I have to calculate the gammas produce due to oxygen activation in the coolant of PWR reactor core. the tally i used is F4 and FM.. but the problem is in core their are different cells within a cell. so how to specify the required cell . i have selected the whole core as a cell and then specify oxygen as a material in FM card with the reaction number 102 for(neutron, gamma) reaction. and then i multiply the result with source strength in core. but the results are 100 times more than actual ... Please help me in this regard.