Sharp Backward Peak inp-He4Elastic Scattering at 1.05 GeV

Abstract

p-^Ue elastic scattering has been measured at 1.05-GeV incident proton energy, close to 180° in the center of mass. The steep backward peak observed at lower energies is still present, but its slope appears to decrease.Our previous results on elastic p-4 H.e scattering at intermediate energies and at angles close to 180° in center-of-mass system revealed the occurrence of a sharp rise in the differential cross section, which became steeper as the energy increased. 1 Various theoretical models have tried to explain this backward peak by either actual nucleon-nucleon interaction in a noneikonal scattering approach, 2 or triton exchange, 3 " 1 or addition of an exchange term in a Glauber model. 6 The data in Ref. 1 were acquired with incident a particles on a hydrogen target to take advantage of the wide coverage of the kinematics: The scattered a's are fast and confined into a cone 14.6° wide. At the higher energy, when cross sections fall to the nanobarn/steradian level, background problems associated with the emptytarget effect become important. However, a useful coverage of the interesting region close to 180° in the cm. system can still be obtained by returning to the more classical method of using a proton beam and a liquid-helium target. Indeed, around 1-GeV proton kinetic energy and above, the a's recoiling from backscattering protons have enough energy to be detected with full efficiency in our spectrometer. Besides, the higher intensity of the proton beam could offset to a certain extent the decrease in cross section. When tried, this method displayed very little background associated with the walls of the target Except for the installation of a liquid helium target, and the use of a proton beam, the apparatus was the same double-focusing spectrometer used in Ref c 1 and described in detail elsewhere. 7 The fact that the recoiling 4 He is detected ensures the elastic character of the reaction The differential cross sections in the cm c system are presented in Fig 0 1, as a function both of c°s #c* m . an< 3 °f * ne invariant quantity u -u rmxo The numerical values are listed in Table I. It is noteworthy to point out that the smallness of the background has allowed us to make a measurement at # Cem . =180°.We show in Fig 0 2 the present data with our earlier results at different energies. 1 ' 8 The results of Comparat et ah 9 at 156 MeV and of Aslanides et al. 10 at 1.05 GeV are also shown. The cross sections are given as a function of t 0 In the following, when comparing different experiments, we will use in the case of incident a particles the equivalent proton laboratory kinetic energy (i.e., giving for pa scattering the same c.m c energy as in ap case); when needed, the cm. momentum will be also given. One can note in Fig. 2 that a sharp peak exists near 180° in the cm. system except at 198 and 438 MeV. Outside the backward region, do/dt is to a large extent energy invariant, as confirmed also by the results at 788 MeV, 11 and it can be described by Glauber-like 12 ' 13 models.At the intermediate energi...

Big bang nucleosynthesis constraints on hadronically and electromagnetically decaying relic neutral particles

“…Here the 4 in the denominator comes from the factor two higher charge of 4 He nuclei compared to 3 H nuclei. Altogether it is found that energetic 4 He contributes only of the order ∼ 6% to the final 6 Li abundance This is also not changed by the existence of a sharp backward scattering peak in N -4 He scattering [74] which, similarly to the possible existence of a backward peak in the 3-nuclei distribution (see above) may increase the 6 Li synthesis due to energetic 4 He by a factor ∼ 1.2.…”

“…[64] a fraction of about 2% of all spallation reactions seem to result in 3 H nuclei at the kinematically highest allowed energy. This corresponds to backward scattering and such peaks in the distribution functions are also observed in 4 He-4 He elastic scatterings [74]. If the four-event backward scattering peak as observed in Ref.…”

“…Table I, App.E.3,E.4) and nuclear spallation reactions (Rec. [74][75][76][77][78][79][80][81][82][83] Table I, App.E.5). This is because the competing processes for thermalization, magnetic moment scattering of neutrons off plasma e ± (Rec.…”