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Spectroscopic information has been extracted on the hole-states of 55Ni, the least known of the quartet of nuclei (55Ni, 57Ni, 55Co and 57Co), one neutron away from 56Ni, the N=Z=28 double magic nucleus. Using the 1H(56Ni,d)55Ni transfer reaction in inverse kinematics, neutron spectroscopic factors, spins and parities have been extracted for the f7/2, p3/2 and the s1/2 hole-states of 55Ni. This new data provides a benchmark for large basis calculations that include nucleonic orbits in both the sd and pf shells. State of the art calculations have been performed to reproduce the excitation energies and spectroscopic factors of the s1/2 hole state below Fermi energy.

We demonstrate an approach to measure the total fusion cross-section for beams of low-intensity light nuclei. Fusion residues resulting from the fusion of 20,16 O+ 12 C are directly measured and distinguished from unreacted beam particles on the basis of their energy and time-of-flight. The time-of-flight is measured between a microchannel plate (MCP) detector, which serves as a start, and a segmented silicon detector, which provides a stop. The two main difficulties associated with the initial implementation of this approach are charge trapping in the silicon detector and slit scattering in the MCP detector. Both these obstacles have been characterized and overcome. To reduce atomic slit scattering in the measurement we have eliminated wires from the beam path by developing a gridless MCP detector. The total fusion cross-section for 16 O+ 12 C in the energy range E CM =8.0–12.0 MeV has been measured in agreement with established literature values (down to the 100 mb level).

Atomic masses play an important role in nuclear physics and astrophysics. The need of experimental mass values for unstable nuclides has triggered the development of a wide range of mass measurement techniques, with devices installed at many laboratories around the world. We have implemented a time-of-flight–magnetic-rigidity ( TOF - B ρ ) technique at the National Superconducting Cyclotron Laboratory (NSCL) that includes a position measurement for magnetic rigidity corrections and uses the A1900 separator and the S800 spectrograph. We performed a successful first experiment measuring masses of neutron-rich isotopes in the region of Z ∼ 20–30, important for calculations of processes occurring in the crust of accreting neutron stars. The masses of 16 nuclei were determined, for 61V, 63Cr, 66Mn, and 74Ni for the first time, with atomic mass excesses of −30.510(890) MeV, −35.280(650) MeV, −36.900(790) MeV, and −49.210(990) MeV, respectively. The mass resolution achieved was 1.8×10−4.

Angular distributions of the Coulomb breakup of radioactive 17F were measured by impinging a 10 MeV/nucleon beam on 208Pb and on 58Ni to study the dynamic polarization effects. The breakup products, oxygen and a proton, were detected in coincidence. First-order perturbation theory significantly overpredicts the breakup cross section for the 208Pb target. Dynamical calculations with a dynamic polarization as the leading order correction were performed. The calculations reproduce the data for 17F on 58Ni but overpredict the breakup of 17F on 208Pb by a factor of two at forward angles.

We [Collaboration members include: F.J. Liang, D. Shapira, C.J. Gross, J.R. Beene, R.L. Varner, P.E. Mueller, A. Galindo-Uribarri, J. Gomez del Campo, D.W. Stracener, Physics Division ORNL, W. Loveland Oregon State Univ., H. Amro, J.J. Kolata Univ. of Notre Dame, J. Bierman Gonzaga Univ. A. L. Caraley State Univ. of New York at Oswego] have measured the formation of Evaporation Residues (ER) at near-barrier energies in collisions between several Te and Sn beams on 58 Ni and 64 Ni targets. The measured excitation functions are sparse but by measuring several systems we try to explore systematic trends in the sub-barrier cross section for ER formation. The data obtained show the expected enhancement in ER production (survival) with the addition of neutrons to the fused system. The overall cross sections, however, depend strongly on the capture cross sections that are the focus of this presentation An analysis of nucleus-nucleus capture in this mass region is attempted in the framework of a WKB formalism that incorporates neutron transfer/flow in a two-step process. The calculated capture cross sections are compared with the new, and other available data in this mass region. We use global potentials in our analysis and do not fit individual data sets but hope that these, or similar, calculations will provide a simple base for the prediction of trends in nucleus-nucleus capture cross sections.

A technique for measuring mass differences has been developed at the Holifield Radioactive Ion Beam Facility (HRIBF) that requires no specialized equipment. Mass differences are measured as position differences between known and unknown-mass isobars, dispersed at the image of the energy-analyzing magnet following the 25 MV tandem post-accelerator, and identified by an energy-loss measurement. The technique has been demonstrated on neutron-rich 77–79Cu and 83–86Ge isotopes produced using the isotope separator online (ISOL) method with the 238U(p,fission) reaction, where a mass accuracy of 500 keV was achieved. These nuclides are well suited to the measurement technique, as they readily migrate out of the production target and to the ion source and comprise the most neutron-rich elements of the isobarically mixed beam. Because modest precision mass values can be obtained with only a few tens of counts of the nuclide of interest among orders of magnitude more of the isobaric neighbors closer to stability, the sensitivity of this technique makes it appropriate for initial mass measurements far from stability.

Initial measurements are presented of the (d,p) reactions on neutron-rich N = 50 isotones along the r-process path of nucleosynthesis with radioactive ion beams of 82Ge and 84Se. Prospects for measurements with unstable 130,132Sn beams are discussed.

A setup for measuring cross-sections of evaporation residues produced in the collision of two heavy ions is described. The system, which detects all reaction products in a narrow angular range around the beam direction, is designed to work best with low-intensity beams ( ⩽ 10 5 ions / s ) and can be highly efficient for reactions studied in inverse kinematic conditions, i.e., heavy-mass beam on lighter-mass targets. The system as presented here has been optimized to study evaporation residues from reactions induced by radioactive ion beams with charges near Z ∼ 50 and energies near 4 MeV/nucleon. Continuous sampling of the beam velocity, energy loss and position allow for the measurement of accurate evaporation residue cross-sections also in situations where the beam contains a mixture nuclides.

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ ) 19 Ne reactions in astrophysical environments depend on the properties of 19 Ne levels above the 18 F + p threshold. There are at least 8 levels in the mirror nucleus 19 F for which analogs have not been observed in 19 Ne in the excitation energy range E x = 6.4 – 7.6 MeV . These levels may significantly enhance the 18 F + p reaction rates, and thus we have made a search for these levels by measuring the 1 H( 18 F, p ) 18 F excitation function over the energy range E c . m . = 0.3 - 1.3 MeV . We have identified and measured the properties of a newly observed level at E x = 7.420 ± 0.014 MeV , which is most likely the mirror to the J π = 7 2 + 19 F level at 7.56 MeV. We have additionally found a significant discrepancy with a recent compilation for the properties of a 19 Ne state at E x = 7.5 MeV and set upper limits on the proton widths of missing levels.

The degree to which the (p, γ) and (p,α) reactions destroy 18F at temperatures ∼ 1 − 4 × 10 8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold ( E x = 6.411 MeV in 19Ne). To gain further information about these resonances, we have used the d(18F,p)19F neutron transfer reaction to selectively populate corresponding mirror states in 19F. The results would suggest 18F(p,γ)19Ne and 18F(p,α)15O reaction rates that are 2-3 times lower than reported previously.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2+ excited states of even-even neutron-rich 132–136Te and 126–134Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134Te beam, from 9Be and 13C targets to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the ν i 13 / 2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

The fusion of protons with radioactive nuclei is important in stellar explosions such as novae and X-ray bursts and for the production of neutrinos in the sun. The Daresbury Recoil Separator and a windowless gas target system have been installed at ORNL's Holifield Radioactive Ion Beam Facility (HRIBF) for measurements of proton capture reactions in inverse kinematics with radioactive ion beams. The performance of the system has been characterized with a number of experiments using stable ion beams. We report on results from these commissioning measurements and plans for measurements of the 1 H( 17 F, 18 Ne) and 1 H( 7 Be, 8 B) reactions.

We have measured the B(E;0+ → 2+) for the first excited 2+ states in the double-closed shell nucleus 132Sn and the two-neutron nucleus 134Sn. The results, based on a preliminary analysis are shown in Fig. 1 along with measurements on the stable Sn isotopes, and earlier results on 126,128,130Sn [1] . The experimental setup developed for the 132,134Sn measurements was also employed in a successful measurement of B(E;0+ → 2+) for the closed-neutron-shell nucleus 82Ge.

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ) 19 Ne reactions at T = 0.1 – 0.4 GK are necessary to understand the emission of positron-annihilation gamma rays expected from novae. Observations of such gamma rays by space-based telescopes would provide direct tests of nova models. These rates, however, are uncertain because of the unknown level structure of 19 Ne above the 18 F+ p threshold. Missing levels in 19 Ne were searched for by stopping a 24-MeV 18 F beam in a thick polypropylene target, and scattered protons were detected in a silicon-strip detector. The 18 F( p , p ) 18 F excitation function has been measured from E c.m. = 0.3 - 1.3 MeV covering most energies of astrophysical interest, and a significant discrepancy was observed with a recent compilation of 19 Ne resonance parameters.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of exciting measurements around the 132 Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2 + excited states of neutron-rich 132,134,136 Te and 126,128,130,132,134 Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134 Te beam, from 9 Be and 13 C targets to populate single-particle states in 135 Te, has also been studied. Results, and plans for future experiments with these neutron-rich beams, are presented.

Evaporation residue cross sections have been measured with neutron-rich radioactive 132 Sn beams on 64 Ni in the vicinity of the Coulomb barrier. The average beam intensity was 2 × 10 4 particles per second and the smallest cross section measured was less than 5 mb. Large sub-barrier fusion enhancement was observed. The measurement is compared to coupled-channel calculations taking into account inelastic excitation and nucleon transfer.

The study of neutron single-particle strengths in neutron-rich nuclei is of interest for nuclear structure and nuclear astrophysics. The distribution of single-particle strengths constrains the effective Hamiltonian and pairing interactions and determines neutron interaction rates that are crucial for understanding the synthesis of heavy nuclei in supernovae via the rapid neutron capture process. Particularly important are the neutron single-particle levels in nuclei near closed neutron shells. Radioactive ion beams from the Holifield Radioactive Ion Beam Facility have been used to study (d,p) reactions in inverse kinematics in order to probe neutron single-particle states in exotic nuclei. The results of a measurement with a 82 Ge beam will be presented.

A method whereby mass differences can be measured using the 25-MV tandem accelerator at the Holifield radioactive ion beam facility (HRIBF) is presented. The technique is unconventional in that mass differences will be measured as position differences at the image of the energy-analyzing magnet following the accelerator and will be made at energies where independent isobaric determination will be possible by a ΔE measurement. The technique is appropriate where the unknown mass is the most neutron-rich isobaric constituent of the beam, such as for neutron-rich fission fragments.

This contribution reviews detectors used for low-intensity radioactive-beam (RB) diagnostics. Emphasis is on diagnostic devices that are based on the detection of secondary products (ions, electrons) resulting from the interaction of the ion beam with a thin detection medium. Devices based on this principle can be tailored to work with low-energy (

B(E2;0+1→2+1) values for the unstable, neutron-rich nuclei 132,134Te were determined through Coulomb excitation, in inverse kinematics, of accelerated beams of these nuclei. The systematics of measured B(E2) values from the ground state to the first excited state have been extended to the N=82 shell closure in the Te nuclei and have been compared with the predictions of different theories. The measurements were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) using the GRAFIK detector. The success of this approach, which couples a 5.7% efficient through-well NaI(Tl) γ-ray detector with thin foil microchannel plate beam detectors, also demonstrates the feasibility for Coulomb excitation studies of neutron-rich nuclei even further from the valley of beta stability, both at present-generation ISOL facilities and at the proposed Rare Isotope Accelerator.

The technique for tagging isobars in a mixed beam using differences in energy lost in an absorber by different isobars has been tested. As expected, isobar separation does improve by allowing more energy loss (thicker absorbers), but such gains could be realized only after achieving good absorber homogeneity. For heavy ions accelerated to low and moderate energies ( MeV / A ) , we found that when homogenous absorbers are used, the largest impediment to achieving good isobar separation rests with uncertainties in energy caused by straggling in the absorber. Measurements of beam energy loss and energy spread were shown to come close to predicted values when accounting for both collisional and charge-exchange contributions to the calculated energy straggling. Reliable prediction of energy straggling then allowed us to study the efficacy of this method for isobar separation when applied to different mass ranges and beam energies. Time-of-flight was used to measure energy loss since this method allows handling of counting rates in excess of 1 MHz and the demands on timing detector resolution and length of flight path were moderate for all cases under study here. Partial separation in a most difficult case, an analyzed beam of A=132 isobars at energies near 3 MeV / A has been demonstrated. The time-of-flight information can be added to the data stream for events of interest, as an additional parameter (tag) to the online data stream. Such event-by-event tagging enables one to study the effect of difference in isobaric mixture in the beam on the reaction outcome even when isobar separation is not complete.

Two 1− levels at 5.35(10) and 6.85(5) MeV were observed for the first time in 20O. The strong direct excitation and subsequent γ-ray decay of these states in virtual photon scattering at 100 MeV/nucleon, along with B(Eλ) predictions for 20O states in this energy region, established their dipole character. The extracted B(E1)↑ values of ≃0.062(16) e 2 fm 2 and ≃0.035(9) e 2 fm 2 for the 5.35 and 6.85 MeV states, respectively, are significantly larger than shell model calculations predict. Such large dipole strengths are not observed for low-lying 1− states in 18O, indicating a shift of dipole strength towards lower energies as one approaches the neutron drip-line.

Excitation of the Isovector Giant Quadrupole Resonance in 208Pb by Coulomb Inelastic Scattering R. L. Varner, J. R. Beene, M. Chartier, ∗ J. F. Liang, D. Shapira, D. Bazin, B. Blank, ∗ B. Sherrill, M. Thoennessen, P. A. DeYoung, and G. F. Peaslee Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6368 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824

A position-sensitive timing detector suitable for detection of low-energy heavy ions is described. The device is based on the detection of secondary electrons emitted following ion impact on a surface. Using a combination of magnetic and electric fields these devices can deliver sub-nanosecond time definition and sub-millimeter position information for ion impact even at incidence rates above 10 6 p / s . When choosing a thin foil for the surface intercepting the heavy ion trajectory, multiple detectors can be used and reliable tracking of heavy ions with moderate energies (≥1 MeV / A ) becomes feasible.

The breakup of radioactive 17 F into a proton and 16 O was measured by bombarding 208 Pb with 170 MeV 17 F. The angular correlations of the fragments and the energy distributions of the protons suggest that the dominant breakup mechanism is a direct process in which a proton is excited into the low energy continuum above the breakup threshold. The breakup cross section measured near the grazing angle is 6.6±0.7 mb/sr for analysis with postacceleration considered and 3.9±0.4 mb/sr without postacceleration. This cross section is small compared to the fusion cross section at energies near the Coulomb barrier, so the breakup can have little influence on the fusion process.

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.

The recently commissioned Recoil Mass Spectrometer (RMS) at the Holifield Radioactive Ion Beam Facility (HRIBF) is described. Consisting of a momentum separator followed by an E-D-E Rochester-type mass spectrometer, the RMS is the centerpiece of the nuclear structure endstation at the HRIBF. Designed to transport ions with rigidities near K = 100, the RMS has acceptances of +/- 10% in energy and +/- 4.9% in mass-to-charge ratio. Recent experimental results are used to illustrate the detection capabilities of the RMS, which is compatible with many detectors and devices. (C) 2000 Elsevier Science B.V. All rights reserved.

The factors affecting the performance of a Position-Sensitive Timing Detector (PSTD), based on the detection of secondary electrons ejected from a thin foil by the passing ion, were investigated. Different contributions to the observed (generally poor) position resolution were investigated. The effects of multiple scattering in the foil were determined first. We then measured the contribution of the position-sensitive detector and signal-handling electronics. Finally, the effect of electron transport, from the point of emission at the foil surface to the micro-channel plate detector (MCP) surface, was measured and evaluated. Our measurements, as well as a simulation of the electron transport, lead us to conclude that for detectors with foil-to-MCP distance ≥5 cm, electron transport limits the devices currently in use to resolution worse than 2 mm (FWHM).

A focal plane detector has been constructed for detecting fusion residues in a gas-filled Enge split-pole spectrograph. It provides position, timing and energy for particle identification. With a timing channel plate detector in the target chamber, the particle time-of-flight can be measured. This detector system is particularly suitable for studying fusion–evaporation reactions in inverse kinematics.

A three dimensional beam profile monitor based on tracking the ionization of the residual gas molecules in the evacuated beam pipe is described. Tracking in position and time of the ions and electrons produced in the ionization enables simultaneous position sampling in three dimensions. Special features which make it possible to sample very low beam currents were employed. The characteristics of this detector make it particularly suitable for sampling beams produced at radioactive beam facilities, provided an auxiliary gas feed can be utilized.

The 17 F( p ,γ) 18 Ne direct capture cross section is important for understanding nucleosynthesis in novae. We have measured cross sections for the proton-transfer reaction 14 N( 17 F, 18 Ne) 13 C in order to determine asymptotic normalization coefficients for the 17 F+ p system and hence the 17 F( p ,γ) 18 Ne direct capture cross section. The technique and preliminary results are presented.

The inclusive single photon spectrum has been measured in the p T range from a few MeV/ c up to approximately 1 GeV/ c at several rapidities in the interval from −2.1 y cm c proton-beryllium and proton-tungsten collisions at the Brookhaven AGS. The measured photon distributions are compared with the expected distribution of photons from π 0 decays and hadronic bremsstrahlung in a search for new sources of “soft” photons at low transverse momentum.

A detector system with a large acceptance, high granularity and a logarithmic response, has been built. It is well suited to detect simultaneously, heavy and light fragments from nucleus-nucleus collisions at energies higher than 10 MeV/nucleon. The angular range spanned by this detector system is ± 20° in-plane and ± 16° out-of-plane with the exclusion of a 2.5° cone around the beam axis. Coverage with a large dynamic range is provided with four large gas filled ionization chambers and behind them a 192-element hodoscope made of individual fast + slow plastic scintillation telescopes in a “phoswich” arrangement.

The back angle scattering of 20 Ne and 24 Mg ions from 12 C display structured excitation functions and oscillatory angular distributions. These measurements bridge the gap between the previously studied 12 C + 16 O and 12 C + 28 Si systems.

The elastic and inelastic scattering of 18 O ions at 120 MeV from a target of 208 Pb have been studied. Cross sections for excitation of the 2 + state at 1.982 MeV in 18 O and of the 3 − state at 2.61 MeV in 208 Pb were measured. In addition, the populations of the m -substates for the 18 O excitation were deduced from the Doppler-broadened line shapes. The data were subjected to a coupled-channels analysis using either Woods-Saxon or folding-model potentials. In addition, the 18 O excitation was found to be described very well by use of a semi-microscopic model. The analyses consistently indicated the presence of a positive static hexadecapole moment of several e · fm 4 for the 2 + state of 18 O. The m -substate population distributions were found to be better fitted if a vector spin-orbit coupling was introduced for the 2 + state of 18 O with a sign opposite to that for the nucleon-nucleus case.

Excitation functions at 7° (lab) have been measured from E c.m. = 5.1 to 11.4 MeV in approximately 114 keV steps for 15 groups of final states in 17 O populated by the 12 C( 9 Be, α) reaction. Statistical tests have been used to locate possible non-statistical structure in the excitation functions. Possible anomalies were found near E c.m. = 6.3, 7.5, 8.9 and 9.7 MeV. Angular distributions were measured at E c.m. = 9.20, 9.71 and 10.23 MeV for the three lowest excited states in 17 O. The data have been compared with Hauser-Feshbach calculations in addition to the following reaction mechanisms: compound plus a single resonance, compound plus interfering resonances and compound plus direct reactions.

The decay times for two new plastic scintillators, the Nuclear Enterprises NE115 and the Bicron BC444, have been measured. They were determined to be 344 and 287 ns respectively. The light yield from an NE115 scintillator was also measured as a function of energy in the range 22.3–59.8 MeV. A good fit to these data points was obtained with the polynomial y = ax 1.06 , indicating minimal nonlinear effects for this scintillator in this energy range.

Cross sections for the 28 Si( 18 O, 18 F) 28 Al reaction have been measured at 19.6 MeV/nucleon. Realistic microscopic DWA calculations using a double folding model with an M3Y effective N-N interaction account for a negligible fraction of the previously measured cross sections at ∼3 MeV/nucleon, but give ∼30% of those observed at 19.6 MeV/nucleon.

Excitation functions were measured for states of 21 Ne populated by the 12 C ( 13 C , α) reaction over the bombarding energy range E lab = 18.2–32.0 MeV (18.4–27.0 MeV) at θ lab = 7°(25°) in in 200 keV steps, and average coherence widths of states and the moment of inertia of the compound nucleus 25 Mg were obtained from these excitation functions. A statistical analysis of these data was performed. Angular distributions for states in 21 Ne to 10 MeV in excitation energy were measured at θ lab = 7°, 18°, 28° and 43° at bombarding energies from 29.0 to 31.0 MeV in 400 keV steps. These data along with Hauser-Feshbach predictions allow us to suggest spins for some states as well as to suggest possible candidates for rotational bands in 21 Ne.

A compact gas cell target for studying fusion and damped collisions of light nuclei has been constructed. The specifications, geometry and other features of the target are described. Typical experimental results obtained with the gas target are presented.