Ground State Band Transitions Research Articles

Effective theory for the nonrigid rotor in an electromagnetic field: Toward accurate and precise calculations ofE2transitions in deformed nuclei

We present a model-independent approach to electric quadrupole transitions of deformed nuclei. Based on an effective theory for axially symmetric systems, the leading interactions with electromagnetic fields enter as minimal couplings to gauge potentials, while subleading corrections employ gauge-invariant non-minimal couplings. This approach yields transition operators that are consistent with the Hamiltonian, and the power counting of the effective theory provides us with theoretical uncertainty estimates. We successfully test the effective theory in homonuclear molecules that exhibit a large separation of scales. For ground-state band transitions of rotational nuclei, the effective theory describes data well within theoretical uncertainties at leading order. In order to probe the theory at subleading order, data with higher precision would be valuable. For transitional nuclei, next-to-leading order calculations and the high-precision data are consistent within the theoretical uncertainty estimates. We also study the faint inter-band transitions within the effective theory and focus on the $E2$ transitions from the $0^+_2$ band (the "$\beta$ band") to the ground-state band. Here, the predictions from the effective theory are consistent with data for several nuclei, thereby proposing a solution to a long-standing challenge.

In-beam gamma ray and conversion electron study ofFm250

We report on a set of in-beam studies of excited states in $^{250}\mathrm{Fm}$. We detected prompt \ensuremath{\gamma} rays by using the JUROSPHERE IV array and conversion electrons by using the SACRED spectrometer. Both devices were used in conjunction with the RITU gas-filled recoil separator located at the University of Jyv\askyl\a. $^{250}\mathrm{Fm}$ nuclei were produced through the $^{204}\mathrm{Hg}$($^{48}\mathrm{Ca}$,$2n)$$^{250}\mathrm{Fm}$ fusion-evaporation reaction. An experimental excitation function gave a maximum reaction cross section of (980 \ifmmode\pm\else\textpm\fi{} 160) nb at an energy of 209 MeV in the center of the target. Tagging techniques were employed, and a number of E2 transitions were observed that connected the ground-state band levels from spin ${4}^{+}$ up to ${18}^{+}$. The highly converted ${4}^{+}\ensuremath{\rightarrow}{2}^{+}$ transition is observed only by use of conversion electron spectroscopy. The observed ground-state band transitions indicate a rotational structure. We deduce a quadrupole deformation parameter of ${\ensuremath{\beta}}_{2}=0.28$ \ifmmode\pm\else\textpm\fi{} 0.02. A low-energy background of apparent nuclear origin is observed in conversion electrons and postulated to arise from the decay of high-K bands. The half-life of $^{250}\mathrm{Fm}$ is measured to be 30.4 \ifmmode\pm\else\textpm\fi{} 1.5 min.

The Study of Intra Gamma and Gamma to Ground State Band Transitions in160Dy

The low energy weak transitions in gamma band of 160 Dy have been investigated. Two new transitions namely 106.72 (4 γ + –3 γ + ) and 132.98 keV (4 γ + –2 γ + ) have been identified in addition to two already reported in this band. The E2 branching ratios have been calculated experimentally and are compared with IBA theory for intra as well as gamma to ground band transitions. E2/M1 mixing ratios calculated from Alaga rules as well as Greiner's theory are in good agreement.

Directional correlations in the decay of 1200y 166Ho

γγ-directional correlations have been measured between the ground state band transitions 6→4, 4→2 and 2→0 and the γ-rays depopulating the levels of the gamma vibrational band and the negative parity states of166Er. Thereby angular momenta could be assigned to the latter states andM2/E1 multipole mixtures determined for the depopulating γ-transitions. TheE2/M1 mixing ratios were obtained for a series ofIγ→I g transitions up toIγ=8+. These are used to investigate the angular momentum dependence of theE2/M1 admixtures.

Average multipolarities of quasi-continuum transitions following the (α, 4n) reaction from measured internal conversion coefficients

The quasi-continuum of electromagnetic transitions following the 160Gd(α,4n) 160Dy reaction exhibits predominant E1 character. This is condluded from conversion electrons observed with a mini-orange spectrometer and corresponding γ-rays, both being selected by coincidences with discrete ground-state band transitions in 160Dy.

Determination of the Charge and Mass Distribution in the Fission ofCf252

A new technique is presented for determining mass and charge distribution in fission. This method is based on obtaining the independent yields of the even-even fission products from the measured intensities of ${2}^{+}\ensuremath{\rightarrow}{0}^{+}$ ground-state band transitions deexciting the prompt-fission products. Transition intensities for members of ground-state bands in 36 even-even fission-product nuclei have been used to determine the centroids (${Z}_{p}$) and widths (${\ensuremath{\sigma}}_{Z}$) of the charge distribution for eight chains of fission products with constant mass and also to determine the centroids (${Z}_{A}$), widths (${\ensuremath{\sigma}}_{A}$), and yields (${Y}_{Z}$) of the mass distribution for 12 chains of fission products with constant charge. The method has been applied to analyze the charge and mass distributions of $^{252}\mathrm{Cf}$ spontaneous fission. The results when compared with the standard radiochemical and $K$ x-ray technique give satisfactory agreement. The discrepancies which exist are predominantly associated with regions influenced strongly by nuclear shells. Examples are given relating the observed ground-state band transition intensities with other observed fission variables such as kinetic energy release and neutron-evaporation systematics.

Measurement and Analysis of Ground-State Band Transitions and2+′States inKr78,80,82,84from (α,xnγ)

Ground-state band levels to ${6}^{+}$ in $^{84}\mathrm{Kr}$, ${6}^{+}$ in $^{82}\mathrm{Kr}$, ${8}^{+}$ in $^{80}\mathrm{Kr}$, and ${10}^{+}$ in $^{78}\mathrm{Kr}$ have been identified with 20-65-MeV ($\ensuremath{\alpha},xn\ensuremath{\gamma}$) reactions on enriched Se targets with $x=2, 3, \mathrm{and} 4$. The data are in-beam and radioactive $\ensuremath{\gamma}$ spectra, angular distributions, relative and absolute excitation functions and \ensuremath{\sim}3-nsec time distributions. All transitions in the ground-state bands are \ensuremath{\gtrsim}95% prompt (\ensuremath{\gtrsim}3 nsec) except for $^{84}\mathrm{Kr}$, in which the 6 \ensuremath{\rightarrow} 4, 4 \ensuremath{\rightarrow} 2, and 2 \ensuremath{\rightarrow} 0 transitions have the same intensities of long-lived (\ensuremath{\gtrsim}200-nsec) component in addition to the prompt component. Multipole mixing ratios of three other transitions in $^{82}\mathrm{Kr}$ are obtained from the angular distributions. The systematics of level energies in the ground-state bands of various Se, Kr, Mo, and Ru isotopes are analyzed with a phenomenological equation. This reveals three limiting types of nuclei.

Conversion Electrons from (α,xn)Reactions on Sm Isotopes and Nuclear Structures of Gd Nuclei

Conversion electrons following 150,152,154 Sm(α, x n ) 150∼156 Gd reactions were measured at 40 and 36 MeV alpha energies. The ground-state band transitions up to spins I ≈12 in the 154 Gd nuclei and a number of prominent transitions in odd mass Gd nuclei were observed.The measurement of the anisotropy was made for the conversion electrons of the ground-state band transitions in the 154 Gd, and the the spin alignment of the ground-state band levels was obtained. The intensities and the angular distributions of the conversion electrons are discussed in terms of the reaction mechanism and the nuclear model.