No abstract

At particle therapy facilities with pencil beam scanning, the implementation of a ripple filter (RiFi) broadens the Bragg peak, so fewer energy steps from the accelerator are required for a homogeneous dose coverage of the planning target volume (PTV). However, sharply focusing the scanned pencil beams at the RiFi plane by ion optical settings can lead to a Moiré effect, causing fluence inhomogeneities at the isocenter. This has been experimentally proven at the Heidelberg Ionenstrahl-Therapiezentrum (HIT), Universitätsklinikum Heidelberg, Germany. 150 MeV u(-1) carbon-12 ions are used for irradiation with a 3 mm thick RiFi. The beam is focused in front of and as close to the RiFi plane as possible. The pencil beam width is estimated to be 0.78 mm at a 93 mm distance from the RiFi. Radiographic films are used to obtain the fluence profile 30 mm in front of the isocenter, 930 mm from the RiFi. The Monte Carlo (MC) code SHIELD-HIT12A is used to determine the RiFi-induced inhomogeneities in the fluence distribution at the isocenter for a similar setup, pencil beam widths at the RiFi plane ranging from σχ(RiFi to 1.2 mm and for scanning step sizes ranging from 1.5 to 3.7 mm. The beam application and monitoring system (BAMS) used at HIT is modelled and simulated. When the width of the pencil beams at the RiFi plane is much smaller than the scanning step size, the resulting inhomogeneous fluence distribution at the RiFi plane interfers with the inhomogeneous RiFi mass distribution and fluence inhomogeneity can be observed at the isocenter as large as an 8% deviation from the mean fluence. The inverse of the fluence ripple period at the isocenter is found to be the difference between the inverse of the RiFi period and the inverse of the scanning step size. We have been able to use MC simulations to reproduce the spacing of the ripple stripes seen in films irradiated at HIT. Our findings clearly indicate that pencil beams sharply focused near the RiFi plane result in fluence inhomogeneity at the isocenter. In the normal clinical application, such a setting should generally be avoided.

To fill up the GSI heavy ion synchrotron (SIS) to its space charge limit with uranium ions, a vacuum arc ion source, based on the metal vapor vacuum arc (MEVVA) IV ion source, has been developed and implemented into operation. The ion source has proven its capability in several long period beam times at the high current injector at GSI. With the ion source it was possible to exceed the space charge limit of 15 mA U4+ ions at the entrance of the linear accelerator (UNILAC). The reliability as well as the noise behavior has been improved to such a degree, that this ion source can be used for injection into an accelerator without objection. In this article we present the improvements of the ion source with the most important operational data. The emission current density of the ion source has been increased from 60 mA/cm2 for the common used GSI-MEVVA to 170 mA/cm2. This results in a full beam ion current of 156 mA at 35 kV with a fraction of fourfold charged uranium ions of 67%. The analyzed U4+ ion beam after dc postacceleration amounts to 25 mA at 131 kV which is 1.7 times higher than the requested ion beam current at the entrance of the radio frequency quadrupole. The reduced power density of the vacuum arc results in a higher efficiency and longer lifetime. Solenoids which are creating magnetic fields to enhance the charge state of the ions are no longer placed inside the vacuum system. This ion source design results in a higher availability after ion source replacement at the injector, and longer lifetime.