Straggling in energy loss of swift hydrogen and helium ions in gases

Straggling in energy loss for 40-keV to 1-MeV hydrogen ions and 100-keV to 2.4-MeV helium ions in a variety of atomic and molecular gases has been measured with an accuracy of 5-7%. When straggling data for hydrogen and helium penetrating the same monatomic gas are compared, significant deviations from the Z12 scaling contained in the Bohr expression for energy straggling Ωsub(B)2 are observed. These deivations are explained by an atomic correlation term stemming from the bunching of electrons into atoms and by an additional straggling term Ωsub(c)2 resulting from charge-state fluctuations. It is shown that the importance of charge-exchange straggling has been substantially overestimated in the past. For hydrogen ions penetrating heavy monatomic gases, the straggling increases with increasing energy and approaches the high-energy Bohr limit, in agreement with theory. The qualitative agreement between these hydrogen results and the electron gas calculations by Bonderup and Hvelplund and by Chu is, however, markedly improved by inclusion of the atomic correlation term. For hydrogen and helium targets, an excess over the Bohr value, caused by finite values of the electron velocities compared to that for the projectile, is observed at velocities where the electronic stopping power has its maximum. For molecular gases, a further increase in straggling is seen, caused by bunching of atoms into molecules, which leads to a further increase in the fluctuations of the number of collisions with electrons. (Auth.)