Search

Radioactive ^{129}Sb, which can be treated as a proton plus semimagic ^{128}Sn core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, B(E2), for the 2^{+}⊗πg_{7/2} multiplet members and candidate πd_{5/2} state were measured. The results indicate that the total electric quadrupole strength of ^{129}Sb is a factor of 1.39(11) larger than the ^{128}Sn core, which is in stark contrast to the expectations of the empirically successful particle-core coupling scheme. Shell-model calculations performed with two different sets of nucleon-nucleon interactions suggest that this enhanced collectivity is due to constructive quadrupole coherence in the wave functions stemming from the proton-neutron residual interactions, where adding one nucleon to a core near a double-shell closure can have a pronounced effect. The enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.

This article reports on the development and experimental results of commercial plasma display panels adapted for their potential use as micropattern gas radiation detectors. The plasma panel sensors (PPS) design an materials include glass substrates, metal electrodes and inert gas mixtures which provide a physically robust, hermetically-sealed device. Plasma display panels used as detectors were tested with cosmic ray muons, beta rays and gamma rays, protons and thermal neutrons. The results demonstrated rise times and time resolution of a few nanoseconds, as well as sub-millimeter spatial resolution compatible with the pixel pitch., Submitted to NIM

The population of evaporation residue entry states in the decay of the compound nucleus 160Er*(54 MeV) is investigated in a cross-bombardment employing the reactions 160 + 144Nd and 64Ni + 96Zr. Evaporation residue cross sections and entry state 7-ray fold distributions of the dominant exit channels were obtained for each reaction, using a 4π 7-ray detection system. An entrance-channel dependence of the 7-ray fold distributions of the xn products is observed. This effect is described successfully by the statistical model making use of compound nucleus angular momentum distributions obtained with a fusion model that provides a good description of the bombarding energy dependence of fusion data for both reactions. In accordance with recent findings on the decay of 164Yb*, it is suggested that the observed differences in the population of the dominant exit channels originate from the primary spin distributions rather than a possible dependence of the compound nucleus decay on the formation mode.

A study of signals originating near the lithium-diffused n+ contact of p-type point contact (PPC) high purity germanium detectors (HPGe) is presented. The transition region between the active germanium and the fully dead layer of the n+ contact is examined. Energy depositions in this transition region are shown to result in partial charge collection. This provides a mechanism for events with a well defined energy to contribute to the continuum of the energy spectrum at lower energies. A novel technique to quantify the contribution from this source of background is introduced. Experiments that operate germanium detectors with a very low energy threshold may benefit from the methods presented herein.

The plasma panel sensor (PPS) is a gaseous micropattern radiation detector under current development. It has many operational and fabrication principles common to plasma display panels. It comprises a dense matrix of small, gas plasma discharge cells within a hermetically sealed panel. As in plasma display panels, it uses nonreactive, intrinsically radiation-hard materials such as glass substrates, refractory metal electrodes, and mostly inert gas mixtures. We are developing these devices primarily as thin, low-mass detectors with gas gaps from a few hundred microns to a few millimeters. The PPS is a high gain, inherently digital device with the potential for fast response times, fine position resolution (

The plasma panel sensor is an ionizing photon and particle radiation detector derived from PDP technology with high gain and nanosecond response. Experimental results in detecting cosmic ray muons and beta particles from radioactive sources are described along with applications including high energy and nuclear physics, homeland security and cancer therapeutics.

The time profiles of Cu, Sn, and Ni ions extracted from a hot-cavity resonant ionization laser ion source are investigated. The ions are produced in the ion source by three-photon resonant ionization with pulsed Ti:Sapphire lasers. Measurements show that the time spread of these ions generated within laser pulses of about 30 ns duration could be larger than 100 μs when the ions are extracted from the ion source. A one-dimensional ion-transport model using the Monte Carlo method is developed to simulate the time dependence of the ion pulses. The prediction of the model agrees reasonably well with the experimental data. To reproduce the observed ion time profiles, we find it necessary to postulate that ion-wall collisions are suppressed inside the ion source by an undetermined ion confinement mechanism and that a substantial fraction of the extracted ions are generated in the vapor-transfer tube rather than the hot cavity. Three-dimensional modeling will be necessary to understand the strong reduction in losses expected from ion-wall collisions which we interpret as evidence for confinement.

A radiation detector technology based on plasma display panels (PDPs), the underlying engine of panel plasma television displays, is being investigated. Emerging from this well-established television technology is the Plasma Panel Sensor (PPS), a novel variant of the micro-pattern radiation detector. The PPS is fundamentally a fast, high-resolution detector comprised of an array of plasma discharge cells, operating in a hermetically sealed gas mixture. We report on the PPS development effort, including proof-of-principle results of laboratory signal observations.

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.

An ISOLDE-type hot-cavity laser ion source based on high-repetition-rate Ti:Sapphire lasers has been set up at the Holifield radioactive ion beam facility. To assess the feasibility of the all-solid-state laser system for applications at advanced radioactive ion beam facilities, spectroscopy and performance tests have been conducted with this source. The results of recent studies on excitation schemes, source efficiency, beam emittance and ion time structure are presented.

In preparation of a laser ion source, we have investigated multi-step laser ionization via Rydberg and autoionizing states for atomic Ni and Ge using a mass separator with an ion beam energy of 20 keV. For both elements resonant three-step excitation schemes suitable for modern Ti:sapphire laser systems were developed. Rydberg series in the range of principal quantum numbers 20 n 80 were localized, assigned and quantum numbers were allocated to the individual resonances. Ionization potentials (IP) were extracted from fits of the individual series and quantum defects of individual levels were analysed for confirmation of series assignment. For Ni the ionization potential could be extracted with significantly increased precision compared to literature with a value of EIP (Ni) = 61 619.77(14) cm −1 . Also, at least one notable autoionizing state above the first IP was discovered for both elements, and the different ionization schemes via Rydberg or autoionizing states were compared with respect to line shape, ionization efficiency and selectivity.

A renewed interest in Accelerator Mass Spectrometry (AMS) from nuclear physics laboratories is emerging in connection with Radioactive Ion Beams (RIBs). At the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL) we are exploring the AMS capabilities of the 25-MV tandem accelerator. Behind this effort is the realization that two fields of research – AMS and RIBs – complement each other in techniques. Development of effective and efficient beam purification techniques is of common interest to both AMS and the RIB program. Two main characteristics of the 25-MV tandem provide unique opportunities for performing the highest sensitivity measurements of AMS; namely (i) the highest operating voltage in the world, and (ii) a folded geometry which involves a 180° magnet in the terminal. For the RIB program, we have used AMS techniques to improve the sensitivity of detection of some radioactive species in the measurement of unknown masses of n-rich nuclei. For AMS, we have concentrated in exploring two important isotopes, 14C and 36Cl, for applications that require the highest sensitivity. We have successfully measured 36Cl/Cl ratios as low as a few times 10−16 in seawater samples demonstrating that our setup has the highest sensitivity for this isotope and proving that 36Cl can be measured at the levels required for a tracer in oceanography.

A gas-filled RF quadrupole ion beam cooler has been significantly improved for cooling negative ion beams. Electron detachment plays a significant role in determining the transmission of negative ions. It has been suggested that collisions with the residual buffer gas in the deceleration and injection region of the ion beam cooler, where the ions still possess relatively large energies, could result in substantial losses in negative ions due to electron detachment. The ion beam cooler is modified to improve the pumping conductance in the quadrupole entrance region for effectively removing the buffer gas and minimizing the residual gas escaping to the deceleration and injection region. This significantly reduces the collisions encountered by the ions before they are retarded to energies below the threshold for electron detachment. Overall transmission efficiencies of more than 50% have been obtained with the modified cooler for some negative ions.

The tin chain of isotopes provides a unique region in which to investigate the evolution of single-particle structure, spreading from N = 50 at 100Sn, through 10 stable isotopes and the N = 82 shell closure at 132Sn out into the r-process path. Direct reactions performed on radioactive ion beams are sensitive spectroscopic tools for studying exotic nuclei. Here we present one experiment knocking out neutrons from tin isotopes that are already neutron deficient and two reactions that add a neutron to neutron-rich 130Sn. Both techniques rely on selective particle identification and the measurement of γ rays in coincidence with charged ions. We present the goals of the two experiments and the particle identification for the channels of interest. The final results will be presented in future publications.

The planned rare isotope accelerator facility RIA in the US would become the most powerful radioactive beam facility in the world. RIA's driver accelerator will be a device capable of providing beams from protons to uranium at energies of at least 400 MeV per nucleon, with beam power up to 400 kW. Radioactive beam production relies on both the in-flight separation of fast beam fragments and on the ISOL technique. In both cases the high beam power poses major challenges for target technology and handling and on the design of the beam production areas. This paper will give a brief overview of RIA and discuss aspects of ongoing conceptual design work for the RIA target areas.

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.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation and single-nucleon transfer. The B(E2;0+ → 2+) values for first 2+ excited states of even-even neutron-rich 132-136Te and 126-130Sn have been measured by Coulomb excitation in inverse kinematics. 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 νi13/2 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.

We describe an experiment optimized to determine the transition probabilities for excitation of the first excited 2+ state in 132Sn. The large excitation energy (4.04 MeV) and consequent small excitation cross-section, together with the modest beam intensity available makes this a challenging experiment. The preliminary result is B(E2; 0+ → 2+) = 0.11 ± 0.03e 2 b 2. The high efficiency and generalized nature of the setup enabled us to also measure the first 2+ state in the two-neutron nucleus 134Sn. We have determined a value of B(E2; 0+ → 2+) = 0.029 ± 0.005e 2 b 2 which shows no sign of the asymmetry with respect to the N = 82 shell closure exhibited by the Te isotopes.

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.

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.

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.

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.

Radioactive ion beams are produced at the Holifield Radioactive Ion Beam Facility using the Isotope Separation On-Line (ISOL) technique where the atoms are produced in a thick target, transported to an ion source, ionized, and extracted from the ion source to form an ion beam. These radioactive ion beams are then accelerated to energies of a few MeV per nucleon and delivered to experimental stations for use in nuclear physics and nuclear astrophysics experiments. At the heart of this facility is the RIB production target, where the radioactive nuclei are produced using beams of light ions (p, d, 3He, α) to induce nuclear reactions in the target nuclei. Several target materials have been developed and used successfully, including Al2O3, HfO2, SiC, CeS, liquid Ge, liquid Ni, and a low-density matrix of uranium carbide. The details of these targets and some of the target developments that led to the delivery of high-quality radioactive ion beams are discussed in this paper.

Evaporation residue cross sections have been measured using neutron-rich radioactive 132Sn beams incident on a 64Ni target in the vicinity of the Coulomb barrier. This is the first experiment using accelerated 132Sn beams to study nuclear reaction mechanisms. The average beam intensity was 2x 104 particles per second and the smallest cross section measured was less than 5 mb. A large sub-barrier fusion enhancement was observed compared to evaporation residue cross sections for 64Ni on stable even Sn isotopes. The enhancement cannot be accounted for by a simple barrier shift due to the change in nuclear sizes. Coupled-channel calculations including inelastic excitation underpredict the measured cross sections below the barrier. The presence of several neutron transfer channels with large positive Q values suggests that multinucleon transfer may play an important role in enhancing the fusion of 132Sn and 64Ni.

The width of the giant dipole resonance (GDR) built on excited states was determined from a measurement of γ -decays in coincidence with 17 O particles scattered inelastically from 120 Sn. The bombarding energy was 80 MeV/u. A width of 4±1 MeV, consistent with the width of the GDR built on the ground state, was found at a temperature T =1 MeV. This result is in disagreement with adiabatic thermal shape fluctuation calculations, indicating an overestimation of the influence of thermal shape fluctuations at low temperature.

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.

Negatively charged atomic ions have been cooled in gas-filled rf-quadrupoles from injection energies between 10 and 150 eV to final energies below 0.5 eV. By choosing the atoms or molecules of the buffer gas to have low masses as compared to the masses of the ions to be cooled, the energy transfer in ion–atom or ion–molecule collisions is minimized. For small enough initial ion energies thus only a small percentage of the ions is neutralized in the collision processes. For Cl− and I− ions cooled in a He buffer gas we have achieved absolute transmission efficiencies of more than 50%.

Studies have been conducted to determine the feasibility of using collisional cooling for reducing emittances and energy spreads in negative-ion beams to levels commensurate with effective isobaric purification with conventional high-resolution electromagnetic isobar separators as required for use at the Holifield Radioactive Ion Beam Facility (HRIBF). We have designed a gas-filled radio frequency quadrupole ion cooler equipped with provisions for retarding energetic negative-ion beams to energies below thresholds for electron detachment at injection and for re-acceleration to initial energies after the cooling process. The device has been used to cool several ion beams with initial energy spreads, ΔE>10 eV to final energy spreads, ΔE∼2 eV FWHM, including O − and F − . Overall transmission efficiencies of ∼14% for F − beams have been obtained. Experimental results show that electron detachment is the major loss mechanism for negative ions.

$\ensuremath{\gamma}$-ray transitions in ${}^{134}\text{Te}$, ${}^{135}\text{Te}$, and ${}^{136}\text{Te}$ were measured from ${}^{9}\text{Be}$-induced reactions with a radioactive ${}^{132}\text{Sn}$ beam at a sub-Coulomb barrier energy of 3 MeV per nucleon using particle-$\ensuremath{\gamma}$ coincidence spectroscopy. The transitions were selected by gating on alpha-like particles in a CsI detector following a combination of $({}^{9}\text{Be},\ensuremath{\alpha}1n)$, $({}^{9}\text{Be},\ensuremath{\alpha}2n)$, and $({}^{9}\text{Be},\ensuremath{\alpha}3n)$ incomplete fusion-evaporation reactions. Distorted-wave Born approximation calculations suggest little to no contribution from the $({}^{9}\text{Be},{}^{7}\text{He})$, $({}^{9}\text{Be},{}^{6}\text{He})$, and $({}^{9}\text{Be},{}^{5}\text{He})$ direct reactions.$\ensuremath{\gamma}$-ray transitions from previously known ${2}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ and ${4}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ multiplet members in ${}^{135}\text{Te}$ are observed. A new $\ensuremath{\gamma}$ ray is observed, assigned to the third-excited state in ${}^{135}\text{Te}$, and new ${2}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ multiplet members are suggested. In addition, spin assignments are made by using recent one-neutron transfer data. The updated experimental data for ${}^{135}\text{Te}$ are compared to shell-model calculations for a relatively complete set of states up to the yrast ${15/2}^{\ensuremath{-}},{4}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ multiplet member at 1505 keV.

Single-neutron states in $^{133}\mathrm{Sn}$ and $^{209}\mathrm{Pb}$, which are analogous to single-electron states outside of closed atomic shells in alkali metals, were populated by the ($^{9}\mathrm{Be}$, $^{8}\mathrm{Be}$) one-neutron transfer reaction in inverse kinematics using particle-$\ensuremath{\gamma}$ coincidence spectroscopy. In addition, the ${s}_{1/2}$ single-neutron hole-state candidate in $^{131}\mathrm{Sn}$ was populated by ($^{9}\mathrm{Be}$, $^{10}\mathrm{Be}$). Doubly closed-shell $^{132}\mathrm{Sn}$ (radioactive) and $^{208}\mathrm{Pb}$ (stable) beams were used at sub-Coulomb barrier energies of 3 MeV per nucleon. Level energies, $\ensuremath{\gamma}$-ray transitions, absolute cross sections, spectroscopic factors, asymptotic normalization coefficients, and excited-state lifetimes are reported and compared with shell-model expectations. The results include a new transition and precise level energy for the $3{p}_{1/2}$ candidate in $^{133}\mathrm{Sn}$, new absolute cross sections for the $1{h}_{9/2}$ candidate in $^{133}\mathrm{Sn}$ and $3{s}_{1/2}$ candidate in $^{131}\mathrm{Sn}$, and new lifetimes for excited states in $^{133}\mathrm{Sn}$ and $^{209}\mathrm{Pb}$. This is the first report on excited-state lifetimes of $^{133}\mathrm{Sn}$, which allow for a unique test of the nuclear shell model and $^{132}\mathrm{Sn}$ double-shell closure.

Single-neutron states in (133)Sn and (209)Pb, which are analogous to single-electron states outside of closed atomic shells in alkali metals, were populated by the ((9)Be, (8)Be) one-neutron transfer reaction in inverse kinematics using particle-γ coincidence spectroscopy. In addition, the s(1/2) single-neutron hole-state candidate in (131)Sn was populated by ((9)Be, (10)Be). Doubly closed-shell (132)Sn (radioactive) and (208)Pb (stable) beams were used at sub-Coulomb barrier energies of 3 MeV per nucleon. Level energies, γ-ray transitions, absolute cross sections, spectroscopic factors, asymptotic normalization coefficients, and excited-state lifetimes are reported and compared with shell-model expectations. The results include a new transition and precise level energy for the 3p(1/2) candidate in (133)Sn, new absolute cross sections for the 1h(9/2) candidate in (133)Sn and 3s(1/2) candidate in (131)Sn, and new lifetimes for excited states in (133)Sn and (209)Pb. This is the first report on excited-state lifetimes of (133)Sn, which allow for a unique test of the nuclear shell model and (132)Sn double-shell closure.

The 16 O + 12 C elastic cross section at 300 MeV has been measured over a wide angular range, displaying the shoulder of a nuclear rainbow. The optical model analysis was done using a microscopic potential as well as phenomenological real potentials. The results of this study confirm the potential characteristics found in previous studies at higher and lower energies.

The proton-stripping reaction from a 11Be radioactive beam incident on a beryllium target demonstrates that only (7±3)% of the 9Li residues in the reaction are in coincidence with the 2.7 MeV γ-ray corresponding to the 9Li first excited state. This implies that the previously observed low-energy neutrons from the decay of the unbound nucleus 10Li represent a direct l=0 transition to the 9Li ground state. Consequently, neutron-unbound 10Li is proven to have the same parity inversion as occurs in the case of 11Be with a 1 s 1 2 intruder state below the natural parity 0 p 1 2 state.

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

Radioactive ion beams of ${}^{17}\mathrm{F}$ were used to study several resonance states in ${}^{18}\mathrm{Ne}$. Clear evidence for simultaneous two-proton emission from the 6.15 MeV state $({J}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1}^{\ensuremath{-}})$ in ${}^{18}\mathrm{Ne}$ has been observed with the reaction ${}^{17}\mathrm{F}{+}^{1}\mathrm{H}$. Because of limited angular coverage, the data did not differentiate between the two possible mechanisms of simultaneous decay, diproton ${(}^{2}\mathrm{He})$ emission or direct three-body decay. The two-proton partial width was found to be $21\ifmmode\pm\else\textpm\fi{}3\mathrm{eV}$ assuming ${}^{2}\mathrm{He}$ emission and $57\ifmmode\pm\else\textpm\fi{}6\mathrm{eV}$ assuming three-body decay. The total width of the ${1}^{\ensuremath{-}}$ state was measured to be $50\ifmmode\pm\else\textpm\fi{}5\mathrm{keV}$. Several additional resonances that decay by single proton emission were also studied.

The time delays in fission induced by bombardment of W with 180 MeV $^{32}\mathrm{S}$, $240\text{\ensuremath{-}}255$ MeV $^{48}\mathrm{Ti}$, $330\text{\ensuremath{-}}375$ MeV $^{58}\mathrm{Ni}$, and 390 MeV $^{74}\mathrm{Ge}$ have been measured by observation of crystal blocking. Nearly all results are consistent with exponential decay with lifetimes of order ${10}^{\ensuremath{-}18}$ s which depend weakly on the atomic number of the composite nucleus. This is inconsistent with the Bohr-Wheeler model of fission from a compound nucleus in statistical equilibrium at each stage in a neutron evaporation cascade and supports a picture of strongly damped quasifission. A simple diffusion model with one-body dissipation reproduces roughly the observed time scale and the exponential decay. It suggests that the outer fission barrier could play a significant role in the observed, very slow decays.

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.

In this report, we define criteria for choosing target materials and for designing, mechanically stable, short-diffusion-length, highly permeable targets for generation of high-intensity radioactive ion beams (RIBs) for use at nuclear physics and astrophysics research facilities based on the ISOL principle. In addition, lists of refractory target materials are provided and examples are given of a number of successful targets, based on these criteria, that have been fabricated and tested for use at the Holifield Radioactive Ion Beam Facility (HRIBF).

We have demonstrated that radioactive beams of Al 17 F + can be produced with intensities as high as 3 × 10 8 ions/s using the conventional Isotope Separation On-Line (ISOL) technique employing fibrous HfO 2 target material and an external source of Al vapor. Commercially available HfO 2 cloth composed of Φ =4–6 μm fibers packed to ∼12% of the bulk density of HfO 2 , was irradiated with ∼2 μA of 44 MeV deuterons from the Oak Ridge Isochronous Cyclotron (ORIC). The target material (1.6 g/cm 2 ) was contained in a high-temperature reservoir which was coupled through a heated transfer line, to a conventional plasma ion source where the radioactive ion beam was extracted. During irradiation, a flux of elemental Al vapor was directed through the fibrous material forming a transportable molecule which was then extracted from the source as Al 17 F + . A detailed description of this technique is given and the results of this initial experiment are discussed. The target system was used to inject 17 F into the tandem electrostatic accelerator at the Holifield Radioactive Ion Beam Facility (HRIBF) and used to study the 17 F(p,p) 17 F excitation function in the 17 F energy range of 10–12 MeV.

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.

The Holifield Radioactive Ion Beam Facility (HRIBF) is a first generation national user facility for nuclear physics and nuclear astrophysics research with radioactive ion beams (RIBs). The reconfiguration, construction, and equipment commissioning phases have been completed and the beam development program is in progress. In this article, descriptions of the facility and newly implemented experimental equipment for use in the nuclear and astrophysics programs will be given and an outline of the initial experimental program will be presented. Special target ion source related problems, endemic to the production of specific short lived RIBs will be discussed. In addition, plans, which involve either a 200 MeV or a 1 GeV proton linac driver for a second generation ISOL facility, will be presented.

The performances of electron cyclotron resonance (ECR) ion sources, in terms of high-charge-state yields and intensities within a particular charge state, can be enhanced by increasing the physical sizes of the ECR zones in relation to the sizes of their plasma volumes. The creation of a large ECR plasma “volume” permits coupling of more power into the plasma, resulting in the heating of a much larger electron population to higher energies, the effect of which is to produce higher charge-state distributions and higher intensities within a particular charge state than possible in present forms of the ECR source. The ECR plasma “volumes” of traditional B-minimum ECR sources can be increased by injecting broadband microwave radiation (multiple-discrete-frequency, variable frequency, or broad-band-width frequency microwave radiation) derived from standard klystron, gyrotron, or traveling-wave-tube (TWT) technologies (frequency domain). To demonstrate that the frequency domain technique can be used to enhance ...

We have measured total nuclear disintegration cross sections for ${}^{208}\mathrm{Pb}$ ions at 33 TeV $(160\ensuremath{-}\mathrm{GeV}A)$ colliding with C, Si, Cu, Sn, and Pb. Using well established theory, we calculate the nuclear electromagnetic, electron electromagnetic, and the hadronic contributions and find that their sum underestimates the measured cross sections. An additive correction term linear in target ${Z}_{T}$ (i.e., ${120Z}_{T}\mathrm{mb}$) is necessary to bring agreement between theory and experiment. The source of this additional term is unknown.

High-energy -rays from the decay of the giant dipole resonance built on excited states were measured in coincidence with fission for the reactions. Both the -ray multiplicity and the -fission angular correlation were determined; the latter quantity is sensitive to the deformation of the fissioning nucleus. This sensitivity has been used in order to estimate the breakdown of dynamical fission time into its pre- and post-saddle components. A statistical model analysis, using a version of CASCADE modified to include pre- and post-saddle delay times in order to simulate the effects of dynamics, can reproduce the data with and for a dynamical fission time .

The $g$ factor and $B(E2)$ of the first excited ${2}^{+}$ state have been measured following Coulomb excitation of the neutron-rich semimagic nuclide ${}^{134}$Te (two protons outside ${}^{132}$Sn) produced as a radioactive beam. The precision achieved matches related $g$-factor measurements on stable beams and distinguishes between alternative models. The $B(E2)$ measurement exposes quadrupole strength in the ${2}_{1}^{+}$ state beyond that predicted by current large-basis shell-model calculations. This additional quadrupole strength can be attributed to coupling between the two valence protons and excitations of the ${}^{132}$Sn core. However, the wave functions of the low-excitation positive-parity states in ${}^{134}$Te up to ${6}_{1}^{+}$ remain dominated by the $\ensuremath{\pi}{({g}_{7/2})}^{2}$ configuration.