Transient-field strength measurements for52Cr traversing Fe hosts at high velocity and polarization transfer mechanisms

Transient-field strengths were measured for⁵²Cr ions traversing polarized Fe hosts at velocities up to 12υ₀ (12υ₀ =c/137 = Bohr velocity). The results are compared with predictions of various transient-field parametrizations and discussed in terms of possible mechanisms by which polarization might be transferred from the Fe host to inner vacancies of the moving Cr ions. Theg-factor of the first 2⁺ state of⁵²Cr was also measured by the transient-field technique and found to be in accord with shell model calculations.     

Systematic measurements of transient fields for W,Os and Pt ions traversing Fe

Transient magnetic fields were measured for W, Os and Pt ions traversing iron hosts with average velocities in the range from approximately 1.6v ₀ to 4.8v ₀ (v ₀=c/137, Bohr velocity). Transient fields for W and Os in Fe are consistent with behaviour found for lighter rare-earth ions and are about 20% stronger than those for Pt in Fe over the majority of the velocity range examined. A measurement was made to confirm that possible heavy-ion beam induced attenuations of the transient field are negligible for low-velocity Pt ions excited by Ni beams. Results are discussed in terms of both empirical and model-based parameterizations of the transient field strength.     

Disparity in transient field characteristics for Au ions traversing Fe and Gd hosts through measurements ofg-factors in197Au

Transient field precessions were measured for levels in the nuclides of^182,184,186W and¹⁹⁷Au as their ions simultaneously traversed polarized Fe and Gd hosts. Consistent sets of gyromagnetic ratios were inferred for the lowest 5/2⁺ and 7/2⁺ states in¹⁹⁷Au using these two ferromagnetic host media, provided that (i) the transient field for Au ions in Gd is calibrated using the W in Gd precessions (simultaneously measured) and, (ii) the transient field for Au in Fe is calibrated using the field strength and velocity dependence for Pt in Fe (previously measured). The presentg-factor results are: g(5/2 ₁ ⁺ ;¹⁹⁷Au)=0.296±0.023, g(7/2 ₁ ⁺ ;¹⁹⁷Au)=0.241±0.021, and g(5/2 ₁ ⁺ ;¹⁹⁷Au)=1.2±0.2.     

Magnetic moment measurements of picosecond states: New results and open problems

The temperature dependence of the precession of the angular correlation of decay gamma rays from swift¹⁵⁰Sm (2 ₁ ⁺ ) ions traversing a gadolinium foil has been found to be proportional to the foil magnetization, supporting the assertion that the transient hyperfine magnetic field acting on these ions is proportional to the magnetization of the hosts (iron or gadolinium). Similar experiments on¹⁹⁴Pt (2 ₁ ⁺ ) ions traversing iron and gadolinium foils are consistent with both the magnetic moment obtained from Rutgers experiments on iron and with a hyperfine field at Pt ions larger for gadolinium than for iron foils, in agreement with the Chalk River parametrization for heavy nuclei traversing gadolinium foils. Finally, the magnetic moments of the 2 ₁ ⁺ states in^144–150Nd,^145,150Sm and¹⁵²Gd have been measured. These data support the evidence of shell closure atZ=64 forN≤88 andZ=50 forN>90.     

The velocity dependence of the transient magnetic field of O in Fe

The transient field precession has been measured for¹⁶O ions in the 3 ₁ ^– excited state recoiling into magnetized iron foils with an initial velocity of 8.0v ₀ (v ₀=c/137). The recoil velocity after passage through the Fe foils has been varied by adjusting the foil thickness. The resulting field strengths show strong deviations from the empirical linear velocity dependence of the transient field. The velocity dependence can be described well on the basis of recently measured probabilities for K-shell vacancy production for O in Fe, with a velocity independent degree of polarization =0.14 for the unpaired electrons in the K-shell.     

Large electron polarization of H-like20Ne-ions traversing gd foils at very high velocities

It is shown that transient magnetic fields in Gd-host for²⁰Ne ions at a mean velocity of 12.5v₀(v₀=c/137), considerably beyond the Bohr velocity of 1s electrons of Ne ions (v_1s=10v₀), are dominated by the Fermi contact field of these electrons. The first excited 2⁺-state of²Ne was used as probe. The derived value for the mean degree of polarization p_1s=0.32(13), is surprisingly large, though still consistent with values obtained at lower velocities. Present calculations of spin exchange cross sections severely underestimate the observed polarization at this high velocity.The authors are grateful to the operating staff of the UNILAC for providing excellent beam conditions. They also thank Dr. P. Maier-Komor for preparing the delicate targets. Support by the BMFT is acknowledged.     

Cross sections for single and double ionization of atomic helium under impact by fast multiply charged ions

Expressions for the cross sections for single and double ionization of atomic helium in collisions with fast multiply charged ions are obtained in the collision parameter range υ ²≫Z≳υ, υ ₀<c, where Z and υ are, respectively, the ion charge and ion velocity, υ ₀ is the characteristic velocity of electrons in the atomic helium ground state, and c is the velocity of light. Zh. Tekh. Fiz. 68, 1–7 (March 1998)     

Transient field measurements on32S(21 +) ions in Gd at the 1s electron Bohr velocity

With the knowng-factor of the Coulomb excited first 2⁺-state in³²S the transient magnetic field was determined for sulphur ions traversing Gd at a mean velocity of 16 ₀ ( ₀=c/137). The degree of polarization deduced for the dominating H-like ions, ¯p_1s =0.10(3), agrees very well with that obtained at lower velocities. In addition, an upper limit of a transient electric field gradient was deduced from the particle--angular correlation which is expected on theoretical grounds.The authors thank the operating staff and in particular Dr. R. Repnow of the accelerator facility at the Max-Planck-Institut für Kernphysik in Heidelberg for providing excellent beam conditions. They are indebted to Profs. D. Habs and U. Schmidt-Rohr for their continuous interest and generous support during the experiment. They are grateful to Dr. P. Maier-Komor and Mrs. A. Meens for preparing the Gd-targets and He-implantation. Support by the BMFT and the Deutsche Forschungsgemeinschaft is acknowledged.     

Disparity between the transient hyperfine fields for Pt and Os in Fe; an electron vacancy sharing interpretation

Precessions of the 2 ₁ ⁺ states in¹⁹⁸Pt and¹⁹²Os have been measured in the enhanced transient hyperfine magnetic field acting on these nuclei as they recoiled through thin polarized cobalt foils. Two separate targets consisting of contiguous layers of¹⁹⁸Pt and¹⁹²Os electrodeposited on ～1 μm and ～4 μm Co foils were employed. The levels of interest were Coulomb excited by 80 MeV³²S and 220 MeV⁵⁸Ni beams and the precessions of the 2 ₁ ⁺ →0 ₁ ⁺ γ-ray angular distributions in both nuclides were measured simultaneously. The results of these studies are compared with recently reported similar studies in which ions of¹⁸⁸Os and¹⁹⁴Pt recoiled through thin polarized Fe foils. It is concluded that (i) the transient field acting on Pt in Fe is singularly anomalous, and (ii) the recent contention that the g-factors of the 2 ₁ ⁺ states in the event Pt isotopes may be substantially lower than had been reported in the literature cannot be sustained. This transient field discontinuity is examined in terms of possible molecular orbital electron vacancy sharing between Pt and Fe.     

Charge state measurements of 23 MeV Cl ions traversing a liquid target

The charge distribution of 23 MeV Cl ions traversing a cloud of silicon oil ((C₆H₅)₄Si₃O₂(CH₃)₄) droplets was measured and was found to extend to very high charge states (17⁺), higher than those obtained for similar stripping in carbon foils.     

Measurements of transient magnetic fields on single-electron Ni-ions

Measurements were performed to determine the transient magnetic field assoclated with polarized is electrons of highly-stripped Ni ions traversing a ferromagnetic Fe layer. Partiele γ angular correlations and spin precessions have been measured with the first excited 2^↓-state in⁶²Ni. The observed precession indicates that the transient field is considerabls weaker than expected.     

Cavity formation at indium impurities in bcc metals

The defect formation in the bcc metals W and Mo above annealing stage III and the influence of rare gases on this process were investigated by means of the perturbed angular correlation technique using¹¹¹In as radioactive probe. In both metals a relatively high electric field gradient (EFG) could be observed at the indium site, characterized by the quadrupole interaction frequencies υ_Q=263 MHz, ν=0 and υ_Q=220 MHz, ν≈0.15 for W and Mo, respectively. The observations are assigned to the growth of threedimensional vacancy clusters at the probe atoms with the indium atoms situated in the inner surface of this cavities, thus experiencing the corresponding surface EFG.     

IMP AC in-beam and out-of-beam;g-factors and pre-equilibrium effects following ion-implantation

An out-of-beam IMPAC technique has been developed to measure theg-factors of the 2 _l ⁺ states in the neutron-deficient Pt isotopes. The hyperfme field for Pt in Fe in those measurements was close to the field obtained in spin-echo and radioactivity measurements, in contrast with in-beam IMPAC measurements for^194,196,198Pt, where smaller field strengths were observed. These data, and static fields from in-beam IMPAC measurements for other ions in the Pt region, show a correlation with the lifetime of the probe state, consistent with the static field being absent for about 10 ps after the ion has come to rest. New and previously published IMPAC data for₇₆Os,₇₇Ir,₇₈Pt and₇₉Au in Fe hosts, are examined to assess whether such measurements may be sensitive to the thermal spike created at the end of the range of an implanted ion.     

Ionization of hydrogen and helium atoms by highly charged fast ions in collisions with small momentum transfer

Ionization of hydrogen and helium atoms is studied for the case of “soft” collisions with highly charged fast ions with v≲Z≪v² and v≫v ₀, where Z is the ion charge, v is the collision velocity, and v ₀∼1 is the characteristic velocity of the electron in the ground state of the atom. Analytical expressions are derived for the singly and doubly differential cross section for ionization of a hydrogen atom accompanied by the ejection of a slow electron v _e≲v ₀, where v _e is the velocity of the ejected electron with respect to the recoil ion). The results are generalized to the case of single ionization of helium. It is shown that soft collisions provide the main contribution to the hydrogen ionization cross section and for all practical purposes determine the cross section for single ionization of helium. The asymmetry in the angular distribution of the ejected slow electrons and the properties of momentum exchange in such collisions are discussed. Finally, a formula for the cross section for single ionization of helium is proposed. Zh. éksp. Teor. Fiz. 112, 1966–1977 (December 1997)     

Investigation of cross sections for the formation and destruction of negative boron ions

The electron loss and electron capture cross sections σ _i,i+m and σ _i,i−m for boron ions and atoms traveling at the velocities V=1.19 and 1.83 a.u. in H₂, He, N₂, Ne, Ar, and Xe are measured. The known experimental data on these cross sections at velocities near the cross-section maximum are analyzed. It is found that the electron loss cross sections can be described by a formula which was previously derived in the free-collision approximation and takes into account features of both the ions and the ambient atoms. As the nuclear charge Z _t of the ambient atoms increases, the cross sections vary nonmonotonically, increasing on average as Zt _t ^1/2 . A formula based on the model of independent electrons is proposed for electron capture by ions with small values of the charge i. It describes the dependence of the electron capture cross section σ _i,i−1 on the mean binding energy of an electron in an ion with the charge i−1. The total electron capture cross section σ _i,i−1 is proportional to the number of vacancies in the unfilled electron shell nearest the nucleus. The cross sections 0σ _i,i−1 exhibit substantially nonmonotonic variation with Z _t, increasing on average as Z _t ^1/3 . Zh. éksp. Teor. Fiz. 116, 1539–1550 (November 1999)     

Balmer Hβ vs. Dβ line intensity in the collisions of H+, H2 +, H3 +, D+, D2 +, and D3 + ions with “realistic” metal surface at energies below 1 keV

The intensities of hydrogen H_β and deuterium D_β spectral lines of the Balmer series were measured as a function of collision energy when H⁺, H₂ ⁺, H₃ ⁺, D⁺, D₂ ⁺, and D₃ ⁺ ions impinged on Al, Ti, Cu, Mo, W, and Pb targets. The collision energies were kept in the 100–1000 eV range. The target surface was contaminated with hydrocarbons from the vacuum pumping system and possibly also by oxygen molecules due to limited vacuum conditions. At projectile velocities above 200 km/s the luminescence of backscattered deuterium atoms is about 30–50% weaker than that of hydrogen atoms.     

TDPAC Measurement of the quadrupole interaction of77Se in arsenic

By TDPAC the quadrupole coupling constant for⁷⁷Se (5/2^–, 248 keV, 9.3 ns) in metallic arsenic was measured to be _Q = 700 ± 60 MHz. The tabulation of electric field gradients (EFGs) for some different sp impurity host combinations shows large values for impurities with 6 valence electrons. If we assume that the selenium impurity is substituted in arsenic, then it is possible to interpret the very high _Q by the assumption of a big EFG and a large quadrupole moment (approximately 1 b) of the excited⁷⁷Se nucleus.     

Velocity and atomic number dependence of the transient magnetic field in iron

Thev- andZ-dependence of the transient magnetic field in iron has been investigated for light ions. The present work on transient fields for²⁰Ne and²⁴Mg at initial velocities up tov _i=8v _o (v _o=c/137) confirms the linearv-dependence of these fields. From the existing and present data a marked atomic shell effect has been found in theZ-dependence forZ26. This dependence can be described by a simple expression if the fields are assumed to be due to polarized electrons in s-shells. This shell effect can be understood qualitatively within the framework of an atomic model.     

The velocity andZ dependence of the transient magnetic field in iron

The velocity dependence of the transient magnetic field in iron has been investigated for¹³⁴Ba at four initial velocities betweenv _i=0.8v ₀ andv _i=3.6v ₀ (v ₀=c/137). The present work confirms the linearv-dependence found for light ions (Z34). Existing data on transient fields were reanalyzed assuming a general validity of the linearv-dependence. The extractedZ-dependence shows a smooth, almost linearZ-dependence for ions withZ12. From the present work, with the calibration of the transient field from systematics, theg-factor of the first excited 2⁺ state in¹³⁴Ba is determined to beg=0.41(6).     

The transient hyperfine field at Si,Pd and Pt ions traversing iron and nickel foils

A measurement of the hyperfine interactions on swift²⁸Si,¹¹⁰Pd, and¹⁹⁴Pt ions recoiling through magnetized iron and nickel foils indicates that for these ions the strength of the transient magnetic field scales with the net magnetization of the ferromagnet.Supported in part by the National Science Foundation.