Intensitätsmessung von Röntgen- und Kerngammastrahlung an einigen schweren Myonenatomen

Absolute intensities of the 5–4 muonic X-ray transition in muonic W, Os, and Bi, and relative intensities of other X-ray transitions have been measured and compared with cascade calculations. The excitation of several levels in W, Os, Ir, Tl, and Bi nuclei resulting from the muonic cascade process has been observed and absolute excitation probabilities have been determined. For the first 2⁺ level of¹⁸⁸Os and the first 3/2⁺-levels of^203,205Tl the excitation probabilities are larger than calculated. The results are discussed.     

Baryon resonances in the mean field approach and a simple explanation of the Θ<Superscript>+</Superscript> pentaquark

We suggest to classify baryon resonances as single-quark states in a mean field, and/or as its collective excitations. Identifying the Roper resonance N(1440, 1/2⁺), the nucleon resonance N(1535, 1/2⁻), and the singlet hyperon Λ(1405, 1/2⁻) as single-quark excitations, we find that there must be an exotic S = +1 baryon resonance Θ⁺ (the “pentaquark”) with a mass about 1440 + 1535 − 1405 = 1570 MeV and spin-parity 1/2⁺. We argue that Θ⁺ is an analog of the Gamov-Teller excitation long known in nuclear physics.     

How indirect (p,t) processes populate the γ-band of 168Yb

Direct (p, t) transfer to the γ-band of ¹⁶⁸Yb is found to be less important than indirect processes involving collective quadrupole excitation and correlated l=0 pair transfer. The 2⁺_γ cross section can be explained by such processes but the 3⁺_γ and 4⁺_γ can not. Hexadecapole excitation to the 4⁺_γ and the role of the unnatural parity of the 3⁺_γ are discussed.     

β-band excitation in inelastic hadron scattering and the mixing of the breathing mode into the low-lying β-vibrations

The excitation of the ²⁴Mg β-band 0⁺ and 2⁺ states at 6.43 MeV and 7.35 MeV, respectively, in inelastic hadron scattering has been considered in the collective model where coupling of these states of the states of the ground state band is via a ß-vibration coupled to the static deformation. It is found that coupled channel effects on the 0⁺ state excitation are rather important and moreover the excitation of the 0⁺ state can be satisfactorily explained only if a monopole breathing mode form factor is included in addition to the monopole β-vibration form factor.     

Reorientation measurements on tungsten isotopes

In a particle-γ coincidence experiment, a thick tungsten target, of natural isotopic abundance, was bombarded with a and ¹⁶O beams. From analysis of the deexcitation γ-rays following Coulomb excitation, the spectroscopic quadrupole moment of the second 2⁺ state (the 22' state) was determined for ¹⁸⁶W and ¹⁸⁴W. In a separate Coulomb excitation experiment a thin, isotopically enriched ¹⁸⁶W target was bombarded with ¹⁶O ions. From analysis of projectiles scattered elastically and inelastically the quadrupole moment of the 2^+' state of ¹⁸⁶W was extracted. The results of the two experiments are in good agreement. The quadrupole moment of the 2^+' state is found to be opposite in sign to that of the first 2⁺ state for both isotopes studied. However, its magnitude decreases rapidly in going from ¹⁸⁶W to ¹⁸⁶W, in contrast to the predictions of the rotation-vibration or asymmetric rotor models. The microscopic theory of Kumar and Baranger does predict the experimental trend, qualitatively. Thus the present results are interpreted as being evidence of strong coupling between β and γ degrees of freedom in the tungsten isotopes, which, according to the theory of Kumar and Baranger, is the source of the reduced value of the quadrupole moment.     

Rotations and intrinsic Kπ = 1+ excitations in deformed rare-earth nuclei

The K^π = 1⁺ intrinsic excitations in deformed rare-earth nuclei are studied in the framework of the random-phase approximation, which excludes the spurious state (zero-energy excitation) related with the rotation of the whole nucleus. We take the Nilsson-plus-pairing, quadrupole and spin-dependent (magnetic dipole or spin-quadrupole) force model. Cranking-model formulae for the moment of inertia and the collective gyromagnetic ratio are derived as the zero-energy limit of the lowest K^π = 1⁺ excitation. They are modified from the usual formulae due to the spin-dependent force. The M1 and E2 transitions from the K^π = 1⁺ excited states to the ground state of ¹⁷⁰Yb are also calculated. The M1 transition strengths are concentrated almost below 6 MeV, while the E2 ones are scattered even up to 12 MeV. Several levels around 12 MeV show rather strong E2 transitions of ΔN = 2 character.     

Three-body continuum structure and response functions of halo nuclei (I): 6He

Three-body scattering states of the Borromean two-neutron halo nuclei are explored in a core + n + n model using the method of hyperspherical harmonics. We analyse the continuum structure (the properties of the continuum wave functions) separately from the continuum response (the magnitudes of one-step transitions from the ground state to the continuum). Predictions are made for the positions and strengths of the isoscalar monopole, electric dipole and quadrupole excitations, as well as for nuclear inelastic and charge-exchange response functions, for the ⁶He nucleus. The known 2⁺ resonance in ⁶He is reproduced. We find 1⁻ strength concentrations at lower energies in the proximity of the three-body threshold, and predict new 2⁺, 1⁺ (and possibly 0⁺) resonances at slightly higher energies in ⁶He.     

Excitation of collective states in spherical nuclei in high-energy proton interactions

The excitation of collective states in spherical nuclei is investigated in interactions with ≈ 1 GeV protons. Collective states of dipole and quadrupole type in a large energy range, including the isoscalar and isovector giant resonances, are considered. The cross sections for proton-nucleus interactions are found within the single-inelastic-scattering approximation of Glauber theory with the use of semi-microscopic nuclear wave functions. The largest probability of excitation is proved to take place for states of isoscalar type. Isovector states are strongly suppressed. It is shown that 1 GeV proton inelastic scattering can be used for the excitation of giant resonances of isoscalar type. The probability of their excitation is of the same order as that for low-lying collective states.     

Microscopic analysis of nuclear collective motions in terms of the boson expansion theory: (I). Formulation

A normal-ordered linked-cluster boson expansion theory, previously worked out by one of the authors (T.K.) and Tamura, has been developed further by reformulating it in a “physical” quasiparticle subspace which contains no spurious particle-number excitation modes. The expansion coefficients of the collective hamiltonian for low-lying quadrupole motions are determined starting from a microscopic fermion hamiltonian including self-consistent higher-order (many-body) interactions derived in our previous work. The contributions from the non-collective states with all possible non-collective one-boson excitations having I^π = 0⁺− 4⁺, which can directly couple to the collective states with one or two phonons, are taken into account in a systematic and compact way.     

Nuclear viscosity and widths of giant resonances

We write down a set of coupled hydrodynamic equations of the Navier-Stokes type which describe the motion of two compressible, viscous nuclear fluids. The solutions of these equations give rise to giant resonances of both isoscalar and isovector type. The viscosity terms in the equations are responsible for the damping of these resonances. Within this framework we obtain expressions for the width of the resonances as a function of the mass number A, and relations between the widths and the excitation energies for various multipolarities (J = 0⁺, 1^?, 2⁺, 3^?, 4⁺), and isospins (T = 0,1). The A dependence of the calculated widths exhibit the experimental trends of the giant dipole and isoscalar quadrupole widths. Also, as a result of the calculation we obtain estimates of the values of viscosity coefficients in nuclei.     

MICROSTRUCTURE ON COLLECTIVITY IN EVEN 116-122Xe ISOTOPES

Using microscopic interacting boson-fermion model formalism, the ground-band, β-band, γ-band and high spins states in even ^116-122Xe isotopes are successfully described. It can explain the recent experimental result that collective structures may coexist with the single-particle states and subshell feature. It predictes the energies of 1⁺ states.     

Fission of muonic 232Th and 239Pu

The neutron emission is studied following the formation of muonic atoms of ²³²Th and ²³⁹Pu. Energy and time distributions are measured. Various processes which contribute to the measured spectra are considered. A collective resonance model of the muon capture is used to calculate the nuclear excitation function. The probability of the radiationless nuclear excitations and the influence of the presence of the bound atomic muon on the fission barrier are discussed. The existing data for the $\text{Γ}{}_{\mathrm{n}}\text{Γ}{}_{\mathrm{f}}$, are analysed. As a result of the analysis the rates of the prompt and delayed fission events (due to the radiationless mu-atomic transitions and the nuclear muon capture, respectively) are deduced from the experimental data to be 0.006/muon and 0.045/muon for ²³²Th and 0.10/muon and 0.49/muon for ²³⁹Pu, respectively. The increase of the fission barrier for muonic atoms is confirmed. The experimental neutron rates can be consistently explained only if it is assumed that in both nucleides the K_α radiationless transitions do not induce fission. The increase of the fission barrier for ²³⁹Pu is hence deduced to be not less than 1.2 MeV. The fate of the atomic muon after the nuclear fission is briefly discussed. Its influence on the interpretation of the present results is found to be small.     

Coulomb excitation of 182, 184, 186W and 166Er with 4He and 16O ions

The Coulomb excitation reaction induced by ⁴He ions selectively excites 2⁺ and 3^? states by direct E2 and E3 Coulomb excitation. In this paper, we present new results from γ-ray spectroscopy with 15 MeV ⁴He ions on a natural abundance target of W. In particular, a 3^? state in each isotope, ^{182, 184, 196}W, was observed by direct E3 excitation. In addition to obtaining B(Eλ, 0 → J = λ) for excitation of each state, information on the reduced transition probabilities for the different decay modes of these states is given. The Coulomb excitation probabilities of the 2'⁺ states at 786 keV in ¹⁶⁶Er and at 737 keV in ¹⁸⁶W were measured with ¹⁶O and ⁴He ions by the backscattered particle-gamma coincidence method in order to determine the static electric quadrupole moment Q_2'. We obtain for ¹⁶⁶Er Q_2' = 2.11 ± 0.37 e· b which is in agreement with (Q_2')_rot and for ¹⁸⁶W Q_2' = 1.17 ± 0.30 e · b which is in agreement with Kumar-Baranger model calculations and is significantly smaller than (Q_2')_rot. This deviation of the measured Q_2' from (Q_2'rot implies in the framework of the Kumar-Baranger calculations a strong coupling between β- and γ-vibrational bands. On the other hand, our observed upper limit for B(E2,2″ → 2′) does not confirm this implication.     

A shell-model description of 0+ intruder states in even-even nuclei

Starting from the nuclear shell structure in medium-heavy and heavy nuclei, the excitation energy for low-lying 0⁺ intruder states is studied. Taking as a simplified model two particle-two hole (2p-2h) excitations across closed shells, the effects of the pairing and the proton-neutron (monopole and quadrupole component) residual interaction on the unperturbed energies are calculated. Application to major closed-shell (fZ = 50, Z = 82) and to subshell (Z = 40, Z = 64) regions is performed. We especially concentrate on 0⁺ intruder states in the even-even Pb nuclei.     

Giant resonances in90Zr and116Sn

The giant resonance region in⁹⁰Zr and¹¹⁶Sn excited by 270 MeV helions has been measured up to about 35 MeV excitation energy. The low and the high energy octupole resonances are seen prominently in addition to the quadrupole and the monopole resonances. The angular distribution data for the various multipoles are satisfactorily explained by the collective model calculations. The percentatge energy weighted sum rule strengths have been determined for all the prominent resonances.     

Collective magnetic dipole transitions: Dependence of the energies and rates on the nuclear effective interaction

The dependence on the properties of the effective interaction of the energies of and excitation strengths to magnetic dipole states in open shell nuclei is studied. In particular the single j-shell for ⁴⁸Ti is used as an example. In this case there are two 1⁺ states with isospin T = 2 and one with isospin T = 3. The conditions for having a strong low-lying collective 1⁺ state are examined. Focus is also given on the excitation strength to the analog T = 3 state since this is of relevance also to β⁺ Gamow-Teller reactions and to double beta decay. It is found that in the rotational limit there is no strength to the T = 3 state and there is an overly strong low-lying 1⁺T = 2 state. This is almost also true for a quadrupole-quadrupole interaction. At the other extreme, as shown by Halse, with a pairing interaction all the strength goes to the T = 3 state. Other interactions considered are pairing plus quadrupole, spin-dependent delta and Kuo-Brown bare and renormalized, and matrix elements taken from the spectrum of ⁴²Sc. It is found that the β⁺ strength can be reduced either by making the two-body J = 2 T = 1 matrix element or J = 1 T = 0 matrix element more attractive, just as was shown by others in heavier nuclei. However such parameter changes have effects on other properties of the 1⁺ spectrum, which can serve as indicators as to whether or not these changes are justified.