4. Atoms & Radioactivity Flashcards
Examples of nucleons?
Protons and neutrons
What is the majority of the mass of an atom?
The nucleus, as the mass of an electron is almost zero compared to the mass of protons and neutrons
What was Democritus’ theory about the atom?
400BC - all matter is made of small invisible units called atoms
What was Dalton’s theory about the atom?
1803 - agreed with Democritus and said atoms of the same element are identical and indestructible
What was Thomson’s theory about the atom?
1904 - Plum Pudding Model
What was Rutherford’s theory about the atom?
1911 - Nuclear Model; that most of the mass of the atom was concentrated in the nucleus which carries a positive charge
What was Bohr’s theory about the atom?
1913 - Bohr’s model, electrons orbit nucleus in certain energy levels
What was Chadwick’s theory about the atom?
1932 - discovered the neutron
Which scientists were involved with the history of the atom?
- Democritus
- Dalton
- Thomson
- Rutherford
- Bohr
- Chadwick
Which was the first sub-atomic particles to be discovered?
The electron, by Thomson
What was the Plum Pudding Model?
A sphere of positive charge (dough) with electrons dotted inside (plums)
What did the discovery of the electron show about atoms?
That atoms were not the smallest thing, as the electron was smaller than the atom
What did the Bohr model show?
- That electrons can only orbit the nucleus in certain energy levels
- Electromagnetic energy is emitted from the atom if an electron jumps from an excited energy level to one that’s closer to the nucleus
- If an electron absorbs energy it can jump to a higher energy state from the nucleus
What did later experiments suggest about the atom?
- the positive charge of any nucleus could be subdivided into a whole number of smaller individual sub-atomic particles called protons
- 1934 - Chadwick used alpha scattering experiment to provide evidence for existence of the neutron
Who was the first to call the nucleus of a hydrogen atom a proton?
Rutherford
In the scattering experiment, how could have the alpha particles been supplied and been moving in the right direction?
From radioactive matter. Surrounded by lead shielding apart from a small opening facing the way they wanted the beam
Why was the chamber evacuated where the scattering experiment took place?
So the alpha particles didn’t change direction and so no other particles would affect the results
Why should alpha particles be repelled by positive atomic material?
They both have positive charges and like charges repel
What happens when isotopes have unstable nuclei?
They break down or disintegrate
What happens when isotopes break down or disintegrate?
They give out radiation said to be radioactive
What is the process when isotopes with unstable nuclei disintegrate?
Radioactive decay
What may nuclear radiation be?
- an alpha particle
- a beta particle
- a gamma ray
- a neutron
What does an alpha particle consist of?
2 neutrons and 2 protons - the same as a helium nucleus
What is a beta particle?
When a high speed electron is ejected from the nucleus as a neutron changes into a proton
What is a gamma ray?
High energy electromagnetic radiation from the nucleus
What are nuclear equations used to represent?
Radioactive decay
What happens to the mass and charge of the nucleus during alpha decay?
Both the mass and charge of the nucleus decease
What happens to the mass and charge of the nucleus during beta decay?
The mass of the nucleus doesn’t change however the charge of the nucleus increases
What happens to the mass and charge of the nucleus during gamma decay?
The emission of a gamma ray doesn’t cause the mass or charge of the nucleus to change
What is an ion?
An atom which has lost or gained an electron
What happens when radioactive radiation strikes atoms in a substance?
They can knock an electron from one atom to another
Properties of alpha particles?
- Relatively big, heavy and slow moving
- Large relative charge (+2)
- Strongly ionising - bash into lots of atoms and knock electrons off them
Properties of beta particles?
- Quite small and move quite fast
* Moderately ionising because they have a smaller relative charge (-1)
What is the relative charge of an alpha particle?
+2
What is the relative charge of a beta particle?
-1
Properties of gamma radiation?
Weakly ionising as gamma is not charged
What is the range of alpha in air?
A few cm
What is the range of beta in air?
A few metres
What is the range of gamma in air?
Infinite
What can alpha radiation be stopped (absorbed) by?
Paper, card, skin
What can beta radiation be stopped (absorbed) by?
Sheet of aluminium (5mm)
What can gamma radiation be stopped (absorbed) by?
Thick sheet of lead or metres of concrete
Which type of radiation is most ionising?
Alpha
Which type of radiation is least ionising?
Gamma
What is the safest type of radiation to have inside the body?
Gamma as it’s the least ionising and escapes easily
What is the most dangerous type of radiation to have inside the body?
Alpha as it’s most ionising and cannot escape
What is the safest type of radiation to have outside the body?
Alpha as it’s least penetrating
What is the most dangerous type of radiation to have outside the body?
Gamma as it’s most penetrating
What is alpha used in?
Smoke detectors
How do smoke detectors work?
Alpha ionises air particles causing a current to flow. If there is smoke in the air, the smoke binds to the ions reducing the number available to carry a current. Current falls and alarm sounds
Why is alpha used in smoke detectors?
- Ionising
- Charged
- Cannot penetrate though a plastic shield
- Can be absorbed by smoke
- Shortest range
Why aren’t beta or gamma used in smoke detectors?
They are less ionising and have a larger range
Are smoke detectors safe given they contain alpha particles?
Yes as alpha has a short range and not penetrating, provided it is not pulled apart
What is beta used in?
Beta emitters test the thickness of thin sheets of metal
Why is beta used to test thickness of metal?
Particles are not immediately absorbed by the material like alpha, and do not penetrate as far as gamma
What is gamma used for?
- Tracers in medicine
- Leak detection in pipes
- Radiotherapy
- Food irradiation
How is gamma used for tracers in medicine?
Gamma radiation should be emitted from the source injected into the patient, so it can be detected outside the body
Why isn’t alpha used for tracers in medicine?
Would be too dangerous as the ionisation it causes could mutate cells and cause cancer
How is beta used for leak detection in pipes?
The radioactive isotope is injected into the pipe. The outside of the pipe is checked with a Geiger-Muller detector to find areas of high radioactivity - where the pipe is leaking
Why is beta used for leak detection in pipes?
It has a short half life so the material doesn’t become a long term problem. It has to be detected through the metal as well as the earth
Why is gamma used in radiotherapy?
Gamma kills cancer cells
Why is gamma used in food irradiation?
Because gamma keeps it fresh for longer as it kills bacteria or fungi
What happens when there are more nuclei in a sample in terms of half life?
When there are more nuclei, more are likely to decay in one half life, so the higher the count rate will be
What is the half life of a radioactive isotope?
• The average time it takes for the number of nuclei of the isotope in a sample to half
or
• The time it takes for the count rate from a sample containing the isotope to fall to half its initial level
What is the net decline of a radioactive sample?
The fraction of nuclei that have decayed in a certain time
How is the net decline of a radioactive sample portrayed?
Fraction, ratio, percentage
doesn’t have a unit
What is the half life required when sterilising medical instruments?
Long - years
What is the half life required when using medical tracers?
Short - days
What is the half life required when using a thickness monitor?
Long - years
What is the half life required when using industrial tracers?
Short - days
What is the half life required when using a smoke alarm?
Long - years
What did Rutherford expect the alpha particles to do in the scattering experiment? Why?
He expected them to go through the thin sheet of atoms easily because they were moving at a very high speed
What was the material of the sheet of atoms in the scattering experiment?
Gold
What was Rutherford expecting to observe in the scattering experiment? Why?
For slight changes in the direction of the alpha particles due to them repelling from the thinly spread positive material of the atoms
Why should alpha particles be repelled by positive atomic material?
They’re both positively charged
Why did Rutherford not expect the electrons to have any noticeable effect on the movement of the alpha particles?
Alpha particles are much larger and have momentum and so need force for them to deflect their direction
What was the set-up that allowed Geiger and Marsden to observe what happened to the alpha particles in the scattering experiment?
A fluorescent screen that when hit by alpha gives off spots of light - looked at using a microscope
During the scattering experiment, which path of the alpha particles agreed with the plum pudding model?
The path which was slightly deflected
During the scattering experiment, which path of the alpha particles disagrees most strongly with the prediction of the plum pudding model?
The path which was reflected over 90 degrees -because alpha have a positive charge and repelled the positive protons in the nucleus of the gold atoms. The nucleus was so dense that it caused the alpha particles to bounce back
During the scattering experiment, only 1 in 10,000 alpha particles were deflected over 90 degrees. What does this suggest about the nucleus of an atom?
It is very small so have very few collisions with alpha particles
When does a nucleus become unstable?
When it possesses either too many or too few neutrons compared to the number of protons
What is radiation detected with ?
A Geiger counter
Define half life using the idea of activity
The time it takes for activity to half from its original value
Define half life using the idea of count rate
The time it takes for count rate to half from its original value
Define half life using the idea of undecayed atoms left in the source
The time it take for half of the nuclei present to decay
How is nuclear radiation used in medicine for imaging?
- radioactive tracers - imaging flows
* gamma cameras - imaging internal organs
What do radioactive tracers image?
Flows inside the body
What do gamma cameras image?
Internal organs
Why is imaging using radioactive radiation important?
To help with diagnosis
Why is nuclear radiation used in medicine?
For treatment; to destroy cancer cells
What processes using nuclear radiation help with diagnosis?
- radioactive tracers
* gamma cameras
What processes using nuclear radiation help with treatment?
- gamma radiotherapy
* radioactive implants
What are radioactive tracers used for?
To trace the flow of a substance through an organ
How do radioactive tracers work?
- a small amount of radioactive material is put into the patients body
- tracer is given time to move through body
- radioactive detector is positioned to detect radiation outside the body
- multiple images are taken to show progress of tracer over time
How is radioactive material put into a patients body to be used as a tracer?
By ingestion or injection
What type of radiation is used in medical tracing?
Gamma
Why is gamma used in medical tracing?
It is the least ionising and the most penetrating, meaning it can leave the body eventually
Why isn’t alpha used in medical tracing?
It can’t be detected outside the body and is very ionising so would damage cells as it moved through the bloody
What are the properties of gamma that ensure medical tracing is not harmful to the patient?
Gamma is not ionising, has a short half life and is not toxic
What does a gamma camera do?
Images internal organs
How does a gamma camera work?
- a radioactive isotope (gamma emitting) is objected into the patient
- the isotope concentrates in the organ
- a gamma camera is positioned over the area emitting gamma
What are gamma cameras made from?
A crystal scintillator (sodium iodide)
What happens when gamma hits a crystal scintillator?
It produces a burst of light
What do photo multiplier tubes do?
Convert energy in electrical signals - which are fed to a computer to produce an image
What does the lead grid do in a gamma camera?
Acts as a collinator - only allows gamma aligned with the holes to hit the crystal, making image sharper
What is a collinator used for in a gamma camera?
To make the image sharper, less blurred
What is the process happening inside a gamma camera to produce an image?
- gamma rays hit a crystal scintillator and crystal produces a burst of light
- bursts of light are picked up by a photo multiplier which convert energy to electrical signals
- the electrical signals are sent to a computer to produce an image
What does ionising mean?
When atoms are turned into ions by losing or gaining electrons
What can ionising radiation do to cells?
Low doses - damage cells
High doses - destroy cells
This can lead to cancer
How are cancer cells destroyed in a tumour?
Gamma radiation in narrow beams
Where is radiation emitted from in radiotherapy?
A radioactive isotope of cobalt
What is the half live of the radioactive isotope of cobalt?
Five years
Why are narrow beams used in radiotherapy?
So that no tissue before the area that is being treated becomes damaged
How do narrow beams of gamma destroy a tumour?
Lots of beams are shone from different angles to overlap on the problem area
Why is gamma used in radiotherapy?
It can damage deep inside the body - it is very penetrating - and can escape the body easily
Why is a half life of five years suitable for gamma radiotherapy?
It lasts for long enough so it doesn’t need to be replaced and the dose each patient receives doesn’t decrease
What are gamma knives used for?
To accurately focus many beams of gamma radiation on one or more brain tumours
What are the beams of radiation in gamma knives life?
- very low intensity
* approximately 200 beams - concentrated on a small volume very accurately
What are radioactive implants used for?
To attack the tumour from inside the body
How do radioactive implants work?
The sources of radiation are placed directly inside the tumour or nearly tissue
How long can radioactive implants stay in for?
Hours or days due to low dose rate sources
What type of radiation can radioactive implants emit?
Beta because they are more local to the tumour and are more ionising than gamma
What is a suitable half life for a radioactive implant?
A few weeks:
- enough time for radiation dose to be low for an extended period
- not long enough to stay active in patient after it’s needed
Advantages of radioactive implants?
- therapy over longer period compared to external beam therapy
- radiation more localised - especially with beta
- patient can spend less time in hospital
Disadvantages of radioactive implants?
- small risk of exposure of radiation to family and friends
* only early stage diseases and small tumours can be treated
What is irradiation?
When the radiation stops as soon as the source of radiation has been removed
Example of irradiation?
When a light is turned on the room is filled with electromagnetic radiation (visible light). When the light is turns off the electromagnetic radiation is gone
What is contamination?
When the source of ionising radiation is transferred
Example of contamination?
When radioactive isotopes in solids, liquids or gases are introduced into the environment
Why is a patient more likely to get cancer through contamination than through irradiation?
In contamination the source of radiation is transferred and will stay in the body for longer. It is inside the body so is closer and therefore more intense.
What is nuclear fission?
The splitting of a large nucleus into two smaller nuclei (‘daughters’) while releasing energy
Which nuclei are split in fission reactions?
Usually uranium-235 but also plutonium-239
What is induced fission?
When a reaction occurs due to a neutron being fired at a uranium nucleus
What do fission reactions release?
- two or three neutrons at high speeds (these are used in further reactions)
- energy in the form of radiation and kinetic energy of the fission neutrons
What form of energy is released in fission?
- radiation
* kinetic - of the neutrons and the fragment nuclei
What happens in a chain reaction?
Each reaction causes more reactions which cause more reactions etc.
What is the energy produced in fission used for?
Heats water and turns it into steam -> turns turbine -> turns generator transforms kinetic to electrical energy
How is the output of a nuclear fission power station made stable?
The energy released must be constant and not grow rapidly like in an uncontrolled chain reaction
In a nuclear reactor, what is the function of the control rods?
To absorb surplus neutrons to keep chain reaction under control
In a nuclear reactor, what is the function of the moderator?
To slow the fission neutrons down
Why is water used as a moderator in a nuclear reactor?
Fission neutrons are slowed down by collisions with the atoms in the water molecules
What is a fission neutron?
A neutron released when a nucleus undergoes nuclear fission
In a nuclear reactor, what is the function of the coolant?
Water - transfers heat so radioactive water is not turned to steam
In a nuclear reactor, why is the reactor core in a thick steel vessel surrounded by concrete walls?
To withstand high temperature and pressure. Walls absorb gamma radiation that escapes steel vessel
In a nuclear reactor, why does the coolant circulate through sealed pipes to and from a heat exchanger?
So contaminated water is not turned to steam and enters the atmosphere
In a nuclear reactor, why may the control rods need to be inserted completely into the reactor core?
So there isn’t too much energy being released from uncontrolled fission
What is nuclear fusion?
When two small nuclei collide and fuse to form a larger nucleus, releasing energy
What process powers the sun and the stars?
Nuclear fusion
How is the gas in a fusion reactor heated?
By passing an electric current through it
What happens when the gas in a fusion reactor gets very hot?
It forms a plasma of small nuclei
How is the plasma contained in a fusion reactor?
Using a magnetic field to prevent it from touching the container walls
What is one of the products when hydrogen nuclei fuse?
Helium
What happens in a fusion reactor is the plasma touches the sides of the container?
The plasma would cool down, and fusion would stop
In fusion, what do nuclei need to fuse?
Enough kinetic energy
In fusion, what happens if nuclei do not have enough kinetic energy to fuse?
They will repel each other and not fuse
Advantages of nuclear fission?
- concentrated source of energy
- no polluting gases - energy is released from nuclear reactions not combustion
- supply of uranium for many years
- reliable - 24/7 (doesn’t depend on weather like renewables)
Disadvantages of nuclear fission?
- produces radioactive waste
- chain reaction must be controlled
- non-renewable
Advantages of nuclear fusion?
- produces very little radioactive waste - safer
- no chain reaction to control - safer
- even more concentrated energy
- fuel readily available (hydrogen in water)
- renewable
- hydrogen is cheap
Disadvantages of nuclear fusion?
• not yet viable - fusion on earth needs more energy input than output (need 5 million amp current!)
Which is a more concentrated source of energy: fission or fusion?
Fusion
Which type of nuclei fuse in the sun?
Hydrogen
Which type of nuclei are formed in the sun?
Helium
