J. G. Carter

Fast gas mixtures for gas-filled particle detectors☆

Abstract Based on knowledge of the electron scattering cross sections at low electron energies, combinations of binary and tertiary gas mixtures have been discovered for which the electron energies and the overall scattering cross sections that correspond to these are reduced, resulting in an increase in the drift velocity, w, of the electrons in the gas mixture for a range of E/P (pressure-reduced electric field) values. Several such mixtures of argon (Ar), carbon tetrafluoride (CF4) and acetylene (C2H2) will be described that provide much faster response in gas-filled detectors compared to P-10 (90% Ar + 10% CH4) over a wide and convinient range of E/P values. The w(E/P) for CF4(100%), Ar(80%) + CF4(20%), Ar(80%) + C2H2(20%), Ar(80%) + C2H2(10%) + CF4(10%) are respectively, ∼15 × 106, ∼12 × 106, ∼5 × 106 and ∼9 × 106 cm s−1 at E/P values of, respectively, ∼6, 0.8–1.5, 0.5–4 and 0.8–3.5 V cm−1 torr−1. Measurements with a conventional proportional counter showed that although the energy resolution of the Ar/CF4 mixtures compares unfavorably with the energy resolution of P-10, that of the C2H2-containing binary and tertiary mixtures compares very favorably.

Cross Sections for Electron Attachment Resonances Peaking at Subthermal Energies

A simple procedure is described whereby both the magnitude and the energy dependence of the cross section for subthermal peaking electron attachment resonances are determined from electron swarm data alone. For an initial energy region it is shown that the attachment cross section can be approximated by σa(e) = Aγ / eγ, and a determination of the power γ of the electron energy e as well as of the corresponding constant Aγ yields both the magnitude and the energy dependence of the cross section at thermal and epithermal energies. The values of Aγ [in (ergs)γ square centimeters], γ, and σa(0.05 eV) (in square centimeters) are 2.5 × 10−29, 1.12, 1.17 × 10−14; 7 × 10−34, 1.412, 2.17 × 10−15; 9.1 × 10−32, 1.148, 1 × 10−16; and 1.66 × 10−29, 1.031, 5.2 × 10−16 for sulfur hexafluoride, 1,4‐naphthoquinone, anthracene, and 1,2‐benzanthracene, respectively. The thermal (T = 298°K) value of the attachment rate for SF6 in the carrier gases N2 and C2H4 is found to be 8.7 × 109 sec−1·torr−1 and 9 × 109 sec−1·torr−1, re...

Low-energy electron interactions with organic molecules: Negative ion states of fluorobenzenes

An electron transmission technique has been employed to determine the positions of the three π‐negative ion states (configurations π12π22π23π41, π21π22π23π15, and π21π22π23π16) of benzene and fluorobenzenes in the vapor phase. These are 1.13 (1.35), 1.13 (1.35), (4.80) eV for benzene; 0.82 (0.91), (1.40), (4.66) eV for fluorobenzene; 0.53 (0.62), (1.41), (4.51) eV for p‐difluorobenzene; (0.77), (0.77), (4.48) eV for 1,3,5‐trifluorobenzene; 0.34, (0.50), (1.29), (4.51) eV for 2,3,5,6‐tetrafluorobenzene; <0.15 (0.36), (1.19), (4.53) eV for pentafluorobenzene; and (0.42), (0.42), (4.50) eV for hexafluorobenzene. The numbers in parentheses are the vertical attachment energies, and those not in parentheses are the 0→0 transitions. On the basis of these data, the first π‐electron affinities (E.A.) of the isolated molecules of these compounds are equal to −1.13, −0.82, −0.53, ≳−0.77, −0.34, ?−0.15, ≳−0.42 eV for benzene, fluorobenzene, difluorobenzene, 1,3,5‐trifluorobenzene, 2,3,5,6‐tetrafluorobenzene, pentaflu...

Low energy electron attachment to SF6 in N2, Ar, and Xe buffer gases

The electron attachment rate constants ka for SF6 have been measured in dilute mixtures of SF6 in high pressure (>1 atm) N2, Ar, and Xe buffer gases at room temperature (T≊300 K) over a wide E/N range (electric field strength to gas number density ratio), corresponding to mean electron energies 〈e〉 from near thermal electron energies (≊0.04 eV) to 〈e〉≊4.3 eV. Particular attention has been paid to the effects of space charge distortion, molecular impurities, and changes in the electron energy distribution function on the measured electron attachment rate constant values at the lower E/N values in these mixtures. The present measured thermal electron attachment rate constants in SF6/N2 and SF6/Xe gas mixtures are in excellent agreement with recent accurate measurements of these parameters in several SF6/buffer gas mixtures. At higher 〈e〉 values, the present SF6/N2 measurements are in fair agreement with previous measurements, while no previous measurements using Ar and Xe buffer gases have been published. T...

Fragmentation of aliphatic chlorocarbons under low‐energy (≲10 eV) electron impact

The fragmentation of ten aliphatic chlorocarbons (CHCl3, 1,1,1‐C2H3Cl3, 1,1,2‐C2H3Cl3, 1,1‐C2H4Cl2, 1,2‐C2H4Cl2, C2Cl4, C2HCl3, 1,1‐C2H2Cl2, cis‐1,2‐C2H2Cl2, and trans‐1,2‐C2H2Cl2) under low‐energy (0–10 eV) electron impact has been investigated with a time‐of‐flight mass spectrometer. Three types of fragment negative ions were observed: Cl−, Cl−2, and (M–Cl)− (parent molecule, M, less one Cl atom)−; for C2Cl4 the parent ion, C2Cl4−*, was also observed at ∼0.0 eV and found to be metastable with an autodetachment lifetime of 14±3 μsec. The Cl− ion was by far the most abundant. The intensities of Cl2− and (M–Cl)− with respect to Cl− depended very strongly on the number and relative positions of the Cl atoms in the molecule. The yield of Cl2− was, as a rule, very much lower when the two Cl atoms in the Cl2− ion originated from the same C atom. The measured Cl− currents as a function of electron energy were corrected for the finite width of the electron pulse, and the resultant unfolded functions exhibited di...

Electron transport studies of gas mixtures for use in e-beam controlled diffuse discharge switches

Gas mixtures for possible use in diffuse discharge switching applications require both high dielectric strength and specific electron transport properties in the conducting and the opening stages of the operation of the switch. In the conducting stage, the electron drift velocity must be large, and the electron loss processes (e.g., due to electron attachment and recombination) must be low so as to maximize the current in the discharge while maintaining low‐discharge impedance. In the opening stage, strong electron attachment along with high dielectric strength is required of the gas mixtures in order to extinguish the discharge as quickly as possible (and, thus, achieve a fast opening time) and to prevent arcing occurring between the switch electrodes due to the high voltages induced across the switch in the opening phase. Measurements of the electron drift velocity, attachment, ionization and transverse diffusion coefficients, and high‐voltage dielectric strengths of several gas mixtures proposed as can...

Electron attachment to perfluorocarbon compounds. III. Fragmentation of aliphatic perfluorocarbons of interest to gaseous dielectrics

Resonance nondissociative and dissociative attachment of slow electrons (0–12 eV) to the perfluorocompounds 2‐C4F6, c‐C4F6, 1,3‐C4F6, c‐C4F8, and 2‐C4F8 have been studied using a time‐of‐flight mass spectrometer. All five compounds capture thermal energy electrons and form long‐lived parent negative ions with autodetachment lifetimes, τa, equal to 6, 7, 9, 6, and 10×10−6 sec for c‐C4F6−*, 1,3‐C4F6−*, 2‐C4F6−*, c‐C4F8−*, and 2‐C4F8−*, respectively. Except for 1,3‐C4F6, the yield for the parent negative ions of these perfluorocarbons was more than a factor of 10 larger than that of the fragment ions. The following fragment negative ions were detected: 2‐C4F6: F−, CF3−, C3F3−; c‐C4F6: F−, C3F3−, C4F5−; 1,3‐C4F6: F−, CF3−, C3F3−, C4F4−; c‐C4F8: F−, CF3−, C2F3−, C3F5−; and 2‐C4F8: F−, CF3−, C2F3−, C3F3−, C3F5−*, C4F6−*, C4F7−*, (where the asterisk indicates that these fragment ions were found to be long‐lived metastables with τa values equal to 70, 17, and 7×10−6 sec, respectively). Groups of negative ion reso...

Electron transport measurements in methane using an improved pulsed Townsend technique

An improved pulsed Townsend technique for the measurement of electron transport parameters in gases is described. The accuracy and sensitivity of the technique have been investigated by performing, respectively, electron attachment coefficient measurements in pure O2 over a wide range of E/N at selected O2 pressures and by determining the electron attachment and ionization coefficients and electron drift velocity in CH4 over a wide E/N range. Good agreement has been obtained between the present and the previously published electron attachment coefficients in O2 and for the drift velocity measurements in CH4. The data on the electron attachment coefficient in CH4 (measured for the first time) showed that with the present improved pulsed Townsend method, electron attachment coefficients up to 10 times smaller than the ionization coefficients at a given E/N value can be accurately measured. Our measurements of the electron attachment and ionization coefficients in CH4 are in good agreement with a Boltzmann e...

Gases for possible use in diffuse‐discharge switches

Desirable electron attachment and electron drift characteristics of gases for possible use in diffuse‐discharge switches are indicated. Gas mixtures for possible use in externally sustained (e‐ beam) diffuse‐discharge switches are suggested on the basis of electron attachment rate constants and electron drift velocities measured as a function of the density‐normalized electric field E/N. Of particular promise are mixtures of Ar and C3F8.

Xe-containing fast gas mixtures for gas-filled detectors☆

Abstract Electron drift velocities are reported for XeCF4, XeC2H2 and XeCF4C2H2 mixtures. For a number of these mixtures the drift velocities are large (4 to 10 × 106 cm s−1) over a range of E P values below 3.5 V cm−1 Torr−1. Such mixtures may find application in gas-filled detectors, especially those involving electromagnetic ionizing radiation.