First results developing time-of-flight proton radiography for proton therapy applications

In proton therapy treatment, proton residual energy after transmission through the treatment target may be determined by measuring sub-relativistic transmitted proton time-of-flight velocity and hence the residual energy. We have begun developing this method by conducting proton beam tests using Large Area Picosecond Photon Detectors (LAPPDs) which we have been developing for High Energy and Nuclear Physics Applications. LAPPDs are 20cm x 20cm area Micro Channel Plate Photomultiplier Tubes (MCP-PMTs) with millimeter-scale spatial resolution, good quantum efficiency and outstanding timing resolution of <= 70 picoseconds rms for single photoelectrons. We have constructed a time-of-flight telescope using a pair of LAPPDs at 10 cm separation, and have carried out our first tests of this telescope at the Massachusetts General Hospital's Francis Burr Proton Therapy Center. Treatment protons are sub-relativistic, so precise timing resolution can be combined with paired imaging detectors in a compact configuration while still yielding high accuracy in proton residual energy measurements through proton velocity determination from nearly mono energetic protons. This can be done either for proton bunches or for individual protons. Tests were performed both in "ionization mode" using only the Microchannel Plates to detect the proton bunch structure and also in "photodetection mode" using nanosecond-decay-time quenched plastic scintillators to excite the photocathode within each of the paired LAPPDs. Data acquisition was performed using a remotely operated oscilloscope in our first beam test, and using 5Gsps DRS4 Evaluation Board waveform digitizers in our second test, in each case reading out both ends of single microstrips from among the 30 within an LAPPD. First results for this method and future plans are presented.