Evolution of deformation in neutron-rich Ba isotopes up to A = 150

All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Evolution of deformation in neutron-rich Ba isotopes up to A=150 Lică, R.; Benzoni, G.; Rodríguez, T. R.; Borge, M. J. G.; Fraile, L. M.; Mach, H.; Morales, A. I.; Madurga, M.; Sotty, C. O.; Vedia, V.; De Witte, H.; Benito, J.; Bernard, R. N.; Berry, T.; Bracco, A.; Camera, F.; Ceruti, S.; Charviakova, V.; Cieplicka-Oryńczak, N.; Costache, C.; Crespi, F. C. L.; Creswell, J.; Fernandez-Martínez, G.; Fynbo, H.; Greenlees, Paul; Homm, I.; Huyse, M.; Jolie, J.; Karayonchev, V.; Köster, U.; Konki, Joonas; Kröll, T.; Kurcewicz, J.; Kurtukian-Nieto, T.; Lazarus, I.; Lund, M. V.; Mărginean, N.; Mărginean, R.; Mihai, C.; Mihai, R. E.; Negret, A.; Orduz, A.; Patyk, Z.; Pascu, S.; Pucknell, V.; Rahkila, Panu; Rapisarda, E.; Regis, J. M.; Robledo, L. M.; Rotaru, F.; SaedSamii, N.; Sánchez-Tembleque, V.; Stanoiu, M.; Tengblad, O.; Thuerauf, M.; Turturica, A.; Van Duppen, P.; Warr, N.

The occurrence of octupolar shapes in the Ba isotopic chain was recently established experimentally up to N = 90.To further extend the systematics, the evolution of shapes in the most neutron-rich members of the Z = 56 isotopic chain accessible at present, 148,150 Ba, has been studied via β decay at the ISOLDE Decay Station.This paper reports on the first measurement of the positive-and negative-parity low-spin excited states of 150 Ba and presents an extension of the β-decay scheme of 148 Cs.Employing the fast timing technique, half-lives for the 2 +  1 level in both nuclei have been determined, resulting in T 1/2 = 1.51(1) ns for 148 Ba and T 1/2 = 3.4(2) ns for 150 Ba.The systematics of low-spin states, together with the experimental determination of the B(E2 : 2 + → 0 + ) transition probabilities, indicate an increasing collectivity in 148-150 Ba, towards prolate deformed shapes.The experimental data are compared to symmetry conserving configuration mixing (SCCM) calculations, confirming an evolution of increasingly quadrupole deformed shapes with a definite octupolar character.DOI: 10.1103/PhysRevC.97.024305

I. INTRODUCTION
Nuclear interactions within the atomic nucleus can be understood in terms of a monopole part that contains the spherical mean field (spherical single-particle levels) and a multipole part that includes everything else [1].The main contributions to the latter term come from the lowest order multipoles (quadrupole and octupole).Hence, the competition between the spherical mean field and these multipole contributions in a particular nucleus produces its intrinsic nuclear shape (spherical, quadrupole and/or octupole deformed, etc.).Barium isotopes (Z = 56) are located in a region of the Segrè chart characterized by a variety of shape-related phenomena, including shape coexistence and possible static octupole deformation.High order deformations can have a strong influence on γ -decay rates and on quasiparticle energies, which are, in turn, inputs for the various theoretical models developed to describe nuclei in this region [2,3].
The evolution of the nuclear shape with the number of nucleons is usually studied from mean-field calculations of potential energy surfaces (PESs) defined along multidimensional deformation spaces.In the Ba isotopic chain (Z = 56) microscopic-macroscopic [4] and fully microscopic Hartree-Fock-plus-BCS and/or Hartree-Fock-Bogoliubov (HFB) [5][6][7] calculations have been carried out, exploring both quadrupole and octupole degrees of freedom.It is noted that stronger octupole correlations are seen in the lightest A = 112-114 and heaviest elements of the chain, starting from A = 144, which corresponds to the well-known region of octupole deformation N = 56 and N = 88 [2].Low-lying negative-parity states have also been found in 122-126 Ba, whose character is understood in terms of octupole correlations [8].
The Ba isotopic chain has been extensively studied in past years up to A = 148, mainly via spontaneous fission of 252 Cf and 248 Cm sources [9,10].Spins and parities of both yrast and side bands could be assigned in most cases through directional correlations from oriented states (DCO) and polarization measurements.Population of excited nonyrast states in Ba isotopes via the β decay of the Cs parent nuclei was also studied prior to the present experiment [11].
Most recently, 144 Ba and 146 Ba isotopes were studied at the CARIBU facility in Argonne.In both cases, the beam was post-accelerated to allow for a safe Coulomb excitation experiment, leading, for the first time, to the measurement of the B(E3; 3 + → 0 + ) transition probabilities [12,13].The β decay of 146 Cs into 146 Ba was also measured with improved statistics, leading to a great extension of the level scheme up to 2.2 MeV [14].
Excitation energies and electromagnetic properties (transition probabilities and moments) cannot be obtained within a pure mean-field description.Therefore, a direct comparison with the experimental data requires further developments.Recently, beyond-mean-field methods including symmetry restorations (particle number, angular momentum, and parity) and configuration mixing within the generator coordinate method (GCM) were implemented with relativistic [15] and Gogny interactions [16].This symmetry conserving configuration mixing (SCCM) method included for the first time the interplay between axial quadrupole and octupole degrees of freedom on an equal footing.One of its first applications was the study of the spectra of 144-146 Ba, providing a good agreement with the experimental data.
This paper presents the first experimental study of the excited structure of 150 Ba and an extension of the level scheme of 148 Ba, together with the determination of transition probabilities in both nuclei.A theoretical interpretation will also be given in terms of quadrupole-octupole coupling.The paper is organized as follows: experimental details are given in Sec.II, while in Sec.III the proposed experimental level schemes for both 148 Ba and 150 Ba are shown.The comparison with SCCM theoretical results is given in Sec.IV and finally conclusions are drawn in Sec.V.

II. EXPERIMENT
In this work we study the 148,150 Ba isotopes, populated in the β decay of their parent 148,150 Cs isobars.Cs isotopes were produced at ISOLDE (CERN) [17,18] by fission of a nanostructured UC x target induced by the 1.4 GeV proton beam delivered by the PS Booster (PSB).The Cs atoms, thermally diffused out of the target matrix, were surface ionized and separated using the ISOLDE General Purpose Separator (GPS).The proton beam current during the run was 1.5-2 μA.Intensities of the exotic beams reaching the experimental setup ranged from 5.5 × 10 2 ions/μC for mass A = 148 to 2 ions/μC for the A = 150 beam (beamline transmission of ∼70%).
Cs ions were implanted on an aluminized mylar tape at the center of the detection setup.The beam extraction was started at the arrival of the proton pulse.The data acquisition recorded the arrival time of each proton pulse, using it as a reference for building decay curves.The tape was moved every 20-25 proton pulses (corresponding to a PSB supercycle of ∼1 minute) in order to remove the unwanted daughter activity.
The experimental setup consisted of the Isolde Decay Station (IDS), equipped with three fast-responding plastic scintillator detectors, to detect the β particles, four high-purity germanium (HPGe) clover detectors for the detection of γ rays following internal decay in the daughter nuclei, and three small-volume conic LaBr 3 (Ce) detectors to perform lifetime measurements of specific nuclear states.The β detection array efficiency was 20% while the total HPGe efficiency, after add-back, amounted to 6% at 0.6 MeV.The digital processing of the energy signals provides resolutions at 1.3 MeV of the order of 2.3 keV for the HPGe detectors and 40 keV for the LaBr 3 (Ce) ones.To increase the efficiency of the γ array, no anti-Compton shield was used, with HPGe crystals placed at few centimetres from the implantation point.The collected spectra show peaks due to partial depositions of energy by γ rays interacting via Compton scattering or pair-production mechanisms.
The setup was optimized to perform the evaluation of the lifetime of specific nuclear states using the fast-timing technique, which is based on the measurement of the time difference between a transition feeding and a transition deexciting a specific nuclear level.The transitions are detected in two different detectors, and the time difference is measured  using a standard TAC (time-to-amplitude converter) electronic module.Further details on the experimental setup and analysis techniques can be found in Ref. [19].

A. 148 Cs β decay
Figure 1 shows the β-gated γ spectrum corresponding to the decay 148 Cs → 148 Ba (T 1/2 = 152(1) ms [19]) recorded in the first 500 ms after a proton pulse, to remove contributions from long-lived activity.The spectrum shows primarily transitions belonging to this decay, together with contributions from the β-delayed neutron-emission channel (β-n) and the granddaughter decay.Newly measured transitions are indicated in bold.
The β decay half-lives of transitions in 147 Ba, which are expected to be populated as the β-n channel, were investigated in order to rule out a possible 147 Cs contamination in the incoming beam.The time behavior of the 109.8-keVtransition in 147 Ba is equivalent to that of the 2 + → 0 + 141.6-keV transition in 148 Ba.This is shown in Fig. 2, where the two spectra have been normalized for comparison.The half-life extracted from the decay curve of the 109.8-keVline is T 1/2 = 158(6) ms, in good agreement with the value for 148 Cs (T 1/2 = 152(1) ms) reported in Ref. [19], while it differs significantly from the adopted one for 147 Cs (T 1/2 = 230(1) ms [20]).
Examples of prompt γ γ spectra are shown in Fig. 3: coincidences with the 2 + → 0 + transition at 141.6 keV are given in panel (a), coincidences with the 4 + → 2 + transition at 281.3 keV in panel (b), while panel (c) shows coincidences with the 1 − → 0 + line at 633.1 keV.
The low-spin structure of 148 Ba, already reported in [9,11], was confirmed, and the level scheme of this nucleus was greatly extended, including up to 38 levels and 74 transitions (out of which 32 levels and 64 transitions are newly observed), as shown in Table I and Fig. 4. The new transitions were identified from coincidences with the previously reported ones, by energy matching and also by analyzing their decay half-lives, when the statistics allowed for it.
The thickness of the lines shown in Fig. 4 scales with the relative intensity of the γ transitions, normalized to the one at 141.6 keV, as shown in column 5 of Table I.The absolute γ -ray intensities were determined using, as normalization, the adopted absolute intensities of both daughter and granddaughter isotopes I abs ( 148 La; 133.5 keV) = 3.9(1)% [21] and I abs ( 148 Ce; 158.5 keV) = 56(1)% [22], assuming no feeding to the ground state of 148 Ce from the 148 La decay.For the 141.6-keV transition a theoretical conversion coefficient of 0.574 (8) has been employed, using BRICC [23], corresponding to a pure E2 multipolarity.The conversion coefficient for Energy (keV)   148 Cs → 148 Ba.The first column from the left reports level energies in keV and the second proposed spins and parities.Columns 3 and 4 show the I β calculated as discussed in the text and corresponding log(f t) values.In the last columns we report the energy of the γ transition deexciting each level, together with its relative intensity (normalized to the 2 + → 0 + transition) and the level to which it decays.Previously known levels and transitions are marked by ( * ).For absolute intensity per 100 β decays, multiply the γ intensities by 0.30 (4).Favored spin assignments are indicated in bold, as discussed in Sec.III C.  2.0(2) 0.0 higher energy transitions is small and was neglected in the further analysis.The absolute β feeding contributions corresponding to each level were extracted by taking into account the feeding and depopulating transitions.The I β towards the ground state were estimated from the missing feeding, once the intensity going to delayed neutron emission was accounted for (subtracted).They are reported in column 3 of Table I and in the left side of the level scheme in Fig. 4. Using the calculated half-life and a Q β of 10.3(6) MeV [24], the corresponding log(f t) values were extracted, as shown in column 4 of Table .I.

TABLE I. Levels populated in the decay
The 148 Cs neutron emission probability of P n = 38(4)% was determined from the ratio between the total number of 147 Ba and 148 Ba nuclei produced, estimated using the adopted values I abs ( 147 La; 167.4 keV) = 15.3(16)% and I abs ( 147 La; 196.1 keV) = 6.7(7)% [25].

B. 150 Cs β decay
Transitions belonging to the decay 150 Cs → 150 Ba were studied for the first time: Fig. 5(a) shows the spectrum following the β decay of 150 Cs, extracted by closing the beam gate after 200 ms and subtracting the long-lived activity.Intense short-lived transitions at 101.1, 217.1, 597.4,613.4,and 945.7 keV are associated with the internal deexcitation of 150 Ba.The lines marked with asterisks belong to the β-n channel being observed in the decay scheme of 149 Ba [19].All these lines correspond to the same decay half-life of 80 (3) ms.An additional strong transition is seen in the spectrum, marked by the "$" symbol: this is coming from the decay 149 Ba → 149 La, identified as originating from the β-n decay branch.Since the presence of contaminant species in the beams has been carefully checked, the presence of transitions belonging to 149 Ba in the decay spectrum of 150 Cs can be traced back to the β-delayed neutron emission channel.
As a comparison, the spectrum associated with the longlived activity is shown in panel (b), where known lines, belonging mainly to the decay of 150  The analysis of γ γ coincidences provided the level scheme shown in Fig. 6: the thickness of each arrows is proportional to its intensity, normalized to the 2 + → 0 + transition, as reported in column 5 of Table II.The transition at 975.9 keV is tentative, and is indicated by a dashed line since it is only seen in coincidence with the 101.1-keVγ line.
The absolute γ -ray intensities were determined using, as normalization, the absolute intensities I abs (97.0 keV) = 30% and I abs (208.7 keV) = 25% [26] from the granddaughter 150 Ce, assuming no feeding to the ground state from the 150 La decay.For the transition at 101.1 keV a conversion coefficient of 1.86(3) [23] has been used, assuming a pure E2 character.Coefficients for the higher energy transitions are small and have been neglected.
Even if a strong contribution from the β-delayed neutron emission branch is seen in the spectrum, the low statistics of  148 Cs → 148 Ba extracted from γ γ coincidences.The I β and logf t are calculated from intensity balance as discussed in the text.For absolute intensity per 100 β decays, multiply the γ intensities by 0.30 (4).
the data and absence of information on the granddaughters populated by the two channels did not allow for a precise estimation of the P n value.The ratio of intensities of the most populated transitions in 149 Ba [19] and 150 Ba nuclei, 316.6and 101.1-keV transitions, respectively, of the order of 44%, suggests a steep increase in the β-delayed neutron emission probability in these exotic members of the Cs chain.This is in agreement with state-of-the-art calculations [27,28] which predict P n to be 41%.We therefore use this theoretical value for the evaluation of the β feeding to the states, reported in Table II and Fig. 6.
The absolute β feeding corresponding to each state was extracted using the procedure described earlier for 148 Cs decay.Given the large Q β window, the extracted I β have to be considered as upper limits.
Table II reports the energy, proposed J π assignments (which will be discussed later in Sec.III C), de-populating γ transitions, and relative intensities of the levels.

C. Spin assignments
The level scheme of 148 Ba has been greatly extended.Starting from the known low-spin level structure, together with considerations involving the decay patterns and systematics, spins and parities of several levels are proposed: (i) The 860.9-keV level is only decaying to the 2 + 1 via the 719.9-keV transition.No feeding transition to this state is seen.Although other assignments are possible, as indicated in Table I, this state is interpreted as 0 + 2 , given the close resemblance to a similar state found in 146 Ba in Ref. [14].(ii) The 980.2-keV level decays to 1 − 1 state through the 293.5-keV transition, to the 4 + 1 via the 557-keV one, and to the 2 + 1 via the 839.6-keV γ ray.This decay pattern closely resembles that of the J π = 3 − state, even if the I β is similar to the one measured for the 1 states, pointing to a J assignment of 3. (iv) The 1522-keV level, decaying with the 473-keV line to the 2 + 2 , and with the 1381-keV transition to 2 + 1 one, is a possible candidate for the 4 + 2 .Its log f t = 6.39( 6) is also comparable to the one measured for 4 + 1 [6.54 (7)].We therefore favou the J π = 4 + assignment.
We do not see high-J states, apart from the 5 − level at 962.3 keV [9], which is populated from high-lying states and not directly fed by β decay.Ground state feeding seems to be very weak in 148 Ba compared to the value reported for 146 Ba (I β = 27%) [14].
The level scheme of 150 Ba is extracted for the first time and, based on the coincidences shown and on systematic comparison with the lighter members of Ba chain, we associate the level at 101.1 keV to the 2 + 1 state, the level at 318.2 keV to the 4 + 1 , while the level at 613.4 keV is proposed as the 1 − 1 level and the one at 698.5 keV as the 3 − 1 state.The decay pattern of these states closely resembles that of 148 Ba: the β decay seems to preferentially populate the 0 + , 2 + , 1 FIG.6. Proposed scheme for the decay 150 Cs → 150 Ba extracted from βγ γ coincidences.The Q β value is taken from systematics [24], while the I β and logf t are calculated from intensity balance as discussed in the text.For absolute intensity per 100 β decays, multiply the γ intensities by 0.12(4).The indicated P n value is taken from [27].
isotopes.At variance from 148 Ba, no feeding to the 4 + state has been obtained.The analysis of the population pattern and a comparison with neighboring nuclei suggest that the state at 1046.8 keV could be the 0 + 2 .No firm assignment can be established for the levels at 1077 and 1178 keV.The systematics of the levels found in the lightest members of the isotopic chain seem to point to a J π = 2 + assignment for the first level, but its decay, showing a strong branch towards the 3 − state and a weaker branch populating the 2 + state, does not fully confirm this hypothesis.
The log(f t) deduced from the β feeding indicate a competition between Gamow-Teller (GT) and first-forbidden (ff) transitions, since both positive-and negative-parity states seem to be populated with similar intensities.This is a typical pattern in the Ba chain, observed also in the case of 146 Ba.We stress that a strong contribution from the Pandemonium effect is expected, given the large Q β window of the decays under study.  15Cs → 150 Ba.The first column from the left reports level energies in keV and the second proposed spins and parities.Columns 3 and 4 show the I β calculated as discussed in the text and corresponding log(f t) values.In the last columns we report the energies of the γ transitions de-exciting each level, together with their relative intensities and the level towards which it decays.For absolute intensity per 100 β decays, multiply the γ intensities by 0.12(4).Owing to the large Q β window, the extracted I β have to be considered as upper limits, and the log(f t) as lower limits, as indicated in Fig. 6.The transition reported in italics is tentative since it is only seen in coincidence with the 101.1-keVγ line.
In Fig. 7 a comparison of low-lying states in 146,148,150 Ba is given.Newly observed transitions and newly proposed spins and parities are marked with italics red labels.The levels are marked by the proposed J π here discussed.

D. Fast-timing measurements
The statistics collected in the two data sets allowed to study the lifetime of the 2 + 1 states in both 148 Ba and 150 Ba.E γ 1 -E γ 2time triple coincidence matrices have been constructed within a coincidence-time window of 50 ns, making use of all possible combinations of LaBr 3 (Ce) detectors, with the additional request of a registered β decay.
The time spectra are obtained by projecting on the time axis the region around the peaks [indicated by the circular polygon in the matrix shown in Fig. 8 (a)].The lifetime of the level is constructed using feeding transitions as start transitions (Y axis) and the decay-out transition as stop transition (X axis).In the specific case the chosen feeding transitions are those at 281-545-633 keV while the deexciting transition is the 141.6-keV 2 + → 0 + transition.
An accurate subtraction of the background is obtained by considering a two-dimensional gate in the energy plane, around the corresponding coincidence peak, as shown in the same panel for the 281-, 546-, and 633-keV transitions.This procedure allows for a better control of contaminant contributions which may have different time characteristics than the average background, and reduces the statistical uncertainty associated with background subtraction by considering a wider two-dimensional background region.The resulting spectrum is shown in panel (c), together with the fit consisting of the convolution between a gaussian and an exponential function.The resulting half-life is also indicated in the panel.
Low statistics did not allow for a similar procedure in the case of 150 Ba and the half-life is determined using a twodimensional E γ -time matrix.The time information is extracted from the β-LaBr 3 (Ce) TAC.Contributions from long-lived daughter activity relative to the proton impact have been subtracted.From this matrix one can also judge the quality of the walk correction applied, since the shape of the time distribution does not change significantly going towards low energies.
The time response of the 2 + → 0 + transition is shown in panel (d).The exponential fit did not include the first 800 ps corresponding to prompt transitions.No contribution from high-lying levels is expected, as it should mainly come from the 4 + state, which is not fed in β decay.Moreover, this state is expected to show a shorter half-life, of the order of tens of ps, which does not affect our result.
The half-lives extracted for the two nuclei, 1.5(1) ns for 148 Ba and 3.4(2) ns for 150 Ba are longer than those extracted for 142-146 Ba, of the order of few hundreds ps [29].This is expected given the lower transition energy, and points towards an increasing collectivity.These results can be translated into B(E2 : 2 + → 0 + ) of 90.4 (25) and 110(4) W.u., for 148,150 Ba, respectively.

IV. DISCUSSION
A qualitative description of the evolution of the deformation, at least in the quadrupole degree of freedom, can be given FIG. 7. Comparison between observed low-lying states and corresponding decaying transitions in 146,148,150 Ba.Transitions reported in this work for the first time and newly proposed J π assignments are highlighted in red italics.Levels and transitions of 146 Ba are taken from Ref. [14].Positive-parity states are grouped in panel (a) while the negative-parity ones are grouped in panel (b).Levels in 142-148 Ba are taken from the literature [9]. the prolate one in the heavier isotopes.For the nuclei 142148 Ba the absolute minima correspond to octupole deformed (β 3 = 0) intrinsic states, although the energy wells around these minima are rather soft in the β 3 direction.This holds also for 150 Ba where the minimum is at β 3 = 0.This is a clear indication that beyond-mean-field correlations should be taken into account, i.e., the restoration of the symmetries broken at the HFB level (particle number, parity, and angular momentum) and the shape mixing (quadrupole and octupole).After performing such symmetry restorations and shape mixing, we obtain both the spectra (excitation energies and electromagnetic moments and transition probabilities) and the collective wave functions (CWFs).These CWFs represent the weights of the different shapes in the different nuclear states.
We can then compare the systematics of the excitation energies of the lowest positive-and negative-parity bands obtained with the present SCCM method.In Fig. 10 we show the theoretical results and the experimental data that include the newly identified excitation energies corresponding to 150 Ba levels.The qualitative behavior of the experimental results is nicely reproduced along the isotopic chain.However, the theory predicts a sharper transition (both in positive-and negative-parity states) from an almost spherical nucleus ( 142 Ba) to quadrupole-deformed ones ( 144-150 Ba).The FIG. 11.Filled symbols show B(E2 : 2 + → 0 + ) in Ba isotopic chain.They are extracted from the measured lifetimes in the case of 148-150 Ba and from Ref. [12,13,29] in the lighter isotopes.The experimental data points are compared to results obtained from the calculations, shown with open symbols.evolution towards larger quadrupole deformations is seen in the lowering of the energies towards mass 150.It has to be noticed that the theoretical spectra are systematically stretched in the positive-parity band.This is a well-known effect in this kind of calculations and it can be solved if triaxial and time-reversal symmetry broken states (cranking states) are included in this framework [34].
The increase in the quadrupole collectivity is also observed in the B(E2 : 2 + → 0 + ) values along the isotopic chain.In Fig. 11 we show a comparison between the theoretical predictions and the experimental data, where we have added the B(E2) values extracted from the newly measured lifetimes for 148-150 Ba.The experimental trend is well reproduced by the present calculations, although theory predicts smaller values for 142 Ba and 148-150 Ba.
Finally, we study the interplay between quadrupole and octupole deformations in these nuclei by analyzing the ground state CWFs obtained with the present SCCM calculations.These CWFs are shown in Figs.9(f)-9(j), where only the β 3 0 quadrant is plotted because they are symmetric under a reflection about the β 3 = 0 axis.The distributions are all peaked at values of β 3 different from zero and not larger than ∼0.2. 142Ba being almost spherical, the rest of the nuclei show a definite quadrupole deformation, which becomes more stable with increasing number of neutrons, with a β 2 parameter around 0.3 in 150 Ba.We observe that the ground state CWFs are peaked at the minimum of their corresponding potential well given in Figs.9(a)-9(e).In each panel the isosurface of the spatial density (0.08 fm −3 ) is shown, in order to better visualise the shape of the isotopes.These densities are computed with the HFB wave functions that correspond to the maximum of each CWF, indicated by the arrows.Pear shapes are clearly seen, 142 Ba being more spherical and 150 Ba more quadrupole deformed.To analyze more quantitatively these results, the mean values and fluctuations of the ground state quadrupole and octupole deformations are shown in Table III.Here we observe that the spreading in the β 3 direction is larger than in the quadrupole.Therefore, SCCM calculations predict both well-established quadrupole and soft octupole deformation in 144-150 Ba ground states.These results are consistent with the small excitation energies for the 3 − states in 142-150 Ba and the large B(E3) values obtained experimentally for 144-146 Ba isotopes [35].

V. CONCLUSIONS
This paper reports on the first measurement of the β decay of 150 Cs → 150 Ba and presents an extension of the partial level scheme for the 148 Cs → 148 Ba decay.They were measured at ISOLDE using the IDS setup which allowed also extraction of the lifetime for the 2 + 1 level in both daughter nuclei, using fast-timing techniques.
The level schemes for the two decays have been extracted from βγ γ coincidences, and spins and parities have been proposed based on their decay pattern, β feeding, and systematics.Low-spin positive-and negative-parity states have been effectively populated in both nuclei, confirming an interplay between GT and ff transitions.
A systematic evaluation of the low-lying levels in the Ba isotopic chain has been presented: the decrease in the excitation energy for the yrast band levels, with a parallel increase of the B(E2 : 2 + → 0 + ) transition probability, confirms an evolution towards increasing quadrupole deformation in these exotic members of the Ba isotopic chain.The low-spin pattern of the level scheme of 150 Ba, showing positive-and negative-parity states connected by strong transitions, suggests the persistence of octupolar correlations, which have been established for the lighter Ba isotopes.
The experimental results have been compared to SCCM calculations with the Gogny D1S EDF including particle number, parity, and angular momentum restoration as well as quadrupole and octupole shape mixing.The calculations describe qualitatively the experimental excitation energies and transition probabilities in the whole isotopic chain ranging from A = 142 to 150.A sharper quadrupole shape transitionfrom almost spherical ( 142 Ba) to well prolate deformed nuclei 144-150 Ba-than that in the experimental data is obtained, and, in particular, theory also predicts a smaller quadrupole collectivity in 142 Ba and 148-150 Ba.Octupole deformed ground states, with some softness along this degree of freedom, have been obtained in all of the isotopes studied here.
Future work to include triaxial and cranking intrinsic wave functions in the calculation is expected to return a better agreement with the experimental data.

FIG. 2 .
FIG.2.Time distribution relative to the proton pulse of the 109.8and141.6-keV transitions from 147 Ba and 148 Ba, respectively.After renormalizing the 141.6-keV time distribution (factor 0.0286), the overlap of the two distributions is in very good agreement with the assumption that levels in 147 Ba are only fed through the β-n decay of 148 Cs (T 1/2 = 152(1) ms) rather than the β decay of 147 Cs (T 1/2 = 230(1) ms).
Pm, are visible.Panels (c), (d), (e), and (f) show βγ γ coincidence spectra obtained by gating on the transitions indicated in each panel.Coincident peaks originating from the Compton scattering of the strong background 511-keV γ activity are visible, and are indicated in the spectra by the "&" symbol.

FIG. 5 .
FIG. 5. (a) 150 Cs decay spectrum acquired within 200 ms from the proton pulse impinging on the UCx target.Long-lived activity (b) has subtracted.Lines belonging to daughter decay are by their energy, while asterisks indicate lines coming from the β-delayed neutron emission channel (i.e. 149Ba) and its subsequent decay (marked by the dollar symbol).Panels (c), (d), (e) and (f) show spectra in coincidence with transitions at 101.1, 217.1, 597.4 and 380.2 keV, respectively.The presence of peaks arising from Compton scattering of the 511-keV γ rays is indicated by "&" symbol.

FIG. 10 .
FIG. 10.Systematic comparison of experimental (filled symbols)and theoretical (open symbols) energy spectra in the Ba isotopic chain.Positive-parity states are grouped in panel (a) while the negative-parity ones are grouped in panel (b).Levels in142-148 Ba are taken from the literature[9].

TABLE II .
Levels populated in the decay

TABLE III .
Mean values and fluctuations of the axial quadrupole (β 2 ) and octupole (β 3 ) deformation parameters calculated for the SCCM ground states given in Figs.9(f)-9(j).