Product development of a condenser dosimeter using a skin-insulated USB-A-substrate with a silicon X-ray diode

To measure integral doses in image-guided radiation therapy, we developed an integral condenser dosimeter comprising a disposable USB-A mini-substrate with a 0.1-µF condenser and a silicon X-ray diode (Si-XD), a microcomputer (mbed) dock, and a personal computer (PC). The Si-XD is a high-sensitivity photodiode selected for detecting X-rays. The USB-A substrate with dimensions of 24 × 14 mm² is inserted into the microcomputer dock, and the condenser is charged to 3.23 V through a 10-kΩ resistor. The condenser charging voltage is subsequently measured directly using an analog-digital converter (ADC) in mbed. When the condenser is fully charged, the microcomputer dock is switched to high impedance, and the substrate is removed. Subsequently, the substrate is exposed to an X-ray source, and the condenser is discharged via the photocurrent flowing through the Si-XD. The substrate is inserted into the dock again, and the charging voltage is measured. The dock is connected to a PC through a mini-USB cable, and integral doses are shown on the PC monitor. The doses were proportional to decreases in the charging voltage, and the calibrated doses corresponded well to those obtained using a typically available ionization chamber.