Purpose: To evaluate the effect on charge collection in the ionization chamber (IC) in proton pencil beam scanning (PBS), where the local dose rate may exceed the dose rates encountered in conventional MV therapy by up to three orders of magnitude. Methods:We measured values of the ion recombination (k s ) and polarity (k pol ) correction factors in water, for a plane-parallel Markus TM23343 IC, using the cyclotron-based Proteus-235 therapy system with an active proton PBS of energies 30-230 MeV. Values of k s were determined from extrapolation of the saturation curve and the Two-Voltage Method (TVM), for planar fields. We compared our experimental results with those obtained from theoretical calculations. The PBS dose rates were estimated by combining direct IC measurements with results of simulations performed using the FLUKA MC code. Values of k s were also determined by the TVM for uniformly irradiated volumes over different ranges and modulation depths of the proton PBS, with or without range shifter. Results: By measuring charge collection efficiency versus applied IC voltage, we confirmed that, with respect to ion recombination, our proton PBS represents a continuous beam. For a given chamber parameter, e.g., nominal voltage, the value of k s depends on the energy and the dose rate of the proton PBS, reaching c. 0.5% for the TVM, at the dose rate of 13.4 Gy/s. For uniformly irradiated regular volumes, the k s value was significantly smaller, within 0.2% or 0.3% for irradiations with or without range shifter, respectively. Within measurement uncertainty, the average value of k pol , for the Markus TM23343 IC, was close to unity over the whole investigated range of clinical proton beam energies. Conclusion: While no polarity effect was observed for the Markus TM23343 IC in our pencil scanning proton beam system, the effect of volume recombination cannot be ignored.

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Theories attempting to explain supralinearity observed in thermoluminescent dosimeters (TLDs) are typically concerned with trap creation [1], track interaction [2], and pre-existing c-hit trap structures [3]. The present work extends the last concept and explores its implications for heavy-ion response.We describe the response of a c-hit detector to gamma-rays by two parameters-the hittedness, c, and the characteristic dose, E 0 , of gamma-rays at which there is an average of one hit per sensitive element. To calculate the response of such a detector to heavy charged particles we also need to know a third parameter-the radius of the sensitive element, a 0 . We take a 0 to be related to the size of the sensitive region around a trapping site. The radial distribution of local dose around the path of a heavy ion is then used as a transfer function, relating the low-LET response of a detector to its high-LET response [4].In earlier work [3] it was suggested that the supralinear response of TL materials for gamma irradiations can be represented by a sum of c-hit components, implying the existence of corresponding (but otherwise undefined) trap structures in this detector. In support of this concept, we note the near-quadratic (2-hit) X-ray response found by Crittenden et al. [5] for peaks 5 and 6 in BDH LiF doped with 80 ppm Mg and Ti, measured at a ramp speed of 20°C min -1 . Peak 6 in the material doped with 3 ppm Ti and measured at 420 nm wavelength appears to have a near 2-hit response saturating at about 800 krad.Decomposition of experimentally measured responses of TLDs to gamma-or X-rays can yield the values of hittedness, characteristic dose, and relative contribution of each component, but cannot provide us with any information on the size of the sensitive element. The value of a 0 must be inferred from the response to high-LET radiations.The response of LiF to fast charged particles has been measured for protons, alpha-particles, and some heavier ions (C, O, and Ne) for TLD-100 [6,7] and TLD-700 [8]. The efficiency of LiF, if plotted as a function of the stopping power of the charged particle, appears to be of the order of that for 60 Co at lower values of LET and then to decrease with increasing LET [6][7][8][9][10].Enhancement of the ratio of heights of peak 6 to peak 5 is observed for LiF irradiated with alpha-particles [9,10] and fast neutrons [II], compared with the ratio for gamma-irradiation. A decrease of supralinearity and loss of sensitivity, relative to gamma-irradiation, are seen for LiF (TLD-700) exposed to 3.7 MeV alpha-particles and 13.3 MeV protons stopping in the material [10]. AbstractTrack theory has been applied to an earlier suggestion, that the supralinearity of TLDs at high gamma-ray doses is due to pre-existing 1-hit and 2-hit trap structures, to calculate high-LET response. Measured 60 Co dose responses for peaks 5 and 6 in LiF (TLD-700) were decomposed to yield parameters characterizing each peak as a two-component c-hit mixture. One value of "trap radius" was assigned to each two...

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