The Stopping of Heavy Ions in Gases

Abstract

The extrapolated ionization range for the ions ${\mathrm{He}}^{+}$, ${\mathrm{N}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{A}}^{+}$ in the stopping gases He, ${\mathrm{N}}_{2}$, air, and A has been determined as a function of energy in the energy range from $\mathrm{ca}$ 20 kev to $\mathrm{ca}$ 250 kev, using monoenergetic heavy ions produced in a Cockcroft-Walton accelerator. A collimated beam of heavy ions is admitted to the range chamber via a three-stage differential pumping system so as to avoid the use of foil windows. The range was determined using a parallel plate ionization chamber mounted so as to permit translation parallel to the axis of the beam. It has been found that the range as measured at a pressure $P$ is not inversely proportional to $P$, and an interpretation of this pressure effect has been given in terms of large angle scattering of the ions into regions outside of the volume swept out by the ionization chamber. A method of correcting for this pressure effect has been developed and applied to the data to yield range-energy curves which are independent of the ionization chamber size. The space distribution of ionization produced in a gas by a collimated beam of heavy ions has been determined for the same combinations of incident ion, stopping gas, and energy as above. The attenuation of the beam of ions is found to be approximately exponential in the axial direction and approximately Gaussian in the lateral direction. A method has been developed for computing the shape and size of a given ionization density contour at a given pressure if it is known at another pressure and the same energy.