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. Author manuscript; available in PMC: 2016 Apr 21.
Published in final edited form as: Phys Med Biol. 2015 Mar 24;60(8):R155–R209. doi: 10.1088/0031-9155/60/8/R155

Figure 3.

Figure 3

Relative fraction of the fluence Φ in a broad beam of protons remaining as a function of depth z in water. The gradual depletion of protons from entrance to near the end of range is caused by removal of protons from nuclear reactions. The rapid falloff in the number of protons near the end of range is caused by ions running out of energy and being absorbed by the medium. The sigmoid shape of the distal falloff is caused by range straggling or by stochastic fluctuations in the energy loss of individual protons.