Scintillators: new ways to fast emission

Transcription

1 Scintillators: new ways to fast emission for Medical and High Energy Physics Applications Rosana Martinez Turtós Tutor: Marco Paganoni Co-tutor: Alessio Ghezzi Submitted for the degree of Doctor of Physics and Astronomy Università degli Studi di Milano Bicocca September 2016

2 DRAFT: 18th September 2016 ii Università degli Studi di Milano Bicocca Rosana Martínez Turtós Scintillators: new ways to fast emission for Medical and High Energy Physics Applications Summary

3 DRAFT: 18th September Chapter 1 Summary 1.1 State-of-the-art and Motivation Over the last few decades, radiation detector research has largely been directed toward the discovery and development of scintillators with improved energy resolution via increased light yield and better proportionality [?], [?]. More recently, generating a prompt response to the passage of ionizing particles has emerged as a critical requirement for next-generation radiation detectors, notably in high energy physics (HEP) and Time-Of-Flight Positron Emission Tomography (TOF-PET) applications. In the search for rare events in the planned high luminosity large hadron collider at CERN (HL-LHC), for example, event discrimination would require a sub-20ps time resolution to mitigate the problem of particle bunch pileup [?]. Similarly, precise time tagging of 511 kev gamma rays is needed in TOF-PET in order to confine the e + -e -annihilation point along the line of response (LOR) to the level of a few millimeters [?], [?]. Current state-of-the-art time resolution values in large multichannel HEP detectors is of the order of 150 ps [?] and < 380 ps in commercial TOF-PET scanners [ref]. Under laboratory conditions, on the other hand, 10 mm long cerium-doped Lu 2 SiO 5 crystals with nanosecond photon emission rates, coupled to modern solid state photodetectors (e.g. silicon photomultipliers, SiPMs) and fast frontend electronics, deliver coincidence time resolution of 117ps for 511keV gammas [?]. If instead the same scintillator was used for the detection of minimum ionizing particles (MIPs with de/dx 1MeV/mm), the resolution improves to 60 ps with single-channel readout [?]. Furthermore, if for the sake of TOF-PET, the CTR could reach levels of 10 ps, the conversion vertex along the LOR (z) could be confined within 1.5mm. This precision, together with the scanners x-y-spatial resolution of 1mm in the plane of the detection pixels, would then lead to true space points and thus, for the first time, allow real-time image reconstruction in PET. An advancement from present state-of-the-art to O(10ps) CTR, however, entails a major technological challenge beyond conventional photon spectroscopy with its inherent scintillation mechanisms. Even a new fast crystal brand, like co-doped LSO:Ce:Ca(0.4%), with novel elementary stoichiometry and lowest emission rise- and decay-times (τ r =20ps, τ d eff =30ns) is still far from reaching the 10-20ps time domain (CTR is 100 ps for 511keV gamma excitation and 34 ps for MIPs and 5mm crystal[?]). Under the approximate assumption ( for τ r τ d ) that coincidence time resolution is parametrized as, CT R = τr τ d N pe (1.1) The photoelectron time density of the crystal-photodetector-frontend ensemble would need to be increased by two orders of magnitude if a CTR of O(10ps) was to be reached. This thesis will concentrate on the impact of the scintillating crystal optical signal as the key factor to reduce timing. The main parameters, i.e. τ r, τ d and light yield (N pe ) will be put

4 DRAFT: 18th September in perspective in order to know their exact contribution to the CTR and therefore, find ways to improve it. Starting with the quantification of the L(Y)SO intrinsic light yield, which sets a limit to the value that N pe could theoretically reach and finishing with the implementation of nanocrystals-based scintillators as a new particle detector, the present contribution proposes a feasible way to overcome the technological challenge that sub-20ps time resolution implies. 1.2 Structure This thesis is organized in the way it follows: Chapter 2, the theoretical background needed to follow the present thesis is being introduced starting with the description of conventional scintillators. The main parameters that characterize the scintillating pulse are described together with its main applications. Chapter 3, the introduction of nanocrystals as a new scintillating ultrafast material is done. Quantum confinement together with carrier multiplication and photoionization effects under optical excitation will be introduced in order to understand the multiexcitonic dynamics exhibit by these systems under ionizing radiation. Special attention is put on nanocrystals that show a severe reduction of Auger recombination. Chapter 4, the validation of the Monte Carlo code used to transport and track optical photons along the scintillator will be done, comparing to light output experimental results. Once the simulations are proven to effectively reproduce the measurements using different coupling/wrapping conditions, estimations of the Intrinsic Light Yield are done, using different approaches. A two separated sections are dedicated to geometrically modified crystals, which bring some gain in light extraction and to the study of high resolution matrices to increase spatial resolution of PET systems. Chapter 5, the experiment designed to measure the Intrinsic Light Yield of LYSO crystals will be presented. Knowledge of this quantity brings complete insight on the optimization limits faced by increasing photostatistic to its maximum value. The idea consist in confine light in a determinate point of the space by electron excitation, extract this light in a well defined cone which will set the solid angle used as a purely geometric correction factor. Chapter 6, in this section a full characterization of the scintillating light rise and decay time will be presented for different types of crystals. The experimental bench uses a pulsed x-rays tube as excitation source, droved by a picosecond laser. Time resolved single photon counting technique allow us to zoom in at the first nanoseconds of the scintillation pulses with a time resolution of 18 ps and IRF between ps sigma. Two rise time components have been identified for most of the crystals, which correspond to direct and delayed excitation of the activator centers. The study included lutetium based scintillating crystals, with and without Ca 2+ co-doping, and a set of garnets (YAG, GaGG, LuAG) with and without Mg 2+ co-doping. Chapter 7 presents and characterize different types of nanocrystals as a potential source of prompt photons in the search for ultrafast radiation detectors. They are CdSe nanoplateletes, CdSe nanoplateletes with a CdS or ZnS shell and CdSe quantum dots with CdS giant shell. As first, their photoluminescence properties in terms of timing will be measured using a 372 nm picosecond laser excitation, followed by pulsed x-rays excitation. Effective decay times measured are of the order of sub-100 ps for CdSe NPLs and sub-1 ns for CdSe/CdS giant shells under ionizing radiation. In order to optimize timing and efficiency performance, the same materials will be coupled to Ag nanoparticles with a plasmon resonance that match the nanocrystals emission. Characterization of this plasmonic layers is done using laser optical excitation. Chapter 8 presents the implementation of nanocrystals as a new particle detector. As first, a dry deposition on one of the faces of a conventional scintillator is done and characterize under x-ray excitation. As a second approach, ZnO:Ga nanopowders are embedded in different hosts and measured in coincidence under gamma and MIPs excitation Chapter 9, conclusions and perspectives.

5 DRAFT: 18th September Discussion Intrinsic light yield measurements done for L(Y)SO concluded in a total of ± 3%(stat) ± 10%(syst) ph/mev [?]. Extrapolating this value to the light output of 20 mm long LYSO crystals, we can concluded that Npe in equation 1.1 could be at best increased by a factor of two if the total amount of light created was extracted from the crystal. Dedicated efforts to improve the energy transfer and quantum efficiency of the luminescent centers could provide another factor of two in Npe before reaching the energy conservation limit. In the other hand, the rise time of a large set of scintillating crystals was also measured using x-ray pulsed excitation and an instrumental response function of ps. Two rise time components have been identified for most of the crystals, which correspond to direct (τr1 O(10ps)) and delayed excitation τr1 O(100ps) of the activator centers. The study included lutetium based scintillating crystals, with and without Ca2+ co-doping, and a set of garnets (YAG, GaGG, LuAG) with and without Mg2+ co-doping. Co-doping with Ca2+ [?] or Mg2+ [?] species was proven to improve rise and decay time of the scintillating signal. However, this method finds poor impact on coincidence time resolution (CTR) measurements [?]. Regarding the decay time and as reported in [?], 16 ns features as a limiting value for lutetium based scintillators. Thus, as will be proven along the Chapters 4 to 7, traditional scintillators with their associated light production and their limitations in photon-extraction and -transfer, build an intrinsic barrier to attaining the sub-20ps domain in time resolution when using 511keV gamma excitation, requiring that new approaches to achieving a prompt photo-response must be explored. Figure 1.1 presents Cra mer-rao lower bound calculations for CTR values as a function of the single photon time resolution (SPTR expressed in sigma) and number of prompt photons produced along with the scintillation emission in the crystal. For the calculations shown, we used the time profile of LSO:Ce, i.e. scintillation rise time of τr = 70 ps and decay time of τd = 40 ns, and crystals of 3 mm and 20 mm lengths with different light transfer efficiencies (LTEs). As shown in Ref. [?] and Fig. 1.1, a coincidence time resolution of 10 ps FWHM can be achieved with a prompt signal of several hundreds of photons produced provided that the SPTR of the SiPM is of the order of 10 ps sigma. Commercially available SiPMs do not provide this value, however, measurements performed on free standing single photon avalanche diodes of the SiPM showed SPTR values below 10 ps sigma. Consequently, the SiPM engineering has to be improved together with light production mechanisms to reach CTR values of 10 ps. a) b) Figure 1.1: Crame r-rao lower bound calculations for CTR using a LSO:Ce scintillator of (a) 3 mm and (b) 20 mm length as function of SPTR and number of prompt photons. Processes such as the Cerenkov effect [?,?,?] and hot-intraband emission [?,?] have been investigated for this purpose, though both suffer from poor light yield. This motivates research not only towards ultrafast sub-nanosecond performance but also to materials that have the

6 DRAFT: 18th September potential to produce prompt photons with sufficiently high yield under ionizing irradiation. A new class of materials, with unparalleled energy conversion efficiency has been found to be a proliferating source of prompt photons under ionizing radiation: nanocrystals. Nanocrystals (NCs), i.e. semiconductors grown at the nanoscale (<100nm) with a sizedependent band-gap structure, are capable of meeting many of the challenges in the current R&D of scintillating detectors. Their tunable optoelectronic properties, combined with recent advances in controlling their size, shape, heterostructure and surface chemistry, have enabled their use in a wide range of photonic applications, such as low-threshold lasing [?], photovoltaic cells [?], single photon sources for quantum information [?,?] and bio-labeling [?]. Their high quantum yields and ultrafast recombination time at the sub-ns level have already generated interest in their use as low-cost, high-performance scintillators for radiation detection. Earlier high energy excitation studies [?,?] have identified several important issues and deficiencies that degrade the light emitting properties of NCs, predominantly non-radiative Auger recombination [?], photo-instability [?], and losses due to the reabsorption of emitted light. However, significant progress has been made to overcome these barriers and to advance this technology for applications outlined above. Auger recombination, for example, has been considerably suppressed with the appearance of 2D-nanoplatelets [?,?] and heterostructured core/shell quantum dots [?,?]. Further improvements were made by embedding ZnO:Ga nanoparticles [?] in a host material leading to the efficient synthesis of composites using dry nanopowders [?,?]. 1.0 ZnO:Ga@PS τd = 518 ps Time (ns) Emission wavelength (nm) Figure 1.2: Illustration of the ultrafast performance of colloidal CdSe NPLs (left) and a ZnO:Gapolystyrene nanocomposite (center) under ionizing radiation. The response of NCs to ionizing radiation was explored using a Hamamatsu streak camera C10910 and a pulsed X-ray tube with an instrumental response function of 70 ps FWHM. Initial studies comprised colloidal CdSe nanoplatelets (NPLs) film of 10 µm thickness deposited on the back of a LSO scintillator. The measurements revealed the CdSe-NPLs ultrafast performance under pulsed X-ray excitation producing 25% of the photons emitted with a decay time of 24ps and 75% with a recombination time of 290ps (τde f f =77ps). Fig. 1.2, left plot, shows a spectral time-resolved image of the light emitted from the CdSe-NPLs-LSO heterostructure generated by pulsed X-rays up to 40 kev. The black line denotes the time-integrated profile for the first 100ps. After normalizing to the total energy deposited in both materials, the NPLs photoelectron density is one order of magnitude higher as compared to LSO. In an attempt to fabricate a feasible detector, ZnO:Ga nanopowder embedded in a polystyrene (PS) matrix was used, where the plastic (polystyrene) scintillator acted as host, transferring non-radiative energy to the nanoparticles. This nanocomposite shows a single-exponential decay time of 518 ps and a rise time of 18ps (see Fig. 1.2, right plot). The calculations shown in Fig. 1.3 give evidence that photon emission in a 1 mm thick ZnO:Ga@PS sample (first-generation nanocomposite production), albeit its significantly lower photoelectron (pe) yield of 33pe for 511keV gammas, reaches the probability threshold of 10

7 DRAFT: 18th September Npe detected (c.d.f.) LSO:Ce:Ca τr=20 ps τd_eff =30 ns Npe = 4000 ZnO:Ga τr=18 ps τd=0.5 ns Npe = 33 CdSe NPLs τr=5 ps τd_eff =77 ps Npe 300 ZnO:Ga τr=18 ps τd=0.5 ns Npe ps 100 ps 200 ps Time after scintillating pulse starts Figure 1.3: Nanocrystals capacity to improve coincidence time resolution using a state-of-the-art photodetection system and 511keV excitation. detected pe at a time of 200ps, only a factor of two inferior than LSO:Ce:Ca (0.4%), as was also confirmed experimentally [ref]. The green curve defining the N pe cumulative density function for CdSe NPLs, which crosses the 10pe line as early as 7ps, assumes a bulk material with time and light yield characteristics given in Fig left. The upper of the two calculated ZnO:Ga-curves assumes a N pe comparable to the radioluminescence yield by nanopowders when not yet embedded in a matrix. These studies demonstrate the large development potential of NC technology and justify the rigorous research in this field to make NCs a viable option for superfast timing in many domains such as HEP and medical imaging.