Calculation of Neutron Production using FLUKA

B. Naumann and E. Grosse

For the estimation of activation and background neutron radiation in physics experiments at ELBE the knowledge of the photoneutron production in structural components of the electron accelerator is required. Detailed calculations of such processes are especially important as at ELBE it is planned to use the neutrons generated from the electron beam [1]. For time of flight studies sub-nanosecond neutron pulses may be obtained [2]. In traditional nuclear physics transport codes as EGS4 [3], GEANT [4] or MCNP [5] photonuclear reactions are not implemented and the neutron production can only be approximated in a two-stage technique by converting a bremsstrahlung photon fluence into a neutron source, as described in [6].
Thus the Monte Carlo radiation transport program FLUKA [7] was implemented for simulating not only electromagnetic interactions of photons and electrons, but also photonuclear reactions and the subsequent neutron propagation. The present status of the FLUKA photonuclear cross-section database is documented in detail in [8]. The photonuclear cross-sections from the FLUKA database were compared with experimental data from [9] for various materials with nuclei of 6 Z 73 and a good agreement was found for nuclei with Z 13.
Using the program FLUKA tracking calculations of the full particle cascade in irradiated materials have been done for the neutron production in various beam dump materials [10]. Here the emitted neutron yield per incident electron and the energy deposition in various materials with widely different Z are presented for 50 MeV electrons (see Fig.1).
For these calculations the size of the target was scaled with the radiation length X0, the radius of the cylindrical target being X0 and its depth 0.3·X0 or 3·X0, respectively. The smooth Z dependence of both plotted quantities, when scaled with their radiation length, allows a straightforward interpolation to other materials.

naumann41.gif


Fig.1  Z-dependence of neutron production and deposited energy in materials bombarded with a pointlike beam of 50 MeV electrons.

References

[1] H. Freiesleben et al., Neutrons at ELBE, Annual report 1997, FZR-215 (1998)14.

[2] B. Naumann and E. Grosse, this Report p. 35

[3] W. R. Nelson et al., The EGS4 Code System, SLAC-Report-265 (1985).

[4] GEANT, CERN Program Library W5013 (1994).

[5] J. F. Briesmeister, Editor, MCNP, Version 4A, LA 12625 M (1993).

[6] B. Naumann and W. Neubert, this Report p. 37

[7] A. Fasso et al., Proc. of the 8th Int. Conf. on Rad. Shielding, Arlington (1994)643-649.

[8] A. Fasso et al., Proc. of the III Spec. Meeting on Shielding Aspects, Sendai (1997)61-74.

[9] EXFOR-Database of the IAEA, http://iaeand.iaea.or.at/exfor.

[10] B. Naumann et al., Strahlfänger für maximale Energien an ELBE, FZR Report FZR-267 (1999).

FZR
 IKH 10/22/99 © B. Naumann