Search Results (26 total)

Measurements of ⁷Be scattering at 87 MeV on a melamine (C₃N₆H₆) target and of ⁸B at 95 MeV on C were performed. For ⁷Be the angular range was extended over previous measurements and monitoring of the intensity of the radioactive beam was improved. The measurements provide a renormalization of the absolute cross section of existing data on the proton transfer reaction ¹⁴N(⁷Be,⁸B)¹³C and lead to improved optical-model potentials used in the incoming and outgoing channels for the distorted-wave Born approximation analysis. The results yield an updated determination of the asymptotic normalization coefficient for the virtual decay ⁸B → ⁷Be+p. The new value, C_tot²(⁸B)=0.466±0.049 fm^-1, is slightly larger than, but consistent with, the previous result. This implies an astrophysical factor, S₁₇(0)=18.0±1.9 eV b, for the solar neutrino-generating reaction ⁷Be(p,γ)⁸B.

¹³N(p,γ)¹⁴O is one of the key reactions which trigger the onset of the hot CNO cycle. This transition occurs when the proton capture rate on ¹³N is faster, due to increasing stellar temperature (≥10⁸  K), than the ¹³N β-decay rate. The rate of this reaction is dominated by the resonant capture through the first excited state of ¹⁴O (E_r=0.528  MeV). However, through constructive interference, direct capture below the resonance makes a non-negligible contribution to the reaction rate. We have determined this direct contribution by measuring the asymptotic normalization coefficient for ¹⁴O→¹³N+p. In our experiment, an 11.8  MeV∕nucleon ¹³N radioactive beam was used to study the ¹⁴N(¹³N,¹⁴O)¹³C peripheral transfer reaction, and the asymptotic normalization coefficient, (C_p1∕2^14O)²=29.0±4.3  fm⁻¹, was extracted from the measured cross section. The radiative capture cross section was estimated using an R-matrix approach with the measured asymptotic normalization coefficient and the latest resonance parameters. We find the S factor for ¹³N(p,γ)¹⁴O to be larger than previous estimates. Consequently, the transition from the cold to hot CNO cycle for novae would be controlled by the slowest proton capture reaction ¹⁴N(p,γ)¹⁵O.

Fusion-evaporation reactions induced by 110  MeV ¹¹B and radioactive ¹¹C on ⁸⁷Rb targets have been studied by measuring evaporation residue–light particle coincidences. The proton to α particle ratio in each reaction has been derived and compared with predictions from statistical model calculations. These calculations account rather well for the experimental data, when a small empirical adjustment of the emission barrier is performed, in agreement with earlier results. No evidence is found for predicted temperature and isospin modification of the binding energies. The possibility of a further study of isospin and temperature dependent effects in fusion-evaporation reactions with radioactive beams is discussed.

The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12), for ²²Mg β decay have been measured with high precision. The latter depended on γ-ray intensities being measured with an HPGe detector calibrated for relative efficiencies to an unprecedented 0.15%. Previous precise measurements of 0⁺→0⁺ transitions have been restricted to the nine that populate stable daughter nuclei. No more such cases exist, and any improvement in a critical Cabibbo-Kobayashi-Maskawa unitarity test must depend on precise measurements of more exotic nuclei. With this branching-ratio measurement, we show those to be possible for T_z=-1 parents. We obtain a corrected Ft value of 3071(9) s, in good agreement with expectations.

We report a study of the β decay of ⁶²Ga, whose dominant branch is a superallowed 0⁺→0⁺ transition to the ground state of ⁶²Zn. We find the total half-life to be 115.84±0.25 ms. This is the first time that the ⁶²Ga half-life has been measured with a purified source. We find that (0.120±0.021)% of the β decays are followed by γ cascades that pass through the ⁶²Zn 2⁺ first excited state at 0.954 MeV. The branching ratio to the first-excited 0⁺ state in ⁶²Zn at 2.33 MeV is <0.043%. We conclude that the branching ratio for the superallowed transition is 99.85_-0.15^+0.05%.

Asymptotic normalization coefficients (ANCs) for ⁸Li→⁷Li+n have been extracted from the neutron transfer reaction ¹³C(⁷Li,⁸Li)¹²C at 63 MeV. These are related to the ANCs in ⁸B→⁷Be+p using charge symmetry. We extract ANCs for ⁸B which are in very good agreement with those inferred from proton transfer and breakup experiments. We have also separated the contributions from the p_1/2 and p_3/2 components in the transfer. We find the astrophysical factor for the ⁷Be(p,γ)⁸B reaction to be S₁₇(0)=17.6±1.7 eV b. This is the first time that the rate of a direct capture reaction of astrophysical interest has been determined through a measurement of the ANCs in the mirror system.

The evolution of very low-metallicity, massive stars depends critically on the amount of CNO nuclei that they produce. Alternative paths from the slow 3α process to produce CNO seed nuclei could change their fate. The ¹¹C(p,γ)¹²N reaction is an important branch point in one such alternative path. At energies appropriate to stellar evolution of very low-metallicity, massive stars, nonresonant capture dominates the reaction rate. We have determined the astrophysical S factor for the ¹¹C(p,γ)¹²N reaction using the asymptotic normalization coefficient for ¹²N→¹¹C+p to fix the nonresonant capture rate. In our experiment, a 110 MeV ¹¹C radioactive beam was used to study the¹⁴N(¹¹C,¹²N)¹³C peripheral transfer reaction and the asymptotic normalization coefficient, (C_peff^12N)²=(C_p1/2^12N)²+(C_p3/2^12N)²=1.73±0.25 fm^-1, was extracted from the measured cross section. The contributions from the second resonance and interference effects were estimated using an R-matrix approach with the measured asymptotic normalization coefficient and the latest value for Γ_γ. We find the S factor for ¹¹C(p,γ)¹²N is significantly larger than previous estimates. As a result, the required density for it to contribute is reduced, and more CNO material may be produced.

We reanalyze the ¹³C(p,γ)¹⁴N radiative capture reaction within the R-matrix approach. The low-energy astrophysical S factor has important contributions from both resonant and onresonant captures. The normalization of the nonresonant component of the transition to a particular ¹⁴N bound state is expressed in terms of the asymptotic normalization coefficient (ANC). In the analysis we use the experimental ANC’s inferred from the ¹³C(¹⁴N,¹³C)¹⁴N and ¹³C(³He,d)¹⁴N reactions. The fits of the calculated S factors to the experimental data are sensitive to the ANC values and are used to test the extracted ANC’s. We find that for transitions to all the states in ¹⁴N, except the third excited state, the ANC’s determined from the transfer reactions provide the best fit. The astrophysical factor we obtain, S(0)=7.7±1.1 keV b, is in excellent agreement with previous results.

We consider the results of two proton transfer reactions, ¹⁰B(⁷Be,⁸B)⁹Be and ¹⁴N(⁷Be,⁸B)¹³C, to obtain a weighted average of the measured asymptotic normalization coefficients for the virtual transition ⁷Be+p↔⁸B. These coefficients specify the amplitude of the tail of the ⁸B overlap function in the ⁷Be+p channel, and are used to calculate the astrophysical S factor for the direct capture reaction ⁷Be(p,γ)⁸B at solar energies, S₁₇(0). In light of recent improvements in the determination of optical-model potentials, including detailed understanding of the correlations between the DWBA analyses of the two experiments, and a new experimental measurement of the asymptotic normalization coefficients for the virtual transition ¹³C+p↔¹⁴N,y we report a weighted average asymptotic normalization coefficient of C_p3/2²=0.388±0.039 fm^-1 for ⁸B ↔⁷Be+p, which implies S₁₇(0)=17.3±1.8 eV b.

We present the results of a search for optical model potentials for use in the description of elastic scattering and transfer reactions involving stable and radioactive p-shell nuclei. This was done in connection with our program to use transfer reactions to obtain data for nuclear astrophysics, in particular for the determination of the astrophysical S₁₇ factor for ⁷Be(p,γ)⁸B using two (⁷Be,⁸B) proton transfer reactions. Elastic scattering was measured using ⁷Li, ¹⁰B, ¹³C, and ¹⁴N projectiles on ⁹Be and ¹³C targets at or about E/A=10 MeV/nucleon. Woods-Saxon type optical model potentials were extracted and are compared with potentials obtained from a microscopic double folding model. Several nucleon-nucleon effective interactions were used: M3Y with zero range and finite range exchange term, two density dependent versions of M3Y and the effective interaction of Jeukenne, Lejeune, and Mahaux. We find that the latter one, which has an independent imaginary part, gives the best description. Furthermore, we find the renormalization constant for the real part of the folding potential to be nearly independent of the projectile-target combination at this energy and that no renormalization is needed for the imaginary part. From this analysis, we are able to eliminate an ambiguity in optical model parameters and thus better determine the asymptotic normalization coefficient for ¹⁰B→⁹B+p. Finally we use these results to find optical model potentials for unstable nuclei with emphasis on the reliability of the description they provide for peripheral proton transfer reactions. We discuss the uncertainty introduced by the procedure in the prediction of the distorted wave Born approximation cross sections for the (⁷Be,⁸B) reactions used in extracting the astrophysical factor S₁₇(0).

The neutron-rich argon isotope ⁴³Ar has been studied by quasielastic and inelastic proton scattering performed in inverse kinematics. The measured inelastic angular distribution for the second excited state is in good agreement with an L=2 transition. Assuming this transition to be E2, yields a β₂ value for this state of 0.25±0.03 when compared with distorted-wave Born approximation calculations. This value is comparable to the one reported for the stable isotope ⁴⁰Ar. Moreover it is similar to those measured by Coulomb excitation for the neighboring even-even isotopes ⁴²Ar and ⁴⁴Ar indicating that the structure of the argon isotopes is stable as a function of neutron number.

The ¹⁴N(⁷Be,⁸B)¹³C reaction was studied using an 85 MeV ⁷Be radioactive beam. The asymptotic normalization coefficients for the virtual transitions ⁷Be+p↔⁸B were determined from the measured cross section. These coefficients specify the amplitude of the tail of the ⁸B overlap function in the ⁷Be+p channel, and were used to calculate the astrophysical S factor for the direct capture reaction ⁷Be(p,γ)⁸B at solar energies S₁₇(0). We find that S₁₇(0)=16.6±1.9 eV b.

Elastic and inelastic proton scattering has been measured in inverse kinematics on the unstable nucleus ⁴⁰S. A phenomenological distorted wave Born approximation analysis yields a quadrupole deformation parameter β₂=0.35±0.05 for the 2₁⁺ state. Consistent phenomenological and microscopic proton scattering analyses have been applied to all even-even sulfur isotopes from A=32 to A=40. The second analysis used microscopic collective model densities and a modified Jeukenne-Lejeune-Mahaux nucleon-nucleon effective interaction. This microscopic analysis suggests the presence of a neutron skin in the heavy sulfur isotopes. The analysis is consistent with normalization values for λ_v and λ_w of 0.95 for both the real and imaginary parts of the Jeukenne-Lejeune-Mahaux potential.

The ⁹Be(p,γ)¹⁰B reaction plays an important role in primordial and stellar nucleosynthesis of light elements in the p shell, but the energy dependence of S(E) has not been well understood. We reanalyze the existing ⁹Be(p,γ)¹⁰B experimental data within the framework of the R-matrix method. The direct capture part of the S factor is calculated using the experimentally measured asymptotic normalization coefficients for ¹⁰B→⁹Be+p. The fitted parameters of the low-lying ¹⁰B resonances are also required to be consistent with previous measurements of ⁶Li(α,γ)¹⁰B. A good simultaneous fit to both radiative capture reactions is found, in contrast to previous analyses. These results demonstrate that experimentally measured asymptotic normalization coefficients, coupled to the R-matrix method, can provide a reasonable determination of direct radiative capture rates, even when the captured proton is tightly bound in the final nucleus.

The ¹⁰B(⁷Be,⁸B)⁹Be reaction has been studied with an 84 MeV ⁷Be radioactive beam. The measured cross section determines the asymptotic normalization coefficients for the virtual transitions ⁷Be+p↔ ⁸B. These coefficients specify the amplitude of the tail of the ⁸B wave function in the two-body channel ⁷Be+p, and may be used to calculate the S factor for the direct capture reaction ⁷Be(p,γ)⁸B at solar energies, S₁₇(0). We find that S₁₇(0)  =  17.8±2.8 eV b.

The ¹⁶O(³He,d)¹⁷F reaction has been used to determine asymptotic normalization coefficients for transitions to the ground and first excited states of ¹⁷F. The coefficients provide the normalization for the tails of the overlap functions for ¹⁷F→¹⁶O+p and allow us to calculate the S factors for ¹⁶O(p,γ)¹⁷F at astrophysical energies. The calculated S factors are compared to measurements and found to be in very good agreement. This provides a test of this indirect method to determine astrophysical direct capture rates using transfer reactions. In addition, our results yield S(0) for capture to the ground and first excited states in ¹⁷F, without the uncertainty associated with extrapolation from higher energies.

The decay structure of the particle-unstable nucleus ¹⁰Li was studied using the method of sequential neutron decay spectroscopy (SNDS) at 0°. The decay energies of ¹⁰Li can be derived from the relative velocity spectrum of the ⁹Li daughter and the neutron measured in coincidence. Evidence for low-lying s-wave strength was observed with a scattering length of <-20 fm, corresponding to a peak energy of <50 keV.

The ¹³C(¹⁴N,¹³C)¹⁴N proton exchange reaction has been measured at an incident energy of 162 MeV. Angular distributions were obtained for proton transfer to the ground and low-lying excited states in ¹⁴N. Elastic scattering of ¹⁴N on ¹³C also was measured out to the rainbow angle region in order to find reliable optical model potentials. Asymptotic normalization coefficients for the system ¹³C+p→ ¹⁴N have been found for the ground state and the excited states at 2.313, 3.948, 5.106, and 5.834 MeV in ¹⁴N. These asymptotic normalization coefficients will be used in a determination of the S factor for ⁷Be(p,γ)⁸B at solar energies from a measurement of the proton transfer reaction ¹⁴N(⁷Be,⁸B)¹³C.

The previously measured decay of the ground state of ¹²O was reanalyzed based on new experimental and theoretical results for the ground state of ¹¹N. In the previous analysis no evidence for diproton emission was found and the measured large decay width was inconsistent with sequential proton decay via the intermediate system of ¹¹N. The recent results on ¹¹N show evidence that the ground state of ¹¹N is at substantially lower energy allowing for a consistent explanation of the two-proton decay of ¹²O in terms of sequential proton emission.

The proton-unbound nucleus ¹¹N has been studied via kinematic reconstruction of the emitted proton in coincidence with the residual ¹⁰C daughter nucleus. Resonances in ¹¹N were populated by using a 40 MeV/nucleon radioactive beam of ¹²N to induce the reaction ⁹Be(¹²N,¹¹N), followed by the proton decay of ¹¹N. The decay energy spectrum was constructed from the energies and separation angle of the ¹⁰C and the proton. In addition to protons from the known 1/2^- state, at 2.24 MeV above the proton decay threshold, another peak is seen near 1.45 MeV. This peak could potentially be due to the predicted 1/2⁺ ground state and/or due to the decay of the 3/2^- state to the first excited state of ¹⁰C

A 39A MeV ³⁸S radioactive beam was used with inverse kinematics to measure angular distributions for elastic and inelastic proton scattering from a CH₂ target. Optical potential and folding model calculations are compared with the elastic distribution. Using coupled channel calculations, the β₂ value for the 2₁⁺ state is determined to be 0.35±0.04. This value, when compared with the corresponding result from a Coulomb excitation measurement, leads to M_n/M_p=(1.5±0.3)N/Z, indicating an isovector contribution to the 2₁⁺ state of ³⁸S.

Distributions of parallel and transverse momenta for ⁷Be fragments formed in the breakup of ⁸B have been measured at 41A MeV. The p_∥ distributions are narrow ( 81±4 and 62±3 MeV/c FWHM for Be and Au targets, respectively), comparable to those of neutron halo nuclei. Reaction mechanisms influence the ⁷Be momentum distributions, so they do not directly reflect the valence proton momentum wave function. We present reaction models that reproduce the distributions.

The lifetime of the neutron-unstable nucleus ¹⁶B was investigated to search for evidence of delayed neutron emission (neutron radioactivity). The lifetime was inferred to be less than 191 ps (68% C.L.) based upon the lack of ¹⁶B fragments observed in the fragmentation of 52 MeV/nucleon ¹⁷C nuclei. © 1996 The American Physical Society.

High energy γ rays from the decay of the giant resonance in hot ¹²⁰Sn nuclei were measured in the excitation energy range of 30–130 MeV. The excited nuclei were populated by inelastic scattering of α particles at 40 and 50 MeV/nucleon. The resonance width was observed to increase monotonically with increasing excitation energy, from 5 MeV at the ground state to ∼12 MeV at the largest excitation energy. Inelastic scattering predominantly populates low angular momentum states, and the observed width increase is thus attributed to fluctuations in the nuclear shape induced by temperature.

The three-body decay ¹²O→2p+¹⁰C was studied following production via single-neutron stripping from a radioactive ¹³O projectile. This is the first observation of two-proton emission from an unbound ground state where the one-proton emission channel is energetically closed beyond the lightest case of ⁶Be. No evidence for ²He emission is seen, despite predictions for a large diproton branching ratio. An upper limit of 7% (95% C.L.) is established for this decay branch. The implications of the small diproton branching ratio observed here and seen previously in ⁶Be are discussed.