Determining the ${}^7$Li($n$,$\gamma$) cross section via Coulomb dissociation of ${}^8$Li

The applicability of Coulomb dissociation reactions to determine the cross section for the inverse neutron capture reaction was explored using the reaction ${}^8$Li($\gamma$,$n$)${}^7$Li. A 69.5 MeV/nucleon ${}^8$Li beam was incident on a Pb target, and the outgoing neutron and ${}^7$Li nucleus were measured in coincidence. The deduced ($n$,$\gamma$) excitation function is consistent with data for the direct capture reaction ${}^7$Li($n$,$\gamma$)${}^8$Li and with low-energy effective field theory calculations.

Figure 1

Partial level scheme of ${}^8$Li and ${}^7$Li. Coulomb dissociation of ${}^8$Li is shown by the blue dashed lines and the right panel displays the spectrum for the neutron capture reaction on ${}^7$Li. Energies are given in MeV. Adapted from Ref. [17]; the $\gamma$-ray branching ratios are from Refs. [18, 19].

Figure 2

Experimental setup. The position and angle of the incoming ${}^8$Li beam were measured by two beam-tracking detectors. A timing detector in front of the target served as the start for the fragment and neutron time-of-flight measurements. Neutrons were detected around 0${}^{\circ }$ with the neutron detector array MoNA [21, 22]. The charged fragments were detected by two CRDCs ($x$,$y$), a thin ($t$, $dE$), and a thick ($E$) scintillator behind a wide-gap sweeper magnet which bent the ${}^7$Li fragment to approximately 40${}^{\circ }$. Unreacted ${}^8$Li beam particles were stopped in a beam blocker located behind the sweeper magnet at a few degrees less than 40${}^{\circ }$ [23, 24].

Figure 3

Neutron energy spectrum gated on events with a decay energy of $E_d<1.5$ MeV.

Figure 4

Sweeper magnet field map. The direction of the incident beam is in the positive $Z$ direction and horizontal deflection of the magnet is in the negative $X$ direction. A typical calculated trajectory through the magnet is shown by the red circles.

Figure 5

Energy loss in the thin scintillator versus momentum. The ${}^7$Li fragments are cleanly separated from some minor contributions of ${}^6$Li fragments.

Figure 6

${}^7$Li energy spectrum gated on events with a decay energy of $E_d<1.5$ MeV.

Figure 7

Overall solid-angle acceptance of the neutron and ${}^7$Li detectors. The circles correspond to the results of the Monte Carlo simulation and the dashed line represents a fit as described in the text.

Figure 8

Measured Coulomb dissociation function for the breakup of ${}^8$Li.

Figure 9

${}^7$Li($n$,$\gamma$)${}^8$Li excitation functions. The data from the present Coulomb dissociation measurement (solid black circles) are compared to previous direct measurements by Imhof et al. (open blue circles, [11]), Nagai et al. (solid green triangle, [13]), Heil et al. (solid red square, [15]), and Nagai et al. (open black squares, [16]). The red solid line corresponds to the inverse velocity (1/$v$) dependence of the cross section which fits the low-energy data of Blackmon et al. [14].

Figure 10

${}^7$Li($n$,$\gamma$)${}^8$Li excitation functions. The nonresonant data are the same as in Fig. 9. They are compared to calculations by Wang et al. (red dotted line, [41]), Huang et al. (green long-dashed line, [42]), and Rupak and Higa (blue short-dashed line, [43]). The best fit to the data is shown by the black solid curve.