2D perovskite-based high spatial resolution X-ray detectors

X-ray radiography is the most widely used imaging technique with applications encompassing medical and industrial imaging, homeland security, and materials research. Although a significant amount of research and development has gone into improving the spatial resolution of the current state-of-the-art indirect X-ray detectors, it is still limited by the detector thickness and microcolumnar structure quality. This paper demonstrates high spatial resolution X-ray imaging with solution-processable two-dimensional hybrid perovskite single-crystal scintillators grown inside microcapillary channels as small as 20 µm. These highly scalable non-hygroscopic detectors demonstrate excellent spatial resolution similar to the direct X-ray detectors. X-ray imaging results of a camera constructed using this scintillator show Modulation Transfer Function values significantly better than the current state-of-the-art X-ray detectors. These structured detectors open up a new era of low-cost large-area ultrahigh spatial resolution high frame rate X-ray imaging with numerous applications.


S1. Thermal and Fast neutron detection using PEALPB
In addition to X-rays, 6 Li-alloyed Phenethylammonium lead bromide (PEALPB) is also responsive to neutrons. The thermal neutrons interact with the 6 Li in PEALPB, and an alpha and triton are produced from each interaction. These charged particles then ionize the scintillator resulting in the generation of photons which, like those generated by X-rays, undergo near-complete internal reflection and leave the detector. For fast neutrons with energy En primarily undergo elastic scattering with the hydrogen in the PEAPLB and produce recoil protons with energy, Ep = Encos 2 θ, θ being the scattering angle. These protons ionize the scintillator and produce the photons that are then detected by the photodetectors.

GEANT4 simulation results
The fast neutron detection efficiency of the microcapillary detector was evaluated by Geant4 Monte Carlo simulations. The detector design architecture is shown in Figure S1. Figure S2 shows the calculated number of photons collected by the imager after firing fast neutrons with an energy of 2.5 MeV, to represent a Deuteron-Deuteron (DD) neutron source, and 14.1 MeV, to represent a DT neutron source, at a series of PEALPB detectors with varying thicknesses practical for radiography. As the thickness of the film increases, more fast neutrons are captured resulting in an increase in the number of photons captured by the imager. The performance of these devices was compared to the performance of stilbene devices of the same thickness (red), and it was observed that PEALPB devices, especially at lower thicknesses, are competitive with stilbene for fast neutron detection.

DD neutron detection performance
The PEALPB devices fabricated with 6 Li were characterized using Neutron irradiation from a Deuterium-Deuterium source. We compared the DD thermal neutron response of the PEAPLB devices with commercial Eljen EJ-410 and EJ-420 fast and thermal neutron detector respectively [1,2]. The neutron characterization set up is shown in Figure S4. All these tests were performed inside a lead cage made with 2-inch thick lead bricks to eradicate any response generated by the X-rays from the DD source and any stray gamma rays. Additional HDPE, Cu sheet, W sheet and Pb sheet shielding is present outside the area shown in this figure. The PMT with the scintillator mated to the top is covered with a light tight packaging to stop any stray light from entering the PMT. DD neutron sources are being used for all the experiments in this paper because these are the industry standard for portable neutron sources used for neutron radiography.
The thermal neutron response of 1-mm thick PEALPB is compared with that of a 6.5mm-thick Eljen EJ-420 neutron only detector in Figure S5. EJ-420 is fabricated using a 6 Li compound dispersed in a ZnS:Ag phosphor matrix. PEALPB has a higher count in the lower energy channels (i.e., for thermal neutrons), and the overall neutron sensitivity as measured by cps/nv is about 30% higher than the thicker EJ-420 detector. For the fast neutron measurements, PEAPB was used without the lithium doping to eliminate thermal neutron detection probability from the 6 Li interactions. Figure S6 shows the response of 1-mm thick PEALPB to fast neutrons and compares it with the response of 16-mm thick EJ-410 fast neutron detector with similar cross-sectional area. At higher channels, the histogram counts are similar for both the scintillators but at lower channels, the aggregate counts of PEAPB is 1.5 times higher than EJ-410. From this data, it is clear that the PEALPB detectors can easily replace the currently used ZnS phosphor-based neutron radiography screens.
With the in-house DD source, we could not acquire neutron images using PEALPB or 6 LiF/ZnS detector plates similar to the X-ray images as discussed in the latter part of this section. This is because the neutron source did not have a high enough flux and directionality required for neutron imaging. Figure S5. Thermal neutron response of 1mm thick PEALPB and its comparison with commercial EJ-420 thermal neutron detectors. Figure S6. Fast neutron response of 1mm thick PEAPB and its comparison with commercial EJ-410 fast neutron detectors.   Figure S8. X-ray radiographs of a safety pin, spring, and magnetic stirrer were acquired using PEALPB thick film. Figure S9. X-ray radiographs of a safety pin and a magnetic stirrer were acquired using a PEALPB infused fabric.   Figure S11. MTF curve of PEALPB detectors with and without extramural absorption layers. This approach provides excellent spatial resolution, however needs high X-ray dose and the images need extensive correction to eliminate the shadows generated by the extramural layers.