external exposure facility PTB Microbeam Paul Schofield The PTB accelerator facility (PIAF) for ion and neutron research at Braunschweig operates a charged-particle microbeam that has been in routine use for over 15 years, using a 3 MV tandem accelerator and compact cyclotron to deliver protons and alpha particles with LET values from approximately 3 to 200 keV/μm. The sub-2 μm focused beam — achieved by quadrupole magnet focusing with a unique 90° bending magnet design that eliminates scattered particles — allows targeted irradiation of individual cell nuclei, cytoplasm or organelles with single counted ions at rates up to 50,000 cells/hour. In addition to microbeam work, the facility offers broad-beam proton and alpha irradiations and neutron field exposures, and has been used extensively in European framework programmes. Located at PTB. K. D. Greif et al., Nucl. Instr. and Meth. B 217 (2004) 505-512 website