Question

how neutron was discovered?

Answer 1

The neutron was discovered until 1932 by James Chadwick used scattering data to calculate the mass of this neutral particle...
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Answer 2

Chadwick discovers the neutron 1932

         For four years, James Chadwick was a prisoner of war in Germany. When World War I ended, he returned 
         to his native England to rejoin the mentor of his undergraduate days, Ernest Rutherford. Now head of 
         Cambridge University's nuclear physics lab, Rutherford oversaw Chadwick's PhD in 1921 and then made him 
         assistant director of the lab.

         Chadwick's own research focused on radioactivity. In 1919 Rutherford had discovered the proton, a positively 
         charged particle within the atom's nucleus. But they and other researchers were finding that the proton did not 
         seem to be the only particle in the nucleus.

         As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the 
         nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the 
         atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. 
         Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding 
         to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as 
         well -- the protons still contributed their mass but their positive charge was canceled out by the negatively 
         charged electrons. So in the helium example, there would be four protons and two electrons in the nucleus to 
         yield a mass of 4 but a charge of only 2. Rutherford also put out the idea that there could be a particle with 
         mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. There was no 
         evidence for any of these ideas.

         Chadwick kept the problem in the back of his mind while working on other things. Experiments in Europe 
         caught his eye, especially those of Frederic and Irene Joliot-Curie. They used a different method for tracking 
         particle radiation. Chadwick repeated their experiments but with the goal of looking for a neutral particle -- 
         one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to 
         determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's. He 
         published his findings with characteristic modesty in a first paper entitled "Possible Existence of Neutron." In 
         1935 he received the Nobel Prize for his discovery.

         His findings were quickly accepted and Werner Heisenberg then showed that the neutron could not be a 
         proton-electron pairing, but had to be its own unique particle -- the third piece of the atom to be found. This 
         new idea dramatically changed the picture of the atom and accelerated discoveries in atomic physics. 
         Physicists soon found that the neutron made an ideal "bullet" for bombarding other nuclei. Unlike charged 
         particles, it was not repelled by similarly-charged particles and could smash right into the nucleus. Before long, 
         neutron bombardment was applied to the uranium atom, splitting its nucleus and releasing the huge amounts of 
         energy predicted by Einstein's equation E = mc2.

         His paper describing his discovery is given below.