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This paper describes the production and characterization of a 26Al beam comprised of both, its 5 + ground state, and its 0 + isomeric state. The 26Al beam was produced in-flight via the p (26Mg, 26Al)n reaction. The isomer fraction of the 26Al beam was maximized by choosing a bombarding energy of 158.5 MeV for the 26Mg primary beam. The resulting beam had an energy of 120 MeV, a total intensity of 2 × 10 5 particles/sec, a purity of 98% and an isomer content of 70%. This high-quality 26Al isomeric beam was used to study the 26Al m (d, p)27Al reaction relevant for understanding the nucleosynthesis of 26Al in the Galaxy.

A large number of (α,p)(α,p) and (α,n)(α,n) reactions are known to play a fundamental role in nuclear astrophysics. This work presents a novel technique to study these reactions with the active target system MUSIC whose segmented anode allows the investigation of a large energy range of the excitation function with a single beam energy. In order to verify the method, we performed direct measurements of the previously measured reactions 17O (α,n)(α,n)20Ne, 23Na (α,p)(α,p)26Mg, and 23Na (α,n)(α,n)26Al. These reactions were investigated in inverse kinematics using 4He gas in the detector to study the excitation functions in the energy range of about 2–6 MeV in the center of mass. We found good agreement between the cross sections of the 17O (α,n)(α,n)20Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the 23Na (α,p)(α,p)26Mg and 23Na (α,n)(α,n)26Al reactions.

Spectroscopic data, such as precise γ-ray branching and E2/M1 multipole-mixing ratios, provide vital constraints when performing multi-dimensional Coulomb-excitation analyses. Consequently, as part of our new Coulomb-excitation campaign aimed at investigating the role of exotic non-axial (triaxial) deformations in the unstable refractory Ru-Mo isotopes, additional beta-decay data was obtained. These measurements make use of ANL’s CARIBU facility, which provides intense beams of radioactive refractory isotopes along with the excellent efficiency and angular resolution of the GRETINA γ-ray tracking array. In this article, we report on the analysis of the A = 110 decay chain, focussing on the identification of previously unreported states in 110Ru following the decay of 110Tc.

The ^{36}Ar(n,γ)^{37}Ar (t_{1/2}=35 d) and ^{38}Ar(n,γ)^{39}Ar (269 yr) reactions were studied for the first time with a quasi-Maxwellian (kT∼47 keV) neutron flux for Maxwellian average cross section (MACS) measurements at stellar energies. Gas samples were irradiated at the high-intensity Soreq applied research accelerator facility-liquid-lithium target neutron source and the ^{37}Ar/^{36}Ar and ^{39}Ar/^{38}Ar ratios in the activated samples were determined by accelerator mass spectrometry at the ATLAS facility (Argonne National Laboratory). The ^{37}Ar activity was also measured by low-level counting at the University of Bern. Experimental MACS of ^{36}Ar and ^{38}Ar, corrected to the standard 30 keV thermal energy, are 1.9(3) and 1.3(2) mb, respectively, differing from the theoretical and evaluated values published to date by up to an order of magnitude. The neutron-capture cross sections of ^{36,38}Ar are relevant to the stellar nucleosynthesis of light neutron-rich nuclides; the two experimental values are shown to affect the calculated mass fraction of nuclides in the region A=36-48 during the weak s process. The new production cross sections have implications also for the use of ^{37}Ar and ^{39}Ar as environmental tracers in the atmosphere and hydrosphere.

International audience; Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5–4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

Reaction rates involving α-particles are known to be important in different astrophysical scenarios. Direct measurements of these reactions at relevant astrophysical energies are experimentally challenging. This is due to the typically small cross sections of these reactions and the experimental difficulties associated with low intensity radioactive beams needed to study them. As a consequence, most of these reaction rates are still unknown. The MUlti-Sampling Ionization Chamber (MUSIC) detector, a simple and highly efficient active target system, has been developed for measurements of (α, n) and (α, p) reactions. Recent results on the direct measurement of (α, p) and (α, n) reactions in the MUSIC detector and future possibilities are presented in this work.

A new large-acceptance RF beam sweeper was designed, constructed and put into operation with the goal to remove the energy-degraded primary beams tails from radioactive beams (RIB) produced by in-flight transfer or charge-exchange reactions. The system makes use of the velocity difference between the RIB beam of interest and the remaining tails of the primary beam after momentum selection by a bending magnet. The time-delayed primary beam components are deflected vertically out of the beam path by the RF sweeper, significantly reducing the stable beam background. Beam purity for the in-flight radioactive ion beams as high as 96% has been achieved with this system.

The existence of $^{26}\mathrm{Al}$ (${t}_{1/2}=7.17\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{ }\mathrm{yr}$) in the interstellar medium provides a direct confirmation of ongoing nucleosynthesis in the Galaxy. The presence of a low-lying ${0}^{+}$ isomer (${^{26}\mathrm{Al}}^{m}$), however, severely complicates the astrophysical calculations. We present for the first time a study of the ${^{26}\mathrm{Al}}^{m}(d,p)^{27}\mathrm{Al}$ reaction using an isomeric $^{26}\mathrm{Al}$ beam. The selectivity of this reaction allowed the study of $\ensuremath{\ell}=0$ transfers to $T=1/2$, and $T=3/2$ states in $^{27}\mathrm{Al}$. Mirror symmetry arguments were then used to constrain the ${^{26}\mathrm{Al}}^{m}(p,\ensuremath{\gamma})^{27}\mathrm{Si}$ reaction rate and provide an experimentally determined upper limit of the rate for the destruction of isomeric $^{26}\mathrm{Al}$ via radiative proton capture reactions, which is expected to dominate the destruction path of ${^{26}\mathrm{Al}}^{m}$ in asymptotic giant branch stars, classical novae, and core collapse supernovae.

International audience; The 12C+12C fusion reaction is one of the most important for nuclear astrophysics since it determines the carbon ignition in stellar environments. Two experiments which make use of the gamma-particle coincidence technique to measure the 12C+12C S-factors at deep sub barrier energies are discussed. Results are presented showing a decrease of the S-factor below Ec.m. = 3 MeV.

The octupole strength present in the neutron-rich, radiocative nucleus $^{146}$Ba has been experimentally determined for the first time using Coulomb excitation. To achieve this, $A$=146 fission fragments from CARIBU were post-accelerated by the Argonne Tandem Linac Accelerator System (ATLAS) and impinged on a thin $^{208}$Pb target. Using the GRETINA $\gamma$-ray spectrometer and the CHICO2 heavy-ion counter, the reduced transition probability $B(E3;3^-$$\rightarrow$$0^+)$ was determined as 48($^{+21}_{-29}$) W.u. The new result provides further experimental evidence for the presence of a region of octupole deformation surrounding the neutron-rich barium isotopes.

Despite the more than 1 order of magnitude difference between the measured dipole moments in ^{144}Ba and ^{146}Ba, the octupole correlations in ^{146}Ba are found to be as strong as those in ^{144}Ba with a similarly large value of B(E3;3^{-}→0^{+}) determined as 48(+21-29) W.u. The new results not only establish unambiguously the presence of a region of octupole deformation centered on these neutron-rich Ba isotopes, but also manifest the dependence of the electric dipole moments on the occupancy of different neutron orbitals in nuclei with enhanced octupole strength, as revealed by fully microscopic calculations.

International audience; A multi-step Coulomb excitation measurement with the GRETINA and CHICO2 detector arrays was carried out with a 430-MeV beam of the neutron-rich $^{110}$Ru (t$_{1/2}$= 12 s) isotope produced at the CARIBU facility. This represents the first successful measurement following the post-acceleration of an unstable isotope of a refractory element. The reduced transition probabilities obtained for levels near the ground state provide strong evidence for a triaxial shape; a conclusion confirmed by comparisons with the results of beyond-mean-field and triaxial rotor model calculations.

Carbon burning plays a crucial role in stellar evolution, where this reaction is an important route for the production of heavier elements. A particle-γ coincidence technique that minimizes the backgrounds to which this reaction is subject and provides reliable cross sections has been used at the Argonne National Laboratory to measure fusion cross-sections at deep sub-barrier energies in the 12 C+12 C system. The corresponding excitation function has been extracted down to a cross section of about 6 nb. This indicates the existence of a broad S-factor maximum for this system. Experimental results are presented and discussed.

This paper presents an update of an on-going collaborative INL-ANL-ISU integral reactor physics experiment whose objective is to infer the effective neutron capture cross sections for most of the actinides of importance for reactor physics and fuel cycle studies in both fast and epithermal spectra. Some fission products are also being considered. The principle of the experiment is to irradiate very pure actinide samples in the Advanced Test Reactor at INL and, after a given time, determine the amount of the different transmutation products. The determination of the nuclide densities before and after neutron irradiation together with the neutron fluence will allow inference of effective neutron capture cross-sections in different neutron spectra.

The $^{23}\mathrm{Na}(\ensuremath{\alpha},p)^{26}\mathrm{Mg}$ and $^{23}\mathrm{Na}(\ensuremath{\alpha},n)^{26}\mathrm{Al}$ reactions are important for our understanding of the $^{26}\mathrm{Al}$ abundance in massive stars. The aim of this work is to report on a direct and simultaneous measurement of these astrophysically important reactions using an active target system. The reactions were investigated in inverse kinematics using $^{4}\mathrm{He}$ as the active target gas in the detector. We measured the excitation functions in the energy range of about 2 to 6 MeV in the center of mass. We have found that the cross sections of the $^{23}\mathrm{Na}(\ensuremath{\alpha},p)^{26}\mathrm{Mg}$ and the $^{23}\mathrm{Na}(\ensuremath{\alpha},n)^{26}\mathrm{Al}$ reactions are in good agreement with previous experiments and with statistical-model calculations. The astrophysical reaction rate of the $^{23}\mathrm{Na}(\ensuremath{\alpha},n)^{26}\mathrm{Al}$ reaction has been reevaluated and it was found to be larger than the recommended rate.

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of ^3He to ^4He at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the 10^(−14) level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of ^3He/^4He in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

We have studied the $^{14,15}\mathrm{C}(d,^{3}\mathrm{He})^{13,14}\mathrm{B}$ proton-removing reactions in inverse kinematics. The $(d,^{3}\mathrm{He})$ reaction probes the proton occupation of the target ground state, and also provides spectroscopic information about the final states in $^{13,14}\mathrm{B}$. The experiments were performed using $^{14,15}\mathrm{C}$ beams from the ATLAS accelerator at Argonne National Laboratory. The reaction products were analyzed with the HELIOS device. Angular distributions were obtained for transitions from both reactions. The $^{14}\mathrm{C}$-beam data reveal transitions to excited states in $^{13}\mathrm{B}$ that suggest configurations with protons outside the $\ensuremath{\pi}(0{p}_{3/2})$ orbital, and some possibility of proton cross-shell $0p\text{\ensuremath{-}}1s0d$ excitations, in the $^{14}\mathrm{C}$ ground state. The $^{15}\mathrm{C}$-beam data confirm the existence of a broad ${2}^{\ensuremath{-}}$ excited state in $^{14}\mathrm{B}$. The experimental data are compared to the results of shell-model calculations.

The neutron-rich nucleus $^{144}$Ba ($t_{1/2}$=11.5 s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers $A$ less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced $E1$ transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multi-step Coulomb excitation of a post-accelerated 650-MeV $^{144}$Ba beam on a 1.0-mg/cm$^2$ $^{208}$Pb target. The measured value of the matrix element, $\langle 3_1^- \| \mathcal{M}(E3) \| 0_1^+ \rangle=0.65(^{+17}_{-23})$ $e$b$^{3/2}$, corresponds to a reduced $B(E3)$ transition probability of 48($^{+25}_{-34}$) W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation., Comment: 5 pages, 2 figures, 1 table, Physical Review Letters 2016

The Argonne Tandem Linac Accelerator System (ATLAS) is the world's first superconducting accelerator for projectiles heavier than the electron. This unique system is a U.S. Department of Energy (DOE) national user research facility open to scientists from all over the world. It is located within the Physics Division at Argonne National Laboratory and is one of five large scientific user facilities located at the laboratory.

A particle- γ coincidence technique has been used to measure the fusion cross-section in the system 12C+12C at low beam energies. Based on these results, it should be possible to measure this important fusion cross-section down to the 10 pb level within a week-long run.

The use of the gas-filled magnet technique for the detection of intermediate mass ( A ∼ 20–40) recoil nuclei produced in (p, α ) reactions in inverse kinematics has been investigated. Through a series of calibration measurements with 27 Al, 28,29 Si and 33 S beams the optimum parameterization for calculating the average charge-state distribution in a gas-filled magnet has been determined. By measuring the magnetic rigidity, the time-of-flight and the differential energy loss of the particles at the focal plane of a gas-filled Enge Split Pole spectrograph it was possible to separate and identify the (p, α ) reaction products from elastically scattered particles at very small scattering angles. This technique was then tested by measuring the p( 33 S, 30 P) α and p( 37 K, 34 Cl) α reactions.

The principle of the proposed experiment is to irradiate very pure actinide samples in the Advanced Test Reactor at INL and, after a given time, determine the amount of the different transmutation products. The determination of the nuclide densities before and after neutron irradiation will allow inference of effective neutron capture cross-sections. This approach has been used in the past and the novelty of this experiment is that the atom densities of the different transmutation products will be determined using the Accelerator Mass Spectrometry technique at the ATLAS facility located at ANL. It is currently planned to irradiate the following isotopes: 232Th, 235U, 236U, 238U, 237Np, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu, 241Am, 243Am, 244Cm and 248Cm.

In this paper we discuss the properties of gas cell targets used for the production of radioactive beams via the in-flight technique as well as gas cell targets used for studies of nuclear reactions in inverse kinematics at the ATLAS heavy ion accelerator.

The near-term radioactive beam capabilities of ATLAS include radioactive beams produced in flight in a gas cell, or starting in the fall of 2009, re-accelerated beams of 252 Cf fission fragments provided by the new CARIBU injector. The availability of such exotic beams will allow for detailed studies of the single-particle aspects of nuclear structure in neutron-rich nuclei reaching out to the astrophysical r-process path by employing light-ion reactions in inverse kinematics. The HELIOS spectrometer is based on a new concept that is especially well suited for such studies. This concept was recently demonstrated using the reactions D (28 Si , p )29 Si with a (stable) 168 MeV 28 Si beam. Since then D (12 B , p )13 B , D (17 O , p )18 O , and D (15 C , p )16 C have been studied successfully. The combination of neutron-rich beams from CARIBU and the HELIOS spectrometer opens a fertile research area of precision studies of the single particle strengths and collective excitations in exotic nuclei, and is likely to have applications in other reactions as well.

Gas catchers allow the transformation of radioactive recoils from various sources into a good optical quality low-energy radioactive beam that is then available for experiments at low-energy or for further acceleration. The CARIBU project uses such a large gas catcher to create beams of neutron-rich isotopes from a Californium source for post-acceleration through the ATLAS superconducting linac to open new research opportunities for nuclear structure physics and astrophysics. The RF gas catcher developed at Argonne has now demonstrated operation at the high intensity required for this application.

A detection technique for high-efficiency measurements of fusion reactions with low-intensity radioactive beams was developed. The technique is based on a Multi-Sampling Ionization Chamber (MUSIC) operating as an active target and detection system, where the ionization gas acts as both target and counting gas. In this way, we can sample an excitation function in an energy range determined by the gas pressure, without changing the beam energy. The detector provides internal normalization to the incident beam and drastically reduces the measuring time. In a first experiment we tested the performance of the technique by measuring the 10,13,15C+12C fusion reactions at energies around the Coulomb barrier. Fil: Carnelli, Patricio Francisco Florencio. Argonne National Laboratory; Estados Unidos. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia Física (Centro Atómico Constituyentes). Proyecto Tandar; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Almaraz Calderón, S.. Argonne National Laboratory; Estados Unidos Fil: Rehm, K. E.. Argonne National Laboratory; Estados Unidos Fil: Albers, M.. Argonne National Laboratory; Estados Unidos Fil: Alcorta, M.. Argonne National Laboratory; Estados Unidos Fil: Bertone, P. F.. Argonne National Laboratory; Estados Unidos Fil: Digiovine, B.. Argonne National Laboratory; Estados Unidos Fil: Esbensen, H.. Argonne National Laboratory; Estados Unidos Fil: Fernandez Niello, Jorge Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia Física (Centro Atómico Constituyentes). Proyecto Tandar; Argentina Fil: Henderson, D.. Argonne National Laboratory; Estados Unidos Fil: Jiang, C. L.. Argonne National Laboratory; Estados Unidos Fil: Lai, J.. Argonne National Laboratory; Estados Unidos Fil: Marley, S. T.. Argonne National Laboratory; Estados Unidos Fil: Nusair, O.. Argonne National Laboratory; Estados Unidos Fil: Palchan Hazan, T.. Argonne National Laboratory; Estados Unidos Fil: Pardo, R.. Argonne National Laboratory; Estados Unidos Fil: Paul, M.. The Hebrew University of Jerusalem; Israel Fil: Ugalde, C.. Argonne National Laboratory; Estados Unidos

The CARIBU facility uses a high-intensity gas catcher system to transform Cf fission fragments into a beam that can then be delivered to experiments at low-energy or at Coulomb barrier energy after post-acceleration through the ATLAS superconducting linac. The method is fast and universal, allowing unique beams of short-lived refractory and reactive isotopes to be extracted and delivered to experiments. The facility hosts an extensive suite of experimental equipment and measurements have been performed with over 100 different neutron-rich isotopes so far. The unique approach used for CARIBU will be described below, together with the present physics program and upcoming upgrades.

International audience; The cross section of the ^12C+^12C fusion reaction at low energies is of paramount importance for models of stellar nucleosynthesis in different astrophysical scenarios, such as Type Ia supernovae and Xray superbursts, where this reaction is a primary route for the production of heavier elements. In a series of experiments performed at Argonne National Laboratory, using Gammasphere and an array of Silicon detectors, measurements of the fusion cross section of ^12C+^12C were successfully carried out with the gamma and charged-particle coincidence technique in the center-of-mass energy range of 3-5 MeV. These were the first background-free fusion cross section measurements for ^12C+^12C at energies of astrophysical interest. Our results are consistent with previous measurements in the high-energy region; however, our lowest energy measurement indicates a fusion cross section slightly lower than those obtained with other techniques.

Total fusion cross section of carbon isotopes were obtained using the newly developed MUSIC detector. MUSIC is a highly efficient, active target-detector system designed to measure fusion excitation functions with radioactive beams. The present measurements are relevant for understanding x-ray superbursts. The results of the first MUSIC campaign as well as the astrophysical implications are presented in this work.

The excitation function for the fusion–evaporation reaction Si 28 + Ni 64 has been measured down to a cross section of 25 nb. This is the first observation of fusion hindrance at extreme sub-barrier energies for a system with a small, negative Q-value ( − 1.78 MeV ). This result is further proof that heavy-ion fusion hindrance, reported earlier only for systems with large, negative Q-values, is a general phenomenon. The measured behavior can be reproduced by coupled-channels calculations with a modified ion–ion potential incorporating the effects of nuclear incompressibility.

The $(d,\ensuremath{\alpha})$ reaction is highly selective, favoring final states in which the removed neutron and proton are completely aligned in a $J=2j$ configuration. We have studied the $^{14,15}\mathrm{C}(d,\ensuremath{\alpha})^{12,13}\mathrm{B}$ reactions in inverse kinematics using the Helical Orbit Spectrometer (HELIOS) at Argonne National Laboratory. In $^{12}\mathrm{B}$, the reaction strongly favors the population of a known ${3}^{+}$ state at 5.61 MeV, and for $^{13}\mathrm{B}$, we observe a possible unreported doublet of states at high excitation energy, probably corresponding to the $^{12}\mathrm{B}({3}^{+})$ state coupled to the $1{s}_{1/2}$ neutron from the $^{15}\mathrm{C}$ ground state. In contrast to single-nucleon transfer, deuteron-transfer reactions have not been widely studied with exotic nuclei.

A new focal-plane detector system has been designed and constructed for measuring very small heavy-ion induced fusion-evaporation cross sections with the Fragment Mass Analyzer (FMA) at Argonne National Laboratory. The system has been tested and successfully used in a fusion-evaporation excitation function measurement for 64 Ni + 100 Mo down to the 5 nb level. The design and performance of the detector system are described in detail with some examples from the latest data. Besides excellent tracking capabilities for each event measured in the detector, suppression of scattered beam background of the order of 4 × 10 - 17 has been demonstrated.

The (8)B(alpha,p)(11)C reaction is part of the network that can bypass the triple a process leading to the production of (12)C. We have measured the astrophysical reaction rate for this reaction by studying the inverse (11)C(p,alpha)(8)B process. The radioactive (11)C beam was produced via the p((11)B,(11)C)n reaction using the in-flight facility at the ATLAS accelerator. The astrophysical reaction rate obtained from the excitation function measured in the energy range E(x)=8.8-10 MeV was found to be a factor of 10-50 higher than previous estimates.

We have studied the elastic scattering of the exotic 17 F nuclei from 208 Pb at Coulomb barrier energies. For the measurement we used a new detector array with pixel structure and a large solid angle. Optical potential analysis shows that 17 F has an imaginary potential much less deep than 19 F. Information on the exclusive 17 F breakup cross section has been deduced too.

The manuscript describes the design, commissioning and performance of a new device for the measurement of bunch time structure of heavy-ion beams. The Bunch Length Detector (BLD) has been developed, constructed and installed at the Argonne Tandem Linear Accelerating System, a superconducting linac. The BLD is a wire scanning detector based on the analysis of secondary electrons produced by the primary beam hitting a target wire. The device provides measuring of bunch phase spectrum for ion beams in a wide range of energies, intensities and species in cw mode with up to 20 ps resolution.

The proposed Rare Isotope Accelerator Facility (RIA) will provide beams of heavy ions with up to 400 kW power in order to produce radioactive beams of nuclei far from stability for research on fundamental questions in nuclear physics and astrophysics. The exact mix of heavy-ion driver beams that will be needed by RIA will depend on the actual experimental programs undertaken, but will surely encompass a wide range of isotopes. We will explore a possible facility operations model and identify the driver beams required. One aspect of that model will be to optimally “illuminate” the r-process path nuclei. Present published ECR ion source performance for the identified mix of beams and assumed charge states will be compared to the requirements of the RIA facility needed to achieve the beam power goals of the facility.

The arbitrary-order map beam optics code COSY Infinity has recently been adapted to calculate accurate high-order ion-optical maps for electrostatic and radio-frequency accelerating structures. The beam dynamics of the superconducting low-velocity positive-ion injector linac for the ATLAS accelerator at Argonne National Lab is used to demonstrate some advantages of the new simulation capability. The injector linac involves four different types of superconducting accelerating structures and has a total of 18 resonators. The detailed geometry for each of the accelerating cavities is included, allowing an accurate representation of the on- and off-axis electric fields. The fields are obtained within the code from a Poisson-solver for cylindrically symmetric electrodes of arbitrary geometry. The transverse focusing is done with superconducting solenoids. A detailed comparison of the transverse and longitudinal phase space is made with the conventional ray-tracing code LINRAY. The two codes are evaluated for ease of optimization, with particular attention to higher-order effects, and future applications are discussed.

A new measurement of fusion reactions down to 10 nb has been performed for the system 64 Ni + 64 Ni. Coupled-channels analyses have been carried out for the systems 60 Ni + 89 Ya nd 64 Ni + 64 Ni. They demonstrate that coupled-channels calculations are unable to reproduce the unexpected steep falloff of the cross sections at extreme sub-barrier energies. Heavy-ion fusion excitation functions are also analyzed in terms of the S factor, as this offers a pragmatic way to study fusion behavior in the energy regime of interest. It is shown that the steep falloff in cross section observed in several heavy-ion systems translates into a maximum of the S factor. The energies where the maximum occurs can be parameterized with a simple empirical formula. The parameterization, which is derived here for rather stiff heavy-ion systems, provides also an upper limit for reactions involving softer nuclei. The asymptotic behavior of reaction cross sections at very low energies is a critical issue for calculating reaction rates of astrophysical interest. Recently, it was pointed out that fusion cross sections for several heavy-ion systems show an unexpected behavior at very low energies, 1) with a much steeper falloff than predicted by coupled-channels calculations, 2) or from Wong’s formula. 3) In the present work, we first present a new result for the excitation function in an open-shell system, 64 Ni + 64 Ni down to the 10 nb level, we then analyse in a coupled-channels description the fusion of 60 Ni + 89 Ya nd 64 Ni + 64 Ni, i.e. the systems which have been measured to extreme sub-barrier energies. Fusion reactions at extreme sub-barrier energies are then studied in a representation in terms of the S factor. This parametrization, which in the past has been successfully applied to fusion reactions with lighter ions at low energies, proves to be an alternative and pragmatic way of characterizing and exploring the behavior of heavy-ion fusion cross sections in the energy domain of interest. To our surprise, a maximum appears systematically in the evolution of S with decreasing energy for all systems that exhibit the steep falloff in cross section described above. This maximum occurs at rather high excitation energies in the compound nucleus.

Among the many production and destruction nuclear reactions that bear on the net 44 Ti yield in the supernova environment, recently highlighted by γ-ray astronomy observations, the α capture on 40 Ca stands out in the α-rich freezeout phase conjectured by astrophysical models. We activate a He-gas target with a 40 Ca beam and implant recoiling 44 Ti nuclei (∼10 6 ) in a Cu catcher. The 44 Ti atoms chemicallyextracted with a nat Ti carrier are counted by accelerator mass spectrometry (AMS). The measurement of the 44 Ti/Ti ratio, together with the amount of Ti carrier added, determine the number of 44 Ti nuclei produced in the activation, independent of chemical yield or transmission efficiency. The resonance strength (7.4±2.5 eV) at E cm ∼1.1 MeV/n determined for two close-by levels of 44 Ti is in excellent agreement with previous prompt-γ measurements. We plan to extend our experiments to the lower energy range.

The 15 O(α,γ) 19 Ne reaction is an important pathway from the hot CNO cycle to the rp process that leads to the synthesis of heavier nuclei. At lower temperatures, the reaction rate is dominated by the transition to the 4.033 MeV 3/2 + state in 19 Ne, the first level above the 15 O(α,γ) threshold at 3.534 MeV. We have performed an indirect measurement of the ratio Λ α Λ γ by populating the 3/2 + state via the 3 He( 20 Ne,α) 19 Ne reaction in inverse kinematics. The α particles were detected in a double-sided Si strip detector telescope while the heavy reaction products, 19 Ne and 15 O, were identified in the focal plane of a magnetic spectrograph. For the 4.033 MeV state in 19 Ne an upper limit Λ α Λ γ of ≤ 7×10 −4 has been obtained.

The bunching system of the ATLAS positive ion injector (PII) has been improved by relocating the harmonic buncher to a point significantly closer to the second stage sine-wave buncher and the injector LINAC. The longitudinal optics design has also been modified and now employs a virtual waist from the harmonic buncher feeding the second stage sine-wave buncher. This geometry improves the handling of space charge for high-current beams, significantly increases the capture fraction into the primary rf bucket and reduces the capture fraction of the unwanted parasitic rf bucket. Total capture and transport through the PII has been demonstrated as high as 80% of the injected dc beam while the population of the parasitic, unwanted rf bucket is typically less than 3% of the total transported beam. To remove this small residual parasitic component a new traveling-wave transmission-line chopper has been developed reducing both transverse and longitudinal emittance growth from the chopping process. This work was supported by the U.S. Department of Energy under contract W-31-109-ENG-38.

This review paper presents the history and status of superconducting RF at the Argonne Tandem Linac Accelerator System (ATLAS) which is a Department of Energy (DOE) National User Facility for research in nuclear physics with accelerated stable and radioactive ion beams.

The Californium Rare Ion Breeder Upgrade (CARIBU) project was conceived to provide neutron rich beams originating from the 3% fission decay branch of a 252Cf source to be accelerated by the Argonne Tandem Linear Accelerator System (ATLAS). This 1Ci 252Cf source will be housed in a movable shielded cask, from which it can be directly transferred into a large helium gas stopper cell. Within the gas stopper, the CARIBU 252Cf source is positioned behind an aluminum degrader foil where the radioactive recoils of interest lose most of their energy before being stopped in the helium gas. To stop recoils over the full fission mass range effectively, three degraders of increasing thickness are required, one to cover the light fission peak and two for the isotopes in the heavy fission peak. The geometry of the source within the gas cell would ideally require a hemispherically shaped degrader foil for uniform energy loss of the fission products. The fabrication of a thin foil of such a shape proved to be exceedingly difficult and, therefore, a compromise “top hat” arrangement was designed. In addition, the ultra-high vacuum (UHV) environment necessary for the gas cell to function properly prevented the use of any epoxy due to vacuum outgassing. Handling, assembling of the foils and mounting must be done under clean room conditions. Details of early attempts at producing these foils as well as handling and mounting will be discussed.

The interaction between neutron-rich nuclei plays an important role for understanding the reaction mechanism of the fusion process as well as for the energy production through pycnonuclear reactions in the crust of neutron stars. We have performed the first measurements of the total fusion cross sections in the systems (10,14,15)C+(12)C using a new active target-detector system. In the energy region accessible with existing radioactive beams, a good agreement between the experimental and theoretical cross sections is observed. This gives confidence in our ability to calculate fusion cross sections for systems which are outside the range of today's radioactive beam facilities.

A technique for producing a high-quality radioactive 56 Ni (T 1/2 =6.1 d ) beam via the two-accelerator method has been developed. Beam intensities of 2×107 56Ni/s were extracted from the ion source and 2×104 56Ni/s were delivered to the target. For a study of neutron transfer reactions in inverse kinematics, a high-efficiency detection system was built consisting of a large solid angle (2.8 sr), high-granularity Si detector array for measuring the outgoing protons in coincidence with the heavy reaction products identified with respect to mass A and nuclear charge Z in the focal plane of a recoil mass separator.

Cross sections for the 44Ti(alpha,p)47V reaction which significantly affects the yield of 44Ti in supernovae were measured in the energy range 5.7 MeV

A method has been developed that can provide beams of many short-lived nuclei of interest in nucleosynthesis along the rp process path. With a {sup 17}F beam (T{sub 1/2}=64 s ) the excitation function of the {sup 17}F( p,thinsp{alpha}){sup 14} O reaction was measured to determine properties of excited states in {sup 18}Ne . These states influence the rate of the {sup 14}O( {alpha},thinspp){sup 17} F reaction which is important for understanding energy generation and nucleosynthesis in x-ray bursts. The present direct measurements yield a pattern of resonances and cross sections which differ substantially from previous estimates. {copyright} {ital 1999} {ital The American Physical Society}