Many-particle decays of α-chain structures in24Mg

We have searched for evidence of exotic cluster configurations in²⁴Mg resembling a linear chain of α particles in various many-particle final states of the¹²C+¹²C system, including¹²C(O ₂ ⁺ )+¹²C(O ₂ ⁺ ) and⁸Be+¹⁶O^*(4α). Such configurations are predicted to occur by a number of different theoretical models of the structure of²⁴Mg. An array of highly segmented double-sided silicon strip detectors permits detailed, high resolution reconstruction of these many-charged-particle final states.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

Neutron polarizations from the9Be(d,n)10B and11B(d,n)12C reactions

Neutron polarization angular distributions were measured for the⁹Be(d,n)¹⁰B and¹¹B(d,n)¹²C reactions for deuteron energies of 5.47 and 5.34 MeV, respectively. Using neutron time-of-flight techniques, polarizations to eight states in¹⁰B and to six states in¹²C have been measured. Neutron polarization energy excitation functions for both reactions were measured for deuteron energies from 3.0 to 5.5 MeV in 0.25 MeV steps at 30° (lab). Distorted wave method calculations carried out to compare theoretical calculations against experimental results were not in good agreement with the data.     

Alpha-particle unstable states in12C

Alpha particle unstable states in¹²C have been investigated using the three reactions¹⁴N(d, α′)¹²C,¹⁴N(d, α′α)⁸Be and¹²C(α, α′)¹²C. Excitation cross sections and angular distributions have been measured for the reactions¹⁴N(d, α′) at 52 MeV and¹²C(α, α′) at 90 MeV. For a few angle pairs the α-decay of excited states in¹²C have been observed in a¹⁴N(d, α′α) correlation measurement. The reactions selectively excite onlyT=0 states. A previously undetected level with a large α-decay width (Γ=1.2 MeV) has been observed at 15.62 MeV excitation. This level shows up clearly in both reactions and is further distinguished from the nearby 14.08 state in the correlation measurement because of the distinctly different energy distributions of the decay products. On the basis of a particle angular distributions the 15.62 MeV level was assigned spin and parity 4⁺ and the level at 14.08 was assigned 3^?. The latter differs from the value suggest by earlier work. Comparison with DWBA calculations indicates that angular distributions of all other prominent levels are in agreement with their earlier assignments. Two levels at 19.20 and 20.30 MeV (both Γ?0.4 MeV) and three further levels at 21.81, 22.7 and 24.24 MeV also decay predominantly by α-emission.     

Search for highly deformed shape isomers in 28Si, 24Mg and 20Ne linked by successive alpha decays

Measurements of the ¹²C(¹⁶O,²⁰Neα)α, ¹²C(¹⁶O,αα)²⁰Ne, ¹²C(¹⁶O,⁸Be)²⁰Ne and ¹²C(¹⁶O,¹⁶Oα)⁸Be reactions have been performed at E_c.m. = 27.0 MeV. For decays proceeding through resonant states in the intermediate ²⁴Mg nucleus, angular correlation measurements have enabled spin assignments to be made and the dominant entrance channel angular momentum to be determined. The results are inconsistent with recent similar experiments which were interpreted as arising from successive α-decays from deformed shape isomeric states in ²⁸Si and ²⁴Mg.     

The structure of25Mg

Gamma-decay modes and spin(-parity) assignments of levels in²⁵Mg have been systematically investigated up to 10 MeV excitation energy by particle-γ-ray angularcorrelation measurements with the²⁴Mg(d, pγ) reaction at 6.5 MeV bombarding energy and with the²²Ne(α,nγ) reaction at 11.8, 12.5, 14.4 and 15.5 MeV bombarding energy. A level scheme has been established which is comprehensive up to 8.3 MeV excitation energy forI≦9/2 and up to 10 MeV for 9/2O d _5/2 — 1s _1/2-O d _3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent. The first intruder states are located near 6.8 MeV excitation energy. The collective properties of²⁵Mg beyond the well established rotational bands are investigated using both the new experimental information and theB(E2)'s obtained from the shell model. The spectrum of²⁵Mg is completely rotational for the first five to six MeV above the yrast line. Shell modelB (M 1)'s reflect the Nilsson model structure of²⁵Mg in great detail. The prospectiveI ^π=9/2^?, 13/2^?, and 15/2^? members of the established negative-parity,K=1/2 band are found in levels atE _x=7801, 9410, and 8896 keV.     

Non statistical reaction contributions in (α, n) excitation functions

The excitation function²⁴Mg(α, n)²⁷Si has been measured from threshold to 27 MeV excitation energy. A survey is given of the experimental excitation functions¹²C(α, n)¹⁵O,¹⁶O(α, n)¹⁹Ne,²⁰Ne(α, n)²³Mg and²⁴Mg(α, n)²⁷Si and HF calculations have been made for them. The agreement is good within a factor of 2. The deviations of the experimental data from the pure statistical cross sections show common structures which can be interpreted as direct contributions.     

Sequential processes in the14N(τ,pα)12C* reaction

α-particle and proton spectra in coincidence with the 4.44 MeVγ-ray from the¹²C first excited state have been taken atE _τ MeV. Several states in¹³N and¹⁶O are seen to contribute to the decay sequenceα?p andp?α respectively. Estimates are given for the branching of these two decay modes and for a possible simultaneous break-up.     

Alpha chain states in 4N-nuclei

We review the experimental evidence for alpha chain states in 4N-nuclei. Bloch-Brink cranked cluster model calculations suggest that such states should exist in all light A=4N nuclei. Recently a strong resonance has been observed in the¹²C(¹²C,¹²C(O ₂ ⁺ ))¹²C(O ₂ ⁺ ) reaction at 33 MeV which has been taken as evidence of a 6-α chain state in²⁴Mg. The angular distributions show strong peaks at 90° due to the simultaneous excitation of several nearly degenerate resonances with a range of L-values. We review our analysis of these data and present the results of calculations for the widths of resonances in hyperdeformed nuclei. We demonstrate that such resonances may have relatively narrow widths despite their high excitation energies. We surmise that the 33 MeV resonance in¹²C+¹²C scattering has a chain state structure.     

16O, 24, 26Mg, 28Si, 32S, 40Ca(α, 12C) and multi-α-cluster transfer reactions

The (α, ¹²C) reaction has been studied on a variety of nuclei, A = 16 to 40, at E_α = 90.3 MeV. The data indicate a rapid fall-off of cross sections with increasing target mass, approximately as A_t^{?5 ± 1}. This and other systematics are used to estimate cross sections for multi-α-cluster transfer reactions in heavy nuclei and suggest σ_T < 10^?34 cm² consistent with present experimental limits. The data for ²⁴Mg(α, ¹²C)¹⁶O has been studied in more detail and indicates a selective population of final states including ¹⁶O g.s., with oscillatory angular distributions in some instances. Finite-range distorted-wave Born approximation calculations for direct ⁸Be pickup have been performed utilizing cluster overlap amplitudes obtained with zero-order SU(3) wave functions. The calculations are in qualitative, and often quantitative, agreement with shapes and absolute magnitudes of the measured angular distributions although the cross sections for certain α-cluster states (2⁺, E_x ≈ 7 MeV; 4⁺, E_x ≈ 10.3 MeV) are greatly overestimated with this model. Other more complicated mechanisms, such as successive α-transfer, cannot be excluded. The systematics of the calculated ⁸Be cluster overlaps and the calculated and measured (α, ¹²C) cross sections are investigated, and implications for multi-α-cluster transfer reactions are discussed.     

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.     

Properties of8Be and12C deduced from the folding-potential model

Theα-α differential cross sections are analyzed in the optical model using a double-folded potential. With the knowledge of this potential bound and resonance-state properties ofα-cluster states in⁸Be and¹²C as well as astrophysical S-factors of⁴He(α,γ)⁸Be and⁸Be(α,γ)¹²C are calculated. Γ_γ-widths and B(E2)-values are deduced.     

16O-8Be break-up states and cluster structure of 24Mg

High-spin states of ²⁴Mg produced in the ¹⁶O + ¹²C interaction and decaying into the ¹⁶O_g.s. + ⁸Be_g.s. channel have been observed in the excitation region between 35 and 52 MeV. Spins have been assigned on the basis of the analysis of the measured angular correlations. Some of these states with positive parity correspond to the known resonances of the ¹²C( ¹²C, ⁸Be_g.s.) ¹⁶O reaction belonging to the ¹⁶O-2α rotational band of ²⁴Mg. Moreover other resonances show up at higher excitation energy with an energy-spin relationship again suggesting a ¹⁶O-2α cluster structure for the associated configuration. Received: 6 July 2001 / Accepted: 16 October 2001     

Study of the structure of16O above 25 Mev by the reaction13C(τ, α)12C*(γ15.1)

The nucleus¹⁶O has been investigated within the excitation range of 25–32 MeV by means of the reaction¹³C(τ, α)¹²C^* _15.1(γ)¹²C. Excitation functions for the 15.1 MeVγ-rays, taken for bombarding energiesE _τ=2.6–12 MeV at 0? and 90?, exhibit some marked structures with widths of 0.5 to 1 MeV. As the (τ, α)-reaction leads to the 15.1 MeV,T=1 state, levels in¹⁶O underlying these structures should have largeT=1 components. Angular distributions of theα-particles coincident to the 15.1 MeVγ-rays show patterns typical for a direct process and are, therefore, compared with DWBA calculations.     

Proton pick-up from nuclei in the middle of the 1p shell

The (d,³He) reaction has been used to excite proton hole states in⁸Li,⁹Be and¹⁰Be. Angular distributions have been measured and have been analyzed in terms of the DWBA to get spectroscopic factors for the considered transitions. Excitation energies and transition strengths are compared with the results of Cohen and Kurath's intermediate coupling shell model calculations, where the two models of the effective interaction produce different results especially for transitions to final states in mass 8 and 10 nuclei. The experimental results are clearly in favour of the (6–16) 2 BME interaction. A positive parity state in¹⁰Be predicted by the calculations has been looked for and found at 9.60 MeV.     

Spin assignments by a high spin selective heavy ion reaction

The reaction¹²C(¹⁴N,d)²⁴Mg is investigated atE _Lab=52 MeV by means or a multigap magnetic spectrograph. Angular distributions of levels between 6.00 and 20.21 MeV are measured using nuclear track emulsions. By quantitative analysis of the angular distributions (FRDWBA- and Hauser-Feshbach calculations), the reaction is shown to proceed almost completely via compound mechanism. The reaction is applied for selective excitation of high spin states in²⁴Mg. Evidence is given by this example that under suitably chosen conditions a complex heavy ion reaction can be a powerful means for spin assignments at high excitation energies.     

Exact finite-range DWBA description of the60Ni(160,12C)64Zn reaction at 60 MeV incident energy

Cross section angular distributions of⁶⁰Ni(¹⁶O,¹²C)⁶⁴Zn reactions leading to three strongly excited states at 60 MeV incident energy and the¹⁶O+⁶⁰Ni and¹²C+⁶⁴Zn elastic scattering at 60 MeV respectively 45 MeV and 54 MeV have been measured using aQ3D magnetic spectrograph. EFR-DWBA calculations assuming the transfer of anα-cluster in its 0s ground state are able to describe the general features of the strongly oscillating experimental angular distributions using a surface transparent optical model potential. The optical model parameters used in the DWBA calculations are obtained from fits of the elastic scattering data of incident and exit channels. The importance of “correct” optical model parameters in the exit channels for relative spectroscopic factors will be discussed and the extracted relative spectroscopic factors will be compared to previous (⁶Li,d) results.     

Study of the 24Mg(α, 12C)16O reaction in the energy range Elab = 24.9–27.76 MeV

The ²⁴Mg(α,¹²C)¹⁶O and ²⁴Mg(α, α')²⁴Mg reactions have been used to search for resonances in the ²⁸Si compound nucleus. The excitation functions were measured in the energy range between 24.9 and 27.76 MeV. Angular distributions of ¹²C nuclei and elastic and inelastic scattering of α-particles have been measured at energies corresponding to extrema of excitation functions. Significant anomalies have been found at E_ex = 31.5 and 33,1 MeV. The spins and parities of these structures are assigned as 9⁻ and 11⁻, respectively.     

The decay of the18O giant dipole resonance to excited residual nuclear states

The decay of the¹⁸O GDR to excited residual nuclear states has been examined by measuring spectra of prompt deexcitationγ-rays. The target was irradiated by bremsstrahlung with endpoint energies of 23.5 and 28 MeV. “Bremsstrahlung-weighted” integrated cross sections are given for the population of the residual nuclear states in¹⁷O,¹⁷N,¹⁶O and¹⁴C. Proton decay to excited states in¹⁷N is remarkably weak. It seems that 1p _3/2 excitations play only a minor role in the¹⁸O GDR.     

The 12C(9Be,n)20Ne reaction

A study of the ¹²C(⁹Be, n)²⁰Ne reaction has been carried out at bombarding energies of 16 and 24 MeV. The spectra at both incident energies are dominated by a consistent set of levels between an excitation energy of 7.3 ± 0.4 and 15.7 ± 0.3 MeV. The rotational band based on the K^π = 0₃⁺ state appears to be strongly populated. Based on this selectivity, additional evidence is provided in favor of identification of the 8⁺ state at 15.9 MeV with this 0₃⁺ band. Angular distributions measured at the higher bombarding energy are compared with statistical compound-nuclear calculations. It appears that a non-statistical mechanism is responsible for the reaction's selective population of states with 8p-4h configuration. Such a mechanism, involving the preferential breakup of the ⁹Be into ⁸Be plus a neutron, is discussed.