Electric Monopole Transitions Research Articles

Pure electric monopole transitions in an odd-mass nucleus.

Six electric monopole (E0) transitions have been observed at low energy in $^{185}\mathrm{Pt}$: These transitions have no observed \ensuremath{\gamma} rays. The observation of pure E0 transitions and of such a large number of E0 transitions in an odd-mass nucleus is unprecedented. The phenomenon is consistent with the mixing of shape isomeric configurations.

Shape coexistence in 190Hg.

Levels in $^{190}\mathrm{Hg}$ were studied by radioactive decay of isotopically separated $^{190}\mathrm{Tl}$. A new band was observed with spin parity (energy, keV) of ${0}^{+}$(1279), ${2}^{+}$(1571), ${4}^{+}$(1975), and ${6}^{+}$(2510). Electric monopole transitions were observed between this band and the ground-state band. Relative B(E2) values for intraband transitions in the new band were found to be \ensuremath{\sim}200 times those for interband transitions. Systematics of shape coexistence in the light Hg isotopes and their possible relevance to recent observations of superdeformation in these nuclei are discussed.

Electric monopole transitions and shape coexistence in nuclei

Electric monopole transitions and shape coexistence in nuclei

02+ and 03+ states in 110Cd.

Electric monopole transitions between low lying ${0}^{+}$ states in $^{110}\mathrm{Cd}$ have been studied by means of electron internal conversion and gamma spectroscopy. Values or upper limits for the ratio ${\mathit{X}}_{\mathit{i}\mathit{j}\mathit{f}}$ between the B(E0;${0}_{\mathit{i}}^{+}$\ensuremath{\rightarrow}${0}_{\mathit{j}}^{+}$) and B(E2;${0}_{\mathit{i}}^{+}$\ensuremath{\rightarrow}${2}_{\mathit{f}}^{+}$) reduced transition probabilities have been determined. The results are discussed in the frame of the proton-neutron-interacting-boson configuration mixing model.

Electric monopole transitions in 122Te.

The first and second excited ${0}^{+}$ states in $^{122}\mathrm{Te}$ have been definitely identified as the 1357 and 1941 keV levels. Electric monopole transitions connecting the ${0}_{3}^{+}$, ${0}_{2}^{+}$, and ${0}_{1}^{+}$ states have been investigated by means of conversion electron and gamma spectroscopy. The ratios X(E0/E2) of reduced E0 and E2 transition probabilities have been determined. The experimental results are compared to the predictions of the neutron-proton interacting boson model.

Electric monopole transitions and isotope shift of nuclear radii of even Sm isotopes in the interacting boson model

The interacting boson model has been used to calculate the isotope shift in146–154Sm isotopes. Fitted model parameters have been used for calculation of the monopole strength parameters ρ(EO) and the branching ratios X(EO/E2) for150–154Sm. Consideration of the effective proton boson number as it reflects the Z=64 subshell closure, is shown to be insignificant in the present calculation.

Electric monopole transitions in76Se

Electric monopole transitions between the 0 2 + , 0 1 + and 2 2 + , 2 1 + levels in76Se, populated in the decay of76Br, were investigated by means of conversion electrons and gammaray spectroscopy. TheK-electron intensity ratios ofE0 andE2 transitions connecting the 0 2 + level to the 0 1 + and 2 1 + levels and of theE0 andE2 components in the 2 2 + →2 1 + transition were determined. The ratioX(E0/E2) of theE0 toE2 reduced transition probabilities and, from the available lifetimes, theE0 strength parameters ρ(E0) were deduced for the two transitions. The results are compared with the predictions of current models.

Electric monopole transitions in 102Ru.

Electric monopole (E0) transitions between low lying levels in $^{102}\mathrm{Ru}$, populated in the decay of $^{102}\mathrm{Rh}$, have been investigated by means of conversion-electron and \ensuremath{\gamma}-ray spectroscopy. Values or upper limits for the ratio X(E0/E2) of E0 to E2 reduced transition probabilities have been derived for the transitions from the ${0}_{3}^{+}$ and ${0}_{2}^{+}$ to the ${0}_{1}^{+}$ and ${2}_{1}^{+}$ states and for the ${2}_{2}^{+}$\ensuremath{\rightarrow}${2}_{1}^{+}$ transition. The experimental results are discussed in the frame of a schematic interacting boson model.

Electric monopole transitions betweenO+states inRu102

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Internal electron-positron pair production from electric monopole transitions

NuclearE0-transitions above the pair production threshold of ω=2mc2∼1 MeV are considered. The rationη of the electron-positron pair creation probability to theK-shell conversion probability is calculated. For92U and ω=1,450 keV a typical value ofη=2·10− is obtained. Good agreement with experimental data is found. Our numerical results for finite size nuclei are compared with analytical expressions utilizing relativistic electron wavefunctions for point nuclei. The energy distributions of emitted positrons are presented for various nuclear charge values and transition energies. The possibility of monoenergetic positron emission is discussed. Of particular interest is the relation of the theoretical results to the recently observed structure in the positron spectra, which have been measured in high energy collisions of very heavy ions.

Electric monopole transitions from excited Kπ = 0+ levels in 172Yb and 174Yb

Electric monopole transitions in 172 Yb and 174 Yb have been studied following thermal neutron capture on 171 Yb and 173 Yb. It is found that the second excited 0 + state in 172 Yb at 1405 keV has a very large X = B( E 0; 0 2 + −0 g + ) B( E 2; 0 2 + −2 g + value of 2.81±0.31. The E0 decay branches for the other 0 + states in 172 Yb were also measured. In the case of 174 Yb no E0 conversion lines were observed from the corresponding 0 + states but upper limits were placed on their intensities. These results indicate that none of the known 0 + states in 174 Yb exhibit such a large X -value. Various interpretations are discussed and it is concluded that the 1405 keV level in 172 Yb may contain large amplitudes of proton pair excitations. A model based on such excitations is presented which accounts qualitatively for the large X (1405) value in 172 Yb as well as for the systematics of large X -values in the rare-earth region.

Study of electric monopole transitions between the ground state and the first excited 0 + state in 40, 42, 44, 48Ca with high resolution inelastic electron scattering

Monopole transitions from the 0 1 + ground states to 0 2 + excited states at 3.353 MeV ( 40Ca), 1.837 MeV ( 42Ca), 1.884 MeV ( 44Ca) and 4.272 MeV ( 48Ca) have been investigated with high resolution inelastic electron scattering (FWHM ≈ 30 keV) at low momentum transfer (0.29 ≦ q ≦ 0.53 fm −1). The respective monopole matrix elements are 2.53 ± 0.41 fm 2, 5.24 ± 0.39 fm 2, 5.45 ± 0.41 fm 2 and 2.28 ± 0.49 fm 2. These results are used together with known ground state charge radii and the average number of holes in the sd shell in the ground state to estimate the number of particle-hole excitations in the wave functions of the excited 0 + states.

Electric Monopole and Quadrupole Matrix Elements for the Kπ=0+ Bands in 154Gd

Electric monopole and quadrupole transitions from the first and second K π =0 + band states in 154 Gd are observed through the decay of 154 Tb with a high resolution iron-free magnetic spectrometer. Electric quadrupole transition matrix elements for the β-band states are deduced from an analysis of ratios Q β g ( I β - I g ')/ eR 2 ρ( I β - I g ). The decrease of the rotation vibration interaction energy for high spin states of the β-band is discussed. Dimensionless ratios X ( I 2 - I λ ) for the second 0 + band are deduced as follows; X (0 2 -0 g )=0.056±0.009 and X (2 2 -2 β )=0.069±0.011. The branching ratio of E0 reduced transition probabilities, ρ(0 2 -0 β )/ρ(0 2 -0 g ), is found to be ≤0.3. The possibility of the second equilibrium deformation is discussed for the second K π =0 + band.

Observation of Electric Monopole Transitions in Tetracyanoquinodimethane

The first direct observation of electric monopole transitions in solids is reported for tetracyanoquinodimethane using an inelastic electron scattering experiment. Good agreement is obtained with a semiempirical self-consistent molecular-orbital calculation which predicts dipole allowed transitions near 3 and 7 eV and optically forbidden monopole transitions near 5 eV. We find an anomalous momentum dependence in the shape of the first dipole transition, possible due to local field effects.

The role of effective two-nucleon potential in the electric monopole transition in C12 and O16

In the present paper the Shiff's model is used to explain the O+-transition mechanism in nuclei. In contrary to the Shiff's study more realistic effective potentials suggested bySiemens andSprung andBanerjee are applied. They give good results.

Electric monopole transitions between0+states inRu100

The electric monopole ($E0$) transitions from the first and second excited ${0}^{+}$ states at 1130.3 and 1740.7 keV, respectively, in $^{100}\mathrm{Ru}$ have been observed from the internal-conversion-electron studies of the decay of $^{100}\mathrm{Rh}$. The monopole strength for the first excited ${0}^{+}$ state $\ensuremath{\rho}(E0;{0}_{1}^{+}\ensuremath{-}{0}_{g}^{+})l0.11$ is deduced as an upper limit. The dimensionless ratios of the reduced $E0$- to $E2$-transition probabilities $X$ are deduced: $X\left(\frac{E0;{0}_{1}^{+}\ensuremath{-}{0}_{g}^{+}}{E2;{0}_{1}^{+}\ensuremath{-}{2}_{1}^{+}}\right)=0.011\ifmmode\pm\else\textpm\fi{}0.001,$ $X\left(\frac{E0;{0}_{2}^{+}\ensuremath{-}{0}_{g}^{+}}{E2;{0}_{2}^{+}\ensuremath{-}{2}_{1}^{+}}\right)=0.6\ifmmode\pm\else\textpm\fi{}0.3$. The observed branching ratio of the monopole strength for the second excited ${0}^{+}$ state is $\frac{\ensuremath{\rho}({0}_{2}^{+}\ensuremath{-}{0}_{g}^{+})}{\ensuremath{\rho}({0}_{2}^{+}\ensuremath{-}{0}_{1}^{+})}=0.73\ifmmode\pm\else\textpm\fi{}0.07$. The experimental results are compared with the theory on the basis of the collective vibrations of an axially and nonaxially deformed nucleus.

Collective Hamiltonian derived from the pairing-plus-quadrupole model: Modification and application to the transitional nuclei, 150, 152Sm

The time-dependent Hartree-Bogolyubov treatment of the pairing-plus-quadrupole model, employed previously for the energy levels and the electromagnetic moments of W, Os and Pt nuclei, has been modified and extended to higher angular momentum states. Comparison with experiment is given for the energy levels, B(E2) values, quadrupole moments, magnetic moments, electric monopole transition moments, isomer shifts, E2/M1 mixing ratios and E0/E2 mixing ratios of 150Sm and 152Sm. Spectroscopic factors for one-proton, one-neutron and two-neutron transfer reactions are given. Microscopic and collective explanations are given for the transition from a vibrational-type spectrum for 150Sm to a rotational-type spectrum for 152Sm, the co-existence of a large Q 2+ with a vibrational-type spectrum and a small cross-over B(E2) in 150Sm, band-mixing anomalies in 152Sm, and signs of the mixing ratios δ( E2/ M1) and ε(E0/E2) in the two nuclei. However, the present model seems to break down in the case of the E2 branching ratios in 150Sm.

Electron capture decay of 238Am and electric monopole transitions in 238Pu

Mass-separated samples of 238 Am(T 1 2 = 98 min) have been used to investigate the EC decay scheme of 238Am. The γ-ray and conversion electron spectra were measured with high-resolution Ge(Li) and Si(Li) spectrometers. Several electric monopole transitions were identified. The K/L ratios of the EO transitions are in agreement with the theoretical values of Church and Weneser. A γ-ray spectrum was measured in coincidence with the electron lines of the 44.1 keV transition in order to distinguish transitions going to the ground state from those terminating at the 2 + or 4 + member of the ground state band. From these data a decay scheme has been constructed. Assignments of levels already known from the β − decay of 2338Np and α-decay of 242Cm have been confirmed. Several other low-spin states have been postulated to account for the observed γ-rays and conversion electrons. From log ft value considerations the ground state of 238Am has been given an assignment of { n[743] 7 2 −; p[523] 5 2 −}1 + and the 962.8 keV level of 238Pu has been assigned to { n[743] 7 2 −; n[622] 5 2 +}1 − . The half-life of 238Am has been measured to be 98±3 min. A 5.94 MeV α-group has been assigned to 238Am decay. The α-branching has been measured to be (1.0±0.4)×10 −4%.

Higher-Order Deformations and Electric Monopole Transitions in Deformed Nuclei

Calculation of electric monopole $X$ ratios in deformed nuclei using the higher-order shape deformations ${Y}_{4}$ and ${Y}_{6}$ yields much better agreement with measured values than is obtained when only pure quadrupole surfaces are assumed. Use was made of recently measured values of ${\ensuremath{\beta}}_{4}$, and ${\ensuremath{\beta}}_{6}$.

Evaluation of the Fermi function; EO competition

It is demonstrated that the usual Fermi function for beta-decay may be evaluated with great accuracy by elementary methods. Competition between electron emission and negaton-positon pair emission in electric monopole transitions is discussed and a figure for its rapid approximate assessment is given.