# HELP: Chadwick's discovery of the neutron (1 Viewer)

#### study1234

##### Member
So this is my understanding of the experiment: using a polonium source, he bombarded alpha particles into beryllium, which knocked off the unknown particles (neutrons), which knocked off protons from a paraffin block which could be detected. Then he used the laws of conservation of momentum and energy (somehow?) to calculate the mass of the unknown particle.

How did he use these laws?? He would be able to know the momentum of both the alpha particles and emitted protons, but how does this result in the mass of the neutron being calculated??

#### tutorhunt

##### Member
Firstly,
Many believed that it was gamma rays knocking off these protons .By analysing the proton-scattering frequencies,Chadwick calculated that to eject protons at such a high velocity, the rays must have an energy of 50 million electron volts, which is impossible.The alpha particles colliding with beryllium nuclei could produce only 14 million electron volts. The cons. of energy states that energy can not be created, nor destroyed, however it seemed as if energy was coming from no-where.
The energy equation of the process is
Mass of B11 + mass of He4 + K.E. of He4 = mass of N14 + mass of n1 + K.E. of N14 + K.E. of n1.
The masses are B11 = 11·00825 ± 0·0016; He4 = 4·00106 ± 0.0006; N14 = 14·0042 ± 0·0028. The kinetic energies in mass units we a-particle = 0·00565 ; neutron = 0·0035 ; and nitrogen nucleus = 0·00061. We find therefore that the mass of the neutron is 1·0067.

If someone can help me out with the cons. momentum bit, that would be much appreciated. I don't really know how to explain it!

Anyway, hope thi helps.

• BLIT2014

#### spatula232

##### Active Member
Old thread but still relevant. How did Chadwick know that the particle was neutrally charged? Is that through the conservation laws as well?

#### mrpotatoed

##### Active Member
isn't affected by an electric/magnetic field and conservation of atomic mass

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#### sy37

##### Active Member
1. Chadwick fired alpha particles towards a beryllium target

2. This interaction between the alpha particles and beryllium target produced radiation (i.e. the alpha particles knocked the neutrons off)

3. This radiation was directed towards a paraffin wax block, here it produced a hydrogen nuclei (protons)

4. This radiation was then directed towards nitrogen gas, producing nitrogen nuclei

5. He knew the masses of the alpha particles, and the speed at which they were ejected. Hence, he was able to set up simultaneous equations involving the known masses and velocities of the alpha particles, and the ejected nuclei

6. He was then able to roughly determine the mass of the unknown radiation, slightly more than a proton (it was a neutron).

7. Initially, he thought it to be gamma rays. However, the simultaneous equations he set up showed the particles had mass and thus momentum and energy, hence cannot be gamma rays, otherwise it would violate the conservation of momentum and energy.