1. Neutron energy spectrum from U and Th traces in the Modane rock simulated with SOURCES (full line). The fission contribution is also shown (dashed line).

2. Sketch of the geometry used as benchmark for comparing the simulations.

3. Neutron energy spectra from U and Th traces in NaCl at rock boundary simulated with GEANT4 (solid line), MCNPX (dashed line) and GEANT3 (dashed-dotted line). The GEANT4 result with uncorrected inelastic cross-section is also shown (dotted line).

4. Neutron energy spectra from rock activity at Modane rock boundary simulated with GEANT4 (solid line) and MCNPX (dashed line).

5. Neutron energy spectra from NaCl after hydrocarbon shielding simulated with GEANT4 (solid lines) and MCNPX (dashed lines).

6. Neutron flux from NaCl above 100 keV (open circles and squares) and above 1 MeV (full circles and squares) as a function of CH 2 thickness simulated with MCNPX (circles) and GEANT4 (squares).

7. Neutron energy spectra from rock activity after lead and hydrocarbon shielding simulated with GEANT4 (solid lines) and MCNPX (dashed lines).

8. Total neutron flux from NaCl above 100 keV (open circles and squares) and above 1 MeV (full circles and squares) as a function of CH 2 thickness obtained with MCNPX (circles) and GEANT4 (squares) after 30 cm of lead.

9. Conclusions and problems Good agreement between MCNPX and GEANT4 for simple geometry. Back-scattering from the walls is smaller in MCNPX than in GEANT4 - may be a problem for a real geometry of a cavern: the spectra may differ by a factor of 2. Effect of the presence of hydrogen is very important: starting with much higher contamination levels for Modane rock than for NaCl, we obtained similar neutron yields from SOURCES (effect of the large dominance of (alpha,n) reactions for NaCl) and ended up with higher fluxes at the rock/cavern surface for NaCl rock. This effect was also observed in Hesti Wulandari’s simulations for Gran Sasso. 55 g/cm 2 of CH 2 are probably ok for 1 tonne-scale experiment (or 20 cm of Pb + 45 g/cm 2 of CH 2 ). Obviously all this has to be calculated for a particular experiment (rock type, geometry, target etc.) to achieve better accuracy.