Pre-clinical studies of the negative pi-meson beam at TRIUMF

The pi- flux from the biomedical channel at TRIUMF increases with increasing channel momentum, while the contaminating electron flux decreases. Since the electrons appear to result from conversion of the high energy gamma-rays produced by pi0 decay in the production target, the electron contamination can be reduced further by target configurations which minimize gamma conversion. The attenuation of pi- beams by in-flight interactions was found to decrease from an initial value of 1.67 +/- 0.02% per g/cm2 at zero depth to 1.48 +/- 0.02% per g/cm2 near the stopping peak. The inactivation of cultured CHO cells by an extended-peak pi- dose distribution has been measured using the gel technique. The survival data have been fitted by a model which characterizes the physical quality of the dose profile by means of measured star densities. This model provides a convenient method of analysis for large sets of survival data and may be useful for prediction of the biological effect of new pi- dose distributions. The RBE value for 50% survival measured at the centre of a 7 cm extended peak was found to be approximately 1.4, in reasonable agreement with recent values obtained at LAMPF and SIN.