Neutron Spectra and Dosimetric Quantities Outside Typical Concrete Shielding of Synchrotron Facilities

Neutron energy spectra at high energy electron accelerators outside concrete shielding of various thicknesses (30 to 240 cm at 65-95 degrees) have been calculated at three electron energies (100 MeV, 1 and 10 GeV) and an optimum Cu target of 30.48 cm long and 5.08 cm in radius with the FLUKA Monte Carlo code. Dose quantities such as the protection quantity of Effective dose E and the operational quantity of Ambient Dose Equivalent H*(10) have been calculated by folding the spectra with the respective ICRP-74 conversion coefficients. For Effective Dose, two conversion coefficients were considered: AP (Anterior-Posterior) and “worst” (at any neutron energy, the highest value among the different available irradiation geometries. The results provide useful information about the neutron dosimetric quantities, the neutron fractions > 20 MeV, and about neutron attenuation lengths in concrete. The results have been compared to those obtained with SHIELD11, an analytical code widely used at electron accelerators. The response of an Andersson-Braun remmeter outside different shield thicknesses has also been evaluated. The results have been examined against typical shielding conditions for current synchrotron facility storage rings (60-120 cm concrete and 1-8 GeV). It is found that: 1) Equilibrium spectra exist for concrete thickness starting at about 100 cm, in which case the spectral shapes (peaks at about 1 MeV and 80 MeV) change little and the dose fraction due to neutrons with energy > 20 MeV increases to a constant of about 65%. 2) E(AP) underestimates E(worst) by about 3% at 60 cm and 5% at 120 cm. 3) H*(10) underestimates E(worst) by about 3% at 60 cm and 14% at 120 cm. 4) The equilibrium attenuation length in concrete is about 115 g/cm, but the attenuation length is only 60 g/cm at 60 cm and 105 g/cm at 120 cm. 5) The source terms and attenuation lengths used in SHIELD11 code for neutrons at three energy groups provide results that are in fair agreement with the FLUKA results. 6) Because the response falls down to nearly zero above 20 MeV, the response of the Andersson-Braun remmeter, when calibrated with Cf or AmBe neutron sources, underestimates H(10) by about 30% at 60 cm and 60% at 120 cm. The practice to double the rem-meter readings in accelerator fields is thus justified. 7) The above (1)-(6) results are nearly independent of the energy between 1 and 10 GeV, but the results for 100 MeV (a typical injector linac energy) are very different. * Corresponding author: [email protected]