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Influence of quasi-elastic channels on fusion

Description: Experiments that measure the strength of different transfer reactions in heavy reaction systems at energies in the vicinity of the Coulomb barrier are discussed. A short discussion is given of experimental techniques that are available in this field and their advantages and shortfalls. The main features of the transfer reactions are summarized. Questions concerning the system dependence and energy dependence of the strongest reaction channels are addressed. A systematic picture of the strength of the neutron transfer cross sections is presented. Some examples for correlations between fusion exchangement and transfer cross sections are given.
Date: March 1, 1986
Creator: Rehm, K.E.
Partner: UNT Libraries Government Documents Department

Nuclear structure in deep-inelastic reactions

Description: The paper concentrates on recent deep inelastic experiments conducted at Argonne National Laboratory and the nuclear structure effects evident in reactions between super heavy nuclei. Experiments indicate that these reactions evolve gradually from simple transfer processes which have been studied extensively for lighter nuclei such as /sup 16/O, suggesting a theoretical approach connecting the one-step DWBA theory to the multistep statistical models of nuclear reactions. This transition between quasi-elastic and deep inelastic reactions is achieved by a simple random walk model. Some typical examples of nuclear structure effects are shown. 24 refs., 9 figs.
Date: January 1, 1986
Creator: Rehm, K.E.
Partner: UNT Libraries Government Documents Department

Transition from quasi-elastic to deep-inelastic reactions

Description: Heavy ion induced transfer reactions are usually considered to fall into two categories. Quasi-elastic processes, on one hand, are characterized by small energy transfers, with one-nucleon transfer reactions being a typical example. These processes are dominant for grazing collisions, and are generally described within simple one-step DWBA calculations. Deep inelastic reactions, on the other hand, occur for more central collisions where the interaction time is longer and subsequently more energy and particles can be exchanged. Quasi-elastic collisions dominate transfer reactions induced by light heavy ions (e.g., /sup 16/O) at energies not too high above the barrier, while deep inelastic collisions are observed mainly in reactions induced by heavier projectiles (Kr, Xe). In this contribution, we discuss the transition between these two processes for the system /sup 48/Ti + /sup 208/Pb. /sup 48/Ti is located between light (/sup 16/O) and heavy (Kr) projectiles and should be well suited for a study of the interrelation between quasi- and deep-inelastic reactions. The experiments were performed with a 300 MeV /sup 48/Ti beam obtained from the Argonne National Laboratory superconducting linac. The outgoing particles were momentum analyzed in a split pole magnetic spectrograph and detected in the focal plane by a position sensitive ionization chamber. The specific energy loss, the magnetic rigidity and the total energy of the outgoing particles were measured enabling mass and Z-identification. The energy resolution was about 3 MeV, determined by the thickness of the /sup 208/Pb target, and thus excluded study of transfer reactions to discrete final states. Angular distributions were measured in the range theta/sub lab/ = 20/sup 0/ to 80/sup 0/ in steps of 5/sup 0/. 8 refs.
Date: January 1, 1986
Creator: Rehm, K.E.
Partner: UNT Libraries Government Documents Department

How well do we understand quasi-elastic reactions at energies close to the barrier

Description: In collisions between too heavy nuclei a wide spectrum of different reaction modes is observed covering the range from simple processes like elastic scattering to complicated multistep transfers and fusion. On the theoretical side heavy ion reactions are usually analyzed using models that were developed first for light ion induced reactions: the optical model for elastic scattering and the DWBA for more inelastic processes like transfer and inelastic scattering. Some of the assumptions going into these approximations, however, are not valid for heavy ion induced reactions. The region between fusion and quasi-elastic reactions is not well understood theoretically. This region is associated with deep inelastic collisions, which are complex multiparticle reactions involving transfer of several protons and neutrons. In this paper, the author discusses to what extent experiments in the field of quasi-elastic scattering are understood within the framework of various theoretical models and in what areas more work is needed.
Date: January 1, 1988
Creator: Rehm, K.E.
Partner: UNT Libraries Government Documents Department

Multi-neutron transfer reactions at sub-barrier energies.

Description: The optimum conditions for multi-neutron transfer have been studied in the system {sup 58}Ni + {sup 124}Sn at bombarding energies at and below the Coulomb barrier. The experiments were performed in inverse kinematics with a {sup 124}Sn beam bombarding a {sup 58}Ni target. The particles were identified with respect to mass and Z in the split-pole spectrograph with a hybrid focal plane detector with mass and Z-resolutions of A/{Delta}A = 150 and Z/{Delta}Z = 70. At all energies the transfer of up to 6 neutrons was observed. The yields for these transfer reactions are found to decrease by about a factor of four for each transferred neutron.
Date: January 20, 1998
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

Possibilities for experiments with neutron-rich radioactive ion beams in nuclear astrophysics.

Description: While prototype experiments with proton-rich radioactive beams have been formed at various existing facilities, investigations with accelerated neutron-rich beams have to await the availability of the next-generation facilities. In this contribution possible future experiments with neutron-rich beams in the areas of nuclear structure and nuclear astrophysics, as well as novel production and detection techniques for these beams are discussed.
Date: September 27, 1999
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

Recent results of radioactive beam experiments at ATLAS.

Description: Various beams of short- and long-lived radioactive nuclei have recently been produced at the ATLAS accelerator at Argonne National Laboratory, using either the so-called In-Flight or the Two-Accelerator method. With these beams many questions which are of interest in the areas of nuclear structure and nuclear astrophysics can be addressed. In this contribution first results from experiments with {sup 44}Ti (T{sub 1/2}=60y) and {sup 17}F (T{sub 1/2}=64s) beams are discussed, which are of interest to the nucleosynthesis in supernovae and X-ray bursts.
Date: December 1, 2000
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

Measurements with radioactive beams at ATLAS.

Description: Reactions of interest to nuclear astrophysics have been studied with radioactive beams at the ATLAS accelerator. Using a modified ISOL technique, beams of {sup 18}F (T{sub 1/2} = 110 min) and {sup 56}Ni (T{sub 1/2} = 6.1d) were produced and the reactions {sup 18}F(p,{alpha}){sup 15}O, {sup 18}F(p,{gamma}){sup 19}Ne, and {sup 56}Ni(d,p){sup 57}Ni have been investigated. The results indicate that the {sup 18}F(p,{gamma}) route is a small contributor to the breakout from the hot CNO cycle into the rp process, while the {sup 56}Ni(p,{gamma}){sup 57}Cu rate is about ten times larger than previously assumed.
Date: August 6, 1998
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

From Simple to Complex Reactions: Nuclear Collisions Near the Coulomb Barrier

Description: Collisions between two heavy nuclei produce a diverse spectrum of reaction modes which is much wider than that observed in light ion studies. For the latter case, two processes are observed: direct reactions and compound nucleus formation. Heavy ion reaction studies on the other hand have identified additional processes such as deep-inelastic scattering, incomplete fusion and quasi-fission reactions. While the boundaries between the various processes are usually not well defined, it is generally accepted that with increasing overlap of the two nuclei the interaction evolves from distant collisions where only elastic scattering and Coulomb excitation processes occur, through grazing-type collisions associated with quasi-elastic reactions to deep-inelastic and fusion-fission processes requiring a substantial nuclear overlap. Varying the bombarding energy is a convenient way to change the overlap of the two nuclei. Measurements of excitation functions can thus probe the onset and the interplay of the various reaction modes. Experiments at bombarding energies in the vicinity of the Coulomb barrier are particularly suited for comparisons with theoretical predictions since the small number of degrees of freedom involved in the interaction greatly simplifies the calculations. In the first part of this contribution a short overview is given on the status of heavy ion reaction studies at energies in the vicinity of the Coulomb barrier. In the second part two experiments, one involving simple and the other studying complex reactions, are discussed in more detail.
Date: January 1, 1992
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

Quasi-electric processes at low energies and their influence on other reaction modes

Description: Quasi-elastic reactions induced by medium-weight projectiles (A=28-80) were measured with good particle resolution using an Enge Split Pole Magnetic Spectrograph. The cross sections for these processes are considerably larger than the ones observed in similar light-ion-induced reactions. The influence of quasi-elastic processes on other reaction modes is discussed. It is found that strong correlations between the various types of reaction exist, requiring a consistent coupled-channels treatment of all reaction modes involved. 46 refs., 14 figs.
Date: January 1, 1989
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

From simple to complex reactions: Nuclear collisions near the Coulomb barrier

Description: Collisions between two heavy nuclei produce a diverse spectrum of reaction modes which is much wider than that observed in light ion studies. For the latter case, two processes are observed: direct reactions and compound nucleus formation. Heavy ion reaction studies on the other hand have identified additional processes such as deep-inelastic scattering, incomplete fusion and quasi-fission reactions. While the boundaries between the various processes are usually not well defined, it is generally accepted that with increasing overlap of the two nuclei the interaction evolves from distant collisions where only elastic scattering and Coulomb excitation processes occur, through grazing-type collisions associated with quasi-elastic reactions to deep-inelastic and fusion-fission processes requiring a substantial nuclear overlap. Varying the bombarding energy is a convenient way to change the overlap of the two nuclei. Measurements of excitation functions can thus probe the onset and the interplay of the various reaction modes. Experiments at bombarding energies in the vicinity of the Coulomb barrier are particularly suited for comparisons with theoretical predictions since the small number of degrees of freedom involved in the interaction greatly simplifies the calculations. In the first part of this contribution a short overview is given on the status of heavy ion reaction studies at energies in the vicinity of the Coulomb barrier. In the second part two experiments, one involving simple and the other studying complex reactions, are discussed in more detail.
Date: December 1, 1992
Creator: Rehm, K. E.
Partner: UNT Libraries Government Documents Department

Evaluation of target purity using various vacuum systems

Description: Gold targets were prepared under various conditions using both a cryopump and a conventional diffusion pump evaporator system. Measurements were made at the Argonne Tandem Accelerator system to determine the purity of these targets.
Date: January 1, 1984
Creator: Thomas, G.E.; Rehm, K.E. & Klimczak, G.W.
Partner: UNT Libraries Government Documents Department

The influence of {sup 18}F induced reactions in the hot CNO cycle

Description: The contribution of the {sup 18}F(p,{gamma}) reaction to the production of {sup 19}Ne, which is an important isotope in connection with the breakout from the hot CNO cycle, has been investigated in experiments with {sup 18}F beams. Measurements of the cross sections for the {sup 18}F(p,{alpha}){sup 15}O and {sup 18}F(p,{gamma}){sup 19 }Ne reactions indicate that the contribution of the {sup 18}F(p, {gamma}) route to the formation or {sup 19}Ne is small.
Date: August 1, 1996
Creator: Rehm, K.E.; Roberts, A.D. & Jiang, C.L.
Partner: UNT Libraries Government Documents Department

The astrophysical reaction rate for the {sup 18}F(p,{alpha}){sup 15}O reaction

Description: Proton and alpha widths for a 3/2{sup +} ({ell}{sub p} = 0) state in {sup 19}Ne at E{sub x} = 7.1 MeV have been extracted using the results of recent measurements of the {sup 18}F(p,{alpha}){sup 15}O reaction. This {ell}{sub p} = 0 resonance dominates the astrophysical reaction rates at temperatures T{sub 9} > 0.5.
Date: March 1, 1996
Creator: Rehm, K.E.; Paul, M. & Roberts, A.D.
Partner: UNT Libraries Government Documents Department

Experimental equipment for an advanced ISOL facility[Isotope Separation On-Line Facility]

Description: This report summarizes the proceedings and recommendations of the Workshop on the Experimental Equipment for an Advanced ISOL Facility which was held at Lawrence Berkeley National Laboratory on July 22--25, 1998. The purpose of this workshop was to discuss the performance requirements, manpower and cost estimates, as well as a schedule of the experimental equipment needed to fully exploit the new physics which can be studied at an advanced ISOL facility. An overview of the new physics opportunities that would be provided by such a facility has been presented in the White Paper that was issued following the Columbus Meeting. The reactions and experimental techniques discussed in the Columbus White Paper served as a guideline for the formulation of the detector needs at the Berkeley Workshop. As outlined a new ISOL facility with intense, high-quality beams of radioactive nuclei would provide exciting new research opportunities in the areas of: the nature of nucleonic matter; the origin of the elements; and tests of the Standard Model. After an introductory section, the following equipment is discussed: gamma-ray detectors; recoil separators; magnetic spectrographs; particle detectors; targets; and apparatus using non-accelerated beams.
Date: March 1, 1999
Creator: Baktash, C.; Lee, I.Y. & Rehm, K.E.
Partner: UNT Libraries Government Documents Department

Nuclear reaction studies with radioactive {sup 18}F beams at ATLAS

Description: The contribution of the {sup 18}F(p,{gamma}) reaction to the production of {sup 19}Ne which is the crucial isotope for the breakout from the hot CNO cycle into the rp process, has been investigated in experiments with {sup 18}F beams. Measurements of the cross sections for the {sup 18}F(p,{alpha}){sup 15}O and the {sup 18}F(p,{gamma}){sup 19}Ne reactions indicate that the contribution from the (p,{gamma}) route to the formation of {sup 19}Ne is small.
Date: July 1, 1996
Creator: Rehm, K.E.; Paul, M. & Roberts, A.D.
Partner: UNT Libraries Government Documents Department

/sup 16/O + /sup 40/Ca inelastic scattering and the elastic scattering wave function

Description: The /sup 16/O + /sup 40/Ca inelastic scattering has been measured at 60-MeV incident energy, and evidence has been found in support of an explicit coupling between direct and elastic channels. The measurements were extended over a large angular range and in particular to very forward angles. The ratio of cross sections between extreme forward and backward angles proved very useful in studying the underlying reaction mechanism.
Date: January 1, 1977
Creator: Rehm, K.E.; Henning, W.; Erskine, J.R. & Kovar, D.G.
Partner: UNT Libraries Government Documents Department

Search for doubly-charged negative ions via accelerator mass spectrometry

Description: The Argonne FN tandem accelerator in conjunction with an Enge split-pole magnetic spectrograph has been used as a highly sensitive mass spectrometer to search for doubly charged negative ions of /sup 11/B, /sup 12/C and /sup 16/O. No evidence for the formation of these ions in an inverted sputter source and the subsequent acceleration in the tandem has been found. The following limits for the ratio of doubly-charged to singly-charged ions were measured: X/sup - -//X/sup -/ < 1 x 10/sup -15/, /sup 11/B; < 2 x 10/sup -15/, /sup 12/C; < 2 x 10/sup -14/, /sup 16/O. A relatively abundant formation of the short lived, metastable He/sup -/ ion in the sputter source has been observed.
Date: January 1, 1983
Creator: Kutschera, W.; Frekers, D.; Pardo, R.; Rehm, K.E.; Smither, R.K. & Yntema, J.L.
Partner: UNT Libraries Government Documents Department

Exploring the {sup 18}F(p,{gamma}){sup 19}Ne gateway to the formation of heavy elements in hot stars

Description: An upper limit to the production of {sup 19}Ne through the {sup 18}F(p,{gamma}) reaction at the recently discovered s-wave resonance has been determined. The limit implies that in a hot stellar environment (T{sub 9} > 0.5) this reaction is not a significant gateway from the hot CNO cycle towards the production of heavier elements in stars.
Date: August 1, 1997
Creator: Rehm, K.E.; Jiang, C.L. & Paul, M.
Partner: UNT Libraries Government Documents Department

Energy dissipation in heavy systems: the transition from quasi-elastic to deep-inelastic scattering

Description: The interaction of medium mass projectiles (A = 28 - 64) with /sup 208/Pb has been studied using a split-pole spectrograph which allows single mass and charge identification. The reaction process in all systems studied so far is dominated by quasi-elastic neutron transfer reactions, especially at incident energies in the vicinity of the Coulomb barrier. In addition to the quasi-elastic component deep inelastic contributions are present in all reaction channels. The good mass and charge separation allows to generate Wilczynski plots for individual channels; for the system /sup 48/Ti + /sup 208/Pb we observe that the transition between the quasi-elastic and deep-inelastic reactions occurs around Q = -(30 to 35) MeV.
Date: January 1, 1984
Creator: Rehm, K.E.; van den Berg, A.; Kolata, J.J.; Kovar, D.G.; Kutschera, W.; Rosner, G. et al.
Partner: UNT Libraries Government Documents Department

Half-life of /sup 60/Fe

Description: The half-life of /sup 60/Fe has been measured to be T/sub 1/2/ = (1.5 +- 0.3) x 10/sup 6/ y, significantly longer than the only previous measurement of Roy and Kohmann which reported a value of 3 x 10/sup 5/ y uncertain by a factor of 3. The present value was obtained from a specific activity and radioisotope concentration measurement of a material produced by spallation of copper with 191-MeV protons. /sup 60/Fe/Fe ratios in the range of 10/sup -8/ were measured with the Argonne FN tandem-superconducting linear accelerator system in conjunction with an Enge split-pole spectrograph. The specific activity was measured through the grow-in of the 1.332 MeV gamma-ray line of the /sup 60/Co daughter activity. 13 references.
Date: January 1, 1984
Creator: Kutschera, W.; Billquist, P.J.; Frekers, D.; Henning, W.; Zeng, M.X.; Pardo, R. et al.
Partner: UNT Libraries Government Documents Department

Observation of the one- to six-neutron transfer reactions at sub- barrier energies

Description: An unambiguous determination of the cross sections for the one- to six neutron transfer reactions has been made in the system {sup 58}Ni + {sup 100}Mo. The cross sections for multi-neutron transfer processes show an exponential falloff in agreement with recent theoretical calculations. Upper limits for the absolute yields to the ground states have been extracted which are smaller by a factor of ten as compared to theoretical predictions.
Date: April 1, 1994
Creator: Jiang, C. L.; Rehm, K. E.; Gehring, J.; Glagola, B.; Kutschera, W.; Rhein, M. et al.
Partner: UNT Libraries Government Documents Department