Question

Why is that the masses for proton and neutron more precisely measured in atomic mass units (u) than in megaelectron volts (MeV)?

Answer 1

Solution:

IN high energy physics it is common .

High energy interactions involve large range of particles. Kg or joules are too far big to express the nucleon mass or energy . So make sensible expression of the quantities involved, u , MeV /c^2 or MeV are used.

Atomic mass unit is the unit used to express the mass of a nucleon ( a proton or aneutron . ).

(The C-12 contains exactly 6 protons and 6 neutrons an 1 amu is exactly the mass of 1 nucleon.

Why only C-12 is chosen is because its mass could be measured more precisely , compared to the other elements . `1 amu = 1 g/mol ) .

Mega electron volts (MeV) are the units of Energy and not of mass. Mass is expressed as Mev /c^2 .

The mass of the nucleons expressed in amu is converted to energy using the Einstein's E= mc^2 (mass energy equivalence) and conveniently converted to MeV . , instead of joules.

1 amu = 1.00 u = 1.66 x 10^-27 kg

E = mc^2 = (1.66 x10^-27 kg ) c^2 where c = 3 x10^8 m/s = speed of light

=> E = (1.66 x10^-27) (3x10^8)^2 = 931.5 MeV

Energy equivalent of 1 u = 931.5 MeV

but mass is 1 u = 931.5 MeV/c^2

It's the convenience of using and expressing the rest mass as MeV/c^2 and rest energy as MeV.