Strong Force & Strong Force & RadioactivityRadioactivityParticle Physics 2Particle Physics 2

Homework Reminder!

Ernest Rutherford

What important experiment did he direct in the early 20th Century?

What did Rutherford conclude from his analysis of the observations?

Today’s ObjectivesToday’s Objectives

The strong nuclear force:The strong nuclear force:             -its role in keeping the nucleus stable;-its role in keeping the nucleus stable;             - short-range attraction to about 3 fm, - short-range attraction to about 3 fm,

very-short range repulsion below about very-short range repulsion below about 0.5 fm0.5 fm

If time:-If time:- Equations for alpha decay and beta - Equations for alpha decay and beta -

decay including the neutrino.decay including the neutrino.

Size of the nucleus...Size of the nucleus...

If the atom were the size of the If the atom were the size of the school canteen, the nucleus would school canteen, the nucleus would be the size of a pea dropped in the be the size of a pea dropped in the middle.middle.

Really small compared to the atom!!Really small compared to the atom!!

For fun?...(don’t worry about the equations)

221

04

1

r

QQF

221

r

mGmF

Repulsive force between two protons.

Gravitational Attraction.

Q1,Q2=eG is gravitational constant.m are proton massesR is distance between the two protons (10-

15m).

Why doesn’t it fly apart?Why doesn’t it fly apart?

The repulsive force between the two The repulsive force between the two charges is much larger than the charges is much larger than the gravitational force.gravitational force.

So why is it stuck together?So why is it stuck together?

We need another force – the We need another force – the STRONG force.STRONG force.

The Strong Force GraphThe Strong Force Graph

Important to NoteImportant to Note

It’s a weird force!It’s a weird force!

At very short ranges, below 0.5 At very short ranges, below 0.5 femtometresfemtometres (0.5 × 10 (0.5 × 10-15-15 m) the m) the strong nuclear force is strong nuclear force is repulsiverepulsive.  . 

It is It is attractiveattractive up to its maximum up to its maximum range of 3 fm (3 × 10range of 3 fm (3 × 10-15-15 m). m).

Strong ForceStrong Force

What would happen if the strong What would happen if the strong force wasn’t repulsive at short force wasn’t repulsive at short distances?distances?

It is about 200 N between two It is about 200 N between two protons – very strong compared to protons – very strong compared to other forces.other forces.

Radioactive DecayRadioactive Decay

Some isotopes are stable but others Some isotopes are stable but others are not.are not.

Those which are not release Those which are not release radiation and change into a more radiation and change into a more stable isotope (normally a different stable isotope (normally a different element).element).

How much do you know? Complete the table (in pencil) that describes the

properties of the three common radiations:-

Radiation

Particle Range in air

Stopped By

AnswersRadiation Particle Range in

airStopped By

Alpha Helium nucleus (P)

Few mm (P)

Paper (P)

Beta High speed electron (P)

Few cm (P)

Aluminium sheet (P)

Gamma Energetic photon (P)

Infinite (P) Several cm lead (P)

Alpha radiationAlpha radiation

Mostly comes from heavy nuclides with Mostly comes from heavy nuclides with proton numbers greater than 82, but proton numbers greater than 82, but smaller nuclides with too few neutrons can smaller nuclides with too few neutrons can also be alpha emitters. also be alpha emitters.

The general decay equation is summarised The general decay equation is summarised below. below.

The term The term QQ stands for the stands for the energyenergy.. Can be written with He as Can be written with He as αα and without and without

the Q.the Q.

Notes on Notes on αα-decay-decay

The alpha particle is a helium The alpha particle is a helium nucleusnucleus (NOT atom). (NOT atom).

Energy is released in the decay. The Energy is released in the decay. The energy is energy is kinetickinetic, with the majority going , with the majority going to the alpha particle and a little going to to the alpha particle and a little going to the decayed nucleus.the decayed nucleus.

The velocity of the alpha particle is much The velocity of the alpha particle is much greater than that of the nucleus. greater than that of the nucleus.

Example DecayExample Decay

The The nucleon numbernucleon number goes down by goes down by 4, the 4, the proton numberproton number by 2. by 2.

A typical alpha decay is: A typical alpha decay is:

  Is this equation balanced? Explain Is this equation balanced? Explain your answer your answer

Beta radiationBeta radiation

Neutron rich nuclei tend to decay by Neutron rich nuclei tend to decay by beta beta minus (b-) emissionminus (b-) emission. .

The beta particle is a The beta particle is a high-speed high-speed electronelectron ejected from the nucleus, NOT ejected from the nucleus, NOT the electron clouds. the electron clouds.

It is formed by the decay of neutrons, It is formed by the decay of neutrons, which are slightly more energetic than a which are slightly more energetic than a proton. proton.

Isolated protons are stable; isolated Isolated protons are stable; isolated neutrons last about 10 minutes. neutrons last about 10 minutes.

Equation for Beta decay.Equation for Beta decay.

The neutron, having emitted an electron, is The neutron, having emitted an electron, is converted to a proton, and this results in converted to a proton, and this results in the proton number of the nuclide going up the proton number of the nuclide going up by 1. A new element is formed. The by 1. A new element is formed. The general equation is: general equation is:

Note the electron can be written as Note the electron can be written as ββ, also , also without the Qwithout the Q

Example Beta DecayExample Beta Decay

NotesNotes The nucleon number remains the same ; The nucleon number remains the same ; The proton number goes up by 1. The proton number goes up by 1. The beta particle is created at the instant of The beta particle is created at the instant of

the decay. the decay. The antineutrino is very highly penetrating The antineutrino is very highly penetrating

and has a tiny mass. It is very hard to detect. and has a tiny mass. It is very hard to detect. A precise amount of energy is released, A precise amount of energy is released,

according to the nuclide. according to the nuclide. That energy is shared among the nucleus, the That energy is shared among the nucleus, the

electron and the antineutrino. electron and the antineutrino.

Question Question

What is the balanced nuclear equation for What is the balanced nuclear equation for the following decays? the following decays?

(a) emission of a beta- particle from oxygen (a) emission of a beta- particle from oxygen 19 19

(b) emission of an alpha particle from (b) emission of an alpha particle from polonium 212 polonium 212

(c) emission of a beta + particle from (c) emission of a beta + particle from cobalt 56 cobalt 56

    Proton numbers O – 8, F – 9, Fe – 26, Co – Proton numbers O – 8, F – 9, Fe – 26, Co – 27, Pb – 82, Po – 84 27, Pb – 82, Po – 84

AnswersAnswers

(a) (a)

(b) (b)

(c) (c)

Summary Summary

A graph of neutron number against proton A graph of neutron number against proton number shows that there are more neutrons number shows that there are more neutrons in larger nuclei in larger nuclei

This is needed to ensure stability of the This is needed to ensure stability of the nuclei. nuclei.

Natural decay occurs with alpha decay Natural decay occurs with alpha decay

Or beta minus decay. Or beta minus decay.