Beam-quality factors and $W_{\mathrm{air}}$ for ion dosimetry
In plain words
Converting a detector reading into dose in a particle beam needs the average energy spent to make one ion pair in air. Its uncertainty, especially for carbon ions, dominates the dose error.
Precise statement
Reduce the uncertainty in the mean energy per ion pair in air $W_{\mathrm{air}}$ and in the beam-quality correction factor $k_Q$ for carbon and other ions to the level needed for a total dosimetric uncertainty below $1.5\,\mathrm{percent}$. ICRU 90 values are $W_{\mathrm{air}}$ of about $34.44\,\mathrm{eV}$ for protons (about $0.4\,\mathrm{percent}$ standard uncertainty) and $34.71\,\mathrm{eV}$ for carbon ions (about $1.5\,\mathrm{percent}$). An answer is a measured value set with reduced uncertainty.
What would settle it
Calorimetric determination of $W_{\mathrm{air}}$ and $k_{Q}$ in clinical ion beams.
Status in the literature
Unverified note
IAEA TRS-398 Rev. 1 (2024) adopted the ICRU 90 data; for carbon ions the $W_{\mathrm{air}}$ and $k_{Q}$ terms still keep the total uncertainty near 3 percent.