Comparison of FLUKA and STAC8 for shielding calculations of the hard X-ray line of the LCLS

This Note describes calculations of the shielding necessary to reduce the dose rate to less than 0.05 mrem/h when synchrotron radiations emitted by the LCLS electrons beam hit a worst type scattering target located in the hard X-ray line. The calculations were performed using the FLUKA Monte-Carlo code and as well as the STAC8 analytical code. The photon spectrum used as a source term corresponds to the spectrum of spontaneous and FEL photons emitted when an electron beam of 13.7 GeV (a 17 GeV electron beam was also considered for some cases) goes through the LCLS undulator system. The calculations have shown that after very thin layer of shielding the shielding requirements are dominated by spontaneous photons with an energy greater than 50 keV. The spectrum used is relevant for shielding calculations downstream of the Front End Enclosure (FEE) since the effect of the C1 collimator and of the two hard X-ray mirrors of the Offset Mirror System (OMS) which significantly reduces the spectrum intensity were taken into account in the spectrum determination. The geometrical configurations considered addressed two different issues: the aim of the first one is to determine the minimum pipe thickness necessary to reduce the dose to 0.05 mrem/h when the beam hits a target enclosed in the pipe considering that personnel may be located at proximity of the beam line. Calculations performed with FLUKA and STAC8 indicate that a standard 4” diameter steel pipe would need to be 6.2 mm thick. The same shielding could be reached by wrapping a standard steep pipe (4” diameter) 0.83” thick with 0.6 mm of lead. The second generic calculations were performed with the same target surrounded by a lateral and downstream slab of shielding material in order to determine the minimum thickness of the lateral and downstream walls when an experiment is being carried out inside a hutch. A very good agreement was found between FLUKA and STAC8 for the lateral wall shielding requirement, namely 0.191 mm of lead, 1.03 mm of steel or 20.5 mm of concrete. For the downstram wall, the results obtained with STAC8 were found to be more conservative and were therefore used. To reduce the dose rate to less than 0.05 mrem/h on contact to the downstream wall it is found that either 0.90 mm of lead or 6.2 mm of steel or 88 mm of concrete are necessary. Those requirements were provided to LCLS during the design phase of the Far Experimental Hall hutches. Comparison of FLUKA and STAC8 for shielding calculations of the hard X-ray line of the LCLS J. Vollaire, A. Prinz