Greetings! We are working on the production of Ac-225 via proton beam hitting Th-232 target. And recently we want to extract the cross section of Ac-225 production from the fluka results. First, we choose the beam intensity as 1e+12 p/s with 1 sec radition time, so that the decay of Ac-225 and other elements can be mostly avoided. The target is set as a cylinder with 3mm thickness. And the material density is set as 11.899. Then at the cooling time 0sec, we found the activity density for Ac-225 is 1.98E+01 Bq/cm^3 (the target volume is set as 1.0 cm^3 in the input file). Since not all the beam protons really involved the inelastic scattering, we count the protons running out from the target, and obtain the ratio 0.98137175136. Also we obtain the total cross section of proton scattering Th-232 as 4713 mb. And the Ac-225 lambda for converting from activity to element number is 8.087234689552827e-07. Therefore, we calculate the cross section for the Ac-225 production as following
sigma = 1.98E+01 / 8.087234689552827e-07 / ( 1e+12 * (1-0.98137175136) ) * 4713 = 6.194276073460916 mb
The problem appears when we change the target density into 118.99. We obtain the changed activity density as 2.66E+01 Bq/cm^3 and the ratio of proton running out from target is 0.00013069056, which seems reasonable with much higher density. Then we calculate the cross section for the Ac-225 production
sigma’ = 2.66E+01 / 8.087234689552827e-07 / ( 1e+12 * (1-0.00013069056) ) * 4713 = 0.155037155568 mb
The obvious change on the cross section surely does not make sense. So we were wondering if we have missed something in our calculation. Thank you.