100 = (4)2
100 = 16
100 = 4
4(I2) = 100
I2 = 25 R/min
By doubling the distance from the x-ray tube, the intensity of the beam can be reduced
to one-fourth its original value--100 R/min to 25 R/min. This is an impressive reduction
in intensity that can be used to advantage by the x-ray specialist in keeping his
exposure to radiation to the barest minimum. As stated in the definition of the inverse
square law, the formula is applicable only to radiation from a very small source such as
the target in an x-ray tube.
4-16. PHOTON ENERGY FACTOR
A factor that influences photon absorption or beam attenuation is the energy level
of the photons. The higher the photon energy, the more penetrating is the x-ray beam,
regardless of the material used for shielding. As kVp is increased, photon energy is
increased, causing more penetration. Figure 4-8 shows three blocks of absorbing
material. As can be seen, more of the high-energy photons penetrate the absorber than
do the low-energy photons.
Figure 4-8. Penetration power of photons of various energy levers.