1. GAN Zaiguo Institute of Modern Physics, Chinese Academy of Sciences Alpha decay of the neutron-deficient uranium isotopes

2. Institute of Modern Physics, Chinese Academy of Sciences Outline  Lanzhou Gas-filled Separator – SHANS  Isotope 216 U and its isomer 216m U  Search for the isotope 215 U   -decay lines of isotope 217 U  Summary 2

3. Institute of Modern Physics, Chinese Academy of Sciences 3 Gas-filled recoil separator in Lanzhou HIRFL-CSR layout SHANS SFC: Sector Focusing Cyclotron  K ~ 70 MeV (E k <10 AMeV)  I ~ 3×10 13 pps (C-Ne) ~ (1-5)×10 12 pps (Mg-Zn) Spectrometer for Heavy Atoms and Nuclear Structure Gas-filled recoil separator – SHANS

4. Institute of Modern Physics, Chinese Academy of Sciences Gas-filled recoil separator – SHANS Fixed target Detection system ConfigurationQv-D-Qv-Qh Trajectory length6.5 m Acceptance 280  mm  mrad (h) 450  mm  mrad (v) Central trajectory radius 1.8 m Bending angle of D 52  Max. magnetic rigidity 2.88 Tm The gas-filled separators use the different magnetic rigidities of the recoils and projectiles traveling through a low-pressure (about 1 mbar) gas-filled volume in a magnetic dipole field.

5. Institute of Modern Physics, Chinese Academy of Sciences Motivation 5  Search for most neutron-deficient uranium isotope 215,216 U   -decay studies provide important info for the nuclear structure

6. Institute of Modern Physics, Chinese Academy of Sciences Excitation function calculated by HIVAP 6

7. Institute of Modern Physics, Chinese Academy of Sciences Experiment  Reaction channels: 40 Ar+ 180 W  216 U+4n; 40 Ar+ 180 W  215 U+5n  Beam: 40 Ar 12+ –Energy: 189.5 MeV ( for 216 U), 103 hours 204.5 MeV (for 215 U), 89 hours 207.6 MeV (for 215 U), 157 hours –Delivered by the SFC of HIRFL in Lanzhou –Typical intensity: 150–250 pnA  Target: enriched 180 W – 180 W (91.4% 180 W and 8.5% 182 W) –thickness:41  g/cm 2 (C)+481  g/cm 2 (W) +14  g/cm 2 (C) –Beam energy at the center of target: 187.2 MeV, 202.4 MeV and 205.5 MeV  Experimental setup: Gas-filled recoil separator SHANS –He pressure:  0.6 mbar  Detection system : 2 MWPC + Si-box + Veto+gamma  Calibration:  source + test reaction  Data acquisition system ： VME –Dead time: <16  s

8. Institute of Modern Physics, Chinese Academy of Sciences EVR - α1 ( 216 U) 8 Vetoed with MWPC and punch-through detectors E cot =187.2 MeV  t (EVR-  1) = 50 ms

9. Institute of Modern Physics, Chinese Academy of Sciences EVR - α1 - α2 ( 216 U) 9  t (EVR-  1) = 50 ms  t (  1-  2) = 6 s

10. Institute of Modern Physics, Chinese Academy of Sciences Measured decay values 10 216g U: E  = 8.384(30) MeV T 1/2 = 4.72 +4.72 -1.57 ms 216m U: E  = 10.582(30) MeV T 1/2 = 0.74 +1.34 -0.29 ms N=124 N=126 L. Ma, Z. Y. Zhang, et al, Phys. Rev. C 91, 051302 (2015)

11. Institute of Modern Physics, Chinese Academy of Sciences I  identification based on reduced  -decay width 11 Phys. Rev. C 75, 054307 (2007) Eur. Phys. J. A 25, 397 (2005) Eur. Phys. J. A 34, 355 (2007) Phys. Rev. C 91, 051302 (2015) [1] J. O. Rasmussen, Phys. Rev. 113, 1593 (1959) [2] Y. Ren, and Z. Ren, Phys. Rev. C 85, 044608 (2012)

12. Institute of Modern Physics, Chinese Academy of Sciences I  = 8 + in N=124 and N=126 isotones 12 [1] K. Hauschild et al., Phys. Rev. Lett. 87, 072501 (2001). [2] E. Caurier et al, Phys. Rev. C 67, 054310 (2003).

13. Institute of Modern Physics, Chinese Academy of Sciences 13 maoshh@impcas.ac.cn α decay with an energy of 8335keV following an implanted recoil nucleus after 5.5ms Physics Letters B 748 (2015) 199–203

14. Institute of Modern Physics, Chinese Academy of Sciences RIKEN 14

15. Institute of Modern Physics, Chinese Academy of Sciences 15 Search for the isotope 215 U

16. Institute of Modern Physics, Chinese Academy of Sciences 16 Search for the isotope 215 U

17. Institute of Modern Physics, Chinese Academy of Sciences Search for the isotope 215 U 17  t (EVR-  1) = 100 ms  t (  1-  2) = 5 s  t (EVR-  1) = 100 ms NuclideEE T 1/2 215 U8.428(30) MeV E cot =205.5 MeV

18. Institute of Modern Physics, Chinese Academy of Sciences Cross sections for the 215 U and 216 U 18

19. Institute of Modern Physics, Chinese Academy of Sciences  -decay lines of isotope 217 U The research status of 217 U 40 Ar+ 182 W-> 217 U+5n O.N. Malyshev et al., Eur. Phys. J. A 8, 295(2000)

20. Institute of Modern Physics, Chinese Academy of Sciences RIKEN Accel. Prog.Rep.45(2012) They tentatively assigned this events to the decays of an isomeric state of 217 U. 18 events 82 Kr+ 138 Ba-> 217 U+3n 8200 keV

21. Institute of Modern Physics, Chinese Academy of Sciences Two alpha energies : 8.215(30)MeV,8.098(30) MeV 21 The 217 U observed in this work However, we prefer to assigned the 8.215(30) MeV to the  decay of 217 U from ground state to ground state transitions (1/2 -  5/2 - ) and 8.098(30) MeV was assigned to the  decay of 217 U populating one low-lying excited state in 213 Th (1/2 -  1/2 - ). In addition, the previously reported value 8.005(20) MeV can be assigned to the decay of 217 U from the ground state to an another excited state in 213 Th (1/2 -  3/2 - ). It is inappropriate to assign this two  particle energies to the decays of isomeric states.

22. Institute of Modern Physics, Chinese Academy of Sciences K.Valli et al., Phys. Rev. 176, 1377(1968) Based on the  -decay schemes of the 125-neutron isotopes ( 215 Th, 213 Ra, 211 Rn and 209 Po), more than one  decay energies (with much large decay branch) should be exist in the decay of 217 U from the ground state. Fact 2 P.Kuusiniemi et al., Eur. Phys. J. A 30, 551(2006)

23. Institute of Modern Physics, Chinese Academy of Sciences K.Valli et al., Phys. Rev. 176,1377(1968) 23 The  particle energies were predicted by extrapolation method

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25. Institute of Modern Physics, Chinese Academy of Sciences 8005(20) keV measured in previous work 8215(30) keV measured in this work Fact 3 The previous reported  -particle energy of 8.005(20) MeV was plotted in the figure. But it deviates from the systematic regularities significantly. If we put the measured value 8.215(30) MeV on the plot, the value fits well into the systematic regularities. Alpha particle energies of ground state to ground state transitions for neutron-deficient Rn, Fr, Ra, Ac, Th, Pa and U isotopes (except 209 Th). Open squares refer to literature values taken from appropriate references. The red solid squares refer to the values measured in this work.

26. Institute of Modern Physics, Chinese Academy of Sciences Summary and outlook  Two new neutron-deficient uranium isotopes 215,216 U were synthesized on the gas- filled recoil separator SHANS  The  -particle energies and half-lives were determined to be 8.384(30) MeV and 4.72(- 1.57/+4.72) ms for the ground state of 216 U and 10.582(30) MeV and 0.74(- 0.29/+1.34) ms for an isomeric state (8 + ). The  -particle energy and half-life of 215 U were determined to be 8.428(30)MeV and 0.73(-0.29/+1.34) ms, respectively.  Two new  lines with energies of 8.215(30) MeV and 8.098(30) MeV were observed for the known isotope 217 U.  The 8.215(30) MeV line was assigned to the  decay of 217 U from ground state to ground state transitions and the 8.098(30) MeV and 8.005(20) MeV are the decays of 217 U populating the low-lying two excited states. In addition, an improved half- life of 20.4(-6.3/+16.5) ms for 217 U was obtained.  However, our conclusions about 217 U (unpublished) need further verification due to the low statistics (five decay chains were observed).

27. Institute of Modern Physics, Chinese Academy of Sciences Collaboration Thank you for your attention! IMP-CAS: Zaiguo Gan, Zhiyuan Zhang, Long Ma, Jianguo Wang, Lin Yu, Huabin Yang, Jian Jiang, Tianheng Huang, Zhong Liu, Xiaohong Zhou Nanjing Univ.: Zhongzhou Ren ITP-CAS: Shangui Zhou

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