Measurement and Application of Topological Information in Physically and Virtually Segmented Scintillators

Abstract

Spatial information about the interaction of radiation and matter within a scintillation detector, also known as event topology, can be obtained from various detector designs. This work explores a new application of event topology for spectroscopy in extreme pileup environments, and a novel technique for measuring event topology through stochastic confinement of scintillation photons.

The determination of the spectral characteristics of a polyenergetic beam of radiation traditionally depends upon resolving the interactions of individual particles, and then correlating features of the resulting distribution of energy deposition to the energy of the incident particles. In environments in which many particles arrive at the detector simultaneously, it can be very difficult to discern the interactions of individual particles. A study detailing a new technique to address this problem is presented, wherein an incident beam of gamma rays is first Compton scattered, and then a physically segmented scintillation detector system is used to measure the topology of scattered gamma rays. The distribution of scattered gamma rays carries information about the incident beam, from which the spectral characteristics can be reconstructed. This technique could be applied to characterize laser-driven inverse-Compton scattering sources, which produce a short but intense burst of collimated, quasi-monoenergetic photons.

In a typical scintillation detector, the scintillation light may reflect many times within the volume before being collected and converted to an electronic signal, usually with a single photosensor. A consequence of this is that the detailed location of interaction is often difficult to reconstruct. Measuring the event topology in scintillation detectors can be accomplished by physically segmenting the volume into voxels, or by recording the spatial and/or temporal distribution of light arriving at multiple photosensors surrounding an un-voxelized scintillator volume. The first method generally requires many channels and can be difficult to fabricate, while the second method can be inefficient and suffer from poor spatial resolution. This thesis investigates a new method for measuring event topology based on opaque scintillators. In opaque scintillation detectors, virtual voxelization can be achieved by repeatedly scattering the scintillation photons, such that they are effectively confined to a small lightball around their origin. Then the photons can be collected by a lattice of wavelength-shifting fibers. The theory and simulation of light transport and collection in such a system are presented, and an experiment is described that measured the absolute efficiency of light collection from an opaque liquid to validate the simulation. Two prototypes are described and characterized, based on the wax-based opaque scintillator and the opaque water-based liquid scintillator. The key result is the demonstration of reconstruction of the position of point-like events with a precision of 4.4 mm, corresponding to approximately 25% of the fiber spacing. The resulting fine voxelization could benefit applications that require topological reconstruction and scaling to large volumes, including antineutrino detection, gamma-ray, neutron, and muon imaging, and positron-emission tomography.