A point source of gamma radiation emits 1.8 × 105 photons uniformly in all directions every second. A flat detector window with an area of 2.5 × 10-3 m2 is positioned normal to the line connecting its centre to the source, at a distance of 1.5 m.
How many gamma photons pass through the detector window every second?
161616
323232
646464
200200200
