Lifetime measurement in 195 Po

. The lifetime of the 17/2 + yrast state in 195 Po has been measured using the recoil distance Doppler-shift technique to be τ = 43(11) ps. The lifetime was extracted from the singles γ -ray spectra obtained by using the recoil-decay tagging method. The present work provides more information of the coupling schemes, shapes and conﬁguration mixing in neutron-deﬁcient odd-mass Po nuclei.


Introduction
In neutron-deficient Po nuclei, two protons away from the Z = 82 shell closure, competing spherical and deformed structures have been found to coexist and mix at low spin [1].When moving away from the N = 126 shell closure towards the neutron mid-shell at N = 104, the protonneutron interaction within the expanding valence space gives rise to quadrupole collectivity and, consequently, particle-hole intruder states are lowered in energy.In the neutron-deficient even-mass Po nuclei such intruding 0 + states have have been observed in α-and β-decay studies and associated with a π(4p − 2h) configuration [2] and an oblate minimum in the potential energy surface [3].
While shape coexistence in the light even-mass Po nuclei is rather well studied, information about the intruder states in corresponding odd-mass nuclei is somewhat scarce.Earlier in-beam and α-decay studies have revealed the low-lying level structure and bands built on the I π = 13/2 + α-decaying isomeric state in 193,195,197 Po [4][5][6].The structure of these bands has been presumed to emerge from the decoupling of the odd i 13/2 neutron from the even-mass core and particularly in the case of 195 Po, from the 194 Po core.The weak coupling picture is demonstrated by the level spacings of the levels above the 13/2 + state in odd-mass Po nuclei being close to the yrast level spacings in their even-mass neighbours.Recent lifetime measurements have revealed the yrast band and the ground state in 194 Po to be predominantly of the oblate intruder character [8].In 195 Po candidates for unfavoured states have been found.The fact that these states lie higher in energy a Electronic address: tuomas.grahn@liverpool.ac.uk than the favoured states further supports the decoupling picture and oblate deformation at low spin [4].
The structure of the 13/2 + state has been deduced to be an admixture of the spherical and oblate configurations in 193,195,197 Po [6], while in 191 Po shape staggering between the isomeric 13/2 + and the 3/2 − ground state has been observed [7].This can be seen as a sign towards the pure intruder configuration of the 13/2 + state in light Po nuclei.
Alternatively, in Ref. [9], it has been suggested that the collectivity of the yrast levels in Po nuclei with A < 199 emerges from harmonic vibrational motion, which is manifested by the near equal level spacings.However, recent lifetime measurements and configuration mixing calculations of the angular momentum projected mean field states in 194 Po [10] and 196 Po [11] support the oblate deformed intruder interpretation of the yrast bands in those nuclei.
To provide further data concerning the low-energy structure of 195 Po, a Recoil Distance Doppler-Shift (RDDS) lifetime measurement has been carried out.The present study provides new data to aid the understanding of the evolution of collectivity in Po nuclei near the neutron midshell.

Experimental details
The present study was carried out at the Accelerator Laboratory of the University of Jyväskylä, where excited 195 Po nuclei were produced in the 113 Cd( 86 Kr,4n) 195 Po reaction at a beam energy of 382 MeV.In the present work experimental conditions were optimised for the study of 196 Po, which has been reported elsewhere [11].The Köln plunger device was combined with the JUROGAM Ge-detector array and the gas-filled separator RITU [12].With a stretched 1 mg/cm 2 113 Cd target on a 1 mg/cm 2 Ta support facing the beam, an initial recoil velocity of v/c = 3.5 % was obtained.A 1 mg/cm 2 thick Mg foil replaced the standard stopper foil of the plunger device, thus allowing the fusionevaporation residues to recoil into RITU.The JUROGAM Ge-detector array consisting of 43 Eurogam Phase I type Compton-suppressed Ge detectors [13] was used to detect prompt γ rays.
The gas-filled separator RITU was used to separate the recoils from the beam and other reaction products and transport them to the GREAT spectrometer [14] at the focal plane of the separator.At the entrance of GREAT recoiling nuclei passed through the MultiWire Proportional Counter (MWPC), which recorded their energy loss and timing information.After the MWPC the recoils were implanted into a pair of Double-sided Silicon Strip Detectors (DSSDs) providing the arrival time of the recoiling nucleus, and information of its subsequent radioactive decay.By using the timing and energy loss information of the MWPC and the timing information of the DSSDs, the selection of the recoils was made.Prompt γ rays were assigned to 195 Po according to principles of the Recoil-Decay Tagging (RDT) method [15].The α decays originating from the isomeric 13/2 + state (E α = 6699 keV, t 1/2 = 1.92 s [16]) were temporally and spatially correlated with the detected recoils.Thus, RDT singles γ-ray spectra of 195 Po were constructed.The data were collected using the triggerless Total Data Readout data acquisition system [17].
RDT singles γ-ray spectra were reconstructed using the GRAIN software package [18] from the data recorded with 13 different target-to-degrader distances (20 µm-6 mm).Due to their suitable efficiency and angular position, only ten of the JUROGAM Ge detectors, positioned at an angle of 134 • with respect to the beam direction, could be used in the final RDDS analysis.The typical separation of the fully Doppler-shifted and degraded components in the final γ-ray spectra was on the order of 2 keV at 319 keV.Sample spectra are shown in Fig. 1.

Results
The lifetime of the 17/2 + state, decaying by a 319-keV γ-ray transition to the α-decaying 13/2 + state in 195 Po [4] was extracted from the singles RDT γ-ray spectra using the Differential Decay Curve Method (DDCM) [19].The intensity of the 319-keV γ-ray transition was normalised by the sum of the respective fully-shifted (I s ) and degraded components (I d ) and a decay curve I s /(I d + I s ) was constructed.The 17/2 + state was fed up to ∼ 60% from the 21/2 + state.As it has been discussed elsewhere [10,11], in certain cases it is reasonable to assume the similar time behaviour for the unobserved feeding than that for the observed one.For the present DDCM analysis, this is justified as the 17/2 + state is fed mainly by the intraband transitions from the 21/2 + state and the feeding time is not particularly long.With this feeding assumption, the final lifetime of τ = 43(11) ps was extracted, which corresponds to the reduced transition probability B(E2) value of 80(20) W.u. The lifetime determination of this state is illustrated in Fig. 2.
By using the rotational model, the absolute values for the transition quadrupole moment Q t and the transition quadrupole deformation parameter β 2 | = 0.17(3) [8]).

Discussion
In the light Po nuclei, and in particular around 192,194 Po, the yrast level patterns of odd-mass nuclei are close to those of the adjacent even-mass ones.The level patterns built on top of the 13/2 + state in odd-mass Po nuclei have been interpreted either as a weak coupling of the odd i 13/2 neutron to the nearly spherical even-mass core [9] or, as a decoupling of the i 13/2 neutron hole from the evenmass oblate core [4].The latter incorporates the particlehole intruder states observed in neighbouring even-mass Po nuclei.Indeed, such behaviour has been observed and discussed in Ref. [6].For the Po nuclei with A > 198 the level patterns can be successfully described by the weak coupling of the odd neutron to the near-spherical core.In the lighter Po nuclei (A < 198) unfavoured states have been observed [1,5].In the odd-mass 191−197 Po nuclei the unfavoured 15/2 + and 19/2 + states lie higher in energy than the corresponding favoured 17/2 + and 21/2 + states.This is consistent with the decoupling scheme.However, in 191 Po the unfavoured states have been observed at lower excitation energies than the favoured states.This can be seen as a sign of a change towards the strong-coupling scheme and prolate deformation in the light Po isotopes [1].In Ref. [6], potential energy surface calculations have been carried out for the light Po nuclei predicting an oblate minimum for 195 Po, associated with the ν1i 13/2 ⊗ π(4p − 2h) configuration.A spherical minimum (ν3p 3/2 ⊗ π(2p − 0h)) has been predicted to lie lower in energy, representing the 3/2 − ground state of 195 Po.However, based on the α-decay hindrance factors, both of these states are deduced to be an admixture of the spherical and oblate configurations.
In Fig. 3, the B(E2) values of the 2 + → 0 + yrast transitions in 194 Po and 196 Po and that for the transition to the 13/2 + isomeric state in 195 Po have been illustrated.Similar level energies and the B(E2) values of the 2 + state in 194 Po and of the 17/2 + state in 195 Po support the decoupling scheme of the odd neutron from the even-mass 194 Po core.Hence, the data in Fig. 3 suggest similar structure for the isomeric 13/2 + state in 195 Po and the ground state in 194 Po, which has been found to be predominantly oblate, with a small admixture of near-spherical configuration [10].

Summary
In summary, the lifetime of the 17/2 + yrast state has been measured in 195 Po.The RDT method provided clean singles γ-ray spectra, thus allowing the extraction of the lifetime using the RDDS technique.A comparison of the B(E2) value, measured in the present work, with the corresponding values in its even-mass neighbours further supports the decoupling of the odd neutron from the evenmass core.

Fig. 1 .
Fig.1.Singles RDT γ-ray spectra recorded at the three targetto-degrader distances with ten JUROGAM detectors at 134 • .Sample fits showing the fully Doppler-shifted (left) and degraded (right) components of the transitions under investigation are also drawn.

2Fig. 2 .
Fig. 2. Lifetime determination of the 17/2 + state in 195 Po.Lifetime values calculated for different distances within the region of sensitivity are presented in the top panel.The decay curve, recorded with ten Ge detectors at 134 • is shown in the middle panel.The bottom panel shows the difference of the quantities Is/(Is + I d ) for the depopulating and direct feeding transitions.The line drawn through the experimental points is the derivative of the decay curve multiplied by the lifetime value.
This work has been supported through EURONS (European Commission contract no.RII3-CT-2004-506065), the Academy of Finland under the Finnish Centre of Excellence Programme 2006-2011 (Nuclear and Accelerator Based Physics contract 213503) and the UK STFC.The UK/France (STFC/IN2P3) Loan Pool and GAMMAPOOL network are acknowledged for the EUROGAM detectors of JUROGAM.CS and PTG acknowledge the support of the Academy of Finland, contract numbers 209430 and 111965, respectively.Discussions with A.N. Andreyev, P.-H.Heenen and M. Bender are gratefully acknowledged.
[11]) values of the 2 + → 0 + transitions in 194 Po[8], and 196 Po[11]and that for the 17/2 + → 13/2 + transition in195Po measured in the present work.The width of the arrow is proportional to the B(E2) value.Level energies and the B(E2) values are given in keV and W.u., respectively.