| In 1930 the two German scientists Walter Bothe and H. Becker bombarded beryllium (94Be) with the alpha particles (the nucleuses of helium). They observed the emission of a very hard radiation. The radiation was not deflected in the magnetic field and could penetrate even a few centimetres thick layer of lead. The scientists assumed that it was a high energetic gamma radiation. But soon they discovered that its energy was much higher than any gamma radiation could have. The scientists also noticed that the radiation knocked protons out the paraffin slug (Irena Joliot Curie and Frederic Joliot Curie). |
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In 1932 James Chadwick noticed that the observed radiation was no gamma radiation but neutral particles - called the neutrons. Their mass was very similar with the mass of the protons. According to him in the experiment conducted by Bothe and Becker the shown below reaction proceed:
(1)
Thanks to the fact that they are neutral the neutrons can penetrate thick layers of different materials (eg. lead). That is because their movement is not disturbed by the positively or negatively charged particles of the penetrated material. The mass of the particles is big enough to cause the movement of the light nucleuses when colliding with them.
It was also discovered that another chemical element boron bombarded with the alpha particles also emits neutrons. The formula describing such reaction is shown below:
(2)
Chadwick could exactly calculate the mass of neutrons knowing the mass of 115B and 147N thanks to the Aston's measurements.
Through many years of the beginning of our century there was a common thinking that atom consisted of a positive nucleus (consisting of some number of positive protons and a smaller number of negative electrons). The positive charge of the nucleus was to by a resultant of the positive and negative charges inside it. What more there were some negative electrons circulating around nucleus keeping the whole atom neutral. Let's take an example of deuterium (an isotope of hydrogen). According to that theory it should consist of the two positive protons and the one negative electron. So finally the nucleus of deuterium had a charge of +1e. That is how the scientists tried to explain the subsistence of isotopes.
When Chadwick discovered the neutral neutron there was a new theory created. According to it the atom nucleus consist of protons and neutrons. The number of the neutrons in the nucleus is the same as the assumed before number of the electrons in it. So now the nucleus of deuterium consist of a one proton and one neutron. The further researches proved that there are no electrons in the nucleus.
The scientists have found some different methods for free neutrons creation. The first method is the one described above. The bombarding the tritium target (31H) with the nucleuses of deuterium (21H) is the second method:
(3)
Another way of producing neutrons is causing interaction between the gamma radiation and the nucleuses of atoms, eg.:
(4)
The slow neutrons can also interact with the nucleuses of different atoms and be absorbed by them. Such absorption is followed by the emission of gamma radiation:
(5)
