Stable and Unstable Nuclei Particle Physics Lesson 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?

Homework Revise for the Skills Test next week. Complete worksheets on Nuclear Physics (during the weekend, after the test). Recap today’s lesson using the slides on the website.

Today’s Objectives To recap specific charge. To know what holds the nucleus together. To know what happens when α, β and γ radiation are emitted.

Problem Solving: Specific Charge Particle Charge Mass Specific Charge Carbon-12 Nucleus 6e 12mP 6e/12mP Carbon-14 Nucleus Calcium-40 Atom Calcium-40 Nucleus Ca2+ Ion e = charge on the electron (=1.60 × 10-19 C) mP = mass of a proton/neutron (=1.67 × 10-27 kg)

Problem Solving: Specific Charge Particle Charge Mass Specific Charge Carbon-12 Nucleus 6e 12mP 6e/12mP Carbon-14 Nucleus 14mP 6e/14mP Calcium-40 Atom 40mP+20me Calcium-40 Nucleus 20e 40mP 20e/40mP Ca2+ Ion 2e 40mP + 18em 2e/40mP e = charge on the electron (=1.60 × 10-19 C) mP = mass of a proton/neutron (=1.67 × 10-27 kg)

The Nuclear Model of the Atom

Question Why don’t the electrons go flying off? Let’s now play with magnets…

What happens in each case?

Van der Graaf Generator Are we still happy with our model?

Size of the nucleus... If the atom were the size of the school canteen, the nucleus would be the size of a pea dropped in the middle. Really small compared to the atom!!

For fun?...(don’t worry about the equations) Repulsive force between two protons. Gravitational Attraction. Q1,Q2=e G is gravitational constant. m are proton masses R is distance between the two protons (10-15m).

Force Comparison Coulomb force of repulsion is about 200N Gravitational Force of attraction is only about 1.9×10-34 N. So Coulomb force of repulsion is much, much stronger!!

Video In Search of Giants (9 of 15) The Weak and Strong Nuclear Forces

Why doesn’t it fly apart? The repulsive force between the two charges is much larger than the gravitational force. So why is it stuck together? We need another force – the STRONG force.

The Strong Force Graph

Important to Note It’s a weird force! At very short ranges, below 0.5 femtometres (0.5 × 10-15 m) the strong nuclear force is repulsive.   what would happen otherwise? It is attractive up to its maximum range of 3 fm (3 × 10-15 m).

Strong Force What would happen if the strong force wasn’t repulsive at short distances? (protons would get pushed together) It is about 200 N between two protons – strong enough to counter the repulsive Coulomb force.

For the exam, you need to know... The strong nuclear force:       -its role in keeping the nucleus stable;       - short-range attraction to about 3 fm, very-short range repulsion below about 0.5 fm Equations for alpha decay and beta - decay including the neutrino.

Radioactive Decay Some isotopes are stable but others are not. Those which are not stable release radiation and change into a more stable isotope (normally a different 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

Answers Radiation Particle Range in air Stopped 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)

What happens to the nucleus? When an α-particle is emitted? When a β-particle is emitted? When a γ-ray is emitted?

Alpha radiation Mostly comes from heavy nuclides with proton numbers greater than 82, but smaller nuclides with too few neutrons can also be α-emitters. The general decay equation is summarised below. The α-particle is also sometimes written as

Notes on α-decay The alpha particle is a helium nucleus (NOT atom). Energy is released in the decay. The energy is kinetic, with the majority going to the alpha particle and a little going to the decayed nucleus. The velocity of the alpha particle is much greater than that of the nucleus.

Example Decay The nucleon number goes down by 4, the proton number by 2. A typical alpha decay is:  Is this equation balanced? Explain your answer

Beta radiation Neutron rich nuclei tend to decay by beta minus (β-) emission. The beta particle is a high-speed electron ejected from the nucleus, NOT the electron clouds. It is formed by the decay of neutrons, which are slightly more energetic than a proton. Isolated protons are stable; isolated neutrons last about 10 minutes.

Equation for Beta decay. The neutron, having emitted an electron, is converted to a proton, and this results in the proton number of the nuclide going up by 1. A new element is formed. The general equation is: Note the β-particle can be written as The denotes a anti-neutrino.

Neutrinos Neutrinos are neutral particles with a very small mass. This makes them very hard to detect. An anti-neutrino is an anti-particle. Don’t stress about this we will be revisiting all this later.

Example Beta Decay Note that the β- particle is assigned a proton number, Z = -1 to make the equation balance. Remember that it is an electron. The anti-neutrino has A=0 and Z=0.

Notes The nucleon number remains the same ; The proton number goes up by 1. The beta particle is created at the instant of the decay. The antineutrino is very highly penetrating and has a tiny mass. It is very hard to detect. A precise amount of energy is released, according to the nuclide. That energy is shared among the nucleus, the electron and the antineutrino.

Question What is the balanced nuclear equation for the following decays? (a) emission of a beta- particle from oxygen 19 (b) emission of an alpha particle from polonium 212 (c) emission of a beta + particle from cobalt 56   Proton numbers O – 8, F – 9, Fe – 26, Co – 27, Pb – 82, Po – 84

Answers (a) (b) (c)

Summary A graph of neutron number against proton number shows that there are more neutrons in larger nuclei This is needed to ensure stability of the nuclei. Natural decay occurs with alpha decay Or beta minus decay.