Mössbauer isomer shifts and quadrupole splittings in osmium compounds

From Mössbauer spectra of the 36.2keV and 69.5keV gamma rays of¹⁸⁹Os, isomer shifts and electric quadrupole interactions for a number of chemical compounds of osmium in the oxidation states +8, +6, +4, +3, and +2 were obtained. The results show that the dependence of the isomer shifts on the oxidation state of Os follows the general pattern established for other transition elements like Ru and Ir. A comparison of the isomer shift data with electron densities at the Os nuclei resulting from relativistic self-consistentfield calculations for free Os ions yieldsΔ 〈r²〉 [36 keV]=?2.0 · 10^?3 fm² andΔ 〈r²〉 [69 keV]=?0.13 · 10^?3 fm³ for the changes of the mean-square nuclear charge radius of¹⁸⁹Os. These values are in qualitative agreement with an interpretation of the low-energy level scheme of¹⁸⁹Os in terms of two rotational bands.     

Effects of nuclear polarizability on isotope shifts in electronic atoms

The isotope shift in electronic atoms is calculated in second order perturbation theory. We consider multipole ordersλ=0, 1 and 2 forZ=20–82 nuclei. To a good approximation it is directly proportional to the electron density at the nucleus. We study whether the polarization effect modifies the extraction ofδ〈r ²〉 values from isotope shifts measurements and find it to be small but not always negligible. Relative to the volume effect, the polarizability is of the same order of magnitude for electronic as well as muonic atoms.     

The early iron age of the Mössbauer era

The demonstration that the 14.4 keV level of⁵⁷Fe was an ideal Mössbauer example produced a veritable explosion in the discoveries of new physical phenomena. Among the important features of⁵⁷Fe are its ready availability and ease of preparation, its magnetic properties in many environments, the high intensity of its Mössbauer spectrum even at elevated temperatures, the relative sharpness of the Mössbauer lines, and the possibility of obtaining the resolution provided by this sharpness by relatively simple techniques. This paper hopes to capture some of the excitement and enthusiasm generated by the discoveries and demonstrations of the nuclear Zeeman effect, together with its polarization properties, the nuclear isomer shift, the quadrupole splitting, the temperature effect, temporal effects, nuclear hyperfine fields, transferred magnetism, the gravitational red shift, and many other new or newly elucidated phenomena.     

In-beam study of74Br and a first cascade in75Kr

An in-beam study of⁷⁴Br has been carried out using γ-ray spectroscopy in connection with the reactions (³He,p2in) and (d,2n) on⁷⁴Se targets. Separate level schemes on top of the two β-decaying isomers of⁷⁴Br are proposed. Two cascades connecting highspin states are assigned to populate theI ^π=4⁽⁺⁾ isomer. In this level scheme, the excited states at 72.1, 188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of⁷⁴Br is tentatively assigned asI ^π=(0^?). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (πg _9/2,νg _9/2) and (πg _3/2,νg _9/2). As a by-product, a cascade of four transitions has been observed during the³He bombardment of⁷⁴Se and tentatively assigned to negative-parity states in⁷⁵Kr.     

Mössbauer isomer shift and hyperfine fields of ordered and disordered Fe3Al alloys

The Mössbauer technique was used to measure the hyperfine magnetic field and isomer shift of⁵⁷Fe nucleus in the ordered and disordered Fe₃Al alloys. The Mössbauer spectra were analyzed to yeld the hyperfine magnetic field distribution curves. A linear correlation has been revealed between the average hyperfine field and the average number of Al atoms in the first two nearest neighbour shells of⁵⁷Fe nucleus,N _Al, for both ordered and disordered alloys. The measured values of the mean isomer shift agree very well with the values expected from the Miedema-Van der Woude model.     

Messung der rückstoßfreien Resonanz-Absorption am 6,2 keV-Niveau in181Ta

A strong recoilless resonance absorption was observed for the extraordinarily narrow 6.2 keV level (T _1/2=6.8 μs) of¹⁸¹Ta. Using sources of¹⁸¹W in W-metal and Ta-metal absorbers, a linewidth of about 9 times the natural width was obtained, with an enormous isomer shift of +(0.85±0.02) mm/sec, which is equivalent to a shift by 260 times the natural linewidth. Recoilless resonance spectra of cold worked and of oxygen doped absorbers showed a high sensitivity of the linewidth against lattice imperfections due to inhomogeneous quadrupole interactions and isomer shifts. Measurements of the residual resistivity ratio and of the lattice constant of the impurity doped absorbers yielded a linear correlation between linewidth and isomer shift.     

Temperature dependence of the electric field gradient of204Pb in hexagonal Tl

The temperature dependence of the nuclear quadrupole interaction of²⁰⁴Pb(4⁺) isomer in hexagonal Tl host has been investigated by the TDPAD technique. It is the first reported EFG which does not respect the T^3/2 law fors-p metals. The chosen approach is the independent treatment of the ionic contribution and of the electronic contribution in the approximation of the additivity of the two contributions, avoiding any assumption of a direct temperature dependence for the total EFG.     

Second-forbidden beta-decay and the effect of (V+A)- andS-interaction admixtures:36Cl

The shapefactor, the logft-value, the electron longitudinal polarization and the distribution of electrons emitted from oriented nuclei of³⁶Cl have been calculated by applying two different nuclear models. Firstly a pure 1d_3/2→1d_3/2 transition has been considered. Secondly a more refined shell model of thes-d shell configuration space has been taken into account. It is shown that there is no great influence of the shell model version to the observables, that the observables depend strongly on the relativistic nuclear matrix elements, that the so-called “two parameter equation” for the shapefactor cannot be used in the case of³⁶Cl and that there is no larger sensitivity of (V+A) admixtures to the electron longitudinal polarization than normally found in allowed decays. Comparisons with the existing experimental data are made.     

^3 \overrightarrow {He} (γ, π°)^3 \overrightarrow {He} as the nuclear probeas the nuclear probe

The state of polarization of the recoil nucleus inγ+³He→π°+³He as well as the asymmetry in the differential cross-section when the initial³He is polarized are studied together with the differential cross-section taking into consideration theS, S′ andD-state admixtures in the nuclear wavefunctions. In view of the considerable spin dependence in the photoproduction amplitudes these observables are found to be quite sensitive to the small admixtures ofS′ andD-states in the nuclear wave-functions.     

Colour transparency and scaling properties of nuclear shadowing in deep inelastic scattering

We present the perturbative QCD analysis of nuclear shadowing in the deep inelastic scattering at smallx in terms of the spatial wave function ofq \(\bar q\) fluctuations of virtual photons. The wave function formalism makes it quite obvious that shadowing is the scaling, rather than the higher twist, 1/Q ², effect, contrary to a numerous recent claims. We demonstrate explicitly how the scaling shadowing comes from the large, hadronic, size quarkantiquark pairs even in the limit ofQ ²→∞, and why it should very slowly, ∞1/log (Q ²/m ²) decrease at very largeQ ². We argue in favor of the scaling triple-pomeron contribution to the nuclear shadowing and present predictions for a cross section of diffraction dissociation of virtual photons and for the mass spectrum of diffraction excitation, which can be checked at HERA and Fermilab. We predict strikingly different scaling properties of diffraction dissociation and nuclear shadowing for the longitudinal and transverse photons. Our, numerial predictions for shadowing are in good agreement with the recent EMC data.     

Measurement of the Σ−→ne −¯v and Σ−→ nμ−¯v rates

The recoilless nuclear resonance absorption of the 90 keVγ-rays of⁹⁹Ru was observed in a variety of chemical compounds of ruthenium in the oxidation states +2 through +8 at 4.2 ° K. For most of the compounds isomer shifts increasing monotonically with the oxidation state of ruthenium were found. This behaviour, which may essentially be attributed to the decreasing number of 4d electrons, and the few exceptions from it are discussed qualitatively on the basis of the molecular orbital theory.     

Measurement of the average polarization of12B in the polarized-muon capture by12C: Magnitude of the induced pseudoscalar form factor

The average polarization of the nucleus¹²B (bound state) produced in the polarized-muon capture reaction¹²C(μ ^?,v _μ )¹²B was measured by the muon spin resonance method in a pulsed muon beam. The polarization of¹²B was maintained completely under a magnetic field of 3 kG in a Grafoil target. After the observed result was corrected for the contribution from the excited states of¹²B, P_av (ground state) was deduced to be 0.462±0.053. This result yieldsg _P /g _A =10.1 _?2.6 ^+2.4 , which is almost consistent with the PCAC prediction.     

A study of coupled-reaction channel effects in the36S +37Cl system,hybridization between single particle orbits

Elastic and inelastic scattering as well as transfer transitions involving a valence proton in thesd- andfp- shell orbits are studied in the interaction of³⁷Cl +³⁶S at E_CM=50 MeV. Experimental angular distributions of single particle states of³⁷Cl (elastic and inelastic transfer) are presented with a CRC analysis. In the CRC calculations the effects of inelastic and transfer couplings are studied using known spectroscopic information. In the CRC analysis six single particle bound states and the collective 2⁺ excitation of³⁶S are included in the coupling scheme. Higher order coupling effects are found to be important. A distinct effect, the mixing of single particle states (of different parity) due to direct and transfer interactions is observed to produce an enhancement of the transfer cross section to the low lying states. This feature is due to a change of the asymptotically defined single nucleon orbits via polarization in the field of the other nucleus, an effect which is analogous to hybridization, known from atomic physics.     

Der Einfluß der Nullpunktschwingungen der Kernoberfläche auf die Formfaktoren der inelastischen Elektronenstreuung

The transition densities from the ground state to exited states of atomic nuclei are calculated from the drop model, taking the zero-point oscillations of the nuclear surface into account. The calculation yields transition densities smeared out over a distance of 2–3 fm near the nuclear surface. In the case ofn-surfon transitions the radial dependence of these densities is proportional to then-th derivative of the ground-state density. The numerical evaluation yields inelastic form factors quite similar to those ofCrannell et al. Compared with the form factors ofWalecka, the higher maxima are less distinct.     

Precise determination of hyperfine interactions and second-order doppler shift in <Superscript>149</Superscript>Sm Mössbauer transition

We have succeeded in precisely determining the hyperfine interactions, particularly the isomer shifts, in the ¹⁴⁹Sm Mössbauer transition. The difference in the nuclear radii between the ground and excited states is critical for the determination of isomer shifts but is relatively small in ¹⁴⁹Sm. Therefore, the precise determination by ¹⁴⁹Sm Mössbauer spectroscopy is difficult. The recent development of synchrotron-radiation-based Mössbauer spectroscopy allows the isomer shifts to be determined more precisely than previously with the help of wellcollimated synchrotron radiation. In particular, the time-window effect assists the precise determination of hyperfine interactions in the ¹⁴⁹Sm Mössbauer transition because this effect enables us to measure spectra with higher energy resolution than natural linewidth determined by the lifetime of the excited states. Meanwhile, highenergy-resolution measurements to determine center shifts by SR-based Mössbauer spectroscopy enable us to observe the second-order Doppler shift, which has not been discussed, particularly for heavy Mössbauer nuclei. We have discussed the precise determination of isomer shifts and the observation of the second-order Doppler shift using ¹⁴⁹Sm synchrotron-radiation-based Mössbauer spectroscopy.     

Relation of contact densities with Mössbauer isomer shifts for 35.46 keV M1 transition of125Te

The internal conversion electron and Mössbauer isomer shifts associated with the 35.46 keV M1 transition of¹²⁵Te were observed for different metal samples into which radioactive¹²⁵I as probe atoms were introduced by means of ion-implantation. From the correlation between the Mössbauer isomer shifts and intensity ratios of O shell to N_I shell conversion electrons, a relation between 5s-electron contact densityp _5s(0) in a.u. and isomer shift δ in mm/s was deduced to bep _5s(0)=106+30.3 δ±4.3[(δ?0.30)²+0.069]^1/2, where δ was measured relative to ZnTe in mm/s. The change of the nuclear charge radius in the 35.46 keV M1 transition of¹²⁵Te was found to be ΔR/R=(0.85±0.12)×10^?4 (corresponding to Δ<r ²>=(3.7±0.5)×10^?3 fm²) when a theoretical 4s-electron contact density of 928 a.u. was used.     

Isomeric yield ratio of fission product148Pr in235U(n th,f)

The independent isomeric yield ratio of¹⁴⁸Pr in thermal neutron induced fission of²³⁵U has been determined experimentally. The fission product¹⁴⁸Pr isomers, extracted directly by on-line mass separation technique, have high-spin (J=4) to low-spin (J=1) isomer ratio of 0.14±0.04 using growth and decay analysis. Statistical model calculation of isomeric yeild ratio using constant initial r.m.s. angular momentumJ _rms can not reproduce either present results or other recent measurements of isomer ratios. TheJ _rms derived from isomer ratio data in all thermal fissioning systems indicate a wide spread ranging from 2? to 13?. No clear correlation betweenJ _rms and isomeric spins or number of neutrons of isomers is found, thus, more model refinements and experimental works should be done in order to evaluate independent isomeric yields correctly.     

Isomer shift calibration of61Ni by lifetime variation measurements in the electron capture decay of57Ni

In the present work, differential ionization chambers are used in order to measure the difference in the electron capture decay rate of⁵⁷Ni nuclei in several pairs of different chemical compounds. Combination of these values with the corresponding Mössbauer isomer shifts provides a reliable experimental method to determine differences in electron densities at the nucleus. This allows to calibrate the isomer shift scale of the concerned nuclide. By using⁶¹Ni isomer shifts given in the literature, this work leads to the first experimental determination of the⁶¹Ni isomer shift calibration constant α=?(1.8±0.9)·10^?3.a ₀ ³ mm.s^?1. This corresponds to a change in the mean-square charge radius Δ(r ²)=?(7.2±3.4)·10^?4 fm² during the 67.4 keV Mössbauer transition of⁶¹Ni.     

Gamma deexcitation of the 180m Ta isomer

Vibrational states built on the K ^π = 9^? isomer and on the ground state (K ^π = 1⁺) in ¹⁸⁰Ta are calculated within the quasiparticle-phonon nuclear model using the ¹⁷⁸Hf nucleus as a core. A procedure for calculating the rates of K-allowed γ-ray transitions from vibrational states built on the isomer to those built on the ground state is presented. The probabilities of two-step processes consisting of a dipole excitation of the isomer and successive E1 and E2 transitions from them to vibrational states built on the ground state of the ¹⁸⁰Ta nucleus are calculated. Two-step transitions from the isomer to vibrational states below 2.7 MeV and to the vibrational states built on the ground state appear to be very weak. There are many E1 transitions from the vibrational states built on the isomer to the vibrational states built on the ground state. They are weak and cannot be responsible for the strong deexcitation of ^180m Ta in the relevant (γ, γ′) reaction. A decisive role is played by collective E2 transitions from dipole excitations in several excitation energy intervals ranging between 2.7 and 4.0 MeV. These highly intense K-allowed two-step γ-ray transitions can be responsible for the strong deexcitation of the ^180m Ta state in the (γ, γ′) reactions.     

Measurement of parity admixtures in nuclear forces in180Hf,181Ta and203Tl

The parity mixing in nuclear states was studied by measuring the circular polarizationP of the γ-radiation emitted from unpolarized nuclei. The analyzer was a “radial transmission magnet” of high symmetry. With 4 NaI scintillation counters and the current integration method 4 independent results were obtained simultaneously. The polarimeter efficiency was determined via the Quadratic Compton effect. Studies of the polarized bremsstrahlung with¹⁷⁰Tm,¹⁷⁷Lu and¹⁹⁸Au sources served as tests for the calculated corrections due to the bremsstrahlung effect. Our results for the circular polarization are:P=?(2.0±0.4) · 10^?3 for the 501 keV transition in¹⁸⁰Hf,p=+(1±4) · 10^?6 for the 482 keV transition in¹⁸¹Ta andP=+(1±7) · 10^?6 for the 279 keV transition in²⁰³Tl. Thus, in the case of¹⁸⁰Hf, the existence of the parity mixing in nuclear states was confirmed.