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Recent lifetime measurements revealed that the B(E2)$_{4^+/2^+}$ ratio is far less than unity in several nuclei in the mass region 160 ≤ A ≤ 170 namely $^{166}$W [1], $^{168}$Os [2], $^{170}$Os [3], and $^{172}$Pt [4]. From a theoretical point of view, the origin and the underlying structure of this anomalous behavior remains unexplained. On the other hand, a quantum phase transition from seniority-conserving structure to a collective regime as a function of neutron number around N ≈ 90 - 94 has been proposed for these nuclei from phenomenological point of view [4]. In the present work, we aimed to extend our investigation of the anomalous B(E2; 4$^+ \rightarrow $ 2$^+$)/ B(E2; 2$^+ \rightarrow $ 0$^+$) ratio phenomenon this mass region in order to provide more data for the future theoretical calculations. We chose $^{164}$W as a good candidate to investigate because, it has a pivotal position with N = 90 to test the hypothesis of a quantum phase transition as the mechanism for the B(E2)$_{4^+/2^+}$ anomaly. We used the DPUNS [5] plunger device in conjunction with the RITU gas-filled separator [6] and the JUROGAM II and GREAT [7] spectrometers for the measurement of mean lifetimes of excited states in $^{164}$W. The fusion evaporation reaction $^{106}$Cd($^{60}$Ni,2p2n)$^{164}$W* at a beam energy of 270 MeV provided an initial recoil velocity v/c of 3.3%. The analysis of the data revealed that 164W has a similar B(E2)$_{4^+/2^+}$ = 0.56(13) < 1 anomaly as in the $^{166}$W, $^{168}$Os, $^{170}$Os, and $^{172}$Pt nuclei. Experimental B(E2) values have been compared to the state-of-the-art beyond-mean-field calculations. However, the theoretical predictions disagree with experimental findings. In the present work, details of the experimental procedure and analysis steps will be explained and the results for the lifetime measurements of first excited 2$^+$ and 4$^+$ states will be presented.
This work has been supported by the UK Science and Technology Facilities Council under grants ST/P004598/1, ST/L005670/1 and ST/L005794/1; the Scientific and Technological Research Council of Turkey (TUBITAK Project No: 117F508); the EU HORIZON2020 programme “Infrastructures”, project number: 654002 (ENSAR2) and by the Academy of Finland under the Finnish Centre of Excellence Programme (Nuclear and Accelerator Based Physics Pro-gramme at JYFL). The UK/France (STFC/IN2P3) Loan Pool and GAMMAPOOL network are acknowledged for the HPGe escape-suppressed detectors of the JUROGAM II array.
[1] B. Saygı et al., Phys. Rev. C96, 021301 (2017).
[2] T. Grahn et al., Phys. Rev. C94, 044327 (2016).
[3] A. Goasduff et al., Phys. Rev. C100, 034302 (2019).
[4] B. Cederwall et al., Phys. Rev. Lett. 121, 022502 (2017).
[5] M. Taylor et al., Nucl. Instrum. Methods Phys. Res. A707, 143 (2013).
[6] J. Saren et al., Nucl. Instrum. Methods. Phys. Res. A. 654, 508 (2011).
[7] R.D. Page et al.,Nucl. Instrum. Methods. Phys. Res. B. 204, 634 (2003).
