Nicolaus Kratochwil

The demand for detectors with a time resolution below 100 ps is at the center of research in different fields, from high energy physics to medical imaging. In recent years, interest has grown in nanomaterials that, benefiting from quantum confinement effects, can feature ultra-fast scintillation kinetics and tunable emission. However, standard characterization methods for scintillation properties–relying on radiation sources with an energy range of several hundreds of keV–are not suitable for these materials due to their low stopping power, leading to a slowdown of this R&D line. We present a new method to characterize the time resolution and light output of scintillating materials, using a soft (0–40 keV energy) pulsed X-ray source and optimized high-frequency readout electronics. First, we validated the proposed method using standard scintillators. Then, we also demonstrated the feasibility to measure the time resolution and get an insight into the light output of nanomaterials (InGaN/GaN multi-quantum well and CsPbBr 3 perovskite). This technique is, therefore, proposed as a fundamental tool for characterization of nanomaterials and, more in general, of materials with low stopping power to better guide their development. Moreover, it opens the way to new applications where fast X-ray detectors are requested, such as time-of-flight X-ray imaging.

The MIP Timing Detector will provide additional timing capabilities for detection of minimum ionizing particles (MIPs) at CMS during the High Luminosity LHC era, improving event reconstruction and pileup rejection. The central portion of the detector, the Barrel Timing Layer (BTL), will be instrumented with LYSO:Ce crystals and Silicon Photomultipliers (SiPMs) providing a time resolution of about 30 ps at the beginning of operation, and degrading to 50-60 ps at the end of the detector lifetime as a result of radiation damage. In this work, we present the results obtained using a 120 GeV proton beam at the Fermilab Test Beam Facility to measure the time resolution of unirradiated sensors. A proof-of-concept of the sensor layout proposed for the barrel region of the MTD, consisting of elongated crystal bars with dimensions of about 3 x 3 x 57 mm$^3$ and with double-ended SiPM readout, is demonstrated. This design provides a robust time measurement independent of the impact point of the MIP along the crystal bar. We tested LYSO:Ce bars of different thickness (2, 3, 4 mm) with a geometry close to the reference design and coupled to SiPMs manufactured by Hamamatsu and Fondazione Bruno Kessler. The various aspects influencing the timing performance such as the crystal thickness, properties of the SiPMs (e.g. photon detection efficiency), and impact angle of the MIP are studied. A time resolution of about 28 ps is measured for MIPs crossing a 3 mm thick crystal bar, corresponding to an MPV energy deposition of 2.6 MeV, and of 22 ps for the 4.2 MeV MPV energy deposition expected in the BTL, matching the detector performance target for unirradiated devices.

A novel technique for testing the timing properties of scintillators is presented. The technique is based on transient absorption (TA) induced in a scintillating material by a selective excitation of the activator ion. A figure of merit to assess the timing properties of scintillators is suggested. This parameter was estimated for a set of cerium doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) bars, which have been fabricated for Barrel Timing Layer sensor of Compact Muon Solenoid detector (CMS BTL) and exhibited different timing properties, and compared with the results obtained by conventional coincidence time resolution (CTR) measurements. The figure of merit applied for the tested bars shows a strong correlation (Pearson's correlation coefficient R = 0.95) with the CTR. These results suggest that the TA technique could be used as an experimental method to expand in a complementary way the extensive qualification procedure of LYSO:Ce crystals that will be performed for the production of the CMS BTL detector.

Achieving good time resolution has become a prerequisite in positron emission tomography (PET) to improve the signal-to-noise ratio and thus the quality of the reconstructed image. A particularly attractive solution for the electronic readout is using low-noise high-frequency circuits (HF), which have demonstrated excellent timing performance in time-of-flight (TOF)-PET applications. With improvements in the achievable coincidence time resolution (CTR), the effect of light transport and gamma-ray depth of interaction (DOI) along the crystal axis becomes a dominant contribution that must be mitigated. To address this issue, a light-sharing mechanism using a matrix of scintillators coupled to an array of SiPMs can be employed to identify the DOI and correct for the induced bias. For the light-sharing method to work, readout integration in a multi-channel scheme is required. The use of HF circuits allows to lower the leading edge detection threshold, enabling the use of the fastest photons produced in the crystals and thereby overcoming the limitation of the NINO amplifier-discriminator chip. Previous studies have shown that the Hamamatsu S13361-3050AE-04 SiPM array and NINO amplifier-discriminator chip-based readout electronics achieved CTR values of 160 ps FWHM using an array of 16 LYSO crystals measuring 3.1x3.1x15 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . In this study, we present the performance improvement achieved by integrating this detector prototype into a 16-channel HF readout board. We outline the current limitations and solutions to overcome them.

Particle physics experiments running at future accelerator facilities will rely on fast timing detectors to cope with high event pileup and to enhance particle identification capabilities. Direct band gap engineered semiconductor nanostructures show a high potential for emission of prompt photons due to quantum confinement, standing out in fast timing and high light yield with potential low cost production, thus triggering interest in the high energy physics community. In this contribution, characterization results of the scintillation properties of some promising nanomaterial scintillators will be presented and an overview of possible light-based detector designs based on these nanomaterials will be given, focusing on the NanoCal Blue Sky project of the European project AIDAinnova, a new generation of shashlik calorimeters. First results, obtained during test beam activities, will be presented as well.

Time-of-Flight (TOF) reconstruction in Positron Emission Tomographic (PET) scanners uses a single kernel to reconstruct all events. However, recently developed detectors combining prompt emission and typical scintillation signal produce output events with different timing spreads. One such detectors technology is based on BGO crystals with Cherenkov photons. Thanks to fast silicon photo-multipliers sensitive in the near-ultraviolet and high-frequency electronic readout, the faint Cherenkov signal produced by the interaction of 511 keV gamma photons can be detected as faster pulse rise times. We present initial results from a Monte Carlo simulation and image reconstruction platform for detectors with multiple timing resolutions in this work. Simulated timing spreads show excellent agreement with the experimental measurements. In addition, the reconstruction software can reconstruct images using listmode and projection data. In terms of contrast recovery, the proposed multi-kernel model with BGO-Cherenkov detectors presents a similar recovery as a typical Gaussian model with LYSO detectors and timing resolution 213 ps. To the best of our knowledge, the use of multiple complex TOF kernels in image reconstruction has not been investigated in the past. However, further optimisations are needed in order to obtain the best possible results.

Das PANDA Experiment bei FAIR in Darmstadt verwendet einen Strahl aus gekuhlten Antiprotonen mit einen Impuls zwischen 1.5 bis 15 GeV die auf Protonen geschossen werden um offene Fragen der starken Wechselwirkung zu beantworten. Das Kernprogramm von PANDA besteht aus Spektroskopie von Charmonium mit genauer Bestimmung von Masse, Zerfallsbreite und Zerfallsmoden, Untersuchung von moglichen exotischen Zustanden, Suche von Modifikationen von Hadronen mit Charm-Quarks in Materie sowie γ Spektroskopie von Hyperkernen. Der Barrel TOF Detektor ist 50 cm vom Antiprotonenstrahl entfernt, umfasst eine Flache von 5.7 m² und deckt einen azimutalen Winkel zwischen 22.5° und 150° ab. Der Detektor wurde entwickelt, um eine Zeitauflosung unter 100 ps (sigma) zu erreichen wodurch eine gute Event Separation und Teilchenidentifikation unterhalb des Cherenkov Limits ermoglicht wird. Der Detektor besteht aus 1920 einzelnen Szintillatoren, welche von Silicium- Photomultiplier (SiPM) an beiden Seiten ausgelesen werden. Diese Arbeit beschreibt die Entwicklung des Szinillator mit SiPM und setzt bisherige Arbeiten am Detektor fort. Dabei wird der Einfluss der Dicke des Szintillators sowie unterschiedliche Materialien auf die Zeitauflosung untersucht. Fur einen auf 5 mm abgeschliffenen Szinillator, der von 4 Hamamatsu SiPM in serieller Schaltung auf beiden Enden ausgelesen wird, wurde eine mittlere Zeitauflosung von 48.2 ±3.4 ps gemessen.

The possibility to improve depth of interaction (DOI) resolution and coincidence time resolution (CTR) of a pixellated PET module by enabling light re-circulation inside it with a light guide is well known. Typically the light guide consists of a non-scintillating material of about 1 mm thickness. In this work, we propose to further extend the concept by replacing the passive light guide with a fast scintillating material, in order to combine the benefits of light re-circulation with a fraction of very fast events, where the DOI is precisely known.Several configurations with such an active layer are proposed and studied in this work by means of Monte Carlo simulations with experimental verification. First, the possibility of replacing the glass light guide with a layer of LYSO is investigated. This configuration allows to reach DOI resolutions beyond the possibilities of a simple glass guide, while retaining comparable performances in terms of energy and timing resolutions. Then, the performance of two fast scintillators (BaF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and BC422) used as light guides, in combinations with crystal arrays made of both LYSO and BGO, is investigated. The fraction of shared events (i.e. those events where the 511 keV gamma ray scatters in the light guide and deposits the rest of its energy in the crystal array) in a 3 mm light guide is found to be around 1% for BC422, and 12.1% for BaF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Therefore, the configuration using the latter material is investigated in depth, and two alternative readout schemes are proposed, to maximize the collection of light produced by BaF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . The results show that ∼ 100 ps FWHM CTR can be reached for shared events using a BaF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> light guide. Finally, the possibility to use such a detector design as a Compton camera is discussed.