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Rapid shape evolutions near A = 100 are now the focus of much attention in nuclear science. Much of the recent work has been centered on isotopes with Z

A semi-thick target method has been used to measure the stopping power for recoils produced in the 120 Sn( 14 N, 5n) 129 La, 120 Sn( 11 B, 4n) 127 Cs and 118 Sn( 11 B, 4n) 125 Cs reactions by γ-ray lineshape analysis. The target quality and thickness (1.2 mg/cm 2 ) were determined by the Rutherford backscattering spectrometry technique. Electronic and nuclear stopping-power parameters were determined for Cs and La ions in Sn and compared with parameters measured with the same method for Pm, Sm and Nd ions in Cd and I ions in Ag. A comparison with the LSS theory was made for the energy range below 0.2 MeV/nucleon, where the Bethe–Bloch formula has no application.

Swedish Research Council; European Community [HPRI-CT 1999-00019]; Spanish MINECO [FPA2015-65035-P]

gamma-ray and fast-timing spectroscopy were used to study levels in Al-30 populated following the beta(-) decay of Mg-30. Five new transitions and three new levels were located in Al-30. A search was made to identify the third 1(+) state expected at an excitation energy of similar to 2.5 MeV. Two new levels were found, at 3163.9 and 3362.5 keV, that are firm candidates for this state. Using the advanced time-delayed (ATD) beta gamma gamma (t) method we have measured the lifetime of the 243.8-keV state to be T-1/2 = 15(4) ps, which implies that the 243.8-keV transition is mainly of M1 character. Its fast B(M1; 2(+) -> 3(+)) value of 0.10(3) W.u. is in very good agreement with the USD shell-model prediction of 0.090 W.u. The 1801.5-keV level is the only level observed in this study that could be a candidate for the second excited 2(+) state.

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10 pags.; 6 figs.; 4 tabs. ; Open Access funded by Creative Commons Atribution Licence 4.0, We have applied the fast timing β(t) technique to remeasure lifetimes of selected states in Ra populated in the β decay of Fr. T = 587(12) ps and 210(13)ps have been obtained for the 3/2 and 5/2 states at 50.1 and 79.7 keV, that are more accurate than the previous values of 630(70)ps and 166(55)ps, respectively. Our |D| value of 0.155(10) e·fm obtained for the K=3/2 configuration together with the available values of |D| for the K=1/2 and K=5/2 parity doublet bands establish the configuration dependence of |D| at low spins in this nucleus. Results of theoretical calculations performed for Ra, using the quasiparticle-phonon model (QPM) with inclusion of the Coriolis coupling, reasonably well reproduce octupole correlations in this nucleus. © The Author(s), 2016, This work was supported in part by the Swedish Natural Science Research Council, Norwegian Scientific Research Committee, the Nansen Foundation, the CICYT “Plan Nacional de Altas Energias” Spain (under contract AEN94-0833-C02-02), the U.K. Engineering and Physical Sciences Research Council, and the Polish Scientific Research Committee.

Lifetime measurements of excited states in 149Nd have been performed using the advanced time-delayed $ \beta$ $ \gamma$ $ \gamma$ (t) method. Half-lives of 14 excited states in 149Nd have been determined for the first time or measured with higher precision. Twelve new $ \gamma$ -lines and 5 new levels have been introduced into the decay scheme of 149Pr based on results of the $ \gamma$ $ \gamma$ coincidence measurements. Reduced transition probabilities have been determined for 40 $ \gamma$ -transitions in 149Nd . Configuration assignments for 6 rotational bands in 149Nd are proposed. Enhanced E1 transitions indicate that the ground-state band and the band built on the 332.9keV level constitute a pair of the $\ensuremath K^{\pi} = 5/2^{\pm}$ parity doublet bands. Potential energy surfaces on the $\ensuremath (\beta_{2},\beta_{3})$ -plane have been calculated for the lowest single quasi-particle configurations in 149Nd using the Strutinski method and the axially deformed Woods-Saxon potential. The predicted occurrence of the octupole-deformed K = 5/2 configuration is in agreement with experiment. Unexpectedly low $\ensuremath \vert D_0\vert$ values obtained for the $\ensuremath K^{\pi} = 5/2^{\pm}$ parity doublet bands may result from cancellation between the proton and neutron shell correction contributions to $\ensuremath \vert D_0\vert$ .

Lifetimes of the yrast levels with spins I π = 23/2− to 39/2− in 129La produced via the 120Sn(14N, 5n) reaction at a beam energy of 77MeV have been measured by the Doppler shift attenuation method. The properties of the studied levels belonging to the πh11/2 band were compared with the theoretical predictions obtained in the framework of the core quasi-particle coupling model. It is suggested that the rigid-rotor model used to describe the cores is too simplified as the even-even nuclei from the A ∼ 130 region are susceptible to shape deformation. Reasonable agreement was obtained with the phenomenological cores based on the experimental data of 128Ba.

Several new subnanosecond lifetimes were measured in Pd-109 using the fast-timing beta gamma gamma (t) method. Fission fragments of the A = 109 mass chain were produced by bombarding natural uranium with 30 MeV protons at the Jyvaskyla Ion Guide Isotope Separator On-Line (IGISOL) facility. Lifetimes were obtained for excited states in Pd-109 populated following beta decay of Rh-109. The new lifetimes provide some insight into the evolution of nuclear structure in this mass region. In particular, the distinct structure of the two low-lying 7/2(+) states occurring systematically across the Pd isotopic chain is supported by the new lifetime measurements. The available nuclear data indicate a sudden increase in deformation at N = 60 which is related to the strong p-n interaction between pi g(9/2) and vg(7/2) valence nucleons expected in this region.

Properties of excited states in Nd-147 have been studied with the multispectra and gamma gamma coincidencemeasurements. Twenty-four new gamma lines and three new levels have been introduced into the level scheme of Nd-147. Using the advanced time-delayed beta gamma gamma(t) method, we measured lifetimes of eight excited levels in Nd-147, populated via the beta decay of Pr-147. We have determined reduced transition probabilities for 30 gamma transitions. Multidimensional potential energy surface calculations performed for Nd-147 suggest two single-quasiparticle configurations with nonzero octupole deformation, with K = 1/2 and K = 5/2. Our calculations also predict a sizable value of the electric dipole moment vertical bar D-0 vertical bar = 0.26e fm for this nucleus, while experimentally, a lower limit of vertical bar D-0 vertical bar >= 0.02e fm has been evaluated for the supposed K = 1/2 parity doublet. In contrast to the theoretical results, we do not observe the parity doublet bands with K = 5/2. This, and the lack of theoretically expected E1 strength in Nd-147,Nd-149 may signal some poorly understood structural effect in the odd-N lanthanides.

Lifetimes of yrast levels with spins Iπ = 23/2--43/2- in 131La populated in the 122Sn(14N, 5n) reaction at a beam energy of 70MeV are measured by the Doppler Shift Attenuation method. A model of side-feeding population is presented. The model parameters are determined in an experiment based on the lineshape of γ-transitions from two highest spin levels and from the intensity distribution along the yrast band. The properties of the h11/2 band in 131La are compared with the theoretical predictions obtained in the framework of the Core-Quasi-Particle Coupling and the self-consistent Total Routhian Surface models.

NESTER; The fast timing $\beta\gamma\gamma(t)$ method was applied to investigate the level lifetimes in $^{30,31,32}$Mg. Levels in Mg have been populated in $\beta$ and $\beta$-delayed neutron emission of Na at the ISOLDE facility. From the $\gamma\gamma$ coincidences a number of new states have been identified and new level schemes were constructed for $^{30,31,32}$Mg. The following preliminary half lives have been determined: T$_{1/2}$ = 3.9(4) ns for the 1789 keV state in $^{30}Mg, T$_{1/2}$ = 133(8) ps and 10.5(8) ns for the 221 keV and 461 keV states in $^{31}$Mg, respectively, and T$_{1/2}$ = 16(4) ps for the 885 keV level in $^{32}$Mg. The 1789 keV level was established as a candidate for the intruder 0$^+$ configuration in $^{30}$Mg with a possible strong E0 branch to the ground state.

Excited states in 110Sn and 111Sn nuclei have been investigated using in-beam γ-ray spectroscopic methods. An 16O beam with an energy of 60-80MeV was used to bombard a 98Mo target. On the basis of the relative excitation functions, γ-ray angular distributions, γ-γ and γ-time distributions, γ-ray multiplicity and total energy data, the level schemes of 110Sn and 111Sn have been studied and extended up to Eexc ∼ 11.5MeV, I = 24ℏ, and Eexc ∼ 11.1MeV and I = (51/2)ℏ, respectively. An extension of the intruder, g.s. and negative-parity bands, as well as 5 new band-like structures are proposed in 110Sn. The nature of newly introduced collective band 8 as well as 10+ states fed in the decay of the collective band is discussed. In the 111Sn nucleus an intruder band based on the 23/2- state has been reinvestigated and the extension of the g.s. band and second negative-parity band is given. Evidence for neutron-core coupling in 111Sn is found. The structure and systematics of excited states in light Sn isotopes is discussed.

The lifetime of levels in 128 Cs and 132 La have been measured in-beam of the Warsaw University Cyclotron using DSA method. The absolute M1 and E2 transition probabilities have been deduced. Two cases have been found: 132 La where the B(E2) values in yrast and partner bands differ up to 30 times and 128 Cs - the B(E2) values in both bands differ only few times. It seems that bands in 128 Cs are better candidates for chiral partner bands than the ones in 132 La .

The He-8 + Pb-208 reaction was studied in the first experiment with the EXOGAM germanium detector array using beam delivered by the SPIRAL facility. gamma-rays from direct and fusion-evaporation reactions were observed with high resolution. gamma-gamma coincidence data were obtained at a beam intensity level of 105 8He particles per second. Specially designed absorbers and beam detectors could further reduce the background radiation by orders of magnitude.

Excited states of the $^{146}\mathrm{Tb}$ nucleus have been studied using $\ensuremath{\gamma}$-ray and electron spectroscopy in off-beam and in-beam modes following $^{112}\mathrm{Sn}$($^{40}\mathrm{Ar}$,$3n3p$) reaction with the use of the OSIRIS-II, HPGe detector array and the conversion electron spectrometer. The multipolarity of the 343 keV transition deexciting the $({7}^{\ensuremath{-}})$ level in $^{146}\mathrm{Tb}$ shows mainly an $E2$ nature and the first excited state above the 23 s isomer is assigned as a $({5}^{\ensuremath{-}},{6}^{\ensuremath{-}})$ state. The log $\mathit{ft}$ values have been deduced for 11 ${\ensuremath{\beta}}^{+}/$EC transitions populating excited states in $^{146}\mathrm{Gd}$. The systematic behavior of spins and parities of the long-lived levels at $0+x$ keV and the first excited states above them in the $N=81$ isotones $^{146}\mathrm{Tb}$, $^{148}\mathrm{Ho}$, and $^{150}\mathrm{Tm}$ is discussed.

The excited states of {sup 148}Ho and {sup 149}Ho isotopes are studied using gamma-ray and electron spectroscopy in off-beam and in-beam modes following {sup 112,114}Sn({sup 40}Ar,xnyp) reactions. Experiments include measurements of single gamma-rays and conversion electron spectra as well as gamma-gamma, electron-gamma, gamma-t, and gamma-gamma-t coincidences with the use of the OSIRIS-II 12-HPGe array and conversion electron spectrometer. Based on the present results, the level schemes of {sup 148}Ho and {sup 149}Ho are revised and significantly extended, up to about 4 and 5 MeV of excitation energy, respectively. Spin and parity of 5{sup -} are assigned to the 9.59-s isomer in {sup 148}Ho based on conversion electron results. Previously unobserved gamma rays feeding the 10{sup +} isomer in {sup 148}Ho and the 27/2{sup -} isomer in {sup 149}Ho nuclei are proposed. Shell-model calculations are performed. Possible core-excited states in {sup 149}Ho are discussed.

Excited states in {sup 219}Ra were studied by means of the {sup 208}Pb({sup 14}C, 3{ital n}) reaction at 65 MeV. The investigation was made with 12 Compton-suppressed Ge detectors and a multicounter array. Transitions in {sup 219}Ra were selected by requiring appropriate conditions on fold and sum energy. The proposed level scheme shows two sidebands with alternating parity structure decaying to the ground-state band. It is consistent with a recent description of {sup 219}Ra in the framework of the reflection-asymmetric mean-field approach, which predicts a {ital K}=1/2 ground-state band and two low-lying bands with quantum numbers {ital K}=3/2 and {ital K}=5/2.

Isotopes of francium have been produced via a spallation reaction of thorium carbide and separated in the ISOCELE facility. The very weak beta branch (0.35%) of 220Fr decay and the beta decay of 222Fr leading to the levels of 220Ra and 222Ra, respectively, have been studied by means of gamma -ray singles and beta -gated gamma - gamma coincidence measurements. A strong evidence has been obtained for the 1- state in 220Ra at 412.9 keV and 15 new levels have been added to the eight excited states known previously in 222Ra. The 3- to 2+ E1 transition in the latter nucleus has been found to be remarkably enhanced compared with analogous transitions in the heavier radium isotopes. This enhancement together with the low energy of the 1- state reflects octupole instability of 222Ra. The essentially higher energy of the 1- state in 220Ra suggests that the latter nucleus already lies at the border of the octupole instability region. In contradiction to experimental indications the theory places 220Ra inside this region. For 222Ra it predicts a Q1 value not far from the authors' experimental result but fails to reproduce Q1, changes in 224,226,228Ra nuclei.

Using the Advanced Time‐Delayed method we have studied transition rates in several neutron‐rich nuclei at the magic shell closures. These include the heavy Co and Fe nuclei just below the Z = 28 shell closure at the point of transition from spherical to collective structures. Of particular interest is 63Fe located exactly at the point of transition at N = 37. A substantial increase in the information on this nucleus was obtained from a brief fast timing study conducted at ISOLDE. The new results indicate that 63Fe seems to depart from a simple shell model structure observed for heavier N = 37 isotones of 65Ni and 67Zn.Another region of interest are the heavy Cd and Sn nuclei at N = 72, 74 and the properties of negative parity quasi‐particle excitations. These experiments, performed at the IGISOL separator at Jyvaskyla, revealed interesting properties of the E2 rates in the sequence of E2 transitions connecting the 10+, 8+, 6+, 4+, 2+ and 0+ members of the multiplet of levels in 122Sn due to neutrons in th...

The high spin state structure of the nucleus220Ac was investigated by means of the209Bi(14C, 3n) reaction. In beamγ ray spectroscopy was carried out using the Château de Cristal multidetector array. Gamma ray multipolarities were established by extracting directional correlation ratios from the Ge—Ge coincidence data and by intensity balance considerations. The level scheme, established up to 18 ħ above the lowest observed state, consists of three alternating parity bands. The data are interpreted in terms of the reflection asymmetric shapes of the nucleus. The analysis of the parity doublet candidates does not show the decrease of the parity splitting predicted for an odd-odd nucleus.

High-spin states in 221Ra were studied by the 210Pb(14C, 3n) reaction at beam energies between 61 and 75 MeV. Excitation functions, multiplicity distributions and γγ-coincidences were measured. The level scheme of 221Ra is found to be composed of two bands of alternating parity, characterized by the simplex numbers s = +i and s = −i, which could be followed up to I π = 27 2 + and I π = 27 2 − , respectively. The presence of parity doublets as well as enhanced E1 transactions suggests a reflection-asymmetric shape for this nucleus.

The beta decay of 215Bi and 216Bi isotopes has been studied by mass separation and nuclear spectroscopy methods using spallation reactions on thorium carbide. For the 7.7+or-0.2 min 215Bi activity the beta decay has been found to populate six of the 215Po levels known previously from the alpha decay of 219Rn. The 216Bi activity has been identified through its beta decay to 216Po, to the 550 keV level known already from the alpha decay of 220Rn and to a new level at 969 keV. Its half-life has been determined to be 3.6+or-0.4 min.

4 pages, 3 figures, 1 table.-- PACS nrs.: 21.10.Tg; 23.40.−s; 27.40.+z., The Advanced-Time-Delayed method was used to measure lifetimes of the states in Ar-41 populated in the β decay of Cl-41. The nuclei Cl-41 were produced at ISOLDE by 1.4-GeV proton bombardment of a thick UCx target and mass-separated as molecular ions, XeCl+. Our measured half-life of the 167.1-keV state, T(1/2) = 315(15) ps, is significantly lower than the previously measured value of 410(30) ps. We have also determined T(1/2) = 260(80) ps and T(1/2) ≤ 46 ps for the 515.9- and 1867.7-keV states, respectively. These are the shortest lifetimes measured so far with the ultrafast timing method using the new LaBr3(Ce) crystals for γ-ray detection., This work was performed as part of an undergraduate research project (ERW) at the Physics Department of the University of Notre Dame. Fast-timing detectors and electronics were provided by the Fast Timing Pool of Electronics. This study was supported in part by the NSF PHY04-57120, Swedish Research Council, BMBF Grant No. 06KY205I, the Alexander von Humboldt Foundation (WBW), Foundation for Polish Science (AK), the European Union Sixth Framework through RII3-EURONS (Contract No. 506065), and the EURTD project TARGISOL (HPRI-CT-2001-50033).

The results of the Doppler-shift attenuation method lifetime measurements in partner bands of 128 Cs and 132 La are presented. Experimental reduced transition probabilities in 128 Cs are compared with theoretical calculations done in the frame of the core-quasiparticle coupling model. The electromagnetic properties, energy and spin of levels belonging to the partner bands show that 128 Cs is the best known example revealing the chiral symmetry breaking phenomenon.

Lifetimes of excited states in {sup 229}Th, populated in the {beta} decay of {sup 229}Ac, have been measured using the advanced time-delayed {beta}{gamma}{gamma}(t) method. Half-lives of 14 states have been determined including 11 of them for the first time. Twenty-seven new {gamma} lines have been introduced into the {beta}-decay scheme of {sup 229}Ac based on results of {gamma}{gamma} coincidence measurements. Reduced transition probabilities have been determined for more than 70 {gamma} transitions in {sup 229}Th. Average |D{sub 0}| values of 0.029(1), 0.077(3), and 0.024(5) e fm have been deduced for the lowest K{sup {pi}}=1/2{sup {+-}},3/2{sup {+-}}, and 5/2{sup {+-}} parity partner bands, respectively. Excited states in {sup 229}Th and experimental transition rates have been interpreted within the quasiparticle-plus-phonon model. The half-life of the 3.5-eV, 3/2{sup +} isomeric state is predicted to be about 10 h. Potential energy surfaces on the ({beta}{sub 2},{beta}{sub 3}) plane for the lowest single quasiparticle configurations in {sup 229}Th have been calculated using the Strutinsky method.

Nuclear structure studies of exotic nuclei via the strength of E2 transitions; advanced time-delayed γγ spectroscopy at the extreme

The decay of the ${K}^{\ensuremath{\pi}}{=8}^{\ensuremath{-}}$ isomer in ${}^{132}\mathrm{Ce}$ with an excitation energy of 2340.2 keV has been investigated using the ${}^{120}\mathrm{Sn}{(}^{16}\mathrm{O}{,4n)}^{132}\mathrm{Ce}$ reaction. A half-life of 9.4\ifmmode\pm\else\textpm\fi{}0.3 ms was determined. Two new decay paths have been found in the deexcitation of this isomer. The hindrance factors for the $E1,$ $M2,$ and $E3$ transitions deexciting the isomer have been determined. The decay properties of the ${8}^{\ensuremath{-}}$ isomers in the $N=74$ isotones are discussed. A band mixing mechanism involving the ground state and $s$ band seems to be responsible for the behavior of the reduced hindrance factors of the $E1$ transitions deexciting the ${K}^{\ensuremath{\pi}}{=8}^{\ensuremath{-}}$ isomers in these isotones. A $K$ mixing, characteristic of the axially asymmetric nuclei, may account for the reduced hindrance factors of the $E3$ transitions to the ${5}_{\ensuremath{\gamma}}^{+}$ states in ${}^{130}\mathrm{Ba}$ and ${}^{132}\mathrm{Ce}.$

Lifetimes of high-spin states in the $N=52\ensuremath{-}54 \mathrm{Ru}$ nuclei have been investigated via the ${}^{65}\mathrm{Cu}{(}^{36}\mathrm{S}{,pyn)}^{96,97,98}\mathrm{Ru} (y=4,3,2)$ reactions using the recoil-distance Doppler-shift technique. The data were collected in coincidence mode. Lifetimes have been extracted for both the positive- and the negative-parity states in ${}^{96}\mathrm{Ru}$ and ${}^{98}\mathrm{Ru}$ and for the positive-parity states in ${}^{97}\mathrm{Ru}.$ The levels studied have lifetimes in the range 2--31 ps. The observed reduced transition probabilities $B(E2)$ and $B(M1)$ are compared with the predictions of the shell model. Theoretical implications of these results are discussed.

The low-lying level structure of the odd-odd nucleus220Ac has been observed for the first time following the alpha decay of 0.8 s224Pa produced by the reaction209Bi (18O, 3n). Alpha-gamma and alpha-electron coincidence measurements were used in the construction of the220Ac level scheme. Opposite parity states with energy splittings similar to those in the recent high-spin nuclear study were observed. However, no levels or gamma transitions were observed in common with those of the high-spin work.

The low-lying levels of the odd-mass nuclei 131 Ce and 129 Ce have been investigated by means of the β + EC decays of 131g+m Pr and 129 Pr, respectively, The Pr nuclei were obtained by bombarding 94,96 Mo targets with a 255 MeV 40 Ca beam. The radioactivities produced in the reactions were transported with a He-jet device and γγt , X γt , e − γt coincidence measurements were performed. Conversion electrons were measured with a magnetic spectrometer and transition multipolarities were deduced. On-line mass separation was used to select the 129 Pr β-decay. The resulting level schemes of 131,129,127 Ce are discussed in connection with level systematics and calculations performed by using the Interacting Boson-Fermion Model (IBFM).

The high-spin state structure of the nucleus 221 Ac was investigated by means of the 209 Bi( 14 C, 2n) reaction at 63 and 65 MeV. Measurements consisted of in-beam γγ coincidences using the Château de Cristal multidetector array and of e − γ coincidences. Two alternating parity bands could be established in 221 Ac up to spin 35 2 h . Almost all of the states may be regrouped into parity doublets and the origin of these doublets is discussed.

Excited states of220Ac have been investigated by means of the209Bi(14C,3n) reaction and using the “Château de Cristal” gamma ray facility. The observation of bands with alternating parity states reveals the reflection asymmetric nuclear shape of220Ac. The extracted B(E1)/B(E2) ratios display a clear spin dependence.

The 115In nucleus was studied by means of γ-ray spectroscopic measurements. This nucleus was populated with the 100Mo(18O, p2n)115In reaction and a charged-particle detector array was used to identify the proton evaporation channel. A level scheme with 17 new transitions is presented and compared with theoretical predictions based on the particle + core model.

Deformed Nilsson and Woods-Saxon potentials were employed for generating single particle states used henceforth for calculating the inertia tensor (cranking model and monopole pairing) and the collective energy surfaces (Strutinsky method). The deformation was parametrized in terms of quadrupole and hexadecapole degrees of freedom. The classical energy expression obtained from the inertia tensor and energy surfaces was quantized and the resulting stationary Schrodinger equation was solved using the approximate method. The secondI π=0 2 + collective level energies were calculated for the Rare Earth and Actinide nuclei and the results compared with the experimental data. The vibrational level energies agree with the experimental ones much better for spherical nuclei for both single particle potentials; the discrepancies for deformed nuclei overestimate the experimental results by roughly a factor of two. It is argued that coupling of the axially symmetric quadrupole degrees of freedom to non-axial and hexadecapole ones does not affect the conclusions about systematically too low mass parameter values. The alternative explanation of the systematic deviations from the 0 2 + level energies could be a systematically too high stiffness of the energy surfaces obtained with the Strutinsky method.

Singles and two-parameter coincidence spectra of γ-rays following the α-decay of 230 Th have been studied using Ge(Li) spectrometers in low-background conditions. Two new energy levels of 226 Ra have been established at 824.2 and 873.5 keV fed by α-transitions with branching ratios (hindrance factors) equal to 3.3 × 10 −6 % (6.1) and 1.4 × 10 −6 % (5.1), respectively. These levels are interpreted as the band head and first rotational state of a new K π = 0 + band. The results for K π = 0 + and K π = 0 − bands in 226 Ra are compared with the analogous data for the neighbouring even nuclei and briefly discussed in terms of nuclear models. In particular, the possible influence of octupole deformation in nuclei in this region is considered.

The γ-rays following the β − decay of 224,226 Fr have been investigated by means of γ-ray singles including multispectrum analysis and γ-γ coincidence measurements using Ge(Li) spectrometers. Most of the observed transitions could be placed in level schemes comprising 40 excited states of 224 Ra and 40 excited states of 226 Ra. The properties of the collective states of 224,226 Ra are discussed. It is concluded that the ground states of these nuclei possess a non-zero octupole deformation.

The γ-rays following the β − -decay of 228 Fr have been studied by means of γ-ray singles including multi-spectrum analysis and γ-γ coincidence measurements using Ge(Li) spectrometers. Most of the observed γ-transitions could be placed in the level scheme of 228 Ra. The accuracy the energy of the first-excited state in 228 Ra has been improved and 35 new excited levels have been established, 11 of them grouped into the ground-state band, the low-lying K π = 0 − band with the head at 474.14 keV and two excited K π = 0 + bands with heads at 721.17 and 1041.9 keV. Candidates for two close-lying K π = 2 + bands have also been found. It is concluded that the ground state octupole deformation, if any, is less pronounced in 228 Ra than in lighter radium isotopes.

High-spin states up to I${=31}^{\mathrm{\ensuremath{-}}}$ have been studied in the $^{218}\mathrm{Ra}$ nucleus and the staggering sequence of levels of positive and negative parity persists up to highest level. This observation supports the idea that in some nuclei the high-spin excitations may primarily be dominated by deformations which do not conserve the intrinsic parity (such as octupole, \ensuremath{\lambda}=3, and higher odd-\ensuremath{\lambda} multipolarities) rather than by the ``usual'' quadrupole (\ensuremath{\lambda}=2) and possibly higher even-\ensuremath{\lambda} multipolarities.

A new isotope 220At has been identified among the mass-separated products of a spallation reaction of ThO2. Its half-life has been found to be 3.71+or-0.04 min. This isotope was identified through its beta decay to four excited levels in 220Rn known previously from the alpha decay of 224Ra.

Strutinsky-type potential-energy calculations are performed using various models for the microscopic as well as macroscopic energy terms. The following conclusions are nearly model-independent: Ra-Th nuclei around A ⋍ 225 have ground-state equilibrium shapes with both quadrupole and octupole deformation; nuclei in the heavy barium region are just barely unstable with respect to octupole distortion of the quadrupole shape, while lighter nuclei are stable with respect to octupole deformation, albeit softness occurs e.g. in the region of the very light and the very heavy Z ⋍ 34 isotopes. Tables of ground-state equilibrium shapes, calculated with the Woods-Saxon potential, are presented for doubly-even nuclei in the Ba and Ra-Th regions of octupole-deformed nuclei.

The γ-rays following the β − decay of 230 Fr have been investigated by means of γ-ray singles including multispectrum analysis and γγ coincidence measurements using Ge(Li) detectors. The half-life of 230 Fr was measured to be 19.1 ± 0.5 s. Most of the observed transitions could be placed in a level scheme comprising 23 new excited states of 230 Ra, ten of them grouped into the K π = 0 + ground-state band, the K π = 0 − band with its 1 − state at 710.9 keV and a K π = 2 + γ-vibrational band with its head at 734.8 keV. It is concluded that 230 Ra is a better rotator than the lighter radium isotopes, and has no ground-state octupole deformation.

Energies up to 300 keV of prominent gamma rays from the decay of 230 Th, 228 Th and some of the 228 Th descendants have been precisely measured by use of Ge(Li) detectors.