Diamond Annual Review 2019/20

100 101 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 A newgeneration of fast, high-sensitivity X-ray detectors Related publication: MykhaylykV. B., Kraus H. & SalibaM. Bright and fast scintillation of organolead perovskiteMAPbBr 3 at low temperatures. Mater. Horiz. 6 , 1740-1747(2019). DOI: 10.1039/C9MH00281B Publication keywords: Organoled perovskite; Cryogenic scintillator; Decay time; Light yield S cintillators are materials that turn ionising radiation into visible light. They’re used to detect radiation and have applications in, for example, cancer diagnosis; scintillators can identify the precise location of a tumour. Fast and efficient materials are highly sought- after for such applications, but little progress has beenmade in identifying new systems in recent decades. Hybrid metal-organic halide perovskites are attracting considerable attention for photovoltaic applications, and for their extraordinary performance in light-emitting and light-detecting devises. They are also very promisingmaterials for radiation detection and, in particular, for scintillation detectors. Researchers wanted to investigate the temperature dependence of decay time and scintillation light output of methylammonium lead bromide (MAPbBr 3 ) after excitation with ionising radiation, as these crystals exhibit very bright emission during cooling. They carried outmeasurements of scintillation decay curves on the Test beamline (B16), usingmonochromatic X-ray radiation for excitation. Their results demonstrate that the performance of this newmaterial exceeds that of the bestmodern commercial scintillators used for X-ray detection. Its properties are indispensable for ultrafast X-ray imaging where sweeping improvement of image quality can be achieved by introducing time-of-flight methods of X-ray detection. The potential benefits from introducing such technique - reduced exposure to ionised radiation and enhanced signal-to-noise ratio - easily outweigh the need to develop a low-temperature scintillation detector. The moderate cooling requirement and the flexibility of the production technology of the metal-halide perovskites make this a very attractive approach. Scintillators detect ionising radiation by converting energy deposited in them to a proportional number of photons. They are omnipresent in many areas of physics, security scanners, or medical applications such as nuclear imaging for cancer diagnostics.The ideal scintillator emits amaximumnumber of scintillation photons per energy deposited, has a high absorption coefficient for gamma quanta, and exhibits a narrow timing profile for its scintillation photons. Brighter and faster scintillator facilitate better timing resolution which is crucial for measuring the time of the radiation interaction with high precision. At present, the dominant limitation of modern scintillators is their timing resolution. The state-of-the-art in the coincidence timing resolution is just breaking the 100 ps barrier with the lowest value of 73±2 ps reported for LSO-Ce scintillators 1 . Recentadvancesindevelopmentofhybridmetal-organichalideperovskites - materials with the general formula MAPbX 3 where MA= methylammonium, and X=Cl, Br, I and remarkable physical characteristics - triggered sharp upsurge of interest to their application for the detection of ionising radiation. The possibility of X-rays detection using intrinsic photoelectric effect has already been demonstrated in MAPbX 3 crystals 2 . It has been swiftly realised that the materials have great promise for the application as scintillation detectors with the key advantage of exhibiting a very fast response time, governed by the probability of radiative decay of excitons, while retaining high conversion efficiency. This notion motivated present study of temperature dependences of the scintillation light yield and scintillation kinetics of relatively easy to synthesise MAPbBr 3 crystals over the 8-295 K temperature range. When excited with X-rays, MAPbBr 3 exhibits narrow, near-edge emission bands peaking at 560 nm with a very pronounced temperature dependence 3 . Fig. 1 shows the scintillation decay curves of MAPbBr 3 crystals measured at pulsed X-ray excitation over 8-132 K temperature range. The decay curves exhibit very fast, non-exponential kinetics that is characteristic of bimolecular recombination of the charge carriers. Quantitative analysis of decay curves revealed that the fast and slow decay time constants in the crystal are about 0.1 and 1 ns at T>50 K. With cooling to lower temperatures, the decay rate of the luminescence kinetics inMAPbBr 3 exhibits steep changes, resulting in a significant increase of the decay time constants. At the same time, the amplitude of the background exhibits a steady rise with cooling indicating that at this temperature the radiative dynamics is dominated by the slow recombination processes due to trapping and release of charge carriers. One of the most important features of scintillation in MAPbBr 3 is that the major fraction of scintillation response from the crystal is released over a nanosecond time interval following an excitation pulse. This is of primary importance for the applications that rely on a fast timing resolution of the scintillator. Evidence of the extraordinary speed of the scintillation response in the crystals under study can be seen in the Fig. 2 where the luminescence decay curves of MAPbBr 3 , and LYSO-Ce scintillator are juxtaposed. It is very clear from these figures that the timing performance of MAPbBr 3 is by far better in comparison with the modern commercial inorganic scintillator LYSO- Ce which exhibits the decay time of 33 ns and exemplifies the one of the best results in the coincidence timing resolution that relies on fast timing. To assess the scintillation performance of the MAPbBr 3 the energy spectra induced by α-particles from 241 Amas a function of temperatureweremeasured. Fig. 3 shows the variation of the scintillation light output of theMAPbBr 3 crystal with temperature together with LYSO-Ce. A clearly measurable scintillation response can be detected when the crystal is cooled to below 180 K. The scintillation efficiency of MAPbBr 3 increases gradually as the temperature is decreased until a plateau is reached at around 60 K. Further increase of the light output by about 20% is observed as the temperature decreases to below 30 K. This rise correlates with the rapid increase of the fractional contribution of the afterglow observed at very low temperatures. Taking the light yield of LYSO-Ce equal to 34,000 ph/MeV at room temperature, the light yield of MAPbBr 3 is determined to be 90,000 ph/MeV at 77 K and 116,000 ph/MeV at T = 8 K. A comparison of the MAPbBr 3 parameters with modern commercial scintillators shown reveals that organolead perovskites are very promising scintillation materials. Of particular interest is the excellent initial photon density calculated as the ratio of light yield to decay time.This is an important parameter that determines the timing precision of the scintillator detector.The higher density of photons in the initial part of the scintillation peak facilitates higher precision in determining the time of interaction. A conservative evaluation shows that this parameter is higher by a factor 20 in MAPbBr 3 compared to the best modern scintillator LaBr 3 -Ce. The stopping power of MAPbBr 3 that is defined by the photoelectric fraction of the absorption coefficient is also very competitive in comparison with other scintillators; only two other materials exhibit higher value. The results of this study place MAPbBr 3 in an excellent position for the development of a new generation of cryogenic, efficient scintillation detectors with nanosecond response time, marking a step-change in opportunities for scintillator-based applications. References: 1. Gundacker S. et al. State of the art timing in TOF-PET detectors with LuAG, GAGG and L(Y)SO scintillators of various sizes coupled to FBK- SiPMs. JINST 11 , P08008 (2016). DOI: 10.1088/1748-0221/11/08/P08008 2. Yakunin S. et al. Detection of gamma photons using solution-grown single crystals of hybrid lead halide perovskites. Nat. Photonics 10 , 585- 589 (2016). DOI: 10.1038/nphoton.2016.139 3. Birowosuto M. D. et al. X-ray scintillation in lead halide perovskite crystals. Sci. Rep. 6 , 37254 (2016). DOI: 10.1038/srep37254 Corresponding author: Dr Vitaliy Mykhaylyk , Diamond Light Source, vitaliy.mykhaylyk@diamond. ac.uk Optics andMetrology Group Beamline B16 Figure 1: Decay curves of X-ray luminescence measured in the MAPbBr 3 crystal at different temperatures in the range 8-132 K (where the emission is most pronounced). The luminescence is excited by 50 ps pulses of monochromatic synchrotron radiation (E=14 keV). Figure 2: Normalised scintillation decay curves observed at excitation by 14 keV X-ray pulses in LYSO-Ce and MAPbBr 3 . Figure 3: Scintillation light yield as function of temperature for the MAPbBr 3 crystal (red) measured at excitation with 5.5 MeV alpha particles from 241Am. The plot also displays the comparison with measurements of the commercial scintillator LYSO-Ce (blue).