MASS DISTRIBUTIONS OF FISSION FRAGMENTS IN THE MERCURY REGION

Transcription

1 MASS DISTRIBUTIONS OF FISSION FRAGMENTS IN THE MERCURY REGION A. V. Andreev, G. G. Adamian, N. V. Antonenko Bogoliubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna, Russia A. N. Andreyev Department of Physics, University of York, UK; ASRC, JAEA, Tokai, Japan; School of Engineering, University of the West Scotland, Paisley, UK 22nd ASRC International Workshop Nuclear Fission and Exotic Nuclei December 3 5, 2014, JAEA, Tokai, Japan 1

2 Outline Introduction Description of the model for binary fission Results of the calculation for the fission of Hg, 194,196 Po, Ir 2

3 Experimental information A. N. Andreyev et al., Phys. Rev. Lett. 105, (2010) A very exotic process of β-delayed fission of 180 Tl was studied by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). The fission-fragment mass distribution of the post-β-decay daughter nucleus 180 Hg is asymmetric. Most probable A values: A L = 80(1) and A H = 100(1) with σ Ai = 4(3) The average total kinetic energy: 134.6(7) MeV The maximum excitation energy of 180 Hg: MeV 3

4 Model of nuclear fission The fission observables are defined by the shape of the fissioning nucleus at the scission point. A i, Z i, β i, d A i, Z i, E Ki, Ei at the scission point at infinity The relative probabilities of formation of different scission configurations are defined by their potential energies. P ({A i, Z i, β i, d}) exp [ U({A ] i, Z i, β i, d}) T 4

5 Driving potential Potential energy of the dinuclear system at the scission point as function of mass asymmetry η = A H A L A H +A L = asymmetric mass distribution = symmetric mass distribution 5

6 Improvements to the scission-point model Use of the double-folding nuclear interaction potential Calculation of the distance between the fragments at scission (internuclear distance is not a parameter) Calculation of the excitation energy and temperature at scission (temperature is not a parameter) 6

7 Potential energy of binary system β i = c i /a i, i = L, H (for small deformations: β 1 β 2 ) U sc (β L, β H, d, l) = UL LD (β L ) + δul sh (β L ) + U zpv L + UH LD (β H ) + δuh sh (β H ) + U zpv H + V C (β L, β H, d) + V N (β L, β H, d) + V rot (β L, β H, d, l) E = E comp + U comp U sc, T = E /a, a = A/12 7

8 Double-folding potential V N (d) = ρ 1 (r 1 )ρ 2 (r 2 d)f (r 1 r 2 )dr 1 dr 2 ρ i (r) = ρ 00 ( s(r) ), 1 + exp a 0i ( ρ 0 (r 1 ) F (r 1 r 2 ) = C 0 F in ρ 00 ρ 00 = 0.17 fm 3 + F ex ( 1 ρ )) 0(r 1 ) δ(r 1 r 2 ) ρ 00 ρ 0 (r) = ρ 1 (r) + ρ 2 (r d) F in,ex = f in,ex + f in,ex N 1 Z 1 N 2 Z 2 A 1 A 2 C 0 = 300 MeV fm 3, f in = 0.09, f ex = 2.59, f in = 0.42, f ex =

9 { ( V N F in F ex (d) = C 0 ρ 2 ρ 1(r)ρ 2 (r d)dr + 00 } +F ex ρ 1 (r)ρ 2 (r d)dr ρ 1 (r)ρ 2 2(r d)dr ) Example: 80 Kr(β = 1.6) Ru(β = 1.7) 9

10 Potential energy surface Without shell corrections 180 Hg 76 Se+ 104 Pd The potential energy is given in MeV relative to the energy of compound fissioning nucleus. 10

11 Potential energy surface 180 Hg 90 Zr+ 90 Zr 180 Hg 76 Se+ 104 Pd The potential energy is given in MeV relative to the energy of compound fissioning nucleus. 11

12 Y (A L ) = Fission-fragment mass distribution l max l=0 l max l=0 (2l + 1) { exp (2l + 1) { exp U(A i,z i,β i,l) T (l) U(A i,z i,β i,l) T (l) } dz L dβ L dβ H } da L dz L dβ L dβ H The fission-fragment mass distribution of the post-β-decay daughter nucleus 180 Hg: The calculated average mass of the light fragment is A L = 79 a.m.u. with σ AL = 5 a.m.u. 12

13 Fission-fragment mass distributions Induced fission of 198 Hg in the reactions: 197 Au(p,f) at E p = 22.4 MeV 194 Pt(α,f) at E α = 50.4 MeV: Exp: M. G. Itkis et al., Sov. J. Nucl. Phys. 52, 601 (1990) 13

14 Fission-fragment mass distributions Solid line: E = B f + 2 MeV Dashed line: E = B f + 20 MeV 14

15 Fission-fragment mass distributions Solid line: E = B f + 2 MeV Dashed line: E = B f + 20 MeV 15

16 Fission-fragment mass distributions β-delayed fission of 194,196 At Exp: L.Ghys et al., Phys. Rev. C 90, (R) (2014) 16

17 Fission-fragment mass distributions Exp: M. G. Itkis et al., Sov. J. Nucl. Phys. 53, 757 (1991) 17

18 Total kinetic energy of fission fragments T KE(A i, Z i, β i, l) = V C (A i, Z i, β i ) + V N (A i, Z i, β i ) + V rot rel (A i, Z i, β i, l) The calculated mean total kinetic energy is: 180 Hg: T KE = 136 MeV, 187 Ir: T KE = 127 MeV, 189 Ir: T KE = MeV, 194,196 Po: T KE = 149 MeV. 18

19 Summary Within the improved scission-point model the existing experimental data on fission-fragment mass distribution and mean TKE in fission of several nuclei in the mercury region are described. For the fission of Hg isotopes we predicted the change of the shape of the mass distribution from symmetric to asymmetric and then to more symmetric with increasing mass number of the fissioning isotopes from 174 to 198. The reactions of induced fission are proposed to check this dependence in the article: A. V. Andreev et al., Phys. Rev. C 86, (2012) In the fission of 194,196 Po the existence of both symmetric and asymmetric fission modes is demonstrated. In the fission of Ir isotopes the change of the shape of the mass distribution from asymmetric to symmetric with increasing mass number of the fissioning isotopes is revealed. 19