Radiolysis particle accelerators

The rise of heavy particle accelerators made it possible to study the radiation chemistry as a function of particle LET with machines such as the Lawrence Berkeley Bevatron and others that allowed the expansion of radiolysis to very heavy ions and very hi h LET. 

The electron itself is frequently used as a primary source of radiation, various kinds of accelerators being available for that purpose. Particularly important are pulsed electron sources, such as the nanosecond and picosecond pulse radiolysis machines, which allow very fast radiation-induced reactions to be studied (Tabata et al, 1991). Note that secondary electron radiation always constitutes a significant part of energy transferred by heavy charged particles. For these reasons, the electron occupies a central role in radiation chemistry. 

During the y-radiolysis of vitreous solutions containing only biphenyl (0.1 M) or only pyrene (0.02 M), the yield of Ph2 and Py- at 77K is high enough for them to be recorded at an irradiation dose of 1019 eV cm-3. At 77 K these particles have been observed to decay spontaneously (Fig. 5), evidently, due to proton transfer from alcohol molecules (the most probable process in the case of Ph2 anion radicals [14]) or to recombination with counterions formed during radiolysis. Naphthalene and pyrene additives to solutions of Ph2 essentially accelerate the decay of the Ph2 anion radical at 77 K which is naturally accounted for by electron transfer from Ph2 to Nh and Py. In agreement with this conclusion the decay of Py in the presence of Ph2 is slower than its spontaneous decay in the absence of Ph2. ... 

As most of the free radicals are short-lived, direct monitoring of their reactions is not an easy task and powerful tools based on fast reaction techniques are required to follow such processes.Thus, fast reaction techniques utilize either short pulses of high-intensity flash of light or laser (in flash photolysis), or short pulses of charged particles and high-energy photons from accelerators (in pulse radiolysis). 

There are several methods used to accelerate charged particle beams for pulse radiolysis. Acceleration requires a force applied by an electric field. The field may be a continuous... 

Time Scales. The time scales measurable by the two techniques are illustrated in Figure 3. The time resolution of pulsed lasers is far superior, reaching to as short as 20 fs, with 200-fs measurements becoming routine in many laboratories. Pulse radiolysis measurements achieve rise times of 20-30 ps in only a few laboratories, while 1-10 ns is more common. Pulse radiolysis is slower because the accelerated particles, usually electrons, repel each other, making it difficult to bunch many of them into a very short pulse. Pulses as short as 5 ps have been reported and new accelerators may achieve 1 ps, but it is likely that pulsed lasers will remain the leader in very high time resolution. 

Relevant to water radiolysis in nuclear reactor, G-values of the water decomposition by fast neutrons have been determined by using a fast reactor at elevated temperatures [59]. Since fast neutron radiolysis is equivalent to proton radiolysis because of the recoil proton formation through the elastic collision of fast neutrons with H2O molecules [60], an alternative approach as a model experiment is the ion beam radiolysis with different LET particles from accelerators at elevated temperatures [61]. 

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