1. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal A modular neutron irradiator based on 241 Am-Be neutron sources P. Buffa, S. Rizzo, E.Tomarchio Nuclear Engineering Department, Palermo University Viale delle Scienze, Ed. 6, I-90128 Palermo, ITALY Coimbra (Portugal), June 7, 2010

2. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Introduction Neutron irradiator facilities with 241 Am-Be sources are worldwide used in order to perform neutron activation analysis (NAA), to investigate materials in different research areas or to test and calibrate neutron detectors and environmental or personal dosemeters. The use of a neutron irradiator is advantageous because of its very stable neutron flux, even it is many orders of magnitude lower than the one of a nuclear reactor or a particle accelerator.

3. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Introduction Most of irradiators are realized with neutron sources located at fixed positions and accordingly the characteristics of neutron spectrum previously assessed do not change. Therefore, an interesting choice is to have a modular facility, capable of varying conditions of irradiation and setting the prevalence of fast or thermal neutron spectrum component.

4. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Introduction In this work we describe the design and the realization process of a new modular 241 Am-Be neutron irradiator in order to obtain a neutron energy distribution useful to test neutron detectors or personal dosemeters. Some of the useful calculations oriented to optimize thicknesses and shape of the moderators and shielding materials were obtained with a Monte Carlo simulation with MCNP5 code.

5. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator The results of the simulations conduct to neutron flux values (F4 Tally) or to dose (F6 Tally). The neutron flux was estimated using the tally F4 (cm - 2 ), which calculates the average flux over a cell (particles·cm -2 ). The absorbed neutron dose rate was obtained through the tally F6 (MeV·g -1 ) which considers the neutron energy deposition average over the cell.

6. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator The intervals of energy considered were: -thermal below the Cadmium cutoff energy (<0.5 eV); two epithermal neutron groups (between 0.5 eV and 0.1 MeV, 0.1 MeV and 0.3 MeV); -three fast neutron groups (0.3 MeV to 1 MeV, from 1 MeV to 5 MeV, and above 5 MeV). The energy distribution from a 241 Am-Be neutron source was given by ISO 8529, with an energy spectrum ranging from 0.025 eV to 12 MeV.

7. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Several geometry configurations are considered: 1)A first simple model, with a neutron source sited at the center of a sphere-shaped moderator of various diameter 2)Four 241 Am-Be sources placed at the edges of a parallepiped Plexiglas structure surrounded by various materials : the sources was more realistically modeled as a cylinder shape of 43 mm lenght and about 20 mm in diameter, coated with a metallic alloy.

8. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator  One of these configurations was realized placing the sources inside Plexiglas tubes held by three square Plexiglas plates of 14 cm side. A Plexiglas pipe placed at the center of the structure functions as irradiation cell. The remaining space inside the container is filled with water which represents both the moderator and the biological shielding.

9. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator The schematic of the Monte Carlo model Configuration is characterized by a prevalence of thermal neutron flux component 241 Am-Be sources moderator/shielding source channel sample channel

10. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator real irradiator geometrical features, adding the remaining significant components After a no-fully positive validation of the simplified model, we decide to carry out the implementation of more detailed model which include real irradiator geometrical features, adding the remaining significant components

11. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator The new model Configuration is characterized by a prevalence of thermal neutron flux component support elements tube lock plate

12. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Experimental validation of the model The experimental test, performed using the "cadmium difference" technique, and the numerical predictions are broadly in agreement, and the results differ by about 15%

13. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Foil A – at source middle plane Foil B – at 5 cm from source middle plane Foil C – at 10 cm from source middle plane

14. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator All the examined configurations do not fully comply with the modularity requirements mentioned in introduction. It was then decided to study a configuration that would allow the use of different type and thickness of moderator or, if desired, completely empting the irradiation cell. To start the MCNP5 simulation process, the following criteria were assumed:

15. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Criteria  A possibility, with simple and easy procedures, to obtain a neutron flux characterized by thermal or fast neutron prevalence by a suitable positioning of components;  A flat behavior of neutron flux within the irradiation cell so that a constant neutron flux density in the sample volume or dosimeter to be irradiated can be assumed, apart sample self-attenuation that must be taking into account;

16. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Criteria  Easy handling of each source, without complicated procedures to its recovery;  Values of maximum effective dose outside the irradiator below population dose limit without any limitation on time, i.e. below 1 mSv per year as provided by the most common national regulations ;

17. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Criteria  Installation flexibility, with the possibility of irradiation of samples of various sizes and also to test individual dosimeters placed on anthropomorphic phantoms.  The features designed with MCNP5 must then be interfaced with construction requirements, and an optimal compromise between two needs may be reached.

18. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal MCNP5 design of a new irradiator Monte Carlo Simulation To draw up the final draft, several tests were made with MCNP5 simulations (more of 100 runs, the minimum number of stories examined for each test: 1  10 8, total processing time not less than 1200 h taking into account the time needed for different personal computers). Based on constraints, taking into account the availability of four 241 Am ‑ Be sources (each of 111 GBq activity, neutron emission: 7  10 6 s -1 ), the final configuration consists of an irradiation box and a separate source recovery box.

19. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal A modular irradiator -Schematic diagram of the final configuration irradiation moduleRest module Source Removable polyethylene Fixed polyethylene Biological shield Boron shield Water LEGEND: Direct long rod Source’s rest positionWatertight crossWater make-up

20. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal A modular irradiator  Irradiation cell is obtained at the center of a polyethylene cube designed so that it is possible to change its geometrical configuration to generate fast or thermal neutron fluxes, and may easily be equipped with filters (or material suitable for conversion). A large portion of the polyethylene moderator can be removed to specifically irradiate large volume sample or test detectors on different phantoms.

21. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal neutron thermal flux prevalence schematic Overall Thermal Normalized neutron flux [cm -2 ] Distance [cm] Source Removable polyethylene Fixed polyethylene Bio shield Boron shield Water LEGEND: Direct long rod irradiation module

22. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Neutron fast flux prevalence schematic Overall Thermal Source Removable polyethylene Bio shield Boron shield Water LEGEND: Normalized neutron flux [cm -2 ] Distance [cm] Direct long rod irradiation module Irradiation pit’s basis

23. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Working drawings : vertical section

24. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Working drawings : horizontal section

25. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal A Modular irradiator : Main features a) respectively fast and thermal pit setting b) respectively exposed sources (direct long rod totally inserted) and rest position sources (direct long rod at maximum extraction) 193162358÷496 b depthhighwidth Maximum overall [cm]  16000  15000  512 430÷490 a Overall Biological shieldWaterPolyethylene Main parts weight [kg] 40x40x40h: 15  : 10 126 Fast flux Prevalence (LxHxP) Thermal flux prevalence ReciprocalPit centre Pit’s features [cm]Sources spacing [cm] Geometric features 5.25·10 4  28·10 6  444 4 Exespected flux in central pit [n·cm -2 ·s -1 ] Emitted neutrons [s -1 ]Total Activity [GBq]Number of sources Nuclear features

26. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal A modular irradiator During the realization The outside part of polyethylene irradiation cell and the removable part before the lavoration

27. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal A modular irradiator The recovery box The recovery box is made of a water- tight container specifically realized in fiberglass surrounded with concrete bricks.

28. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal In progress activities  The irradiation assembly is nearing completion;  The sources must be transferred into the rest module;  A comparison between simulate and experimental flux values will carry out by means of bubble detectors, bubble spectrometry system (BDS), TLD detectors and so on.  It is programmed a large energy group spectroscopy by using activated materials (with a threshold energy).

29. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal What do we expect?  Uniformity of neutron flux values inside the irradiation cell;  Easy to get a neutron flux distribution from thermal prevalence to fast prevalence with a few operations.  An useful facility to NAA applications, dosemeters testing and calibration.

30. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Conclusions Monte Carlo simulation is confirmed as an useful technique to study neutron irradiators with different configurations and to preliminary assess the characteristics of neutron flux originating from various sources. The studied model can still be used for planning research experiences or particular applications of neutron irradiation, by identifying the most promising facility assessment.

31. UNIVERSITA’ DEGLI STUDI DI PALERMO DIN DIPARTIMENTO DI INGEGNERIA NUCLEARE CHERNE 2010 - 6th Workshop on European Collaboration for Higher Education and Research in Nuclear Engineering and Radiological Protection - 7-9 June 2010 – University of Coimbra – Coimbra, Portugal Thank you for your attention ! Coimbra (Portugal), June 7, 2010