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Now showing items 1 - 16 of 26

  • Analysis of Cu and O in high-Tc superconductors by 7–9 MeV protons

    J. Keinonen   J. R?is?nen  

    The use of proton-induced gamma-ray emission for the simultaneous analysis of Cu and O in high- Tc superconductors is demonstrated. Utilizing 7-9 MeV protons, the ratio of O relative to Cu can be determined reliably to a few percent accuracy in homogeneous bulk samples and films thicker than 130 mum using standard bulk samples of O and Cu. Results of the present method are compared with those of the analysis of YBa 2Cu 3O 7-x done by the Rutherford scattering of alpha-particles and the non-Rutherford scattering through the 16O(p,p 0) 16O reaction
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  • Defects in H implanted GaAs studied by ion-beam and low-energy positron techniques

    J. Keinonen   E. Rauhala   J. R?is?nen   K. Saarinen   P. Hautoj?rvi   C. Corbel  

    Ion-beam and low-energy positron-beam techniques have been used to study damage and implanted ion distributions and their annealing behavior in semi-insulating GaAs after the room temperature implantation of 3times10 15-1times10 17 60 keV H + cm -2. The redistribution of the implanted H during annealing was observed to be connected to the migration of implantation-induced defect-complexes. A huge increase in the displaced atom concentration in the region of the H concentration was observed after annealings. A monovacancy overlayer, dissociation of H-vacancy complexes, and formation of stable vacancy-H agglomerates were observed in the different parts of the slowing-down region of the implanted H
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  • Hydrogen decoration of defects produced by Na implantation into polycrystalline Cr and Ta

    J. Keinonen   A. Kehrel   K.P. Lieb   M. Uhrmacher  

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  • Velocity dependence in low-velocity electronic stopping power of heavy ions

    J. Keinonen   A. Kuronen   M. Hautala   V. Karttunen   R. Lappalainen   M. Uhrmacher  

    The velocity dependence in the low-velocity electronic stopping power of heavy ions has been studied for Al and Si ions in Ta in the velocity region 0.4nu 0-4nu 0 (nu 0 is the Bohr velocity). The first experimental information is obtained from the range distribution and Doppler-shift-attenuation data
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  • Short lifetimes in 24Mg for test of rotational collectivity in shell-model wave functions

    J. Keinonen   P. Tikkanen   A. Kuronen   á.Z. Kiss   E. Somorjai   B.H. Wildenthal  

    Mean lifetimes of levels in 24Mg have been measured using the Doppler-shift-attenuation method in conjunction with the reactions 23Na( p,gamma) 24Mg and 12 C( 14N,pn) 24Mg. The lifetime values of limits were determined for 27 bound levels below the excitation energy of 11 MeV; the lifetimes of 3 levels are reported for the first time. For the effective stopping of recoils, the targets were prepared by implanting 23Na and 12C into Ta backings. The Monte Carlo method and the experimental stopping power were used in the DSA analysis. The experimental transition strengths are compared with recent shell-model calculations
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  • Effective small-volume [18O]water target for the production of [18F]fluoride

    J. Keinonen   A. Fontell   A.-L. Kairento  

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  • Diffusion of nitrogen in ion-implanted chromium and tungsten

    J. Keinonen   J. R?is?nen   A. Anttila  

    The diffusion of N in the group VI B metals Cr and W has been studied in the previously uninvestigated temperature ranges 300deg-550degC (Cr) and 600-800degC(W) using ion-beam techniques. Diffusion couples were created by ion-implantation. The time-dependent diffusion profiles were monitored by the use of the Nuclear Resonance Broadening (NRB) technique. The linear Arrhenius plots extracted from the measured diffusivities indicate that the diffusivity of implanted N in Cr and W can be described by the activation energy Q=1.39plusmn0.06 eV and 2.32plusmn0.16 eV and the pre-exponential factor D0=(7.0plusmn7.2)times10 -4 cm 2/s and 4.3plusmn8.3 cm 2/s, respectively. The solubilities of N in Cr and W from the implanted distributions were found to deviate from those obtained using conventional metallographical methods
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  • Isoscalar E2 strengths in the A = 34 mass triplet

    J. Keinonen   A. Luukkainen   M. Bister  

    The mean lifetimes tau=47plusmn5 and 7.5plusmn1.7 fs of the 2.16(J pi=2 1+, T=1) and 3.38(2 2+,1) MeV states in 34Cl( Tz =0), respectively, have been measured using the Doppler-shift attenuation method and the reaction 33S(p,gamma) 34 Cl. The relative intensities of the isoscalar E2 transitions 2.16(2 1+,1)rarr0(0 +,1) and 3.38(2 2+,1)rarr0(0 +,1) MeV have been determined to be (16.3plusmn0.5)% and (2.1plusmn0.7)%, respectively. The transition strengths are compared with the analogue transitions in 34S( Tz=+1) and 34Ar( Tz=-1) as well as with theoretical calculations
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  • Coulomb excitation and lifetime measurements of the 21+ states in 76, 82, 84Kr

    J. Keinonen   K.P. Lieb   H.P. Hellmeister   A. Bockisch   H. Emling  

    A mean lifetime of tau=35plusmn3 ps of the 2 1+ state in 76Kr has been measured with the recoil distance method via the reaction 63Cu( 19F,alpha2n) 76Kr. The B(E2; 0 1+rarr2 1+) values and lifetimes of the 2 1+ states in 82,84Kr have been measured via Coulomb excitation using the 1.4 MeV/ A UNILAC krypton beams. The intensities of the gamma-rays from the Coulomb excited levels of 82,84Kr were interrelated with those of the target nuclei 27Al, 64,66 Zn and 70,72,74,76Ge and yielded the values B(E2; 0 1+rarr2 1+ )=0.255plusmn0.009 and 0.122plusmn0.005 e2.b 2 for 82,84Kr, respectively. In turn, these B (E2) values and the B(E2; 0 1+rarr2 1+) values of the even Ge and Zn isotopes from the literature were used in a Doppler-shift attenuation analysis to obtain experimentally lacking electronic stopping power for Kr ions slowing down in Al, Zn and Ge, for Ge ions in Ge and for Zn ions in Zn
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  • Diffusion of nitrogen in vanadium and niobium

    J. Keinonen   J. R?is?nen   A. Anttila  

    The diffusion of N in the group VB metals V and Nb has been studied in the previously uninvestigated temperature range 300-500degC using ion-beam techniques. Diffusion couples were created by ion implantation. The time-dependent diffusion profiles were monitored by the use of the nuclear resonance broadening (NRB) technique. New values for the solubility of N in Nb were obtained. The diffusion rates presented support recent observations of the diffusivity of interstitial impurities in body-centered cubic metals in which positive deviations from Arrhenius behaviour have been seen at high temperatures
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  • Hydrogen burning of 24, 25, 26Mg in explosive carbon burning

    J. Keinonen   S. Brandenburg  

    The absolute resonance strengths of the 24-26Mg(p,gamma) 25-27Al reactions have been studied. The absolute values Sc.m.=1.03plusmn0.11, 2.3plusmn0.2 and 2.2plusmn0.2 eV for the frequently used standard resonances at Ep=823, 434 and 454 keV in the reactions 24-26Mg(p,gamma) 25-27Al, respectively, have been determined. In addition, absolute values for use as standards were determined for the 684 keV 25Mg(p,gamma) 26Al and 840 and 1966 keV 26Mg(p,gamma) 27Al resonances. New relative values were determined for the strengths of the 25 Mg(p,gamma) 26Al resonances at Ep=0.3-1.7 MeV and the 26Mg(p,gamma) 27Al resonances at Ep =0.3-2.1 MeV. The resonance energies and total widths obtained in the present work are also given. The branching ratios and angular distributions of the gamma-rays used in the absolute strength determinations were obtained in the same set-up as the absolute strength measurement. Using the revised resonance strengths, the astrophysical reaction rates of hydrogen burning of 24-26Mg in explosive carbon burning in the stellar temperature region of T=(0.1-5.0)times10 9K are compared with Hauser-Feshbach calculations. The present reaction rates are also discussed in the frame of the MgAl cycle
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  • Annealing behaviour of C-implanted α-Hf

    J. Keinonen   J. R?is?nen  

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  • First-principles simulation of collision cascades in Si to test pair-potentials for Si-Si interaction at 10 eV–5 keV

    J. Keinonen   A. Kuronen   K. Nordlund   R.M. Nieminen   A.P. Seitsonen  

    Interatomic potentials for Si-Si interaction are tested at energies of 10 eV-5 keV for Si ions in ion-beam amorphized Si by simulating range distribution data with the molecular dynamics method. The range profile of 1times10 16 10 keV 30Si + cm -2 implanted into originally crystalline silicon was measured using a nuclear reaction technique. An interatomic repulsive potential from first-principles calculations is proposed for the Si-Si interaction. The dependence of the number of vacancies produced in low-energy collision cascades on the potential is demonstrated by simulations of the cascade dynamics
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  • Analogue E2 transitions in the A = 26 nuclei

    J. Keinonen   A. Anttila   A. Luukkainen  

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  • Gamma-transition strengths of T = 1 states in 10B

    J. Keinonen   A. Anttila  

    The 1.74 ( Jpi=0 1+, T =1) and 5.16 (2 2+, 1) MeV states in 10 B have been studied through the reaction 6Li(alpha,gamma) 10B. The lifetimes tau m =7plusmn3 and <6 fs, respectively, have been obtained with the DSA method. The gamma-ray decay scheme of the 5.16 MeV state has been revised and the ambiguous isoscalar E2 branch to the 1.74 MeV state has been clarified and shown to have an intensity of (0.7plusmn0.2)%. The angular distributions of gamma-rays from the 5.16 MeV state have been measured and mixing ratios determined. The M1 transition 1.74(0 +,1)rarr0.72(1 +,0) MeV is compared with the analogue beta-decay from the ground state of 10C(0 +,1) to the 0.72 MeV state and is discussed in terms of shell-model calculations. The E2 strength of the 5.16(2 +,1)rarr1.74(0 +,1) MeV isoscalar transition is compared with its analogues in the neighbouring T=1 nuclei 10B and 10C in the light of isospin selection rules
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  • Lifetimes of the unbound 4?1, 5+1 and 3+2 states in 14N

    J. Keinonen   A. Anttila   M. Bister  

    The lifetimes 18.8plusmn1.1, 105plusmn5 and 13plusmn5 fs of the unbound 8488 (4 1-), 8961 (5 1+ ) and 9130 (3 2+) keV states in 14N, appearing as midget resonances at Ep=1012, 1523 and 1701 keV in the 13C(p,gamma) 14N reaction, respectively, have been determined for the first time with the improved DSA method. In the DSA analysis the experimental correction factors of the nuclear and electronic stopping powers were used in combination with Monte Carlo calculations. By the use of the new lifetimes and revised branching ratios, the transition strengths are discussed in terms of shell-model calculations
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