Energy dependence of11Li dissociation cross section

Abstract

The Coulomb breakup cross section of l~Li is calculated as a function of its bombarding energy. Comparison is made to cross sections at 790MeV/nucleon and 30 MeV/'nucleon. Low energy reactions on a high-Z target show a greatly enhanced Coulomb breakup cross section that is more sensitive to the distribution of dipole response strength than high energy reactions thus providing more structure information. PACS: 21.60.-n; 25.70.NpThe structure of the neutron-rich nucleus 11Li has been the subject of a great amount of recent discussion. Kobayashi et al. [13 have measured the breakup and total reaction cross sections for 800 MeV/nucleon 11Li on a variety of targets. More recently, Anne et al. [2] have also measured these cross sections at a lower beam energy of 30 MeV/nucleon. On a low-Z target (where the Coulomb effects are negligible), the unusually large total reaction cross section is a clear signal of an extended density distribution in 11Li. This is the so called "neutron halo". On a high-Z target, Coulomb excitation also contributes to the cross sections. The Coulomb breakup cross section can be directly related to the electric dipole response of l~Li [3, 43, and this can give much useful information on the nuclear structure.The two-neutron separation energy for ~Li is $2, = 0.25_+ 0.I0 MeV. This very weak binding allows these two valence neutrons to be found at large distances from the more tightly bound 9El core, thus forming the neutron halo. Furthermore, l~ is unbound by 0.80+ 0.25 MeV. Thus, the interaction between the two valence neutrons is as important as the mean field potential for the stability of ~ ~Li. Because of this, one expects a correlation to exist in the wave function of these valence neutrons. To examine the effect of this correlation, one must go beyond a mean field description. Cluster models [5][6][7] which consider a two body problem consisting of 9El core plus dineutron and three body models [-8, 9] which treat the 9Li + n + n system have been proposed.In this report, we will not focus too closely on specific structure models, but instead concentrate on what information can be extracted from the type of cross section measurements that have been made. Let us first consider the nuclear contribution to the cross sections. For reactions of ~lLi at 790 MeV/nucleon, we have previously developed a diffractive eikonal model [10] to calculate the nuclear contribution to the total reaction and 9Li + 2n breakup cross sections. It uses a purely imaginary eikonal phase determined from the target density and nucleon-nucleon cross section to calculate the absorption of the projectile wave function after passing the target at a given impact parameter. For reactions on a t2C target, where the Coulomb contributions are negligible, it does an excellent job of reproducing the experimental cross sections. Now we wish to consider the much lower beam energy of 30 MeV/nucleon. At this energy, one must allow for a real part of the eikonal phase in addition to the imaginary part. Consider a nucleon in an initial...

Electromagnetic excitation ofLi11

“…[31] does not include a scaling term for Coulomb excitation. Adding an extracted Coulomb dissociation cross section of 3.8+0.8 b [32] yields a total reaction cross section o. t,t = 9.5+1.1 b, in good agreement with our result. A somewhat less quantitative comparison can also be made to a result of Blank et aL [33], which yielded 7.23 6 0.78 b for iiLi + Pb at an average iiLi energy of 70 MeV/nucleon.…”

“…This value is listed in Table I. The magnitude of o, determined here is consistent with cr, determined from the 1n-Li data (cr, =3.2+0.6 b) and from cr, determined from the difference o,=cr2"cr"« --3.9 + 0.3 b, where cr2"=5.1+0.3 b was measured with the telescope, and cr"« --1.2 b from a calculation [32]. The statistical accuracy of the decay energy spectrum shown in Fig.…”

“…The quantity o ", was calculated in Ref. [32] and o', was determined from the difFerence between cr2" and cpu, . The events shown are due to Li breakup in the target, Si AE detectors, and CsI.…”

Importance of nuclear effects in the dissociation ofHe6andLiBalamuth1, Griffioen2, Bush3 et al. 1994Phys. Rev. Lett.172100View full textAdd to dashboardCite

“…Furthermore, calculations for ttLi at 800 MeV/nucleon suggest a significant nuclear contribution to the breakup cross section [9]. At energies between 30 and 100 MeV/nucleon, where much recent work has been done and where the breakup cross section is more sensitive to the distribution of E1 strength [10], the relative importance of Coulomb and nuclear processes re-quires further investigation.…”

Coulomb dissociation ofLi11

“…The energy at the center of the Pb target was 28 MeV/nucleon. At this energy, Coulomb dissociation on Pb is known to dominate over nuclear dissociation due to the large number of low-energy virtual photons [3,13,14]. Also, the coincidence requirement of two neutrons at small angles diminishes the nuclear contribution [12].…”