Evaluating gamma-ray induced defect dynamics in HB-grade pencil graphite for radiotherapy-relevant dosimetry: an ESR and structural study

Abstract

The present work evaluates the dosimetric properties of commercial HB-grade pencil graphite (2 mm diameter) by analyzing radiation-induced paramagnetic defects in the radiotherapy-relevant dose range. For this purpose, HB pencil lead graphite was ground into a fine powder (20–100 μm) and then exposed to ⁶⁰Co gamma radiation at clinical radiotherapy levels ranging from 0 to 20 Gy. The structural and magnetic alterations were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and temperature-dependent X-band electron spin resonance (ESR) spectroscopy (20–300 K). XRD analysis confirmed the stability of the long-range graphitic lattice post-irradiation, while Raman spectroscopy showed evidence of radiation-induced structural disorder. ESR spectra exhibited a narrow Lorentzian signal, consistent with localized paramagnetic centers associated with the heterogeneous graphite-clay composite. Low-temperature ESR measurements revealed Curie-type paramagnetism, with line broadening at lower temperatures suggesting strong dipole-dipole interactions between localized spins. Additionally, gamma exposure induced systematic variations in both the ESR g-factor and spectral linewidth. Furthermore, a linear dose-response relationship was observed over the investigated dose range, with R² ≈ 0.99 between the ESR signal intensity and the absorbed gamma dose. Consequently, the investigated graphite-clay composite shows potential for further dosimetric investigation, provided that future studies address the native background and broader statistical validation.