The new proton therapy facility is being assembled at the Paul Scherrer Institute (PSI). The beam delivered by the PSI sector cyclotron can be split and brought into a new hall where it is degraded from 590 MeV down to an energy in the range of 85-270 MeV. A new beam line following the degrader is used to clean the low-energetic beam in phase space and momentum band. The analyzed beam is then injected into a compact isocentric gantry, where it is applied to the patient using a new dynamic treatment modality, the so-called spot-scanning technique. This technique will permit full three-dimensional conformation of the dose to the target volume to be realized in a routine way without the need for individualized patient hardware like collimators and compensators. By combining the scanning of the focused pencil beam within the beam optics of the gantry and by mounting the patient table eccentrically on the gantry, the diameter of the rotating structure has been reduced to only 4 m. In the article the degrees of freedom available on the gantry to apply the beam to the patient (with two rotations for head treatments) are also discussed. The devices for the positioning of the patient on the gantry (x rays and proton radiography) and outside the treatment room (the patient transporter system and the modified mechanics of the computer tomograph unit) are briefly presented. The status of the facility and first experimental results are introduced for later reference.

Antiprotonic X-rays from the helium isotopes have been observed at pressures of 36, 72, 375 and 600 mbar. The antiproton beam from LEAR with momenta of 309 and 202 MeV/c has been stopped at these pressures using the cyclotron trap. The X-rays were detected with Si(Li) and intrinsic Ge semiconductor detectors. Absolute Xray yields were determined and the strong-interaction 2p shifts and the 2p and 3 d broadenings measured to be Ezp=(-17_+4)eV, F2p=(25_+9)eV and Fad=(2.14 _+0.18)meV for /~3He and ezp=(-18+_2)eV, F2. =(45_+5) eV and Fad =(2.36 +0.10) meV for/54He.

The ionization of antiprotonic noble-gas atoms due to formation and Auger deexcitation was investigated. The experiment was performed at low target pressures (p & 50 hPa) in order to prevent electron refilling from neighboring atoms. For this reason, the cyclotron trap was used. This device is designed to stop as many particles as possible in a small stop volume at even low gas pressure.The degree of ionization was determined in antiprotonic neon, argon, and krypton by means of measuring yields of circular radiative transitions at relatively high principal quantum numbers. X rays were observed between 1.6 and 21.0 keV using a Si(Li) detector. Complete ionization was found in all three cases.

X-rays of Balmer and Lyman transitions in antiprotonic hydrogen and of Balmer transitions in antiprotonic deuterium were observed at pressures below 300 hPa using Si(Li) semiconductor detectors. The measurement was performed at the LEAR-facility at a beam momentum of 202 MeV/c. In order to stop antiprotons in a low pressure gaseous target with high efficiency, a novel technique, the cyclotron trap has been used. Absolute yields were determined and compared with cascade calculations. A distinct difference in the cascade of antiprotonic hydrogen and deuterium is found. The parameters of strong interaction in antiprotonic hydrogen are determined to be 81~ = -(620 + 100)eV, Fls= (1130 _+ 170) eV and F2p = (32-t-10) meV.