Resonant breakup of ${}^8\mathrm{Be}$ in ${}^{112}\mathrm{Sn}({}^7\mathrm{Li},{}^8\mathrm{Be}\to 2\alpha )$ reaction

Background: Reactions involving weakly bound projectiles with $\alpha +x$ cluster structure are known to produce a large number of $\alpha$ particles, where transfer-induced breakup is one of the possible origins. Detailed investigation on each of these channels is desirable to understand the underlying reaction mechanism.

Purpose: Our purpose is to measure the $+1p$ transfer-induced breakup in the ${}^{112}\mathrm{Sn}({}^7\mathrm{Li},{}^8\mathrm{Be}\to 2\alpha )$ reaction to understand the possible modes of ${}^8\mathrm{Be}$ breakup and their contribution to inclusive $\alpha$ production.

Methods: $\alpha -\alpha$ coincidence measurements have been carried out for the above reaction at $E_{\mathrm{beam}}=30$ MeV. Projectile-like fragments were detected using five sets of Si-strip-detector telescopes and five sets of single Si-detector telescopes. Optical model analysis of elastic scattering data, coupled reaction channels (CRC), and continuum discretized coupled channels (CDCC) calculations were performed to understand the measured cross sections.

Results: The experimental cross sections for $+1p$ transfer-induced breakup in (${}^7\mathrm{Li},{}^8\mathrm{Be}\to 2\alpha$) reaction through different resonance states of ${}^8\mathrm{Be}$ have been obtained. Relative energy distribution and Monte Carlo simulation confirm the observation of breakup of ${}^8\mathrm{Be}$ from its $4^{+}$ (11.35-MeV) resonant state for the first time along with its well-known $0^{+}$ (92-keV) and $2^{+}$ (3.12-MeV) resonances. Simultaneous description of elastic scattering, transfer, and breakup cross sections have been made using CDCC plus CRC formalism.

Conclusions: Production of $\alpha$ particles in $+1p$ transfer-triggered breakup is found to proceed mainly through three resonance ($0^{+},2^{+}$, and $4^{+}$) states of ${}^8\mathrm{Be}$.

Figure 1

The schematic diagram of the experimental setup showing strip detector array, single telescopes and monitors inside the scattering chamber.

Figure 2

Typical two-dimensional ($\mathrm{\Delta }E$ vs $E_{\mathrm{total}}$) energy calibrated spectrum acquired in one of the vertical strips at $\theta ={75}^{\circ }$ for a beam energy of 30 MeV. The inset shows the total coverage in $\theta$ and $\varphi$ by the strip detector array.

Figure 3

Relative energy spectra corresponding to $\alpha +\alpha$ breakup channel at $E_{\mathrm{beam}}=30$ MeV, showing the breakup proceeding through different excitations of ${}^8\mathrm{Be}$. Dotted lines in panel (c) represent Gaussian fits to the resonance states to obtain the experimental peak positions and widths.

Figure 4

Typical plots of $E_{\mathrm{rel}}$ versus $E_{\alpha }$ corresponding to the events when two $\alpha$ fragments detected at (a) ${\theta }_1={55}^{\circ }$ and ${\theta }_2={57}^{\circ }$, (c) ${\theta }_1={55}^{\circ }$ and ${\theta }_2={90}^{\circ }$, and (e) ${\theta }_1={60}^{\circ }$ and ${\theta }_2={128}^{\circ }$ respectively. Respective one-dimensional projections of the above events on $E_{\alpha }$ are given in panels (b), (d), and (f).

Figure 5

Differential cross sections in center-of-mass frame for sequential breakup of ${}^7\mathrm{Li}\to {}^8\mathrm{Be}\to \alpha +\alpha$ corresponding to (a) $0^{+}$, (b) $2^{+}$, and (c) $4^{+}$ states of ${}^8\mathrm{Be}$ along with (d) the elastic scattering, measured at $E_{\mathrm{beam}}=30$ MeV. The hollow circles in panels (a), (b), and (c) represent $2\alpha$ breakup cross sections corresponding to no target excitation, whereas the filled circles represent total breakup cross sections corresponding to both ground state as well as excited states of the target like nuclei. Lines represent the results of the CRC calculations using fresco. Dashed lines represent the theoretical cross sections only for g.s. to g.s transition and solid lines represent total cross sections for g.s. and excited states of ${}^{111}\mathrm{In}$.

Figure 6

Measured (circles) elastic scattering cross sections have been compared with the fresco calculations (lines) showing the effect of coupling of direct and resonant breakup and transfer channels.

Figure 7

Experimental and theoretical cross sections for sequential breakup of ${}^7\mathrm{Li}\to {}^8\mathrm{Be}\to \alpha +\alpha$ corresponding to (a) $0^{+}$, (b) $2^{+}$, and (c) $4^{+}$ states of ${}^8\mathrm{Be}$ corresponding to no target excitation. Dashed and solid lines represent the results of fresco calculations using CRC only and CDCC+CRC formalisms respectively.

Figure 8

Experimental cross sections for $1p$ stripping corresponding to g.s. of ${}^6\mathrm{He}$ and (a) g.s., (b) first excited state, and (c) second excited state of ${}^{113}\mathrm{Sb}$. Solid lines represent fresco calculations using the CDCC+CRC formalism.

Figure 9

Experimental cross sections for $1n$ stripping corresponding to g.s. of ${}^6\mathrm{Li}$ and (a) g.s. and (b) 0.738-MeV excited state of ${}^{113}\mathrm{Sn}$. Solid lines represent fresco calculations using CDCC+CRC formalism.