s- and p-wave neutron spectroscopy Xf(ii). Intermediate structure in the 28Si + n reaction: Doorway state calculation

A core-particle calculation developed to describe the low-lying structure of ²⁹Si is extended above neutron threshold energy to yield information on the structure of doorway states indicated by the ²⁹Si + n reaction. The recent experimental evidence for a $\text{J =}\text{3}\text{2}{}^{\mathrm{?}}$ doorway state common to the ²⁸Si + n and the ²⁹Si + γ channels is supported by the calculation which also reproduces correctly the magnitudes of the neutron escape widths and the E1 radiative strengths of the $\text{3}\text{2}{}^{\mathrm{?}}$ doorway states.     

s- and p-wave neutron spectroscopy Xc. Intermediate structure: 88Sr

Neutron total cross section measurements of natural Sr were made from 50–875 keV using a high resolution proton beam and the ⁷Li(p, n) reaction as a neutron source. These data were analyzed with the help of an R-Matrix code to extract resonance (energies and other) parameters up to about 850 keV. 2p-1h and particle-vibration doorway interpretation of the s-, p- and d-wave resonances is attempted in terms of the sum rule Σγ_n² = γ_d². Predictions based on both of these models agree with the experimental results. As expected the p-wave resonances are stronger than either s- and d-wave structure. Theory accounts for the p-wave strength remarkably well. Possible location of the p-wave s.p. resonance is reproduced with a real potential and its damping due to the imaginary potential is calculated.More fragmentation of the strong p-wave doorways is observed than was expected for a compound nucleus so near ⁹⁰Zr, but a larger strength function is observed apparently due to the p-wave giant resonance.     

s- and p-wave neutron spectroscopy: Part VII. Widths of Neutron Resonances

Neutron reduced widths Γ_n⁰ and Γ_n¹ are reported for about 200 resonances observed in neutron total cross sections of Ca^{40, 44}, Ti⁴⁸, Cr^{50, 52, 54}, Fe^{54, 56}, Ni^{58, 60}, Sr⁸⁸, Y⁸⁹, Sn¹²⁴, Te¹³⁰, Ba^{136, 138}, and Pb^{206, 207, 208}, in the energy region 1 to 200 ^kev. Average parameters $\text{Γ}{}_{\mathrm{n}}{}^0$, $\text{Γ}{}_{\mathrm{n}}{}^0\text{D}$, and $\text{Γ}{}_{\mathrm{n}}{}^{\mathrm{(1)}}\text{D}$ have been derived and the Wigner distribution for local spacings and the Porter-Thomas distribution for reduced widths are verified for the resonances in the even-even nuclei Ca⁴⁰, Fe⁵⁶, Ni⁵⁸, and Ni⁶⁰. A simple method of area analysis which is less tedious and time consuming than the method reported before in Part III is also described.     

s- and p-wave neutron spectroscopy: Part V. Level Spacings in Heavy Nuclei

Total neutron cross sections were measured for natural rubidium and thallium and for the separated isotopes, Pb²⁰⁸, Pb²⁰⁷, Pb²⁰⁶, Tl²⁰³, and Rb⁸⁵. Approximate s-wave resonance spacings were estimated for Pb²⁰⁷, Y⁸⁹, Rb⁸⁷, Rb⁸⁵, Tl²⁰³, and Tl²⁰⁵. D₀ = D_J(2J + 1) = 2D_S(2I + 1) = 50, 24, 8, 4, and 30 kev, respectively, where D_s is the average (s-wave) level spacing for all channels. The spacings (D₀) of Pb²⁰⁶ and Pb²⁰⁸ were found to be of the order of hundreds of kew; there is also evidence that resonance spacings are very wide for Sr⁸⁶ and Ba¹³⁶. It is concluded that, in all compound nuclei with a neutron number silightly less than the magic numbers 50, 82, and 126, the resonance spacings are usually not much less than when the magic number is exceeded slightly. Since neutron excitation energies of these sub-magic nuclei are higher than the average, the observed wide level spacings below the magic numbers must be due to the properties of the nearly closed shells and can not possibly be caused only by low excitation energy of the compound nucleus. In considering these comparisons it is shown that, for s-wave resonances, the relation $\text{D}{}_{\mathrm{J}}\text{=}\text{D}{}_0\text{(2J + 1)}$ is a useful approximation in that a plot of D₀ is a much less erratic function of A than is the observed spacing. We also discuss the Bethe-Hurwitz effect, i.e., the influence on resonance spacing (apart from the 2J + 1 factor) of an unpaired nucleon in the target nucleus. We estimate that $\text{α ≧ 30}$ in the equation $\text{D}{}_0\text{(0)}\text{D}{}_0\text{(W)}\text{=}\text{exp}\text{(αW)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ where D₀(0) is the energy and spin independent spacing parameter, and W is the excitation energy of the compound nucleus.     

25Mg (α, n)28Si and 26Mg(α, n)29Si as neutron sources in explosive neon burning

The yields of neutrons from the reaction ²⁵Mg(α, n)²⁸Si and of γ-rays from the reaction ²⁵Mg(α, nγ)²⁸Si have been measured as a function of bombarding energy over the range 1.8–6.3 MeV, and the yield of neutrons from ²⁶Mg(α, n)²⁹Si has been measured over the range 1.8–6.0 MeV. Cross sections for ^{25, 26}Mg(α, n)^{28, 29}Si were extracted from the data and compared with global statistical-model calculations. The agreement is very good. Thermonuclear reaction rates under stellar conditions appropriate for explosive neon burning are calculated and their significance for the nucleosynthesis of rare neutron-rich nuclei is discussed.     

Nuclear structure and direct reaction mechanism in neutron scattering on 28Si in the energy range 6.8 to 14.8 MeV

The structure of excited states in ²⁸Si as well as the energy dependence of the reaction mechanism are investigated. Angular distributions related to the 0₁⁺, 2₁⁺, 4₁⁺, 0₂⁺, 3₁⁺, 2₂⁺ + 2₃⁺, 3₁⁻ + 4₂⁺ levels in the ²⁸Si(n, n′) reaction were measured at incident energies 6.8, 7.0, 8.0 … 12.0 MeV. The analysis was extended up to 14.8 MeV bombarding energy. The experimental cross sections are described by means of a coupled-channel calculation including compound nucleus contributions. The low-lying excited states can be interpreted by a rotational model with prolate but also with oblate g.s. deformation. For the higher-lying states various coupling schemes have been tested. Especially, a second rotational band could be described adopting an oblate deformation.     

High resolution gamma-ray spectroscopy of the27Al(p,γ)28Si resonance reaction

High resolution gamma-ray spectra have been measured from the²⁷Al (p,γ)²⁸Si reaction for the resonances atE _p=2·482, 2·511 and 2·735 MeV at ϑ _pγ=0°, 30°, 55° and 90° using a Ge (Li) gamma spectrometer. From the spectra and the angular distributions the properties of the resonance states have been obtained. These states are the isobaric analogues of the levels at 4·69, 4·75 and 4·93MeV levels respectively in the parent nucleus A²⁸l.     

New source of decoupling of E1 strength from the GDR: boundary condition mixing for s- and p-wave neutron states near threshold

Non-statistical effects in neutron capture reveal that E1 strength of neutron orbits of low l-value is partially decoupled from the GDR. Particle-hole diagonalisations with realistic forces do not show this effect. We show that it can arise from a new source, boundary condition mixing, which operates when fine-structure states are introduced. This may also explain the ‘pygmy dip’ in E1 strength reported for masses ～ 195.     

The structure of28Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

²⁸Si level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approximately 250 levels which are almost completely characterized according to the quantum numbersI, π, T of the levels. The properties of positive-parity states are compared to the predictions of shell model calculations within the completes-d basis space using the unifieds-d shell Hamiltonian. A spectrum of 48 experimentalT=1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculation of radiative widths and γ-decay modes which uses free-nucleong-factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in²⁸Si. The detailed shell model analysis of theT=0 spectrum is extended to the limiting energy whereT=1 wave function admixtures, not contained in the theory, become important experimentally. This happens at 6–8 MeV above the yrast state, depending on the spin value. Altogether it appears that a spectrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intruder states from outside thes-d shell space are observed atE _x =10 945 keV (I ^π=4⁺) and 12 860 keV (I ^π=6⁺) and are interpreted as members of aK ^π=0⁺ rotational band.     

Impurity absorption spectroscopy in 28Si: the importance of inhomogeneous isotope broadening

We report high-resolution infrared absorption spectra of the neutral donors phosphorus and lithium, and the neutral acceptor boron, in isotopically pure 28Si crystals. Surprisingly, many of the transitions are much sharper than previously reported in natural Si. In particular, the 2p(0) line of phosphorus in 28Si has a full width at half maximum of only 4.2 microeV, about 5 times less than the narrowest 2p(0) line previously reported for natural Si, making it the narrowest shallow impurity transition yet observed. The widely held assumptions that the impurity transitions previously reported in high quality samples of natural Si revealed the true, homogeneous linewidths, are thus shown to be incorrect. The sharper transitions in 28Si also reveal new substructures in the boron and lithium spectra.     

Analysis of two neutron (proton) transfer reaction data in the Zr-region

The spectroscopic amplitudes, form factors, angular distributions and total cross-sections for two nucleon transfer reactions in Zr-region in the zero range distorted wave Born approximation are calculated using consistent set of shell model wave functions. A single normalisation factor gives a good fit to all the two neutron transfer reaction data whereas the corresponding fit for the two-proton transfer reaction data is less satisfactory.     

The reaction n + p → d + γ and the isospin structure of the strangeness: Conserving nonleptonic weak Hamiltonian

The circular polarization P_γ and angular asymmetry α of the photons emitted in the reaction n + p → d + γ are computed in a covariant formalism within the framework of a composite-particle theory of the deuteron. The most important magnetic and electric dipole transition amplitudes in this radiative capture process are determined through a low-energy theorem. It is found that some of the proposed models for the strangeness-conserving nonleptonic weak Hamiltonian give variations on the ratio $\text{α}\text{P}{}_{\mathrm{\gamma }}$ of one or two orders of magnitude.     

Study of the excitation functions of27Al(n, α)24Na,27Al(n,p)27Mg and28Si(n,p)28Al reactions

Cross sections for the reactions²⁷Al(n, p)²⁷Mg and²⁸Si(n,p)²⁸Al were measured by activation method between 13.40 and 14.83 MeV neutron energy. An accuracy of about 4% was achieved using the²⁷Al(n, α)²⁴Na reaction as a reference at 14.1 MeV where the relative excitation function has also been measured. Results obtained were compared to a recent compilation and that calculated by the Hauser-Feshbach model. Using the back-shifted level density formula and taking into account the contribution of the separated levels, the calculations were extended to the energy range from the threshold to 18 MeV. A structure was observed in the²⁷Al(n, α)²⁴Na reaction cross section curve around 14 MeV neutron energy.     

Measurement of the ground state 2n pickup probability for 28Si+68Zn and its role in sub-barrier fusion enhancement

The ground state and excited state transfer yields for the 2-neutron pickup channel in the ²⁸Si+⁶⁸Zn system have been measured explicitly. The recoil mass separator at the nuclear Science Centre, New Delhi was used for the measurement. A NaI(T1) detector was used for detecting the deexcitation γ’s from the transfer products. The kinematic coincidence technique was employed for the transfer measurement. Simplified coupled channels calculations show that out of all transfer channels the major contribution to the sub-barrier enhancement comes from the ground state 2 neutron pickup channel with a ground state Q-value of+1.83 MeV.     

Yrast feeding and multiplicity of gamma transitions in the reaction 124Sn(28Si, 5n) 147Gd at 139, 146 and 152 MeV

The population of high-spin yrast and near yrast states of ¹³⁴⁷Gd and the associated multiplicity of feeding transitions have been studied in the ¹²⁴Sn (²⁸Si, 5n) reaction at 139, 146 and 152 MeV. The yrast feeding is found to be independent of the initial angular momentum distribution and very weak for states of spin greater than 30 ħ. The average multiplicity of γ-transitions is found to increase with increasing beam energy. The experimental results are interpreted in terms of collective excitations within rotational bands that channel the γ- decay towards the known oblate single-particle yrast states below ~30 ħ.     

First study of the (14C, 12C) reaction: Selectivity of the reaction and the energy levels of 28Mg and 30Si

The first measurements are reported for (¹⁴C, ¹²C) two-neutron stripping reactions. Energy spectra up to an excitation energy of 12 MeV have been measured at 69 MeV for the ²⁶Mg(¹⁴C, ¹²C) and ²⁸Si(¹⁴C, ¹²C) reactions. A strong selectivity of this reaction is observed. Using this selectivity, the comparison of the spectra suggests J^π assignments for several ³⁰Si and ²⁸Mg states. The results from the other two-neutron stripping reactions, (t, p) and (¹⁸O, ¹⁶O) are compared with those of the (¹⁴C, ¹²C) reaction.