The β-ray surface density measurement instrument is widely used for the online measurement of the surface density of electrode sheets in the coating process of lithium batteries. A significant measurement error was observed when the source-to-film distance (SFD) varied under fixed source-to-detector distance (SDD).

This study aims to investigate and minimize the impact of SFD variations on the accuracy of β-ray surface density measurements.

Firstly, experimental tests were conducted using the β-ray surface density measurement device with a fixed SDD to quantify the influence of SFD changes (4~8 mm) on copper film surface density measurements. Then, Geant4 Monte Carlo simulations were performed to model β-ray scattering effects under three collimator configurations: detector-side single collimator, source-side single collimator, and dual collimators. Finally, the optimized source-side single collimator geometry was experimentally validated by reproducing SFD variations and recalculating measurement deviations.

Initial experimental results reveal that 1 mm SFD variation causes a maximum measurement deviation of 13.3% in surface density values. Monte Carlo simulation results demonstrate that the source-side single collimator configuration minimizes scattering effects, showing the smallest mean deviation among all three geometries tested. Experimental validation confirms this improvement with the maximum deviation decreasing from 13.3% to 3.5% after device optimization.

Results of this study show a 73.7% reduction in measurement error caused by SFD variations is achieved by adopting a source-side single collimator, providing a practical solution for enhancing online surface density measurement accuracy in lithium battery production.