Lifetimes of high-spin levels in39Ar

The high-spin states in³⁹Ar at 2651, 3992, 4543, and 5536 keV were excited in oxygen-induced fusion-evaporation reactions. Mean lifetimes ofτ _m =1.0±0.2, 1.2±0.3, 1.6±0.3, and <1 ps were determined for these levels, respectively, by means of the recoil distance Doppler shift technique. The transition rates are discussed in terms of the Bansal-French weak-coupling model.     

Dynamics of photoluminescence in GaInNAs saturable absorber mirrors

Femtosecond time-resolved photoluminescence experiments have been used to study the nonlinear dynamics of novel monolithic GaInNAs/GaAs semiconductor saturable absorber mirrors at 1.08 and 1.55 m. The mirror structures were grown using molecular-beam epitaxy followed by Ni-ion implantation and thermal annealing. We present photoluminescence measurements showing the critical role of post-growth processing on the response time of GaInNAs/GaAs absorber mirrors. PACS 42.65.Re; 42.50.Nn; 42.70.Nq     

Self-starting stretched-pulse fiber laser mode locked and stabilized with slow and fast semiconductor saturable absorbers

A self-starting stretched-pulse mode-locked erbium-doped fiber laser that uses fast and slow semiconductor saturable absorbers is described. By using two absorbers, we obtained reliable operation at a fundamental repetition rate with 250 pJ of pulse energy without multiple-pulse breakup. External chirp compensation was used to compress the highly chirped pulses to durations of 135 fs.     

Average g-factors for high-spin states in 78Kr

Inverse reactions of ^{63, 65}Cu beams on ^{18, 16}O targets have been used to populate states of ⁷⁸Kr by fusion-evaporation reactions. The excited nuclei recoiled at high velocity v/c ≈ 7 % through a polarized iron (⁵⁴Fe) layer and were stopped in a copper layer. During the period in iron, 0.05–0.65 ps, the nuclei were subjected to the intense transient magnetic field (initially ～ 3500 T). The resulting precession of the high-spin nuclear states populated during this time was determined by measuring the time integral rotation angle of the discrete γ-ray transitions at low spin.The average g-factor at low spin 2 ≦ J ≦ 8 compared to that at higher spin 8 ≦ J ≦ 12 in ⁷⁸Kr was found to be identical within the experimental uncertainties of ～ 15 %. This result implies that either there are no rotational alignment effects at the backbend in ⁷⁸Kr or more plausibly, proton (g ≈ 1) and neutron (g ≈ 0) aligned bands are equally competitive and both populated in the reaction. It is then likely that the resulting g-factor represents an average over many populated proton and neutron aligned bands.     

Hydrogen burning of 24, 25, 26Mg in explosive carbon burning

The absolute resonance strengths of the ^24–26Mg(p, γ)^25–27Al reactions have been studied. The absolute values S_c.m.=1.03 ± 0.11, 2.3 ± 0.2 and 2.2 ± 0.2 eV for the frequently used standard resonances at E_p=823, 434 and 454 keV in the reactions ^24–26Mg(p, γ)^25–27Al, respectively, have been determined. In addition, absolute values for use as standards were determined for the 684 keV ²⁵Mg(p, γ)²⁶Al and 840 and 1966 keV ²⁶Mg(p, γ)²⁷Al resonances. New relative values were determined for the strengths of the if²⁵Mg(p, γ)²⁶Al resonances at ifE_{p = 0.3–1.7 MeV} and the ²⁶Mg(p, γ)²⁷Al resonances at E_p = 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 γ-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) × 10⁹ K are compared with Hauser-Feshbach calculations. The present reaction rates are also discussed in the frame of the MgAl cycle.     

Lifetimes of low-lying levels in 18F

Mean lifetimes of levels in ¹⁸F have been measured using the Doppler-shift attenuation method and the inverse reaction ³He(¹⁶O, p)¹⁸F. Targets of ³He implanted into Al, Nb, and Au foils were employed in the measurements. The Doppler-broadened lineshapes observed at 0° to the beam were analyzed to obtain the following lifetime values: 0.971 ± 0.030, 0.605 ± 0.029 and 0.435 ± 0.041 ps for the 1.70(1⁺), 2.52(2⁺) and 3.36(3⁺)MeV members of the K^π = 1⁺ rotational band, 5.12 ± 0.56, 0.403 ± 0.018 and 1.91 ± 0.17 ps for the 2.10(2^?), 3.13(1^?) and 3.79(3^?) MeV members of the K^π = 0^? bands, and 〈1.2, 2.7^+4.1_?2.7 and 20 ± 2 fs for the 3.06(2⁺, T = 1), 3.72(1⁺) and 3.84(2⁺) MeV states, respectively.     

Coulomb excitation and lifetime measurements of the 21+ states in 76, 82, 84Kr

A mean lifetime of τ = 35 ± 3 ps of the 2⁺₁ state in ⁷⁶Kr has been measured with the recoil distance method via the reaction ⁶³Cu(¹⁹F, α2n)⁷⁶Kr. The B(E2; 0⁺₁ → 2⁺₁) values and lifetimes of the 2⁺₁ states in ^{82, 84}Kr have been measured via Coulomb excitation using the 1.4 MeV/A UNILAC krypton beams. The intensities of the γ-rays from the Coulomb excited levels of ^{82, 84}Kr were interrelated with those of the target nuclei ²⁷Al, ^{64, 66}Zn and ^{70, 72, 74, 76}Ge and yielded the values $\text{B(}\text{E}\text{2; 0}{}^{\mathrm{+}}{}_1\text{→ 2}{}^{\mathrm{+}}{}_1\text{= 0.255±0.009}\text{and}\text{0.122 ± 0.005}\text{e}\text{2 ·}\text{b}\text{2}\text{for}{}^{\mathrm{82,\; 84}}\text{Kr}$, respectively. In turn, these B(E2) values and the (E2; 0⁺₁ → 2⁺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.     

Defects in H implanted GaAs studied by ion-beam and low-energy positron techniques

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 3 × 10¹⁵−1 × 10¹⁷ 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.     

Diffusion of nitrogen in vanadium and niobium

The diffusion of N in the group VB metals V and Nb has been studied in the previously uninvestigated temperature range 300–500 °C 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.     

Large transient magnetic fields for rare-earth ions recoiling in gadolinium and g-factors of high-spin states in 156, 158, 160Gd

We have determined the transient magnetic field for Coulomb-excited rare-earth nuclei recoiling with velocities in the range between 0.7ν₀ and 6ν₀ into ferromagnetic gadolinium cooled to a temperature T = 80 K. Measured and calculated g-factors in ¹⁶⁹Tm have been used as calibration standards. The transient field is found at first to increase with increasing recoil velocity, and then to level off, approaching a nearly constant value of 5.5 kT at ν = 6ν₀. At the higher velocities (3ν₀ < ν < 6ν₀) the transient fields for ¹⁶⁹Tm recoils in gadolinium are a factor of 1.42 ± 0.12 larger than those in iron, whereas the densities of polarized electrons are the same in both ferromagnets. This result demonstrates that an explanation of the transient field must take into account the atomic structure of the host (and probably also that of the recoil). The transient field is too large to be described only in a statistical picture in which inner-shell (ns) vancancies are filled by capture of polarized (4f) electrons. Possible mechanisms may involve either polarization transfer from the outer shells by spin-flip interactions, or direct vacancy polarization by diabatic molecular orbitals.The transient field calibration has been corroborated making use of known g-factors of low-spin states in ^{156, 158, 160}Gd populated by Coulomb excitation of thick Gd single crystals. For the high-spin states in these nuclei, the g-factors are found to decrease slightly, with the ratio $\text{g(10}{}^{\mathrm{+}}\text{)}\text{g(2}{}^{\mathrm{+}}$) reduced to 0.89±0.12, 0.83±0.11, and 0.93±0.13, respectively. Similar decreases have been observed previously for other N = 90?96 nuclei.