Radiation sensitisers

Mitchell, J.B., Cook, J.A., Krishna, M.C., DeGraff, W., Gamson, J., Fisher, J., Christodoulou, D., and Wink, D.A. (1996). Radiation sensitisation by nitric oxide releasing agents. Br. J. Cancer. Supplement 27, S181-184. 

For the radiation rapid cure experiments, appropriate resin mixtures containing oligomers, monomers, flow additives and sensitisers (UV) were applied to the substrate as a thin coating, the material placed on a conveyor belt and then exposed to the UV and EB sources. The time taken to observe cure for each of the samples was then measured on a relative basis. The UV system used was a Primarc Minicure unit with lamps of 200W per inch. Two EB facilities were utilised namely a 500KeV Nissin machine and a 175KeV ESI unit. 

When the source of initiation is altered from ionising radiation to UV, analogous additive effects to those previously discussed have been found. For reasonable rates of reaction, sensitisers such as benzoin ethyl ether (B) are required in these UV processes. Thus inclusion of mineral acid or lithium perchlorate in the monomer solution leads to enhancement in the photografting of styrene in methanol to polyethylene or cellulose (Table V). Lithium nitrate is almost as effective as lithium perchlorate as salt additive in these reactions (Table VI), hence the salt additive effect is independent of the anion in this instance. When TMPTA is included with mineral acid in the monomer solution, synergistic effects with the photografting of styrene in methanol to polyethylene are observed (Table VII) consistent with the analogous ionising radiation system. 

The exchange of electronic excitation between two atoms frequently results in sensitised fluorescence and one of the earlier examples was the discovery of emission of the fluorescence of atomic sodium, which occurs when a mixture of sodium and mercury vapour is irradiated with mercury resonance radiation at 2537 A... 

In the sensitised UV copolymerisation, radical pathways similar to the ionising radiation process are available for grafting (Equations (10-12,4,U0 + being representative sensitiser). 

Most of the experimental techniques for thermal systems apply here. One important exception is that for experiments using Hg 2537A radiation, the vacuum system must be completely free from mercury, i.e. no mercury diffusion pumps or McLeod gauges should be used, unless mercury-sensitised reactions are being studied. Even the use of iodine, gold or similar amalgamating metals does not completely free a particular section of the vacuum system from mercury. There is an excellent treatise on photochemical reactions by Calvert and Pitts. ... 

The most common sources of radiation for kinetic photochemical studies are mercury lamps of which there are three types. The low-pressure lamp is used mainly for mercury-sensitised studies. For general photochemical studies the most useful lamp is the medium-pressure lamp having several lines of reasonable... 

Typical thermal and photolytic techniques may be used but a common method is to fill an ampoule, fitted with a break-seal, with the reactant or reactants, dose, quench in liquid nitrogen, and reattach to the vacuum. This is followed by conventional analysis. Facilities for heating the sample at the radiation source can be employed. RV s are constructed from Pyrex or metal " and may in some cases be coated with graphite. However, with different surfaces, different products can result . Thin-walled RV s have to be used for a-particle-induced reactions . In the radon-sensitised reaction of Hj and O2, Lind has shown that the rate coefficient is inversely proportional to the square of the diameter of a spherical RV. 

Photochemical redox reactions can generate fuel, including H2, CH4 and CH3OH. The splitting of water to produce hydrogen has been the focus of particular attention. H2O is transparent to solar UV or visible radiation, and therefore sensitisation is required to drive the water-splitting process. In the early attempts at the photoredox splitting of water of Heidt and McMillan (1953), the process was sensitised by solution redox species such as... 

Finally, in several cases the radiation has a frequency lower than required for the direct excitation of reactants. In this case excitation is still possible if a sensitiser is present this can absorb the radiation, be converted from a singlet to a triplet excited state and transfer energy to a reactant, say A (the respective triplet excitation energies being, of course, in the correct relation). A simple scheme in this case (S is the sensitiser) is... 

Investigation of the mechanism of photocycloadditions can be carried out with the help of the above or similar schemes, including analysis of the effect of varying reactant concentrations, and of adding quenchers and sensitisers, on the quantum yield of the processes at different wavelength of the radiation employed. A few cases will be mentioned. 

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