LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 2
MATERIALS AND METHODS
primary proton beam from the FOTIA was collimated using an adjustable slit to reduce the
fluence and then diffused using a gold foil. The diffused beam was channeled to the exit window
made of 20 micrometer titanium foil of 3 cm diameter to get uniformly distributed irradiation
area. The samples were thus irradiated under normal atmospheric pressure at 24 o C. The target to
be irradiated was positioned at a distance of 11 mm from the exit window, that being the closest
possible place to mount the target. Before the irradiation, a silicon surface barrier (SSB) detector
was positioned at the same position where samples were irradiated and the beam energy as well
as the uniformity was measured. Another SSB detector placed inside the scattering chamber at a
forward angle of 40 o to the primary beam, after the gold foil, served as a monitor detector. The
ratio of the monitor detector counts to that of the flux measured using SSB detector at the sample
position was measured by multiple trials and the calibration factor was obtained. Monitor
detector counts and the measured ratio were used for delivering the required fluence to the
samples. Flux was calculated to be 10 8 and the fluence was kept such that each cell is hit by at
least 100 proton particles ensuring that bystander effect is kept to minimum. Fluence was then
used to calculate dose with the help of specific ionization curve constructed for the given energy
distribution and composition of the cellline. The average energy of the proton beam used in this
study was measured to be 3.2 MeV and corresponding estimated LET is 12.5 keV/µm within the
cell layer. Initial portion of Bragg’s curve was used for dose estimation since the cell layer used
for irradiation was in monolayer geometry with thickness less than 10µm, which helped to avoid
steep increase in LET within the sample.
The cells were grown in monolayer geometry on 35mm petri dishes. Media was removed and
petridishes were mounted on the exit window vertically for the irradiation, after which they were
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