AA-T1: Chadwick and the discovery of the neutron

James Chadwick (20 October 1891 – 24 July 1974) was the person to win a Nobel Prize for the discovery of the neutron inside the nucleus. 

The background to Chadwick’s work began at 1920 Ernest Rutherford’s theory that there was neutrons with the nucleus, because he found that the atomic number and the atomic mass would be explained from the existence of neutrally charged particle.

Further on in 1930 Walther Bothe and his student Herbert Becker were the first scientists to confirm Rutherford’s theory of the neutron.  While doing their experiments they found out an unusual type of gamma radiation. It was unusual because it had a much great ability to penetrate than any gamma radiation known. Bothe’s and Becker’s experiment was based on bombarding beryllium with polonium alpha particles.

In 1932 Irene and Fredric Curie did another experiment involving this unknown radiation to discover that it was absorbed by the paraffin wax and at the same time ejecting what they thought to be was a proton of high energy.                              

In 1932 Chadwick researched the radiation of boron and beryllium. Chadwick and Rutherford worked at Cambridge University together. Chadwick came up with an idea for an experiment to discover the existence of the neutron. What Chadwick did was use the unusual radiation to determine its properties after a series of tests.

A sample of beryllium was bombarded with alpha particles (this was a type of radiation which naturally occurred).This caused a strange new type of radiation, which confused physicists earlier on, to be emitted.  He did tests on this new type of radiation.  He showed that this type of radiation was not affected by magnets so it wasn’t positive or negative and it didn’t invoke the photoelectric effect (this was when photons a type of particle hits a metal surface and ejects electrons). This couldn’t prove the photoelectric effect because it didn’t contain electrons.

We can see that when the alpha particles hit the beryllium nuclei it ejects what was thought to be a proton or this unusual radiation at the time.

Alpha radiation consists of two electrons and two neutrons and it hits a beryllium atom has 4 protons and 5 neutrons. They join together to form 6 protons and 6 neutrons while giving away a neutron. The new product was in fact a carbon atom which consisted of 6 protons and 6 neutrons and therefore produced one neutron extra, which Chadwick eventually discovered from this experiment.  

In the same year he finally discovered, that this unusual radiation consisted of particles that were almost identical to the mass of protons. He assumed that the radiation contained one electron and one proton making it “neutral” or has Rutherford had mentioned,” the neutron” at Bakerian lecture in 1920. In 1935 he was awarded the Nobel Prize for his discovery of the neutron. 

His discoveries changed the shape of the atom and this lead to the development of the atomic bomb and nuclear fission. In 1943 – 1946 he was involved in the Manhattan Project (which was the construction of the atomic bomb) as Head of the British mission.

Chadwick’s article in Nature (10^th May 1932: “the existence of the neutron)

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http://en.wikipedia.org/wiki/James _Chadwick

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The discovery of the neutron (James Chadwick’s remarkable experiment)

http://www.suite101.com/content/the-discovery-of-the-neutron-a46060

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Cambridgephysics.com

http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron5_1.htm

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