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A chloral hydrate-agar gel has been used for depth-dose measurements with x-rays and electrons. HCl is formed from the chloral hydrate by the radiation and thus changes pH and electrical conductivity of the gel. Radiation beams can be visualized by adding an acid-base indicator. Quantitative measurements are made by spectrophotometry or by probing the gel with a pH electrode or a conductivity electrode pair. Spatial resolution of perhaps 0.2 mm has been obtained with conductivity electrodes. Doses of 2000 rad or more are required to obtain satisfactory records, the lower limit being set by inhomogeneities in the unexposed gel.

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