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

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

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.)

Experimental difficulties - Restricted choice of systems Available targets  stable or long-lived nuclei Secondary beams  no beams above 238 U 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 A and Z of both fission fragments simultaneously

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.

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)

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.

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

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

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.

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 Z f and not in N f. - 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.

Standard deviation

Position of fission channels in Z and N

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

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