Radioactivity

W Richards

The Weald School

Structure of the atomA hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…

I did an experiment (with my colleagues Geiger and Marsden)

that proved this idea was wrong. I called it the “Scattering

Experiment”

Ernest Rutherford, British scientist:

The Rutherford Scattering ExperimentAlpha

particles (positive

charge, part of helium

atom)

Thin gold foil

Most particles passed through, 1/8000 were

deflected by more than 900

Conclusion – atom is made up of a small, positively charged nucleus surrounded by

electrons orbiting in a “cloud”.

The structure of the atomELECTRON –

negative, mass nearly

nothing

PROTON – positive,

same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

Atoms are roughly 10-10m in diameter, while the nucleus is 10-15 – 10-14m

The structure of the atomParticle Relative Mass Relative Charge

Proton 1u (1.7x10-27kg) +1.6x10-19C

Neutron 1u (1.7x10-27kg) 0

Electron 0 -1.6x10-19C

MASS NUMBER (A) = number of protons + number of neutrons

SYMBOL

No. of neutrons N = A - Z

PROTON NUMBER (Z) = number of protons (obviously)

IsotopesAn isotope is an atom with a different number of neutrons:

Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.

Notice that the mass number is different. How many neutrons does each isotope have?

A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

Quarks

Pe-

Low energy scattering

We can investigate the structure of protons by bombarding them with electrons:

High energy scattering

Elastic collision. Electrons and protons behave as expected.

Pe-

Inelastic collision. Energy is “absorbed” by the proton and increases its internal energy. This is Deep Inelastic Scattering and suggests that the proton is made of smaller particles called quarks.

Introduction to RadioactivitySome substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation:

Radiation

The nucleus is more stable after emitting some radiation – this is called “radioactive decay”.

IonisationRadiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by giving them enough energy to “knock off” electrons:

Alpha radiation is the most ionising (although short range). Ionisation causes cells in living tissue to mutate, usually causing cancer.

The Geiger-Muller Tube

Metallic case (cathode)

Mixture of argon and halogen

Central anode

Mica end window

Types of radiation1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)

2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle.

3) Gamma – after or decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.

Unstable nucleus

Unstable nucleus

Unstable nucleus

New nucleus

New nucleus

New nucleus

Alpha particle

Beta particle

Gamma radiation

Words – frequency, proton, energy, neutrons, helium, beta

Changes in Mass and Proton NumberAlpha decay:

Am241

95Np

237

93α

4

2+

11

5

0

+1C

11

6B β+

90

39Sr

90

38Y β

0

-1+

Beta - decay:

Beta + decay:

“positron”

Blocking RadiationEach type of radiation can be blocked by different materials:

Sheet of paper (or 6cm of air

will do)

Few mm of aluminium

Few cm of lead

SummaryProperty Alpha Beta - Beta + Gamma

Charge

Rest mass

Penetration

What is it?

Ionising ability

Deflection by Magnetic FieldsAlpha and beta particles have a charge:

++

-

2 protons, 2 neutrons, therefore charge =

+21 electron, therefore

charge = -1

Because of this charge, they will be deflected by electric and magnetic fields:

+

-

+

Background Radiation

Radon gas

Food

Cosmic rays

Gamma rays

Medical

Nuclear power

13% are man-made

Nuclear fission

Uranium nucleus

Unstable

nucleus New nuclei (e.g. barium

and krypton)

More neutron

s

Neutron

Chain reactions

Each fission reaction releases neutrons that are used in further reactions.

Radioactive DecayRadioactivity is a random process. The number of radioisotopes that will decay clearly depends on the number of radioisotopes present at that point in time:

Activity (in Bq) = λN

λ = “The decay constant” and has units of s-1. It is constant for a particular radioisotope.

Half LifeThe decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…

At start there are 16 radioisotope

s

After 1 half life half have

decayed (that’s 8)

After 3 half lives another

2 have decayed (14 altogether)

After 2 half lives another

half have decayed (12 altogether)

= radioisotope = new atom formed

A radioactive decay graph

Time

Count

1 half life

1 half life

1 half life

Half LifeTo calculate half life there are a few methods:

1) Read from a graph

2) Calculate using an equation

t½ = ln2

λ

Half Life questions

100s

1) The graph shows the activity of a radioisotope. Determine the half life and decay constant.

2) If there are 106 atoms present right now calculate how many will decay over the next second.3) What percentage of a sample of radioactive material will exist after 200 years if the half life is 50 years?

4) Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.