Recently the reactions H 2 (7r"~,y)2n and H 2 (w, p)2n were investigated with the aim of determining the neutron-neutron l S 0 scattering length, a nn . The preferred value for a nn extracted from the first reaction is 1 ' 2 -17 F, unfortunately with an uncertainty of about 5 F. Several investigations 3 " 5 of the second reaction gave values around -22 F. While the theoretical analysis of the reaction H 2 (7T~,y)2« can be performed with an accuracy of ±1 F, 8 the reactions with more than two nucleons in the final state are considerably more difficult for interpretation.In a reaction a+A-~b +n + n the spectrum of the particle b reflects the neutron-neutron final-state interaction. However, distortive effects arise from at least two sources 7 :(i) Interaction between b and the system 2w. It is reasonable to expect that this depends upon the type of the particle b and on the relative b-2n energy.(ii) The interference between the n-n and the n-b resonances 8 and the interference between resonant and nonresonant processes. 9 The study of the reaction H 3 (n,d)2n offers another possibility 4 to extract a nn . The comparison of the reaction E 2 (n f p)2n and E B (n,d)2n allows one also to investigate the influence of the various distortive effects.In order to determine kinematically such a reaction one has to measure five parameters. The measurement of the energy of the charged particle at 0° at a fixed incident energy is, thus, an incomplete measurement, and only that region of the spectrum which corresponds to the strong final-state interaction of the two neutrons is liable to meaningful interpretation.The deuteron spectra from the reaction H 3 (n, d)2n at E n = 14.4 MeV were measured with different detecting systems. The targets used were (1) a solid Cu-Ti-H 3 target containing 0.3 mg of H 3 and 10% of He 3 ; (2) two gas targets, one containing 0.1 mg and the other 1.3 mg of H 3 , with the He 3 content less than 0.4%. The charged particles were detected with a semiconductor counter telescope 10 in conjunction with multidimensional analyzers. 11 The telescope consisted of two thin AE counters and an E counter depleted to detect 11.5-MeV protons.Two pulses (from the A£ 2 and E counters) were analyzed by a lOOxlOO-channel analyzer. The resulting three-dimensional graphs gave proton, deuteron, and triton &E 2 -vs-E spectra simultaneously. 12 In a set of measurements three pulses (from the AE lf &E 29 and E counters) were analyzed by a lOOxlOOxlOO-channel analyzer. All those events for which AE X and AE 2 pulses did not satisfy specific criteria were rejected. 13

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Angular distributions of the B.e z (n,d)T> and He 3 (w,^>)T processes with 14.4-MeV neutrons have been measured using a gas He 3 target and a counter-telescope system. The differential cross sections for elastic scattering, B.e s (n,n)He 3 , have been obtained by the same technique, detecting recoil He 3 particles in the range from 0° to 40° (lab). The theoretical analysis of the data has been made within the framework of existing theories. The Legendre-polynomial-expansion curves fitted to the data are also presented.

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