An instrument on a space probe uses a point source of gamma radiation to calibrate its sensors. A detector with a flat window of area 0.18 m2 is mounted at a distance of 2.4 m from the source, such that the face of the window is normal to the line joining it to the source. The source emits gamma photons isotropically (uniformly in all directions) at a constant rate of 8.0 × 104 s-1. Assuming no attenuation of gamma rays by any intervening medium, how many gamma photons enter the detector window per second?
200 s−1200\text{ s}^{-1}200 s−1
400 s−1400\text{ s}^{-1}400 s−1
800 s−1800\text{ s}^{-1}800 s−1
2500 s−12500\text{ s}^{-1}2500 s−1
