Energy Loss of Swift Protons in Liquid Water: Role of Optical Data Input and Extension Algorithms - Radiation Damage in Biomolecular Systems - page 252

The width and intensity of the ELF in the damped Ritchie model [ 21 ] broadens
and diminishes, respectively, as k increases, but not enough to agree with the
experimental data.

15.4 Results and discussions

We present in this section the main magnitudes characterising the electronic energy
loss of proton beams through liquid water. Our framework is the plane-wave (first)
Born approximation (PWBA), which provides accurate output over a substantial
portion of the electronic regime. The most important advantage of the PWBA
consists of the fact that, essentially, the scattering problem is reduced to find suitable
descriptions for the target ELF over the complete range of energy- and momentum-
transfer, i.e. the Bethe surface.
The influence of the experimental set of data for the OELF into the stopping
power and the energy-loss straggling of liquid water is discussed, as well as, the
relevance of the different methodologies used to extend the valence excitation
spectrum to non-zero momentum transfers. All the results that follow use the
GOS model (in the hydrogenic approach) to describe the oxygen K-shell electron
excitations.
Finally, the depth dose distributions of protons in liquid water are evaluated
for the different models of the Bethe surface, and the corresponding results are
compared and discussed.

15.4.1 Magnitudes characterizing the energy loss distribution:

Sand ˝ 2

A proton moving through a material experiences charge-exchange processes that
modify its charge state. Therefore, the evaluation of the total stopping power, S ,
Eq. 15.1 , requires the knowledge of the stopping power for each one of the projectile
charge states, which are H C and H 0 for a proton beam.
In Fig. 15.4 we show the stopping power of H C and H 0 in liquid water obtained
by the MELF-GOS model [ 19 , 22 ] for a wide range of incident energies, where the
IXSS data [ 16 ] for the OELF of liquid water are employed. The inset of Fig. 15.4
shows the energy dependence of the charge fractions of H C and H 0 in liquid water,
as obtained from a parameterization to experimental data [ 25 ]. It can be seen that
neutral hydrogen dominates at low projectile energies, whereas bare protons are
most abundant at higher energies. In the intermediate region, both charge states are
almost equally probable.
Although S C and S 0 look quite similar, the different behaviour of the charge
state fractions C

and 0 as a function of the projectile energy significantly