1. P. Buffa, S. Rizzo, E. Tomarchio
Fig. 2 – Photograph of the plexiglass devices used for irradiating bare and cadmium covered gold foils.
Fig. 2 – Photograph of the plexiglass devices used for irradiating bare and cadmium covered gold foils.
Fig. 5 – Schematic diagram of the final configuration of a modular neutron irradiator.
Application of Monte Carlo simulation to design a modular 241Am-Be neutron irradiator
P. Buffa, S. Rizzo, E. Tomarchio
Dipartimento di Ingegneria Nucleare, Università degli Studi di Palermo
Viale delle Scienze, Parco d’Orleans, Edificio 6, I Palermo, Italy
address :
DIN
UNIVERSITA’ DEGLI STUDI DI PALERMO
DIPARTIMENTO DI
INGEGNERIA NUCLEARE
INTRODUCTION: Neutron irradiator facilities with 241Am-Be sources are worldwide used to perform neutron activation analysis (NAA), to investigate material characteristics or to test and calibrate neutron detectors and dosemeters. The use of a neutron irradiator is advantageous because have a very stable neutron flux, even it is many orders of magnitude lower than the one of a nuclear reactor or a particle accelerator.
Most of irradiators have fixed source positions, so that the characteristics of neutron flux are well established. In some applications, may be very interesting to have the choice of the prevalence of thermal, epithermal or fast flux components. For this goal, a new neutron irradiator with 241Am-Be sources able to modulate neutron flux components is designed and realized. The design was carried out by means of a Monte Carlo simulation with the code MCNP5 and taking into account various moderators and other shielding materials. Validation of Monte Carlo modeling was carried out by simulating an irradiator with four sources of 241Am-Be in fixed locations in water.
RESULTS AND DISCUSSION: A last model was assumed as the final design of a new neutron irradiator. To draw up the final draft, several tests were made with MCNP5 simulations (100 runs, minimum number of stories examined for each test: 5 107, total processing time 1200h taking into account the time needed for different personal computers). In Fig. 2 are given the almost flat behaviours of normalized neutron fluxes evaluated by MCNP5 simulation in some representative points of the irradiation cell. Schematic diagram of the final configuration is shown in Fig.3.
MCNP5 MODELLING OF A IRRADIATOR: Monte Carlo code MCNP, version 5, is used to verify the efficacy of various materials and to optimize thicknesses. The neutron flux was estimated using F4 tally, which calculates the average flux over a cell (particles·cm-2) whereas dose was estimated using F6 tally. Some models are considered: a first, with a neutron source located at the center of a sphere-shaped moderator of various diameter, was initially adopted to study various parameters. In a second geometry, four 241Am-Be sources, 43 mm lenght and about 20 mm in diameter, made of a metallic alloy, immersed in various moderator materials (water, graphite, polyethylene and grains of polyethylene), were modeled. One of these configurations was realized inside a room of the Department, placing the sources on the edges of a parallelepiped structure of 14 cm side while the irradiation cell was put at the center of the system. The experimental validation of the model MCNP5 was carried out by irradiating 6 gold foils, diameter 1.3 cm and weighing approximately 0.6 g each, housed in a Plexiglas device, bare and covered with cadmium (see Fig.1). The gold samples were then subjected to gamma spectrometry by using an ORTEC HPGe detector with relative efficiency 35%. The values of thermal and epithermal-fast flux component were determined by applying the "cadmium-difference“ method. The differences between the flux values derived from simulation and experimental ones are all <20%.
Fig. 2 – Behaviours of normalized neutron fluxes with respect to distance from the center of the irradiation cavity Thermal (at left) and fast neutron (right) flux prevalence.
Fig. 3 – Schematic diagram of the final configuration of a modular neutron irradiator. 
Fig. 1 - Plexiglass devices
used for irradiating bare and cadmium covered gold foils.
CONCLUSIONS: The availability of a modular neutron irradiator apparatus gives an alternative to perform NAA outside a nuclear reactor, at lower cost and with greater flux stability, well as a calibration facility for neutron detectors and dosemeters. Actually, the modular neutron irradiator is under realization.
Poster No. 269
RadChem th Radiochemical Conference
18 – 23 April 2010, Mariánské Lázně , Czech Republic