# How Chadwick concluded that the particles are neutrons but not photons?

James Chadwick conducted an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. How he concluded that the radiation was made up of neutrons and not photons?

• The paper is at royalsocietypublishing.org/doi/epdf/10.1098/rspa.1932.0112 May 20, 2022 at 19:07
• p 342 of his Nobel lecture. He observed that the knock-off atom maximum velocities were fit very well by an elementary non-relativistic kinematics law, namely elastic billiard-ball to billiard-ball collision, compared to billiard-ball to bowling-ball collision, very different than Compton scattering. This should probably be posted in HSM. May 20, 2022 at 20:03
• Can I get specific link. Actually I am just a grade 12 student, I know about collision but have not heard billiard-ball collision and Compton scattering. May 21, 2022 at 4:19
• I assume you have been able to derive the elastic collision formulas referred to with Newtonian mechanics? Compton scattering is relativistic photon-electron scattering. The neutron discovery and determination of its mass indicated is summarized in WP. Essentially, a mosquito cannot push a billiard ball, but another billiard ball can. It can even knock off a bowling ball! May 21, 2022 at 13:37

Let us suppose that the radiation consists of particles of mass M moving with velocities up to a maximum velocity V. Then the maximum velocity which can be imparted to a hydrogen atom, mass 1, by the impart of such a particle will be $$U_p={2M\over M+1} V$$ and the maximum velocity imparted to a nitrogen atom will be $$U_n= {2M\over M+14 }V$$ Then $$\frac{M+14}{M+1}=\frac{U_p}{U_n}$$ The velocities $$U_p$$ and $$U_n$$ were found by experiment. The maximum range of the protons ejected from paraffin wax was measured and also the ranges of the recoil atoms produced in an expansion chamber filled with nitrogen. From these ranges the velocities $$U_p$$ and $$U_n$$ can be deduced approximately: $$U_p\approx 3.7 \times 10^9$$ cm/sec, $$U_n\approx 4.7 \times 10^8$$cm/sec. Thus we find M = 0.9. We must conclude that the beryllium radiation does in fact consist of particles, and that these particles have a mass about the same as that of a proton.