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Water Calorimetry of High-Energy Electron Beams at Room Temperature Under Steady State Conditions

R Tosh

R Tosh*, F Bateman, H Chen-Mayer, National Institute of Standards & Tech, Gaithersburg, MD

TH-E-BRB-8 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom B

Purpose: The present work summarizes preliminary dosimetry measurements, obtained via room-temperature water calorimetry, of high-energy electron beams available from a Clinac 2100C. Our objective is to assess any limitations to the use of the existing photon standard with electron beams at 6, 9, 12, 16 and 20 MeV.

Methods: Vertical 12-, 16- and 20-MeV electron beams, provided by a Clinac 2100C, were preset to deliver a nominal dose rate at dmax in water of 2.4 Gy/min. The calorimeter system, whose operation has been described earlier, was used essentially as it has been used with photon beams; however, for this preliminary work the sensor depth was fixed at 5 cm (thus, SDD = 105 cm, SSD = 100 cm). With this arrangement, we were able to vary both the dose rate and the gradient in the falloff region overlapping the vessel, and thereby subject the calorimeter to conditions that approximate normal dosimetric conditions (at near-reference depth) but also, in the case of 12 MeV, test the importance of gradient effects on the calorimeter response. Several runs were conducted at each beam quality, and for each run, 20 cycles of radiation (each cycle consisting of one minute on, one minute off) were delivered.

Results: Dose measurements obtained from the calorimeter at each of the three qualities generally follow expectation, based upon depth-dose profiles of the respective beams. More detailed comparison with preliminary results from runs we conducted with a Farmer-type cylindrical chamber show small but significant discrepancies, indicating a possible systematic error within the calorimeter system.

Conclusions: Follow-on studies are currently underway to address these discrepancies both in terms of the calorimeter response and reference dosimetry with the ionization chamber.

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