Design and Implementation of a Digital Positron Annihilation Lifetime Spectrometer for Measurements in Graphite

Abstract

A digital Positron Annihilation Lifetime Spectrometer was designed and used to perform measurements on three graphite materials, reactor grade graphite, pyrolytic graphite, and a foam graphite developed by Oak Ridge National Laboratory. Positrons are a useful probe of the microstructural features of matter since they are attracted to open-volume pores and defects where the electron density is lower than in other parts of the material. Various types of graphite were studied because of their importance in nuclear technology, including as a moderator in nuclear reactor cores.

A lifetime spectrometer consists of a scintillation detector, photomultiplier tube, and equipment to perform timing analysis on the detected radiation. This equipment can either consist of analog pulse shaping and timing electronics or a system that digitizes and processes the radiation pulses. A new type of scintillation material, Lanthanum Bromide [LaBr3(Ce)], was tested and compared to the scintillator usually used for lifetime experiments, Barium Fluoride. The Lanthanum Bromide was expected to perform somewhat better than BaF2 based on its scintillation properties, and this was confirmed.

The digital system was tested and its performance optimized. The digital lifetime spectrometer shared some similar equipment with a standard analog spectrometer and as such, both could be used simultaneously to take measurements. The digital spectrometer showed improvement in its timing resolution over the analog system. The measurements on graphite were more conclusive for the digital system than for the analog system, as results from the former matched published data well and were more consistent in general. This was due to the greater flexibility in timing methods and opportunity for optimization afforded by the digital system.

Measurements on the graphites supported other work in the literature showing a lifetime of approximately 200 ps in the reactor grade and pyrolytic graphites with a second lifetime on the order of 410 to 425 ps. In the reactor grade graphite the second lifetime was about 410 ps while for the pyrolytic graphite it was about 425 ps. The second lifetime is higher in the pyrolytic graphite and it is attributed to its greater disorder. The 200 ps lifetime is explained as the lifetime of positrons in a perfect graphite crystalline structure while the 400 ps lifetime is explained as the lifetime of positrons at grain boundaries or defects between different regions in the graphite. Data fits involving a third lifetime for the materials were not satisfactory in terms of fitting statistics or results related to physical phenomena.

Measurements on the foam graphite yielded two lifetimes of approximately 125 ps and 340 ps. The first value matches the lifetime of para-positronium, a bound state of an electron and positron known to form in porous materials. The theoretical lifetime of this type of positronium is 129.3 ps. The 340 ps lifetime is most likely the result of positron annihilation in the graphite structure of the foam reflecting positron annihilation in both the perfect crystal structure of the graphite and in grain boundaries. This lifetime is close to the 334 ps mean lifetime found in the reactor grade graphite. Further work and improvements in the experimental technique and equipment could provide more insight into the measurements on graphite.