Partial Atomic and Ionic Stopping Powers of Gaseous Hydrogen for Helium and Hydrogen Beams

Abstract

The partial stopping power of a target material for an ion or an atom is the energy lost per target particle per cm 2 in all types of collisions in which the ion or the atom is the projectile, with the exception of those collisions in which the projectile leaves with its charge changed. Partial atomic stopping powers of target gases for atoms are measured by imposing a strong transverse magnetic field on the stopping cell. For partial ionic stopping-power measurements, the ion beam is held in a circular orbit, closely defined by vanes and slits, as it traverses the stopping cell. The partial stopping powers e 0 (for He 0 traversing H 2 gas), ei (for He + ), c 2 (for He ++ ) have been measured in the kinetic energy range 40-460 keV, and new measurements of the ordinary, or total stopping power of H 2 gas for helium from 40 to 220 keV have been made. Using the known charge composition of He beams in H 2 , it is possible to deduce the fraction of the stopping power losses due to charge changing collisions, and this varies from 37% at 140 keV to 27% at 400 keV. At 120 keV, the total energy loss in completing the charge changing cycle He + -> He 0 -* He + is 95±9 eV, rising to 117=b 10 at 160 keV. . 5 In a simple capture and loss cycle involving He 0 and He + in a beam of kinetic energy E electron volts, the number of cycles completed in an absorber is easily shown to be <r c | AJE | /e where | AE \ is the magnitude of the energy decrement in eV, e is the standard or total stopping power, and <r c -cr owio/(<roi+<rio) is the cross section for cycle completion [cf. Ref. 4, Eq . (III-3)]. In the lower energy range specified above, if \AE\/E~0.1 t the number varies from 20 to 50.

Stopping power for helium in aluminum

“…17 The Auger and resonant capture and loss cross sections are found from the imaginary part of the self-energy associated with the Henucleus-bound-electron composite. 18 The energy loss per unit path length in the shell capture process is obtained 19 in terms of the target atomic density, the energy of the electron bound in the nth shell of the Al atom e n , the binding energy of the captured electron e$, and the cross section <r n o for the n-• 0 transition. In the resonant capture and loss processes it is given by…”

Molecular electronic stopping cross section for H and He beams colliding with water: projectile charge state contribution

“…The Bethe theory for structured projectiles within the First born approximation Experimentally, it is known that a beam contains all the charge states as a function of the beam energy [28]. As proposed by Allison et al [28,29,45], the electronic stopping cross section for all charge fractions in the beam is given by…”

“…Now, we turn to the study of the beam charge fraction as required by equation (1). For the case of a hydrogen beam, the charge fractions are determined from the electron capture, σ 10 , and stripping cross sections, σ 01 , as formulated by Cuevas et al [29], i.e.…”

“…Nevertheless, while penetrating matter, the projectile ion may carry, at least temporarily, some bound N 1 Z 1 electrons, since electron capture and loss by the projectile are processes with a high probability [9,[25][26][27]. Thus, for a given projectile velocity, the ion beam may consist of projectiles with varying number of electrons [28,29].…”

Charge-exchange straggling in equilibrium

“…Charge-exchange straggling has been mentioned early [2] and discussed in the experimental literature [3][4][5]. Direct evidence emerges from state-specific energy-loss measurements [6][7][8][9] and subsequent work by these teams.…”