Operation and testing of the PULSTAR reactor intense slow positron beam and PALS spectrometers

An intense slow positron beam has been established at the 1-MW PULSTAR nuclear reactor. The beam is operational generating mono-energetic positrons with an energy of 1-keV. The maximum measured intensity slightly exceeds 10(9) e+/s. The beam is operated routinely with an intensity of approximately 5x10(8) e+/s. The positrons are generated through gamma-ray pair production interactions in two back-to-back banks of tungsten converter/moderators. The gamma-rays are produced in the PULSTAR core and by thermal neutron capture in a cadmium shroud that surrounds the tungsten. The primary utilization of the PULSTAR positron beam is the characterization of nanoscale structure in materials. Consequently, the beam has been equipped with two state-of-the-art PALS spectrometers. The first spectrometer is dedicated to measurements in materials such as metals and semiconductors. This spectrometer is based on pulsing and bunching of the primary beam and is currently operating with a timing resolution of approximately 390 picoseconds. The second spectrometer is dedicated to measurements in materials where positronium formation is promoted. The timing resolution of this spectrometer is designed to be similar to 0.5 nanosecond with an on-sample spot size of 1-2 mm. For both spectrometers, the energy of the positrons can be varied to allow depth profiling with on-sample intensity exceeding 10(6) e+/s.