Measuring the coincidence time resolution of the detectors in a TOF-PET test setup at CERN laboratory in Switzerland. Credit: Stefan Gundacker
An international collaboration led by CERN, European Organization for Nuclear Research, studied the efficiency of next-generation silicon photomultipliers (SiPMs) using barium fluoride crystals when excited with X-ray and gamma photons. Their findings on scintillation kinetics, published in the journal Physics in Medicine & Biology, describe a fast cross-luminescence emission, an ultrafast emission, and the ability to effectively suppress a slow 800 nanosecond emission. The work opens a unique path for the development of ultrafast timing applications in medical diagnosis, range monitoring in ion therapy and high-energy physics.
“The intention is to bring cross-luminescence materials such as barium fluoride (BaF2) and these new SiPMs, with further improved performance, to the system level. There are of course still many open questions, especially for the integration in a system, but the prospects are promising. Besides the application in positron emission tomography (PET), we think that BaF2 might have the highest impact for new medical X-ray imaging technologies, such as time-correlated single-photon counting computed tomography,” said Stefan Gundacker, lead study author and group leader for time-of-flight detectors at Experimental Molecular Imaging at RWTH Aachen University in Germany.
A fundamental application
The researchers tested state-of-the-art vacuum ultraviolet silicon photomultipliers (VUV-SiPMs) from Fondazione Bruno Kessler using BaF2 scintillators. Scintillators are materials which can convert high energy radiation such as X-rays or gamma rays to a near visible or visible light. One key finding was an ultrafast emission with sub-100 picosecond decay time in BaF2, observed with 511 keV excitation and ultrafast VUV excitation at FinEstBeAMS beamline at MAX IV. The slow self-trapped exciton (STE) band emission was possible to significantly suppress through doping with yttrium, cadmium or lanthanum. This suppression is important to avoid a loss of information in a detector caused by pile-up effects.
Previous work by Gundacker and colleagues found that standard SiPMs and scintillators are approaching their limits in timing performance. The new vacuum ultraviolet silicon-photomultipliers may offer a significant improvement in time resolution using BaF2 detectors.
“The development of these new SiPMs was pushed much forward by large experiments, with the search for new physics and dark matter,” explained Gundacker. “I think this is a perfect example where fundamental research efforts not only bring answers to questions on how the universe works, but also can have a direct impact on people’s lives, by implementing new technologies in novel medical devices for example.”
BaF2 crystals are cost effective compared with standard Lutetium-based scintillators, for example cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) or lutetium orthosilicate (LSO) according to the study.
The power of photon detection sensitivity in the deep-UV range coupled with fast cross-luminescence emission in BaF2 scintillators holds promise to elevate positron-emission-tomography (PET) and time-of-flight computed tomography (TOF-CT) for high-rate, ultrafast timing at low cost.
The researchers will continue investigations on larger BaF2 crystal samples, new VUV-SiPM technologies as well as the survey of other cross-luminescence materials with different properties. Investigation of optical coupling, especially below 200 nm wavelength, will be another challenging future endeavour. The coupling would require long-term stability and reliability to achieve commercialization potential.
S Gundacker et al. Vacuum ultraviolet silicon photomultipliers applied to BaF2 cross-luminescence detection for high-rate ultrafast timing applications. 2021 Phys. Med. Biol. 66 114002. https://doi.org/10.1088/1361-6560/abf476