MeV State Of 12C Research Articles

The photodisintegration of13C through excited states12C and12B

Gamma ray spectra from the reactions 13C( gamma ,n gamma ') and 13C( gamma ,p gamma ') have been measured simultaneously in a Ge(Li) detector at 112 degrees using bremsstrahlung with end-point energies in the range 15-44 MeV in order to study the decay of the 13C giant resonance through excited states of the residual nuclei. The cross sections for neutron emission to the 4.44 and 15.1 MeV states in 12C and for proton emission to the 0.95 MeV state in 12B have been measured. The integrated 13C( gamma ,2n) cross section up to 40 MeV has been determined and found to be very small in comparison with the ( gamma ,n) cross section. The ( gamma ,n) and ( gamma ,p) cross sections for feeding the ground states and also certain excited states which do not decay by gamma emission have been deduced. The cross sections are compared with other measurements and theoretical calculations. The structure of the 13C giant resonance is discussed. The results support the evidence for isospin splitting of the giant resonance as predicted in a number of theoretical calculations.

The elastic scattering of 33 MeV polarized 3He particles by beryllium and carbon

Angular distributions of 3He polarization in elastic scattering from 9Be and 12C, and in inelastic scattering to the 4.44 MeV state of 12C were measured using a 33 MeV polarized 3He beam. The polarization and cross-section data were successfully analysed in terms of the standard optical model. Selectively of the 3He elastic polarization for various potential sets was established in both optical model and coupled-channel analyses. While it is possible to determine unambiguously the geometry of the spin-orbit term, its strength is strongly correlated with other optical model parameters.

3α decay of the 10.84 and 11.83 MeV states of 12C

The theoretical density distributions for the 12C-3α break-up have been compared with those obtained experimentally for the 10.84 and 11.83 MeV states of 12C. The spin and parity assignment of the 11.83 MeV state of 12C has been deduced from the analysis of data.

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) 12C ∗ 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) 12C ∗(γ) 12C reaction.

Inelastic scattering of α-partlcles from 12c below 18.0 MeV

We have studied the inelastic scattering of α-particles leading to the 7.65 MeV and the 9.65 MeV states of 12C from lab energy of 18.0 MeV down to 2 MeV above the threshold for each reaction. Angular distributions were obtained at energies where significant structure appeared in the excitation functions. The measured total cross sections were used to evaluate the effect of inelastic scattering processes on the rate of the 3α → 12 C reaction in stellar interiors.

Final state interactions among the decay products of the hypernuclei Λ8Li and Λ12B

The results of a study of the decay of the Λ 8Li and Λ 12B hypernuclei are presented. The Λ 8 Li→π −+ 8 Be ∗→π −+ 4 He+ 4 He and Λ 12 B→π − + 12 C ∗→π −+ 4 He+ 4 He+ 4 He modes have been investigated in detail. New information on the decay mechanism of the 16.11 MeV state of 12C has been obtained.

A study of the mechanism of the 11B(d, n) 12C reaction by means of neutron-gamma correlation measurements

Reaction plane neutron-gamma angular correlations have been measured for the neutron group to the first excited state of 12C at neutron detector angles of 0° and 162° and five deuteron energies between 1.6 and 3.3 MeV. The results are analysed within the framework of the spin-dependent and spin-dependent DWBA theories of stripping reactions. Possible contributions due to competing reaction mechanisms are discussed. In addition correlation measurements are reported of the group to the 15.1 MeV state of 12C at θ n = 0° and E d = 2.88 and 3.31 MeV .

Alpha particles following the β-decay of 12B and 12N

The 10.3±0.3 MeV state in 12C, populated by the β-decay of 12B and 12N, decays by α-particle emission to the ground state of 8Be; its width is 3.0±0.7 MeV. The population and decay of the 10.3 MeV state limit the spin to J π = 0 + or 2 +. The 12.7 MeV state in 12C, populated by the β-decay of 12N, decays by α-particle emission to the J π = 2 + first excited state of 8Be. The formation and decay of the 12.7 MeV state uniquely determine the spin as J π = 1 +. The estimated value of log ft = 3.7 +0.3 −0.5 for the β-decay of 12N to the 12.7 MeV state suggests that this state is closely related to the 12N ground state.

The mechanism of the reaction12C(n, n )3 at 14.18 MeV neutron energy

A detailed study in photoemulsions of the reaction 12C(n, n)3α excited by neutrons of energy 14.18 ± 0.05 MeV has provided information on the reaction mechanism. About 50% of the events are attributed to the mode 12C(n, n)12C*, 12C*(α)8Be* -> 2α, most of the remainder being due to 12C(n, α)9Be*, 9Be*(n)8Be* -> 2α and 9Be*(α)5He*(n)α. However, the reaction 12C(n, 5He*)8Be* cannot be excluded. Four-body break-up, suggested by some authors, is unnecessary to explain the results. The 10.8 and 11.8 MeV states of 12C occur in the (n, n) reaction and the data allow the assignment 1- to the former and suggest 2- for the latter.

Photoalpha reactions below 21.5 MeV with 14N and 16O

Cross-section measurements integrated to 21.5 MeV have been made for the 14N(γ, α) 10B reaction (59 events), the 16O(γ, α) 12C reaction (48 events) and the 16O(γ, 4α) reaction (53 events), with the assumption that 10B and 12C were left in their ground states. The values for the single-alpha events are 0.74±0.11 MeV · mb for 14 N and 0.055±0.008 MeV · mb for 16O . The value for the four-alpha events is 0.124±0.017 MeV · mb. Partial cross-sections for the distinct modes of 16O breakup into two 8Be nuclei both in the ground state, and via the 7.7 MeV state of 12C, are 0.015±0.006 MeV · mb and 0.053±0.011 MeV · mb, respectively. The distribution in angle between the alpha and the photon direction for the 16O(γ, α) 12C reaction ( 12C assumed to be in its ground state) gives an E2 E1 intensity ratio of 0.3 while for the 7.7 MeV state the ratio is 0.2. The smallness of the cross-section for the photodisintegration of 16O yielding an alpha or four alphas, being 0.18±0.02 MeV · mb, agrees with the strongly inhibiting effects of ΔT = ±1 for E1 absorption and for no change in T for alpha decay.

The population ratio of the 4.433 MeV and 7.656 MeV states in12C in the reaction9Be(α, n)12C, and the parameters of the 7.656 MeV level

A pulsed beam neutron time-of-flight spectrometer has been used to study the reaction9Be(α, n)12C* at bombarding energies of 5.53, 5.76 and 5.97 MeV. Angular distributions of neutron groups leading to the ground state and first two excited states of12C were measured, and the population ratio of the 4.433 and 7.656 MeV levels has been established as 10.7±0.8 at ∼5.5 MeV. In conjunction with other evidence the measured value of population ratio establishes the alpha particle decay width of the 7.656 MeV level as approximately the Wigner limit, the spin and parity as 0+, and allows estimates to be made of the probabilities of pair and γ decay of this state.

Alpha-particle decay of the 15∙1 MeV state in12C

A search was carried out for the α-particle decay of the 15∙1 MeV state in12C. The state was made by the10B (3He,p)12C reaction. The protons feeding the 15∙1 MeV state were seen in proton singles spectra and in coincidence with 15∙1 MeVγ-rays, but none were seen in coincidence withα-particles. Comparison with a nearby state known to decay virtually solely by α-particles established that the 15∙1 MeV stateα-decays less than 20 % of the time. Combining this result with the results of γ-ray resonance scattering we find that ┌α< 15 eV. The sensitivity of the experiment was limited by a continuum from the break-up of the10B +3He system.

The Decay of the 7·68 MeV State in 12C

A polonium-beryllium source has been used to investigate the radiations in coincidence with the 4�43 MeV ?-ray leading to the ground state of 12C. The coincident radiation consisted of a continuum due to neutrons and a ?-ray of energy 3�05 MeV and intensity 0�12?0�08 that of the 4�43 MeV ?-ray.