Ionization of the Noble Gases by Protons: Jesse Effects as a Function of Pressure

Abstract

A new method has been developed to measure W, the ratio of the energy lost by a proton to the number of ion pairs produced when these particles interact with a gas. In this method, protons are Rutherford scattered from a beam by a thin gold foil into a W cell where total gas ionization is measured. The number of protons scattered into the W cell is measured with a silicon surface barrier detector, and the energy lost to the gas is calculated from stopping power data. Most of the protons pass through the thin gold foil and enter an emission cell where emission spectra are studied with a vacuum ultraviolet scanning monochromator. At 400 torr for pure gases the following values of W (eV per ion pair) were found for 3.6-MeV protons: He, 4S.2±0.9; Ne, 39.3±0.8; Ar, 26.6±0.S; Kr, 23.0±0.S; Xe, 20.S±1.2; N2, 36.6±0.7. No appreciable pressure dependence was observed for the pure gases. The Jesse effect, which is the increase in ionization observed when impurities are added to certain gases, was studied as a function of concentration and pressure for mixtures of C2H., C2H2, and CO2 with Ar. At small concentrations of CJI. or C2H2 the Jesse effect decreases as the total gas pressure increases. These data, along with other facts cited in this paper, tend to confirm an energy pathways model for argon. Based on a comparison of our data with this model, we deduce the values 6.SXlO-1o and 1.1X1Q-9 cm S seC I for the rate constants for producing the Jesse effect from the IPI (1048 A) resonance state of argon by collisions with CJI. and Ct H2, respectively. W values were also studied as a function of pressure and concentration for mixtures of Ne, Ar, Kr, and Xe with He. The dependence of W on pressure and concentration was similar to that of argon. PARKS, HURST, STEWART, AND WEIDNER MONOCHROMATOR

The w-value in air for proton therapy beams

“…For some other measurements stopping power data were required to determine (w air ) p -values (Bakker and Segre´, 1951;Hiraoka et al, 1989;Moyers et al, 2000;Parks et al, 1972) and arguments can be made for correcting these data to PETRA calculations, but for stopping powers in the gases used for the measurements, not for the PETRA/ICRU water-to-air stopping power ratios. Denis et al (1990) and Petti et al (1986) measured the energy loss in the gas and did not require stopping power data.…”

“…those made by measuring the total ionization due to protons stopping in or crossing gas-filled cavities) by Parks et al (1972) and by Thomas and Burke (1985) were mentioned in ICRU (1998) but were not considered in the evaluation of (w air ) p . These measurements were made at low energies (o4 MeV) and the values for air have been deduced here from measurements in argon (Parks et al, 1972) and nitrogen (Thomas and Burke, 1985) using appropriate conversion factors, viz., (W air /W Ar ) p ¼ 0.963 and (W air / W N 2 ) p ¼ 1.313. These ratios are the means of those measured by Hiraoka et al (1989) and Larson (1958).…”

“…No additional uncertainties were assigned to the data because of the use of these ratios. In the case of Parks et al (1972), who measured W in several noble gases, the argon data were used since measurements in argon are more consistent than those in other gases (ICRU, 1979). Their data analysis required the use of argon stopping powers that were taken from Janni (1982).…”

Heavy-ions induced scintillation experiments

“…where W exc is the energy required to produce an excited atom in a medium taking into account all phenomena such as primary electron excitation, electron collision cascade, heavy-ion excitation, etc... [3] W exc in rare gases excited by alpha particles and fission fragments has been measured by several authors and values about 53.8 eV for Argon [4]. Fore pure rare gases, no substantial pressure effect on the W-values have been measured [7] Recalling equation 2, about 5.16E+13 argon atoms can be excited every second. The range of 5.56 MeV alpha particles in argon at STP (Standard conditions for Temperature and Pressure) is about 37 mm [5]: a nuclear-excited plasma of argon at atmospheric pressure of the shape of a half sphere if the excitation source is punctual will therefore have a volume of about 100 m .…”

Directional Detection of Galactic Dark Matter

“…La présence d'impuretés dans le mélange gazeux est contrôlée par un système de circulation du gaz, mis en fonctionnement après une étape de mise sous vide (10 −6 mbar). En effet, l'effet de la pureté du gaz sur la valeur de W (l'énergie moyenne pour produire une paire électron-ion) a été mise en évidence dans [119].…”