Turner J E, Hamm R N
Health Sciences Research Division, Oak Ridge National Laboratory, TN 37831-6123, USA.
Health Phys. 1995 Sep;69(3):378-84. doi: 10.1097/00004032-199509000-00009.
Heavy charged particles travel in essentially straight lines in matter, while electrons travel in tortuous paths. Frequent multiple elastic Coulomb scattering by atomic nuclei is often cited as the reason for this electron behavior. Heavy charged particles also undergo multiple Coulomb scattering. However, because they are massive, significant deflections occur only in rare, close encounters with nuclei. In contrast to heavy particles, the inelastic interaction of an electron with an atomic electron represents a collision with a particle of equal mass. In principle, therefore, repeated inelastic scattering of an electron can also produce large-angle deflections and thus contribute to the tortuous nature of an electron's track. To investigate the relative importance of elastic and inelastic scattering on determining the appearance of electron tracks, detailed Monte Carlo transport computations have been carried out for monoenergetic pencil beams of electrons normally incident on a water slab with initial energies from 1 keV to 1 MeV. The calculations have been performed with deflections due to (1) inelastic scattering only, (2) elastic scattering only, and (3) both types of scattering. Results are presented to show the spreading of the pencil beams with depth in the slab, the transmission through slabs of different thicknesses, and back-scattering from the slab. The results show that elastic nuclear scattering is indeed the principal physical process that causes electron paths to be tortuous; however, the smaller effect of inelastic electronic scattering is far from negligible.
重带电粒子在物质中基本上沿直线传播，而电子则沿曲折路径传播。原子核频繁的多次弹性库仑散射常被引为这种电子行为的原因。重带电粒子也会经历多次库仑散射。然而，由于它们质量较大，只有在与原子核发生罕见的近距离碰撞时才会发生显著的偏转。与重粒子不同，电子与原子电子的非弹性相互作用相当于与一个等质量粒子的碰撞。因此，原则上，电子的反复非弹性散射也能产生大角度偏转，从而导致电子轨迹的曲折。为了研究弹性散射和非弹性散射在确定电子轨迹外观方面的相对重要性，对初始能量从1 keV到1 MeV垂直入射到水板上的单能笔形电子束进行了详细的蒙特卡罗输运计算。计算分别考虑了由(1)仅非弹性散射、(2)仅弹性散射和(3)两种类型散射引起的偏转。给出的结果展示了笔形束在水板中的展宽、通过不同厚度水板的透射以及从水板的背散射。结果表明，弹性核散射确实是导致电子路径曲折的主要物理过程；然而，非弹性电子散射的较小影响也远非可以忽略不计。
