Charged particle emission from the capture of negative pions: energy spectra, LET distributions and W-value

Abstract

The yields of charged particles that are emitted following the capture of negative pions have been measured. By using various combinations of Si and CsI(T1) detectors, the counter telescope was able to measure yields of protons, deuterons, tritons and helium nuclei down to 2 MeV and heavier nuclei down to 5 MeV. Targets of graphite, water, muscle-equivalent solution and rigid bone substitute were used, and data were taken a t a scattering angle of 90" at both the peak and the plateau of the depth-dose distribution and additionally at 45' in the plateau. The observed thick target yields showed substantial numbers of deuterons and tritons in all targets as well as helium and lithium nuclei. For the graphite target only, beryllium nuclei also were observed. The number ratio of protons to deuterons and tritons was determined from the thick target spectra in carbon to be 1.27. This disagrees with the value 3.1 calculated by Guthrie et al. Our yields show agreement however with the results of Castleberry et al. and Budyashov et al. over the narrower energy range used by these workers. A W-value for a chamber at the peak containing nitrogen is calculated to be 35.8 eV per ion pair, indicating that an ionisation chamber calibrated against low LET radiation will underestimate the peak pion dose by 3.5%. Initial secondary particle spectra and dose : LET distributions are calculated from the measured yields. No significant difference is observed in the LET : dose distributions in muscleequivalent solution and rigid bone substitute. I n muscle-equivalent solution it is found that 46% of the total dose delivered by charged particles a t the peak has an LET > 10 keV pm-l and about 29% with an LET > 30 keV pm-l. The comparable figures for the plateau are 42% and 22% respectively.

Instrumentation for treatment of cancer using proton and light-ion beams

“…14 Clinical advantages of negative pions (m1t === 267 me) are also being studied as the penetrating pions deposit a large amount of energy at the end of their flight path due to the Bragg peak and nuclear "star" formation. [5][6][7] The negative pions are the lightest member of the heavy charged-particle family. Even though the dose distributions of fast neutron beams do' not have the dose localizing advantage, 8 clinical trials are also underway using beams of fast neutrons of tens of MeV to exploit their radiobiological advantages over photons due to the fastneutron's high LET (linear energy transfer) characteristics.…”