X-ray imaging with gaseous detectors using the VMM3a and the SRS

The integration of the VMM3a Application-Specific Integrated Circuit (ASIC) into RD51's Scalable Readout System (SRS) provides a versatile tool for the readout of Micro-Pattern Gaseous Detectors (MPGDs). With its self-triggered high-rate readout, its analogue part that allows to get information on the deposited energy in the detector, and its so-called neighbouring-logic that allows to recover information on the charge distribution, this new system has features of particular interest for digital X-ray imaging. In the present article, we want to emphasise the capabilities of VMM3a/SRS by presenting results of X-ray imaging studies. We will highlight the advantages on the energy and the spatial resolution provided by the neighbouring-logic. In the first part, we focus on spatial resolution studies. We show how segmented readout structures introduce a repeating pattern in the distribution of the reconstructed positions (using the centre-of-gravity method) and how this behaviour can be mitigated with the neighbouring-logic. As part of these studies, we explore as well an alternative position reconstruction algorithm. In the second part of the article, we present the energy resolution studies.Peer reviewe

Timing performance of a Micro-Channel-Plate Photomultiplier Tube

The spatial dependence of the timing performance of the R3809U-50 Micro-Channel-Plate PMT (MCP-PMT) by Hamamatsu was studied in high energy muon beams. Particle position information is provided by a GEM tracker telescope, while timing is measured relative to a second MCP-PMT, identical in construction. In the inner part of the circular active area (radius r5.5 mm) the time resolution of the two MCP-PMTs combined is better than 10 ps. The signal amplitude decreases in the outer region due to less light reaching the photocathode, resulting in a worse time resolution. The observed radial dependence is in quantitative agreement with a dedicated simulation. With this characterization, the suitability of MCP-PMTs as t0 reference detectors has been validated.Peer reviewe

Precise charged particle timing with the PICOSEC detector

The experimental requirements in near future accelerators (e.g. High Luminosity-LHC) has stimulated intense interestin development of detectors with high precision timing capabilities. With this as a goal, a new detection concept called PICOSEC,which is based to a “two-stage” MicroMegas detector coupled to a Cherenkov radiator equipped with a photocathode has beendeveloped. Results obtained with this new detector yield a time resolution of 24 ps for 150 GeV muons and 76 ps for single pho-toelectrons. In this paper we will report on the performance of the PICOSEC in test beams, as well as simulation studies andmodelling of its timing characteristicsPeer reviewe

Towards robust PICOSEC Micromegas precise timing detectors

The PICOSEC Micromegas (MM) detector is a precise timing gaseous detector consisting of a Cherenkov radiator combined with a photocathode and a MM amplifying structure. A 100-channel non-resistive PICOSEC MM prototype with 10x10 cm^2 active area equipped with a Cesium Iodide (CsI) photocathode demonstrated a time resolution below 18 ps. The objective of this work is to improve the PICOSEC MM detector robustness aspects; i.e. integration of resistive MM and carbon-based photocathodes; while maintaining good time resolution. The PICOSEC MM prototypes have been tested in laboratory conditions and successfully characterised with 150 GeV/c muon beams at the CERN SPS H4 beam line. The excellent timing performance below 20 ps for an individual pad obtained with the 10x10 cm^2 area resistive PICOSEC MM of 20 MOhm/sq showed no significant time resolution degradation as a result of adding a resistive layer. A single-pad prototype equipped with a 12 nm thick Boron Carbide (B4C) photocathode presented a time resolution below 35 ps; opening up new possibilities for detectors with robust photocathodes. The results made the concept more suitable for the experiments in need of robust detectors with good time resolution

Precise timing with the PICOSEC-Micromegas detector

This work presents the concept of the PICOSEC-Micromegas de-tector to achieve a time resolution below 30 ps. PICOSEC consists of a two-stageMicromegas detector coupled to a Cherenkov radiator and equipped with a photo-cathode. The results from single-channel prototypes as well as the understanding ofthe detector in terms of detailed simulations and preliminary results from a multi-channel prototype are presented.Peer reviewe

Charged particle timing at sub-25 picosecond precision : The PICOSEC detection concept

The PICOSEC detection concept consists in a “two-stage” Micromegas detector coupled to a Cherenkov radiator and equipped with a photocathode. A proof of concept has already been tested: a single-photoelectron response of 76 ps has been measured with a femtosecond UV laser at CEA/IRAMIS, while a time resolution of 24 ps with a mean yield of 10.4 photoelectrons has been measured for 150 GeV muons at the CERN SPS H4 secondary line. This work will present the main results of this prototype and the performance of the different detector configurations tested in 2016-18 beam campaigns: readouts (bulk, resistive, multipad) and photocathodes (metallic+CsI, pure metallic, diamond). Finally, the prospects for building a demonstrator based on PICOSEC detection concept for future experiments will be discussed. In particular, the scaling strategies for a large area coverage with a multichannel readout plane, the R&D on solid converters for building a robust photocathode and the different resistive configurations for a robust readout.Peer reviewe

A large area 100 channel Picosec Micromegas detector with sub 20 ps time resolution

The PICOSEC Micromegas precise timing detector is based on a Cherenkov radiator coupled to a semi-transparent photocathode and a Micromegas amplification structure. The first proof of concept single-channel small area prototype was able to achieve time resolution below 25 ps. One of the crucial aspects in the development of the precise timing gaseous detectors applicable in high-energy physics experiments is a modular design that enables large area coverage. The first 19-channel multi-pad prototype with an active area of approximately 10 cm$^2$ suffered from degraded timing resolution due to the non-uniformity of the preamplification gap. A new 100 cm$^2$ detector module with 100 channels based on a rigid hybrid ceramic/FR4 Micromegas board for improved drift gap uniformity was developed. Initial measurements with 80 GeV/c muons showed improvements in timing response over measured pads and a time resolution below 25 ps. More recent measurements with a new thinner drift gap detector module and newly developed RF pulse amplifiers show that the resolution can be enhanced to a level of 17~ps. This work will present the development of the detector from structural simulations, design, and beam test commissioning with a focus on the timing performance of a thinner drift gap detector module in combination with new electronics using an automated timing scan method

Progress on the PICOSEC-Micromegas Detector Development : Towards a precise timing, radiation hard, large-scale particle detector with segmented readout

This contribution describes the PICOSEC-Micromegas detector which achieves a time resolution below 25 ps. In this device the passage of a charged particle produces Cherenkov photons in a radiator, which then generate electrons in a photocathode and these photoelectrons enter a two-stage Micromegas with a reduced drift region and a typical anode region. The results from single-channel prototypes (demonstrating a time resolution of 24 ps for minimum ionizing particles, and 76 ps for single photoelectrons), the understanding of the detector in terms of detailed simulations and a phenomenological model, the issues of robustness and how they are tackled, and preliminary results from a multi-channel prototype are presented (demonstrating that a timing resolution similar to that of the single-channel device is feasible for all points across the area covered by a multi-channel device).Peer reviewe

Application of gas scintillation properties in optically read out GEM-based detectors

Starke Signalverstärkung, hohe Ortsauflösung und niedriges Materialbudget zählen neben der Anwendbarkeit in Umgebungen mit hohen Teilchenflüssen zu den wichtigsten Vorteilen von gasbasierten Detektoren und machen diese zu einer attraktiven Technologie für die Messung von Strahlung und für bildgebende Verfahren. Die Auslese von Szintillationslicht, welches während der lawinenhaften Vervielfachung von Elektronen emittiert wird, mittels moderner Bildsensoren liefert akkurate Visualisierungen der einfallenden Strahlung. Die anpassbaren Signalverstärkungsfaktoren von Strukturen wie gasbasierten Elektronenvervielfachern (GEMs) erlauben die Messung von Strahlung über einen weiten Bereich von Energien von minimal ionisierenden Teilchen bis zu niedrigenergetischen Röntgenstrahlen und stark ionisierender Strahlung. Die Szintillationseigenschaften von Gasmischungen für optisch ausgelese GEM-basierte Detektoren wurden untersucht. Die Lichtausbeute und Szintillationsspektren des emittierten Szintillationslichtes wurden in verschiedenen Konfigurationen mit unterschiedlichen Gasmischungen und elektrischen Signalverstärkungsfeldern gemessen um die optimalen Parameter für optisch ausgelesene Detektoren zu bestimmen. Optisch ausgelesene GEM-basierte Detektoren wurden über längere Zeiträume hinweg geschlossen betrieben, wobei sich die Intensität der gemessenen Signale nur minimal verminderte. Die präsentierten Untersuchungen der Szintillationseigenschaften und darauf basierender Detektorkonzepte ebnen den Weg für Anwendungen in bildgebenden Verfahren bis hin zu Kernphysik und Hadronentherapie. Optisch ausgelesene GEM-basierte Detektoren optimiert für Röntgenaufnahmen und Tomografie wurden entwickelt. Der Betrieb in einem proportionalen Bereich der Signalverstärkung und ausreichende Sensitivität zur Aufnahme einzelner Röntgenstrahlen ermöglichten die Aufnahme von Röntgenfluoreszenz und die darauf basierende Unterscheidung verschiedener Materialien mit 2D Auflösung. In Verbindung mit Interaktionszeitinformationen, welche von schnellen Photonendetektoren oder von ergänzenden elektronischen Ausleseverfahren stammen können, wurden Teilchenbahnen in einer optisch ausgelesenen Zeitprojektionskammer (TPC) aufgenommen und zu 3D Repräsentationen rekonstruiert. Eine optisch transparente, segmentierte Anode wurde entwickelt um die simultane Anwendung von optischen und elektronischen Ausleseverfahren zu erlauben und die Rekonstruktion von komplexen Teilchenbahnen in der optisch ausgelesenen TPC zu ermöglichen. Ein planisphärischer GEM-basierter Detektor mit radial fokussierten Feldlinien im aktiven Detektionsvolumen wurde entwickelt um Parallaxe zu minimieren und die deutlich verbesserte Ortsauflösung von Detektoren mit dicken Detektionsvolumen basierend auf diesem Konzept für Anwendungen in der Röntgenfluoreszenzanalyse und Kristallographie wurde demonstriert. Die gute Ortsauflösung, welche mit optischer Auslese erreicht werden kann, und das niedrige Materialbudget gasbasierter Detektoren wurden für einen Detektor für Protonenstrahlen kombiniert. Die Möglichkeiten der Aufnahme von 2D Profilen der deponierten Dosis sowie der Beobachtung der Profile und Intensität von Protonenstrahlen wurden in einer klinischen Einrichtung für Protonentherapie demonstriert.Strong signal amplification, high achievable spatial resolution and low material budget as well as applicability in high-rate environments as key advantages of micropattern gaseous detectors make them an attractive candidate for radiation detection and imaging. Reading out scintillation light emitted during electron avalanche multiplication with modern imaging sensors provides accurate visualisations of incident radiation. The adjustable gain of amplification structures such as Gaseous Electron Multipliers (GEMs) enables radiation detection over a wide range of energies from minimum ionising particles to single low-energy X-ray photons and highly ionising radiation. Scintillation characteristics of gas mixtures for optically read out GEM-based detectors were investigated. Light yield and scintillation spectra of the emitted scintillation light in a range of operating conditions with variable amplification fields and different gas mixtures were studied to determine optimum operation conditions for optically read out detectors. Long term sealed mode operation of optically read out GEM-based detectors was achieved with a minimal degradation of signal strength. The presented gas scintillation studies and detector concepts based on optically read out GEMs effectively pave the way for applications ranging from radiation imaging to high energy physics and hadron therapy. Optically read out GEM-based detectors optimised and employed for X-ray radiography and tomography were developed. Operating in a proportional high-sensitivity regime, single X-ray photon sensitivity could be used for X-ray fluorescence imaging and material distinction with 2D resolution. Augmenting images of particle tracks with timing information, which can be obtained with fast photon detectors or complementary electronic readout, 3D reconstructed trajectories in an optically read out Time Projection Chamber (TPC) could be obtained. A transparent multi-pad anode was developed to combine simultaneous optical and electronic readout to extend the track reconstruction capabilities of optically read out TPCs. A planispherical GEM-based detector employing radially focused field lines in the conversion volume to minimise parallax-induced broadening was developed and shown to permit significantly improved spatial resolution for X-ray fluorescence applications of gaseous detectors with thick conversion layers. Taking advantage of the high spatial resolution achievable with optical readout and the low material budget of gaseous detectors, a proton beam monitoring detector was developed. 2D dose imaging as well as accurate beam profile and intensity monitoring were demonstrated at a clinical proton therapy facility.13