Dose-dependent refractive index changes in megavoltage X-ray-irradiated high-density polyethylene measured by terahertz time-domain spectroscopy

This note investigates terahertz time-domain spectroscopy (THz-TDS) as a non-destructive technique for detecting radiation-induced changes in high-density polyethylene (HDPE). HDPE samples were irradiated with megavoltage X-rays at doses from 100 Gy to 800 Gy using a medical linear accelerator at a dose rate of 6.00 Gy/min. The refractive index, measured with THz-TDS in the range 0.3-2.5 THz, showed an overall positive linear association with absorbed dose (R2=0.83), corresponding to a sensitivity of 7×10⁻⁶ Gy⁻¹ with a 95 % confidence interval of 4.10-9.84 × 10⁻⁶ Gy⁻¹. The absorption coefficient shows no systematic dependence on dose within the measurement uncertainty. These results provide preliminary evidence that THz-TDS can detect dose-dependent changes in the refractive index of HDPE and may provide a basis for further investigation of polymer-based dosimetry.