Study of 11B at high excitations with the 9Be(3He,p)11B and 9Be(α, d)11B reactions

The nucleus ¹¹B has been studied over the excitation energy range from 8.5 MeV to 21.5 MeV with the ⁹Be(³He, p)¹¹B / reaction at / E_{³He = 38 MeV}. The analogs of the parent states in ¹¹Be have been located at 12.56, 12.92, 14.40, 16.44, 17.69, 18.0, 19.15 and 21.27 MeV. A complementary measurement with the ⁹Be(α, d)¹¹B reaction at E_α = 48 MeV demonstrates that the 16.44, 17.69, 18.0 and 19.15 MeV resonances have rather pure isospin $\text{T}{}_{\mathrm{f}}\text{=}\text{3}\text{2}$. The 14.40 MeV state is a strongly isospin-mixed analog of the $\text{5}\text{2}{}^{\mathrm{+}}\text{1.78}\text{MeV}$ state in ¹¹Be. It is argued that spin S = 1 transfer is involved in the excitation of the 16.44 MeV state and its 3.887 MeV parent in ¹¹Be in a two-step stripping process. The $\text{T}{}_{\mathrm{f}}\text{=}\text{1}\text{2}$ states and the lowest three $\text{T}{}_{\mathrm{f}}\text{=}\text{3}\text{2}$ states are compared with the predictions of DWBA utilizing shell-model form factors. It is concluded that the $\text{T}{}_{\mathrm{f}}\text{=}\text{1}\text{2}$ strength is more strongly fragmented than is implied by the calculations of Teeters and Kurath.     

The 10B(α, n) 13N13N1 cross section for α-energies from 1.0 TO 5 MeV

The three neutron groups, populating the ground state, the 2.366 MeV state and the 3.51, 3.55 MeV doublet of ¹³N were studied. Differential cross section measurements between E_α = 1.0 and 5.0 MeV at 0, 90 and 160° showed fifteen resonances of which nine had not been seen before in this reaction. Angular distributions were taken at six α-energies. These confirmed the J^π = 3^? assignment of the 12.690 level in ¹⁴N and permitted a tentative assignment of J^π = 3^? to the 13.166 MeV level. Compound nucleus formation involving overlap of several resonances appears dominant, except for the neutron group to the doublet in ¹³N shows evidence for a direct reaction mechanism.     

The 26Mg(14N, 10B)30Si and 26Mg(14N, 11B)29Si reactions

Reactions induced by ¹⁴N on ²⁶Mg at bombarding energies of 60–95 MeV have been studied. Angular distributions for states populated in ²⁹Si by the (¹⁴N, ¹¹B) reaction and in ³⁰Si by the (¹⁴N, ¹⁰B) reaction have been compared with Hauser-Feshbach and DWBA calculations to determine the reaction mechanism and to deduce spectroscopic information. The cross sections for the states populated in ²⁹Si and ³⁰Si are in poor agreement with statistical model calculations, indicating a non-compound nucleus mechanism.     

Nuclear reactions of 24 MeV 7Li on 10B

The charged-particle reaction channels accessible in the bombardment of ¹⁰B targets by ⁷Li nuclei of 24 MeV incident energy are investigated by measuring all ejectiles up to Be. Except for tritons, the reactions with light ejectiles proceed mainly via the formation of a compound nucleus, whereas some of the reactions with ejectiles heavier than the α-particle can be described by a direct mechanism.     

Angular distributions of neutron polarization from the 14C(p,n)14N and 11B(α, n)14N reactions and R-matrix analysis oF 15N in the excitation-energy range between 11.5 and 12.5 MeV

Angular distributions of neutron polarization from the ¹⁴C(p, n)¹⁴N and ¹¹B(α, n)¹⁴N reactions have been studied for the particle energies E_p = 1.788, 2.025, 2.272 and 2.450 MeV, and E_α = 2.049 MeV. The polarization was derived from the left-right asymmetry induced by elastic scattering from ⁴He. Together with existing measurements of angular distributions and total cross sections for several reaction channels leading to ¹⁵N with an excitation energy between 11.5 and 12.5 MeV, these data were used to deduce from R-matrix analysis a set of resonance parameters for the ¹⁵N levels in this energy range.     

Elastic and inelastic scattering of 17.5 and 40.0 MeV 3He particles from 11B

The ³He scattering cross sections have been measured at 17.5 and 40.0 MeV from the ground state and first four excited levels of ¹¹B. The results are analysed with the SCA assuming ¹¹B to be a symmetric rotator and including the K-band mixing introduced by the Coriolis force. A systematic attempt is made to deduce the hexadecapole moment and the sign of the quadrupole deformation of ¹¹B.     

Quasi-free scattering in 7(p,pd)5He and 12C(p,pd)10B reactions at 670 MeV

The reactions ⁷Li(p, pd)⁵He and ¹²C(p, pd)¹⁰ were investigated at 670 MeV in quasi-free scattering geometry at large angles. In transitions to low-lying as well as to highly excited states of the residual nuclei the energy distributions of the scattered protons are in agreement with the mechanism of quasi-free scattering on quasi-deuterons in the p- and s-shells of ⁷Li and ¹²C.     

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.     

The compound nucleus reaction 12C(11B, 2α) 15N

Energy spectra and differential cross sections of nitrogen products formed in the reaction 28 MeV ¹¹B + ¹²C have been measured using a ΔE?E counter telescope. The energy spectra are smooth and therefore indicate that the nitrogen products were formed by a compound nucleus mechanism, via the formation and decay of the compound nucleus ²³Na. The experimental results are compared with statistical model calculations and good agreement is obtained. This result provides further evidence for the importance of the compound nucleus mechanism in heavy ion reactions with light nuclei and also gives added validity to the statistical model for light compound systems.     

Excited states of 10B from the 6Li(α, γ) and 9Be(p, γ) reactions

Excitation functions for the (α, γ) and (p, γ) reactions leading to ¹⁰B have been measured at θ = 0° in the energy range from E_x = 6.7 to 7.6 MeV. Two resonances corresponding to levels at 6.88 and 7.44 MeV are observed. Branching ratios extracted from γ-ray spectra are the same in both reactions for the 6.88 MeV (1^?, T = 0 + 1) level, but different at 7.44 MeV. The T = 0 + 1 level at 7.44 MeV (Γ = 90±10keV) is assigned 2^? or 2⁺ from its strong branch to the 3⁺ ground state. We find no evidence for a second isospin mixed 1^? state.     

Strong coupled-channels effects in the 9Be(α, t)10B reaction

Differential cross sections were measured for the reactions ${}^9\text{Be}\text{(α, α')}{}^9\text{Be}\text{,}{}^9\text{Be}\text{(α,}\text{t}\text{)}{}^{10}\text{B and}{}^9\text{Be}\text{(α,}{}^3\text{He}\text{)}{}^{10}\text{B at}\text{E}{}_{\mathrm{\alpha }}\text{= 65}\text{MeV}$ for angles ranging from θ_lab = 6° to 48°. Optical-model analysis was performed for elastic α-scattering from ⁹Be at E_α = 48, 65 and 104 MeV, and DWBA and CC calculations were done for the inelastic α-scattering at E_α = 65 MeV. DWBA calculations for the ⁹Be(α, ³He) reactions do not fit the transfer data so well and extracted spectroscopic factors are in disagreement with those of Cohen and Kurath and with values obtained from other reactions. Full CRC calculations assuming a band structure for the low-lying states of ¹⁰B and employing a modified set of Cohen and Kurath spectroscopic factors yield globally better fits both in shape and in absolute cross section for differential cross sections to low-lying states in ¹⁰B obtained in ⁹Be(α, t)¹⁰B at $\text{E}{}_{\mathrm{\alpha }}\text{= 65}\text{MeV and}{}^9\text{Be}\text{(}{}^3\text{He, d}\text{)}{}^{10}\text{B at}\text{E}{}_{\mathrm{<mtext>d</mtext>}}\text{= 17}\text{MeV}$. In general, strong coupled-channel effects mainly affecting the distorted waves are observed both in entrance and exit channels.     

Levels of 14N near 13.7 MeV excitation from the analysis of Doppler-broadened γ-line shapes in the 10B(α, pγ)13C reaction

Excitation functions have been measured in the ¹⁰B(α, pγ)¹³C reaction by means of γ-detection. An enriched target and a Ge(Li) detector were used. The Doppler-broadened γ-line shape has been analysed, resulting in the angular distributions of protons for each bombarding energy. From a R-matrix analysis of these distributions, the parameters corresponding to three resonances have been determined.     

Final state interaction and direct features of the 9Be(τ, α)αα and 10B(d, α)αα reactions

Alpha-particle spectra and α-α correlations have been measured from the ⁹Be(τ, α)αα and ¹⁰B(d, α)αα reactions at several energies between 2.9 and 10.0 MeV. Significant difierences in the spectra from the reactions were observed at equal c.m. energies and momentum transfer values. However the continuous energy spectra have been successfully analyzed with two-body interactions as obtained from elastic α-α scattering, within a formalism adequate for a three-body state. DWBA fits have also been obtained for both reactions but they are satisfactory only for the ⁹Be(τ, α)⁸Be reaction.     

A study of the 2H(d,p)3H AND 2H(d,n)3He reactions and the excited state of 4He at 23.9 MeV

The relative differential cross sections have been measured for the ²H(d, p)³H and the ²H(d, n)³He reactions from 300 keV to 700 keV in 50 keV steps. Angular distributions of all charged particles from the reactions were taken from 20° to 160° in the laboratory system. Energy-dependent asymmetry coefficients from the expansion of the centre-of-mass angular distributions in terms of even powers of cos θ were obtained as were the branching ratios between the two reaction modes. The different energy dependences of the moments of the two cross sections were used to test the need for the existence of a recently reported T = 0 state in ⁴He.     

A study of the 26Mg(12C, 12B)26Al charge-exchange reaction

The reaction ²⁶Mg(¹²C, ¹²B)²⁶A1(5⁺, 3⁺) has been studied using a beam of 102 MeV of ¹²C. Shell-model, microscopic direct model and finite-range coupled reaction channel (CRC) calculations including recoil effects, have been performed, for comparison with the experimental data. DWBA calculations were performed for the intermediate states of interest in the ¹¹B + ²⁷Al and in the ¹³C + ²⁵Mg channels and these results were also compared with the experimental ones. The dominant reaction mechanism for ²⁶Mg(¹²C, ¹²B)²⁶Al(5⁺, 3⁺) appears to be the sequential mode.     

An experimental study of the 9Be(p, π+)10Be and 9Be(p, π−)10C reactions at 185 MeV

A magnetic spectrometer was used for the momentum analysis of pions produced by 185 MeV protons on a ⁹Be target. The obtained energy resolution (0.55 MeV FWHM) made it possible to resolve a number of discrete final states in ¹⁰Be and ¹⁰C. Energy spectra were measured in the angular region 15°–130° (lab). Angular distributions for six peaks in ¹⁰Be and four peaks in ¹⁰C are presented in tables and graphs. A peak corresponding to a not previously reported level in ¹⁰Be was observed at 11.75 ±0.11 MeV excitation energy. The measured $\text{π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}$ ratio for the ground state analogues was found to be angular dependent and varied from 30 at 35° (lab) to 2 at 125° (lab). The results are compared with theoretical predictions and discussed in terms of one- and two-nucleon models.     

Multiparticle break-up of 10B induced by fast neutrons

The reaction $\text{n}\text{+}{}^{10}\text{B}\text{→ α + α +}\text{d}\text{+}\text{n}$ induced by 14.4 MeV neutrons has been investigated in a kinematically complete experiment. Several final-state interactions have been observed in the sequential decay of the n + ¹⁰B system. The angular distribution of the ¹⁰B(n, ⁶Li₁)⁵He₀ reaction, leading to two unstable particles and consequently measurable only in a kinematically complete experiment, has been measured. It has been analysed in the framework of DWBA as a pick-up process with the transfer of the five-nucleon cluster ⁵Li. The presence of quasifree scattering and quasifree reaction with the deuteron as spectator has also been observed.     

Two-proton pickup in the zirconium region using the (6Li, 8B) reaction

Spectra at θ_lab = 8° have been measured for the ^{92, 94, 96, 98, 100}Mo(⁶Li, ⁸B)^{90, 92, 94, 96, 98}Zr and ^{90, 92, 94}Zr(⁶Li, ⁸B)^{88, 90, 92}Sr two-proton pickup reactions at 90 MeV. The strongest transitions to levels in the Zr isotopes are the 0⁺ → 0⁺ transitions to the g.s. and first excited 0⁺ states. The salient feature of the five Zr spectra is the large increase in 0⁺₂ transition strength relative to that of the g.s. which is observed in the two heaviest isotopes, ^{96, 98}Zr. In ⁹⁰Zr, the 0⁺₂ transition is weak whereas it is the dominant transition in ⁹⁸Zr with twice the g.s. strength. These large variations in relative cross section, which are reproduced by DWBA calculations, are attributed to changing g.s. proton configurations in Zr. In the Sr spectra, which are distinctly different from the Zrspectra, the strongest transitions to excited states are to 2⁺ rather than 0⁺ states. The (⁶Li, ⁸B) reaction seems to be adequately described as a one-step cluster transfer of a T = 1, S = 0 proton pair. The suitability of this reaction for measurements of two-proton pickup is discussed.     

Proton capture by 11B above the giant resonance

The cross sections for proton capture by ¹¹B to the ground and excited states of ¹²C have been measured in the proton energy interval between 18 and 43 MeV. The ground-state cross section shows good agreement with theoretical calculations including correlations. Capture photons have also been observed to all the residual 1p-1h states of ¹²C having a dominant $\text{1}\text{p}{}^{\mathrm{?1}}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ hole: the corresponding cross sections systematically show a giant resonance whose energy increases with the increasing excitation energy of the “background” level. The resonances at 27.4, 31, 33.2, 37 and 43 MeV, seem to show observable interference effects.