Design and Testing of a Prototype Slow Positron Beam at the NC State University PULSTAR Reactor

Abstract

Slow positrons have proven to be a powerful tool for the analysis of materials. This nondestructive probe is capable of reconstructing the electronic environment below the surface of materials utilizing the conservation of energy and momentum on the annihilation quanta. Radioactive sources are capable of producing slow positron rates on the order of 10⁶ Hz, but the pair production mechanism is necessary to produce more intense beams. The environment in the vicinity of nuclear reactor cores is capable of producing copious amounts of photons with energy above the threshold for the pair production mechanism.

MCNP simulations were performed which modeled North Carolina State University's PULSTAR reactor to calculate the photon flux above the pair production threshold energy. It was determined that the addition of cadmium to the end of the beam port would enhance the photon flux by a factor of 3. Further simulations demonstrated that core optimization could increase the flux by an additional factor of 2.

A positron beam was designed to focus positrons produced in the vicinity of the core into a solenoid which would allow transport through the biological shield. SIMION simulations were performed to demonstrate the effectiveness of the proposed beam and concluded that the beam design could focus positrons with a wide range of energies.

A prototype beam was completed and placed within a beam port of the PULSTAR reactor. Tests immediately demonstrated the production and control of slow positrons. Additional tests established the dependence of the count rate on the axial magnetic field of the solenoid, with a maximum count rate achieved by the prototype beam being 33x10⁷ slow positrons per second.

The knowledge gained from the prototype can be applied to future work to improve the overall beam effectiveness. Future works includes the bending of the solenoid and implementation of core optimizations. The size of the moderator can also be increased from that of the prototype to increase the beam intensity.