The response of the silicon-based linear energy transfer spectrometer (RRMD-III) to protons from 1 to 70 MeV

Experiments have been performed to measure the response of the silicon-based linear energy transfer (LET) spectrometer (RRMD-III) to protons with energies ranging from 1 to 70MeV. The beam energies incident upon RRMD-III were obtained using plastic polycarbonate degraders with mono-energetic proton beams that were extracted at 70, 40 and 25 MeV from a cyclotron. The energy loss in the degraders never exceeded 70% of the incident energy in order that the energy spread of the emerging protons created by straggling and scattering effects could be suppressed. Estimates of the LET were influenced by the asymmetric energy distribution of the degraded beam. Protons with energies less than 12.2 MeV did not reach the second Si detector in RRMD-III and were not measured. Measured values of LET were within ±7% of the expected value for incident protons that had an energy sufficient to penetrate the second Si detector in RRMD-III (E > 15.4MeV). The LET of stopping particles was computed using a lookup table listing LET vs E, and a simple stopping power formula for a particle with charge Z and energy E_R, where E _R is the energy of the stopping particle measured in the second Si detector.