why is there several sudden decrease in the mass absorption coefficient at certain wavelength of radiation?

Answer 1

My guess would be because some types of materials or shielding its passing through or being absorbed into are more or less resistant than others. An uneducated guess but a guess none the less. Otherwise I would say it could be because of the attenuation coefficient which is the number characterizing the extent of interaction of photons of specified gamma (or X) rays in their passage through a material. The linear attenuation coefficient is a measure of the photon interaction per unit thickness of material and is stated in units of reciprocal length (or thickness). The mass attenuation coefficient is equal to the linear attenuation coefficient divided by the density of the material. Any way thats my guess. Its all in what the material is and the intensity of the wavelength of the radiation going through it.