Dipole Decay Research Articles

On the decay of compound nuclei following alpha-particle and12C induced reactions

Multiple coincidence rates have been measured using a detector system consisting of a Ge(Li) spectrometer and eight NaI(Tl) or eight liquid scintillators. Reactions induced byα-particles with energies of 51–55 MeV and 118 MeV12C ions are studied. The data are analysed to give the first and second central moments of the distribution of the number ofγ-rays feeding individual levels in the final nuclei. When these numbers are compared to spin distributions calculated with the statistical model code GROGI the relative importance of dipole and quadrupole deexcitation modes can be ascertained. In particular, in the122Te(α, 4n)122Xe reaction theγ-decay prior to the entry into the ground band is well described as a statistical process proceeding to 50% by dipole and 50% by quadrupole radiation. In the166Er(α,4n)166Yb and192Os(α,4n)192Pt reactions the relative amount of quadrupole radiation is larger and it seems that the dipole and quadrupole decay takes place via separate cascades. In the164Dy(12C, 7-8n) reactions the average multiplicity is independent of spin, suggesting that the nucleus forgets the spin of the entry state before the process enters into the ground band. In the176Yb(12C, 8n)180Os reaction, finally, the nucleus definitely retains memory of the entry state during the decay. In this last case the multiplicity measurement is combined with aγ-ray singles measurement to give an average excitation energy prior to theα-decay and the average moment of inertia characterising the decay of the high-spin states.

Quantitative description of the geomagnetic field during the Matuyama-Brunhes polarity transition

A model of the reversing geodynamo based on the assumptions (1) that reversals start in a localized region of the core and (2) that upon its onset this reversed region extends, or “floods”, both north-south and east-west until the entire core is affected, has recently been shown to provide a generally successful simulation of existing paleomagnetic records of the Matuyama-Brunhes transition (Hoffman, 1979). In this paper the modelled solution is analyzed so as to reveal the behavior of the dominant Gauss coefficients during the transition. At the time of total axial dipole decay the controlling components are found to be a zonal octupole ( g 3 0) and a non-axisymmetric quadrupole ( g 2 1, h 2 1). Given the distribution of sites corresponding to the available records of the Matuyama-Brunhes, the existence of a significant zonal quadrupole field component cannot be ruled out; however, the role of any equatorial dipole component can be neglected. Due to the presence of a significant low-order non-axisymmetric term in the analyzed transition field, the predicted minimum intensity experienced during the Matuyama-Brunhes is found to be dependent on both site latitude and longitude. In particular, over a mid-northern circle of latitude, the predicted minimum intensity is found to vary by more than a factor of three, averaging about 10% of the full polarity field strength. Although not a unique solution, the applicability of the findings from this analysis is not tied to the phenomenological model from which they were derived. More specifically, whether the above two-component non-dipole transitional field arises from assumed configurational changes of the reversing geodynamo (as is the case for the flooding model) or, alternatively, is considered to be a stationary (non-reversing) portion of the field during axial dipole decay and regeneration, has little effect on either the calculated path locality of the virtual geomagnetic pole or the minimum intensity experienced at a given site. These two possible situations, in principle, should be distinguishable given the future attainment of detailed paleomagnetic data corresponding to back-to-back (R → N and N → R) polarity transitions.

Application of relativistic Coulomb Green's function to the calculation of transition probabilities in an electric field

The probability of transitions between the levels of multiply charged ions in a strong electric field are expressed in terms of the matrix elements of the relativistic Coulomb Green's function. The probability of magnetic dipole decay of the 2s1/2 level of a hydrogenlike ion in a field F=107 V/cm is calculated.

The excited4Poand4P states of lithium

Absolute term values of Li(1s2snp)4Po and (1s2pnp)4P states (n=3,4,5) and their corresponding transition probabilities for electric dipole decay are calculated by means of configuration interaction calculations approximately corrected for relativistic, radiative and mass polarisation effects. Only 1s2pnp to 1s2snp transitions (n=2,3,4,5) possess relatively high decay rates to be readily observed in the beam-foil source; however, the n=5 transition at lambda =5405.2(0.6) AA has not yet been detected. Unobserved weak and very weak lines are predicted. An earlier assignment of lambda =8504 AA to 1s2s3p is not confirmed ( lambda =8605.0(2.5) AA at very weak intensity but within the range of detectability). Comparison of theoretical E1 decay rates with experimental mean lifetimes suggests that autoionisation is by far the principal mode of decay for all these even-parity 4P states except 1s2p2, thus explaining the scarce participation of 4Po to or from 4P transitions in Li quarter spectra.

ProposedE1−M1Interference Experiment in Hydrogenic Ions to Measure Spin Polarization

It is shown that in the Stark quenching of a spin-polarized beam of metastable hydrogenic ions, a measurable right-left asymmetry can be produced in the emitted radiation which is proportional to the degree of spin polarization. The effect, which has previously been observed in absorption in neutral cesium and thallium, is due to interference between spontaneous magnetic dipole and induced electric dipole decay modes. Asymmetry measurements offer a simple means of determining the degree of spin polarization in the heavier hydrogenic ions.

Configuration-interaction studies of transition energies and oscillator strengths for the lowest quartet states of neutral lithium

Configuration-interaction calculations for the lowest $^{4}S$, $^{4}P^{o}$, $^{4}P$, $^{4}D^{o}$, $^{4}D$, $^{4}F^{o}$, and $^{4}F$ states of neutral lithium have been carried out with a basis of $10s8p8d6f3g2h1i$ Slater-type orbitals. The nonrelativistic-energy upper bounds converge to within 4-45 ${\mathrm{cm}}^{\ensuremath{-}1}$ of the expected nonrelativistic energies. An explanation is given for the fact that transitions from $^{4}F$ states have not been observed in beam-foil experiments. Some aspects involved in the assessment of the reliability of calculated oscillator strengths are discussed, and the calculated mean lifetimes for electric dipole decay are compared with experiment.

Optical coherent transients: Generalized two-level solutions

Pulsed radio-frequency transient phenomena of resonant two-level spins are often described by the Bloch equations where the phenomenological dipole (${T}_{2}$) and population (${T}_{1}$) decay times appear. A similar description has been carried over recently into the microwave and infrared regions as well. At visible-ultraviolet wavelengths, however, a more general solution is required because the transition levels do not constitute an isolated system and also spontaneous emission can no longer be disregarded. As a consequence, the lower (1) and upper (2) transition levels are characterized by vastly different decay rates (${\ensuremath{\gamma}}_{1}\ensuremath{\ne}{\ensuremath{\gamma}}_{2}$) and the lifetime ${T}_{1}$ is no longer a defined quantity. In view of laser-induced coherent transient phenomena now being observed, we present general solutions of the density-matrix equations of motion for an optically resonant two-level system in contact with a multilevel reservoir where ${\ensuremath{\gamma}}_{1}\ensuremath{\ne}{\ensuremath{\gamma}}_{2}$. Particular and perturbative solutions involving intensity-dependent damping and frequency shifts are included with applications to echo and nutation phenomena.

Resolution of apparent SU(6) violation in vector meson magnetic dipole decays

We correct the M1 vector meson electromagnetic decays for recoil effects of the final pseudoscalar and for several smaller effects, using results from the FKR and bag models of meson structure. The apparent SU(6) violations in these decays seen in naive SU(6) are then found to be artifacts of recoil neglect, with the single exception ϱ - → π- γ.

Comment on the Magnetic Dipole Decays of Mesons

It is shown that a realistic quark model is consistent with current experimental evidence on the magnetic dipole decays of the vector and pseudoscalar mesons. (AIP)

Nuclear response in the continuum

Abstract Continuum effects may be easily treated in the RPA particle-hole theory of excitations, if the calculation is done with the response function formalism. We apply the theory to the low multipolarities in 16 O, 40 Ca, and 208 Pb. We confirm that the width of the giant dipole state in light nuclei is accounted for by the continuum. In heavy nuclei the dipole decay width is relatively small. The giant quadrupole state is predicted to be much narrower than the dipole since it is concentrated in a single state. The 3ħω) octupole state is quite spread out in light nuclei, but has a width of only 1 MeV in 208 Pb.

Dimerization of impurity-vacancy dipoles in alkali halides

The aggregation of impurity-vacancy dipoles in KCl and NaCl doped with various divalent impurities is shown to follow initial second-order kinetics. This is accomplished by including dissociation as well as formation of dimers in the theory of dipole decay. An apparent initial third-order process, reported by many workers over the past dozen years, arose because dissociation was neglected in the analyses previously made.

Mean Life of the Metastable2 P13State of the Two-Electron Fluorine Ion

By measuring the radiative decay in flight of a foil-excited heliumlike fluorine beam, we have determined the lifetime of the metastable $2 ^{3}P_{1}$ state of the two-electron ion ${\mathrm{F}}^{7+}$. Our data yield the singlet-triplet intercombination transition probability $A(1 ^{1}S_{0}\ensuremath{-}2 ^{3}P_{1})=(1.77\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, which is in excellent agreement with theoretical calculations of a strongly $Z$-dependent spin-orbit-induced electric dipole decay, and extends our earlier measurements for ions in this isoelectronic sequence. Because this transition is observed in the solar spectrum for all abundant heliumlike ions from carbon to iron, we also discuss the relevance of this measurement to solar physics.

Magnetic-quadrupole decay of the low-lying n3P2states of helium like ions

Variational calculations are presented from the n3P2-11S0 magnetic-quadrupole decay rates, with n=2,5, for members of the helium isoelectronic sequence up to Z=10. The magnetic-quadrupole decay rates are compared with the competing n3P2-23S1 electric dipole decay rates to determine the importance of the magnetic- quadrupole transitions in the radiative decay of the n3P2 states.

Geomagnetic models from satellite surveys

The use of satellite scalar magnetic field data to produce geomagnetic field models is reviewed. There have been nine separate spacecraft that have acquired observations of the geomagnetic field from low (<1500 km) satellite altitudes since 1958. The magnetic field models produced from such data have not been sufficiently compared with surface vector data to firmly, establish their validity. One comparison has indicated that satellite‐derived models are as valid over‐all as those produced from surface data. Variations in the field components are shown to be larger than those in scalar field by factors of 4–10. Recent satellite models indicate an increase in dipole decay to −27 γ/year, a value that may be plausible, judging from extrapolation of earlier surface‐derived models. Also noted is a recent slowing of eccentric dipole westward drift.

SU(6) and Superconvergence of Pion Photoproduction

The superconvergence sum rules of pion photoproduction amplitudes are studied to predict the electromagnetic properties of the nucleon and 3-3 nucleon resonance. Exactly the same results are obtained for the $\frac{D}{F}$ ratio of nucleon magnetic moment and magnetic dipole decay width of the 3-3 resonance as from static SU(6). The problem of saturation of the intermediate-state summation by low-lying resonances is discussed.