Search Results (94 total)

A new experimental approach was developed that can reduce the uncertainties in astrophysical rapid proton capture (rp) process calculations due to nuclear data. This approach utilizes neutron removal from a radioactive ion beam to populate the nuclear states of interest. Excited states were deduced by the γ-decay spectra measured in a semiconductor Ge-detector array. In the first case studied, ³³Ar, excited states were measured with uncertainties of several keV. The 2 orders of magnitude improvement in the uncertainty of the level energies resulted in a 3 orders of magnitude improvement in the uncertainty of the calculated ³²Cl(p,γ)³³Ar rate that is critical to the modeling of the rp process. This approach has the potential to measure key properties of almost all interesting nuclei on the rp-process path.

The proton-rich nuclei ²³Al and ²⁷P were studied using the ²⁴Mg(⁷Li,⁸He)²³Al and ²⁸Si(⁷Li,⁸He)²⁷P reactions, respectively. Several energy levels are observed in each nucleus. The mass excess of ²³Al was measured to be 6.773(28) MeV and the mass excess of ²⁷P was measured to be -0.670(41) MeV. The first excited state of ²³Al has been resolved from the ground state for the first time at an excitation energy of E_x=0.550(20) MeV. The first excited state of ²⁷P has been measured for the first time at an excitation energy of E_x=1.199(19) MeV. The astrophysical implications of these results for the reaction rates of ²²Mg(p,γ)²³Al and ²⁶Si(p,γ)²⁷P for the nucleosynthesis of ²²Na and ²⁶Al in Ne novae will be discussed.

In an attempt to clarify the situation regarding the low-lying structure of ¹⁰Li, a spectroscopic measurement of the structure of ¹⁰Li with the ⁹Be(⁹Be,⁸B)¹⁰Li reaction at E(⁹Be)=40.1(1) MeV/nucleon was performed using the S800 spectrograph at Michigan State University. The data are fit best with a single p-wave resonance at -S_n=0.500(60) MeV with a width of Γ=400(60) keV. No higher lying states were observed and there is no evidence of a state at ∼250 keV in the data. An excess strength at threshold was observed, but cannot be definitively attributed to a state.

We have measured the parallel momentum distributions of outcoming fragments in the one-neutron breakup of the odd-mass carbon isotopes ^19,17,15C and ¹⁴B. Owing to their low neutron separation energies, the study of these nuclei is of particular interest regarding the appearance of the halo phenomenon. Discrepancies between experiment and theory observed for ¹⁵C and ¹⁴B indicate that their halos are not as prominent as in ¹¹Be or ¹¹Li, and that core excitations start playing a role in the breakup mechanism. In ¹⁷C the halo appears hindered by a d-wave neutron ground state configuration. Finally, the data on ¹⁹C suggests an s-wave neutron around the 2⁺ excited state in ¹⁸C.

The energy at which collective transverse flow in the reaction plane disappears, the balance energy E_bal, is found to depend on the isospin of the system using the reactions ⁵⁸Fe+⁵⁸Fe and ⁵⁸Ni+⁵⁸Ni. The more neutron-rich system exhibits higher balance energies for all measured impact parameters, in agreement with the predictions of a transport model which incorporates an isospin dependent mean field and isospin dependent in-medium nucleon-nucleon cross sections.

Collective transverse flow of nuclear matter was measured as a function of the ratio of neutrons to protons ( N/Z) of the interacting system for the first time. The collisions of three isotopically pure beams of A  =  58 nuclei with two A  =  58 targets were studied at 55 MeV/nucleon. The results for the flow variables demonstrate the sensitivity of transport models to elementary aspects of the nucleon-nucleon collisions.

The ¹H(⁶He, ⁶Li)n reaction was studied at 0° with the NSCL A1200 fragment separator in the energy loss mode. A ⁶He secondary beam at E / A=93 MeV was used to measure the Gamow-Teller and Fermi strengths between the ground state of ⁶He and the ground and excited states of ⁶Li, in inverse kinematics. At 0° the ground-state cross section is measured to be dσ_GS / (dΩ)=43±16 mb/sr, which is dominated by systematic error in the secondary beam flux. The ratio of Gamow-Teller to Fermi strength is not sensitive to this error and is found to be (87±6)% of that expected from (p, n) systematics and β decay. Angular distributions have been measured between 0° and 10° in the center of mass.

To gain a better understanding of the production of exotic isotopes and provide information on the stability of nuclei along the path of the rapid-proton capture process, isotopic cross sections from the reaction ⁷⁸Kr + ⁵⁸Ni at 75 MeV/nucleon were measured at 0° using the A1200 fragment separator. Most notably the particle stability of ⁶⁹Br was thoroughly probed during this experiment and it appears to be particle unstable. The experimental production cross section data are compared to previous krypton isotope fragmentation data to explore the dependence of the N/Z ratio of the projectile on the observed isotopic distributions (‘‘memory effect’’) as well as with an intranuclear cascade code developed for higher energies (≳ 200 MeV/nucleon) and a semiempirical parametrization derived from high energy systematics. © 1996 The American Physical Society.

An attempt was made to find the very neutron-rich isotope ²⁶O among the fragmentation products of a 90 MeV/nucleon ⁴⁰Ar beam. This isotope has been predicted to be bound but was not observed in a previous experiment by Guillemaud-Mueller et al. As part of the search, the momentum distributions of all the oxygen isotopes in the range from ¹⁷O to ²⁴O were carefully determined so that the optimum separator setting for ²⁶O could be used. From an extrapolation of the counting rates of the lighter oxygen isotopes, we expected to observe several hundred events of ²⁶O during the measurement. However, no events could be attributed to ²⁶O, thus indicating the particle instability of this isotope. The results for the production cross sections of 72 neutron-rich isotopes, ranging from ³⁸P to ¹¹B, are presented and compared to predictions. © 1996 The American Physical Society.

Differential cross sections have been measured near 0° for the dominant channels in the mirror reaction ¹³C(¹³N,¹³C)¹³N at E/A  =  57 and 105 MeV. The cross sections of the peaks in the excitation spectrum are discussed in terms of Gamow-Teller and Fermi transition strength. The cross section per unit Gamow-Teller strength is found to be enhanced relative to that for unit Fermi strength when compared with previous results from (p,n) reactions. The present work represents the first use of a mirror-symmetric projectile and target system in a charge-exchange reaction as well as the first application of the developing radioactive nuclear beam field to this area.

Momentum spectra have been measured for the reaction ¹¹B(⁷Li,⁸B)¹⁰Li at 130 MeV and laboratory angles of 5° and 3.5°. There is strong evidence for the existence of a broad state in the unstable nucleus ¹⁰Li, corresponding to a single p-wave neutron resonance unbound to neutron decay by 538±62 keV with wdith Γ_lab=358±23 keV. There is also weak evidence that the ¹⁰Li ground state may be either an s- or a p-wave resonance barely unbound to neutron decay with S_n≥-100 keV and Γ_lab<230 keV. The measured Q values for the reaction leading to these states are -32 908±62 keV and greater than -32 471 keV, respectively. A comparison is made with previous measurements and theoretical predictions of the low-lying structure of the neutron-unstable nucleus ¹⁰Li.

The mass of the nucleus ¹¹Li has been determined from a measurement of the Q value of the reaction ¹⁴C(¹¹B,¹¹Li)¹⁴O at E / A≈32 MeV. The results, which indicate a two-neutron separation energy of S_2n(¹¹Li)=295±35 keV, put this basic quantity on a firm basis for use in theoretical models of the halo nucleus ¹¹Li.

The rp process provides a mechanism whereby energy is generated and proton-rich nuclei are synthesized in various astrophysical environments. Understanding this process requires knowledge of the half-lives, binding energies, and reaction cross sections for the nuclei along its path, which for A≳60 lies near the proton-drip line. Using the A1200 radioactive-beam facility at the National Superconducting Cyclotron Laboratory, we have performed an experiment to measure the β-decay half-lives of several T_z=-1/2 nuclei in this mass region by identification and implantation of projectile fragments followed by a rapid beam cutoff and observation of the β decay. Using this technique, the half-lives of ⁶¹Ga, ⁶³Ge, and ⁶⁵As (0.15±0.03 s, 95_-20⁺²³ ms, and 0.19_-0.07^+0.11 s, respectively) have been measured and are found to be consistent with expectations from β-decay theory for decay between mirror states with Q_β∼9 MeV. In addition, the proton-rich nucleus ⁶⁶Se has been observed for the first time. The experimental method used for this measurement and the implications of these results for the rp process are discussed.

Previous measurements of the half-life of ³²Si, which is produced by cosmic-ray spallation of ⁴⁰Ar and of interest for geophysical dating, display large discrepancies. We present a new method for half-life measurements of this and other isotopes. Two samples containing 6.7×10⁸ and 7.4×10^{8 32}Si nuclei, respectively, were prepared by implantation of separated projectile fragments. The half-life of ³²Si was calculated from the two independent measurements of the specific activity to be 132±13 yr.

The absolute cross section and angular distributions for γ-ray production in the reaction ¹⁴N+^natAg at 35 MeV/nucleon have been measured. Calculations of the expected γ-ray yield using an enhanced version of the nucleon exchange transport model have been performed. It has been observed that use of a sharp ground state momentum distribution in the γ-ray calculations does not lead to reproduction of the experimental yields. Diffuse momentum distributions have been incorporated into the model, and the calculations with these momentum distributions give good agreement with experimental yields. Similar calculations have been performed for the production of preequilibrium neutrons and protons for the same system.

A ⁹²Mo beam with an energy of E/A=70 MeV has been used to produce new isotopes near the proton drip line. The Michigan State University National Superconducting Cyclotron Laboratory A1200 fragment separator was used to detect the new isotopes ⁷⁸Y, ⁸²Nb, ⁸⁵Mo, ⁸⁶Tc, and ^89,90Ru.

Energy spectra and angular distributions have been measured for high energy gamma rays (E_γ≥20 MeV) from proton-nucleus reactions at 104, 145, and 195 MeV on targets of C, Zn, and Pb. Gamma rays were observed with energies up to 170 MeV. The spectra showed differences from the typical exponential shape that is observed in gamma ray production from heavy-ion reactions. The angular distribution of the gamma ray is forward peaked in the laboratory, which is consistent with emission from a moving source. A comparison is made with previous measurements at 72, 140, 168, and 200 MeV. The experimental evidence indicates that first-chance incoherent proton-neutron bremsstrahlung is the main production mechanism.

Energy spectra and angular distributions have been measured for high energy gamma rays (E_γ≥20 MeV) from the p+d reaction at 145 and 195 MeV. Gamma rays were observed up to the maximum energy allowed by kinematics. A comparison is made with previous measurements for the p+d system at 140, 197, and 200 MeV. Below the free pnγ threshold the general shape of the energy spectra and angular distributions are in reasonable agreement with a recent calculation of the free pnγ elementary process. However, the magnitude of the predicted cross section is not in good agreement with the present data.

An E/A=65 MeV ⁷⁸Kr beam has been used to produce six new isotopes near the proton-drip line. The newly commissioned A1200 beam-analysis device was used to observe the astrophysically interesting isotope ⁶⁵As, as well as ⁶¹Ga, ⁶²Ge, ⁶³Ge, ⁶⁹Br, and ⁷⁵Sr. Implications of the observation of these nuclei are discussed in terms of the astrophysical rp process.

Results are presented from a search for high-energy gamma rays (E_γ>140 MeV) resulting from the decay of the delta resonance produced in heavy-ion collisions. The energy spectrum of gamma rays in the energy range 15<E_γ<180 MeV was measured at a laboratory angle of 90° for the reaction ¹⁴N+Zn. The incident beam energy was E/A=75 MeV. The energy spectrum in the region 50<E_γ<180 MeV is exponential with an inverse slope parameter of 26 MeV. There is no indication of an enhancement in the spectrum above 140 MeV due to the decay of the delta resonance.

The (²⁰Ne,¹⁹Ne), (²⁰Ne,²¹Ne), and (²⁰Ne,²¹Na) reactions on ⁹⁰Zr and ²⁰⁸Pb have been investigated at 500 and 600 MeV incident energies. Experimental spectra have been compared with predictions of distorted-wave Born approximation calculations. The different shapes observed for proton and neutron pickup spectra are explained quite well by the calculations. In particular, the broad peak observed at about 6.5 MeV in the (²⁰Ne,²¹Ne) spectra is shown to originate from ejectile excitation in the 1d_3/2, 1f_7/2, and 2p_3/2 orbitals. An important component of the (²⁰Ne,¹⁹Ne) cross section is due to three-body processes, such as the elastic breakup of ²⁰Ne into ¹⁹Ne and one neutron. In addition, bumps are observed at about 1.5 and 10 MeV excitation energy in ²⁰⁹Pb and 2.5 and 14 MeV in ⁹¹Zr. The low energy peaks are shown to originate mainly from neutron transfer to high spin orbitals, i.e., 1i_11/2 and 1j_15/2 in ²⁰⁹Pb and 1g_7/2 and 1h_11/2 in ⁹¹Zr. Because of the selectivity of the (²⁰Ne, ¹⁹Ne) reaction for large angular momentum transfer, it is proposed that the structures observed at 14 MeV in ⁹¹Zr and 10 MeV ²⁰⁹Pb are due to a neutron transfer to high spin orbitals, such as 1i_13/2 and 1k_17/2 in ⁹¹Zr and ²⁰⁹Pb, respectively.

Azimuthal distributions of fragments with respect to the reaction plane are studied in the Ar+V system as a function of beam energy. Light charged particles are found to exhibit an enhanced emission in the reaction plane which increases with the mass of the detected particle. As the beam energy is increased, the asymmetry decreases. Possible mechanisms behind the asymmetry, such as rotational collective motion and directed transverse momentum, are discussed.

Excited state production has been measured for ⁷Li, ⁷Be, and ¹⁰B fragments emitted in the ⁴⁰Ar+¹²C reaction at E/A=8, 10, and 12 MeV. The use of reverse kinematics and a system with low rotational kinetic energy permitted a more direct test of the excited state method for temperature determination than had previously been possible. The results show that the population of excited states reflects the temperature expected from Fermi gas calculations at the appropriate excitation energy for the compound nucleus and is consistent with the temperature determined from the slope of the fragment spectra.

The reaction of ⁴⁰Ar with ¹²C has been studied at a bombarding energy of E/A=8 MeV. Fragments ranging from lithium to titanium isotopes were detected in charged-particle detectors placed at a small forward angle (θ=11°). The coincident gamma rays were observed also and used to determine the populations of various bound excited states. The products could be generally described as either complex fragments or evaporation residues. An analysis of fragment velocities and a limited number of charged-particle coincidence events suggests that the complex fragments are emitted in a binary-decay process from the compound nucleus. The extent of statistical equilibrium of the compound nucleus was studied by analyzing the population distributions of both the complex fragments and the evaporation residues. The deduced nuclear temperatures of the complex fragments were found to be in general agreement with that expected for the compound nucleus. The observed (relative) populations of states in a typical evaporation residue were compared with predictions of a statistical evaporation code. The good agreement achieved suggests that heavy fragments with masses close to the total mass in the reaction are produced from the compound nucleus by the statistical emission of light particles, such as nucleons and alpha particles. The populations of states in nuclei with A≤35, somewhat beyond the mass range of statistical evaporation, are consistent with thermal excitation, and these nuclei must be the reaction partners of the light complex fragments. Thus, all of the detected fragments are consistent with decay of an equilibrated compound nucleus by either sequential light-particle emission or binary decay into large complex fragments.