Neutron-fragment coincidence measurements inN14+Ho andN14+Ni reactions at 35 MeV/nucleon

We have studied ${}_{}{}^{14}{}_{}{}^{}$N${+\mathrm{}}^{165}$Ho and ${}_{}{}^{14}{}_{}{}^{}$N+Ni collisions at 35 MeV/nucleon by detecting neutrons at ten angles from 10° to 160° in coincidence with light fragments at six angles between 7° and 30°. For a given fragment, the seven spectra of coincident neutrons whose angles are not near the fragment angle were fitted with two moving, thermal sources. One source is a slowly-moving, targetlike source; the other is a faster (approximately half-beam-velocity), hotter source. The latter source, an intermediate-rapidity source, reproduces a left-right asymmetry in which the fragment-neutron cross sections are larger for emission to opposite sides of the beam axis than for emission to the same side. The temperature of the targetlike source is about 3.5 MeV with the Ni target and about 2.5 MeV with the Ho target. The corresponding values for the intermediate-rapidity source are 10 and 8.5 MeV. These source temperatures are approximately independent of fragment angle, but the measured neutron multiplicities increase with fragment angle. For the targetlike source the largest multiplicities, 1.6 for Ni and 9 for Ho, are about (3/4) the values computed with a statistical code. The multiplicities of the intermediate-rapidity source are about (1/4) the multiplicities of the targetlike source. The necessity for an intermediate-rapidity source is also demonstrated in terms of ‘‘missing momentum’’ in the final state of an assumed two-body, ${}_{}{}^{14}{}_{}{}^{}$N+target inelastic collision. The mass number of this source is estimated in peripheral collisions and in central collisions. The approximate independence of source parameters on fragment angle may indicate that the neutrons and the fragments are emitted at different times during the reaction. The neutron multiplicities of the two sources have a parallel upward trend with fragment angle, indicating that the two sources may be correlated. We speculate that a small, hot source evolves into the cooler, large targetlike source.