3. Recent HRIBF Research - Measurement
of the 134Te(d,p) Reaction for the Study of the Single-Particle
Structure of 135Te
(S. D. Pain, spokesperson)
The study of the single-particle structure of neutron-rich nuclei adjacent to shell closures yields important information for understanding the evolution of nuclear structure away from stability. Furthermore, the properties of neutron-rich nuclei near shell closures are significant to the understanding of neutron-capture processes that are responsible for the formation of the heavy elements. The low-level densities and low neutron-separation energies in such nuclei result in low neutron-capture rates, with significant components from direct capture into low lying states and isolated resonances. The properties of these single-particle states are crucial for determining direct capture rates. Transfer reactions are well established as a powerful spectroscopic tool for studying the single-particle structure of nuclei, and the (d,p) reaction preferentially populates the low-spin single-particle levels that are important for neutron capture.
The neutron-rich nucleus 135Te (Z=52, N=83) is one neutron beyond the N=82 closed shell, and two protons beyond the Z=50 shell closure; consequently, the properties of many low-lying states are predominantly of single-neutron structure. Its proximity to these shell closures makes it of particular importance to nuclear structure and astrophysics. Astrophysically, the structure of low-lying levels in 135Te is of significance to the isotopic abundances of xenon measured in pre-solar diamond grains, which indicate an overabundance of light (124,126Xe) and heavy (134,136Xe) isotopes compared to intermediate mass Xe isotopes, relative to the solar abundances. These anomalous isotopic ratios cannot be explained satisfactorily by addition of the p and r process xenon abundances, as observed in the solar system, as the relative excesses of 134Xe and 136Xe do not correspond to the isotopic ratios of the average r process .
Proposed explanations for this heavy isotope anomaly include the rapid separation of xenon from its precursors (iodine and tellurium) in supernova ejecta [1, 2], processes in which the rate of neutron capture is somewhere between the s and r processes, or a low-entropy r process . The relative abundances of 134Te and 136Te in such an astrophysical environment has a direct bearing on the final abundances of the β-decay daughters 134Xe and 136Xe. Therefore, knowledge of the properties of the low-lying neutron states with low ℓ in 135Te will add constraint to the range of conditions under which such isotopic ratios of xenon can be formed.
Figure 3-1: Proton energy vs laboratory angle for a single strip of a forward angle detector (gated on protons), showing loci corresponding to the 134Te(d,p)135Te reaction populating a number of states in 135Te. The intense group at the lower-left of the plot is due to elastically scattered protons. The red dotted lines are expectations for the population of states in 135Te and the elastically scattered protons.
We have measured the 134Te(d,p)135Te reaction in inverse kinematics, utilizing a beam of 134Te incident at 643 MeV on a 100 μg/cm2 CD2 target (effective thickness due to rotation). The proton ejectiles were measured in an array of position-sensitive silicon detector telescopes, comprised of an early implementation of the Oak Ridge Rutgers University Barrel Array (ORRUBA) augmented by the Silicon Detector Array (SIDAR). The setup covered the angular ranges 55° to 125° with ORRUBA detectors, and 145° to 165° with SIDAR detectors. The ORRUBA detectors subtending angles forward of 90° were set up as charged particle telescopes, to enable the separation of proton ejectiles from elastically scattered deuterons and carbon nuclei from the target.
A preliminary plot of proton energy vs angle, for one strip of a single forward-angle ORRUBA telescope, is shown in Fig. 3-1, gated on protons from the ΔE-E measurement. Loci can be seen corresponding to transfer to the ground state and a number of excited states in 135Te, along with the locus from protons scattered elastically from the target.
Figure 3-2 shows a Q-value spectrum for the data shown in Figure 3-1. The presence of a strongly populated state near 1.8 MeV, along with the relative strength of the peak at about 1 MeV (relative to the ground and first-excited state peaks) is suggestive that the majority of the f5/2 strength lies at 1.8 MeV, rather than at about 1 MeV, as previously supposed . Further analysis will yield angular distributions, enabling the determination of spin assignments and the extraction of spectroscopic factors for the levels populated.
Figure 3-2: Q-value spectrum for the 134Te(d,p)135Te reaction, calculated event-by-event, for a single forward-angle strip.
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