Radiation physics design of the new low emittance injector tunnel for LCLS-II-HE upgrade

The LCLS-II High Energy (HE) project at SLAC National Accelerator Laboratory extends the existing LCLS-II linac energy from 4.5 GeV to 8.2 GeV, enabling photon production at shorter wavelengths and higher energies for next-generation X-ray science. To achieve the required beam brightness and stability, a new superconducting Low Emittance Injector (LEI) is being constructed in a dedicated tunnel parallel to the existing linac. The LEI incorporates a high-gradient superconducting RF gun and low-mean-transverse-energy photocathodes to deliver electron beams with emittances significantly below those of the existing injector. This paper presents the radiation physics design and safety considerations for the LEI integration, including a structured methodology developed to identify worst-case credible loss points, assess field-emission-driven radiation using standardized FLUKA routines, and design shielding in areas complicated by large penetrations. Monte Carlo simulations performed with FLUKA demonstrate that the combined passive shielding, access-control systems, and beam-containment instrumentation satisfy SLAC and DOE radiological limits for normal operation, mis-steering, and Maximum Credible Incident conditions, while enabling safe reciprocal operation of the two injectors.