E. Bolle

Test beam results of 3D silicon pixel sensors for the ATLAS upgrade

Results on beam tests of 3D silicon pixel sensors aimed at the ATLAS Insertable B-Layer and High Luminosity LHC (HL-LHC) upgrades are presented. Measurements include charge collection, tracking efficiency and charge sharing between pixel cells, as a function of track incident angle, and were performed with and without a 1.6 T magnetic field oriented as the ATLAS inner detector solenoid field. Sensors were bump-bonded to the front-end chip currently used in the ATLAS pixel detector. Full 3D sensors, with electrodes penetrating through the entire wafer thickness and active edge, and double-sided 3D sensors with partially overlapping bias and read-out electrodes were tested and showed comparable performance.

The AX-PET concept: New developments and tomographic imaging

The Axial PET (AX-PET) concept proposes a novel detection geometry for PET, based on layers of long scintillating crystals axially aligned with the bore axis. Arrays of wavelength shifting (WLS) strips are placed orthogonally and underneath the crystal layers; both crystals and strips are individually readout by G-APDs. The axial coordinate is obtained from the WLS signals by means of a Center-of-Gravity method combined with a cluster algorithm. This design allows spatial resolution and sensitivity to be decoupled and thus simultaneously optimized. In this work we present the latest results obtained with the 2-module AX-PET scanner prototype, which consists of 6 radial layers of 8 LYSO crystals each (crystal size: 3 × 3 × 100 mm3). The WLS arrays comprise 26 strips (3-mm wide) per layer. The estimated energy resolution from point-like measurements is 11.8% (FWHM at 511 keV). The intrinsic spatial resolution was measured for the two modules in coincidence at two different configurations using point-like sources, showing very little degradation when the modules were placed oblique to each other. The axial spatial resolution was 1.5 mm (FWHM) in all the studied cases. Tomographic data of extended phantoms filled with fluorine-18 have been acquired. Imaging a larger transaxial Field-of-View (when compared to the previous measurement campaign) was possible thanks to implementing secondary motion of one of the modules. We have also developed various reconstruction approaches which take into account the particular nature of AX-PET data, as well as a count rate model which allowed us to develop an acquisition protocol able to compensate for count losses. The reconstructed phantom images confirm the imaging capabilities of AX-PET, and the recent advancements in the DAQ let us expect significant improvements for future campaigns.

COMPET — A preclinical PET scanner implementing a block detector geometry with high resolution, high sensitivity and 3D event reconstruction

COMPET is an innovative implementation of a small animal PET scanner using a novel block detector geometry, allowing for a high resolution and high sensitivity. One detector block is built up from layers of long LYSO crystals. Perpendicular and interleaved between the crystals, Wave Length Shifting (WLS) fibers are used. The scintillation light created by a gamma ray interacting with a crystal is measured with Geiger mode Avalanche Photo Diodes (GAPDs) at one end of the crystals. A small part of the scintillation light escapes the crystals and enters the WLS, where it has a certain probability to be absorbed and re-emitted at a longer wavelength. This light is measured at one end of the WLS by a GAPD. With this setup, the point of interaction (POI) of the gamma ray is deduced, allowing for the 3D reconstruction of the interaction point between the gamma ray and the detector. Thus, not only photoelectric interactions are used to reconstruct the line of responses (LOR) for each event, but also Compton scattered gamma-rays are included. Using 4 such modules, the total detector comprises a total amount of 1080 readout channels, where 600 are used for the crystals and 480 for the WLS. The central point source resolution was deduced from Monte Carlo simulation to be below 1 mm FWHM in transaxial direction. The sensitivity to detect coincident gamma rays emitted at the centre of the field of view is up to 16%. With its compact geometry, high point source resolution, high sensitivity and its low amount of readout channels, the COMPET detector geometry provides a promising detector layout for future preclinical PET scanners.

A Monte-Carlo based model of the AX-PET demonstrator and its experimental validation

AX-PET is a novel PET detector based on axially oriented crystals and orthogonal wavelength shifter (WLS) strips, both individually read out by silicon photo-multipliers. Its design decouples sensitivity and spatial resolution, by reducing the parallax error due to the layered arrangement of the crystals. Additionally the granularity of AX-PET enhances the capability to track photons within the detector yielding a large fraction of inter-crystal scatter events. These events, if properly processed, can be included in the reconstruction stage further increasing the sensitivity. Its unique features require dedicated Monte-Carlo simulations, enabling the development of the device, interpreting data and allowing the development of reconstruction codes. At the same time the non-conventional design of AX-PET poses several challenges to the simulation and modeling tasks, mostly related to the light transport and distribution within the crystals and WLS strips, as well as the electronics readout. In this work we present a hybrid simulation tool based on an analytical model and a Monte-Carlo based description of the AX-PET demonstrator. It was extensively validated against experimental data, providing excellent agreement.

AX-PET: Concept, proof of principle and first results with phantoms

AX-PET is a novel PET concept based on long crystals axially arranged and orthogonal Wavelength shifter (WLS) strips, both individually readout by Geiger-mode Avalanche Photo Diodes (G-APD). Its design was conceived in order to reduce the parallax error and simultaneously improve spatial resolution and sensitivity. The assessment of the AX-PET concept and potential was carried out through a set of measurements comprising individual module characterizations and scans in coincidence mode of point-like and extended sources. The estimated energy and spatial resolutions from point-like measurements are RFWHM=11.6% (at 511 keV) and 1.7–1.9 mm (FWHM) respectively as measured with point-like sources placed in different positions of the FOV. First results from scans of extended phantoms confirmed our expectations.

Long axial crystals for PET applications: The AX-PET demonstrator and beyond

The usage of long, axially oriented scintillator crystals in a PET scanner has been shown by the AX-PET Demonstrator as a possible solution for a high resolution and high sensitivity PET detector. In the AX-PET implementation, arrays of wavelength shifting (WLS) strips, placed orthogonally behind every crystal layer, are used to define the axial coordinate. After extensive characterization measurements, the AX-PET Demonstrator has been successfully used for the reconstruction of several phantoms and a few rodents. Possible extensions of the AX-PET concept towards Time Of Flight capabilities have been investigated, using Philips digital SiPMs as alternative photodetector. Promising CRT values equal to 211 ps have been demonstrated, using long crystals with dual sided readout and mean timing method. Finally, we report about an alternative way to reconstruct the axial coordinate: exploiting the light sharing of 100 mm long crystals with special surface treatment resulted in axial resolutions of the order of 4 mm FWHM.

Highly scalable asynchronous trigger system for PET using UDP/IP

Asynchronous coincidence triggering in PET can be realized by using UDP over 1 Gbps Ethernet. This will bypass problems related to point-to-point connections and tree-structures in real-time coincidence trigger units. By using UDP over Ethernet, the trigger system is easy to scale, and off-the-shelf components can be used. However, due to unknown and network load dependent delay times of UDP communication packages, and due to possible package losses, the system must react tolerantly towards these occurrences. In our FPGA based readout system, event packages (consisting of an event time, energy, and channel number) are stored in a FIFO. The event times are sent to a Central Trigger Unit (CTU) through a UDP/IP link where trigger decisions are made and broadcasted back to the readout cards. The broadcasted trigger times are compared to existing event times to decide if the event package is discarded or sent to a computer farm. Since the UDP packages with event times and trigger times are self-contained there is no need for sorting algorithms usually associated with UDP networking. The UDP/IP stack is implemented solely in hardware without using large amount of resources.