Why couldn't the mysterious rays coming out of Beryllium foil in Chadwick's experiment on being hit by alpha particles have been gamma rays?
Gamma photons are massless so we can't expect them to hit and displace a heavier particle like proton with such high energy.
In his experiment, Chadwick observed some mysterious uncharged rays originating from the Beryllium target and hitting the wax and ejecting protons.
These rays were earlier believed to be gamma as they were neutral but Chadwick reasoned that gamma was just another electromagnetic radiation/photon, and he said that it was impossible for a photon to expel such a heavy particle such as proton itself with this much high energy.
Understand by this analogy.
If I tell you that something hits a billiard ball and it moves with the very high comparable velocity.
See which of the below mentioned cases are possible.
Case I
A dust speck (very tiny particle) is thrown towards a billiard ball with very high velocity. Can their velocities be interchanged? Can after collision the billiard ball have as large energy as compared to the minute dust particle?
Case II
A billiard ball with same mass hits the given billiard ball and interchanging of velocity happens. Elastic collision, remember.
Because of this Chadwick had his doubts and this lead to the discovery of neutron, a particle with similar mass as proton but no charge.
FYI, EM waves don't get deflected in magnetic or electric field and gamma rays also travel in straight line that's why the term " rays".