Point source, energy fluence rate and photon question

[johnq2k7]

Imagin that point source is positioned at the origin of a sphere. Assume a monoenergetic isotropic photon source, emitting 10^9 photons s^-1, each with a frequency of 2.7 x0 10^20 Hz.


a.)Calculate the energy fluence rate [MeV cm^-2 s^-1] at a radial distance of 20 cm from the source. Calculate the corresponding photon fluence crossing a spherical surface elemental area at a radial distance of 20 cm over a time period of 10 seconds.


b.) Calculate the number of photons reaching a detector subtending polar angles between 20 and 25 degrees, placed at a radial distance of 5 cm, after a collection time of 1 minute.




need a lot of help here, please help!

 

[BigJDubs]

a.) The energy fluence rate can be calculated as: Energy Fluence Rate = (10^9 photons/s) * (2.7 x 10^20 Hz) * (1 MeV / 6.242 x 10^18 Hz) = 4.51 x 10^3 MeV cm^-2 s^-1 The corresponding photon fluence crossing a spherical surface elemental area at a radial distance of 20 cm over a time period of 10 seconds can be calculated as: Photon Fluence = (4.51 x 10^3 MeV cm^-2 s^-1) * (10 sec) = 4.51 x 10^4 MeV cm^-2 b.) The number of photons reaching a detector subtending polar angles between 20 and 25 degrees, placed at a radial distance of 5 cm, after a collection time of 1 minute can be calculated as: Number of Photons = (4.51 x 10^3 MeV cm^-2 s^-1) * (60 sec) * (cos(25) - cos(20)) = 1.17 x 10^5 photons