In 1932, John Cockcroft and Ernest Walton carried out an experiment in which they used high-energy protons to split a lithium nucleus.
Cockcroft and Walton experiment Apparatus
Cockcroft and Walton’s apparatus
Hydrogen discharge chamber, evacuated tube, high d.c. voltage (anode and cathode), lithium target, phosphorus screen
Procedure
Protons produced in a hydrogen discharge chamber are injected into the tube.
700 kV
Protons are accelerated towards the lithium target by the high d.c. voltage.The electric field exerts a force on the protons.The voltage is noted so that the energy of the incident proton can be calcu- lated.
Evacuated tube
After the protons strike the lithium target, the positions where the alpha particles (produced in the nuclear reaction) hit the zinc sulphide screen are noted.
High voltage
The velocities and masses of the alpha particles are also noted.
Observation
There are scintillations seen on the zinc sulphide screen.
0 V -particle -particle Conclusion
Some lithium nuclei are transmutated when struck by a proton, changing into two helium nuclei (alpha particles) in accordance with the equation:
1 1H 3 7Li : 2 4He 2 4He
Zinc sulphide screens Lithium target Fig 23.3: Apparatus for the Cockcroft and Walton experiment Microscope
By analysis of position and momentum of the alpha particles, Einstein’s mass-energy equivalence equation (E mc2) can be proven.
