Pulsed Electron Beam Dosimetry

The use of pulsed electron sources capable of delivering doses of tens of megarads in periods of 10 to 40 nanoseconds presents unique problems from the standpoint of dosimetry. To conduct experiments in this environment it is usually desirable to measure the spatial variation of the incident flux as well as the absorbed dose. At present the larger available pulsed electron machines may give poor exposure reproducibility at constant source-to-sample distance. This requires that dosimetry be performed with each exposure and necessitates the use of a "thin" in-line dosimeter. Measurements of absorbed dose are readily obtained by "thin calorimeter" techniques. A practical limitation to this type of measurement is its insensitivity to point variations in the incident flux. Changes in transmittance, or optical density, of materials such as blue cello-phane or red Cinemoid provide a means of mapping the exposure with reasonable precision. Comparative dosimetric studies were made with titanium, aluminum, and copper calorimeters with thickness as a variable. An in-line pair calorimeter experiment showed that a calorimeter can be used to measure the surface dose to a sample provided the effects of scattering(including back-scattering) and absorption are considered. The change in transmission of the dyed films for a given amount of energy absorbed showed a strong dependance on the ambient oxygen concentration. The oxidation effect is obvious when one compares pulsed and steady-state irradiation in air and in vacuum. Therefore, ambient conditions must be considered in a calibration.