This thesis aims to characterise the accuracy of a new active CMOS pad detector for dosimetry in HDR Brachytherapy. A 1x1x1 mm3 detector is doped with high resistivity n-type silicon, thickness of 100 μm (Wafer10) and 48 μm (Wafer20) epitaxial substrate and built in standard CMOS along with a transimpedance amplifier. This work investigates and examine this detector’s sensitivity and signal to noise ratio in air for use in High Dose Rate Brachytherapy (HDR BT) dosimetry. To understand operational characteristics of the detector, a current-voltage (I-V) characteristic curve was created, and the depletion voltage of the junction observed by using a Keithley Programmable Voltage Source to sweep the voltage and measure the current collected at the pad. Applying the established depleted voltage, the detector sensitive volume was tested by irradiating 5.5 MeV alpha beam with field size 1 mm x 1 mm at 1000 Hz event rate to observe the impact of radiation on the electronic and collection e