Stopping cross-sections of liquid water and water vapour for alpha particles within the energy region 0.3 to 5.5 MeV.

Phase effects in the stopping power of heavy charged particles are important in dosimetry and, in particular, in cavity chamber theory and in the design and operation of homogeneous gas-filled ionisation chambers for neutron dosimetry. A single apparatus has been designed to allow measurement of stopping cross-sections of materials in liquid and vapour phase for alpha particles. Experimental stopping powers of water vapour are presented for energies from 0.3 to about 5.5 MeV and of liquid-phase water from 1.75 to 4.75 MeV. The molecular stopping cross-sections of the vapour tend to be greater than those for the liquid. The differences between the stopping powers for the two states of H2O indicate phase effects of no more than about 4% in the energy region of comparison (down to 1.75 MeV), with an increasing trend towards lower energies. Major fluctuations in stopping power ratio are not observed in this energy region.