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Autoradiography and Fluorography 313<br />

8. X-ray films that are not pre-exposed to light respond to radiation in a sigmoidal fashion,<br />

because the halide crystals of the emulsion are not fully activated (10). In contrast, in a preexposed<br />

film, the response becomes linear and proportional to the amount of radioactivity,<br />

therefore, allowing precise quantitative measurement of radioactivity by scanning the<br />

autoradiography (11). In addition, pre-exposure (preflashing) of the film results in a twoto<br />

threefold increase in sensitivity, for levels of radioactivity near the minimum level of<br />

detectability (7), enabling autoradiography of gels containing 14 C and 3 H radioisotopes to<br />

be performed at room temperature, instead of –70°C. To pre-expose a film, a stroboscope<br />

or a flash of light (

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