Liquid ionization chambers for radiation therapy

Liquid ionization chambers are described to be used as monitors in radiation therapy. The development focuses on two areas of application, an on-line monitor for intensity modulated radiation therapy and the measurement of the energy deposit of heavy ions in tissue. This requires a thin detector with two-dimensional read-out and an absorbing chamber with three-dimensional read-out, respectively. Both detectors are realized using similar techniques. First measurements with prototype chambers are described. The liquids isooctane, isononane (TMP), and tetramethylsilane (TMS) are used in a high purity grade in order to realize high current signals for electronic read-out in parallel at frequencies exceeding 10 Hz. Signals of more than a factor 4 with respect to isooctane, analysis grade, are obtained. With an electrode structure of 400 pads in a two-dimensional geometry an uniformity in efficiency within 1.2 % has been measured.