Hello. I'm a little unsure of how to proceed on this problem... Here it is: A 3-Gev proton flux is monitored by measuring 24Na activity induced in 25 microm (6.85 microg/cm2) aluminum foil via 27Al(p, 3pn)24Na reaction (for 3 Gev protons, cross section = 9.1 mb). Exactly 15 hr after the end of 15-minute irradiation, the activity in the aluminum monitor is measured with a calibrated end-window counter (efficiency= 0.037 in the geometrical arrangement used). The net counting rate is 27,430 counts/min. What is the average proton flux through the sample during the irradiation? So what I did first was correct for efficiency and found the actual counts to be 741351 cpm or 12355 cps. Then since the count was made 15 hrs after bombardment, the activity at the end of the bombardment was 24710 cps. At this point, I'm stuck because I don't know how to handle the areal density and convert it into the number of atoms in the beam. Can this problem even be solved without knowing the area of the beam? I pretty sure I need to find the number of atoms of Co that are in the beam, right? Any help is appreciated. Thanks.