Radiolysis of Aqueous Solutions with Pulsed Ion Beams. 4. Product Yields for Proton Beams in Solutions of Thiocyanate and Methyl Viologen/Formate

Abstract

The yields of radicals from water decomposition produced in the radiolysis of two types of aqueous solutions, thiocyanate and methyl viologen (MV 2+ ), were determined using proton pulses of 5.2 MeV energy. Aerated thiocyanate solutions in the concentration range of 0.001-0.75 M gave yields of (SCN) 2 •-, formed from scavenging OH radicals, that were lower than those for high-energy electrons and higher than those for 4 He ions of 21 MeV energy. The (SCN) 2 •yield increased with increasing thiocyanate concentration, but the decay of thiocyanate radicals through intratrack reactions appears to be substantial in proton radiolysis. Methyl viologen radical cations (MV •+ ) formed by scavenging e aq -, H atoms, and OH radicals were measured in deaerated 0.5 mM MV 2+ solutions containing formate. The MV •+ yields agreed with the results of steadystate proton beam radiolysis, which confirms earlier results that this system is a suitable chemical dosimeter for ion beam pulse radiolysis. The yields of MV •+ in deaerated MV 2+ solutions containing formate and formate/ tertiary butanol were used to determine the yields of OH radicals and the sum of the e aqand H atom yields. Both sets of yields for proton beams were lower than the corresponding ones for high-energy electrons and higher than those for 21 MeV 4 He ions. The predicted hydroxyl radical yields for proton beams increase with increasing scavenging capacity and approach the value found for high-energy electron radiolysis. The sum of the e aqand H atom yields is about 1.8 molecules/100 eV and nearly independent of the scavenging capacity for OH radical. Intratrack reactions were simulated using a deterministic diffusion kinetic model, and the results qualitatively predict the observed yields in the thiocyanate and the MV 2+ solutions.

A nanosecond pulse radiolysis study of the hydrated electron with high energy ions with a narrow velocity distribution

“…Literature data concern experiments with 3 MeV proton pulses of 1 ns duration (the mean LET was 29 eV/nm) [3,4], microsecond pulses of various ions [1,2,13,14] or continuous irradiation followed by the analysis of the final products resulting from reactions with various scavengers [17,25,26]. It is of interest to compare values obtained under similar LET conditions if one assumes that the concentrations of the primary radical species are only determined by the deposited energy in the ionization track.…”

“…A few studies have been performed with high energy pulsed heavy ions to depict the effects of the linear energy transfer (LET) on the radiolysis of liquid water [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. For many years the physical-chemistry occurring in the ionisation tracks of heavy ions has been described in a general way [15,16].…”

“…This trend is a result of a more efficient recombination of the radical species to form molecular species in the tracks since the density of the ionisation events is greater with high LET particles. This assumption has been deduced mainly from studies using the chemical scavenging method [11][12][13][14]17] which is based on concentration measurements of the final stable reaction products. In this method the time dependence of the g-values is determined from the scavenger concentration and the bimolecular rate constant of the scavenger and the radical species (see for instance [17]).…”

Concentration and Incident Energy Dependences of Transient Species in Water by H⁺ Ion Irradiation

“…The characteristic effects have been investigated by using water and aqueous solutions in the past because water is considerable substance of the radiation biology. There are two methods to clarify reactions of the reactive species; the scavenging method [1][2][3] and the time-resolved spectroscopic method [4][5][6][7]. The scavenging method has been used to estimate the yields of the reactive species in water samples irradiated with heavy ions.…”

“…•-by using the transient absorbance measurement [6]. In the series of our researches to understand the irradiation effects of MeV-class heavy ions, we constructed the time-resolved spectroscopic system [7].…”