Chapter 13 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

Radiation imaging in used to locate, and in many cases also to quantify, radionuclide or a radionuclide-bearing compound from solid material. There are two basic types of imaging techniques: planar imaging giving information of radionuclide distribution at two dimensions and tomography giving three-dimensional information. The latter technique is only briefly described at the end of the chapter. Imaging techniques are typically used in biological and medical applications to locate target molecules. To enable the location of these molecules they have been labelled with a radionuclide, typically a beta-emitting radionuclide in planar imaging and a gamma-emitting radionuclide in tomography. Radiation emitted by these radionuclides is then detected by autoradiography or using technique based on CCD camera filming in case of planar imaging and by an array of gamma detectors in case of tomography.

Autoradiography can be divided into two categories, film autoradiography and storage phosphor screen autoradiography. The prefix auto means that the source of radiation is within the sample unlike in other types of radiographies in which the sample is exposed to an external radiation source, such as X-rays. Autoradiography dates back to late 19^th century when Henri Bequerel discovered in 1896 that uranium salts produced an image on photographic plates (Figure XIII.1).

Figure XIII.1. Image of a uranium salt on a photographic plate (autoradiogram) determined by Henri Bequerel in 1896 (http://www.japanfocus.org/-elin_o_hara-lavick/3196/article.html).

In film autoradiography a film is apposed to a radionuclide-bearing sample. The sample should be flat and as smooth as possible, for example pressed plant or polished rock surface. The film consists of a 0.2 mm polymeric (polyester or cellulose acetate) support plate coated with an emulsion comprising fine silver halide (AgCl, AgI, AgBr) grains in gelatin. The outer surface facing the sample can have a very thin protective cover. Radiation, typically beta particles but also alpha particles, emitted from the sample pass the surface cover and ionize silver atoms in the emulsion layer, which is typically 10-20 µm thick. The released electrons travel in the emulsion and after losing their kinetic energy reduce Ag+ ions into metallic silver Ag forming a latent, invisible image of the radionuclide distribution on the sample. These latent metallic silver centers comprise only of a few silver atoms. When the film is developed in a reducing liquid, Ag⁺ ions around the latent silver metal centres reduce and the amount of metallic silver in the crystal increases by a factor of 10⁸-10¹⁰ making them visible either by eye (macro autoradiography) or by microscope (micro autoradiography).

Figure XIII.2. Principles of autoradiography (http://lifeofplant.blogspot.fi/2011/12/autoradiography. html).

The autoradiogram seen on a film gives a qualitative picture of the distribution of the target radionuclide, or the compound/material bearing the radionuclide, in the sample. Depending on the type and thickness of the sample and the type and energy of radiation the image represents the radionuclide distribution either on the surface of the sample or also in its bulk. If, for example, the sample is rock, the density of which is about 2-3 g/cm³ only radiation originating from the sample surface (alpha particles), or very close to it (beta particles), can be detected on the film due to self-absorption of radiation in the sample at higher sample depths. In the case of imaging a plant sample, having a much lower density, much larger fraction of the radiation comes from the inner part of the sample giving thus also information of radionuclide distribution in its bulk. From the sensitivity point of view autoradiography technique is best suited for tracers utilizing beta emitters of an intermediate energy, such as ¹⁴C (E_max = 156 keV) and ³⁵S (E_max = 167 keV) for which the energy is high enough to avoid self-absorption but low enough to avoid penetration of beta particles through the reactive gel layer.

An important parameter in autoradiography is the resolution, which means the ability of the system to differentiate two individual points in the sample. A typical resolution range is from 5 µm to 50 µm. The resolution is dependent on the following factors, in the order of importance:

1. Distance between the film and the sample. Closer contact to the sample can be obtained by using a fluid silver halide emulsion without the polymeric support, which improves resolution by 5-7 times at maximum.
2. Energy of radiation. The lower the beta energy the better the resolution due to a shorter range of emitted beta particles. The resolution with the low energy beta emitter ³H (E_max = 18 keV) is about ten times better than with the high energy beta emitter ³²P (E_max = 1710 keV). Resolution with the intermediate energy beta emitter ¹⁴C (E_max = 156 keV) is in between these two.
3. Thickness of the sample, the resolution being the better the thinner the sample is.

Figure XIII.3 shows an autoradiogram of a rock impregnated with polymethylmetacrylate labelled with ¹⁴C. The dark areas represent pores (the method is described in detail later in the chapter). In addition to the sample, also standards with known activities of the target nuclide are prepared and their autoradiograms are determined in an identical way as that of the sample. These standards are used to quantify the radionuclide distribution in the sample. The darkness or grey level distribution of the autoradiogram is measured with an optical densitometry measuring the absorption of exposed light at various points of the autoradiogram. The absorption values are converted to optical densities, which are compared point by point to those observed with standard samples and relative activity values can thus be determined at various points at 5-50 µm resolution. The autoradiogram can also be scanned and the grey level values at various points, pixels, are determined with a computer. Exposure times of autoradiographic films vary in a wide range up to weeks, mostly depending on the activity levels. Finding a suitable exposure time requires optimization and experience.

Figure XIII.3. An autoradiogram of the surface of a rock impregnated with polymethylmetacrylate labelled with ¹⁴C. Standard series with varying ¹⁴C activities are at the bottom.