E0 transition strength in 160 Yb

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2 E0 transition strength in 160 Yb N. Blasi 1, F.Camera 1,2, B. Melon 3, A. Nannini 3,4, M. Ottanelli 3,4, M. Rocchini 3,4, A. Saltarelli 5, J.J. Valiente-Dobon 6, O.Wieland 1 1 INFN-Sezione di Milano, Italy 2 Dipartimento di Fisica, Università di Milano, Italy 3 INFN Sezione di Firenze, Italy 4 Dipartimento di Fisica e Astronomia, Università di Firenze, Italy 5 Scuola di Scienze e Tecnologie- Sezione di Fisica, Università di Camerino and INFN Sezione di Perugia, Italy 6 INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy. Abstract: The study of E0 strength in the A~ mass region was undertaken by our group few years ago searching for X(5) signatures. Within this study, we performed measurements on 156 Dy, 160 Er and 162 Yb, and a measurement on 164 Yb was approved and scheduled in autumn. The proposed measure on 160 Yb is needed to confirm the expected trend of the E0 strength trend in the Yb isotope chain. The experiment aims to complete the study on the E0 transitions in the Yb light isotopes, and is part of a larger study of E0 strengths in the A~ region. Within this project, we measured the E0 transition from the first 0 + excited state to the ground state in 156 Dy and 160 Er [1]. These nuclei were proposed [2, 3] as possible candidates for the X(5) symmetry, on the base of the properties of their low lying spectra. The X(5) dynamic symmetry was introduced few years ago [4], as the shape transition point between the spherical and the axially deformed shapes which can be described analytically. The experimental signatures for the X(5) symmetry are the following : (i) the energy of the yrast states and the transition strength between them show a characteristic behavior lying between those of a vibrator and a rotor; (ii) the position of the first excited 0 + state is 5.67 times the energy of the first 2 + level; (iii) the non yrast states based on the first excited 0 + state have larger energy spacings than the yrast sequence; (iv) inter-sequence B(E2) values show a characteristic pattern. In this context, another observable that should be taken into consideration is the E0 transition strength from the first excited 0 + state to the ground state, which has particularly large values in the transitional region between spherical and deformed nuclei [5, 6]. For example, 154 Gd, which shows all the signatures of a X(5), has the largest monopole strength ρ 2 value among the Gd isotopes. The critical symmetry point X(5) can be approximated by numerical calculations, which have the advantage of describing the transition from spherical to rotational through the chain of isotopes identifying the position of the critical point when it lies between two neighboring nuclei. Unfortunately, ρ 2 values are quite difficult to measure, for its determination requires the accurate knowledge of the lifetime of the initial level. A more common quantity related to the E0 strength

3 is the X(E0/E2) value, defined as the ratio between the reduced B(E0; ) and B(E2; ) transition probabilities.. As can be seen in figure 1, the X(E0/E2) value in a X(5) nucleus (see the best X(5) example, 154 Gd, black dot in the grey square in the figure) has an intermediate value between the spherical (minimum) and deformed (maximum) value. In the same figure, also the known X(E0/E2) values for the Dy and Er isotopes are shown. For the nuclei highlighted by circles, 156 Dy, 160 Er and 162 Er, the E0 strength have been obtained by our group, using the mini orange spectrometer in LNS [7,8,9], the other values are taken from the literature. The results obtained for the two X(5) candidates, 156 Dy and 160 Er, were different, although both were expected to lie very close to the critical point: the measured factor X(E0/ E2, ) was found to be 0.061±0.011 for 156 Dy [7,8], a value close to the 154 Gd one, and 0.11 ±0.03 for 160 Er [7]. Figure 1. X(E0/E2) values in the A~150 region for Gd (Z=64), Dy (Z=66), Er (Z=68) isotopes. 154 Gd (value within the grey square) is the best known example of X(5) nucleus. Values highlighted by circles were measured by our group at LNS. Theoretical calculations indicate that in this region the change in shape is quite abrupt from the spherical limit to the critical point and smoother from this point to the deformed rotor limit. It turns out that the 156Dy nucleus has a rather spherical shape, while 160 Er results already deformed, although still quite close to the critical point. This seems to explain the difference in the experimental X(E0/ E2, ) values. From these results, it appears that the critical point is quite a sharp point that could occur in between two isotopes, and therefore hardly observable. In a recent paper [3], McCutchan et al. investigated the low lying structure of 162 Yb (Z=70), clarifying few inconsistencies in the level scheme. They suggest that 162 Yb represents a rather promising X(5) candidate, since the yrast states energies and the position of the first excited 0 + state well agree with the predictions of the model. On the other hand, other properties, such as B(E2) values for the yrast band, are in disagreement. No X(E0/E2) value was known in the Yb isotopes previous our measurement in 162Yb using the mini orange spectrometer in LNS [10]. The value X(E0/E2) = 0.078±0.022 we obtained [11] well compares with the values found for 154 Gd and 156 Dy. To draw a definitive conclusion on the 162Yb properties, it would be important to study the trend of the X(E0/E2) values from 160 Yb to 166 Yb. In figure 2, the measured X(E0/E2) value for 162 Yb is compared to two IBA calculations perfomed for the Yb

4 isotope chain (see ref [11] for details). Figure 2. The X(E0/E2) value measured for 162 Yb at LNS with the MOS is compared to two IBA calculations for the Yb isotope chain (see ref [11] for details). A new measurement on 164 Yb was approved already by the PAC, and is scheduled in the last part of the year. To complete the systematic, we propose to measure the branching between the (E0, ) and the (E2, ) transitions in 160 Yb. The first excited 0 + state in this nucleus lies at 1086 kev excitation energy [12]. This state decays to the 21 + level via the 843 kev γ ray [12]. We will measure the gamma transitions with a germanium detector, while conversion electrons will be measured at the same time with the mini orange spectrometer. The transition can only proceed via electron conversion, therefore no gamma transition is expected at 1086 kev. The state in 160 Yb will be populated via beta decay of 160 Lu, produced by the 144 Sm( 19 F, 3n) reaction at 85 MeV. According to the code PACE4, the reaction populates 160 Lu with a cross section σ of ~130 mb, and 160 Yb directly with σ ~ 90 mb. This accounts for about 67% of the total cross section. The half lifetime of 160 Lu is ~36 sec, therefore we will use the beampulsing device with a pulsing time of 1 minute, alternating irradiation and measuring period (cycles). 160 Yb is also unstable and decays with a half lifetime of 4.8 min to 160 Tm, which decays to 160 Er in a half lifetime of 9.4 min. This implies that it will be necessary to it will be necessary to introduce a waiting time regularly, in order to clean the spectrum from the presence of 160 Er. At the same time, it has the advantage of giving us the opportunity to measure also 160 Er, which has few known E2 transitions in the energy region of interest, as an efficiency calibration and a normalization check point. For that purpose, we plan to measure one full day with the pulsing time of the beam-pulsing device set at 10 minutes. In this way, we will enhance the population of 160 Er and reduce to negligible the population of 160 Yb. The MOS apparatus is mounted on the 60 beam line. Gamma rays will be detected by a HPGe detector, while conversion electrons will be detected by the MOS. The expected cross section for populating 160 Lu is smaller to that we had in the previous experiment on 162 Yb. Based on that experience, the required beam time is 8 days (24 BTU) plus 1 day (3 BTU) for the calibration run. Before beam time, at least six days are needed for preparation with free access in the experimental hall in order to align the MOS, pumping, cooling the detectors and make calibrations.

5 References 1 N. Blasi et al., Phys. Rev. C 90, (2014) 2 R.M. Clark et al., Phys. Rev. C 68, (2003) 3 E.A. McCutchan et al., Phys. Rev. C 69, (2004) 4 F. Iachello, Phys. Rev. Lett. 87, (2001) 5 K. Heyde and R.A. Meyer, Phys. Rev. C 37, 2170 (1988) 6 P. von Brentano et al., Phys. Rev. Lett. 93, (2004) 7 Lo Bianco et al., LNS Activity Report 2007, p Lo Bianco et al., LNS Activity Report 2009, p S. Nardelli, Tesi di Dottorato, Università di Camerino N. Blasi et al., LNS Activity Report 2011/12, p N. Blasi et al., Phys. Rev. C 88, (2013) 12 C.W. Reich, Nucl. Data Sheets 105, 557 (2005)

6 Status report of the studies on E0 transition strength in A~ nuclei The study on E0 strength in nuclei in the transitional region around A~160 initiated few years ago, with the aim of searching for X(5) nuclei or phase transitions. The following measurements were performed at LNS: E0 strength in 156 Dy; reaction: 148 Sm( 12 C, 4n) at 72 MeV; 24 BTU E0 strength in 160 Er; reaction: 150 Sm( 14 N, 4n) at 72 MeV; 24 BTU E0 strength in 162 Yb; reaction: 147 Sm( 19 F, 4n) at 95 MeV; 24 BTU The following measurement has been approved in 2016 to be performed at LNS: E0 strength in 164 Yb; reaction: 150 Sm( 19 F, 5n) at 105 MeV; 24 BTU Results have been published in the following papers: N. Blasi et al., Phys.Rev. C 88, (2013) N. Blasi et al., Phys.Rev. C 90, (2014) During the data analysis a method to measure the transmission curves using continuous and discrete calibration sources has been developed and published in: L.Guerro, N.Blasi, A.Saltarelli, Nucl. Instr. And Methods A739, 32 (2014) With this method, the transmission curve can be measured with an error of few percent and this reduces the error on the measured transition strengths. The following figures show the measured transmission curves for 156 Dy (left) and 162 Yb (right). The points indicate known transitions used as calibration points. The results of the analysis on the E0 strengths in this mass region are summarized in the following figures for Dy and Er, where the X(E2/E0) values (ratios between E0 and E2 strength) are plotted.

7 The red open circles are the values we measured. The value for 162 Er was a side product of the measurement on 162 Yb. Experimental values are compared to IBA calculations (lines) with different assumptions, as explained in the cited papers. The result for 162 Yb is shown in the following figure: The measured value is the only one known in the Yb isotopes in this region. It is compared with IBA calculations (lines) as in the previous case.