Low-pressure measurements of energy depositions in nanometre objects with a single ionisation chamber in a 60Co beam using the variance–covariance method

Dose-mean lineal energies were measured using the variance–covariance method and commercial ionisation chambers in a therapy level 60Co field at several different air pressures. The results confirmed the trend from previous studies and the experimental data was extended down to a simulated object diameter of 4.2 nm using a low-noise electrometer. The variance–covariance method was adapted to consecutive charge integrations from a single ionisation chamber and used to correct for signal variations due to pressure drifts in the ion chamber. Monte Carlo simulations using MCNP 6.2 were performed, and the results follow the trend of the experimental values in the nanometric region.