Why did James Chadwick use berryllium to discover neutrons? In Rutherford's alpha particle scattering experiment he used gold foil because it's malleable and can be drawn into very thin sheet.
Why did James Chadwick use Berryllium while discovering neutrons then?
He could have used Gold as well.
 A: Chadwick didn't discover the neutron on purpose, of course. After the discovery of the nucleus by Rutherford in 1911, alpha particles were used to probe its structure. These kind of experiments were pioneered by Rutherford himself (as an example, he discovered in 1917 that the nitrogen nucleus contains hydrogen nuclei, i.e. protons). In 1930, Bothe and Becker had discovered that when hit by a stream of alpha particles, Beryllium emitted some kind of electrically neutral radiation. At first, it was thought that it was regular gamma radiation. Then in 1932 Irène and Frédéric Joliot-Curie showed that the properties of the radiation emitted by Beryllium couldn't be explained in terms of gamma rays, so Chadwick set up an experiment to confirm Joliot-Curie's observations, and he succeeded to do so.
Now let's have a look at the neutron separation energies ($S_{n}$) of the first, let's say, ten stable nuclei of the periodic table (see for example here).
Helium: 20577 keV
Litium: 7251 keV
Beryllium: 1665 keV
Boron: 11454 keV
Carbon: 18721 keV
Nitrogen: 10553 keV
Oxygen: 15664 keV
Fluorine: 10432 keV
Neon: 16865 keV
As you can see, Beryllium has the lowest neutron separation energy amongst the lightest stable nuclei. Gold has
Gold: 8072 keV
I'm not able to check the data about each and every stable element of the periodic table, but according to the plots in this document (see page 110), there are no light stable elements with $S_{n}$ lower than that of Beryllium. Light elements were used for those kind of experiments (in order to probe the nuclear structure, it's easiest to start from the bottom). It's easy to guess why the first element to be found to release neutrons was Beryllium.
A: In 1930 two German physicists Walther Bothe and Herbert Becker dicovered that when alpha particles were directed at beryllium some form of ionising radiation was produced, which they thought was gamma rays. Later Irene Joliot-Curie (Marie Curie's daughter) and her husband discovered that this radiation could eject protons from paraffin wax. Chadwick's advance was to measure the energies of the incident alpha particles and the produced protons very carefully, and show from this that the radiation couldn't be gamma rays as the energy required would be too high. The only solution was that the radiation was a neutral particle, which of course was the neutron.
So Chadwick used beryllium because he already knew it worked. The next question is why Bothe and Becker used beryllium ...
