Radioactivity photographic methods

The simplest arrangement to measure radioactivity involves measurement of the ionization caused by radiation in a gas filled chamber. The number of ionized particles generated in the gas by direct interaction (primary interaction) with radiation can be Increased several fold by various methods discussed below. The photographic methods of determination of radioactivity also depend upon ionization action of the rays which makes the grains of silver bromide sensitive to the developer. 

Photographic Detection Radioactive substances affect photographic plates. Although the intensity of the affected spot is related to the amount of radiation, precise measurement by this method is tedious. 

Cellular autoradiography techniques using radioactive nucleic acid probes have several features in common with nucleic acid immunocytochemistry. The method is based on the hybridization of radioactive probes to cellular targets and the subsequent exposure of photographic emulsion, which, when developed, reveals blackened (exposed) silver grains close to the site of hybridiza-hon. Hence, cellular autoradiography techniques permit excellent specihcity and localizahon of the hybridized probe—to 1 qm when tritium is the label used in the autoradiography-based method (9). 

In addition, a very simple and low cost method can be used to detect surface flaws. This is the use of penetrants that are typically fluorescent dyes. Usually, a three-step procedure is used. The ceramic part is first soaked in a fluorescent dye. Then the part is dried and cleaned in a very controlled manner to remove the dye from smooth surfaces but not from the surface defects. When the part is examined under ultraviolet li t, the surface defects such as cracks and porosity retain the dye and show up brilliantly. This method is used widely for surface inspection of ceramics and is frequently included as part of a quality assurance certification. Penetrants are effective for nonporous ceramics. With open porosity, the penetrant will enter all the pores of the ceramic, giving fluorescence to the whole ceramic piece, thus preventing detection of surface flaws. Not all penetrants are fluorescent dyes. Radioactive krypton can be used as a penetrant. It is retained in cracks or other defects and can be detected by either a Geiger counter or by carefully wrapping the ceramic piece in photographic film. After development of the film, the location of cracks emd pores can be detected. 

Various methods can be used to measure the intensity of radioactive emissions. These exploit the ability of radiation from radioactive isotopes to cause ionisation (Geiger-Miiller counting), to excite fluorophores (scintillation counting), or to cause exposure of light-sensitive photographic emulsion (autoradiography) (Slater 1990 Rickwood et al. 1993). 

There are several methods for directly depicting metal distributions in plants, including autoradiography and X-ray fluorescence (XRF or XFS). The former method - essentially the procedure by which Becquerel discovered the phenomenon of radioactivity in 1896 -means using and detecting (by photographical means) emission by some radionuclide (often non-metals such as "C, but also Ee and Cd (Figs. 2.7 and... 

You learned earlier that Becquerel discovered radioactivity because of the effect of radiahon on photographic plates. Since this discovery, several other methods have been devised to detect radiation. The effect of radiation on photographic film is the same as the effect of visible light on the film in your camera. With some care, film can be used to provide a quantitative measure of radioactivity. A film badge is a device containing a piece of radiation-sen-sihve film that is used to monitor radiation exposure. People who work with radioactive substances carry film badges to monitor the extent of their exposure to radiation. 

Emanations from radioactive substances affect photographic plates just as ordinary visible light does. Becquerel s discovery of radioactivity resulted from the unexpected exposure of such a plate, wrapped in black paper, by a nearby enclosed sample of a uranium-containing compound, potassium uranyl sulfate. After a photographic plate has been developed and fixed, the intensity of the exposed spot is related to the amount of radiation that struck the plate. Quantitative detection of radiation by this method is difficult and tedious. 

The principal detection methods involve the use of either photographic film to create an image of the location of the radioactive substance or a counter that allows the measurement of intensity of radiation emitted from some source by converting the radiation energy to an electrical signal. 

It was soon realised that this radioactivity is an atomic property with an intensity directly proportional to the concentration of the element yielding it and entirely independent of the state of chemical combination of that element. Not only do the rays affect a photographic plate, but they induce ionisation in air and thus assist the discharge of an electroscope. Hence a radioelement can be detected electroscopically no matter what chemical process it undergoes. This enormously simplifies the method of detection which is both rapid and delicate. 

In this method a second photographic plate is placed in contact with the original collector plate. The radiations emitted by any radioactive nuclide cause, after a suitable exposure, a blackening on the second plate and thus makes possible the identification of the nuclide or nuclides responsible for the radioactivity. An alternative method is to scan the first plate with a... 

The first detailed investigation of [n, p) and [n, 2n) reactions with neutrons of 9 Mev and above, was made by Waffler. He measured the cross sections for the n, p) process for a number of elements irradiated by the neutrons emitted in the forward direction from Li and B, bombarded by 500 kev deuterons. In the forward direction the former reaction, in addition to producing many low energy neutrons, produces also neutrons of 10 and I3 Mev the latter reaction produces neutrons of 9 and 13-5 Mev. The flux of neutrons was determined by a photographic plate method. The cross sections were found by measuring the radioactivities of the product nuclei. Since the neutron sources produced a rather varied energy spectrum, Waffler s results represent average cross sections, and. a strict comparison with theoretical values is difficult. His results, however, show that the n, p) process is consistently more probable than theory will allow. 

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