The relationship of x-ray energy in the beam with thickness of matter is exponential: this means a fixed percentage of x-ray photons will be removed with each centimeter of thickness.

For example, for a 100 keV x-ray beam, 1 cm of water will attenuate 17% (0.17) of the x-ray photons in the beam.

If we start with 100 x-ray photons in the beam, 17 will be removed in 1 cm of water, leaving 83. for the second cm of water, 17% of the remaining 83 will be removed, 14.1, leaving 68.9, etc.

An exponential relationship is where equal changes of one quantity (thickness of material in this case) give equal percentage changes in another quantity (number of x-ray photons transmitted). If a graph were plotted of thickness of material against the logarithm of the transmitted x-ray photons, a straight line would be produced.

MATERIAL	DENSITY	ELECTRON DENSITY	EFFECTIVE ATOMIC NUMBER	PHOTON ENERGY	SIZE="2" FACE="SYMBOL" >m/cm as percent attenuated
Water	1.0	3.343	7.5	100 keV	0.17
				10 MeV	0.03
Bone	1.65	3.19	12.3	100 keV	0.3
				10 MeV	0.04
Lead	11.35	2.38	82	100 keV	62
				10 MeV	4.3

The charts below list the x-ray absorption for each of two photon energy levels; 10.0 MeV and 5.0 MeV. Each line entry is for a thickness of water (depth) expressed in centimeters. Note the depth of water required for absorption of 50% of the x-ray dose for each of the two energy levels of x-rays.

Thickness of Water cm	Number x-ray photons attenuated %	Number of x-ray photons transmitted %
0	0	100
1	3	97
2	5.9	94.1
3	8.7	91.3
-
-
-
-
-
-
23	51.2	48.8

Thickness of Water cm	Number of x-ray Photons Attenuated %	Number of X-ray Photons Transmitted %
0	100	100
1	4.5	95.5
2	8.8	91.2
3	12.9	87.1
4	16.8	83.2
5	20.5	79.5
6	24.1	75.9
7	27.5	72.5
8	30.8	69.2
-	-	-
15	49.8	50.2