HEAVY-ION TRANSFER REACTIONS

Douglas Cline

University of Rochester

 

The study of light-ion induced few-nucleon transfer reactions has contributed significantly to our understanding of nuclear structure. Single-nucleon transfer is a direct probe of single-particle shell structure while two-nucleon transfer is a direct probe of pairing correlations in nuclei. The study of heavy-ion induced transfer reactions offers unique opportunities to investigate many intriguing aspects of physics [1]. For example, it can be used to selectively populate high-$j$ orbits in actinide nuclei, or inelastic excitation accompanied by pair-transfer, allows one to study the spin dependence of pairing correlations in two-nucleon transfer reactions. Diabolic pair transfer [2,3,4], which is a manifestation of Berry phase [5] in nuclei, is a special case of pair transfer across a band crossing. Enhancement for multinucleon-transfer [6] between two colliding superfluid nuclei, at subbarrier energies, could lead to the nuclear Josephson effect [7].

The Rochester-UT collaboration initiated this heavy-ion transfer research program in 1982, using the Spin Spectrometer at Oak Ridge. This program now exploits the considerable advance in sensitivity provided by Gammasphere/CHICO. The early Spin Spectrometer work discovered enhanced correlated transfer of nucleon pairs in grazing collisions between two superfluid spherical Sn nuclei. The enhancement factor was $\sim 500$ relative to DWBA values for 2n transfer between the 0$^{+}$ ground states of even-A Sn nuclei [1,8]. This strong enhancement is in agreement with transfer calculations including pairing [6,9].

For heavy-ion transfer reactions involving strongly-deformed nuclei, the accompanying Coulomb excitation excites the deformed nucleus to high spins in the ground band prior to pair transfer, allowing study of pairing correlations as a function of rotational frequency. A study of enhanced correlated 2n transfer for strongly deformed Dy nuclei identified strongly-enhanced cold 2n pair transfer for extreme peripheral collisions. This explained the so-called slope anomaly in the dependence of 2n transfer probability as a function of distance of closest approach [10]. Furthermore, this work discovered an interference dip in the angular dependence of the excitation probability to individual states that results from the influence of deformation on the pair transfer [8,10]. This controversial result has been confirmed by experiments using a new magnetic spectrometer technique at Rochester [11], and also using the Crystal Ball [12]. This strong enhancement for two-neutron transfer bodes well for study of quenching of pair correlations at high spin, and possible identification of the nuclear Josephson effect or diabolic pair transfer. Our first search for diabolic pair transfer was unsuccessful but now this result is understood [4]. We are going to continue to pursue the search for diabolic pair transfer and the measurement of pairing strength after the first band crossing.

Our work at Eurogam and Rochester measured the population distribution to individual states in heavy-ion induced one-neutron transfer. It showed that the receptor receives a substantial fraction of the total excitation energy and the partition of excitation energy between the receptor and the donor is primarily determined by the spectroscopic strength distribution. DWBA analysis reproduces the general trend although discrepancies exist when comparing populations of individual final states [13]. Also we showed that many side bands, including the $S$-band are populated in the one-neutron pickup channel [14]. This single-nucleon transfer work, and the strongly enhanced pair transfer we have observed, show that heavy-ion transfer reactions can be used as sensitive and quantitative probes of single-particle structure and pairing correlations. Our demonstration that the $\gamma $-ray deexcitation transitions from both reaction partners can be completely resolved, even when both are strongly deformed nuclei, allows use of a wide range of nuclear species in the entrance channel [15] to optimize the Q-value for transfer to specific exit channels.

Heavy-ion induced transfer reactions have many applications to nuclei spectroscopy beyond their use as a selective probe of single-particle structure and pair correlations. For example, we have used heavy-ion transfer, as well as more complicated reactions, to study nuclear structure of neutron-rich nuclei in the rare-earth and actinide regions that are inaccessible by other techniques. In one of our recent experiments, the rotational bands based on the high-$j$ orbitals, such as 7/2$^{+}$[633] in $^{169}$Er and 7/2$^{-}$[743] in $^{237}$U were populated via one-neutron transfer for reactions between $^{170}$Er and $^{238}$U in addition to the population of neutron-rich nuclei, $^{171,172}$Er. Experiments using proton-transfer reactions are planned to extend nuclear structure studies to the existing boundary of neutron-rich nuclei in actinides.

Deep-inelastic reactions have been explored recently to populate the superdeformed band in $^{100}$Mo and neutron-rich nuclei in the $A\sim 180-190$ region at bombarding energies $\sim 13-25\%$ above the barrier. The combination of Gammasphere and CHICO for such experiments improves the sensitivity for channel selection by identifying the deep-inelastic events using the pseudo-Q-value deduced from the measured quasi-two-body kinematics. This collaborative research program will to be exploited further.

References to the early work can be found in Ref. [1]. This program has resulted in six Ph.D. theses. They are S. Juutinen (Jyvaskyla, 1988), X.T. Liu (Knoxville, 1988), W.J. Kernan (Rochester, 1989), M.A. Stoyer (Berkeley, 1990), K.G. Helmer (Rochester, 1992) and M. Devlin (Rochester, 1995).

 

References

1) C.Y. Wu, W. von Oertzen, D. Cline, M.W. Guidry, Annu. Rev. Nucl. Part. Sci, 40, 285 (1990).

2) R.S. Nikam and P. Ring, Phys. Rev. Lett. 58, 980 (1987).
3) L.F. Canto, P. Ring, Y. Sun, et al., Phys. Rev. C 47, 2836 (1993).

4)

 K.G. Helmer, C.Y. Wu, D. Cline, A.E. Kavka, W.J. Kernan, E.G. Vogt, M.W. Guidry, X.L. Han, R.W. Kincaid, X.T. Liu, H. Schecter, J.O. Rasmussen, A. Schihab-Eldin, M.A. Stoyer, and M.L. Halbert, Phys. Rev. C 48, 1879 (1993).

5) M.V. Berry, Proc. R. Soc. London, Ser. A 392, 45 (1984).
6) J.H. Sorensen and A. Winther, Phys. Rev. C 47, 1691 (1993).

7) K. Dietrich, Ann. Phys. 66, 480 (1971).
8)
W.J. Kernan C.Y. Wu, X.T. Liu, D. Cline, T. Czosnyka, M.W. Guidry, M. Halbert, A.E. Kavka, R.W. Kincaid, S.P. Sorensen, E.G. Vogt., Nucl. Phys. A524, 344 (1991).

9) H. Weiss, Phys. Rev. C 19, 834 (1979).
10)
C.Y. Wu, X.T. Liu, W.J. Kernan, D. Cline, T. Czosnyka, M.W. Guidry, A.E. Kavka, R.W. Kincaid, B. Kotlinski, S.P. Sorensen, E. Vogt., Phys. Rev. C 39, 298 (1989).

11) M. Devlin, D. Cline, R. Ibbotson, M.W. Simon, and C.Y. Wu, Phys. Rev. C 53, 2900 (1996).

12) T. Hartlein, H. Bauer, D. Pansegrau, and D. Schwalm, Eur. J. A. 4, 41 (1999).

13) C.Y. Wu, D. Cline, M. Devlin, K.G. Helmer, R.W. Ibbotson, M.W. Simon, P.A. Butler, A.J. Cresswell, G.D. Jones, P.M. Jones, J.F. Smith, R.A. Cunningham, and J. Simpson, Phys. Rev. C 51, 173 (1995).

14)

 A.J. Cresswell, P.A. Butler, D. Cline, R.A. Cunningham, M. Devlin, F. Hannachi, R. Ibbotson, G.D. Jones, P.M. Jones, M. Simon, J. Simpson, J.F. smith, and C.Y. Wu, Phys. Rev. C 52, 1934 (1995).

15) C.Y. Wu, D. Cline, M.W. Simon, R. Teng, K. Vetter, M.P. Carpenter, R.V.F. Janssens, I. Wiedenhover, Phys. Rev. C 61, 021305(R) (2000).

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