GEF 2017/1.1

General description of fission observables. Released September 24, 2017.

Properties of GEF 2017/1.1: GEF 2017/1.1 is the result of a new (preliminary) fit of the model parameters to experimental data. In particular, the mass distribution of 235U(nth,f), the system for which the most accurate data exist, was considered with special care. The description of these data could appreciably be improved by discarding the yield of A=129 from ENDF/B-VII. After careful studies, we came to the conclusion that this value is not correct and that it considerably disturbed previous fits. Moreover, in contrast to previous GEF versions that used theoretical masses, GEF 2017/1.1 uses experimental masses and appropriately chosen level densities in the evaporation routine in order to better model the fine structure in the fission-fragment yields. By these measures, the reduced Chi-squared for the mass distribution of 235U(nth,f) decreased from 7.6 in 2016 to 2.4 (see figure below).
Moreover, a few inconsistencies in the description of the fluctuations in the division between TKE and TXE were corrected.
GEF 2017/1.1 should be considered as preliminary, because a comprehensive fit of the model parameters regarding all fission quantities over a large range of fissioning systems has not yet been made.

Mass distribution of 235U(nth,f)
The mass distribution of 235U(nth,f) from GEF-2017/1.1 (red points) in comparison with the ENDF/B-VII evalution (black symbols with error bars). Note the appreciably reduced value of Chi-squared compared to previous GEF versions. The calculated contributions from different fission channels are traced in green. 10 million events have been calculated. The data point for A=129 is taken from H. Thierens, D. de Frenne, E. Jacobs, A. de Clercq, P. D'Hondt, A.J. Deruytter , Nucl. Instrum. 134 (1976) 299-308.

An extended version of GEF-2017/1.1 that includes delayed processes (output of delayed-neutron multiplicities, delayed-neutron emitters, cumulative fission-fragment yields in ENDF format) is available on demand.
It provides also an option for producing random files of fission-fragment yields in ENDF format.

We are happy about any feed-back (mail to schmidt-erzhausen<at> This helps to correct errors, to improve the quality of the model and to better respond to the needs and preferences of the users.

Use of the GEF code is subject to the GNU GENERAL PUBLIC LICENSE agreement that you find here: <License>.

Stand-alone version (Monte-Carlo method)

The stand-alone version of the GEF code is written in FreeBASIC (a). The FreeBASIC compiler produces binary code from the same source on Windows (b) and on Linux. The executable uses the C library.  For  the Windows version, a GUI is provided, written in JustBasic (c). The Windows version runs also on Linux with Wine (d).  The Windows version should also run on OS X (e) with Wine (d). (Did somebody try it?)

a) FreeBASIC is available from with no cost.
b) Windows is either a registered trademark or a trademark of Microsoft Corporation in the United States and/or other countries.
c) JustBasic is available from with no cost.
d) Wine is a windows compatibility layer for Linux and Mac OS X (
e) OS X is a trademark of Apple Inc., registered in the U.S. and other countries.

Quick start on Windows:

Quick start on Linux:

Windows version

Input mask of the graphical user interface:

Input maks of GEF GUI

For Windows and Linux

Complete package


Readme file (Technical information, list of relevant publications)
JEFF-Report 24, part I (Comprehensive documentation of the GEF model.)
JEFF-Report 24, part II (Technical information on the GEF code.)

Source (for Windows and Linux)

Source files


Input files for extended input options
Distribution of entrance energies (example)
List of input parameters (example)
See Readme file for more detailed information.
Data tables in XML format (example)
List-mode file (example)   (short sample of most complete event list)
Raw data for the multi-variate distribution of fission-fragment yields from calculations with perturbed parameters (example)
See Readme file for more detailed information.

You may consider using the Windows version of GEF on Linux with Wine in order to profit from the more comfortable input handling offered by the graphical user interface. This option may also solve problems of missing libraries under Linux.

Subroutine (Folding method) 

The subroutine aims for being used in combination with other nuclear-reaction codes. For a given fissioning nucleus with excitation energy E* and angular momentum I it calculates complete distributions of a number of fission observables before emission of prompt neutrons and prompt gamma radiation with the GEF model.


Technical description of the subroutine

FreeBASIC version

In preparation

FORTRAN version

In preparation