Perturbed angular distribution of recoil implanted F19-nuclei in a nickel-environment

Magnetic hyperfine fields acting on F¹⁹ in a Nickel lattice have been investigated. Time-dependent spin rotation has been observed following the excitation and recoil implantation with a pulsed proton beam using the reaction F¹⁹(p,p′)F¹⁹?. Two hyperfine fields were detected at 290 °K:H _hf ⁽¹⁾ =+17.6 ± 0.5 kGH _hf ⁽²⁾ =+91 ± 3 kG. The variation of the effective fields with the external polarizing field was studied. The mean life of the 197 keV level has been remeasured as τ=128±2 nsec.     

Time differential perturbed angular correlations of Gd160 implanted into ferromagnetic Gd and of Yb176 into Fe and Ni

Time differential perturbed angular correlation measurements of Gd¹⁶⁰ in ferromagnetic Gd at various temperatures and of Yb¹⁷⁶ in Fe and Ni have been performed following Coulomb excitation with a pulsed beam and recoil implantation. Using the theory of combined static magnetic and electric hyperfine interaction the measured time spectra are reasonably well reproduced, the magnetic fields and electric field gradients being in agreement with other works. A phase shift of the Larmor precession, however, points to anomalous hyperfine fields acting on the nuclei in a very short time interval after the beam pulse.     

Untersuchung gestörter Winkelverteilungen an F19 in ferromagnetischen Gittern

The perturbation of the 197 keV transition angular distribution in F¹⁹ was investigated by time-dependent spin rotation measurements following excitation with a pulsed beam. The recoil implantation technique was used to determine the internal magnetic fields for F¹⁹ in Fe, Co and Ni lattices. The results are:H _HF(F¹⁹ in Fe)=+(95700±500) Oe,H _HF(F¹⁹ in Co)=+(59500±1500) Oe,H _HF(F¹⁹ in Ni)=?(21830±350)Oe. The temperature and field dependence of the effective fields was studied. Strong satellite fields due to lattice perturbations were detected. The half life and the gyromagnetic ratio of the 197 keV 5/2⁺ state in F¹⁹ were redetermined asT _1/2=(80.2±0.5) nsec andg=+1.436±0.007.     

Spin and parity assignments for 23Na levels from the 19F(α, p)22Ne reaction

Resonances were found in the 120° and 160° excitation functions for the ¹⁹F(α, p_o)²²Ne reaction between E = 2.5 and 5.0 MeV corresponding to ²³Na levels at excitation energies between 12.56 and 14.51 MeV. Twenty-one on-resonance angular distributions were analysed with single-level and two-level theory to extract ²³Na spin and parity information. The results of an earlier experiment were analysed by the same procedure, extending the diagnostics down to a ²³Na excitation energy of 11.55 MeV. The analysis incorporated an optical potential for the α-particle consistent with previous channel-spin-$\text{1}\text{2}$ reaction studies and α-induced reaction data.     

Pulsed magnetic field method for measuring polarization of radioactive beams

A new method has been developed for measuring the magnitude of nuclear spin polarization of a secondary, radioactive beam by making a pulsed magnetic field measurement that does not require advance knowledge of the nuclide's magnetic moment. Using a standard β NMR apparatus, a magnetic double ratio is determined from the counting rates in 0° and 180° β detectors for magnetic field on and off conditions. This ratio provides direct information on the induced spin polarization of a radioactive beam. A demonstration of the method was performed using spin polarized ¹²B nuclei produced by fragmentation of an 80 MeV/nucleon ¹⁸O beam in a Nb target. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence of the hyperfine fields for fluorine in ferromagnetic iron,nickel and gadolinium

The temperature dependence of the magnetic hyperfine fields at the sites of F nuclei implanted into ferromagnetic Fe, Ni and Gd has been studied in the temperature range from 77 K to 670 K. A pulsed proton beam was used to observe the time-differential precession of the 5/2⁺ state in¹⁹F. Deviations from the bulk magnetization were found for Fe and Ni. The damping of the two observed fields in Ni was interpreted in terms of a field distribution caused by an induced radiation damage. The occupation sites for F and possible mechanisms of the anomalous temperature dependence are discussed.     

A study of the (α, t) reaction on 19F, 27Al, 51V and 59Co

The angular distributions of tritons from the (α, t) reaction on ¹⁹F, ²⁷Al, ⁵¹V and ⁵⁹Co nuclei corresponding to the (0⁺) ground states and (2⁺) excited states in the final nuclei have been measured in the angular range between 15° and 170° at α-particle energies of 25 MeV. For reactions on ²⁷Al and ⁵¹V nuclei, the differential excitation functions have also been obtained at different angles of outgoing tritons at E_α from 20 to 25 MeV. The experimental angular distributions are analysed by the DWBA approximation on the assumption of a nucleon stripping mechanism. The analyses of the present results and the data obtained earlier for the (α, t) reaction on the 1 p shell nuclei, A ? 30, reveal that the distinguishing feature of the reaction under study is the presence of backward angle peaks in the reaction cross section, which appear to be associated with exchange processes. For the (α, t) reaction on the heavier nuclei (A > 30), the dominant mechanism is nucleon stripping.     

Radio-frequency μSR experiments at the ISIS pulsed muon facility

This paper explores the use of pulsed radio-frequency (RF) techniques to remove the frequency limitations imposed on conventional transverse muon spin rotation (μSR) experiments at a pulsed muon source by the finite muon pulse width. The implementation of the 90° pulse technique is demonstrated by observing the free precession signal of diamagnetic muons implanted in polythene, the change in signal amplitude as a function of RF pulse length is plotted and the precise condition for a 90° pulse determined. The technique is evaluated by comparing measurements made using conventional spin rotation experiments to those employing pulsed RF methods. The potential for applying standard NMR multiple-pulse methods to the μSR experiment is considered and the use of two-pulse RF sequences (90° _x ?τ?90° _x and 90° _x ?τ?180° _x ) to form a muon spin echo demonstrated.     

7Li(α,n)10B differential cross-section measurements from threshold to Eα = 5.1 MeV

Differential cross sections for the ⁷Li(α, n)¹⁰B reaction have been measured at lab angles of 0°, 20°, 31°, 50°, 60°, 70°, 80°, 90°, 100° and 114° for α-particle energies between 4.385 and 5.1 MeV. A thick natural lithium target was bombarded with a 5.2 MeV, nanosecond-pulsed ⁴He⁺ beam and neutron velocity spectra at each angle were measured by time-of-flight techniques. These data have been converted to cross sections at 10 keV intervals in α-particle energy. Angular distributions have been fitted with a series of Legendre polynomials. Angle-integrated cross sections, the 0° excitation function, and angular distributions are compared to past measurements and R-matrix calculations.     

X-band Electron Spin Relaxation Times for Four Aromatic Radicals in Fluid Solution and Comparison with Other Organic Radicals

X-band electron spin relaxation times of BDPA (1:1 α,γ-bisdiphenylene-β-phenylallyl), galvinoxyl 2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy, DPPH (2,2-diphenyl-1-picrylhydrazyl) and thianthrene radicals in fluid solution were measured by electron spin echo and inversion recovery at ambient temperature. Tumbling correlation times are estimated to be in the range of 20–30 ps. In this fast tumbling regime T ₁ ~ T ₂. Relaxation times are compared with previously reported values for symmetrically substituted triarylmethyl, semiquinone, and nitroxide radicals. The concentration dependence of spin lattice relaxation for neutral BDPA in toluene is about 10³ times greater than for anionic trityl radicals in water. T ₁ decreases in the order carbon-center BDPA > galvinoxyl > DPPH > thianthrene. The dominant relaxation mechanisms are proposed to be a local mode for BDPA, spin rotation, local mode and modulation of anisotropic proton hyperfine coupling for galvinoxyl, modulation of anisotropic nitrogen hyperfine for DPPH, and spin rotation plus modulation of anisotropic proton hyperfine coupling for thianthrene.     

Energy levels in 23Na from the 19F(α, n)22Na reaction

The 0° differential cross sections for the production of four neutron groups, populating the ground state and the four lowest excited states of the residual ²²Na nucleus were measured. For incident α-energies of up to 4.7 MeV more than sixty resonances, corresponding to levels in ²³Na were seen; most of them had not been reported previously. Angular distributions were measured at eight α-energies. The excitation curve for the ¹⁹F(α, P₁γ)²²Ne reaction was also obtained and eighteen additional levels were identified in ²³Na.     

Mass distributions of fusion products in reactions between 19F and 12C

Nuclear reactions between ¹⁹F and ¹²C target nuclei were studied at beam energies of 50, 63.2 and 76 MeV. Reaction products in the mass range from A = 11 to 31 emitted in forward direction (4°–20°) were identified with a time-of-flight telescope. Mass distributions of the fusion products were obtained. Their characteristic structure is interpreted as a consequence of the superposition of nucleon and α-particle emission. Qualitative conclusions are drawn on the relative intensities of the different decay modes of the compound nucleus. Fusion cross sections and estimates of the contribution of direct channels to the total reaction cross section are discussed.     

Observation of muon level-crossing resonance in antiferromagnetic MnF2

We report the observation of μ⁺ Level-Crossing Resonances (LCR's) in the ordered phase of an antiferromagnetic material. Two LCR's were observed in MnF₂ as a function of longitudinal magnetic field in the temperature range between 10K and 65K. Both are attributed to a muon in an interstitial octahedral-like site. The low field resonance is attributed to a muon-nuclear spin flip-flop transition involving the two nearest neighbour¹⁹F nuclei. The high field resonance occurs when the applied field cancels the local hyperfine field on the muon. The positions and widths of the LCR's were seen to scale with the sublattice magnetization.     

Relaxation studies using selective excitation double Mössbauer techniques

We have developed a technique for studying time dependent hyperfine interactions using selective excitation double Mössbauer (SEDM) procedures. The measured relaxation time between the two spin orientations of the iron atoms in α-Fe₂O₃ at the Morin transition is 1.1 ± 0.2 × 10^-7 sec.     

Spin-parity combinations in 16N from the 18O(d,α)16N reaction

The reaction ¹⁸O(d, α)¹⁶N has been studied using a tensor-polarized deuteron beam at incident energies of 8.50, 9.25, 10.00, 10.30, 10.75 and 11.30 MeV. From measurement of the tensor analysing power of the reaction when the α-particles are emitted near 0°, spin-parity combinations have been deduced for thirteen levels in ¹⁶N up to 6.2 MeV excitation. These results, together with previous work, lead to new and unambiguous spin and parity assignments for four of these levels.     

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°.     

Spin and parity assignments for 35Cl levels from the 31P(α, p)34S reaction

³⁵Cl states at excitation energies between 9.9 and 11.8 MeV have been identified through sharp resonances in the ³¹P(α, p_o)³⁴S excitation functions at 25°, 105° and 155° for E_α = 3.25–5.50 MeV. Forty-eight on-resonance angular distributions, normalized to an absolute cross section scale, have been subjected to single-level and two-level analyses resulting in spin and parity assignments for each resonance. Approximately half the resonances were of the pure single-state type, having unique angular distribution shapes. Data from 12 resonances of an earlier experiment ¹) were analyzed with the same theory, extending the diagnostics down to a ³⁵Cl excitation energy of 9.1 MeV. A set of optical potentials consistent in all four reactions that this experimental program encompasses has been incorporated in the present analysis. Validity of the optical potential is demonstrated for α-particles elastically scattered by ³¹P.     

The Formation of Lithiated Ti-Doped α-Fe2O3 Nanocrystalline Particles by Mechanical Milling of Ti-Doped Lithium Spinel Ferrite

Perturbed Angular Correlation method has been used to study the hyperfine magnetic field in the Heusler alloy Pd₂MnSb(Sn). Ion implantation of the recoil ¹¹¹In nuclei following heavy ion nuclear reactions ¹⁰⁸Pd(⁷Li, 4n)¹¹¹In and ¹⁰⁸Pd(⁶Li, 3n)¹¹¹In has been used to great advantage in the present case resulting in large implantation efficiency. Only a few hours of irradiation time with moderate beam current of the order of 400–500 nA resulted in sufficient implanted ¹¹¹In activity on the sample for good quality measurements. The hyperfine field was measured at ¹¹¹Cd probe nuclei substituting Mn and Sb(Sn) sites as a function of temperature. The fraction of ¹¹¹Cd nuclei occupying Mn atom sites was found to increases with the annealing of sample at higher temperatures.     

Measurement of the neutron spectra from the decay of the giant quadrupole resonance in 238U

We have performed the coincidence experiments (α, α'f) and (α, α'nf) on ²³⁸U for a beam energy Eα = 120 MeV and for α-scattering angles Θα' = 9°, 15°, and 17°. The 9° and 17° data correspond to maxima in the angular distribution for the isoscalar giant quadrupole resonance, whereas the 15° data lie close to a minimum. Using a set of fission counters which covered roughly 80% of 4π, we were able to reject neutrons associated with fission in the (α, α'n) spectrum. We present a first look at these spectra.     

The magnetic structure on the ground state of the equilateral triangular spin tube

The ground state of the frustrated equilateral triangular spin tube CsCrF₄ is still hidden behind a veil though NMR spectrum broaden into 2 T at low temperature. In order to investigate the spin structure in an ordered state by ¹⁹F-NMR, we have determined the anisotropic hyperfine coupling tensors for each three fluorine sites in the paramagnetic state. The measurement field was raised up to 10 T to achieve highest resolution. The preliminary analysis using the obtained hyperfine tensors has shown that the archetypal 120°-type structure in ab-plane does not accord with the NMR spectra of ordered state.