The upgraded intense positron beam facility at the PULSTAR reactor

Abstract

The intense positron beam user facility at the PULSTAR reactor of the North Carolina State University has undergone recent upgrades to address issues caused by radiation damage over the years to the Tungsten moderator and the main beam structure. The two-bank Tungsten moderator is replaced with a four-bank Platinum moderator. The effect of moderator geometry has been systematically studied using SIMION and COMSOL multiphysics and the positron transport efficiency is optimized. Initial tests showed a positron rate of nearly 6×108 e+/s. The two associated spectrometers were also modified to improve their performance. The magnetically guided spectrometer with an RF buncher (e+-PAS) has been upgraded to include a new pre-buncher and a modified chopper to significantly improve the signal to noise ratio for PALS spectroscopy. In addition, a sample changer with heating and cooling capabilities is under development to address the sample turn-around efficiency and evacuation time. Meanwhile, the electrostatically focused Ps-PAS spectrometer is under an upgrade to further shrink the beam spot size and improve the beam brightness. Magnetic shielding is being implemented between the two remoderator stages and the electrostatic lenses have been redesigned to achieve better beam focusing. After the first-phase of modifications, the on-sample spot size is expected to be improved from ∼2 mm to ∼0.5 mm. The second-phase of modifications involves the implementation of a third stage of brightness enhancement and a final magnetic focusing lens. The new features will allow the beam to be focused down to less than 100 μm and enable measurement with a spatial resolution of several tens of micrometers.