Observation of two-photon emission in nuclear pion capture

The emission of two photons above 25 MeV energy was observed in the capture of stopped pions by beryllium and carbon with a rate of (1.0 ± 0.1) × 10⁻⁵, respectively (1.4 ± 0.2) × 10⁻⁵ per capture. These rates are about two to four times greater than various free-nucleon estimates based on the π⁻+π⁺→γγ annihilation mechanism alone.     

Neutron spectra coincident with discrete gamma rays in π− capture on 165Ho and 181Ta

Neutron spectra from π^?-capture in medium-heavy nuclei are presented in coincidence with discrete nuclear gamma rays to exhibit possible correlations of the spectra to neutron multiplicity and residual nuclear states. A discrete component in the pre-compound neutron spectra is resolved for the first time, and its correlation to the population of high spin nuclear states is observed.     

Electric quadrupole moments and strong interaction effects in pionic atoms of 165Ho, 175Lu, 176Lu, 179Hf and 181Ta

The effective quadrupole moments Q_eff of the nuclei of ¹⁶⁵Ho, ¹⁷⁵Lu, ¹⁷⁶Lu, ¹⁷⁹Hf and ¹⁸¹Ta were accurately measured by detecting the pionic atom 5g-4f X-rays of the elements. The spectroscopic quadropole moments, Q_spec, were obtained by correcting Q_eff for nuclear finite size effect, distortion of the pion wave function by the pion-nucleus strong interaction, and contribution to the energy level splittings by the strong interaction. The intrinsic quadrupole moments, Q₀, were obtained by projecting Q_spec into the frame of reference fixed on the nucleus. The shift, ε₀, and broadening, Γ₀, of the 4f energy level due to the strong interaction between the pion and the nucleons for all the elements were also measured. Theoretical values of ε₀ and Γ₀ were calculated and compared to the experimental values. The measured values of Q₀ were compared with existing results in muonic and pionic atoms. The measured values of ε₀ and Γ₀ were also compared with existing values.     

One- and two-nucleon emission following the nuclear radiative pion capture

Decay channels in the radiative pion capture reactions are discussed and their respective yields calculated. Specific consequences of a recently suggested quadrupole excitation mode in the ( ^–,) reaction are found and their experimental investigation is urged. A yield of about 19% of the total decay was estimated for the two-nucleon decay channel even for the light nuclei like¹⁶O.On leave of absence fromthe Institute of Nuclear Physics, Czechosl. Acad. Sci., 250 68 e, Czechoslovakia.We a re grateful to the members of Lausanne-Münich-Zürich collaboration, particularly to C.Joseph, G.Strassner and P.Trüol for supplying us with the experimental results prior to publication. M. G. also thanks them and R.Wünsch for interesting discussions. We are grateful to R. A.Sakaev for his help at an early stage of this work.     

Complex shape effects in nuclear rotational spectra

We study the evolution of collectivity in the structure of nuclear rotational bands based on complex quadrupole-octupole shapes. We apply an extended version of a quadrupole-octupole rotation model capable of reproducing both the low-lying states of alternating parity bands interpreted on the basis of octupole vibrations and the higher spin states considered as members of a single octupole rotational band. In such a way, the complicated odd-even staggering effects observed in light actinide nuclei are described successfully. The implemented model analysis suggests a unified mechanism in which the octupole band structure is formed as the result of the transition from an octupole vibration (soft) mode to a rotation of a shape with a stable quadrupole-octupole deformation.     

Towards a better understanding of O nuclear structure: Muon capture and radiative pion capture reactions

Muon capture and radiative pion capture reactions on ¹⁶O have been analysed with the aim to extract more information on the importance of higher components of the A = 16 nuclear ground and excited states. Good agreement with the experimental data for both reactions may be achieved if at least 2p2h components of the nuclear wave functions are taken into account. The positive parity excited states in ¹⁶N are shown to contribute 43 % to the radiative π⁻ capture but only ≈ 10 % to the muon capture rate.     

Finite particle number effects in high-energy nuclear collisions: Implications on pion spectra

We study pion production from high-energy nuclear collisions by means of a simple statistical model. The shapes of the observed spectra exclude that all pions result from freely decaying delta resonances. Rather, they have to participate in kinetic equilibration processes. Finite particle number effects are found to be very important: equilibration does not occur globally but rather in groups of only a few particles. The pion production rates cannot be explained in terms of a chemical equilibrium.     

Analysis of175Lu NMR data on dilute alloys of Lu in Tb and Dy

Results of Sano, Shimizu and Itoh on NMR frequencies of¹⁷⁵Lu in ferromagnetic Lu_0.02Tb_0.98 and Lu_0.02Dy_0.98 alloys are fitted by use of a model with five adjustable parameters. Two of these are related to the magnitude and asymmetry of the electric-field-gradient (EFG) tensor at a Lu nucleus, and three others specify the magnitude of the magnetic hyperfine field and its orientation with respect to the principal axes of the EFG tensor. For LuDy it is found that (i) the hyperfine field is tilted with respect to these axes, and (ii) the asymmetry of the EFG tensor differs in magnitude from published calculated values based on a point-change model. For LuTb at least one of these two statements is shown to be true. Tilting of the hyperfine field is consistent with inferences from neutron scattering data that the 4f moment is tipped away from the b axis in Tb metal.     

Energy levels and transitions in165Ho from neutron scattering

The¹⁶⁵Ho (n, ń) and¹⁶⁵Ho (n, ń γ) reactions were utilized to obtain information on the energy levels of¹⁶⁵Ho up to about 1400 keV excitation. A Ge (Li) detector was used in the time-gated mode to measure the de-excitation γ-rays. Neutrons were detected by a conventional time-of-flight spectrometer. Results from the two sets of measurements are combined to construct an energy level scheme for¹⁶⁵Ho. The level scheme is compared with results from other sources and some new levels are proposed.     

Hyperfine structure measurements in metastable states of165Ho

The hyperfine structure constants and the electronic g _J factor of the state 4f ¹¹ 6s ^{2 4} I _11/2 belonging to the holmium ground multiplet have been measured using the atomic beam magnetic resonance technique combined with a state-selective laser-induced detection of the resonant atoms. By the same method the g _J factor of the level 4f ¹¹6s ²⁴ I _9/2 was determined, while the hyperfine structures of this metastable state and of two high-lying even parity states have been investigated by high resolution laser spectroscopy. The results for the experimentalA andB factors of all four members of the ground multiplet allow a least-squares evaluation of the three magnetic dipole and the three electric quadrupole effective radial parameters for the configuration 4f ¹¹ 6s ² of holmium, yielding an accurate value for the spectroscopic nuclear electric quadrupole moment:Q _hfs(¹⁶⁵Ho)=2.716(9)b (uncorrected for quadrupole shielding). From a comparison to the quadrupole moments measured in mesic holmium atoms the shielding factor could be estimated.     

Partial transitions in radiative pion capture on light atomic nuclei

The partial transitions in radiative pion capture on light nuclei are studied within the shell model with intermediate coupling. The probabilities of capture from s- and p-states of a mesoatom and the total yield of γ-quanta have been calculated and compared with experimental data.     

Importance of momentum dependent terms in radiative pion capture

The contribution of the momentum dependent terms in radiative pion capture from bound atomic orbits is investigated. A shell model calculation for ⁶Li shows that the momentum dependent terms represent ≈ 2% of the 1s capture rate and up to 40% of 2p capture rates.     

Hexadecapole interaction in the atomic ground state of165Ho

Using the atomic beam magnetic resonance method several hyperfine transitions in the ground state of¹⁶⁵Ho were measured at magnetic fields near zero Oe. Including a hexadecapole interaction constantD a perfect fit of the seven hyperfine separations was possible giving the following results:A′=800.583645 (6)MHz,C′=?1.504 (37)kHzB′=?1668.00527 (33)MHz,D′=?0.137 (14)kHz. These interaction constants have been corrected for second order hyperfine interaction within the⁴I ground multiplet. The corrected constants are the following:A=800.583173 (36)MHz,C=?0.249 (140)kHzB=?1668.078 70 (330) MHz,D=?0.148 (16) kHz. Using a value for 〈r _4f ^/?5 〉_Ho of Fraga a nuclear hexadecapole moment can be calculated:Π(¹⁶⁵Ho)=0.89·10^?48cm⁴. Because of severe uncertainties still present in the theory for calculating the electronic matrix elements this value can be only regarded as highly speculative.     

Intrinsic and rotational excitations in 175Lu

The γ-ray cascades following the ¹⁷⁶Yb(p, 2n)¹⁷⁵Lu reaction are studied with Ge(Li) detectors and interpreted within the level scheme of ¹⁷⁵Lu. According to this interpretation the $\text{1}\text{2}$⁺[411], $\text{1}\text{2}$^?[541], $\text{5}\text{2}$⁺[402], $\text{7}\text{2}$⁺[404] and $\text{9}\text{2}$^?[514] rotat $\text{15}\text{2}$⁺, $\text{21}\text{2}$^?, $\text{13}\text{2}$⁺, $\text{19}\text{2}$⁺ and $\text{19}\text{2}$^? members, respectively. Fu band head is determined to be 626.6 keV. The data for the rotational bands are discussed within the particle-rotor model and compared with the available information on the corresponding bands in the adjacent Lu isotopes.     

Multipole mixing ratios of transitions in175Lu populated from the decay of175Hf

Electron capture decay of¹⁷⁵Hf(70 d) provides a possibility for unambiguous determination of the sign and magnitude of multipole mixing ratios for 89 keV and 114 keV transitions in¹⁷⁵Lu. This is because both gamma rays are in coincidence with the 230 keV transition which is of a pureE2 character. The results of theγ —γ directional correlation measurements, in terms ofE2/M1 mixing ratiosδ, areδ ₈₉ = 0.11±0.07 andδ ₁₁₄ = 0.465±0.005. In addition the directional correlation measurements of the 89γ- 343γ and 319γ-114γ cascades yieldδ ₃₄₃ = 0.08±0.07 andδ ₃₁₉ = ?0.152±0.015.Radioactivity ¹⁷⁵Hf from¹⁷⁴Hf(n, γ); measuredγ —γ(θ) deducedδ(E2/M1); Ge(Li) detectors.