Double differential cross sections for proton induced electron emission from molecular analogues of DNA constituents for energies in the Bragg peak region

Abstract

For track structure simulations in the Bragg peak region, measured electron emission cross sections of DNA constituents are required as input for developing parameterized model functions representing the scattering probabilities. In the present work, double differential cross sections were measured for the electron emission from vapor-phase pyrimidine, tetrahydrofuran, and trimethyl phosphate that are structural analogues to the base, the sugar, and the phosphate residue of the DNA, respectively. The range of proton energies was from 75 keV to 135 keV, the angles ranged from 15° to 135°, and the electron energies were measured from 10 eV to 200 eV. Single differential and total electron emission cross sections are derived by integration over angle and electron energy and compared to the semi-empirical Hansen-Kocbach-Stolterfoht (HKS) model and a quantum mechanical calculation employing the first Born approximation with corrected boundary conditions (CB1). The CB1 provides the best prediction of double and single differential cross section, while total cross sections can be fitted with semi-empirical models. The cross sections of the three samples are proportional to their total number of valence electrons.

Proton-impact-induced electron emission from biologically relevant molecules studied with a screened independent atom model

“…The fact that the IMM-AR results become slightly smaller than the IAM-PCM calculations above E = 1000 keV cannot be explained on geometrical grounds. It points either to an underestimation of the overlap effect by the IAM-PCM or to a small inconsistency between the (atomic) TC-BGM results and the experimental cross section data used in the IMM-AR calculation of [9]. Given that no details about the latter were provided, we cannot offer a more definitive explanation for the (small) discrepancy.…”

“…One can imagine how the magnitude of the overlap effect varies as a function of orientation and collision energy and that even for pyrimidine, the smallest of the four molecules, the orientation-averaged IAM-PCM cross section will be smaller than the zero-overlap limit corresponding to the IAM-AR. (3) and (4) as described in the text; CB1: first Born approximation with corrected boundary conditions within the CNDO approach [9]; experiments: Bug14 [32] (see also [28]) for electron impact using equivelocity conversion, Wolff14 [8], Rudek16 [9] for proton impact.…”

“…The three experimental proton-impact data points of [9] We compare them with equivelocity electron-impact measurements from [32] only, since we are not aware of other theoretical calculations or measurements for proton impact. Consistent with the pyrimidine case, the agreement is excellent in the energy region above the conditions within CNDO approach [9]; experiments: Bug14 [32] (see also [28]) for electron impact using equivelocity conversion, Rudek16 [9] for proton impact. conditions within CNDO approach [9]; experiments: Bug14 [32] (see also [28]) for electron impact using equivelocity conversion, Rudek16 [9] for proton impact.…”

“…While progress has been made on both experimental (see [2][3][4][5][6][7][8][9] for proton-impact collisions) and theoretical [10][11][12] fronts, the complexity and multitude of molecules of interest suggest that there is a role to be played by simplified models which are easily applicable to a wide range of collision systems.…”

“…Classical arguments or quantum-mechanical first-order perturbation theory represent natural starting points for modeling electron removal in ion-molecule collisions. Making use of these ingredients, a number of analytical cross section formulae have been proposed and applied (see, e.g., [13][14][15] and the discussion in [9]). Their advantage is simplicity-typically they only require the binding energies of the target electrons as physical input-but their success depends on (additional) parameters which are determined (semi-) empirically.…”