γ-Ray emission from oriented nuclei in a multiaxis nuclear spin system: 166mHo165Ho

A new method is proposed to measure multiaxis nuclear spin structures using γ-ray emission from oriented radioactive nuclei. This method, which depends only on angular momentum theory, is presented for helical spin structures. The atomic magnetic structure can also be obtained when the nuclear magnetism is produced by hyperfine interaction. Measurements on ^166mHo in a single crystal of ¹⁶⁵Ho metal show that the spin axes form a single cone with a half-angle of 80.4±0.4°.     

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.     

A study of charge,energy and angular momentum transfer in the 56Fe + 197Au and 56Fe + 107, 109Ag reactions at 7.2 and 8.3 MeV/nucleon

Kinetic energy spectra, charge and angular distributions have been measured for thirty elements produced in the reactions of 401 and 460 MeV ⁵⁶Fe + ¹⁹⁷Au and in the reaction of 470 MeV ⁵⁶Fe + ^{107, 109}Ag. In addition, γ-ray multiplicities were measured at the 470 MeV bombarding energy for both targets at a limited number of angles. The charge distributions for the deep-inelastic component of these systems increase monotonically with atomic number in the measured angular range, whereas, those for the quasielastic component are skewed toward Z-values below the projectile. The angular distributions for the Fe-induced reactions show a smooth evolution from a side-peaked to forward-peaked distributions with increasing mass transfer. This side peak is more intense and more persistent for mass transfers from the projectile to the target. In the quasielastic region the γ-ray multiplicity is observed to increase almost linearly with decreasing Q-value whereas for large negative β-values it is essentially constant and independent of the exit channel mass asymmetry. Finally, angular distributions, angle-integrated charge distributions and γ-ray multiplicities have been compared with a diffusion model in which the dynamics of shape evolution, N/Z equilibration, angular momentum and energy exchange occur via one-body forces.     

Sequential fission angular distributions from mass-asymmetric heavy-ion reactions

The angular distributions of sequential fission fragments have been measured for the reactions of ⁴⁰Ar with ¹⁹⁷Au and ²³⁸U as a function of reaction Q-value and charge transfer. These angular, distributions are used to study the angular momentum and alignment of the deep-inelastic products which undergo fission. All of the fission fragment angular distributions are strongly focused into the plane defined by the beam and the projectile-like fragment velocity vectors. The in-plane angular distributions from reactions with uranium are isotropic for small energy losses and become anisotropic as the energy loss increases. For large negative Q-values, the in-plane anisotropy increases as the deep-inelastic products become more symmetric. The variation of the in-plane anisotropy with mass asymmetry for the two systems in this work was compared to a compilation of previous work and a consistent pattern was found. These alignment data are compared to equilibrium statistical calculations.     

Observation of nuclear rotational spectra in pion capture on 165Ho and 175Lu

In X-ray spectra of pionic atoms of ¹⁶⁵Ho and ¹⁷⁵Lu we observed nuclear γ-rays of Dy and Yb isotopes, respectively, arising from the pion capture reactions ¹⁶⁵Ho(π^?,xn^165?xDy¹, and ¹⁷⁵Lu(π^?xn^175? Yb¹, where x = 1, 3, 5, 7, 9, 11.     

Polarized neutron capture by polarized 165Ho nuclei used to assign spins to 166Ho levels

The reaction ¹⁶⁵Ho(n, γ) has been investigated using polarized neutrons and polarized ¹⁶⁵Ho nuclei. Spin values could be assigned to 11 levels of ¹⁶⁶Ho on the basis of angular distribution measurements on primary γ-transitions. In the case of four other levels, spins are restricted to two values.     

Identification of Single-Particle Configurations at Very High Spin in Some Rare-Earth Nuclei

The nuclear behaviour at high angular momenta is studied by γ-rays emitted in (HI, Xn) reaction. The study of very-high spin states in ¹⁵³Ho via discrete-line γ-ray spectroscopy and a comparison between the partial decay schemes of ¹⁵³Ho and the neighbouring ¹⁵²Dy and ¹⁵⁴Er nuclei are discussed in the context of the high spin structure. Second, we present comments on some phenomena observed at high spin states in ¹⁵³Ho and ¹⁵²Dy nuclei.     

Intrinsic fragment spins generated in the reactions of 20Ne with 197Au and 238U at 12.6 MeV/nucleon

The average magnitude and alignment of the intrinsic spin of the heavy partner from the reaction of 252 MeV ²⁰Ne with ¹⁹⁷Au and ²³⁸U were determined as a function of Q-value. These spin values were extracted from sequential fission angular distributions obtained in coincidence with projectile-like products. For all Q-values a large out-of-plane anisotropy was observed, while for large negative Q-values an in-plane anisotropy was observed. A very large entrance-channel mass-asymmetry was chosen to provide a stringent test of equilibrium statistical model predictions for the spin alignment. The importance of determining the direction of the line-of-centers of the dinuclear system at scission is discussed. Large values of P_ZZ were deduced for all Q- values. P_XY was observed to be positive in the quasielastic region and negative in the deep-inelastic region. The extracted alignment data are compared to equilibrium statistical model calculations.     

165Ho fast neutron cross sections

Total neutron cross sections of¹⁶⁵ Ho were measured from 0.1 to 1.5 MeV with resolutions of ? 2.5 keV. The observed total cross sections varied slowly with energy and displayed no significant structure. Differential neutron elastic and inelastic scattering cross sections were determined at intervals of ?50keV from 0.3 to 1.5 MeV. The inelastic excitation of states in¹⁶⁵Ho at; 98, 214, 371, 460, 517, 586, 712, 824, 995, 1104 and 1143 keV was positively observed with probably identification of several additional states. The observed excited structure and the respective cross sections were correlated with known single-particle and collective states and with excited structure postulated from systematics. The measured cross sections were compared with calculated values based upon spherical and deformed optical-potentials, and compound-nuclear processes. Total cross sections were best described by a spherical potential while the differential elastic angular distributions were better represented by deformed-potential calculations. Resonance interference effects were found small and, at the energies of the present experiments, the contribution of direct processes was not large.     

Analysis of the properties of excited states in 165Er

An analysis of ¹⁶⁵Tm decay has shown that precision measurements of the γ-ray and internal conversion electron intensities, in combination with angular correlation measurements, provide additional data useful for the determination of the mixing and intensity ratios of γ-transitions with a small energy difference. The spin of the 589.868 keV state is found to be $\text{I}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}{}^{\mathrm{?}}$. Doublet opposite-parity transitions from states at 1103.495, 920.632 and 745.968 keV to levels near 590 keV with an energy difference ΔE = 108 ± 22 eV are identified.     

Study of complete and incomplete fusion for loosely bound projectiles 6He and 6Li on 165Ho and 166Er targets

Complete fusion (CF) and incomplete fusion (ICF) reactions were studied for the cases of the loosely bound ⁶He and ⁶Li projectiles bombarding ¹⁶⁶Er and ¹⁶⁵Ho targets at energy of about 10 MeV/nucleon. Experiments were carried out to test an approach exploiting the measured intensities of ?? rays emitted at the transitions between the yrast-band levels of reaction products formed after the termination of neutron evaporation. Partial waves feeding the CF reactions ¹⁶⁵Ho (⁶Li, ⁵ n) ¹⁶⁶Yb and ¹⁶⁶Er(⁶He, ⁶ n) ¹⁶⁶Yb as well as ICF ¹⁶⁵Ho (⁶Li, ??3n) ¹⁶⁴Er and ¹⁶⁶Er(⁶He, ??4n) ¹⁶⁴Er reaction channels were revealed from the obtained ??-ray data. The method of exit channel identification via the triple coincidence (??1-??2-light charged particle) was employed for these reactions study.     

The hyperfine interaction of gadolinium and holmium in cubic Ho: (Gd,Ce)Fe2 compounds

The low-temperature hyperfine parameters of the ferrimagnetic C15 intermetallic system Ho_0.03Ce_xGd_0.97-xFe₂ have been measured at Gd using the 87 keV γ-ray of ¹⁵⁵Gd and at Ho using spin echo nmr of ¹⁶⁵Ho. A linear decrease of the hyperfine field B_t with increasing x is observed, the slope being almost the same for both nuclei. The observed slope (dB_t/dx ? -37T) is nearly three times greater than that found for ¹⁵⁵Gd in the analogous H₀:(Y, Gd)Fe₂ system; this is attributed to the influence of the increased conduction electron density when trivalent lanthanide ions are replaced by Ce⁴⁺.     

Q- and Z-dependence of angular momentum transfer in deeply inelastic collisions of 86Kr with 209Bi

The dependence of the in-plane and out-of-plane angular correlations of fragments from fissioning heavy products on the kinetic energy and Z of the light reaction partner have been measured. From the dependence of the angular correlations on Q-value and hence energy loss, together with existing data from which the total angle-integrated cross section as a function of energy loss can be extracted, we have determined the dependence of the angular momentum transferred to the heavy product on the initial orbital angular momentum or impact parameter. The resulting dependence is qualitatively consistent with the sticking limit for a reaction intermediate of touching deformed fragments. More specific nuclear models generally underestimate the angular momentum transfer, although the one-body proximity-friction model accounts for the major fraction of the angular momentum transfer. A recent model incorporating both one-body proximity friction and collective excitations accounts quite well for the observed angular momentum transfer. The Z-dependendence of the anisotropy shows the importance of angular momentum fractionation for the less probable events, where the Z of the fissioning system is appreciably less than that of the target. The transferred angular momentum is shown to be fairly strongly aligned along the perpendicular to the reaction plane, with alignment values of 0.6 to 0.8. The component of angular momentum not along the perpendicular to the reaction plane is found to be primarily oriented perpendicular rather than parallel to the recoil direction. The absolute fission probabilities are found to be qualitatively consistent with J-dependent calculations using the J-values deduced from the angular correlations.     

Magnetic moment of the 3/2 + state in 165Ho

The Larmor precession for the 362 keV state in ¹⁶⁵Ho(I ^π ?=?3/2^?+?, T _1/2?=?1.512 μs) in Dy₂O₃ with an external magnetic field of 0.3 T was determined to be ??32.3 ± 0.6 MHz by use of the perturbed angular correlation technique, intending to determine the magnetic moment and apply it to the measurement of the hyperfine field at Ho in Fe. The magnetic moment for this state was tentatively deduced under the assumption that the paramagnetic correction factor for a free Ho^3?+? ion is applicable to the present case. The independent A ₂₂ measurement for the 633???362 keV γ cascade for the sign assignment of the Larmor frequency is inconsistent with that from known multipolarities and mixing ratios for this cascade, implying that the mixing ratios may be different from the reported values.     

Gamma-ray multiplicities in α-induced compound and precompound reactions

Measurements of γ-ray multiplicities from the¹⁵⁰Sm(α, xn) reactions at beam energies from 40–140 MeV show that the maximum angular momentum transfer occurs at an energy between 12 and 18 MeV higher than that of the maximum of the excitation function for the respective exit channel. At still higher bombarding energies, where precompound processes dominate, a decrease of the multiplicity is observed.     

Study of the165Ho(γ,n)164Ho reaction

The spectrum of neutrons following photo-excitation of¹⁶⁵Ho with 8,999 keVγ-rays has been observed. The data indicate a neutron separation energy of 7,987 (2) keV. Eleven excited states are identified, with excitation energies generally in good agreement with those observed in the neutron pickup reaction. Significant discrepancies between observed and theoretical branching are noted.     

Test of time reversal invariance in 2-MeV neutron scattering from aligned165Ho

We describe a search for the time reversal non-invariant five-fold correlation (s·Ixk)(I·k) in the scattering of 2 MeV polarized neutrons from an aligned¹⁶⁵Ho target. Here s is the neutron spin,I is the target spin, andk is the neutron momentum vector. Differences in transmission were sought when s was reversed. The target was a single crystal of holmium, cryogenically aligned with a specially built dilution refrigerator. Rotating the target axis of alignment while cold permits the parity-even, five-fold correlation to be identified among other spin-dependent correlations in the forward elastic scattering amplitude. An analyzing power was found to be (1±6)×10⁻⁴, consistent with time reversal invariance.We obtain a bound of 5×10⁻³ for α_T, the ratio of T-odd to T-even couplings in the effective nucleon-nucleon interaction.     

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.     

Angular momentum transfer in 12C-, 20Ne- and 40Ar-induced fission

Angular momentum transfer in a variety of ¹²C-, ²⁰Ne- and ⁴⁰Ar-induced fission reactions has been investigated using γ-ray multiplicity techniques. Fission fragments were detected in coincidence using a pair of solid-state detectors. The fragment masses were deduced from the kinetic energies and emission angles using two-body kinematics. The γ-ray multiplicities (M_γ) of the fission fragments were measured utilizing an array of eight NaI detectors. For most of the systems studied, M_γ is nearly independent of the exit-channel mass asymmetry. The strongest dependence on mass is observed in the systems ¹⁵⁴sm + 240 MeV ⁴⁰Ar, where a minimum exists at symmetry, and ¹⁹⁷Au + 164 MeV ²⁰Ne, where nuclear structure effects are suggested by the data. For all the reactions the quantity M_γ tends to decrease gradually with increasing fragment kinetic energy. The magnitude of M_γ generally appears to be larger than expected on the basis of rigid rotation, suggesting a spin enhancement effect. The data are compared with a simple model which assumes the statistical excitation of a variety of angular momentum bearing collective modes. Reasonable agreement is obtained with the experimental results. The roles of other collective effects, such as shape fluctuations and angular momentum fractionation, are also considered.     

Superdeformed bands in153Ho

In a recent experiment carried out with theEurogam Phase II γ-ray spectrometer three new superdeformed (SD) bands have been established in¹⁵³Ho. The properties of these SD bands are discussed in terms of proton particle-hole excitations and are compared to the SD bands in the adjacent nuclei. This is the first observation of SD structures in aZ=67 nucleus.