Spectroscopy of 32P (II)

Spins, parities and transition probabilities of levels in ³²P at E_x > 3.5 MeV have been determined with the ²⁹Si(α, pγ)³²P reaction at bombarding energies of E_α = 12.80, 12.93 and 16.30 MeV. Proton-gamma angular correlation experiments and DSA lifetime measurements lead to six unambiguous spin assignments and to many spin limitations. The measured mixing and branching ratios yield many transition strengths for dipole and electric quadrupole transitions. Five, sofar unknown, energy levels are reported. A doublet at 4.03 MeV excitation energy and a high-spin state (E_x = 4.27 MeV; J^π = 5^?) were observed. Shell-model predictions have been compared to the present experimental results.     

Elastic magnetic electron scattering from 29Si and 31P

Elastic magnetic electron-scattering form factors from ²⁹Si and ³¹P have been measured in the range of momentum transfer from 1.0 to 2.8 fm^?1. Analysis of the data using a particle-core-coupling model yields $\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ occupation probabilities for the valence nucleon of 0.46 ± 0.01 for the neutron in ²⁹Si and 0.48 ± 0.01 for the proton in ³¹P. The radius of the $\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ proton orbit in ³¹P has been found to be (5.3^+2.6_?1.5)% larger than the corresponding neutron orbit in ²⁹Si. Recent large-basis shell-model calculations of the form factors do not accurately reproduce the experimental results.     

The α-transfer reactions 27Al(6Li,d)31P, 29Si(6Li,d)33S and 31P(6Li,d)35Cl at 36 MeV

The α-transfer reactions ²⁷Al(⁶Li, d)³¹P, ²⁹Si(⁶Li, d)³³S and ³¹P(⁶Li, d)³⁵Cl have been studied at a ⁶Li energy of 36 MeV. Absolute cross sections and angular distributions have been measured and an exact finite-range distorted-wave Born approximation analysis assuming a direct cluster transfer has been used to extract from the data α-particle spectroscopic strengths for levels populated in ³¹P, ³³S and ³⁵Cl in the three reactions respectively. The results show that in the case of most of the low-lying excited states of ³¹P, a single value of L of the transferred α-particle contributes, though a multiplicity of L-values are allowed by angular momentum selection rules. It is also found that the α-particle spectroscopic strength of the ground state of ³¹P is a factor of 2 more than the strengths of the ground states of ³³S and ³⁵Cl. The α-spectroscopic strengths of ground states of these, as well as other odd-A s-d shell nuclei, are compared with the presently available shell model calculations.     

Microscopic and macroscopic DWBA analysis of the reactions 28Si(α, d)30P, 32S(d, α)30P and 32S(α, d)34Cl studied at Eα = 50MeV and Ed = 40MeV

The reactions ²⁸Si(α, d)³⁰P, ³²S(d, α)³⁰P and ³²S(α, d)³⁴Cl have been studied at E_α = 50 MeVand E_d = 40 MeV. The angular distributions have been analysed in terms of the single-step, zero-range DWBA with microscopic as well as macroscopic form factors. By requiring an almost identical shape in the microscopic and macroscopic radial form factors for all L-values with L ? 6 the size parameters of the Woods-Saxon well in which the transferred cluster is bound to the nucleus were determined as r₀ = 1.15 fmanda = 0.76 fm. Despite the differences between the two approaches with these parameters the shape of the microscopic angular distribution is well reproduced and the corresponding strengths agree to within 25%. The method has been applied to the three reactions and relative two-nucleon spectroscopic factors have been deduced. A comparison is made with the results of two shell-model calculations.     

The 20Ne(12C,p)31P, 20Ne(12C,d)30P and 20Ne(12C, α)28Si reactions in the region of the coulomb barrier

The excitation functions of the ²⁰Ne(¹²C, p)³¹P, ²⁰Ne(¹²C, d)³⁰P. and ²⁰Ne(¹²C, α)²⁸ Si reactions were measured at bombarding energies between 6.9 and 16.9 MeV (c.m.) by steps of 156 keV, at an average lab angle of 2.82°. The average coherence width of states in the compound nucleus, ³²S, populated in the reactions was deduced through the analysis of the fluctuations in the measured excitation functions. The result agrees with the average compound nucleus width predicted by the Hauser-Feshbach expression. The fluctuation analysis shows that these reactions proceed mainly through the formation of a compound nucleus. A cross correlation analysis revealed that the fluctuations in the different excitation functions are statistically independent and that there is no evidence of intermediate structure resonances.     

Four-nucleon transfer with the 32S(16O, 12C)36Ar reaction

Angular distributions have been measured for the ³²S(¹⁶O,¹²C)³⁶Ar reaction at 45.5 MeV leading to excited states between E_x, = 0 and 8 MeV. Experimental cross sections are compared with exact finite-range DWBA calculations in combination with extensive shell-model calculations, which include sd- and fp-shell configurations. Transitions to low-energy positive-parity states and bound negative-parity states are well reproduced. The calculations, however, fail to describe some high-energy positive-parity states. Calculations with a complete cluster expansion for the four transferred nucleons give about 50% larger cross sections, but can not explain the observed discrepancies. Possible interference of reaction processes other than direct α-transfer and special structure effects are discussed.     

Reaction and fusion cross sections for 32S on 27Al and 48Ti

Elastic scattering and evaporation residues have been measured for the system ³²S + ²⁷Al at E_c.m. = 66.4, 73.2 MeV and ³²S + ⁴⁸Ti at E_c.m. = 96.0 MeV. Reaction cross sections have been obtained by use of the optical theorem and are found to be about 60 % larger than the fusion cross sections.     

Spectroscopy of intermediate 16F states with the reaction 14N(3He,np)15O

Excited states of ¹⁶F have been investigated with the reaction ¹⁴N(³He, np)¹⁵O at E = 10.5 and 12 MeV in kinematically complete experiments. Proton groups corresponding to the decays of intermediate ¹⁶F states were observed at various angles with counter telescopes in time coincidence with the associated neutrons detected at θ_n^lab = 0° with a time-of-flight spectrometer. Excitation energies and decay widths Γ_p0 of these states have been extracted from the proton spectra. Lower limits for the orbital angular momentum in the decay channel and for the spin of the states have been deduced from the obtained angular correlations. By comparison with the reaction ¹⁴N(³He, pp)¹⁵N measured at E = 13 MeV, pairs of $\text{T = 1}{}^{16}\text{F}$ parent/¹⁶O analog states have been identified. J^π assignments and shell-model configurations are discussed on the basis of the selectivity of the reactions measured.     

Spectroscopy of 42Ca from 41Ca(d,p)

Energy levels in ⁴²Ca up to 7.8 MeV have been studied in the neutron capture reaction ⁴¹Ca(d, p)⁴²Ca with 12 MeV bombarding energy. Ninety-four excited states have been identified and angular distributions have been measured in the interval from 5° to 110° by means of a broad-range magnetic spectrograph. The angular distributions together with DW calculations have been used to determine I_n values and spectroscopic factors. The $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ strength sum agrees with shell-model expectations if the f_{$\text{7}\text{2}$} spectroscopic factors are renormalized by $\text{1}\text{0.75}$, in line with other $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$. transfer experiments on ⁴⁰Ca and ⁴¹Ca. A similar renormalization of the l_n = 1 spectroscopic factors brings this strength sum in accordance with the shell-model calculations. The effective ($\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}{}^2$) matrix elements for ⁴²Ca are compared with the corresponding matrix elements of ⁴²Sc and ⁴⁸Sc. The differences between the three sets of matrix elements are of the order of a few hundred keV or less. The monopole centroid energy of the ($\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{)}{}^2$ multiplet is shifted downwards in the mass-42 nuclei compared to ⁴⁸Sc, possibly indicating the importance of the monopole pairing force near ⁴⁰Ca.     

Formation and deexcitation of the 32S compound nucleus via the 20Ne+12C and 16O+16O entrance channels

Reaction-product cross sections following ²⁰Ne+¹²C and ¹⁶O + ¹⁶O collisions at several incident energies have been measured with a E-ΔE counter telescope. They are compared to statistical model predictions. Fair agreement is obtained for the high-Z evaporation residue cross sections, but a strong discrepancy is observed for the lower-Z reaction products. Possible explanations are discussed. It is shown that the compound nucleus formation does not depend on the structure of the colliding ions in the entrance channel and also that it is not limited by the ³²S yrast line.     

Spectroscopy of A = 16 from 13C(6Li,t)16O

The ¹³C(⁶Li, t)¹⁶O reaction has been studied at 34 MeV. Selective population of narrow states is observed up to 21 MeV excitation in ¹⁶O. This reaction populates strongly both unnatural-and natural-parity states that have little or no ¹²C + α₀ width. The measured angular distributions are compared with Hauser-Feshbach and finite-range DWBA calculations. Reasonable agreement with the DWBA calculations is found for most of the states strongly populated. The widths of the narrow states populated in the 16–20 MeV excitation region are presented. Comparison of the present data with that from medium-energy inelastic scattering and other multiparticle transfer reactions is made.     

Search for intermediate structure in 36Ar via the 24Mg(12C, α)32S reaction

The ²⁴Mg(¹²C,α)³²S reaction was investigated in the energy range E_c.m. = 11.9–19.4 MeV by measuring excitation functions of the α₀ and α₁ groups. Angular distributions (θ_c.m. = 12–97°) were also measured at a number of energies. The excitation functions were subjected to a statistical analysis by means of evaluating correlation and deviation functions; no statistically significant anomalies were found. The α₀ angular distributions display fairly high angular-momentum selectivity as pairs of Legendre polynomials provide acceptable fits to most of them: however, only one, at E_c.m. = 18.1 MeV, is strongly dominated by a single partial wave, l = 11. Excitation functions as well as angular distributions of both α₀ and α₁ cross sections were found to be in good qualitative agreement with Hauser-Feshbach calculations throughout the energy range studied. Thus, the analysis of the data shows that intermediate resonant structures, if present, are weak and interfere strongly with the statistical compound-nucleus background, which effectively prevents their clear observation and identification in the present study.     

Lifetime measurements in 22Ne, 28Si and 31P relevant to the interpretation of the Z2 variation in low velocity DSAM results

Accurate lifetimes have been measured for low-lying levels in ²²Ne, ²⁸Si and ³¹P by bombarding ⁴He implanted targets with beams of ¹⁹F and ²⁸Si ions. Mean lifetimes determined by fitting Doppler-broadened γ-ray lineshapes were $\text{(E}{}_{\mathrm{<mtext>x</mtext>}}\text{in MeV}\text{, τ}\text{in ps}\text{):}{}^{22}\text{Ne}\text{(1.275, 5.15 ± 0.31; 3.357, 0.324 ± 0.009),}{}^{28}\text{Si}\text{(1.779, 0.667 ± 0.035),}{}^{31}\text{P}\text{(1.266, 0.70 ± 0.07; 2.234, 0.363 ± 0.024}$). The lifetime values for the 3.357 MeV level in ²²Ne and the 2.234 MeV level in ³¹P are used to calibrate low velocity DSAM lifetime data for these two levels and to obtain scaling factors to theoretical electronic stopping powers for Ne and P ions.     

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.     

A study of the 18O(6Li,d)22Ne reaction at 32 MeV

Results from a study of the ¹⁸O(⁶Li, d)²²Ne reaction at a ⁶Li energy of 32 MeV are reported. The L-dependence of the shapes of the measured angular distributions provide a check on recent J^π assignments for some of the high-lying levels in ²²Ne. A finite range distorted wave analysis assuming a direct cluster transfer has been used to extract from the data α-particle spectroscopic strengths for most of the natural parity levels populated below 8 MeV of excitation. These strengths are compared with theoretical predictions for those few states for which a definite correspondence can be made between the calculated and experimental levels of ²²Ne. For transitions to the members of the ground-state band, the observed strengths disagree with the predictions. This disagreement has also been observed in the ¹⁶O(⁶Li, d) reaction and its cause is not understood. It is in marked contrast with the good agreement found for (⁶Li, d) reactions on targets of mass 20 ≦ A ≦ 24.     

Spectroscopy of two coincident charged particles emitted following bound pion absorption in 12C, 59Co and 197Au

Energy spectra and angular distributions of two coincident charged particles emitted following stopped negative pion absorption in ¹²C, ⁵⁹Co and ¹⁹⁷Au were measured. Most of the data can be understood within the framework of a pair absorption model including final-state interactions. Ground-state transitions in the missing mass spectra of ⁴H and ⁴He deduced from the αα and tα spectra of ¹²C, respectively, show that reactions involving a large part of the nucleus also occur.     

Spectroscopy of 75Se studied with the 72Ge(α, nγ) reaction

States in ⁷⁵Se have been excited by the reaction ⁷²Ge(α, n)⁷⁵Se and a level scheme built up from the observed γ-ray spectra. Gamma-ray angular distribution measurements have been analysed to yield spin assignments, mixing ratios and branching ratios. Comparison of the positive parity spectrum with the levels predicted by a Coriolis coupling calculation shows good agreement. In particular the model can successfully predict the occurrence of “anomalous” states with $\text{J}{}^{\mathrm{\pi }}\text{=}\text{5}\text{2}{}^{\mathrm{+}}\text{and}\text{7}\text{2}{}^{\mathrm{+}}$ which lie close to the $\text{9}\text{2}{}^{\mathrm{+}}$ state in this nucleus, and which ar general feature of many neighbouring nuclei. Calculated E2 and M1 transition rates among these levels agree with available experimental data to within a factor of two. A strong case can therefore be made for regarding the low-lying levels in ⁷⁵Se as arising from an odd neutron coupled to a deformed prolate core.     

32S-induced reactions at 10 MeV/u bombarding energy

The deep-inelastic processes of the reactions ³²S + ²⁸Si, ^natS, ⁴⁰Ca, ⁵⁸Ni, ⁷⁴Ge are studied at 10 MeV/u bombarding energy employing a kinematical coincidence spectrometer. From the measured energies, momenta, masses and atomic numbers of two heavy fragments the corresponding parameters for the unobserved reaction products and the reaction Q-values are deduced. It is found that the reactions generally show the pattern of a normal deep-inelastic process which is followed by the evaporation of several light particles. But with much less intensities other processes also seem to occur: three-fragment exit channels and incomplete energy damping which is correlated with the emission of a few light particles of high momenta.