Febetron

Shimamori and Hatano (1976) describe a Febetron-injected microwave cavity apparatus for measuring electron concentration following pulse irradiation. Its application to thermalization in Ar and CH4 is similar to the method of Warman and Sauer (1975). In a related experiment, Hatano et al. (private communication) measure the electron collision frequency directly. 

The importance of these studies extends beyond the ecific realm of radiation-induced polymerisation since many of these species are formed or thought to be formed in other types of cationic polymerisation. The development of even faster irradiating instruments such as the Febetron, which allows nanosecond pulses, will provide an even deeper insight into these processes. 

Almost all the work in pulse radiolysis is based on the use of three types of electron accelerators linear accelerators (linacs). Van de Graaff accelerators, and Febetrons. The first accelerator used by Keene at Manchester was a 4-MeV linac with pulses of 0.2-2 ps duration [47a] this was replaced in 1967 with an 8-12-MeV linac capable of delivering pulses from 5 ns to 5 ps duration [93]. Further improvements made to the Manchester system up to 1989 have been documented [93]. Similarly, the 13-MeV linac used at Argonne in 1960 by Matheson and Dorfman produced pulses of 0.4 to 5 ps duration [46], whereas in 1989 the equipment comprised a 20-MeV linac, capable of producing pulses from 25 ps to 10 ps duration, and a 3-MeV Van de Graaff accelerator, which is dedicated to EPR and magnetic resonance studies (see below) [95, 98]. 

Some experiments may require the use of different doses. Depending on the accelerator, this can be done by a change of the length of the pulse (if the experiment allows it), the focusing of the beam and/or the beam current. Alternatively, the amount of radiation that reaches the sample cell can be varied by means of scatter plates of variable thickness or, particularly with Febetrons, metal shields with holes of different sizes [105]. 

The high intensity pulsed electron source used was a Febetron 705 system (Field Emission Corp., McMinnville, Ore.). In this source the discharge from a stack of capacitor-inductance modules is applied to the field emission cathode of a vacuum tube to give an electron pulse through the tube window with energies from 0.5-1.6 Mev. at currents 4000 amp. The current waveform is shown in Figure 1. 

Figure 1. Current waveform of electron pulse from Febetron 705...

A Febetron tube window B Filling tube, connected to Hoke valve... 

A Febetron tube window B Platinum foil, 0.0025 cm. thick C Stainless steel plates, 0.15 cm. thick... 

The results at the lower Febetron dose rates of 1024-1025 e.v./gram sec. show a decrease in the ratio of cyclohexene to bicyclohexyl. This may be caused by a reduction in the reaction of thermal hydrogen atoms with radicals and indicate a return to the low dose rate (i.e., 1016 e.v./gram sec.) mechanism. However, dosimetry at these dose rates at the present time is not sufficiently accurate to warrant further discussion. 

In the Febetron irradiations, the fractional lowering of the hydrogen yield by benzene is the same as at low dose rates, but there is less effect on the cyclohexene and bicyclohexyl yields, and the ratio of cyclohexene... 

The cyclohexene and bicyclohexyl yields in the Febetron irradiations at N20 concentrations above 0.1M are. — 3.5 and — 1.9, respectively. Since the unimolecular yield is decreased by N20, the extra yield of cyclohexene that must presumably come from Reaction 8 (H + CeHn -> C6Hio + H2), is. — 0.8. Since this is the same as the extra yield in pure cyclohexane it appears that the yield of thermal hydrogen atoms is not affected by adding N20. 

The effect of benzene in the Febetron irradiations is markedly different from that of the electron scavengers. The difference between the cyclohexene and bicyclohexyl yields is much smaller and implies that benzene decreases the yield of thermal hydrogen atoms. Thus, benzene is not acting as an electron scavenger as has been suggested (17). 

Pulsed Electron Accelerator. The very high intensity pulsed electron accelerator used was a 705 Febetron (Field Emission Corporation, McMinnville, Oregon, U.S.A.). This is nominally a 2 Mev. accelerator. Its mode of operation has been described previously (16). 

Irradiation Cells and Calorimeters. The cell used for gas phase irradiations with the Febetron was made of stainless steel. The gas volume was a cylinder 4.0 cm. in diameter and 3.0 cm. deep behind a 0.0127 cm. stainless steel electron window. The electron window was welded around the edge of the cell. Welded to the rear of the cell was a small filling tube with a stainless steel Hoke vacuum valve. The whole assembly could be pumped and baked. Gases were frozen into the filling tube volume with liquid nitrogen and allowed to expand into the cell on warming. 

The radical reactions were initiated by irradiation of the gas mixtures in the cell with a 30 ns pulse of 2 MeV electrons from a Febetron 705B field emission accelerator. The irradiation dose was varied by inserting stainless steel attenuators between the accelerator and the cell. The doses are given relative to the maximum dose, which is set to unity. SFe was used as diluent gas and the experiments were carried out at a total pressure of 1 bar. The pulsed irradiation is used to rapidly (<1 ms) produce a high concentration (10 " - 3 x 10 cm" ) of radical species e.g. OH and F atoms. 

At this time possible transients have not been observed yet. Consequently monoelectronic oxidations have been carried out using pulse radiolysis technique (Febetron 708) and fast kinetics absorption spectrophotometry. 

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