Possibilities of cryogenic autoradiography
Possibilities of cryogenic autoradiography. Pankratov D.A., Korobkov V.I. //Journal of Analytical Chemistry. 2014. V. 69. Is. 7. P. 632–637
Photographic properties of the nuclear photographic detector BioMax MR Film from KODAK are studied at the temperature of liquid nitrogen. The characteristic curves obtained at room and cryogenic temperatures indicate that the detector retains its physical and photographic properties, and its possibilities can be expanded to studies of deeply frozen samples. The data obtained point to an increase in the sensitivity of the photographic material frozen to cryogenic temperatures at short exposures.
Autoradiography is a traditional instrumental method for the investigation of the local chemical heterogeneity of materials. It is used in quite different fields of science and engineering: microbiology, medicine, geology, materials science, and in solving astronomy and space science problems. The method is based on recording the ionization radiation of a radionuclide by photographic detectors. A visible picture of the distribution of radionuclides in the surface layer of a material (sample), an autoradiogram (ARG), is usually obtained after the photographic development of the detector. In addition to other conditions, an important condition for obtaining high-quality images with a good resolution is the immovability of the radionuclides in the studied material relative to the detector material within its exposure. This requirement restricts the choice of samples that can be investigated by autoradiography to materials in which the diffusion or motion (for example, due to Brown or convective motion) of the components or phases present is hindered or even impossible. Thus, autoradiography is usually limited by the study of only solid substances, whereas liquid solutions and dispersed systems with liquid dispersive media remain outside the possibilities of this technique. However, in many cases researchers can obtain and will suffice information on the distribution of components in a sample frozen to low temperatures. For example, the problem of the distribution of the 57Co radionuclide in frozen solutions is important in their study by Mössbauer spectroscopy.
The application autoradiography to the study of frozen samples was from time to time mentioned in the literature. The main objects of study by low temperature autoradiography are biological tissues frozen to rather low temperatures: from –10, –20, and –30 to –76 and –80°C. However, these works were occasional and did not include the analysis of the possibility of changes in the properties of photographic materials used as detectors at low temperatures.
It is clear that, if a latent image is formed at a significantly reduced exposure temperature, this, undoubtedly, should be reflected in the qualitative properties of the detector. It is known that many events of the formation of latent images and also of accompanying and competing processes are thermally activated and their rates depend on temperature. On examples of traditional photographic materials it was known that a change in exposure temperature can result both in horizontal and possible vertical shifts of the isodensity curve in the reciprocity failure diagram. As a result, temperature effects can be separately distinguished among a number of photographic effects; these can be expressed as a monotonous increase in the optical density of a photographic material with decreasing temperature at low illumination intensity or with increasing temperature at high illumination intensity and as complicated nonmonotonous temperature dependence at moderate illumination intensities. This is associated with the complex temperature dependence of the photosensitivity of traditional photographic materials. We could not find in the literature any description of the effect of exposure temperature on the photodetectors used in autoradiography. It is clear that, for a particular detector–ionization source pair, such temperature effects cannot be qualitatively and quantitatively predicted without special experiments.
The aims of this work were the study of the effect of low temperatures on the registration of audioradiographic images of the Mossbauer 57Co radionuclide by one of widely used nuclear detectors (BioMax MR Film from KODAK), the development of a procedure of cryogenic autoradiography, and the design of corresponding devices and facilities.
Thus, it was found experimentally that the nuclear photographic detector BioMax MR Film from KODAK at the temperature of liquid nitrogen retains its physical properties (the base does not crack, does not undergo curling, the photolayer does not flake). It was shown that this detector can be used to record the radiation of 57Co both at a room temperature and at the temperature of liquid nitrogen. The characteristic curves for the studied nuclear photographic detector obtained at room temperature and at the temperature of liquid nitrogen indicate that the detector as a whole retains its properties at cryotemperatures, and can thus be used to study deeply frozen samples. In addition, an increase in the sensitivity of the frozen detector at low exposures allows the registration of sample areas containing very small amounts of radionuclide atoms.