Initial measurements and studies for the commissioning of an on-line dose monitoring system installed in the new free electron laser facility SwissFEL

The Swiss Free Electron Laser (SwissFEL) is the new large-scale research facility presently under commissioning at the Paul Scherrer Institut (PSI). It has been designed to produce X ray pulses with repetition rate up to 100 Hz and wavelengths in between 1 and 70 ˚Angstrom. X rays pulses are generated by accelerating bunches of electrons up to an energy of 7 GeV and setting them in transversal oscillation thanks to arrays of magnets. The shielding structure, designed in an early stage of the SwissFEL project and weakened by the presence of cable ducts, ventilation or cooling shafts, can cope with full beam losses for less than few seconds. This, jointly with the possibility for workers and public to access several of the areas surrounding the more than 700 long accelerator tunnel, lead to challenges in ensuring that the radiation level in these areas remains in compliance with legal constraints on the maximum allowed dose regulated by the Swiss Federal Office of Public Health (FOPH). For this purpose an on-line survey system, named Dose Rate Protection System (DRPS), is being installed. It monitors the ambient dose arising from neutrons inside the accelerator tunnel and, by opportunely calibrating these readings, derives the dose outside the tunnel. The environment the DRPS detector units have to face is challenging from a radiation protection point of view. It is characterized by secondary radiation composed by pulsed neutron fields (PNF) with high a relevant component of neutrons with energy above 20 MeV accompanied by a high photon field. This thesis work, carried out in a time window during which both SwissFEL and the DRPS were under commissioning, contributed to the development and the calibration of the DRPS by means of experimental measurement campaigns and specific studies of the radiation protection aspects of the SwissFEL shielding structure. In particular this work focused on four main aspects: Shielding verification. Beam operation was necessary for the commissioning of both the DRPS and the SwissFEL accelerator. The FOPH required to ensure that the legal constraints on the maximal allowed dose were respected even before the DRPS was in operation. Dedicated measurements were then performed for several positions along the beam tunnel considered as worst cases from a radiation protection point of view in order to verify the compliance with law and allow SwissFEL operation with limited beam parameters. i ii Survey instruments capability verification. In order to prove their acquisition capabilities in the SwissFEL beam tunnel radiation environment, the DRPS monitoring units dose indications were compared to the ones given by two other commercial neutron survey devices in a dedicate measurement campaign. Pulsed Neutron Fields were produced by intentionally letting the electron beam impinging on a safety component placed at the end of the SwissFEL injector section. Results were benchmarked by Monte Carlo simulations. Dark current. Within this thesis project, it was also performed the first experimental evaluation of the neutron and photon emissions due to field emission of electrons in the proximity of several structures of the SwissFEL injector section in order to derive useful information of radiation protection relevance for the DRPS development. Shielding categorization. Considering the complexity of adequately calibrating the DRPS, an overall categorization of the SwissFEL building according to radiation protection relevant parameters, like guidance values, shielding attenuation factors, distances and thicknesses of the material constituting the shielding was performed as part of this thesis. This categorization became the basis for the first evaluation of the detector positioning and alarm threshold setting and is to be verified while the SwissFEL commissioning goes on.

Il presente lavoro affronta alcune delle problematiche riguardanti la radioprotezione dello Swiss Free Electron Laser (SwissFEL), la nuova sorgente di raggi X progettata per l’uso in un ampio spettro di studi scientifici, attualmente in fase di messa in opera presso il Paul Scherrer Institut (Villigen, Switzerland). Al fine di garantire il rispetto dei limiti imposti dalla legge sulla dose prodotta da tale facility in tutte le aree ad essa circostanti, un sistema di sicurezza chiamato Dose Rate Protection System (DRPS) è in fase di messa in servizio. Nell’ambito di questo lavoro di tesi, effettuato in una finestra temporale nella quale sia SwissFEL che il DRPS erano in fase di commissioning, ci si è soffermati sugli studi premliminari che hanno portato alla definizione della procedura di calibrazione del DRPS e su alcune delle campagne di misura necessarie per l’implementazione del DRPS. In particolar modo, è stata condotta una prima campagna di misura al fine di verificare le indicazioni di dose delle unità di rivelazione del DRPS, i LUPIN, in presenza di campi neutronici pulsati, tramite il confronto con altri neutron REM counter, presso SwissFEL. Nell'ambito del commissionamento di SwissFEL, una seconda campagna di misura è stata condotta allo scopo di verificare le proprietà di attenuazione della radiazione da parte della struttura schermante del tunnel che ospita l'acceleratore. Infine, è stata per la prima volta effettuata una misurazione della radiazione secondaria prodotta dalla dark current generata dalle diverse sezioni di cui è composto l'iniettore di SwissFEL.