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Positron Annihilation Detection Beyond the Limits

Periodic Reporting for period 1 - PALADIN (Positron Annihilation Detection Beyond the Limits)

Reporting period: 2015-09-01 to 2017-08-31

Given the growth and aging of the world population, innovative solutions for sustainable energy and health are needed. Novel technologies play an enabling role in the realization of such solutions. Better devices for the detection of annihilation quanta resulting from positron annihilation are an example. In material science these detectors are needed for research on renewable energies and innovative energy storage using Positron Annihilation Lifetime Spectroscopy (PALS). In medicine they are required for diagnosis, staging, and treatment monitoring of diseases using Time-of-Flight Positron Emission Tomography (TOF-PET).

In both fields time resolution is a key parameter. Sub-100 picosecond resolution is needed for improved sensitivity and signal-to-noise ration but is not yet available. The goal of this project was finding of ways to improve resolving times of radiation detectors used for PALS and TOF-PET and not deteriorating other performance parameters at the same time. In order to realize that goal two different approaches were investigated in the project, i.e. the application of big monolithic scintillators using digital silicon photomultipliers and extracting information of the radiation by reconstruction of its interaction and the improvement of the time resolution via the use of a hybrid scintillation/Cherenkov materials.
As the best suited scintillator had to be used for the radiation detector, an overview study of potential inorganic scintillators was performed to find the best suitable material. As the focus of PALADIN is put on timing, ultrafast L(Y)SO:Ce (co-doped with rare-earth elements) was investigated in combination with silicon photomultipliers as photodetectors. In detail, four potential materials were compared for their properties. During these studies additional materials. e.g. the relatively cost-efficient BGO, were tested in order to further improve performance of such detectors. In this material also the emission of a low but detectable number of Cherenkov photons was predicted. As the Cherenkov emission is known to be very fast, it was investigated if detection the few Cherenkov photons could have an impact on the coincidence resolving times and if scintillation light could provide information of the annihilation radiation at the same time . After a successful study the a search for more potential materials for that hybrid detector approach revealed the promising candidate BSO. It could be shown with this material the timing performance could be further improved although the theory predicted a worse performance when only scintillation emission was considered.

As a very low number of emitted Cherenkov photons is deteriorating the precision of the timing information and about half of these few photons is lost during propagation towards the photo detector, it was investigated if the photon transport could be improved in an additional study. As a result, it could be shown that basic modifications of the scintillator surface could have significant impact on the photon transport and by optimizing the surface parameters the signal ratio of scintillation (energy) to Cherenkov emission (timing) and consequently the overall detector performance could be even further improved.

Results of the described studies were presented at several international conferences and workshops. Moreover, an international patent application was filed about the hybrid scintillation and Cherenkov emission readout using BSO and related materials. The research project was presented in events like the ScienceDay of TU Delft or a guest lecture at TU Vienna. An interinstitutional project initiated in the frame work of PALADIN was awarded with the ScienceDATE grant of the faculty of Physics of TU Delft.
For the initial studies on L(Y)SO:Ce scintillators coincidence resolving times (CRT) better than 80 ps FWHM could be achieved using silicon photomultipliers, representing best ever measured results for the dimensions of the scintillators. Furthermore, for the first time a hybrid approach using a combination of scintillation light and an additional Cherenkov emission could be applied for this class of radiation detectors using the scintillator BGO. Also these experiments resulted in best ever reported timing performance of BGO. New technologies in photon detection, i.e. the digital silicon photomultiplier, made it possible to extract precise time information in the ps-domain out of just a few photons. Given these results, BGO offers several advantageous characteristics when compared to L(Y)SO:Ce, e.g. better stopping power and lower cost. Furthermore, the timing performance could be even further improved by applying the even lower-cost-material BSO. Within these measurements it was demonstrated that minor modifications of the scintillator surfaces can be used to optimise not only the performance of the hybrid radiation detectors but also of pure scintillation based systems. Eventually, these unprecedented results led to an international patent application. Furthermore, as it was not yet completely understood if only Cherenkov emission is responsible for the good timing performance the detailed luminescence characteristics of BGO and BSO were investigated. To do so a very simple but precise implementation of the time-correlated photon counting method based on digital silicon photomultipliers has been developed and introduced to the community.
The PALADIN detector: 6 sided readout of a monolithic scintillator using digital SiPM.