Forward-backward asymmetries in the direct breakup of6Li

Abstract

The (a+d) breakup of 60 MeV 6Li scattered from 2~ has been measured inside the grazing angle for c.m. energies of the fragments between 100 keV and 1.5 MeV. The distinctly different anisotroples of the sequential and direct breakup components show that Coulomb dissociation cannot be related in a straightforward manner to the astrophysically relevant 4He(d, 7)6Li capture reaction.In 1985 it was proposed to study photodisintegration processes A(x, y z)A in the Coulomb field of a nucleus A in order to extract cross sections for the corresponding radiative capture reaction y(z,f0)x at low relative energies Ey,, which are of particular astrophysical interest [1]. An example to test the feasibility of this method is the breakup process 2~ a d)2~. The experiments have to investigate which corrections (for example those clue to the final state interaction between the fragments and the presence of a "catalyst.") must be applied to the standard Coulomb excitation theory in order to describe such breakup experiments with sufficient accuracy.Our experiment was performed by bombarding a 5.4 mg/cm 2 thick 2~ target with a 6Li beam of 60 MeV provided by the Heidelberg boosted MP tandem. The experimental setup is shown in Fig. 1. Coincident a-d events were measured by two semiconductor telescopes mounted in horizontal geometry so that the beam and the trajectories of a particle and deuteron shared roughly The AEI-zXE2-E telescope for the deuterons (T2) had two 250 #m thick crossed strip detectors $1,2 as AE counters, and a 1.5 mm E-detector combining good angular resolution (A0=0.5 ~ ) with a large total solid angle of 8 msr. It was protected against the huge number of elastically scattered ~ Li projectiles by an aluminium foil 500 #m thick. In the lab system, our detector arrangement covered relative angles between a particle and deuteron of 2.5 ~ _< 0=a _< 8 ~ The average center-of-mass angle of the decaying 6Li* projectile thus amounts to about 15 ~ which is well below the grazing angle, i.e., in a regime where Coulomb excitation has been proven to dominate [2].Figs. 2aand2b show for 0aa = (3.5• ~ and (7z~1) ~ the deduced 47r integrated a-d breakup cross sections and their statistical errors versus the relative energy E~, that is the sum-energy of a particle and deuteron in their center-of-mass system. This relative energy Ecm ad was calculated from the measured a energy Ea, the beam energy and the relative angle 0~d , asuming twobody kinematics for excitation and decay. There are two kinematical solutions for fixed relative angle 0=a and fixed relative energy Ecm which are associated czd with different values of E~. Events with higher E~, corresponding to forward emission of the a particle, are plotted in the right part of the figures; emission of the a particle in backward direction leads to lower values of E=, and those events are plotted in the left part with reflected Ecr~ axis. The 47r integration of the measured ad coincidence cross sections assumed isotropic decay in the center-of-mass system of the excited ...

Observation of nonresonant coulomb break-up of 156 MeV6Li projectiles

“…The asymmetry is defined by the left-rightdifference normalized to the sum of the triple differential cross sections. Evidently we do not see any conspicuous asymmetry like that reported in [6]. Moreover the data are consistent with a Coulomb excitation mechanism followed by a E2 decay mode leading to no asymmetry in the considered energy range.…”

“…Recently a Heidelberg group [6] explored the direct 6Li break-up at 60 MeV and reported conspicuous and peculiar "forward-backward" asymmetries for the nonsequential emission of the break-up fragments from the ct + d center-of-mass system. The extent of these asymmetries can be hardly explained by the Coulomb break-up theory, at least not within the standard "quasi-sequential" approach.…”

Nuclear reaction rates and primordial6Li

“…In order of decreasing low-energy S-factor, the calculations are from Coulomb-breakup measurements of Kiener et al [32], and the models of Mohr et al [25], Ryzhikh et al [26], and Typel [23]. The 53 keV upper limit is from Cecil et al [30], and all other points are from Robertson et al [24] In [31,32,33]. In this scheme, 6 Li nuclei are Rutherford-scattered off some high-Z nucleus.…”

“…However, a number of difficulties arise in treating the data [30,34] which produce additional uncertainties in this approach, mostly because contributions from the various partial waves are not the same in Coulomb breakup as they are in radiative capture. One group [33] reports anomalous angular dependence in the data. The cross sections inferred from the breakup measurements of Kiener et al [32] are significantly higher than any of the theoretical estimates, perhaps suggesting interference from the nuclear force, even at small scattering angles, or perhaps supporting a higher than anticipated low-energy cross section.…”

A complete picture of the breakup in6,7Li-induced reactions

“…The first being direct (non-resonant) breakup [8] where differential nuclear forces between the target and the projectile fragments [5,9] were believed to be the dominant contributor. The second breakup mode was sequential (resonant) breakup [10,11] which proceeds sequentially by first exciting the nuclei, via Coulomb excitation, to it continuum state which then dissociates into its cluster fragments. Further observations showed breakup triggered by nucleon transfer [12][13][14] also played an important role.…”