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RICH detectors, detectors for medical imaging, photosensors
and read-out electronics

Advances in our understanding of elementary particles and their forces are tightly connected to the progress in detection techniques; novel instruments and methods can, in turn, be applied to other fields, most notably to advances in medical imaging. In the Photodetector development lab we are doing both, developing advanced particle identification detectors, mainly Ring Imaging Cherenkov detectors (RICH), and novel detectors for medical imaging with positron emission tomography (PET).


Our expertise

The members of the Photodetector development laboratory come from the Department for experimental particle physics (F9) of the Jožef Stefan Institute (JSI), and Faculty of Mathematics and Physics, University of Ljubljana. JSI is by far the largest research institute in Slovenia, covering a broad range of disciplines, from physics, chemistry and biochemistry to electronics and computing. Its Department for experimental particle physics (F9) has strong involvements in the ATLAS experiment at CERN, Belle and Belle II experiments at KEK in Tsukuba, Japan, and the Pierre Auger Project at FERMILAB in the USA. Several of the members of the team from the Faculty of Mathematics and Physics are also members of the Medical Physics research group.

In the Photodetector development lab we are:

Developing novel ring imaging Cherenkov detectors

We collaborate in large international collaborations, where particle identification is crucial part of the analysis of decays. Our work is focused on the design and development of Ring Imaging Cherenkov Counters. We were involved in the design and operation of HERA-B RICH, and two Belle II particle identification detectors: Aerogel RICH and Time-Of-Propagation Counter. HERA-B RICH used a C4F0 gas radiator and a large 24 m2 spherical mirror for imaging Cherenkov rings. Single photons were detected by 2240 Hamamatsu multi-anode photomultipliers with about 27,000 channels. It was the first Ring Imaging Cherenkov Counter that used multi-anode photomultipliers for photon detection. The strong contribution of teh Ljubljana team and coordination of the research activities by Peter Križan resulted in fully reached design goals.

The Belle II spectrometer incorporates two-particle identification systems, both located inside a strong 1.5 T magnetic field: Ring Imaging Cherenkov detector with Aerogel radiator in its forward end-cap and Time-Of-Propagation Counter in the barrel region. The first one consists of an aerogel radiator and a detector plane with 420 Hybrid Avalanche Photo Detectors. Samo Korpar coordinated the design and the installation of the sub-system. The complex front-end readout electronics, supported by the reconstruction software developed and tested in our laboratory, contribute to the excellent and smooth operation of the particle identification detector. The Time-Of-Propagation Counter consists of large quartz panels, in which the emitted light is internally reflected and detected by microchannel plate photomultipliers. Our lab members contributed to the design and installation of the detector. Marko Starič is also responsible for the important part of the reconstruction and calibration software.

We study photon sensors for the new generation of Cherenkov ring detectors (RICH). Due to HL-LHC upgrade, the particle fluxes will increase so that multi-anode photomultipliers currently used for detecting photons in LHCb RICH, will have to be replaced by a photon sensor with increased granularity. Hybrid Avalanche Photon detectors of Belle II RICH with an Aerogel radiator will also not be able to operate after the Belle II long term upgrade. In our laboratory we are exploring new detection concepts and possibilities for both upgrades. We are developing a silicon photomultiplier, single-photon sensor that will be very fast, will have fine granulation, will be sensitive to light of long wavelengths and withstand radiation load, mainly due to neutron flux. As an alternative, we study Large Area Picosecond Photon Detector, based on microchannel PMT technology. We design and construct the prototypes of RICH detectors, test them in our laboratory and evaluate them In the high-energy charged particle test beams at CERN, Geneva, DESY, Hamburg and KEK, Japan.

Developing novel detection methods for positron emission tomography, PET and TOF PET

Experimental particle physics strives to develop and master state-of-the-art technology. Innovations from our laboratories can be usefully transferred to other areas. Nuclear medicine is a successful example where we are introducing advances in photodetectors and reading electronics to improve detector technology.

Leveraging our experience from the design and deployment of Cherenkov detectors in experimental particle physics, we have developed a novel positron emission tomography (PET) detector based on prompt Cherenkov photons. Cherenkov PET detectors can be built from low-cost lead fluoride (PbF2) crystals, and enable excellent time-of-flight (TOF) resolution. In 2011, we experimentally demonstrated a TOF resolution of 95 ps FWHM. We have shown that by using advanced detector geometries, the fundamental limit to Cherenkov TOF PET resolution due to physical process in the crystal is only 22 ps FWHM. We produced two multi-channel Cherenkov TOF PET modules, integrated with fast readout electronics, and experimentally demonstrated Cherenkov PET image reconstruction for the first time. Our latest simulation studies show that whole-body Cherenkov TOF PET scanners produce image quality at least equal to state-of-the-are clinical scanners [G. Razdevšek et al., see Publications].

We are studying a flat-panel TOF PET device using fast scintillators and custom-designed photodetectors. Ultra-fast detection makes PET imaging possible in open geometry, using a significantly smaller amount of detection crystal, thus enabling a lower cost and greater flexibility. Through precise simulations, we have shown that such an approach achieves image quality comparable to the best current commercial devices while also enabling more affordable simultaneous imaging of the whole body. We are involved in international cooperation to build such a PET device, including the University of Barcelona, Fondazione Bruno Kessler, the University of Davies in California and Harvard University.

Investigating novel photosensors with associated electronics

Advanced photon detectors play an important role in different areas. We focus on single-photon detection for charged particle identification in HEP experiments to fast photon detection for medical applications and applications in life sciences. We are studying the properties of the different vacuum, solid-state and vacuum detectors. In different experimental setups, we measure the position dependence of the time and charge response of the detectors to short laser light pulses.
We have been working with multi-anode photomultipliers since 1996. We have successfully deployed 16 and 4 channel versions in the HERA-B RICH, used them in a variety of environmental applications and continued their exploration with 64 channel devices in the LHCb RICH.

Silicon photomultipliers started their rise two decades ago. As they have much higher detection efficiency reaching above 50% and are much easier to handle, we are investigating their response, evaluating samples from different producers and exploring the operation ranges to employ them in high radiation environments. Within the AIDAInnova consortium, we are leading the task of the development of neutron hardened devices. We are also exploring their usage in Time-Of-Flight Positron Emission Tomography and for detection of fluorescence light for life sciences applications.

Hybrid avalanche Photo Detectors are hybrid vacuum devices where photo-electron from the photo-cathode is accelerated towards the segmented avalanche photodiode. The device works in the strong perpendicular magnetic field, making it an ideal sensor for Belle II RICH with an aerogel radiator. In the laboratory, we have studied the device and developed a complex multi-layer readout board for reading out the signals.

Microchannel plate PMT is currently the fastest single-photon detector. We are evaluating different devices, from 1 inch devices to 2” and larger 20 cm LAPPD. We are studying the effect of high voltage variation at different multiplication stages on the timing and position resolution of the detector.

Developing detectors for life sciences

In recent years much effort has been spent developing readout electronics and photodetectos for the needs of future high luminosity high energy physics experiments. Big improvements in performance and cost-efficiency of such technologies were achieved, which can also benefit other applications. We have developed a novel fluorescence lifetime measurement system based on the latest silicon photomultiplier photodetectors and signal digitization approaches. Fluorescence lifetime measurements are used in a wide range of applications in life sciences, for example, imaging of cell structures in biology, detection of cancer cells in medicine and protein binding assays in pharmacy. However, due to intrinsic limitations of the typically used time-correlated single-photon counting (TCSPC) method, the fluorescence lifetime measurements require sophisticated instrumentation and a certain time for acquisition.

By detecting the fluorescence emission with a fast photodetector and sampling the whole signal, our novel approach enables lifetime measurement from even a single pulse of excitation light – real-time acquisition – while reducing instrumentation complexity and cost. With the first device, we have demonstrated fluorescence lifetime acquisition with an accuracy comparable to a commercial TCSPC system. We have incorporated the whole acquisition system in a compact and portable form, enabling experimental studies on the locations of potential users. For these developments, we have received the Best innovative projects within public research organizations for the economy in 2021 prize at the 14th International Technology Transfer Conference 2021 organized by the Jozef Stefan Institute Center for Technology Transfer and Innovation.


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