E2 component in subcoulomb breakup of 8B

Abstract

We calculate the angular distribution and total cross section of the 7 Be fragment emitted in the break up reaction of 8 B on 58 Ni and 208 Pb targets at the subCoulomb beam energy of 25.8 MeV, within the non-relativistic theory of Coulomb excitation with proper three-body kinematics. The relative contributions of the E1, E2 and M 1 multipolarities to the cross sections are determined. The E2 component makes up about 65% and 40% of the 7 Be total cross section for the 58 Ni and 208 Pb targets respectively. We find that the extraction of the astrophysical S-factor, S 17 (0), for the 7 Be(p,γ) 8 B reaction at solar energies from the measurements of the cross sections of the 7 Be fragment in the Coulomb dissociation of 8 B at sub-Coulomb energies is still not free from the uncertainties of the E2 component.KEYWORDS: Coulomb dissociation of 8 B, radiative capture of p and 7 Be, Astrophysical S-factors. PACS NO. 25.70.De, 25.40.Lw, 96.60.Kx * Work supported by EPSRC, UK, grant no. GR/K33026. † E-mail address: shyam@tnp.saha.ernet.in Recently von Schwarzenberg et al. [17] have measured the breakup of 8 B on the 58 Ni target at the beam energy of 25.8 MeV, well below the Coulomb barrier, where the E2 component of the breakup is expected to be dominant. In contrast to the experiments reported in Refs. [10,16] where 7 Be and p were measured in coincidence, these authors detect only the 7 Be fragment. In their analysis of the data, they have used the non-relativistic theory of Coulomb excitation [18] and the radiative capture cross sections of Kim, Park and Kim (KPK) [12] to estimate the E1, E2 and M1 component of the breakup cross sections. However, the final state has been approximated as a two body system by these authors. This implies that the measured angles of 7 Be were equated to those of the 7 Be-p center of mass

Breakup of 8B and the 7Be(p, γ)8B reaction

“…[15] used the Alder-Winter's semiclassical theory of Coulomb excitation, where the final state is treated as a two-body system, thus assuming that the measured angles of 7 Be were equal to those of the 7 Be-p center of mass. The inadequacy of this assumption has been demonstrated in [8]. Furthermore, the total breakup cross section reported in this experiment could not be reproduced within the Alder-Winther theory even if a wide variety of structure models of 8 B were used [21].…”

“…As discussed in [8], the angles of 7 Be can be related to those of 8 B * . A given θ 7Be gets contributions from a range of generally larger θ 8 B * .…”

“…The 8 B isotope produced in the Sun via the radiative capture reaction 7 Be(p,γ) 8 B is the principal source of the high energy neutrinos detected in the Super-Kamiokande (SK) and 37 Cl detectors [1]. In fact the calculated rate of events in SK as well as SNO detectors [3] is directly proportional to the rate of this reaction measured in the laboratory at low energies (∼ 20 keV) [3].…”

“…This makes the rate of the reaction 7 Be(p, γ) 8 B the most poorly known quantity in the entire nucleosynthesis chain leading to the formation of 8 B [4]. It may be noted that the rate of the 7 Be(p,γ) 8 B reaction is usually given in terms of the zero-energy astrophysical S-factor, S 17 (0).The Coulomb dissociation (CD) method provides an alternative indirect way to determine the cross sections for the radiative capture reactions at low energies [5,6,7,8,9]. In this procedure it is assumed that the break-up reaction a+Z → (b+x)+Z proceeds entirely via the electromagnetic interaction; the two nuclei a and Z do not interact strongly.…”

E1-E2interference in the Coulomb dissociation of8B

“…Also, it is possible to calculate very exclusive observables up to the level of the triple differential cross section. Previous calculations on the breakup of 8 B were done by assuming that the angular distribution of fragments is isotropic in the projectile rest frame [12][13][14]. This approximation gives…”