Physics P5 Radioactivity

What is an atom?

A positively charged nucleus surrounded by negatively charged electrons, with almost all of the mass in the nucleus

Describe Dalton's Atomic Model 1803

1st

Based on observations from chemical reactions he suggested all matter was made of invisible ATOMS

Atoms are the smallest building block

All atoms of the same element are the same

Describe Thomson's Atomic Model 1904

aka plum pudding model; a large positively charged circle with negatively charged particles arranged randomly inside

Simplified:

Positive atom

Negative electrons can be removed

Explains +/- ions

Why was Thomsons model (plum pudding model) introduced

As they discovered the electron

How small is an atom

1 × 10^10m

Describe the arrangement of particles in an atom

Nucleus made of Protons and Neutrons with a positive charge, with most of the atoms mass

Surrounded by negatively charged electrons

What is an isotope?

Atoms with the same number of protons but a different number of neutronsSS

Atomic number, mass number definitions

Relative Charge and Relative Mass of

Protons

Neutrons

Electrons

Describe the Geiger-Marsden Experiment

An alpha radiation source passes through a Collimator (To make a thin beam of alpha particles)

It is in a vacuum to ensure the alpha particles are not absorbed or deflected by air particles

The alpha particles are directed at a very thin gold foil sheeet

The angle of deflection is measured by the fluorescent screen

What were Geiger-Marsdon's experiment results

Most alphas passed straight through

Some alphas deflected at a small angle

1 in 10,000 were scattered back at large angles

Describe Rutherford's Model

Most of the atom is empty space so most alphas pass straight through

But most of the mass is concentrated in the nucleus with a Positive charge which repels the positive alphas

Describe Bohr's Model

In this model electrons were in particular shells at a fixed radius

What is ionising radiation

Radiation that can ionise atoms by removing electrons from them

What what can be interpreted from the alpha scattering experiment

1. There is a nucleus containing all the positive charge and most of the mass of the atom

2. The nucleus is very small and most of the atom is empty space

Rough size of molecules

But molecules can be big up to about 10nm

Nucleon number

The number of protons and neutrons in the nucleus

Why are some materials Radioactive

In some isotopes the combination of neutrons and protons makes the nucleus unstable

Some occur naturally other are man-made

The atom wants to become more stable by emitting gamma radiation and Kinetic energy

This not a chemical change it takes place inside the nucleus of an atom, it is not affected by any physical or chemical changes

What are the 4 types of radiation

Alpha

Beta

Gamma

Neutron

What is alpha decay

It occurs in nuclides that are too heavy

The heavy nuclide will emit 2 protons and 2 neutrons or 4/2 He

What is Gamma decay

Occurs when the nucleus has too much energy. Nothing changes but the substance has less energy

Why is radioactive decay random

Radioactive decay is random because we can never tell which nucleus will decay.

What does activity mean

The rate (Emissions per second) at which radiation is emitted from a radioactive material

Measured in Becquerel (Bq)

What does the term Half-Life mean

Is the time taken for the activity of a sample to drop by halfThe shorter the Half-Life the greater the activity, but the sample decays away Much quicker

How to find the Half-Life from an activity-time graph

How to calculate the net decline after a given number of half-lives

What is Irradiation

Irradiation happens when a person or object is exposed to readiation emmited by radioactive materalIrradiation stops as soon as the source is removed or shielded

Simplified: Exposure to radiation

Risks of Contamination and Irradiation in different types of radiation

What does being exposed to radiation lead to?

The ionisation of living cells. This can damage DNA which leads to cell death,mutation and possibly cancer

What are some saftey precautions when using radioactive materials

Shield sources e.g. lead lined boxes

Use tongs

Wear gloves and aprons

Why is no dose of radiation safe

As a single ionisation could cause cancer however the risk is significantly lower

What is nuclear Fission

Is the process of splitting one large nuclei into two maller nuclei, energy is released in the form of KE and gamma radiation

What is nuclear Fusion

Is the process of fusing two small nuclei to form one large nuclei, energy is released in the form of KE and Gamma radiation

Describe chain reactions in nuclear Fission

- only one extra neutron is required to induce a uranium-235 nucleus to split by fission

- during the fission, it produces two or three neutrons which move away at high speed

- each of these new neutrons can start another fission reaction, which again creates further excess neutrons this is a chain reaction

Problems with Fusion power stations

very hard to give Hydrogen nuclei enough energy to overcome electrostatic repulsive forces

Need temperatures of over 100 million degrees

It is hard to control and sustain reaction

Where does some of the mass go in nuclear fusion

to energy

What are some uses of nuclear radiation

As medical tracers

gamma emitters are used as alpha and beta would not escape the body to be detected they would be absorbed and cause damage

Radioactive dating

We use radioactive decayed to date fossils and rocks

To control thickness of a substance eg metal foil

IF the foil got too thick the count would go down and if it was too thin the count would go up helping monitoring the thickness of materials

Beta radiation is used as alpha would be completely blocked by the foil and gamma would pass right through

Sterilization

Food and medical equipment can be sterilized by irradiating them with nuclear radiation

Gamma rays are used as the ionize microbes leading to their death, Alpha or Beta would not penetrate the container of the food so Gamma radiation is used

Benefits of fusion power stations

Lots of hydrogen fuel available in water

Produces helium which is safe

Lots of energy is released from each fusion event

Why was Daltons model introduced

introduced

Due to evidence from chemical reactions

Give some examples of ionising radiation

UV

X-Ray

Gamma

Alpha

Beta

What is Nuclear radiation

Radiation emitted by radioactive nuclei, it is also ionising

How can Nucleur radiation by detected

Geiger-Miller tube connected to an electronic counter

Describe the these of Alpha particles

Symbol

Ionising power

Range in air

What it is

It is the nucleus of helium-4

High ionising power

2-3cm in air

Describe the these of Beta particles

Symbol

Ionising power

Range in air

What it is

It is a fast moving electron

High ionising power

30-50cm in air

Stopped by 3mm of aluminium

Describe the these of Gamma particles

Symbol

Ionising power

Range in air

What it is

It is the highest frequency EM wave

Low ionising power

Very large (infinite) range in air

Why was Rutherfords model of the atom introduced

Due to the Alpha-scattering experiment

ion defenition

An atom that has become charged by either gaining or losing an electron

What is Beta Decay

Happens in nuclides hat have too namy neutrons, one of those neutrons turns into a proton and an electron

What is neutron emmision

If a nuclei has too many neutrons atoms can decay by shedding a neutron

What is contamination?

When the radioactive source itself enters your body or gets on your skin

What are the main sources of background radiation

Radon gas roughly 50%

Rocks

Cosmic rays

Foods

Why is nuclear fusion hard

As it takes a lot of energy to bring the nuclei very close together.And the temperature must be over 100 million degrees Celcius

Explain Nuclear power stations using fission

Where is the majority of the mass in an atom

the Nucleus