|MadSci Network: Physics|
Theoretically, it is not possible to stop X or gamma rays completely by any thickness. Photons in X ray or Gamma rays region have infinite range. The interaction processes are always stochastic and hence, they can travel without interaction deep into the materials even with heavy density and thicknesses. The intensity of radiation reduces exponentionally as Intensity passed=(original intensity)*exp(-ux) Here, u is stopping power and x is thickness of the material. u depends on many factors, prominently on density and atomic number of the elements in the material However, the penetration capability increases with increase in energy or in other words, with increase in frequency or with decrease in wave length. The thickness required to stop X rays is always mentioned in half value or tenth value layers (HVL and TVL). Suppose, the material is two half value layers, the X ray intensity is reduced by a factor of four and if it is three layers, the intensity is reduced by a factor of eight (i.e., in the order of 2^n). The values of HVL and TVL for different materials and X ray energies can be obtained from ICRU publications. As you rightly mentioned, a single photon can escape any limit of thickness. Theoretically, it requires infinite thickness to surely stop a X ray photon.
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