Investigations into the DNP characteristics of electron and proton irradiated polyethylene and polypropylene

Abstract

To achieve a significant enhancement of the nuclear polarisation of solid state target materials one must use the method of dynamic nuclear polarisation. This necessitates unpaired electrons, that can be created as paramagnetic structural defects by the irradiation of materials. To this end polyethylene (PE) and polypropylene (PP) materials were irradiated by 13MeV protons at a temperature of 120K and over a temperature range of 87K to 210K with ~20MeV electrons. The identification of the radical types that are created, as well as the quantification of the unpaired electron density, was made in a conventional ESR spectrometer. Samples were polarised in a 2.5 T magnetic field at a temperature of 1 K.<br /> It was found that the irradiation temperature has an effect on the nuclear relaxation times of the materials. Higher irradiation temperatures result in lower radical yields and consequently longer relaxation times. Both materials exhibit an increase of polarisation values for an increase in irradiation temperature. The largest polarisation values were obtained in the PE materials of around 20% when irradiated at a temperature of 210 K, but PP only reached a maximum of 8% at the same irradiation temperature. Better results for the PP materials, in terms of polarisation values, were obtained by the heating of the samples at room temperature post irradiation. Here it was possible to obtain results of 14% polarisation.