Crystal Clear, 30 years on

Apr 28, 2021

The Crystal Clear collaboration (experiment RD-18) began in 1991 as part of the R&D programme run by the Detector Research and Development Committee (DRDC) to address the formidable challenges posed by the future LHC. The objective was clear: identify the most suitable scintillating crystals to pave the way for the discovery of the Higgs boson. Now, 30 years on, it is clear that the collaboration has exceeded all expectations. Not only did it contribute to one of the greatest physics discoveries of the twenty-first century, but it also went on to help drive innovation in the medical technology sector.

Amidst the large-scale R&D efforts to develop the detectors for the future LHC, the collaboration quickly set to work in studying scintillating crystals whose scintillation mechanisms were still a mystery. In 1994, that research led to the recommendation to use lead tungstate (PbWO4 or PWO), a material combining the advantages of high density, fast scintillation and good resistance to radiation with relatively low manufacturing costs, for the construction of the CMS electromagnetic calorimeter and the ALICE PHOS detector. That recommendation was followed, as both detectors are made from PWO crystals.

The purpose of an electromagnetic calorimeter is to measure the energy of photons, electrons and positrons. The particles’ energy is transformed into light as they pass through the crystals and is then detected by a photodetector whose signal is analysed to identify the original particle. Notably, it was in the heart of the CMS electromagnetic calorimeter that the Higgs boson was identified by its decay into two photons.

Starting in 1995, in parallel with its R&D work on scintillators for high-energy physics, the Crystal Clear collaboration branched out into medical applications with the development of several positron emission tomography (PET) devices for imaging in nuclear medicine. PET uses scintillating crystals for the coincidence detection of pairs of photons resulting from electron–positron annihilation. The collaboration started by developing ClearPET prototypes, PET cameras for small animals(1), then moved on to ClearPEM prototypes for detecting breast cancer(1) and, more recently, the EndoTOFPET-US prototype for detecting pancreatic and prostate cancer.

Today, the collaboration’s efforts to improve the coincidence time resolution (CTR) of these tomography machines continue, the target being a CTR of 10 picoseconds (as against more than 200 picoseconds for commercial PET cameras), which would improve image quality while reducing the time spent in the scanner and the dose administered to the patient(2). To this end, the collaboration is exploring new detection concepts, including the development of scintillating nanomaterials.

The Crystal Clear collaboration is also currently pursuing its initial R&D work on future detectors. “Detectors for future accelerators will have to deal with unprecedented constraints on their components. Developing fast, radiation-resistant crystals and coming up with new ways to use them will be vital to designing detectors based on the scintillators of tomorrow,” says collaboration spokesperson Étiennette Auffray, laying out a vision for the future of Crystal Clear.

The technologies developed for high-CTR PET cameras, known as time-of-flight PET cameras, inspired the insertion of a layer of LYSO (lutetium-yttrium oxyorthosilicate) crystals, called the “barrel timing layer”, in the CMS central barrel between the tracker and the electromagnetic calorimeter, which will measure the time of flight of each particle. A “Spaghetti Calorimeter”, or “SpaCal”, made up of an absorber and scintillating crystal fibres, is also being studied as part of the EP department’s R&D programme. It could replace the central part of the current LHCb electromagnetic calorimeter.

Unfortunately, the current health situation prevents the members of the Crystal Clear collaboration from celebrating its 30th anniversary, at least for now. But despite that, Crystal Clear, which has always moved with the times, is looking resolutely forward to a bright future for scintillating crystals.

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(1) See CERN Courier July/August 2013 p.23
(2) Paul Lecoq et al, 2020 Phys. Med. Biol. 65 21RM01


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