gan zaiguo institute of modern physics, chinese academy of sciences alpha decay of the...

GAN ZaiguoInstitute of Modern Physics, Chinese Academy of Sciences

Alpha decay of the neutron-deficient uranium isotopes

Institute of Modern Physics, Chinese Academy of Sciences

Outline

Lanzhou Gas-filled Separator – SHANS

Isotope 216U and its isomer 216mU

Search for the isotope 215U

-decay lines of isotope 217U

Summary

2

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 (Ek<10 AMeV)

I ~ 3×1013 pps (C-Ne)

~ (1-5)×1012 pps (Mg-Zn)

Spectrometer for Heavy Atoms and Nuclear Structure

Gas-filled recoil separator – SHANS


Gas-filled recoil separator – SHANS

Fixed target

Detection system

Configuration Qv-D-Qv-Qh

Trajectory length 6.5 m

Acceptance

280mmmrad (h) 450mmmrad (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.

0 00 1/3 1/3

0

( / ) ( / )( ) 0.0227 0.0227 0.0227 [T m]

( / )

ave

A v v A v v AB

q Z v v Z


Motivation

5

Search for most neutron-deficient uranium isotope 215,216U a-decay studies provide important info for the nuclear structure


Excitation function calculated by HIVAP

6


Experiment

Reaction channels: 40Ar+180W216U+4n; 40Ar+180W215U+5n Beam: 40Ar12+

– Energy: 189.5 MeV ( for 216U), 103 hours

204.5 MeV (for 215U), 89 hours

207.6 MeV (for 215U), 157 hours– Delivered by the SFC of HIRFL in Lanzhou– Typical intensity: 150–250 pnA

Target: enriched 180W – 180W (91.4% 180W and 8.5% 182W)– thickness:41g/cm2(C)+481g/cm2(W) +14g/cm2(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


EVR - α1 (216U)

8

Vetoed with MWPC and punch-through detectors

Ecot=187.2 MeV

Dt (EVR-a1) = 50 ms


EVR - α1 - α2 (216U)

9

Dt (EVR-a1) = 50 msDt (a1-a2) = 6 s


Measured decay values

10

216gU: Ea = 8.384(30) MeV

T1/2 = 4.72+4.72-1.57 ms

216mU: Ea = 10.582(30) MeV

T1/2 = 0.74+1.34-0.29 ms

N=124 N=126L. Ma, Z. Y. Zhang, et al, Phys. Rev. C 91, 051302 (2015)


Ip identification based on reduced a-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)


I p = 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).


[email protected] [email protected]

α decay with an energy of 8335keV following an implanted recoil nucleus after 5.5ms

Physics Letters B 748 (2015) 199–203


RIKEN

14



17

Dt (EVR-a1) = 100 msDt (a1-a2) = 5 s

Dt (EVR-a1) = 100 ms

Nuclide Ea T1/2

215U 8.428(30) MeV 1.330.290.73 ms

Ecot=205.5 MeV


Cross sections for the 215U and 216U

18


-decay lines of isotope 217U

21.31/2 5.7

8.005(20) MeV

15.6 ms

E

T

The research status of 217U

40Ar+182W->217U+5n

O.N. Malyshev et al., Eur. Phys. J. A 8, 295(2000)


RIKEN Accel. Prog.Rep.45(2012)

They tentatively assigned this events to the decays of an isomeric state of 217U.

18 events

82Kr+138Ba->217U+3n

8200 keV


Two alpha energies : 8.215(30)MeV,8.098(30) MeV

The 217U observed in this work

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

Institute of Modern Physics, Chinese Academy of SciencesK.Valli et al., Phys. Rev. 176, 1377(1968)

Based on the -decay schemes of the 125-neutron isotopes (215Th, 213Ra, 211Rn and 209Po), more than one decay energies (with much large decay branch) should be exist in the decay of 217U from the ground state.

Fact 2

P.Kuusiniemi et al., Eur. Phys. J. A 30, 551(2006)


K.Valli et al., Phys. Rev. 176,1377(1968)

The particle energies were predicted by extrapolation method


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 209Th). Open squares refer to literature values taken from appropriate references. The red solid squares refer to the values measured in this work.


Summary and outlook

Two new neutron-deficient uranium isotopes 215,216U 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 216U 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 215U 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 217U.

The 8.215(30) MeV line was assigned to the decay of 217U from ground state to ground state transitions and the 8.098(30) MeV and 8.005(20) MeV are the decays of 217U populating the low-lying two excited states. In addition, an improved half-life of 20.4(-6.3/+16.5) ms for 217U was obtained.

However, our conclusions about 217U (unpublished) need further verification due to the low statistics (five decay chains were observed).


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

gan zaiguo institute of modern physics, chinese academy of sciences alpha decay of the...

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