Radiation-induced conductivity

F.W. Davies J.E, Shrader, The Radiation Induced Conductivity of Laad Azide (U) , Proceedings of the Fifth Symposium, Nuclear Survivability of Propulsion and Ordnance Systems, Menlo Park (Oct 1975) (SRED), DNA 4032P, contents of paper not classified... 

One of the most important experimental methods of studying the electron-ion recombination processes in irradiated systems are measurements of the external electric field effect on the radiation-induced conductivity. The applied electric field is expected to increase the escape probability of geminate ion pairs and, thus, enhance the number of free ions in the system, which will result in an enhanced conductivity. 

Considerable disagreement still prevails as to the mechanism of crosslinking in polymers. Is it an ionic or a radical process While the author admits the existence of some ionic species (as revealed by ESR at low temperature and by radiation-induced conductivity), his present view is that the ionic contribution to crosslinking in solids and liquids is only minor. This attitude is based inter alia on the following evidence. 

Three types of reactive species are formed under irradiation and may become trapped in polymers ionic species, radicals, and peroxides. Little is known about the role of ions in the chemical transformations in irradiated polymers. Long-lasting ions arise, as demonstrated by radiation-induced conductivity, and may become involved in postirradiation effects. The presence of trapped radicals is well-established in irradiated polymers, but certain problems remain unsolved concerning their fate and particularly the migration of free valencies. Stable peroxides are produced whenever polymers are irradiated in the presence of oxygen. Both radicals and peroxides can initiate postirradiation grafting, and the various active centers can lead to different kinetic features. 

However, ionic species are undoubtedly present in irradiated polymers and persist for a considerable time after irradiation. This is demonstrated by the radiation-induced conductivity observed in most plastic insulators and by the slow decay of this conductivity, which may still be noticeable several months after irradiation (7). 

Wintle, H. J. Radiation induced conductivity in irradiated polyethylene. Polymer 2, 444 (1961),... 

However, the absence of radiation-induced conductivity data at relevant doses and dose... 

Warman, J. M. de Haas, M. P. Wentinck, H. M. The study of radiation induced conductivity changes in microheterogeneous materials using microwaves, Radiat. Phys. Chem. 1989, 34, 581. 

Keywords Compact fuel cell Proton conductive oxide Radiation induced conductivity Radiation enhanced diffusion... 

Figures 2(a) and (b) show radiation induced conductivity (RIC) for the specimens with and without H as a function of ionizing dose rate at the first and second cycles, which were calculated using Ohm s law from the experimental data between from 0 to +10 V and the specimen volume. The conductivity increased with the increase of the ionizing dose rate. The values at the ionizing dose rate of 0.5 kGy/s became higher by two orders of magunitude than that without radiation, namely it is 0 Gy/s. The RIC with H is higher compared with that without H. Therefore, the RIC may be caused by electronic excitation as well as enhanced diffusion of H due to ionization irradiaion. After irradiation for 29 reactor full power days, the second reactor irradiation experiment was carried out for 27 days. The RIC with H at first cycle is almost same as that at second cycle. The RIC without H at several ionizing dose rate decreased by about one order of magnitude.

Johnson and Willson interpreted the main feature of the observations on solid polyethylene doped with aromatic solutes in terms of an ionic mechanism it was analogous to that proposed for irradiated frozen glassy-alkane-systems in which ionization occurred with G = 3 — 4 [96], The produced charged species, electron and positive hole, were both mobile as indicated by the radiation-induced conductivity. The production of excited states of aromatic solutes was caused mainly by ion-electron neutralization. The ion-ion recombination was relatively slow but it might contribute to the delayed fluorescence observed. On the basis of Debye-Simoluchovski equation, they evaluated the diffusion coefficients of the radical anion of naphthalene and pyrene as approximately 4 x 10 12 and 1 x 10 12 m2 s 1 respectively the values were about three orders of magnitude less than those found in typical liquid systems. 

What could be observed in the later experiments was a radiation-induced conductivity at water contents considerably lower than observed in the initial measurements. Thus, in the region of 12 to 22 water molecules per nucleotide (corresponding to the A DNA structure) a water-concentration-independent conductivity was observed. It was concluded that this does not arise from ice-like regions but from a layer of water molecules that is more intimately connected with the structure of the bio-molecule. The nature of the mechanism underlying this conduction remains to this day a mystery. 

The majority of studies using PR-TRMC in recent years have involved complex organic materials such as conjugated polymers or discotic liquid crystals. Inorganic semiconductor materials have however also received attention over the years. The first of these was a study of the radiation-induced conductivity in a powder sample of the high-Tc superconductor DyBajCujOj,. The superconductivity transition at 88 K on cooling was found to be accompanied by a dramatic, sudden increase in the radiation-induced conductivity an effect which... 

An extensive series of measurements was also carried out on the anisotropy of the radiation-induced conductivity by carefully orientating single PDA crystals within the microwave cavity. Charge transport was found to be favored by up to two orders of magnitude along, as opposed to perpendicular to, the directors of the polymer chains. 

Warman JM, de Haas MP, Rupprecht A. (1995) Radiation-induced conductivity in oriented DNA. In Hagen U, Harder D, Jung H, Streffer C (eds.), Radiation Research 1895-1995, vol 2. pp. 212-215. Universitatsdruckerei H. Stiirtz, Wurzburg, Germany. 

Frey H, Moller M, de Haas MP, Zenden NJP, Schouten PG, van der Laan GP, Warman JM. (1993) Radiation-induced conductivity in poly(methylphenylsily-lene) and poly(di- -hexylsilylene) studied by time-resolved microwave conductivity. Macromol26 89-93. 

The radiation-induced conductivity in hydrated samples of aligned versus non-aligned solid DNA fibers was compared by Warman et al. [166]. The dry fibers were found to be essentially nonconducting, as a water content of 10-15 waters per base pair is necessary to observe conductivity. Under these conditions, the conductivity of the aligned and nonaligned films was found to be similar. It was concluded that conductivity in solid B-DNA is due to highly mobile charge carriers within the outer mantle of the biopolymer chains rather than within the base-pair cores. ... 

Mobilities from Radiation Induced Conductivity Measurements 107... 

Conductivity measurements on irradiated solids (2, 4, 5) revealed the similarity of the mechanisms of photo- and radiation-induced conductivity and indicated the importance of the role of traps and recombination centers which may exist at positions of disorder in the liquid or solid structure. Radiation-produced impurities can also act as traps. 

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