IMPROVEMENTS IN THE THERMAL NEUTRON CALIBRATION UNIT, TNF2, AT LNMRI/IRD

Area of Science:

• Nuclear Physics
• Radiation Metrology
• Neutron Physics

Background:

• The standard thermal neutron flux unit (TNF2) at the Brazilian National Ionizing Radiation Metrology Laboratory (LNMRI/IRD) underwent a rebuilding process.
• Neutron fluence is generated by moderating four 241Am-Be sources (0.6 TBq each).

Purpose of the Study:

• To redesign and characterize the rebuilt TNF2 facility.
• To evaluate the neutron fluence quality, including intensity, spectrum, and cadmium ratio.
• To validate the system's performance for traceability in radiation metrology.

Main Methods:

• Facility simulation and redesign using MCNPX code with graphite core and paraffin-added graphite blocks.
• Evaluation of neutron fluence quality through simulations.
• Assembly of the system based on simulation results.
• Experimental measurements of neutron spectrum, fluence, and ambient dose equivalent (H*(10)).

Main Results:

• Optimized geometric arrangements of moderator materials and neutron sources were identified through simulations.
• The rebuilt TNF2 system was characterized at central and external points.
• Validation of spectral measurements, fluence, and H*(10) was performed.

Conclusions:

• The rebuilt TNF2 facility demonstrates improved neutron fluence quality.
• The characterization and validation ensure traceability for radiation metrology applications.
• The study provides a robust methodology for characterizing neutron flux units.