Question

why boron absorbs neutrons? Why other elements don't show this kind of property?

Answer 1

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning.

Answer 2

t's boron-10 that is the good neutron absorber. Boron-11 has a low cross section for neutron absorption.

The size of the nucleus isn't terribly relevant because neutrons are quantum objects and don't have a precise position. The incident neutron will be delocalised and some part of it will almost always overlap the nucleus. What matters is the energy of the reaction:

10B+n→11B

and the activation energy for the reaction.

I'm not sure we understand nuclear structure well enough to give a quantitative answer to this. However neutrons, like all fermions, like to be paired and 10B has 5 neutrons while 11B has 6 neutrons. So by adding a neutron we are pairing up the neutrons and completing a neutron shell. We would expect this to be energetically favourable.

This argument would apply to any nucleus with an odd number of neutrons, but 10B is a light nucleus so we expect the effect to be particularly big. The lightest such nucleus is 3He, with one neutron, and that has has an even bigger neutron absorption cross section. However practical considerations rule out the use of 3He as a neutron absorber. 6Li, with three neutrons, also has a reasonably high cross section, though it is less than boron and helium.