Nuclear Forces

Nuclear Forces

Principles of Nuclear Physics

NPE-503

Lecture by: Zahra Ali

Fundamental Forces

Electrostatic Force

protons have a positive electric charge and neutrons

have no electric charge at all, there must be some sort of

extra force a force even stronger than the electromagnetic force to hold these nuclei together.

a proton and neutron themselves are not fundamental, but

composed of even smaller, fractionally-charged particles

known as quarks.

shown or r 3 fm for the pp force.

Nuclear Forces

Characteristics Of The Nuclear Force.

short range:

It falls to zero so abruptly with inter-particle separation.

The interior nucleons are completely surrounded by

other nucleons with which they interact.

For nuclear force binding energy is B/A

const done by short range and saturation of nuclear forces.

More accurate form of nuclear force

potential can be obtained by scattering

of nucleons on nucleons or nuclei.


Strongly Attractive and Strongly repulsive

The nuclear force is a short range force. Maximal range

~1.7 fm. Two nucleons within about 2 fm of each other

feel an attractive force.

The inter nucleon potential has a hard core that prevents the nucleons from approaching each other

closer than about 0.4 fm.

charge independent 1. The only difference between the np and pp potentials is the

Coulomb potential s

2. The nn system is more difficult to study because free

neutrons are not stable from analyses of experiments.

3. cross sections of nucleon scattering are not dependent on

their electric charge. For nuclear forces neutron and proton are two different states of single particle - nucleon.

The term nucleon refers to either neutrons or protons because the neutron and proton can be considered different charge states of the same particle.


Nuclear Potential The angular distribution of neutron classically scattered by

protons.

Neutron + proton (np) & proton + proton (pp) elastic.

The nuclear potential

H. Yukawa proposed corresponding potential based on short range of nuclear forces and nonzero rest mass of intermediate particles. where

m is mass of intermediate particle and /mc is its Compton wave length.

Nature of Nuclear Forces

Expect strong interaction electric force influences also. Nucleus has positive charge and for positive charged particle this force produces

Coulomb barrier (range of electric force is larger than this of strong force).

r

eV(r)

r mc

The strong nuclear force is created between nucleons

by the exchange of particles called mesons.


Pion exchange mechanisms:

n n + 0 p + 0 p n p + - p + - n

p n + + n + + p Nucleons swap identities in about 50% of the events Pion exchange could also provide an explanation for the

magnetic moment of the uncharged neutron Protons and neutrons emit and absorb mesons. Why their masses are not changed?

Exchange nature of nuclear forces:


Uncertainty principle:

Et violation of energy conservation is allowed if t is shorter then /E.

Maximal range of nuclear forces is R = 1.7 fm.

Then the smallest time of nucleon transit is:

t = R/c.

Value of violation of energy conservation is during

emission of meson with mass m:

E = mc2.

If time of violation will be t we obtain for maximal possible energy violation (meson mass):

mc2 = c/R (the same as earlier shown)


Summary Very strong, short-range force between nucleons

Strongly repulsive at very short separations

Strong force is spin-dependent and charge symmetric

Interaction potential between nucleons comprises

central and non-central components

Yukawa model views the inter-nucleon force in

terms of the exchange of pions (p-mesons)

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Nuclear Forces