Positron lifetimes in five phases of BEA

Positron lifetimes have been measured as a function of temperature in 4-butyloxybenzal-4′-ethylaniline (BEA). BEA has been previously reported to have two liquidcrystalline phases (smectic and nematic) with transition temperatures as follows: solid → smectic, 40.5° C; smectic → nematic, 51.0° C; and nematic → isotropic liquid, 65.5° C. Positron life time spectra were resolved into two components, with the shorter component τ₁ remaining approximately constant in all phases, and τ₂ exhibiting reversible changes at all of the above transitions. In addition, an irreversible discontinuity in the τ₂ lifetime was observed in the vicinity of 28° C, indicating the presence of a new phase (phase X) of BEA. The τ₂ andI ₂ values obtained for the various phases of BEA are: solid (1.25 nsec, 7.1%), phase X (1.97 nsec, 26.2%), smectic (2.36 nsec, 23.6%), nematic (2.72 nsec, 28.3%), and isotropic liquid (2.69 nsec, 29.8%).     

Bestimmung der Lebensdauer des 3P-Zustands von ionisiertem Helium aus der Linienbreite von Hochfrequenzübergängen

The dependence of the half-widths of 3S _1/2(m=?1/2)?3P _1/2(m=?1/2)-transition signals in ionized helium on rf power has been measured. By linear extrapolation of the squared half-widths towards zero power, a value of τ_3P =(0.324±0.011) nsec for the lifetime of the 3P-state has been deduced (theoretical value 0.329 nsec).     

Resonanzstreuung am 202,8 keV-Niveau im J127

Using a gaseous Xe¹²⁷ source as exciting radiation resonance fluorescence from the 202.8 keV level in I¹²⁷ has been observed. The detection of the weak resonance effect in the presence of a strong 375 keV radiation was possible because of the high energy resolution of a Li-drifted Germanium detector. Assuming a transition energy ofQ _ECO =543 keV for the Xe¹²⁷-I¹²⁷decay the mean lifetime of the 202.8 keV level was determined to be τ=(0.56±0.09)nsec.     

Positron-annihilation investigation of high-temperature neutron-irradiated molybdenum

Molybdenum samples have been irradiated in EBR II at temperatures ranging from 600 to 1500°C. Both lifetime and Doppler-broadening measurements of the positron annihilation characteristics have been performed and the effects of voids on these measurements determined. Good correlation of both measurements with the suspected swelling behavior has been obtained consistent with a peak in the swelling between 0.3 and 0.4 T_m, where T_m is the melting-point temperature. These positron techniques appear to be a much more sensitive probe of void swelling than other investigative methods. At temperatures greater than 600°C, a change in the positron lifetime is attributed to the possible arrival of impurities at the void surface. A third, as yet unresolved lifetime component, appears to be present that causes the values of τ₁, the shorter lifetime associated with the free-lattice annihilations, to be too large relative to an annealed sample. A possible explanation is the presence of radiation-induced dislocation loops that can act as additional positron trapping sites.     

用正电子湮没技术研究形变铁的恢复

本文介绍应用正电子湮没寿命和多普勒加宽方法研究60％形变铁等时退火各阶段中缺陷的恢复行为,实验结果说明正电子湮没平均寿命τ及多普勒加宽的S参数都是退火温度的函数,完整晶体中湮没寿命τ_f=111±lPS,缺陷中湮没寿命τ_d=162±lps,本文还根据捕获模型计算了正电子的捕获速率及τ₁,结果证明与理论预期的完全一致,经计算得出捕获正电于的相对缺陷浓度的范围为10^-7-10^-4数量级。 关键词：     

Lebensdauer des 5s5p 3 P 1-Zustands von Strontium

The lifetime of the 5s5p ³ P ₁ state of Sr has been determined with optical double resonance by observation ofrf-transitionsΔm _J =±1 between Zeeman levels of the even isotopes. The measurement yields the value τ(5³ P ₁, Sr)=2.1(3) · 10^?5 sec. A new type of narrowing of optical double resonance signals due to a time-of-flight effect of the radiating atom has been observed and compared with the theory.     

Lifetime of the Z1 centre in KC1:Sr

Data on the radiative lifetime τ(Z₁) of the relaxed excited state of the Z₁ centre in KC1:Sr in the 63–135 K range are reported for the first time [τ(Z₁)= 194 ± 10 nsec at 63 K]. The activation energy of deexcitation (ΔE = 0.132 ± 0.008 eV) for the luminescence process is in agreement with the value obtained by Paus from a quantum yield analysis.The results are discussed on the basis of an analogy with the F-centre behaviour.     

Excited state absorption dynamics of fast relaxing dye no. 5

The excited state absorption dynamics of the fast mode-locking dye No. 5 for Nd-glass lasers is investigated. The excited state absorption cross-section σ_ex for 1.053 μm picosecond pulse excitation is determined from energy transmission measurements. The excited state absorption populates a higher lying singlet state S₄. The population lifetime τ₄ of this state is determined from two-step excited fluorescence quantum yield measurements. The obtained values are σ_ex= (5.5±1)×10^-17 cm² and τ₄=60±20 fs.     

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.     

Helium bubbles in neutron-irradiated copper-boron studied by positron annihilation

The influence of helium, introduced by the ¹⁰B(n, α)⁷Li reaction, on the evolution of defect structure in copper containing a few hundred ppm boron has been studied by detailed positron lifetime and two-photon angular correlation measurements, supplemented by TEM studies. In the as-irradiated state of Cu-B, two lifetime components have been resolved. The shorter lifetime, τ₁, = 167 ps of 97% intensity, has been understood as due to positron trapping at small helium-vacancy complexes, while the longer lifetime τ₂ = 450 ps of 3% intensity is explained as due to helium-free voids. Marked changes in the annihilation characteristics observed at 670 K are interpreted in terms of the nucleation of microbubbles, controlled by thermally activated helium migration to vacancy traps. Corroborative evidence for the onset of helium clustering is obtained from the change in the average size of positron traps as deduced from the smearing of the measured angular correlation spectra. Helium bubbles and helium-free voids coexisting in the system have been distinguished by a three-component analysis of the lifetime spectra. Bubbles are found to be stable beyond the temperature of dissociation of voids. The size and concentration of bubbles, determined independently by TEM measurements, are in accordance with the positron annihilation results in the growth stage. The observed positron lifetime at higher annealing temperatures has been analysed by relating the annihilation rate to helium atom density and helium pressures in bubbles evaluated. These pressures are in satisfactory agreement with the estimates of equilibrium pressures, leading to the conclusion that bubble relaxation occurs by the mechanism of thermal vacancy condensation.     

用正电子湮没技术研究USY沸石的“二次孔”结构

测量了NaY及USY沸石的正电子寿命谱．得到NaY沸石有四个寿命分量：两个短寿命分量和两个长寿命分量．两个长寿命分量中，τ₃，τ₄分别是o-Ps在β笼和超笼中的湮没．USY沸石有五个寿命分量：两个短寿命分量和三个长寿命分量，三个长寿命分量中，τ₃为o-Ps在笼中的湮没，τ₄，τ₅分别为o-Ps在“二次微孔”和“二次中孔”中的湮没．与NaY沸石相比，在真空中，USY的τ₄增加到 关键词：     

Hyperfine structure of the 4s4p1P1-state of67Zn by level crossing spectroscopy

The magnetic dipole coupling constantA of the 4s 4p¹P₁ state of⁶⁷Zn was measured in a level crossing experiment. The result is A(4s4p¹P₁ ⁶⁷Zn)= 17.7(5) MHz. In a modified Breit-Wills theory thisA factor leads to a parameter λ_P= 0.71(3), which describes the difference in the radial wave functions of thep-electron in 4s4p¹P₁ and³P₁ states respectively. The lifetime of the 4s4p¹P₁ state was remeasured by Hanle-effect experiments. The result τ=1.41± 0.04 nsec is in good agreement with other measurements.     

Determination of the lifetime of the 4p 1/2-state of potassium by Hanle-effect

The lifetime of the first excited 4P _1/2-state of potassium has been determined to be τ=27.3(3) nsec by investigating the intensity of the scattered resonance light of the potassiumD ₁-line as a function of a magnetic field. From this result the absorption oscillator strength of theD ₁-line of potassium could be deduced to bef=0.326(4).     

Zum Elektroneneinfang des185Os

The relative probabilities forK-capturePK ₉₃₁/PK ₆₄₆=0.109±0.005 andPK ₈₈₀/PK ₆₄₆=0.708±0.010 were measured yielding a total transition energyQ _EC =(1015.0 ±0.7)keV andQ _EC =(1016±3)keV, rsp. The reported 5 keV-transition in the decay of¹⁸⁵Os between the 880 keV- and 875 keV-level was not confirmed by comparing the measured relativeK-capture probabilityPK ₈₇₅/PK ₆₄₆=0.732±0.007 with the calculated value. This is in accord with the measured upper limit for the lifetime of the 880 keV-level,τ<3 nsec.     

New measurement of the neutron lifetime with a large gravitational trap

The lifetime of the neutron is one of the key physical quantities used to determine the weak interaction parameters and to test predictions of the theory of primary nucleosynthesis. The lifetime of the neutron has been measured in the reported experiment by the method of storing neutrons in a material trap with a gravitational valve. Fomblin grease UT-18 hydrogen-free fluorine polymer has been used as coating. The resistance of the coating to repeated cooling down to 80 K combined with heating up to 300 K has been studied. The probability of losses in the trap is as small as 1.5% of the neutron decay probability. The lifetime of the neutron τ_n = (881.5 ± 0.7_stat ± 0.6_syst)s obtained at the new step is in good agreement with a commonly accepted value of (880.2 ± 1.0) s presented by the Particle Data Group.     

Richtungskorrelation für Kernresonanzfluoreszenz und Lebensdauer des 412 keV-Niveaus von198Hg

Using the method of thermal Doppler-broadening of the emission line, the effective differential cross sections for nuclear resonance scattering for 412 keV photons by¹⁹⁸Hg has been measured for well defined scattering angles of 120° and 160°. The results indicate a partial mean lifetime for this energy level of τ_γ=(5·12±0·23)·10^?11 sec. This result differs appreciably from the values reported by other workers. Possible reasons for these discrepancies are discussed. The ratio of the differential cross sections for resonance scattering at angles of 120° and 160° is consistent with a 0-2-0 transition.     

Flame spectroscopy of TiO: Radiative lifetimes and oscillator strengths of the α (C3Δ-X3Δ) system

Lifetimes of the C³Δ state of TiO have been measured for vibrational levels v′ = 0 to 2 from the decay of photoluminescence excited in a chemiluminescent flame by a short-pulsed (5 nsec) tunable dye laser. Radiative lifetimes of τ_v′=0 = 37 ± 9 nsec, τ_v′=1 = 29 ± 7 nsec, τ_v′=2 = 28 ± 7 nsec were obtained and were used to estimate oscillator strengths which were found to be in good agreement with earlier values based on intensity measurements.     

Evidence for the coexistence of spherical and deformed states in38Ar

The³⁸Ar levels at E_x=5350, 7289 and 9341 keV have been investigated using the³⁵Cl(α,pγ) reaction at E_α=l4 and 14.5 MeV. From particle-γ-ray angular correlations the spin assignments J(5350)=4, J(7289)=6,4 and J(9341)=8,6,4 have been obtained. Life-time measurements using the Doppler-shift attenuation method yielded τ(9341)=106±25 fs and τ(7289)=77±30 fs, while the lifetime τ(5350)=200±50 fs was known previously. All levels have positive parity and decay by enhanced E2 transitions. Hence we propose that they are the J^π=4⁺, 6⁺ and 8⁺ members, respectively, of a K^π=0⁺ rotational band which has the E_x=3377keV, J^π=0⁺ and the 3937 keV, J^π=2⁺ levels as further members. The enhancement of inband E2 transitions is 30 _?6 ⁺¹⁰ W.u. (4→2), 35 _?14 ⁺³⁰ W.u. (6→4) and 20–36 W.u. (8 → 6), respectively, yielding an average intrinsic quadrupole moment Q₀=850 _?125 ⁺²⁰⁰ mb. The moment of inertia is given by h²2θ=92 keV. The present data are in good agreement with the predictions of a deformed state in³⁸Ar that coexists with the spherical states.     

Nuclear resonance fluorescence in121Sb

Usingγ quanta from a⁵⁶Co source, resonance fluorescence from a 3,452 keV level in¹²¹Sb was observed. Four deexcitingγ transitions were identified. The profile of theγ emission line, investigated by means of a high speed centrifuge, yields with a novel variety of Doppler shift analysis a lifetime of τ_4,298=(110±50)fs for theγ emitting 4,298 keV level in⁵⁶Fe. The lifetime of the 3,452 keV level was determined to beτ _3,452 =(200±100)fs.     

Radiative lifetimes and transition moments in MgO

An experimental and theoretical study of several transitions in the MgO molecule is reported. The radiative lifetime of the MgO B¹Σ⁺v'=0 level is measured to be 32.7±1.7 nsec, while that of the electronic state(s) excited by 372 nm radiation (d³Δ and/or f³Π) is 11.8±0.5 nsec. The B-X and A-X transition moments are calculated using state averaged multiconfiguration self-consistent field and configuration interaction methods. With an estimate of the B-A transition moment derived from an earlier calculation, the radiative lifetime of the B¹Σ⁺v'=0 level is computed to be 24 nsec. A value of 0.23 msec is calculated for the lifetime of A¹Πv'=0.