Speaker
Description
Quasielastic (QE) scattering in the vicinity of the Coulomb barrier serves as a highly sensitive probe of reaction dynamics in heavy-ion collisions. Within the coupled-channels (CC) framework, the interplay between relative motion and intrinsic nuclear excitations leads to a distribution of effective barriers, commonly referred to as barrier distributions. These distributions encode valuable information about nuclear structure and reaction mechanisms and can typically be extracted from QE measurements [1,2]. However, observed deviations from standard CC predictions suggest that additional weakly coupled channels such as nucleon transfer and non-collective excitations may play a significant role in shaping the measured barrier structure [2].
Experimental studies have shown that QE barrier distributions $D_{qe}$ for several systems are smoother than predicted by standard CC calculations [3,4,5,6,7]. This behavior has been observed in reactions involving $^{20}$Ne with $^{92}$Zr [3], $^{61}$Ni [4], and $^{94,95}$Mo [6], and $^{24}$Mg with $^{92}$Zr [5]. It is interpreted as a consequence of dissipative dynamics arising from coupling to a large number of weak non-collective states. Although individually weak, these couplings collectively redistribute kinetic energy into intrinsic degrees of freedom, resulting in a damping of the barrier distribution structure. To describe such effects, the coupled-channels approach extended with random matrix theory (CCRMT) has been developed, incorporating statistical couplings to single-particle excitations [8]. These results have motivated further investigations of this dissipation mechanism using different strongly deformed projectiles and target systems with varying single-particle level densities.
In this context, quasielastic scattering measurements for the $^{24}$Mg + $^{92,94,95}$Mo systems were performed at the China Institute of Atomic Energy (CIAE), Beijing. The experiment was carried out at the HI-13 tandem accelerator, which delivered a $^{24}$Mg beam with an average current of approximately 50 enA over an energy range of 71–93 MeV in 1 MeV increments. Enriched self-supporting targets of $^{92}$Mo (99.93%), $^{94}$Mo (98.97%), and $^{95}$Mo (95.40%), each with a thickness of about 100 $\mu$g/cm$^{2}$, were employed. QE events were recorded using an array of silicon detectors arranged in five concentric rings spanning backward angles of 139$^\circ$ to 170$^\circ$, supplemented by forward-angle detectors positioned at 30$^\circ$. This geometry ensured extensive angular coverage, enabling reliable extraction of quasielastic barrier distributions. An increase in single-particle level density from $^{92}$Mo to $^{95}$Mo is anticipated to strengthen dissipative effects, resulting in a progressive smoothing of the barrier distributions [6]. The analysis of the collected data is currently in progress and preliminary results will be presented during the conference.
REFERENCES
[1] M. Dasgupta et al., Annu. Rev. Nucl. Part. Sci. 48, 401 (1998).
[2] H. Timmers et al., Nucl. Phys. A 584, 190 (1995).
[3] E. Piasecki et al., Phys. Rev. C 80, 054613 (2009).
[4] A. Trzcińska et al., Phys. Rev. C 92, 034619 (2015).
[5] A. Trzcińska et al., Phys. Rev. C 102, 034617 (2020).
[6] G. Colucci et al., Acta Phys. Pol. B 17, 3-A23 (2023).
[7] E. Piasecki et al., Phys. Rev. C 100, 014616 (2019).
[8] S. Yusa et al., Phys. Rev. C 82, 024606 (2010).