Direct Coulomb Breakup ofLi7

Abstract

The direct breakup of 70-MeV 7 Li scattered from a l20 Sn target is investigated at forward angles. Inside the grazing angle, it is found that the breakup is dominated by the Coulomb interaction between projectile and target.PACS numbers: 25.70.NpThe study of the breakup of light-ion projectiles such as 6 Li and 7 Li is of special interest since the simple cluster nature of these nuclei considerably simplifies calculations concerning the breakup process. Thus using the a-t cluster description for 7 Li in an adiabatic calculation, Thompson and Nagarajan 1 concluded that the direct breakup of 70-MeV 7 Li into the a + t channel was mainly due to the differential nuclear force between the 208 Pb target and the projectile fragments. However, a subsequent study of inelastic scattering of 68-MeV 7 Li from 208 Pb, where a similar cluster-adiabatic calculation was used, showed that the Coulomb interaction becomes increasingly important for smaller angles. 2 This therefore raises the question as to the importance of the Coulomb interaction for projectile breakup at forward scattering angles. In this Letter we present new evidence that, at forward angles, the direct breakup of 70-MeV 7 Li on 120 Sn is primarily due to the Coloumb interaction between projectile and target.The experiment was undertaken by use of the 20-MV NSF tandem accelerator at Daresbury. Since we specifically wished to study small relative energies between the emitted a and t fragments, the two solid-state detector telescopes were placed in close vertical geometry. 3 The collimators were 10 mm x 8 mm and the vertical separation between their centers was 15 mm. The collimators were placed at 115 mm from the target, except at the most forward angle of 11.5°, where they were at 150 mm. The target was isotopically pure 120 Sn of 4 mg/cm 2 thickness.The energy spectra of a particles for events where the total energy of the coincident a and t particles corresponded to the target being left in its ground state are shown in Figs. Kb)-1(d). The spectra for the more backward angles are dominated by a pair of peaks associated with the kinematic solutions for the reaction 120 Sn( 7 Li, 7 Li4. 63 -• a + /) 120 Sn gs .. The center-of-mass (cm.) energy of the a-rpair, e r , for a given a energy, is shown on the top axis. 500 400 ^300 ^200 100 0 400 ^300 D O ^200 100 0 80 £60 D O ^40 20 0 20 £15 D O <-> 10 e (MeV)2. 16 2. 16 310 ITO 0129 f. 0 3T6 a) Monte-Carlo S i mu Lat i on 120c ,7. 7. * s 120c bn ( L i . Li, ,,->a + t) bn ' 4, 63 gs E, =70MeV lab 8 =22.0° ,, 120c 7 , 7 , * . 120c b) Sn (Li. LI -»ot + t) bn E, =70MeV 6, =22. 0° c)6lab=:15. 0° 20. 0 30. 0 40. 0 50. 0 E (MeV) 60.0 FIG. 1. (a) a-energy spectrum for sequential breakup of 7 Li using a Monte Carlo simulation, (b)-(d) Experimental a-energy spectra of the reaction 120 Sn( 7 Li, 7 Li* -a + r) 120 Sn g . s . at 22°, 15°, and 11.5°.

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

“…For the 7 Li reaction, breakup into α + t is prominent, with a wide E rel distribution, indicating essentially all prompt breakup. The largest contribution to prompt breakup for 7 Li, however, is from prompt breakup of 8 Be, i.e. α+α breakup with higher E rel .…”

“…3(b, c)), the E rel spectra for the α + α breakup mode show a sharp peak at 92 keV corresponding to the slow 8 Be ground-state decay. This comprises ∼half of all the α + α yield.…”

“…As an example, E rel distributions have been calculated for breakup of 8 Be, from a nominal 1 MeV excitation energy, in the Coulomb field of 208 Pb at 29.0 MeV of energy. Fig.…”

“…However, breakup into α+p contributed to five distinct peaks in the spectrum, matching the expected Q-values for neutron stripping from the projectile and forming the unbound 5 Li, and the five identifiable energy states that 209 Pb could populate. The small α + α yield results from pick-up of a neutron and a proton, forming 8 Be which subsequently decays into two α-particles.…”

“…Two different breakup modes were identified. 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.…”