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2. Recent HRIBF Research - (d,p) Reactions on 130Sn and 132Sn in Inverse Kinematics
(R. L. Kozub, spokesperson RIB-134; K.L. Jones, spokesperson RIB-132)

The r-process is thought to be responsible for the synthesis of about half of the nuclear species heavier than Fe [Bu57], but little experimental information is available for nuclear structure and the r-process near the N=82 closed neutron shell and the A~130 abundance peak. The r-process is most productive where neutron-rich nuclei far from stability have high level densities and find themselves in an environment of sufficiently high neutron density that the rate of neutron capture exceeds the rate of beta-decay. In such cases, statistical (Hauser-Feshbach) methods can be used to predict the cross sections for reaction rate calculations by averaging over resonances. However, for nuclei far from stability that are near the magic numbers N=50, 82, and 126, the level densities are too low for a statistical approach [Ra97], and it is thus necessary to input the single-particle structure of individual levels. The (d,p) neutron transfer reaction has proven to be an excellent tool for the measurement of these single-particle properties, which are also critically needed for the development of structure models for all nuclei. We report here some very preliminary results from two recent experiments in which beams of 130Sn and doubly magic 132Sn were used to measure single-particle properties of 131,133Sn via the 2H(130,132Sn, p)131,133Sn reactions.

The information known to date on 133Sn has come from gamma-ray transitions following the 134In(beta)134Sn*(n)133Sn* decay sequence [Ho96] and the fission of 248Cf [Ur99], both of which favor the population of states having J≥3/2. Low-lying pf-shell states and the h9/2 state have thus been located, but there is no single-particle information available. Further, not even the location has been firmly established for the expected p1/2 state.

Yrast cascades in 131Sn involving states with J≥11/2 have been studied by Bhattacharyya, et al. [Bh01], and some of the low-lying, positive-parity hole states have been assigned tentatively [Se94] from beta-decay experiments. In the (d,p) reaction, however, one expects the strongest states to be l=1 and l=3 transfers coupled to the 130Sn ground state, i. e., negative-parity 1p-2h states, none of which has been identified.

In the present work, radioactive beams of 630-MeV 132Sn and 130Sn ions bombarded an 80 micrograms/cm2-thick CD2 foil. In order to detect protons near 90° in the laboratory, the target surface was placed at 30° with respect to the beam axis, so the effective thickness was 160 micrograms/cm2. Protons from the reactions were detected with an array of position-sensitive silicon strip detectors,including 14 of the new ORRUBA detectors, with half of SIDAR and a 5x5cm position sensitive telescope.

Some samples of the online data from these experiments are shown in Figs. 2-1 and 2-2. In Fig.2-1, four states can be observed in 133Sn, presumably the f7/2 ground state, and the p3/2, p1/2, and f9/2 excited states. All but the p1/2 appear to have kinematics in agreement with published data for the mass/excitation energies. The first result from this experiment is expected to be the location of the p1/2 single-particle state in 133Sn . A similar pattern at roughly similar Q-values (higher excitation energies in 131Sn) is observed in the 2H(130Sn,p)131Sn reaction (Fig. 2-2). Angular distributions will be extracted, pursuant to determining J assignments and measuring spectroscopic factors. Analysis of the data is ongoing.

Figure 2-1: Energy versus angle plot of 2H(132Sn,p)133Sn events from one of the "forward" position-sensitive strip detectors. Most elastic scattering events were stopped by the 65-μm-thick ΔE detector in front of this detector. Also shown are expected 2H(132Sn,p)133Sn kinematics loci for known states in 133Sn. Excitation energies shown are in keV.

Figure 2-2: Energy versus angle plot of 2H(130Sn,p)131Sn events from one of the "forward" position-sensitive strip detectors. All elastic scattering events were stopped by the 140-μm-thick ΔE detector in front of this detector. The gain is approximately the same as in Fig.2-1.

[Bu57] E. M. Burbidge et al., Rev. Mod. Phys. 29, 547 (1957).
[Bh01] P. Bhattacharyya, et al., Phys. Rev. Letters 87, 062502 (2001).
[Ho96] P. Hoff et al., Phys. Rev. Lett. 77(6) 1020 (1996).
[Ra97] T. A. Rauscher et al., Nucl. Phys. A621, 327c (1997).
[Se94] Yu. V. Sergeenkov, et al., Nuclear Data Sheets 72, 487 (1994).
[Ur99] W. Urban et al., Eur. Phys. J. A5, 239 (1999).

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