NEA-1864 GEF 2021/1.1.

GEF 2021/1.1

GEF is a computer code for the simulation of the nuclear fission process. The GEF code calculates pre-neutron and post-neutron fission-fragment nuclide yields, angular-momentum distributions, isomeric yields, prompt-neutron yields and prompt-neutron spectra, prompt-gamma spectra, and several other quantities for a wide range of fissioning nuclei from mercury to seaborgium in spontaneous fission and neutron-induced fission. Multi-chance fission (fission after emission of neutrons) is included. For neutron-induced fission, the pre-compound emission of neutrons is considered. Output is provided as tables and as values of fission observables on an event-by-event basis.

Specific features of the GEF code:

• The mass division and the charge polarisation are calculated assuming a statistical population of states in the fission valleys at freeze-out. The freeze-out time considers the influence of fission dynamics and is not the same for the different collective variables.

• The separability principle [1] governs the interplay of macroscopic and microscopic effects.

• Five fission channels are considered. The strengths of the shells in the fission valleys are identical for all fissioning systems. The mean positions of the heavy fragments in the asymmetric fission channels are essentially constant in atomic number, as suggested by experimental data [2].

• The stiffness of the macroscopic potential with respect to mass asymmetry is deduced from the widths of measured mass distributions [3].

• The excitation-energy-sorting mechanism [4,5,6,7,11] determines the prompt neutron yields and the odd-even effect in fission-fragment yields of even-Z and odd-Z systems.

• Prompt neutron emission from the fragments is calculated with a Monte-Carlo statistical code using level densities from empirical systematics [8] and binding energies from mass tables with gamma competition included.

• Spectra and multiplicities of prompt gamma emission are provided. Non-statistical gamma emission is calculated with a dedicated VMI model.

• Model uncertainties, covariances and correlation coefficients are determined by a series of calculations with perturbed parameters. Covariances and correlations of fission yields from two different systems are available.

The official GEF websites are http://www.khschmidts-nuclear-web.eu/GEF.html and https://www.lp2ib.in2p3.fr/nucleaire/nex/gef/

The Monte-Carlo method is used.

Uncertainties are deduced from perturbed calculations.

A typical calculation with 100 000 events takes about 5 seconds on one processor of a recent Intel i7 CPU (2.80GHz). Calculations with perturbed parameters and calculations at higher excitation energies, where multi-chance fission occurs, require somewhat more time.

The main routines are written in FreeBASIC (http://www.freebasic.net/). FeeBASIC produces compiled binary code using the C run-time library. Graphics output is based on the X11 library. A graphical user interface is provided for Windows, written in JustBasic (http://www.justbasic.com/), which has a specific run-time library. The Windows version of GEF runs also under WINE on LINUX.

1. Experimental evidence for the separability of compound-nucleus and fragment properties in fission, K.-H. Schmidt, A Kelic, M. V. Ricciardi, Europh. Lett. 83 (2008) 32001

2. Nuclear-fission studies with relativistic secondary beams: analysis of fission channels, C. Boeckstiegel et al., Nucl. Phys. A 802 (2008) 12

3. Shell effects in the symmetric-modal fission of pre-actinide nuclei, S. I. Mulgin, K.-H. Schmidt, A. Grewe, S. V. Zhdanov, Nucl. Phys. A 640 (1998) 375

4. Entropy-driven excitation-energy sorting in superfluid fission dynamics, K.-H. Schmidt, B. Jurado, Phys. Rev. Lett. 104 (2010) 212501

5. New insight into superfluid nuclear dynamics from the even-odd effect in fission, K.-H. Schmidt, B. Jurado, arXiv:1007.0741v1 [nucl-th]

6. Thermodynamics of nuclei in thermal contact, K.-H. Schmidt, B. Jurado, Phys. Rev. C 82 (2011) 014607

7. Final excitation energy of fission fragments, K.-H. Schmidt, B. Jurado, Phys. Rev. C 83 (2011) 061601(R)

8. Inconsistencies in the description of pairing effects in nuclear level densities, K.-H. Schmidt, B. Jurado, Phys. Rev. C 86 (2012) 044322

9. General description of fission observables, K.-H. Schmidt, B. Jurado, Ch. Amouroux, JEFF Report 24, NEA of OECD, 2014

10. Revealing hidden regularities with a general approach to fission, K.-H. Schmidt, B. Jurado, Eur. Phys. J. A 51 (2015) 176

11. Influence of complete energy sorting on the characteristics of the odd-even effect in fission-fragment element distributions, B. Jurado, K.-H. Schmidt, J. Phys. G: Nucl. Part. Phys. 42 (2015) 055101

12. General description of fission observables: GEF model code, K.-H. Schmidt, B. Jurado, C. Amouroux, C. Schmitt, Nucl. Data Sheets 131 (2016) 107

13. Review on the progress in nuclear fission - experimental methods and theoretical descriptions, K.-H. Schmidt, B. Jurado, Rep. Progr. Phys. 81 (2018) 106301

GEF can be compiled and installed in Windows and Linux, using exactly the same sources files. Specific executables are provided for the two systems. GEF was tested on Windows and Linux.

Memory < 250 Mbyte; Disc < 100 Mbyte, with higher requirements for outputs with more events generated.

1. Windows XP or newer.

2. Any Linux distribution, 32-bit or 64-bit. Some additional libraries need to be installed, see www.freebasic.net -> documentation -> using the FreeBASIC compiler -> Installing FreeBASIC. It is recommended to use the 32-bit version of FreeBASIC on a 64-bit system for better numerical stability. Please install the appropriate libraries.

Multi-chance fission is supported.

The results on neutron emission prior to fission and prompt-neutron emission between saddle and scission, and from the fragments are given separately. The sequence of the events in the list-mode output is sorted by energy at fission in the case of multi-chance fission in order to save computing time. An optional enhancement factor may be specified. A value >1 increases the statistics of the Monte-Carlo calculation and hence reduces the statistical uncertainties and fluctuations of the results. Default value is 100 000 events. This is adapted for a rough overview, but the number of events should be appreciably increased when accurate results are required. This is particularly useful to compare different systems, to study systematic trends and to determine reliable covariances.

GEF provides all results event by event in a list-mode file on demand.

K.-H. Schmidt

Erzhausen, Germany

Beatriz Jurado

CENBG

CNRS/IN2P3

Chemin du Solarium

B.P. 120

F-33175 Gradignan, France