Nuclear Physics

AQA unit 2

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The Atom - The basics

Everything is made up of atoms - even us. Atoms are extremely small however are built up of even smaller particles called subatomic particles. These are protons, neutrons and electrons. Protons have a relative mass of 1 and a (+) charge. Neutrons have a relative mass of 1 and have a neutral charge. Electrons have a negligible mass and a (-) charge. The number of protons always equals the number of electrons and so have equal and opposite charges which balance or cancel out forming an atom with no overall charge. Protons and neutrons are found in the nucleus whilst the electrons orbit outside in shells.

When an atom loses or gains electrons it becomes an ion, a charged particle. If it loses electrons it becomes positively charged if it gains electrons it becomes negatively charged as the number of protons and electrons is no longer the same.

An Isotope is an atom of the same element which has the same number of protons but different number of neutrons. It is still the same element as the proton number, which determines what element it is, does not change. The nuclei of some isotopes can be radioactive and can decay, spitting out perticles and releasing radiation.These are called radioactive isotopes. There are three types of radiation.


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Unstable Nuclei

Sometimes the nuclei of isotopes is unstable and can decay in an attempt to become more stable. This is called radioactive decay. In this process an atom can emit a particle and radiation into the atmosphere. Radioactive isotope can emit Alpha, Beta or Gamma radiation. If an atom emits Alpha or Beta radiation the atom becomes an atom of a different element as its nucleus changes into that of a different element. This is as the proton number, which determines which element it will be changes and so it becomes a different element.

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Alpha Decay

An Alpha particle is a Helium Nucleus. A helium nucleus contains 2 protons and 2 neutrons meaning it has a +2 charge. There are no electrons as it is just the Helium nucleus involved and electrons orbit the nucleus. When an atom emits an Alpha particle it loses 2 protons and 2 neutrons - which means that its mass number goes down by 4, as four subatomic particles are lost from the nucleus and its atomic number goes down by 2, as only 2 protons are emitted.

A little biography on Alpha D

- Alpha is slow and large [Think fat man]

-Alpha doesn't penetrate very far into the atmosphere and is stopped very easily by e.g. skin or paper 

-Alpha is very ionising as due to its size it easily knocks electrons off of on atoms turning them into ions which is why they are bad inside the body as they can ionise cells which can cause cancer

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An example of Alpha Decay


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Beta Decay

A Beta paticle is a high enegry electron. It is emitted from the nucleus of an unstable isotope when a neutron in the nucleus changes into a proton and an electrons. This causes the mass number to stay the same but the atomic number to increase by one, as there is now one extra proton. As electrons don't belong in nucleus it is immidietly emitted out.

A little biography on Beta D

- Beta is fast moving and extremely small [Think hungry mice]

- Beta is moderatly penetrating and ionising

- Beta can be stopped by a thin sheet of aluminium

- Beta was born in Greece (Don't need to know)

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An example of Beta dacay

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Gamma is an electromagnetic wave with a short wave length. Gamma radiation is released sometimes when an atom emits an Alpha or Beta radiation. As the nucleus of the atom becomes more stable its nucleus produces excess energy which it needs to get rid of. The nucleus gets rid of this energy in the form of a gamma ray. This causes no change in the atoms mass or atomic number.

A little biography about Gamma D

- Gamma is very penertating and can travel through most materials without being stopped

- It is not very ionising as gamma tends to pass through atoms instead of bumping into them

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Background Radiation

Background radiation comes from naturally occuring unstable isotopes as well as man made sources. Thec largest contributer of background radiation comes from natural sources such as the air, rocks, food and drink etc.

Manmade sources contribute to a small portion of this.

Read on for more detail.

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Sources of Radiation - Rocks

The largest percentage of background radiation comes from rocks. Some rocks give out a radioactive gas called Radon which can be dangerous in confined spaces. Rocks are in the soil which plants that we eat grow in, they are used in the bricks for our houses, our houses are built upon different types of rock and so on. So basically we are exposed to some background radiation from rocks but we can;t stop eating or move to get away from it

Some people live on rocks which produce more radon gas and so are more radioactive. If this is the case then they are exposed to higher levels of radiation and at risk of radiation penetraing through there skin and ionsing cells producing cancer - lung cancer in paritcular. People who live on more radioactive rock have to have well ventilated homes so the concntration of radon gas is not as high.

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Cosmic Rays

Cosmic rays are constantly coming from space to earth all the time. Cosmic rays can come from the sun. This isn't much of a problem for us as by the time the rays get through Earth's atmosphere only a small amount of cosmic rays remain however this is a problem for people at high altitudes e.g. pilots or God. Since they are at high altitudes quite often they are exposed to more cosmic rays than us at low altitudes and so are more at risk of the radiation penetrating through there skin and ionising cells producing cancer.

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Manmade Sources

Manmade sources contribute to the smallest portion of background radiation.They come from processing or the use of radioactive materials such as unstable isotopes in nuclear reactors.

Radiation comes from nuclear weapons testing, radioactive waste form nuclear reactors and medical waste such as x-rays etc.

Medical waste contributes to the largest portion of manmade sources of background radiation.

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What increases our background radiation??

Occupation - some occupations such as a radiographer, working in a nuclear power plant, miner and a pilot can expose you to more background radiation

High alltitudes - means that we are closer to space and so exposed to more cosmic rays from e.g. the sun

Underground mines - means that you are surrounded by rocks and exposed to more radon gas.

Where you live - you can live on rocks which emit more radon gas than others

Quick Note: It is difficult to identify when a person has died from radiation poisoning as radiation passes straight out of your body and cannot be detected.  lso we do not know if other factors contributed towards there death which makes it difficult to calculate the number of people who die from radiation poisoning each year. 

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The story of the atom

John Dalton put forward the idea that all matter consisted of particles called atoms which were the smallest things in existence and could not be broken down.

100 years later, along came J J Thompson who discovered that an atom could be broken down as he discovered electrons. He knew that electrons were negatively charged and for an atom to stay intact he also knew there should be positive charges in an atom. This led him to put forward the theory that atoms were like plum puddings - a sphere of positive charge with small negatively charged electrons dotted around inside like plums in a pudding.

But then came Ernie Rutherford, after he discovered the Alpha particle of a 2+ charge. Ernie decided to do an experiment....

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Ernest Rutherford's scattering experiment

Ernie decided to test J J Thompsons theory by firing Alpha particles at a thin gold sheet (as you do). Ernie expected to see the Alpha particles go straight through however some were deflected. He repeated his experiment and found the same thing happened again. This led him to the conclusion that as most of atoms went straight through, atoms are mainly empty space. Some atoms were deflected as the +2 charge of the alpha particle was repelled by the positive charged nucleus of the gold atoms causing the alpha particles to be deflected allowing him to conclude that an atoms nucleus has a positive charge. As only a few particles were deflected this allowed him to conclude that the positive charge was concentrated in the atom in a small space - the nucleus. This information allowed Ernie Rutherford to put forward the nuclear model of the atom which is the structure of the atom as we know it today...Good old Ernie.

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Scaterring experiment


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Nuclear Fission

Nuclear fission in which a large nuclei is split into two smaller nuclei and 2 or 3 neutrons - this forms a lot of energy in the form of heat. Nuclear reactors and nuclear submarines use this process to generate elecrticity. The 'fuel' or isotopes used are uranium-235 or plutonium-239. These have rather large nuclei which are easier to split. Inside a nuclear reactor a slow moving neutron is fired at a uranium-235 nucleus. The nucleus absorbs this neutron which causes it to split into two smaller nuclei or daughter nuclei, 2 or 3 extra neutrons and a lot of energy. The 2 or 3 neutrons can then go on to be absorbed by other uranium-235 nuclei causing it to split and release another 2 or 3 neutrons and so on and so on - this is called a chain reaction. The energy released when the nucleus split can be used as heat energy to heat up water and produce steam, which drives a turbine, which drives a generator and produces electricity.

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Controlling a nuclear reactor

A nuclear reactor can be dangerous at times as if to many uranium-235 nucleus's split forming energy in the form of heat the reactor is at risk of over heating and going into meltdown this is why nuclear reactors have control rods - to control the rate of the reaction.

Control rods are good at absorbing neutrons, and so if there are too many nuclei splitting in a nuclear reactor, and too much heat energy being produced then the control rods are lowered into the core or reactor vessel to absorb neutrons to prevent a chain reaction from occuring as quickly. If more neutrons are absorbed then less neutrons are available to split nuclei and give out heat energy and so the reactor is able to cool down.

However if the reaction is happening to slow, then the control rods are lifted out of the core or reactor vessel so less neutrons are absorbed and so more neutrons are available to split more nuclei and take part in a chain reaction - producing more heat and so more electricity is produced.

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How control rods work

( image) Shows neutron being absorbed by conrol rod. (Right image) Control rods are lowered more neutrons absorbed so reactor cools down, less energy is made.Control rods rise less neutrons are absorbed rector gets hotter, more energy is made. 

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Nuclear Fusion

Nuclear Fusion is a process used to combine two small nuclei to make one large nucleus - this produces more energy than fission or any chemical process such as burning fossil fuels.

The process in which fusion occurs:

- Two nuclei usually Deuterium and Tritium isotopes of hydrogen are used.

- The two both have positive charges and so repel naturally

- A temperature of 150 million degrees is required to make the isotopes combine together against their natural repulsion

- Once they attract they form Helium nuclei and for a second become helium-5 as they have one extra neutron in the nucleus

- This nucleus is emitted out of the nucleus as is a lot of energy

- Products are Helium nucleus, a neutron + energy

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Nuclear Fusion - Why a high temerature is needed

An extremely high temperature is requried to overcome the natural repulsion of the two positively charged nuclei in order to get them close enough to attract each other - as when the get close enough to each other something called the strong force attracts them both towards each other forming a large nucleus.

There is nothing on earth that can withstand that heat without melting and so the reaction takes place within a magnetic field. However at the minute the process requires more energy to produce than it give out.

Scientists think that fusion is the way forward as it is zero carbon, very little radiactive waste, there is a continuous supply of hydrogen isotopes and it produces a lot of energy.

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Nuclear Fusion

Isotopes of hydrogen Deutrium and Tritium are found in sea water. Deutrtium can be found in sea water wheras tritium has to be extracted as lithium and broken down into tritium. Even so the supplies of sea water are more abundant than fossil fuels or uranium.


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Bethany Cunningham

They're really good :D

On flash card 10 you put 'cosmin rays '

Nelema Uddin

Thanks Beth **

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