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14 декабря, 2021
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The 7Li System
The 6Li(n, t) reaction is a standard neutron cross section from thermal energies to I MeV [36]. In this energy range is a well developed § resonance
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Figure 6: Differential cross section of the reaction 3H(p, n)3He calculated for Ecm = 3.0 MeV. The data are for 4.101 MeV protons from Perry. The full line represents the R-matrix analysis, the dashed one the full calculation using the Bonn potential, the dot-dashed one the small calculation, and the dotted one the semi-realistic calculation. The open circles denote the full calculation with the 3P2 matrix element replaced by the corresponding R-matrix one, for details see [38]. |
around 2.40 MeV neutron energy [45]. An RGM calculation using realistic NN-forces is no more feasible. Even a calculation using a semi-realistic potential form [26] poses a major task, due to the many possible fragmentations, like 4He-3H and 6Li-n for the standard reaction, but additional fragmentations 6Li (excited)-n, 5He-d and 5Li-(nn) [45] are necessary to reproduce the position of the § — resonance reasonably well [46, 26]. The the first task is to determine the internal wave functions of all fragments together with the excited states such that the many thresholds are reproduced reasonably well. As all effective forces share a reduced core, they tend to overbind, in case the model space is increased by using more and more width parameters or configurations with internal orbital angular momenta. Therefore any such calculation is a compromise between reproducing the size of the various fragments, the relative threshold energies and the total binding energy of the system, in this case the 7Li ground state § and first excited state , well below the 4He — 3H threshold. Increasing the model space too far, leads to overbinding and to tiny fragments, reducing the binding energy might yield too large fragments and then too strong interactions at low energies. Typ-
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Figure 7: As fig. 6, but data and R-matrix results (crosses) are compared and calculations employing AV18 in the small model space (av18), adding negative parity distortion channels (av18n), for the large model space (av18-1) and adding negative parity distortion channels (av18n-1). |
ical examples for the charge conjugate system 7Be are given in [47]. The potential is described in [26]. What can be achieved for the corresponding elastic scattering 6Li(n, n) is displayed in fig. 8, taken from [48].
It should be noted, however, that this effective potential overbinds neutron halo nuclei, like 6He and 8He [49].
To cure this problem a realistic NN interaction has to be used for the NN — P-wave configurations, see [49], whereas for the positive parities still the effective force [26] with the reduced core is taken. A calculation using this potential also in the 7Li-system is under way, see [50].