Population of bound excited states in intermediate-energy fragmentation reactions

Fragmentation reactions with intermediate-energy heavy-ion beams exhibit a wide range of reaction mechanisms, ranging from direct reactions to statistical processes. We examine this transition by measuring the relative population of excited states in several sd-shell nuclei produced by fragmentation with the number of removed nucleons ranging from two to sixteen. The two-nucleon removal is consistent with a non-dissipative process, whereas the removal of more than five nucleons appears to be mainly statistical.

Figure 1

γ-ray spectrum in coincidence with ${}^{21}\mathrm{Na}$ residues produced from ${}^{24}\mathrm{Mg}$. The data points are shown with statistical error bars. The individual components of the simulated spectrum corresponding to individual γ rays, the exponential background, and their sum (thick solid line) are presented.

Figure 2

Level-scheme of ${}^{21}\mathrm{Na}$. Experimental data (left) are compared to shell-model calculations (right).

Figure 3

Relative population of excited states in ${}^{21}\mathrm{Na}$ from ${}^{24}\mathrm{Mg}$ and ${}^{34}\mathrm{Ar}$. For each reaction, the predictions $P_k^{\mathrm{sm}}$ for a nondissipative process and the fit with the population $P_k=P_k^{\mathrm{sm}}+P_k^s$ are shown.

Figure 4

Relative population of excited states of ${}^{18}\mathrm{Ne}$ from ${}^{34}\mathrm{Ar}$. The predictions from nondissipative population $P_k^{\mathrm{sm}}$ (dotted-dashed line), statistical population $P_k^s$ with a temperature $T=1$ MeV (dotted line) and $T=3$ MeV (dashed line), and best fit from Eq. (5) (solid line).

Figure 5

Relative production of excited states of ${}^{26}\mathrm{Ne}$ from ${}^{28}\mathrm{Mg}$ [2]. Experimental data are compared to theoretical predictions, namely the population calculated from Eq. (4) ($P_k^{\mathrm{sm}}$), a complete stripping calculation (knockout) and a statistical population ($P_k^s$) with $T=1$ MeV and $T=4$ MeV.

Figure 6

Correlation between the weight α of direct process (see text) and the temperature-like parameter T of the statistical part.