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. SchmidtGSI, Darmstadt

Global view on fission channels

Low-energy fission - Complex structuresLow-energy fission - Complex structures

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

Theoretical descriptionTheoretical 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 difficultiesExperimental 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 A and Z of both fission fragments simultaneously

What to do?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?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 channelsRelative 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 Position of fission channels in AA

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

Standard 1 Standard 2

Position of fission channels in Position of fission channels in ZZ and and NN

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

Standard 1 Standard 2

Position of fission channels in Position of fission channels in ZZ and and NN

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 outlookConclusions 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 Zf and 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.

Standard deviationStandard deviation

Position of fission channels in Z and NPosition of fission channels in Z and N

Relative yield of fission channelsRelative yield of fission channels

N/Z(132Sn) = 1.64

Shells of fragmentsShells of fragments

Importance of spherical and deformed neutron and proton shells

Wilkins et al. PRC 14 (1976) 1832