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This work presents a direct measurement of the $^{96}$Ru($p, \gamma$)$^{97}$Rh cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating $^{96}$Ru ions interacted repeatedly with a hydrogen target. The $^{96}$Ru($p, \gamma$)$^{97}$Rh cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the $\gamma$-ray strength function as well as the level density model can be pinned down with the measured ($p, \gamma$) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the $\gamma$-ray strength function and the level density have been removed. As a result, a constrained $^{96}$Ru($p, \gamma$)$^{97}$Rh reaction rate over a wide temperature range is recommended for $p$-process network calculations.

Coulomb breakup at high energy in inverse kinematics of proton-rich Cl-31 was used to constrain the thermonuclear S-30(p,gamma)Cl-31 capture reaction rate under typical Type I x-ray burst conditions. This reaction is a bottleneck during rapid proton-capture nucleosynthesis (rp process), where its rate depends predominantly on the nuclear structure of Cl-31. Two low-lying states just above the proton-separation threshold of S-p = 296( 50) keV in Cl-31 have been identified experimentally using the (RB)-B-3-LAND setup at the GSI Helmholtzzentrum fur Schwerionenforschung GmbH. Both states are considered to play a key role in the thermonuclear S-30( p,gamma)Cl-31 capture reaction. Excitation energies of the first J(pi) = 1/2(+), 5/2(+) states have been extracted and the reaction rate for proton capture on S-30 under typical rp-process temperatures has been investigated.

We have measured production yields and longitudinal momentum distributions of projectilelike fragments in the reaction ${}^{129}$Xe+${}^{27}$Al at an energy of ${E}_{\mathrm{lab}}$=790$A$ MeV. Production cross sections higher than expected from systematics were observed for nuclei in the neutron-deficient tails of the isotopic distributions. A comparison with previously measured data from the fragmentation of ${}^{136}$Xe ions shows that the production yields strongly depend on the neutron excess of the projectile with respect to the line of $\ensuremath{\beta}$ stability. The momentum distributions exhibit a dependence on the fragment neutron-to-proton ratio in isobaric chains, which was not expected from systematics so far. This can be interpreted by a higher excitation of the projectile during the formation of neutron-deficient fragments.

At the ALADIN-LAND setup at GSI the unbound nucleus Be-13 has been produced in one-neutron knockout reactions from a 304 MeV/nucleon relativistic beam of Be-14 ions impinging on a liquid hydrogen target. An analysis of the data including all available information about Be-13, and in particular recent data from a similar experiment performed at RIKEN, has been performed. A consistent description is reached. It is found that the excitation spectrum is dominated by s-waves at low energy, which solves problems from previous seemingly contradictory interpretations. A possible interference between two s-states in Be-13 is also discussed. The results indicate that the ground-state wave function of Be-14 is dominated by valence neutrons in the s-shell contributing with 60-75% of the total neutron knockout cross section.

Previously unknown states in ${}^{18}$Na and ${}^{19}$Mg have been studied by measuring the trajectories of their decay products with microstrip detectors. Analyzing angular correlations of the fragments provided information on decay energies and widths of the parent states. The ground state of ${}^{18}$Na has been detected and its one-proton decay energy of 1.23(15) MeV determined. Four previously unknown states in ${}^{19}$Mg at 2.1, 2.9, 3.6, and 5.2 MeV have been observed. The competition between simultaneous and sequential two-proton emission of states in ${}^{19}$Mg is discussed, and the conclusion of a direct mechanism of $2p$ radioactivity of the ${}^{19}$Mg ground state is confirmed.

Using a primary beam of $^{58}\mathrm{Ni}$ at 650 MeV/nucleon impinging on a beryllium target, production cross sections of proton-rich fragments from projectile fragmentation have been measured at the projectile-fragment separator FRS at Gesellschaft f\"ur Schwerionenforschung Darmstadt m.b.H. The experimental data ranging from nickel to scandium for isotopes close to stability as well as for fragments at the proton drip line are compared to predictions of the microscopic ISApace code, to calculations with a revised abrasion-ablation model and with the statistical abrasion model, and to the results of the empirical parametrization EPAX. Besides these systematic measurements, evidence for the particle stability of $^{50}\mathrm{Ni}$ has been found for the first time, whereas the nuclei $^{49}\mathrm{Co}$ and $^{54}\mathrm{Cu}$ are shown to be unbound. These observations are compared to mass predictions.

The isotope [sup 52]Ni has been produced by fragmentation of a [sup 58]Ni beam at 68 MeV/nucleon on a nickel target and separated using the LISE spectrometer at GANIL. We have measured the half-life of [sup 52]Ni and the energies of [beta]-delayed protons emitted during the decay. Two proton lines have been observed at [ital E][sub [ital p]]=1.06[plus minus]0.05 MeV and 1.34[plus minus]0.06 MeV with branching ratios of 0.06[plus minus]0.01 and 0.11[plus minus]0.01, respectively. The measured half-life of [sup 52]Ni is [ital T][sub 1/2]=38[plus minus]5 ms. A partial decay scheme is proposed.

One-neutron knockout data for N18-22 are analyzed in the eikonal approximation of the Glauber model. The role of the s-d shell and the crossing of the N = 14 neutron subshell are discussed. Of particular interest is the nucleus N-22, where the knockout data provide a sensitive experimental test for a possible halo structure of its ground state. The observation of a narrow momentum distribution of the N-21 fragments, together with an essential 1s(1/2) contribution needed to describe the observed longitudinal-momentum distribution, allow the firm conclusion that the ground state of N-22 is a well-developed nuclear halo. The results also show that the N = 14 subshell in N-22 is somewhat reduced as compared to that of O-23.

The N/Z dependence of projectile fragmentation at relativistic energies has been studied with the ALADIN forward spectrometer at the GSI Schwerionen Synchrotron (SIS). Stable and radioactive Sn and La beams with an incident energy of 600 MeV per nucleon have been used in order to explore a wide range of isotopic compositions. For the interpretation of the data, calculations with the statistical multifragmentation model for a properly chosen ensemble of excited sources were performed. The parameters of the ensemble, representing the variety of excited spectator nuclei expected in a participant-spectator scenario, are determined empirically by searching for an optimum reproduction of the measured fragment-charge distributions and correlations. An overall very good agreement is obtained. The possible modification of the liquid-drop parameters of the fragment description in the hot freeze-out environment is studied, and a significant reduction of the symmetry-term coefficient is found necessary to reproduce the mean neutron-to-proton ratios /Z and the isoscaling parameters of Z, 23 pages, 29 figures, published in Physical Review C

Cross sections for the electromagnetic dissociation of [sup 197]Au targets by 1.7 GeV/nucleon [sup 20]Ne and 1 GeV/nucleon [sup 86]Kr, [sup 197]Au, and [sup 209]Bi beams have been measured by an activation method. We observe systematic discrepancies between the experimental data and a simple Weizsaecker-Williams calculation: For the heaviest projectiles, the calculation overpredicts the 1[ital n] cross sections, whereas the 2[ital n] and 3[ital n] cross sections are systematically underpredicted. The deviations can be significantly reduced, however, by calculations including isoscalar and isovector giant quadrupole excitations as well as the possibility to excite multiphonon states. The most important experimental result of the present study is the observation of large cross sections for three-neutron removal from [sup 197]Au with [sup 197]Au and [sup 209]Bi projectiles which can only be explained by a dominant contribution from two-phonon giant dipole excitation. For an almost quantitative reproduction for all [ital xn] channels observed, we have to assume a higher excitation probability by roughly a factor of 2 for the double giant dipole resonance as compared to the harmonic approach.

The recently claimed observations of non-negligible amounts of $^{6}\mathrm{Li}$ in old halo stars have renewed interest in the Big Bang nucleosynthesis (BBN) of $^{6}\mathrm{Li}$. One important ingredient in the predicted BBN abundance of $^{6}\mathrm{Li}$ is the low-energy $^{2}\mathrm{H}$($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)$^{6}\mathrm{Li}$ cross section. Up to now, the only available experimental result for this cross section showed an almost constant astrophysical $S$ factor below $400$ keV, contrary to theoretical expectations. We report on a new measurement of the $^{2}\mathrm{H}$($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)$^{6}\mathrm{Li}$ reaction using the breakup of $^{6}\mathrm{Li}$ at 150 $A $ MeV. Even though we cannot separate experimentally the Coulomb contribution from the nuclear one, we find clear evidence for Coulomb-nuclear interference by analyzing the scattering angular distributions. This is in line with our theoretical description, which indicates a drop of the ${S}_{24}$ factor at low energies as predicted also by most other models. Consequently, we find even lower upper limits for the calculated primordial $^{6}\mathrm{Li}$ abundance than before.

A powerful method of investigating proton-unbound nuclear states by tracking their decay products in flight is discussed in detail. To verify the method, four known levels in 15 F , 16 Ne , and 19 Na were investigated by measuring the angular correlations between protons and the respective heavy-ion fragments stemming from the precursor decays in flight. The parent nuclei of interest were produced in nuclear reactions of one-neutron removal from 17 Ne and 20 Mg projectiles at energies of 410–450 A MeV. The trajectories of the respective decay products, 14 O + p + p and 18 Ne + p + p, were measured by applying a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of the fragments, which provided information on energies and widths of the parent states. In addition for reproducing properties of known states, evidence for hitherto unknown excited states in 15 F and 16 Ne was found. This tracking technique has an advantage in studies of exotic nuclei beyond the proton drip line measuring the resonance energies and widths with a high precision although by using low-intensity beams and very thick targets.

Two high-lying states in F15 and Ne16, unbound with respect to one-proton (1p) and two-proton (2p) emissions, have been observed in the fragmentation of Ne17 at intermediate energies. They undergo mainly sequential emissions of protons via intermediate states in O14 and F15 and have decay energies of 7.8(2) and 7.6(2) MeV, respectively. The widths of the newly observed states in F15 and Ne16 are much smaller than the Wigner limits for single-particle configurations, of 0.4(4) and 0.8(+8-4) MeV, respectively. In addition, narrow widths of 0.2(2) MeV are derived for two other high-lying states in F15 with Qp of 4.9 and 6.4 MeV, which match features of the recently predicted narrow odd-parity F15 states with two valence protons in the sd shell. All energies and widths have been obtained by analyzing angular correlations of the decay products, p-p-O14 and p-p-N13, whose trajectories have been measured by a tracking technique with silicon microstrip detectors. © 2009 The American Physical Society.

Proton-proton correlations were observed for the two-proton decays of the ground states of {sup 19}Mg and {sup 16}Ne. The trajectories of the respective decay products, {sup 17}Ne+p+p and {sup 14}O+p+p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the sd shell.

Interaction cross sections ({sigma}{sub I}) of {sup 72,76,80}Kr on a C target were measured at the projectile fragment separator FRS at GSI. Effective matter radii of the nuclei were deduced by a Glauber model analysis so as to reproduce the experimental {sigma}{sub I}. An increase of matter radii toward the proton drip-line is observed. We compare the extracted radii to theoretical predictions, which are based on Skyrme-Hartree-Fock-Bogoliubov (SHFB) and on relativistic mean field (RMF) approaches. The enhancement of the matter radius observed in {sup 72}Kr is discussed.

We have measured cross sections for the formation of actinide transfer products in the reaction of 72-MeV $^{13}\mathrm{C}$ projectiles with $^{254}\mathrm{Es}^{\mathrm{g}}$ targets. The pattern of nuclide yields is similar to those observed in the reactions of heavier ions with $^{254}\mathrm{Es}^{\mathrm{g}}$. We have constructed the primary element yields from these results and show that the total cross section for transfer reactions is 58 mb. The total reaction cross section is about 300 mb. Lawrencium isotopes are formed with larger cross sections than are consistent with the trends of the transfer-product distributions; we explain this in terms of massive transfer, and model the lawrencium yields with an evaporation code.

By exploiting Coulomb dissociation of high-energy radioactive beams of the neutron-rich nuclei $^{129\ensuremath{-}132}\mathrm{Sn}$ and $^{133,134}\mathrm{Sb}$, their dipole-strength distributions have been measured. A sizable fraction of ``pygmy'' dipole strength, energetically located below the giant dipole resonance, is observed in all of these nuclei. A comparison with available pygmy resonance data in stable nuclei ($^{208}\mathrm{Pb}$ and $N=82$ isotones) indicates a trend of strength increasing with the proton-to-neutron asymmetry. On theoretical grounds, employing the RQRPA approach, a one-to-one correlation is found between the pygmy strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skins. On this basis, by using the experimental pygmy strength, parameters of the nuclear symmetry energy (${a}_{4}=32.0\ifmmode\pm\else\textpm\fi{}1.8$ MeV and ${p}_{o}=2.3\ifmmode\pm\else\textpm\fi{}0.8$ MeV/fm${}^{3}$) are deduced as well as neutron-skin thicknesses ${R}_{n}\ensuremath{-}{R}_{p}$ of $0.24\ifmmode\pm\else\textpm\fi{}0.04$ fm for $^{132}\mathrm{Sn}$ and of $0.18\ifmmode\pm\else\textpm\fi{}0.035$ fm for $^{208}\mathrm{Pb}$, both doubly magic nuclei. Astrophysical implications with regard to neutron stars are briefly addressed.

Production cross sections for the projectile fragmentation of a 1.05A GeV {sup 80}Kr beam on a Be target were measured at the projectile fragment separator FRS at GSI. Cross sections were obtained for isotopes of the elements Ge to Kr close to the proton drip line. These data are compared to the results of the empirical parametrization EPAX and to abrasion-ablation calculations. We also compare the results to predictions of intranuclear-cascade calculations. The intranuclear-cascade calculations allow us to also compare well measured one-proton pickup cross sections.

An exclusive measurement of the Coulomb breakup of {sup 8}B into {sup 7}Be+p at 254A MeV was used to infer the low-energy {sup 7}Be(p,{gamma}){sup 8}B cross section. The radioactive {sup 8}B beam was produced by projectile fragmentation of 350A MeV {sup 12}C and separated with the FRagment Separator (FRS) at Gesellschaft fuer Schwerionenforschung in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S{sub 17} factors exhibit good agreement with the most recent direct {sup 7}Be(p,{gamma}){sup 8}B measurements. By using the energy dependence of S{sub 17} according to the recently refined cluster model for {sup 8}B of P. Descouvemont [Phys. Rev. C 70, 065802 (2004)], we extract a zero-energy S factor of S{sub 17}(0)=20.6{+-}0.8(stat){+-}1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated by Descouvemont to be about 5%.

A systematic study of the population probabilities of nanosecond and microsecond isomers produced following the projectile fragmentation of U-238 at 750 MeV/nucleon has been undertaken at the SIS/FRS facility at GSI. Approximately 15 isomeric states in neutron-deficient nuclei around A similar to 190 were identified and the corresponding. isomeric ratios determined. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). This model represents an upper limit for the population of isomeric states in relativistic projectile fragmentation. When the decay properties of the states above the isomer are taken into account, as opposed to the sharp cutoff approximation, a good agreement between the experimental and calculated angular momentum population is obtained.

Electromagnetic and nuclear inelastic scattering of the halo nucleus ${}^{11}\mathrm{Be}$ have been investigated by a measurement of the one-neutron removal channel, utilizing a secondary ${}^{11}\mathrm{Be}$ beam with an energy of 520 MeV/nucleon impinging on lead and carbon targets. All decay products, i.e., ${}^{10}\mathrm{Be}$ fragments, neutrons, and $\ensuremath{\gamma}$ rays have been detected in coincidence. Partial cross sections for the population of ground and excited states in ${}^{10}\mathrm{Be}$ were determined for nuclear diffractive breakup as well as for electromagnetically induced breakup. The partial cross sections for ground-state transitions have been differentiated further with respect to excitation energy, and the dipole-strength function associated solely with transitions of the halo ${2s}_{1/2}$ neutron to the continuum has been derived. The extracted dipole strength integrated from the neutron threshold up to 6.1 MeV excitation energy amounts to ${0.90(6)e}^{2}{\mathrm{fm}}^{2}.$ A spectroscopic factor for the $\ensuremath{\nu}{2s}_{1/2}{\ensuremath{\bigotimes}}^{10}\mathrm{Be}{(0}^{+})$ single-particle configuration of 0.61(5) and a root-mean-square radius of the ${2s}_{1/2}$ neutron wave function of 5.7(4) fm have been deduced.

The excitation of the double-phonon giant dipole resonance was observed in heavy projectile nuclei impinging on targets of high nuclear charge with energies of 500‐700 MeV/nucleon. New experimental data are presented for 136 Xe and 238 U together with further analysis of earlier data on 208 Pb. Differential cross sections ds/dE* and ds/du for electromagnetic excitations were deduced. Depending on the isotope, cross sections appear to be enhanced in comparison to those expected from a purely harmonic nuclear dipole response. The cumulative effect of excitations of two-phonon states composed of one dipole and one quadrupole phonon, of predicted anharmonicities in the double-phonon dipole response, and of damping of the dipole resonance during the collision may account for the discrepancy. In addition, decay properties of two-phonon resonances were studied and compared to that of a statistical decay.

Fragmentation of secondary beams of neutron-rich, unstable ${}^{19,20,21}\mathrm{O}$ isotopes at beam energies near 600 MeV/nucleon was studied by measuring the production cross sections for carbon, nitrogen, and oxygen fragments. Data for stable ${}^{17,18}\mathrm{O}$ beams were obtained as well. The measurements serve to illuminate the isospin dependence of the fragmentation process. The experimental results are compared to those from empirical parametrization and those from abrasion-ablation models.

We have measured production cross sections of projectilelike fragments in the reaction of neutron-deficient 112Sn with a 9Be target at an energy of Elab=1A GeV. Results have been obtained for neutron-deficient products along the N=Z line down to the subnanobarn range. We find very good agreement with the recently revised empirical parametrization EPAX. In addition, a few one- and two-proton pickup cross sections have been measured. The former ones are well reproduced by intranuclear-cascade calculations.

New experimental data obtained mainly at the GSI/FRS facility allow to modify the empirical parametrization of fragmentation cross sections, EPAX. It will be shown that minor modifications of the parameters lead to a much better reproduction of measured cross sections. The most significant changes refer to the description of fragmentation yields close to the projectile and of the memory effect of neutron-deficient projectiles., Comment: 10 pages, latex file, 11 postscript figures, submitted to Phys. Rev. C

Negative results for the production of superheavy elements in damped collisions between $^{238}\mathrm{U}$ projectiles and $^{248}\mathrm{Cm}$ targets are reported. This reaction was believed to permit a closer and more widespread approach to the predicted island of stability near Z=114 and N=184 than any practical fusion reaction. Aqueous and gas phase chemistry techniques were used to isolate superheavy element fractions. The fractions were counted for spontaneous fission activity, fragment kinetic energies, and neutron multiplicities. Cross-section limits for half-lives from hours to several years are g4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{2}$.

Excitation functions have been measured for production of isotopes of Bk through Fm in bombardments of /sup 248/Cm with 234- to 294-MeV /sup 40/Ca ions and with 239- to 318-MeV /sup 48/Ca ions. The maxima of the isotopic distributions for these elements occur at only 2 to 3 mass numbers larger for /sup 48/Ca than for /sup 40/Ca reactions. The shapes of the distributions and the half-widths of about 2.5 mass numbers are quite similar to those observed previously for reactions of /sup 16/O, /sup 18/O, /sup 20/Ne, and /sup 22/Ne with /sup 248/Cm. In general, the excitation functions for /sup 40/Ca show maxima near the Coulomb barrier while those for /sup 48/Ca are about 20 MeV above the barrier. The cross sections decrease rather slowly with increasing projectile energy over the energy range studied, indicating that the additional projectile energy is not manifested as excitation energy of these actinide products.

We report radiochemically determined cross sections for the heaviest known actinides produced in transfer reactions of /sup 16,18/O and /sup 22/Ne with /sup 254/Es as a target. A comparison with data for similar transfers from /sup 248/Cm targets is made. Transfer cross sections are extrapolated for the production of the unknown, neutron-rich isotopes of elements 101 through 105, and the unique potential of /sup 254/Es as a target to make these exotic nuclei accessible is demonstrated.

We have measured the mass and kinetic-energy distributions from the spontaneous fission of {sup 258}Fm, {sup 258}No, {sup 259}Md, {sup 260}Md, and {sup 260}(104). All are observed to fission with a symmetrical division of mass. The total-kinetic-energy distributions strongly deviated from the Gaussian shape characteristically found in the fission of all other actinides. When the total-kinetic-energy distributions are resolved into two Gaussians, the constituent peaks lie near 200 and 233 MeV. We conclude that both low- and high-energy fission modes occur in four of the five nuclides studied. We call this property bimodal fission.'' Even though both modes are possible in the same nuclide, one generally predominates. We offer an explanation for each mode based on shell structures of the fissioning nucleus and of its fragments. The appearance of both modes of fission in this region of the nuclide chart seems to be a coincidence in that the opportunity to divide into near doubly magic Sn fragments occurs in the same region where the second fission barrier is expected to drop in energy below the ground state of the fissioning nucleus. Appropriate paths on the potential-energy surface of deformation have been found by theorists, but no physical grounds have beenmore » advanced that would allow the near equal populations we observe traveling each path. We suggest that this failure to find a reason for somewhat equal branching may be a fundamental flaw of current fission models. Assuming the proposed origins of these modes are correct, we conclude the low-energy, but also mass-symmetrical mode is likely to extend to far heavier nuclei. The high-energy mode will be restricted to a smaller region, a realm of nuclei defined by the proximity of the fragments to the strong neutron and proton shells in {sup 132}Sn.« less

Yields for isotopes of Rn through Pu have been measured in the reaction /sup 48/Ca+/sup 248/Cm at an energy of 248--263 MeV (1.04--1.10 times the Coulomb barrier). Despite the low bombarding energy, high and essentially constant integral yields of about 1 to 2 mb for the elements Rn through U were observed. There is evidence that these nuclides are produced with little excitation energy.