Accurate discrimination and energy measurement of alpha particles remain a key challenge in proton boron fusion driven by high-intensity laser-plasma interaction due to the complex mix of ions generated in these extreme conditions. We present a novel implementation of a high-accuracy, low-background technique involving a CR-39 enhanced image plate that was used with a Thomson parabola spectrometer (TPS) and differential filtering. This technique demonstrated a strong reduction in background contamination arising from plasma ions compared to standard CR-39 and allowed for the generation of a contaminant-free alpha particle energy spectrum from a boron foam target irradiated by a 10 J, 800 fs laser pulse with an intensity of $\sim$$2\times10^{19}\ Wcm^{-2}$. The laser pulse was from a hybrid Ti:Sapphire-Nd:glass system generated from the Chirped Pulse Amplification (CPA) mode. The spectrum covered an energy range of 3–8 MeV with a corresponding energy resolution of 0.1–0.5 MeV. The developed filtering technique has the potential to measure even lower energy ranges, further extending its applicability compared with existing methods. The differential filtering solution strongly reduces the signal from carbon ions that would otherwise overlap the alpha particle trace on the TPS, providing a quasi-contaminant-free signal, while the CR-39 enhanced the detection sensitivity compared to other detectors.