Silicon drift detectors and readout electronics for high throughput spectroscopy applications

X-ray Absorption Fine Structure (XAFS) spectroscopy studies the absorption coefficient of materials in the region close to the absorption edges. After-edge oscillations of the absorption coefficient contain information about the chemical and physical structure that surrounds the excited atom. Fluorescence mode XAFS spectroscopy makes it possible to obtain the XAFS for thick or very diluted samples measuring their emission spectrum that, in the dilute limit, is proportional to the absorption. In this technique, the sample is excited by a beam of photons, whose energy can be finely tuned using a monochromator. The energy of the beam is changed at small steps starting in the region immediately before the absorption edge and ending 1 keV after it. The fluorescence produced by the sample is acquired by an X-ray radiation detector. Solid State Detectors (SSDs) are generally used for XAFS as they guarantee high energy discrimination. Among SSDs, the best performances in terms of count rate and energy resolution are provided by Silicon Drift Detectors (SDDs). SDDs, that have been introduced by E. Gatti and P. Rehak in 1983, are characterized by a signal-charge collecting electrode with a very low capacitance, independent from the device active area. These detectors have the high energy resolution required by the applications (in an 55-Fe spectrum, the Ka line should have FWHM<200 eV) to distinguish among the different spectral lines of interest. As in SSDs each photon is revealed independently and its processing takes a finite amount of time, there is a finite dead time after each pulse in which the detector is not able to acquire another pulse. This sets a limit on the maximum count rate the detector can achieve. Synchrotron beamlines are experiencing in these years an increment in brilliance (i.e. in fluorescence flux) that asks for an improvement of the existing detectors in terms of count rate. My doctoral activity took place in the framework of the ARDESIA experiment. ARDESIA (ARray of DEtectors for Synchrotron radIation Applications) is an experimental project funded by the INFN National Scientific Committee V, whose goal is the realization of an X-ray spectrometer for synchrotron experiments based on arrays of SDDs. The project has started in 2015 and it is expected to end in 2018. The spectrometer has been designed and manufactured in all its parts, from the detection module to the mechanical structure and the readout electronics. Chapter 1 is the introduction of this thesis. It gives first some insights on XAFS spectroscopy, to locate ARDESIA inside the correct frame. After that, it introduces SSDs, with emphasis on SDDs that are described with some details. In this part of the chapter, the basic concepts of detector signal processing are recalled, as they are used multiple times during the thesis. Finally, the main requirements of a spectrometer for synchrotron radiation applications are obtained and commented. Chapter 2 and Chapter 3 describe ARDESIA detection module. Chapter 2 is focused on the preliminary studies to optimize detector geometrical properties (i.e. detector size and shape). Two main aspects are considered. First, the finite drift time of the charge inside a SDD degrades the energy resolution at short pulse processing times and it depends on detector size. Second, the signal to background ratio worsens in case of pixelated detectors with dead areas among channels. Chapter 3 describes ARDESIA detection module and its characterization. The first part describes the detection module in all its components, while the second part deals with its characterization. The detection module has good performances in terms of both energy resolution and high count rate capability. The SDDs array has been realized in two different shapes, square and circular, that gives similar results. Chapter 4 describes ARDESIA spectrometer. The spectrometer interfaces the detection module and the scattering chamber, providing the correct environment for the detection module to work into. It provides the detection module with all the needed bias voltages and it brings the detector outputs to further readout electronics stages. In particular, the spectrometer accounts for detection module cooling and it sustains static vacuum, preventing ice formation when the temperature falls below 0 °C. The first part of the chapter presents the design of the spectrometer while the second part the results of the characterization. Chapter 5 and Chapter 6 describe the development of a dedicated analog readout electronics for ARDESIA spectrometer. Chapter 5 reviews analog pulse processing techniques for high count rates. In particular, the chapter focuses on the comparison among different families of analog filters at short shaping times, on the comparison among different pile-up rejection techniques and on the study of channels multiplexing strategies. Chapter 6 presents TERA (Throughput Enhanced Readout ASIC). This ASIC has been designed on the basis of the studies described in Chapter 5 and it is meant to be an analog readout solution for ARDESIA spectrometer, providing high count rate while keeping good energy resolution.

La spettroscopia XAFS (X-ray absorption fine structure) studia il coefficiente di assorbimento dei materiali nella regione vicina ai bordi di assorbimento. Le oscillazioni post-bordo contengono informazioni sulla struttura fisico-chimica che circonda la specie atomica sotto analisi. La spettroscopia XAFS in fluorcenza permette di ottenere la XAFS per campioni spessi o molto diluti. Questa tecnica misura lo spettro di emissione del materiale che, nel limite di alta diluizione, è proporzionale all'assorbimento. Il campione viene eccitato da un fascio di fotoni, la cui energia può essere selezionata con precisione tramite un monocromatore. L'energia del fascio viene cambiata a piccoli passi, partendo dalla regione immediatamente precedente al bordo di assorbimento fino a circa 1 keV dopo di esso. La fluorescenza emessa dal campione viene acquisita tramite un rivelatore di raggi X. Nella spettroscopia XAFS si impiegano normalmente rivelatori di radiazione a stato solido (SSD) perché garantiscono una elevata discriminazione energetica. Tra i rivelatori a stato solido, gli SDD (Silicon Drift Detector) garantiscono le migliori prestazioni in termini di tasso di conteggio e risoluzione energetica. Gli SDD, che sono stati introdotti da E. Gatti e P. Rehak nel 1983, sono caratterizzati da un anodo dalla piccolissima capacità, indipendente dalla area attiva del dispositivo. Poichè nei rivelatori a stato solido ogni fotone è rivelato in modo indipendente ed il suo processamento richiede un tempo finito, per ogni fotone rivelato c'è un tempo morto nel quale il rivelatore non è in grado di rivelare un altro evento. Questo impone un limite al massimo tasso di conteggi che il rivelatore può raggiungere. Le beamlines dei sincrotroni stanno sperimentando in questi anni un incremento nella brillanza (cioè nel flusso di fluorescenza) che richiede un miglioramento dei rivelatori esistenti in termini di tasso di conteggio. La mia attività di dottorato si è svolta nell'ambito del progetto ARDESIA. ARDESIA (ARray of DEtectors for Synchrotron radIation Applications) è un progetto sperimentale finanziato dall'INFN, il cui obiettivo è la realizzazione di uno spettrometro a raggi X basato su matrici di SDD per esperimenti di sincrotrone. Nel corso della mia attività di dottorato, lo spettrometro è stato progettato e realizzato in ogni sua parte, a partire dal modulo di rivelazione fino alla struttura meccanica ed alla elettronica di lettura.