[LHM]

It is found that when a 6.3*10^-5 g sample of radium-226 is placed in a vacuum and exposed to a screen of area 1.0 mm² placed 1.5 m away from the sample, an average of 46 alpha particles hit the screen in 10.0 minutes. During this short time ew can assume that only the first step in the decay chain occurs. Calculate th total number of decays per gram of radium-226 per second.

The solution starts aout by saying that there are 46/(10*60*6.3*10^-5) alpha particles emitted by 1 g in 1 second and hitting the screen. But then they multiply that by 4*pi*(1.5*10³)², but where do they get that from?

[Borek]

Sphere surface.