# Studies and Rejection of Intercrystal Crosstalk on FPGA in a High-Energy Photon-Counting System
## 1 Introduction  
Advances in photon-counting detectors have made it possible to utilize the time, position, and energy information of the detected photon signal, which is useful to distinguish intercrystal scatter due to Compton scattering [1, 2, 3]. Intercrystal scatter has a bad effect on system sensitivity and can lead to a degradation of spatial resolution [4]. Unfortunately, system sensitivity and spatial resolution are two key factors in radiation detection systems, especially for positron emission computed tomography (PET) systems that require accurate positioning. It has been suggested that intercrystal scattering is the primary cause of event mispositioning in high-resolution PET systems [5]. Moreover, the tendency to utilize smaller crystal sizes leads to a higher fraction of intercrystal scatter [6]. Many experiments have been performed to look at the impact of intercrystal crosstalk on positioning error and the implications for the spatial resolution of PET detectors [7, 8, 9, 10, 11]. Simulations also suggest that these effects will ultimately lead to image blurring and a lossin contrast at the system level [12]. The conventional method used to reduce the effect of intercrystal scattering is setting a proper energy threshold mostly on the photopeak. However, this method is somewhat limited when the energy resolution is poor, as is the case for the high-energy system presented in this study, and inevitably, object scatter will be partly lost. Other attempts appeared to improve the structure of the detector. Multi-layer detectors with depth-of-interaction (DOI) detection capability can extract the position information of photons. Intercrystal scattering events can be discriminated by reading out each layer independently and utilizing the pulse information obtained [13; 14]. Also, utilizing a thin intercrystal septa has been proven to be effective [6]. But it will greatly increase the cost and complexity in large-scale systems with hundreds of detectors, and the septa, like lead strips, may introduce additional interference, especially in high-energy systems. Many researchers have proposed different filters concerning the light distribution of intercrystal crosstalk, which have the capability to improve image quality significantly [15; 16; 17]. However, some complicated filters, for instance, the one based on maximum likelihood algorithms [18], require high computational effort and are therefore very challenging to implement directly in the field-programmable gate array (FPGA) data processing module. Moreover, with an object to be detected in our system, the energies of photons will spread after penetrating the object, while PET systems detect coincidence events of 511 keV photons, which is a further difference [19]. As can be seen above, although many publications dealing with intercrystal scatter interactions in PET detectors already exist, there are few relevant studies in other radiation detection systems. Here, we investigate the intercrystal scatter phenomenon in a high-energy large container inspection \(\gamma\)-ray photon-counting system, and the details of the studied system can be found in reference [20]. The system uses a \({}^{60}\)Co radioisotope source, which emits photons of 1.17 MeV and 1.33 MeV, with an average energy of 1.25 MeV [21; 22]. Theoretically, photons with higher energy are more likely to scatter with the lutetium yttrium oxyorthosilicate (LYSO) [23] scintillation crystal (see Table 1). Experimentally, according to the Geant4 [24] simulation results of our system, as the photon energy increases, crosstalk is more likely to occur and to be observed in detectors that are far from the incident one (see Figure 1). In this paper, we study the effect and properties of the measured intercrystal scatter interactions and reject them using a time window of 5 ns and a detector window of one or two accordingly.