M.E. Ortiz

Study of resonant reactions with radioactive ion beams

Abstract A fast and efficient method to study (p,p) and (p,α) resonances with radioactive beams in inverse kinematics is described. It is based on the use of thick targets and large area double-sided silicon strip detectors (DSSDs) to detect the recoiling light-charged particles and to determine precisely their scattering angle. The first nuclear physics experiments with the technique have been performed recently at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge with stable beams of 17 O and radioactive beams of 17 F . The high-quality resonance measurements obtained demonstrate the capabilities of the technique. Pure 17 F beams from HRIBF were produced by fully stripping the ions and separating the interfering and more abundant 17 O ions by the beam transport system. The removal of interfering isobars is one of the various common challenges to both accelerator mass spectrometry (AMS) and radioactive ion beam (RIB) production. Experiments done with RIBs will benefit from the use of the most efficient techniques for production, isobar separation, transport and detection.

Yrast levels in 27Al suggested by resonance structure in the 12C(15N, α) reaction☆

Abstract A large number of correlated resonances with ∼400 keV (c.m.) widths were observed in addition to statistical fluctuations in the 12 C( 15 N,α) reaction. The resonant portion of the cross section suggest the population of high-spin levels close to the yrast line of the compound nucleus.

Study of the coherence widths Γ in 28Si measured by the 12C(16O, α) reaction

Abstract States in 24 Mg were populated by the 12 C( 16 O, α) 24 Mg reaction. Excitation functions were measured in the bombarding energy range E c.m. = 17.1–19.7 MeV for states in 24 Mg with excitation energies from 0 to 12 MeV. A fluctuation analysis of the excitation functions was made and the coherence widths Γ of the compound nuclear states were extracted. By comparing theoretical calculations of Γ with present and previous measurements values for the moment of inertia and the spin cutoff parameter of 28 Si were extracted.

Proton resonance reactions using thick targets in inverse kinematics

Abstract A technique to study proton resonance reactions using heavy ion beams has been tested with 13C beams on a 20 μm polypropylene target. Continuous target motion is maintained to avoid hydrogen loss due to damage by the heavy ion beams and high quality excitation functions can be obtained in a single exposure. This technique can be very valuable for studies with radioactive beams.

Coherence widths in 27Al measured with the 12c(15n, α) reaction

Abstract Excitation functions for the 12 C( 15 N, α) reaction have been measured at θ lab = 7°,in200 keV steps from E 15 N = 21.4–39.0 MeV. A total of forty-one states were observed in 23 Na with excitation energies from 0 to 12.5 MeV. The analysis of the excitation functions reveals the presence of statistical and non-statistical components. The coherence widths Γ were extracted from the statistical fluctuations. By comparing measured and theoretical widths, values for the spin-cutoff and moment-of-inertia parameters were extracted for the compound system 27 Al. It is also shown that the choice of the Hauser-Feshbach denominator for the calculation of Γ does not affect significantly the slope of the curve Γ versus the excitation energy in the compound system if E x .

Study of the 14964Gd → 14963Eu → 14962Sm decay chain

A study has been made of the gamma-ray and conversion-electron spectra in the decay chain 149Gd(9.4 d) → 149Eu(93.1 d) → 149Sm, following the electron-capture decay of 149Tb, Gamma-ray and conversion-electron emission probabilities were measured with a double-focusing magnetic spectrometer, and internal conversion coefficients, multipolarities and mixing ratios were deduced for several transitions of 149Eu and 149Sm. These values have been compared with previously published data. Gamma-ray emission probabilities have also been measured for 145Sm, 145Pm and 145Nd, which originated via the alpha-decay branch of 149Tb.

Preliminary Results with the Reaction 84Kr on 27Al at E/A = 15 MeV Using the HILI

The study of heavy-ion collisions in the energy regime between 10 and 100 MeV/u is a useful tool to explore the mechanisms that arise as the bombarding energy increases. It has been reported that the complete fusion cross section which is dominant at low energy central collisions decreases to the benefit of processes of more binary character[1] This feature leads to the following questions. At what energy does such transfer from fusion mechanism to binary mechanisms occur? Is this “transition energy” system dependent? To what extent does the binary process persist? Evidences of binary processes at energies around 30MeV/u and higher have been reported in experimental studies[2, 3, 4, 5, 6] Another feature that appears when the bombarding energy of the reaction is increased is the production of intermediate mass fragments (IMF’s). The mechanisms and sources reponsible for IMF production are still open questions. With our study of the 79Br on 27A1 at 11.8 MeV/u and 84Kr on 27A1 at 15 MeV/u systems we intend to explore some of these questions.

Energy Calibration of Intermediate Mass Fragments from the 930-MeV 79Br on 27Al Reaction

The intermediate mass fragments produced in the reaction 79Br on 27Al at 930 MeV have been characterized using the Heavy Ion Light Ion (HILI) detector.1 Their laboratory energy has been determined using two different approaches.

Coincidences between light particles, evaporation residues, and complex fragments emitted in the reaction {sup 58}Ni + {sup 58}Ni at 500 MeV bombarding energy

Light particles (protons and alphas) were measured in coincidence with complex fragments (4 40) using the large detector array HILI. A {sup 58}Ni beam of 500 MeV extracted from the HHIRF tandem accelerator was used to bombard a {sup 58}Ni target of 99% enrichment. A good account of the proton and alpha spectra in coincidences with the residues can be achieved only by including in the statistical model calculation the emission of complex fragments and allowing a small emission of a dinuclear configuration formed prior to fusion. The relative kinetic energy spectra between the complex fragments and the residues show a typical Coulomb peak consistent with emission from the compound nucleus and the out of plane angular correlation shows that the emission is coplanar.

Fusion cross sections for the /sup 12/C+/sup 15/N system

Fusion cross sections were measured for the /sup 12/C+/sup 15/N system at two /sup 12/C bombarding energies, 98 and 117 MeV. The extracted fusion cross sections are 946 +- 74 mb for E(/sup 12/C) = 98 MeV and 889 +- 89 mb for E(/sup 12/C) = 117 MeV. The critical angular momenta for fusion deduced from the cross section measurements equal, within 1h, the values deduced from /sup 14/N+/sup 13/C and /sup 10/B+/sup 17/O at the same excitation energies, thus indicating a compound nucleus limitation for fusion.