Nonstatistical structures in the excitation functions of20Ne(16O,12C)24Mg and20Ne(16O,20Ne)16O reactions

The data on the excitation functions of²⁰Ne(¹⁶O,¹²C)²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(1.37, 2⁺),²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(4.12, 4⁺+4.24, 2⁺) +²⁰Ne(¹⁶O,¹²C^*(4.44, 2⁺))²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(6.01, 4⁺+6.43, 0⁺),²⁰Ne(¹⁶O,²⁰Ne)¹⁶O,²⁰Ne(¹⁶O,²⁰Ne^*(1.63, 2⁺))¹⁶O, and²⁰Ne(¹⁶O,²⁰Ne^*(4.25, 4⁺))¹⁶O reactions atθ _lab=13° fromE _c.m.=22.8 to 38.6 MeV have been subjected to a statistical analysis comprising of the calculations of the distribution of cross sections, deviation functions, cross-correlation functions, summed excitation functions, cross-channel correlation coefficients and coherence widths. The analysis confirms the existence of nonstatistical structures atE _c.m.=24.6, 27.8, 31.7 and 35.5 MeV, and identifies a new structure of the same nature atE _c.m. =25.6 MeV.     

Resonant structures in24Mg+28Si elastic and inelastic scattering

The data on the excitation functions of²⁴Mg+²⁸Si elastic and inelastic (2⁺ ?0⁺, 2⁺ ?2⁺, 4⁺ ?0⁺ and 4⁺ ?2⁺) scattering fromE _c.m.=48.97 to 57.21 MeV have been subjected to a statistical analysis consisting of the calculations of deviation function, cross-correlation function, summed excitation function, cross-channel correlation coefficients, coherence widths, and the distribution of cross sections. Based on the outcome of the analysis resonant structures atE _c.m.=49.23, 50.02, 50.51, 52.10, 52.53, 53.27 and 54.14 MeV have been confirmed and three new structures of the same nature have been identified atE _c.m.=51.42, 54.88 and 55.60 MeV.     

Phenomenological analysis of the 24Mg(12C, α)32S resonant reaction studied at low incident energy

The ²⁴Mg(¹²C, α)³²S reaction has been studied in the region of the Coulomb barrier. Three angular distributions for the α-particle groups populating the ground and first-excited 2⁺ state of ³²S have been measured spanning the apparent resonant structure observed at E_c.m = 14.20 MeV. Regge-pole and phase-shift analyses suggest a spin assignment of 8 for the corresponding resonance at E_x = 30.51 MeV in ³⁶Ar.     

Investigation of high spin states in23Na and23Mg and the12C+12C resonance atE c.m.=19.3 MeV

The resonance in the¹²C+¹²C system atE _c.m.=19.3 MeV has been studied in the neutron and proton decay channels. From(p, γ), (n, p) and (n, γ) coincidence measurements high spin states could be localized inA=23 nuclei, in²³Mg up to possibly 21/2⁺. These states were resonantly populated. The present data favorJ ^π=17/2⁺ for the 9.61 MeV state in²³Mg and therefore also for the probable parent state at 9.81 MeV in²³Na. AJ ^π=12⁺ assignment to the resonance is supported and evidence was found that the resonant excitation of the lowest 15/2⁺ and 17/2⁺ states in mass 23 nuclei is caused by a change of the intrinsic structure of the Yrast levels aroundJ ^π=15/2⁺ towards larger deformation.     

A comparative study of the electric giant dipole resonance of 24−26Mg

The electric giant dipole resonance of ^24?26Mg has been explored up to 30 MeV excitation energy with bremsstrahlung. ΔE, E spectra of charged photo-particles and spectra of prompt deexcitation γ-rays from excited residual nuclear states were obtained at various bremsstrahlung endpoint energies. The ²⁵Mg(γ, p₀), (γ, d), ^24,25Mg(γ, α) differential cross sections as well as ^24?26Mg(γ, xγ′) integrated cross sections are presented. The results are discussed in terms of one-particle, one-hole excitations and isospin composition of giant resonance states. A comparison with calculations for ²⁴Mg gives poor agreement. Excitations from deeper shells were found in the giant dipole resonance of ²⁴Mg, but do not seem to be concentrated at higher energies. In ²⁵Mg, only weak excitations of this kind were found, and they are completely absent in ²⁶Mg.     

Fissionlike exit channel in the 5.8 MeV/u84Kr on24Mg reaction

Fissionlike (FL) phenomena for systems around mass 100 have been shown previously interpretable in terms of dynamical fission. This appears to be dependent on the entrance channel mass asymmetry. We have studied the fissionlike exit channel in the⁸⁴Kr+²⁴Mg reaction atE(Kr)=5.8 MeV/u. This energy is just above the threshold of fission, so that a clear separation between the Deep Inelastic Component (DIG) and the FL one can be achieved. The bombarding energy corresponds to the same excitation energy in the composite system¹⁰⁸Cd (E ^*=101 MeV) as in the³²S (158 MeV) on⁷⁶Ge reaction we studied earlier and for which fusion-evaporation along with fission-like contributions were measured. The width of the FL distribution confirms the previously observed dependence with the mass asymmetry in the entrance channel which is consistent with the extra push model.     

Optical model and coupled-channel analysis of 60–90 mev deuteron scattering from24Mg and89Y

Angular distributions of deuterons scattered from²⁴Mg and⁸⁹Y in the angular range 12 °→100 ° have been measured atE _d=60, 77 and 90 MeV. With some exceptions the elastic and inelastic data were found to be described consistently by the optical model approach and by coupled-channel analysis, respectively. Collective model features of²⁴Mg are derived from the inelastic scattering results.     

Unelastische Elektronenstreuung an24Mg bis 11,5 MeV Anregungsenergie

From experiments at low momentum transfer (E ₀<59 MeV, 104°<Θ< 153°) ground state radiation widths for levels at 9.85, 9.97, 10.72 MeV (1⁺), 1.37, 4.23, 7.36, 9.00, 9.30, 10.36, 10.94, 11.47 MeV (2⁺), 7.60, 8.38 MeV (3^?) and ground state matrix elements for levels at 6.44, 10.7 MeV (0⁺) have been determined. Transition radii are also given.     

New aspects of the rotational model in25Mg

Spin assignments have been made to the²⁵Mg levels in theE _x=5–6 MeV region fromγ-ray angular distributions measured in the²²Ne(α, n y) reaction atE _α=8.0 and 8.8 MeV and fromp-γ angular correlations measured in the²⁴Mg(d, p γ) reaction atE _d=6 MeV. Unique spin assignments ofJ=7/2, 5/2, 5/2, and 9/2 could be made to the levels atE _x=5005, 5511, 5851, and 5967 keV, respectively. Ambigious assignments have been made to the levels atE _x=5245, 5524, 5785, 6032 keV (J=11/2, 7/2), 5455 keVJ=13/2, 9/2), and 5738 keV (J=3/2, 5/2). The present data confirm previous assignments ofJ=1/2 to the levels atE _x=5108 and 5466 keV, respectively. Lifetime estimates have been obtained, using the Doppler-shift attenuation method, for the levels atE _x=5245 keV (τ=30–60 fs), 5785 keV (τ=50–100 fs), 5967 keV (τ=50–100 fs), and 5455 keV (τ>1ps). All other levels in theE _x=5–6 MeV region haveτ<60 fs. A breakdown of theK-selection rule has been observed in theγ-decay of some high spin states, indicating a deviation from the strong coupling model.     

Excitation of ground and gamma bands in the 24Mg(α, α')24Mg reaction at 120 MeV

The cross sections for excitation of the members of the ground state (g.s.) band 0^?(g.s.), 2⁺ (1.37 MeV) and 4⁺ (4.12 MeV) and the γ-band 2⁺_γ (4.24 MeV), 3_γ⁺(5.24 MeV) and 4_γ⁺(6.01 MeV) in ²⁴Mg have been measured in inelastic α-scattering at E_α = 120 MeV. The excitation of these states are found to be well described by a coupled-channel calculation (CCBA) performed in the framework of the asymmetric rotational model. Two sets of parameters are found to give excellent fits to the data, but in both a direct coupling between the ground state and the 4⁺ state is found necessary.     

Gamma-rays from the24Mg(p,γ)25Al reaction for proton energiesE p=800 keV toE p=1,830 keV

The yield curve and resonance strengths of resonances in the²⁴Mg(p, γ)²⁵Al reaction for the proton energy rangeE _p=800 keV toE _p=1,830 keV, have been determined and the existence of a resonance atE _p=1.184 MeV confirmed. Single energy spectra as well as decay schemes and branching ratios are given for these resonances. An upper level of 2.0×10^?15 sec for the lifetime of the 3.079 MeV level in²⁵Al results from a Doppler shift attenuation measurement carried out on the 2.624 MeV gamma line originating in the 3.070 MeV→0.455 MeV transition in²⁵Al.     

Alpha decay of isobaric analogue states in24Mg and28Si

Theα-decay of isobaric analogue states (which are forbidden by isospin selection rules) excited in²⁴Mg and²⁸Si through preton capture by²³Na atE _p=677 and 739 keV and by²⁷Al atE _p=295, 327 and 405 keV, respectively, have been studied using solid state track detectors. The ratio ofα-decay widths of the two resonance states in²⁴Mg to the state at 1.632 MeV (2⁺) in²⁰Ne yields the value 0.065 for the mixing parameterε and the value 4.01 keV for the Coulomb matrix element responsible for the isospin mixing in²⁴Mg. In²⁸Si the measurement of theα-decay widths of the three resonance states resulted in the determination of the proton, gamma and alpha partial widths which comprise the total width of the resonance states. Limits have been set for the value of the two mixing parameters involved in this case. Upper limits of 16 and 39 keV have been obtained for the Coulomb matrix elements responsible for the isospin mixing in²⁸Si.     

Resonant and intrastate transitions to the 2+ state in8Be

We have studied resonant, intrastate and non-resonant bremsstrahlungγ-ray transitions leading to the 2⁺ state in⁸Be atE _x=2.94 MeV. We find aγ-width of Γ _γ =0.45 eV for the 4⁺ state in⁸Be, corresponding to anE 2 strength of 19 W.u. For the intrastateγ-ray transition within the 2⁺ resonance we predict a maximum cross section of 2.5 nb atE≈3.3 MeV. An experimental observation of this novel type ofγ-ray emission, however, appears to be difficult due to the dominance of competing resonant and direct capture processes.     

The isoscalar strength distribution in 24, 26Mg, 28Si and 40Ca obtained from inelastic alpha scattering at 120 Mev

The inelastic α-scattering reaction at E_α = 120 MeV with an energy resolution of 90–150 keV has been used to investigate isoscalar strength distributions in ^{24, 26}Mg, ²⁸Si and ⁴⁰Ca. For ^{24, 26}Mg and ²⁸Si the E2 strength between E_x = 14 and 27 MeV is strongly fragmented. In ⁴⁰Ca the E2 strength is mainly concentrated near $\text{E}{}_{\mathrm{<mtext>x</mtext>}}\text{~ 65 A}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ MeV, although here the onset of fragmentation can be observed. The sum rule strength for the different multipolarities was obtained by applying for each nucleus an L-dependent normalization procedure. In this way we observed in total in ^{24, 26}Mg, ²⁸Si and ⁴⁰Ca for excitation energies up to 27 MeV an amount of (61⁺⁸_?6), (50⁺⁹_?8), (38⁺⁸_?6) and (94 ± 14)%, respectively, of the isoscalar E2 energy weighted sum rule (EWSR) of which (36⁺⁷_?5), (28⁺⁸_?7), (24⁺⁷_?5) and (74 ± 12)% was found between E_x = 14 and 27 MeV. In addition isoscalar E0, E3 and E4 strength was observed in this excitation energy region. A detailed comparison has been made between the isoscalar quadrupole strength distribution observed in the ^{24, 26}Mg(α, α') reaction and the E2 strength excitation function obtained from radiative α-capture measurements. In the low excitation energy region coupled channel effects have been observed, especially for the excitation of the 3⁺ states. Moreover, a considerable percentage of the 1?ω isoscalar dipole and octupole strength has been observed for excitations below 14 MeV.     

The 12C+24Mg elastic scattering: An example of anomalous transparency at coulomb barrier energies

Fifteen complete angular distributions of the elastic scattering of ¹²C+²⁴Mg were measured at energies around the Coulomb barrier (E_cm = 10.67–16 MeV). The angular distributions are strongly oscillating and could be well described by an optical potential family, whose real part was determined without continuous ambiguity. The imaginary part of this optical potential is very shallow. At four energies the inelastic scattering angular distributions leading to the 2⁺ state of the ²⁴Mg were also measured and analysed with coupled-channels calculations. The volume integrals of the optical potentials used in the coupled-channels calculations present the threshold anomaly in their energy dependence, with a clear Q-value dependence.     

Energy dependence of the 24Mg(16O, 12C)28Si reaction

Excitation functions for the reaction ²⁴Mg(¹⁶O, ¹²C)²⁸Si(g.s., 2⁺₁) were measured at 5°(lab) in the energy range 32 < E_c.m. < 49 MeV. Although the resonant structure, previously observed at lower energies, becomes progressively weaker, three new correlated maxima have been observed near E_c.m. = 37.5, 40.2 and 43.5 MeV. Angular distribution measurements at these energies yield spin assignments, from P²_j(cos θ) comparisons, of 27, 29 and 31, respectively. Attempts to find a consistent optical-model fit to the elastic scattering in the entrance channel and an exact finite-range DWBA fit to the four-nucleon transfer reaction in this energy range were unsuccessful. Such a failure is to be expected if strong couplings between the elastic channel and inelastic channels of either the initial or final system are important. The features of the resonance phenomena in the transfer reaction are discussed within a band crossing model framework.     

Exact finite range CCBA results for the reaction 24Mg(d,p)25Mg at 14 and 15 MeV

The differential cross-sections for the ${}^{24}\text{Mg(d, p)}{}^{25}\text{Mg}\text{5}\text{2}{}^{\mathrm{+}}\text{(}\text{g.s.}\text{)}$ reactions at E_d = 15 Mev and $\text{7}\text{2}{}^{\mathrm{+}}\text{(1.61}\text{MeV state}\text{)}$ reaction at E_d = 14 and 15 MeV have been calculated using the full finite range CCBA code OUKID. It is shown that finite range effects are very important for these light ion reactions and that the deuteron D-state makes a negligible contribution at these energies.     

Neutron scattering from 26Mg

Elastic and inelastic cross sections have been measured for 24 MeV neutrons incident on ²⁶Mg using the time-of-flight technique. These cross sections and existing proton data were analyzed in terms of a Lane-consistent optical potential. Distorted-wave and coupled-channel calculations are presented. Deformation parameters for the first 2⁺ state (1.81 MeV) and the second 2⁺ state (2.94 MeV) derived from both the (n, n') and (p, p') data are used to obtain the ratio of neutron and proton transition matrix elements M_n(E2) and M_p(E2) for these states. Comparison of results for $\text{M}{}_{\mathrm{<mtext>n</mtext>}}\text{(}\text{E}\text{2)}\text{M}{}_{\mathrm{<mtext>p</mtext>}}\text{(}\text{E}\text{2)}$ with those obtained from pion work, from electromagnetic (EM) rates and theoretical evaluations are presented.