Investigation of one-nucleon transfer reactions between complex nuclei at incident energies between 3 and 8

Angular distributions of the ground state transitions in the reactions ¹¹B(¹⁴N, ¹⁵O)¹⁰Be, ¹¹B(¹⁴N, ¹³C)¹²C, ¹²C(¹¹B, ¹²C)¹¹B and ¹³C(¹²C, ¹³C)¹²C have been measured with a magnetic spectrometer as an identification system. The first two reactions (angular momentum transfer l = 2) measured at 41, 77 and 113 MeV show a clear damping of the oscillatory structures in the angular distributions. This effect was qualitatively reproduced by DWBA calculations which take into account a recoil phase, thus showing that the damping of the structures is due to the recoil of the transferred particle on the system. The reaction ¹¹B(¹²C, ¹¹B)¹²C (l = 2) at 87 MeV with Q = 0 is well reproduced by the calculations, whereas the ¹³C(¹²C, ¹³C)¹²C reaction as the only l = 0 transition is in complete disagreement with theoretical expectations.     

A new determination of the partial widths of the 16.11 MeV state in 12C

The partial widths of the second T = 1 state of ¹²C, at 16.11 MeV excitation energy, have been determined by measuring the ¹¹B(p, γ) and ¹¹B(p, α) cross sections at the E_p = 163 keV resonance corresponding to this state. These measurements result in the new values of Γ_p = 21.7 ± 1.8 eV and Γ_γ = 21.6 ± 3.3 eV, for the partial widths of this state; approximately 3 times smaller and larger, respectively, than the present values in the literature. The new result for the proton width eliminates a serious discrepancy found in an earlier comparison of the partial widths of the T = 1 analogue states of the A = 12 system. Measurements were also made of the ¹¹B(d, n)¹²C¹ reaction to compare the proton widths of the 15.11 and 16.11 MeV T = 1 states; these measurements confirm the new, smaller proton width for the 16.11 MeV state. An attempt was also made to determine the γ-width of the 16.11 MeV state by measuring the γ-branching ratio in the ¹⁰B(³He, p)¹²C¹(γ)¹²C reaction.     

Excitation functions of nuclear reactions producing11C

Excitation functions of the reactions⁹Be(³He,n),¹⁰B(d,n),¹¹B(p,n),⁹Be(α,2n),¹¹B(d,2n) and¹²C(n,2n), all leading to the residual nucleus¹¹C, were measured with activation techniques. Projectile energies have been chosen to populate the composite systems¹²C^* and¹³C^* in energy ranges overlapping for 31MeV≦E ^*(¹²C)≦38MeV and 26 MeV≦E ^*(¹³C)≦33Mev, respectively. The attainable thick target yields are highest forp+¹¹B. Statistical model calculations fail to quantitatively reproduce the experimental data although preequilibrium decay modes have been taken into account.     

Fusion and neutron-transfer cross sections for 13C + 10B and 13C + 11B reactions at subbarrier energies

The cross sections for the ¹⁰B(¹³C, ¹²C)¹¹B neutron-transfer reaction, leading to the ¹¹B 4.45 and 6.74 MeV and ¹²C 4.44 MeV excited states, and for ¹³C + ¹⁰B fusion have been measured by the characteristic and total γ-ray yield methods, respectively, over the energy (c.m.) interval 2.4–5.8 MeV. For ¹³C + ¹¹B, with no transfer reactions present, the fusion cross sections have been measured between E_c.m. = 2.3 and 6.4 MeV. The fusion cross sections for ¹³C + ¹⁰B and ¹³C + ¹¹B are found to be almost equal and slightly enhanced with respect to those for ¹²C + ¹⁰B and ¹²C + ¹¹B.     

Total reaction and transfer cross sections for 11B + 9Be and 13C + 9Be reactions at low energies

The total reaction cross sections for ¹¹B + ⁹Be and ¹³C + ⁹Be have been measured by the total γ-ray yield method over the energy intervals E_c.m. = 1.4–4.4 MeV and E_c.m. = 2.0–5.2 MeV, respectively. The cross sections for the neutron transfer reactions ¹¹B(⁹Be, ⁸Be)¹²B, leading to the ¹²B 0.953 and 1.674 MeV states, and ¹³C(⁹Be, ⁸Be)¹⁴C, leading to the ¹⁴C 6.094, 6.728 and 6.902 MeV states, have been obtained from the yields of the characteristic γ-rays. The α-transfer reaction ¹¹B(⁹Be, ⁵He)¹⁵N, leading to many unresolved ¹⁵N states, has been observed with large cross section. There is, however, no evidence for the ¹³C(⁹Be, ⁵He)¹⁷O transfer process in the ¹⁷O + nα channels. This different behaviour of the ¹¹B + ⁹Be and ¹³C + ⁹Be systems seems to indicate that the α-transfer reaction at sub-barrier energies is not a direct transfer process, and that it probably occurs via molecular state formation.     

An eikonal treatment including two-step mechanisms for the (p,n) reaction on light nuclei

The reactions ⁷Li(p, n)⁷Be(g.s.), ⁷Li(p, n)⁷Be(0.43 MeV), and ¹³C(p, n)¹³N(g.s.) are treated in an eikonalized distorted-wave impulse approximation (DWIA), including the contribution of a two-step mechanism involving isovector correlations for the last reaction. Our calculation uses the free on-shell nucleon-nucleon t-matrix, with spin and isospin dependence, and incorporates antisymmetrization of it. We find agreement at roughly the 20% level with the forward experimental charge-exchange results in the energy range 120 to 200 MeV. The two-step contribution for the ¹³C case is rather small, being of the order of 10% at 0°; it interferes constructively, thus improving somewhat the agreement with experiment.     

A study of the reaction (π+, π+ p) in 12C and 6Li

We have measured the angular and momentum distributions of the scattered pions from the reaction ¹²C(π⁺, π⁺p)¹¹B and ⁶Li(π⁺, π⁺ p)⁵He in a coincidence experiment. We compare our results with the plane and distorted wave impulse approximations.     

New isomeric states in the stable nucleus 185Re

High-spin states in ¹⁸⁵Re have been studied using the ¹⁷⁶Yb(¹³C,p3n) reaction at 65 MeV. Levels up to spin (37/2) h? at E _x = 4.799 MeV have been identified. Among them two levels were observed with half-lives of 6 ± 2 ns and 123 ± 23 ns.     

The11B(p,p′)11B* reaction: An extended coupled-channels treatment

The predictions of a multi-configurational shell model continuum calculation for the¹¹B(p, p) and¹¹B(p,p′) reaction channels are discussed. In the calculated excitation function for the (p, p ₁) channel theT=1, 2^? resonances play a dominant role in the region of 22 MeV to 24 MeV excitation in¹²C. These model predictions are consistent with the known parent states in the¹²B system. Corroborative evidence is also obtained by comparing theory with the differential cross section data.     

High resolution study of the reactions 54, 56, 58Fe(16O, 12C)58, 60, 62Ni and a comparison with (6Li,d) α-transfer spectroscopy

Spectra and angular distributions for the reactions ^{54, 56, 58}Fe(¹⁶O, ¹²C)^{58, 60, 62}Ni (E_x = 0.0–4.5 MeV) have been measured at 50 MeV with an energy resolution of 45–80 keV using a Q3D spectrograph. The selectivity of the (¹⁶O, ¹²C) reaction is found to be very similar to the (⁶Li, d) reaction. The close correspondence recently noted between the (⁶Li, d) spectra and levels strongly excited in (t, p) and (³He, n) two-nucleon transfer reactions is also observed to be present for the (¹⁶O, ¹²C) reaction. Relative α spectroscopic factors for (¹⁶O, ¹²C) and (⁶Li, d) obtained in a DWBA analysis assuming direct one-step α-cluster transfer are in very good quantitative agreement. Unnatural parity states, whose excitation is forbidden in the DWBA α-cluster approximation, are observed to be very weakly populated. These results, together with previous work on s-d shell and Ni targets, strongly suggest that the spectroscopic information provided by the (¹⁶O, ¹²C) and (⁶Li, d) reactions is essentially the same and that this information may be reliably extracted by DWBA analysis.     

Angular-momentum analysis of pionic nucleon capture

The pionic capture reaction A(N, π)(A + 1) is analyzed in terms of the Mandelstam model for pion production by two nucleons. The results are applied to the proton-capture reactions ¹²C(p, π⁺)¹³C and ⁴He(p, π⁺)⁵He, and comparisons are made with experiments.     

12C(p,n)12N and 16O(p,n)16F reactions at Ep = 35 and 40 MeV: Reliability of the information obtained from DWBA analysis of low-energy (p,n) data

A high-resolution study of the ¹²C(p, n)¹²N and ¹⁶O(p, n)¹⁶F reactions was made at E_p = 35 and 40 MeV. The low-lying states in ¹²N(1⁺, 2⁺ and 2⁻) and in ¹⁶F(0⁻, 1⁻, 2⁻ and 3⁻) were clearly resolved, and their angular distributions were measured. Extensive DWBA analysis was made and compared with the data. The calculated angular distribution shapes are found to be in agreement with the data and insensitive to the choice of the parameters involved. On the other hand, the magnitudes of the DWBA cross sections depend strongly on the bound state parameters in the case of a transition from a tightly bound state to a loosley bound state. In the other cases the overall uncertainty of the DWBA cross section magnitudes was estimated to be about ±30%. Within this uncertainty the experimental cross sections for the ¹²C(p, n) reaction were explained by the calculation, but those for the ¹⁶O(p, n) reaction were not: the observed strengths were about a half of the calculated values. Since these results agree with those at intermediate energies, the origin of the discrepancy is considered to be in the structure of the mass 16 nuclei rather than in reaction dynamics. In general, the present results compare well with those at intermediate energies, indicating that the structure information extracted from low-energy, high-resolution (p, n) data is basically sound if careful analysis of the data is made.     

The role of the 14N(n,p)14C reaction in neutron irradiation of soft tissues

The cell-killing potential of the ¹⁴N(n,p)¹⁴C reaction was considered with regard to neutron absorption in human nuclear DNA and respiratory phosphates for: (A) 10¹² thermal neutrons in 1 kg of soft tissue, (B) a mono-energetic beam of 2 MeV neutrons incident in 1 kg of soft tissue such that the total collision kerma was 10 J/kg, and (C) an evenly distributed 0–66 MeV neutron beam, also incident in 1 kg such that the total collision kerma was 20 J/kg. For case (A) 0.0017 ¹⁴N(n,p)¹⁴C reactions could be expected per DNA double strand, case (B) 0.053, and case (C) 0.0039. The probabilities that a proton emitted outside the nucleus would cross nuclear DNA were estimated from ¹⁴N tissue content for adult skeletal muscle, liver, and kidney tissues, for (1) nuclear DNA being concentrated in a sphere of 1.8 μm diameter, and (2) nuclear DNA being evenly distributed in a spherical nucleus 5 μm in diameter. It was concluded that even in a nitrogen-rich tissue exposed to a collision kerma of 20 J/kg by a 0–66 MeV fast neutron beam, the ¹⁴N(n,p)¹⁴C reaction directly kills at most 10 cells in every 1000, 4 of these by DNA nitrogen absorption and 6 by the ¹⁴N(n,p)¹⁴C protons emitted elsewhere in the cell. However, the dose due to the ¹⁴N(n,p)¹⁴C reaction should be measured where exposure to thermal neutron fluxes is significant. For therapeutic neutron doses the number of respiratory phosphate molecules in which the ¹⁴N(n,p)¹⁴C reaction occurs is insignificant, and doses from ¹⁴C-decay after neutron therapy are also negligible.     

Nuclear clustering aspects in astrophysics

Atomic nuclear clusters play a crucial role in nucleosynthesis in the universe, especially in the main sequence of heavy element synthesis. Cluster aspects in nucleosynthesis are briefly discussed based on a Cluster-Nucleosynthesis Diagram proposed here. Two recent topics on critical α-induced thermonuclear reactions are reviewed; the first one is the¹²C(α, γ)¹⁶O reaction for the He burning stage and the other one is the⁶ Li(α, n) ¹¹B reaction for the big bang nucleosynthesis. A new field of nuclear astrophysics using radioactive nuclear beams is also discussed.     

Lifetime of the 3.854 MeV state in 13C by the recoil-distance method

The lifetime of the 3.85 MeV state in ¹³C has been remeasured by the recoil-distance method using the ¹²C(d, p)¹³C reaction. The value of the mean life determined is τ = 13.0 ± 0.4 ps. This result is compared to previous measurements, both by the Doppler-shift attenuation method and the recoil-distance method.     

Spectroscopy of 13Be

Six quasi-stationary states of ¹³Be populated in the ¹⁴C(¹¹B,¹²N) ¹³Be reaction at E_lab = 190 MeV are reported. A Q-value = −39.60(9) MeV and a mass excess, M.E.= 33.95(9) MeV, have been found for the lowest observed spectral line. The ground state is unstable with respect to one-neutron emission by 0.80(9) MeV. Excitation energies of 1.22(10), 2.10(16), 4.14(12), 5.09(14) and 7.0(2) MeV have been obtained for the observed spectral lines.     

KeV astrophysics with GeV beams; blazing a new trail on the summit of Nuclear Astrophysics

GeV beams of light ions and electrons are used for creating a high flux of real and virtual photons, with which some problems in Nuclear Astrophysics are studied. GeV ⁸B beams are used to study the Coulomb dissociation of ⁸B and thus the ⁷Be(p,γ)⁸B reaction. This reaction is one of the major source of uncertainties in estimating the ⁸B solar neutrino flux and a critical input for calculating the ⁸B solar neutrino flux. The Coulomb dissociation of ⁸B appears to provide a viable method for measuring the ⁷Be(p,γ)⁸B reaction rate, with a weighted average of the RIKEN1, RIKEN2, GSI1 and MSU published results of S ₁₇(0)=18.9±1.0 eV-b. This result, however, does not include a theoretical error estimated to be ±10%. GeV electron beams on the other hand, are used to create a high flux of real and virtual photons at TUNL-HIGS and MIT-Bates, respectively, and we discuss two new proposals to study the ¹²C(α,γ)¹⁶O reaction with real and virtual photons. The ¹²C(α,γ)¹⁶O reaction is essential for understanding Type II and Type Ia supernova. It is concluded that virtual and real photons produced by GeV light ions and electron beams are useful for studying some problems in Nuclear Astrophysics.