Treatment units for radiosurgery, like Leksell Gamma Knife and adapted, or dedicated, linear accelerators use small circular beams of ionizing radiation down to 4 mm in diameter at the isocenter. By cross-firing, these beams generate a high dose region at the isocenter together with steep dose gradients of up to 30% per mm. These units are used to treat small complex shaped lesions, often located close to critical structures within the brain, by superimposing several single high dose regions. In order to commission such treatment units for stereotactic irradiations, to carry out quality assurance and to simulate treatment conditions, as well as to collect input data for treatment planning, a precise dosimetric system is necessary. Commercially available radiation dosimeters only partially meet the requirements for narrow photon beams and small field sizes as used in stereotactic treatment modalities. The aim of this study was the experimental determination of the output factors for the field defining collimators used in Gamma Knife radiosurgery, in particular for the smallest, the 4 mm collimator helmet. For output factor measurements a pin point air ionization chamber, a liquid ionization chamber, a diode detector, a diamond detector, TLD microcubes and microrods, alanine pellets, and radiochromic films were used. In total, more than 1000 measurements were performed with these different detection systems, at the sites in Munich and Zurich. Our results show a resultant output factor for the 4 mm collimator helmet of 0.8741 +/- 0.0202, which is in good agreement with recently published results and demonstrates the feasibility of such measurements. The measured output factors for the 8 mm and 14 mm collimator helmets are 0.9578 +/- 0.0057 and 0.9870 +/- 0.0086, respectively.

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An atomic beam of tungsten and photoexcitation with monochromatized synchrotron radiation were used to measure the first photoion yield spectra of W+ and W + in the region of 30 -60 eV. The comparison with calculated photoionization cross sections shows that the resonance structure can be explained by discrete Sp~Sd, 6s and 4f~Sd transitions if one takes into account the thermal population of the initial states 5d 6s Dp & 234 and 5d 6s 53.

An arrangement is described for determining the relation between the energy imparted to ferrous sulphate (Fricke) solution by total absorption of 5.6 MeV electrons and the yield of ferric ions produced in the solution. Since the energy imparted is derived from measurements of basic quantities, the method represents an absolute determination of the absorbed dose to water. By means of ampoules filled with calibrated Fricke solution the beam of a cobalt source was measured to serve as a PTB standard for the quantity absorbed dose to water under specified conditions. The total uncertainty of this method of determining the absorbed dose to water for electrons and photons in the energy range above 1 MeV appears to be smaller than that of the calorimetric method, which is also applied in this laboratory for photon radiation.

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