# A Large Area Pixelated Silicon Array Detector for Independent Transit In Vivo Dosimetry
## 2 Materials and Methods  
In this work, a new large area pixelated silicon detector, named \"Magic Plate-987\" (MP987) is presented and used to perform transit dosimetry. To verify the suitability of the MP987 detector and its readout system as a dosimetry tool in EBRT, a comprehensive set of basic characterisation measurements were undertaken to evaluate the performance of the device. Once completed, the MP987 was tested to evaluate its performance in transit dosimetry measurements.  
The MP987 detector presented in Figure 1a, comprises 987 diodes, ultrasonically bonded to a 0.5 mm thick fibreglass printed circuit board (PCB). Assembling was achieved by using the patented \"drop in\" technology adopted from the \"Magic Plate\" detector [15]. The diodes used to populate the detector have a surface area of 1.5 \(\times\) 1.5 mm\({}^{2}\) and are presented in Figure 1b. They have a low resistivity p-type substrate with a 40 \(\upmu\)m thick n-type epitaxial layer. The n-p junction extends as close as possible to the edges of the die, having a sensitive surface area of 1.27 \(\times\) 1.27 mm\({}^{2}\). The large surface area of the sensitive volume and shallow epitaxial depth has three intended purposes: increase the signal to noise ratio for low dose rate measurements, increase radiation hardness and significantly reduce the effect of trapped charge at Si-SiO\({}_{2}\) interfaces after exposure to radiation [16].  
The MP987 utilises two different detector pitches to maximise the spatial resolution of the detector. The central area consists of a 13 \(\times\) 13 diode formation at a 5 mm pitch, with the surrounding area having a pitch of 7.5 mm, giving a total detector area of 22.5 \(\times\) 21 cm\({}^{2}\), as displayed in Figure 1a. This allows a balance of high spatial resolution, for measuring small fields at the centre while extending the active area for larger and more conventional field sizes. The dual pitch avoids having to add multiple layers to the PCB, which would be required for routing the pixel connections to the electronic interface if the full surface was at the smaller pitch. Additional layers would increase the PCB thickness, requiring ground planes to avoid cross-talk, which could reduce the water equivalence of the detector and increase the attenuation for the EPID imager. The new PCB results in the MP987 having more than 8 times the number of active pixels and greater than 4.7 times the active area of the original \"Magic Plate\".  
To handle the large number of active pixels a custom data acquisition system (DAQ) was developed to be capable of reading out up to 1024 channels, on a pulse-by-pulse basis. The system developed was based on the 64-channel analogue front end chip, the AFE0064 (Texas Instrument, Dallas, TX, USA) and the field programable gate array (FPGA) Spartan-6 from Xilinx (USA) [17]. A custom front-end board was developed for the MP987 which consisted of four AFE0064 chips, making each module capable of handling 256 channels. Four 256 channels modules are connected in a star-like network and readout simultaneously by a single FPGA using a common master bus for communication (Figure 2). The FPGA was connected to a computer through a USB 2.0 interface. Custom software and firmware were developed to communicate with the FPGA, receive, decode, and display data in real-time.  
Figure 1: (**a**) MP987 PCB populated with diodes; (**b**) Schematic of the top surface of diodes used to populate MP987. Blue area represents active n+ region 1.27 \(\times\) 1.27 mm\({}^{2}\), total chip size 1.5 \(\times\) 1.5 mm\({}^{2}\). At the right corner is a contact pad to p+ substrate.