 3 He)2n gives a nn =-16.45± 1.0 fm and r m = 3.1 ±0.5 fm. This application has been criticized 10 ' 29 * 30 and these values are not reliable independent determinations of a nn and r nn .Neutron-neutron QFS was studied by bombarding a cylindrical 2-in.x2-in. C 6 D 6 target with the 14.1-MeV neutron beam defined by associated a particles. Two outgoing neutrons were detected at ^n(3) ==^M (4) = 30°>

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