why lead is better shield against radiations than water of equal coloumn?
Answers
I assume you’re referring to ionizing radiation. It’s a little complicated.
Alpha, Beta, and Gamma radiation, as well as other charged particles, such as Positrons, are not effectively shielded by water. Lead provides an effective shield for Gamma radiation.
Materials with a low atomic number Z are appropriate as beta particle shields. With high Z materials bremsstrahlung (secondary radiation – X-rays) results. This radiation is created during slowing down of beta particles while they travel in a very dense medium. Plexiglass, thick cardboard or thin aluminum plate will provide protection from beta radiation and will prevent production of bremsstrahlung.
Alpha radiation can be shielded by a piece of paper, so no problem.
Lower energy (slow) Neutrons may be shielded with materials that will absorb them (such as Cadmium or Hafnium). However, these materials are made radioactive by the neutrons, so additional shielding must be added to shield the radiations (Beta or Gamma, for instance) subsequently emitted.
High energy (fast) neutrons must be slowed down before they can undergo absorption. This is done by the fast neutrons losing energy by undergoing many collision type interactions with particles near the size of the neutron, like a Hydrogen nuclei. So materials like water or paraffin, which contain a lot of hydrogen atoms, are suitable for this.