The isovector monopole state and the escape width of double analog resonances

An irreducible vector operator is used to generate isovector monopole states with welldefined values of T and T_z in N > Z nuclei. Reduced transition strengths are calculated in two different ways and the results seem compatible. The Wigner-Eckart theorem is used to derive expressions for the mixing of the monopole states with the ground state and its analog states. Expressions for the escape widths of the monopole states are also derived. An application is made to double analog resonances, and it is found, contrary to expectations, that the mixing with the T?1 monopole state is unable to account for the observed elastic partial width of these resonances in medium and heavy nuclei. The reason for this is the small escape width of the monopole state.     

Atomic capture of negative mesons: (I). The Fermi-Teller model

We develop the Fermi-Teller model for describing the initial atomic capture and subsequent deexcitation of negative mesons. This enables us to predict an “initial” distribution for the lowest, quantum-mechanical part of the cascade. Our results are fairly close to the traditional ad hoc “statistical” distribution. Advantages and shortcomings of the Fermi-Teller model are discussed.     

The reaction 36S(p,γ)37Cl: (I). Excitation energies and γ-ray branchings of bound states deduced from resonances in the range Ep = 500–2000 keV

Yield curves of the reaction ³⁶S(p, γ)³⁷Cl have been measured over the range E_p = 500–2000 keV with a highly enriched (81%) ³⁶S target. Proton energies, with a precision of typically 0.3 keV, and strengths are presented for the nearly 200 observed resonances. Several previously reported resonances, among which the well-known $\text{J}{}^{\mathrm{\pi }}\text{=}\text{7}\text{2}{}^{\mathrm{?}}\text{, E}{}_{\mathrm{p}}\text{= 1887}\text{keV}$ analogue resonance, are proven to be multiplets.At 75 selected resonances in the ranges E_p = 500–1200 and 1800–2000 keV the decay schemes have been studied. These measurements also provide rather detailed information on the γ-ray branching ratios of more than 50 bound states of which the majority has not been observed previously. Precision excitation energies have been determined; for the levels with E_x < 5 MeV the median uncertainty amounts to 30 ppm. The reaction Q-value is Q = 8386.34 ± 0.23 keV.These precision data invalidate several previous spin and parity assignments to low-lying bound states of ³⁷Cl. They also provide a basis for the lifetime measurements and spin and parity assignments to be discussed in the following paper.     

Heavy-ion one-neutron transfer reactions involving 13C: (I). Spectroscopic factor products

The total cross sections of some heavy-ion one-neutron transfer reactions involving ¹³C, which lead to excited states of one of the final nuclei, have been measured at energies below the Coulomb barrier. Products of the neutron spectroscopic factors in the initial and final states have been extracted using a Coulomb wave Born approximation, and have been compared with theory.     

Isospin-forbidden particle decays in light nuclei: (I). T = 32 levels of 9Be, 9B

Absolute branching ratios have been measured for the isospin-forbidden nucleon decays of the lowest $\text{T =}\text{3}\text{2}$ levels of ⁹Be and ⁹B to the ground state and first excited state of ⁸Be. The $\text{T =}\text{3}\text{2}$ levels were populated by the ⁷Li(³He, n) and ⁷Li(³He, p) reactions. The subsequent decay products were observed in coincidence with the neutrons or protons, detected at 0°. Branching ratios of 0.028 ± 0.021 and 0.50 ± 0.11 were determined for the neutron decays of the 14.39 MeV, $\text{T =}\text{3}\text{2}$ level of ⁹Be to the ground state and 2.9 MeV, 2⁺ first excited state of ⁸Be, respectively. Branching ratios of 0.11 ± 0.04 and 0.33 ± 0.09 were determined for the analogous proton decays of the ⁹B, 14.67 MeV $\text{T =}\text{3}\text{2}$ level. The similarity of the decay properties of these analogue levels is in contrast to the large asymmetries observed for $\text{T =}\text{3}\text{2}$ levels in other A = 4N + 1 light nuclei.     

Photoexcitation of 209Bi and natHg using neutron-capture γ-rays from cobalt

Using neutron-capture γ-rays from cobalt, the ²⁰⁹Bi(γ,γ') and ^natHg(γ,γ') reactions were studied using a 53 cm³ Ge(Li) detector and the reactor facility at the Rhode Island Nuclear Science Center. Resonant scattering was observed at 4.924±0.005 MeV for ^natHg. Resonant scattering was observed at 5.609±0.005 MeV in ²⁰⁹Bi, with an effective cross section of 348±70 mb. The ratio of intensity scattered at 150° to that scattered at 100° was found to be 1.18±0.12 for ²⁰⁹Bi. This is consistent with a dipole transition to a $\text{9}\text{2}$ state but does not eliminate other transitions and spin assignments. No significant inelastic structure was found in the spectra from either sample. The improved energy specification in the present experiments has led to different energy assignments for the resonant states and hence, different incident intensities are producing the observed scattering. These differences lead to higher effective cross sections than those previously reported. Minimum ground state level widths for ²⁰⁹Bi and ^natHg were estimated to be 0.032 eV and 0.43 eV respectively. The need for high-resolution survey data to re-establish systematics is suggested.     

Dispersion relations and low-energy elastic scattering of charged particles from nuclei: (II). Application to N-4He scattering

A “Coulomb-modified” dispersion relation is applied to low-energy p-⁴He forward elastic scattering. To check the validity of the modification, the results are compared with those deduced from n-⁴He elastic scattering. Completely analogous information is obtained for the two processes. The exchange of three bound nucleons is found to contribute strongly to N-⁴He forward scattering. The corresponding ⁴He-³H-p and ⁴He-³He-n coupling constants are evaluated as R_p = 3.8±0.3 and R_n = 3.0±0.3, respectively. These constants are related to the strength of the asymptotic wave function of nucleons in ⁴He and thus to the tail of the nucleon distribution. A comparison of R_p with the empirical proton distribution in ⁴He as deduced from e-⁴He elastic scattering shows excellent agreement. From R_p and R_n the effective ranges of the singlet ³H-p and ³He-n interactions in the ground state of ⁴He are determined to be equal, in accordance with charge symmetry, and to have a value of r_eff = 1.072±0.006 fm.     

Electromagnetic properties of the πNN system: (II). The γD → Dπ° reaction

By computing the non-relativistic limits of the direct and the pion-nucleon rescattering amplitudes, we have obtained a good accounting for the available experimental data, both at threshold and in the Δ(1236) region. The effects of the Fermi motion were fully taken into account. At threshold the pion-nucleon rescattering diagram is very important, and we have studied the sensitivity of the results to the different ingredients of the calculation: the deuteron wave function, the pion propagator, the elementary operators, etc. In the Δ(1236) region the rescattering effects are small and the cross section is mainly sensitive to the direct term. Here the Fermi motion effects are important and the kinematical ambiguities in the elementary amplitudes are avoided in our treatment. The good overall agreement with the data gives us some confidence in the methods we are using in this series of papers which study the electromagnetic properties of the πNN system.     

The reaction 36S(p, γ)37Cl: (II). Lifetimes,spins and parities of 37Cl levels

Analysis of the γ-ray angular distributions measured at 16 ³⁶S(p, γ)³⁷Cl resonances yields the spins and/or parities of 21 bound states of ³⁷Cl in addition to the resonance spins. Among the latter are four $\text{J =}\text{1}\text{2}$ resonances. For several other bound states the possible spins have been restricted. Multipolarity mixing ratios have been deduced from the same data. Lifetimes (or lifetime limits) of 29 bound states have been deduced from DSA measurements.This extensive experimental information allows a comparison with the results of a shell-model calculation for both the even- and odd-parity states of ³⁷Cl. In this calculation either zero, one or two particles are promoted from the 2s_{$\text{1}\text{2}$} or 1d_{$\text{3}\text{2}$} to the 1f_{$\text{7}\text{2}$} or 2p_{$\text{3}\text{2}$} subshells. Up to an excitation energy of about 5 MeV a one-to-one correspondence is found between experimental and calculated levels.     

Classical many-body model for heavy-ion collisions: (III). Ne-Ne and Ca-Ca calculations

The classical many-body model, previously introduced, has been employed to perform numerical calculations of systems consisting of 20 on 20 and 40 on 40 nucleons. Comparison is made with the 800A MeV data of Nagamiya, et al. Microscopic time development of the system and central compressions are displayed for Ca on Ca. Microscopic comparions with the fireball/firestreak models are presented; although qualitative agreement is found, interesting and expected shortcomings in the latter were seen. In particular, the classical many-body model exhibits shear viscosity and incomplete thermalization.     

The systematics of nuclear single-particle states (II). the region A < 35

The energies of single-particle states in nuclei with A < 35 are obtained as eigenvalues of a local Saxon-Woods potential with depth depending linearly on A and on the nuclear symmetry parameter.     

Nuclear structure studies in the Te isotopes: The Te(d, p)Te reaction

The reaction ¹²⁵Te(d, p)¹²⁶Te has been studied at a bombarding energy of 7.5 MeV in order to get information about the ¹²⁶Te nuclear level scheme. Forty-two levels were observed below an excitation energy of 5.1 MeV. The angular distributions of 25 of the emitted proton groups were compared with DWBA calculations to determine the angular momentum of the captured neutrons. Contributions from more than one l_n value were observed for two of the transitions. Transition strengths were extracted and compared to pairing-theory calculations.     

Two-fluid model description of isovector giant resonances in fast rotating nuclei

Isovector giant resonances of arbitrary multipolarity in fast rotating nuclei are studied by solving the inviscid two-fluid equation of relative motion in a rotating frame of reference. Both Coriolis and centrifugal forces are taken into account. The resulting expressions display in a quite simple way general features of giant multipole resonances of fast rotating nuclei, in addition to a good agreement with other calculations for the giant dipole resonance. Typical values for the resonance energies and their fragmentation due to nuclear deformation and rotation are given. In particular, enormously large resonance splitting should occur in the superdeformed states.     

A microscopic description of the negative parity resonances in the mass-4 system by the coupled-channels method (II)

The coupled-channels method for the treatment of the continuum is used to study the negative parity excited states of the ⁴He nucleus in the Tamm-Damcoff approximation. It is shown that the splittings within the SU(4) supermultiplet are well reproduced in this calculation when the spin-dependent forces responsible for the removal of the degeneracy are taken appropriately into account. The location of the T = 0, 1^? resonance and the admixture of spurious c.m. excitation in the intrinsic excitation are discussed. In addition to interpreting the observed spectrum, the available experimental data for the particle channels are well described by the theory.     

Direct reactions to many-channel, many-level final states : (II). (d, p)n and (He, d)p

In an earlier paper, the cross section for a direct reaction to a generalised positive-energy final state, described by an R-matrix wave function, was derived. Here a distinction is emphasised between two classes of such a reaction, depending on whether it can or cannot decay back to the target state. A major contribution in the latter case can be via direct break-up. This is computed for the reaction ⁷Li(³He, d)⁸Be(p)⁷Li, treated by a stripping mechanism. The dependence of the cross section on the final-state (⁸Be) channel energy is discussed. The present work uses shell-model wave functions for R-matrix basis states, and this is shown to be appropriate and useful for direct reactions.