A fast neutron irradiation facility at the Portuguese Research Reactor

The Portuguese Research Reactor (RPI) is a pool-type 1 MW reactor designed by AMF Atomics and built during the period of 1959/61. It has currently a maximum thermal neutron flux of 3x1013 n/cm2/s. The use of the RPI was never high, due to a combination of several factors, but has steadily increased in recent years. As expected, the focus is now in activities that do not depend strongly on a high neutron flux and where the RPI can be competitive. A dedicated facility for the irradiation of electronic components with fast neutrons was recently installed. Fast neutron fluxes of up to 1x109 n/cm2/s are available in the facility. The simultaneous gamma dose rate is 20 Gy/h for a neutron flux of 2x108 n/cm2/s. The gamma dose rate was tailored to keep the neutron/gamma ratio expected at the location of the signal conditioners for temperature measurement at the LHC facility at CERN. Initial irradiations of components were performed inside a special container immersed in the pool, but this required operating the RPI at a reduced power of only 2 kW. It was then decided to build a dedicated facility using a beam tube, to allow normal operation at 1 MW, with access for other users and a decrease of costs. The new facility, shown schematically, was built around the 6'' beam tube E4. A dry irradiation cavity with 100 x 60 x 60 cm was created at the end of the tube. In order to allow irradiations at higher neutron fluxes, a 100 cm long cylinder with 15 cm diameter was placed inside the beam tube, thus creating an inner cavity where the circuits can also be placed. This ensures a good volume to place the circuits and the cables running to the measuring instruments. The intensity of the gamma field is reduced by a 3 cm long Pb filter and the thermal component of the beam is cut with a boral filter. The components under test are mounted on several boards, inside boxes. Normally each irradiation campaign runs from Monday to Friday with approximately 13 hours of irradiation followed by 11 hours of stand-by per day, due to the two-shift per day operation. The goal is to achieve a neutron fluence of 5x1013 n/cm2 for the central board during one week of operation of the reactor (about 60 hr). This fluence exceeds the one expected for 10 years of operation in the LHC. In this paper the irradiation facility is described and the characterization of the neutron and gamma fields is presented