Transcript 3 Alpha decay process

第十四届核结构会议，2012年4月11-16，湖州师范学院
超重核的阿尔法衰变
张鸿飞
兰州大学核科学与技术学院
Outline
1
Introduction
2
The Method
3
Alpha decay process
4
Alpha decay of SHN
5
Summary
1
Introduction
Alpha deacy is a typical quantum barrier tunneling problem, which was
solved by WKB method several tens years ago. ( G.Gamow, Z.Phys.51
(1928)204; E.U. Condon and R.W.Gurney, Nature, 122 (1928) 439).
The question how alpha particle emission from the mother nucleus is
still open. The cluster-like theories suppose that alpha particle is
preformed in the nucleus with a certain preformation factor, while the
fission-like theories approaches consider that alpha particle is formed
progressively during the very asymmetric fission of the parent nucleus.
The experiment can not distinguish the two formation modes at this
moment. (Royer and Zhang, PRC 77 (2008) 037602).
Alpha decay is the main decay mode of the superheavy nuclei (SHN).
The investigation of properties of these nuclei is extremely intriguing for
exploring the position of the island of stability of the SHN, and
understanding some new, unexpected features of nuclear structure.
2
The method
(1) WKB approximation
(2) Alpha decay potential barrier
E  EV  ES  EC  EN
The generalized liquid drop model (GLDM)
(Royer, JPG 26 (2000) 1149)
Two improvements were made for the usual liquid drop model
(1) Quasi-molecular shapes
(2) Proximity energy
The nuclear energy EN has been introduced to take into account
the finite-range effects of the nucleon-nucleon force between the
close surfaces . (Royer, NPA 444, (1985) 477)
  0.9517 (1  2.6 I d2 )
is surface parameter.
The introduction of proximity force lowers the barrier of 7.3 MeV, and shift the
peak 2.1 fm towards a more external position for 264Hs.
3
Alpha decay process
There are two different decay modes for alpha decay
in the market, cluster-like and fission-like modes.
Cluster-like mode
Fission-like mode
The experimental investigation can not unambiguously
distinguish these two decay modes up to now.
In the framework of cluster-like mode
(Zhang and Royer, PRC 77, (2008), 054318)
(1)
(2)
(3)
(4)
The preformation factor P0 of an alpha cluster inside
the mother nucleus can be extracted from Eq. (1).
The penetration probability
plays the most important role
to determine the life-time of
alpha decay.
The preformation factor and
assault frequency reflect the
nuclear structure features of
the mother nucleus.
Clearly the closed shell structures
play the key role for the preformation
mechanism, and more the nucleon
number is close to the magic nucleon
numbers, the more the preformation
of alpha cluster is difficult inside the
mother nucleus.
Zhang et al., PRC 80, (2009) 057301
In the framework of fission-like mode
(Wang,Zhang, Zuo and Li, CPL 27, (2010), 062103)
log10v0
An approach to deal with the assault frequency
Zhang, Royer, Li, PRC 84 (2011) 027303
G is global quantum number.
 4, for( N , Z )  82

g i  5, for82  ( N , Z )  126
 6, for( N , Z )  126

The alpha decay process is rather a radioactivity emission process of a cluster
preformed on the surface of the nucleus but before the potential penetration.
4
Alpha decay properties of SHN
Zhang, Gao, Wang, Li, Zhao, Royer, PRC, 85 (2012) 014325
To find out the reasonable
alpha decay energy and
the features of SHN.
MMM results form
Wang et al., PRC 81 (2010)
044322, PRC 82 (2010) 044304.
Experimental data from Audi 2013.
The RMS deviation with respect
to 2149 measured nuclear masses
Is 0.441 MeV ( the corresponding
results with FRDM is 0.566 MeV).
Another impressed improvement is
the RMS deviation of 46 SHN is
reduced 0.263 MeV ( 0.566 MeV
for FRDM )
One proton separation energies and alpha decay energies of 162 isotones
and 184 isotones.
Potential energy surface of 270Hs and 298114 by Constrained Relativistic Mean Field
theory with the parameters NL3
The nucleus 270Hs is a double sub-magic nucleus and 298114 is a spherical
double magic nucleus.
Alpha decay life-times of Hs and Z=114 isotopes with WKB penetration
probability, and the potential barrier is constructed by the GLDM.
The calculated alpha decay half-live of 264Hs is 23.33 second with MMM Q,
15.14 second with experimental Q. For 298114, the calculated alpha decay
half-live is 1537588.07 seconds ( about 18 days).
5
Summary
• The alpha decay can be described by GLDM.
• The alpha decay process is rather a preformed cluster emission
mode.
•
270Hs
is a deformed double nucleus whose calculated alpha decay
half-life is 23.33 seconds, and 298114 is a spherical double nucleus
whose calculated alpha decay half-life is about 18 days. The
spontaneous fission is another challenge for superheavy nuclei,
which may be the main decay mode for nuclei far away from fission
stability line.
Thank you !