Global view on fission channels

1. Global view on fission channels C. Böckstiegel, S. Steinhäuser, H.-G. Clerc, A. Grewe, A. Heinz, M. de Jong, J. Müller, B. VossInstitut für Kernphysik, TU Darmstadt A. R. Junghans, A. Kelić, K.-H. Schmidt GSI, Darmstadt

2. Low-energy fission - Complex structures - Similar kind of complexity seen in e.g. TKE, mass-dependent neutron yields or gamma-ray multiplicities.

3. Theoretical description • Strutinsky-type calculations of the potential-energy landscape • (e.g. Pashkevich, Möller et al.) • Statistical scission-point models • (e.g. Fong, Wilkins et al.) • Statistical saddle-point models • (e.g. Jensen et al., Duijvestijn et al.) • Dynamical approach based on the solution of Langevin equations of motion • (e.g. Asano et al., Aritomo et al.) • Time-dependent Hartree-Fock calculations with GCM • (e.g. Goutte, Dubray et al.)

4. Experimental difficulties • - Restricted choice of systems • Available targets  stable or long-lived nuclei • Secondary beams  no beams above 238U by fragmentation • Reaction products  limited N/Z range in heavy-ion fusion • - Physical limits on resolution • Z and A resolution difficult at low energies • Scattering in target/detector at low energies (tails in A/TKE • distribution) • - Technical limits on correlations • No experimental information available on AandZ of both fission • fragments simultaneously

5. What to do? An empirical overview on the observed structures in low-energy fission.  Common features behind the large variety of the complex structures seen for the different fissioning systems.  Use as a test of different theoretical approaches. Same kind of approach done by other authors (e.g. Unik et al, Rochester (1973); Brosa et al, Phys. Rep. (1990); Dematte et al, Nucl. Phys. A (1997); Mulgin et al, Phys. Lett. B (1999)), but for a limited range of N/Z of the fissioning nuclei.

6. Which kind of empirical overview? - Method of independent fission channels (Super-long, Standard 1, Standard 2) as proposed by e.g. Brosa et al, Phys. Rep. (1990). Böckstiegel et al, Nucl. Phys. A (2008) - Analysed data:Z and A distributions measured in EM-induced fission of secondary beams, low-energy particle induced fission and spontaneous fission; for refs. to data see Böckstiegel et al, Nucl. Phys. A (2008)

7. Relative yield of fission channels - Superlong channel decreases with increasing A - For given Z of the fissioning system, Standard 1 channel increases with increasing A and Standard 2 decreases.

8. Position of fission channels in A Standard 1Standard 2 - For a fixed Z of fissioning system, average positions of Standard 1 and Standard 2 are increasing with increasing mass of the system.

9. Position of fission channels in Z and N Standard 1Standard 2 - For both fission channels, position in Z is stable, while position in N increases with A of the fissioning system.

10. Position of fission channels in Z and N Calculations based on macro-microscopic approach using input from shell model: N=82 and Z=50 as responsible for Standard 1, and N=88 as responsible for Standard 2. Something beyond shells.

11. Conclusions and outlook • - "Lesson" from the fission-channel study: • The parameters of the fission channels vary in a smooth and • systematic way from Ac to Cf. • Position of St1 and St2 "stable" in Zfand not in Nf. - Still, we need experimental data with much better quality, especially information on N and Z of both fission fragments simultaneously  Need for new experimental set-ups, like e.g. ELISe at FAIR.

12. Standard deviation

13. Position of fission channels in Z and N

14. Relative yield of fission channels N/Z(132Sn) = 1.64

15. Shells of fragments Importance of spherical and deformed neutron and proton shells Wilkins et al. PRC 14 (1976) 1832