Radioactivity And Particles

Question 1

What is the nucleus?

Answer 1

-The nucleus of an atom contains protons and neutrons
-It makes up most of the mass of the atom
-It is tiny
(A whole atom is mostly empty space)

Question 2

What is an isotope?

Answer 2

• Isotopes are atoms with the same number of protons but a different number of neutrons
• Usually each element only has one or two stable isotopes e.g. Carbon 12
• The other isotopes tend to be radioactive (the nucleus is unstable, so it decays and emits radiation) e.g. Carbon 14 is an unstable isotope of carbon

Question 3

What is radioactive decay?

Answer 3

• A random process

- Completely unaffected by physical conditions such as temperature or by any sort of chemical bonding etc.

Question 4

What happens during radioactive decay?

Answer 4

• When the nucleus does decay it spits out one or more of the three types of radiation (alpha, beta, gamma)
• In the process the nucleus often changes into a new element
• Alpha and beta particles and gamma rays are ionising radiation emitted from unstable nuclei in a random process

Question 5

Describe background radiation

Answer 5

There is (low-level) background nuclear radiation all around us all the time it comes from:

1. Substances on Earth: some radioactivity comes from air, food, building materials, soil, rocks..
2. Radiation from space (Cosmic Rays): these come mostly from the sun
3. Living things: there is a little bit of radioactive material in all living things
4. Radiation due to human activity: e.g. Fallout from nuclear explosions, or nuclear waste (though this is usually a tiny proportion of the total background radiation)

Question 6

Describe nuclear radiation

Answer 6

1. Causes ionisation
2. The further the radiation can penetrate before hitting an atom and getting stopped, the less damage it will do along the way and so the less ionising it is

Question 7

How can you detect nuclear radiation?

Answer 7

Photographic film or a Geiger-Muller detector

Question 8

Describe Alpha Radiation

Answer 8

1. Made up of two protons and two neutrons (Helium Nuclei)
2. Big, heavy and slow moving
3. Therefore they are strongly ionising
4. They are positively charged 2+
5. Therefore alpha particles are deflected by electric and magnetic fields
6. Weakest penetrating power

Question 9

What and how is beta radiation formed?

Answer 9

1. A high energy electron which has been emitted from the nucleus of an atom when a neutron turns into a proton and an electron
2. When a Beta particle is emitted, the number of protons in the nucleus increases by 1, so the atomic number increases by 1 but the mass number stays the same

Question 10

Describe Beta Radiation

Answer 10

1. Beta particles are high energy electrons
2. They have mass but so still travel slowly but not as slow as alpha, they move quite fast and are quite small
3. Therefore are moderately ionising
4. They are charged (-1)
5. Therefore beta particles are deflected by electric and magnetic fields
6. Penetrate moderately before colliding

Question 11

Describe Gamma Radiation

Answer 11

1. They are very short wavelength electromagnetic waves
2. They have no mass they are just energy in the form of an EM wave
3. They can penetrate a long way into materials without being stopped
4. This means they are weakly ionising because they tend to pass through rather than collide with atoms. But eventually they hit something and do damage
5. Gamma rays have no charge so they are not deflected by electric of magnetic fields
6. Gamma emission always happens after beta of alpha decay. YOU NEVER GET JUST GAMMA RAY EMITTED.
7. Gamma ray emission has no effect on the atomic or mass numbers of the isotope. If a nucleus has excess energy, it loses this energy by emitting a gamma ray

Question 12

What was the plum pudding model?

Answer 12

1. In 1804 John Dalton agreed with Democritus that matter was made up of tiny spheres (“atoms”) that could not be broken up. He reckoned that each element was made up of a different type of “atom”
2. Nearly 100 years later J J Thomson discovered that electrons could be removed from atoms. So Dalton’s theory was not quite right that atoms could not be broken up
3. Thomson suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them like plums in a plum pudding

Question 13

What was the Rutherford scattering experiment?

Answer 13

1. That “plum pudding” theory did not last very long though as in 1909 Hans Geiger and Ernest Marsden tried firing alpha particles at thin gold foil
2. Most of them went straight through, and were detected when they hit a zinc sulphide screen and gave off a tiny flash of light (a scintillation)
3. However some were knocked a bit off course but a very small proportion (about 1 in 8000) actually seemed to bounce off the gold foil

Question 14

What were the implications of the Rutherford scattering experiment?

Answer 14

1. If the plum pudding model was right then alpha particles would just pass straight through the gold foil
2. Rutherford realised that most of each gold atom must be empty space, but he knew atoms did have a mass
3. Therefore the rare event of a rebound meant that an alpha particle had run into something very massive and so he realised that the mass must be concentrated in a very tiny volume at the centre of the atom (the nucleus)
4. The fact that some of the alpha particles bounced back meant that inside the atoms there must be a small positively charged nuclei, which repel the positive charges of the alpha particles

Question 15

What does Rutherford’s nuclear model of the atom say?

Answer 15

1. Most of the mass must be concentrated at the centre of the atom, and most of the atom must be empty space
2. The nucleus must be small since very few alpha particles are deflected by much
3. It must be positive to repel the positively charged alpha particles

Question 16

What does the amount of deflection depend on?

Answer 16

1. The faster an alpha particle is travelling the less it will be deflected by a nucleus
2. The more positively charged a nucleus (i.e. the higher the atomic number) the more an alpha particle will be deflected
3. The closer an alpha particle passes to the nucleus the more it will be deflected (the electric force diminishes with the square of the distance

Question 17

Describe half life

Answer 17

1. The activity of a radioactive source always decreases over a period of time, and is measured in Bequerels
2. Each time a decay happens and an alpha or beta particle or gamma ray is given out, it means one more radioactive nucleus has disappeared
3. As the unstable nuclei all disappear the activity as a whole will decrease, so the older a sample becomes, the less radiation it will emit
4. How quickly the radiation drops off varies a lot, for some isotopes it only just a few hours before nearly all the unstable nuclei have decayed whilst others last for millions of years
5. The problem with trying to measure this is that the activity never reaches zero, which is why we have to use the idea of half life to measure how quickly the activity drops off

Question 18

What is the definition for half life?

Answer 18

• The half-life of a radioactive sample is the time taken for the activity of the sample to half
• The half-life of a radioactive sample is the average time taken for half of the original mass of the sample to decay
• Half life is different for different radioactive isotopes

Question 19

What does a short half life mean?

Answer 19

-The activity falls quickly because lots of the nuclei decay quickly

Question 20

What does a long half life mean?

Answer 20

-The activity falls more slowly because most of the nuclei don’t decay for a long time, they just sit there basically unstable but kind of binding their time

Question 21

How is nuclear radiation used for medical tracers?

Answer 21

Medical tracers use beta or gamma radiation

1. A source which emits beta or gamma radiation is injected into the patient or swallowed
2. The radiation penetrates the body tissues and can be detected externally
3. As the source moves around the body, the radiographer uses a detector to monitor its progress or to get a ‘snapshot’ of its distribution
4. A computer converts the reading to an on-screen display showing where the radiation is coming from
5. Doctors use this method to check whether the organs of the body are working as they should
6. The radioactive source has to have a short half life, so you can use less of the radioactive source but still get a reading on your detector
7. You could not use an alpha source because it would be stopped by the body’s tissues so you would never detect it externally and it has a strong ionising power which makes alpha radiation really harmful if it gets inside you

Question 22

How is gamma radiation used in industrial tracers?

Answer 22

1. You squirt a gamma source into the pipe, let it flow along, and go along the outside with a detector
2. Gamma radiation will penetrate through a metal pipe, but some of it gets absorbed, exactly how much depends on the thickness of the pipe and what it is made of
3. If there is a crack in the pipe, the gamma source will collect outside the pipe, and your detector will show extra high radioactivity at that point
4. The isotope used must be a gamma emitter, so that radiation can be detected even through rocks or earth surrounding the pipe , alpha and beta radiation would be too easily blocked
5. It should also have a short half-life so as not to cause long-term hazard if it collects somewhere

Question 23

How is radiation used for radioactive dating of rocks?

Answer 23

1. The discovery of radioactivity and the idea of half life have scientists their first opportunity to accurately work out the age or rocks, fossils and archaeological specimens.
2. By measuring the amount of radioactive isotope left in a sample, and knowing its half-life, you can work out how long the thing has been around
3. Igneous rocks contain radioactive uranium which has a very long half-life. It eventually decays to become stable isotopes of lead , so a clue to a rock’s sample age is the relative proportions of uranium and lead isotopes
4. Igneous rocks also contain radioactive potassium-40, some of which decays to stable argon gas. So you can date rocks by the relative proportion of potassium-40 to trapped argon gas

Question 24

How can you use radiation to see how old dead things are?

Answer 24

1. Carbon-14 makes up about one ten millionth of the carbon in air. The level stays fairly constant in the atmosphere
2. The same proportion of C-14 is also found in living things
3. When they die, the C-14 becomes trapped inside wood, bones, wool etc. C-14 is a beta emitting radioactive isotope, so the C-14 inside them decays over time and its radioactivity decreases
4. Comparing the activity of the sample to living tissues let you make an estimate of how many half-lives of C-14 has passed, thus giving you an idea of how long ago the animal or plant died
5. The ratio of C-14 to C-12 in living materials is fixed , so comparing the ratio in the sample can also help you estimate the age.

Question 25

How does gamma and beta radiation damage cells and tissues?

Answer 25

1. Beta and gamma radiation can penetrate the skin and soft tissues to reach the delicate organs inside the body. This makes beta and gamma sources more hazardous than alpha when OUTSIDE the body
2. If they get inside the body e.g. by being swallowed or breathed in their radiation mostly passes straight out without doing much damage

Question 26

How does alpha radiation damage cells and tissues?

Answer 26

1. Alpha radiation cannot penetrate the skin but it is very dangerous if it gets inside the body
2. Alpha sources do all their damage in a very localised area

Question 27

What is ionising radiation and what are some of the dangers?

Answer 27

1. When radiation enters your body , it will collide with molecules in your cells. These collisions cause ionisation, which damages or destroys the molecules
2. Lower doses tend to cause minor damage without killing the cell . This can cause mutations in cells which then divide uncontrollably which can form a tumour which can lead to cancer
3. Higher doses tend to kill cells completely, causing radiation sickness if a large part of your body is affected at the same time
4. The extent of the harmful effects depends on how much exposure you have to the radiation and its energy and penetration

Question 28

How is radiation used to treat cancer?

Answer 28

1. Once cancer has started patients can be given radiotherapy to kill the cancer cells and stop them dividing
2. This involves a high dose of gamma rays, carefully directed to zap the cells in the tumour while minimising the dose to the rest of the body

Question 29

Why is radioactive waste difficult to dispose of safely?

Answer 29

1. Most radioactive waste from nuclear power stations and hospitals is ‘low-level’ meaning it is slightly radioactive, such as clothing and syringes
2. This kind of waste can be disposed of by burying it in secure landfill sites
3. High-level waste is really dangerous and a lot of it stays highly radioactive for tens of thousands of years, and so has to be treated very carefully
4. It is often sealed into glass blocks, which are then sealed in metal canisters, These could then be buried deep underground
5. However it is difficult to find suitable places to bury high-level waste. The site has to be geologically stable (e.g. not suffer from earthquakes), since big movements in the rock could disturb the canisters and allow radioactive material to leak out. If this material gets into the groundwater, it could contaminate the soil, plants, rivers etc.. and get into our drinking water

Question 30

What protection should you take when working with radioactive materials?

Answer 30

1. It is important to minimise your exposure when you are working with radioactive sources
2. Never allow skin contact with a source and always hold tongs and at arm’s length
3. Keep the source pointed away from the body and avoid looking directly at it
4. Store radioactive sources in a sealed lead box whenever they are not being used

Question 31

If you work regularly with radioactivity what precautions do you need to take?

Answer 31

1. Medical workers who use radioactivity need to wear lead aprons and stand behind lead screen during procedures
2. Industrial nuclear workers wear full protective suits to prevent tiny-radioactive particles being inhaled or lodging on the skin or under fingernails etc.
3. Workers can also use remote-controlled robot arms to carry out tasks in highly radioactive areas

Question 32

What is nuclear fission and where is it used?

Answer 32

• Most power stations get the energy they need to drive the generators by burning fuel (e.g. coal) or from the natural motion of something (e.g. waves, tides). Nuclear power stations do it a bit differently…(Nuclear power stations use nuclear fission)
  1. Nuclear fission is the splitting of an atom which releases energy
  2. Nuclear power stations are powered by nuclear reactors
  3. In a nuclear reactor, nuclear fission is carried out in a controlled chain reaction, uranium or plutonium atoms split up releasing loads of energy

Question 33

Describe a fission chain reaction (you can split more than one atom)

Answer 33

1. The “fuel” that is split in a nuclear reactor (and bombs) is usually uranium-235, the uranium-235 is called a fissile material because it goes through the splitting process easily
2. If a slow moving neutron gets absorbed by a uranium-235 nucleus, the nucleus can split and this process releases energy in the form of KINETIC ENERY of the fission products
3. Each time a uranium-235 (U-235) nucleus splits up, it spits out a small number of neutrons. These might go on to hit other uranium-235 nuclei, causing them to split and release even more neutrons, which hit even more nuclei, and so on and so on
4. This process is known as a chain reaction

Question 34

Describe the products of the chain reaction

Answer 34

1. When uranium-235 splits in two it will form two new daughter nuclei, these two daughter nuclei are lighter elements than uranium
2. These new nuclei are usually radioactive because they have the “wrong” number of neutrons in them. This is the big problem with nuclear power, it produces huge amounts of radioactive waste which is very difficult and expensive to dispose of safely
3. Each nucleus splitting gives out a lot of energy, this energy is in the from of kinetic energy of the fission products (the daughter nuclei and the neutrons)
4. This energy can be converted to heat energy in the reactor by collisions with other atoms.

Question 35

What is a nuclear bomb?

Answer 35

A nuclear bomb in an uncontrolled fission chain reaction

Question 36

Describe the inside of a Gas-Cooled Nuclear Reactor

Answer 36

• This describes a gas-cooled nuclear reactor, but there are many other kinds
  1. Free neutrons in the reactor ‘kick-start’ the fission process
  2. The atoms produced then collide with other atoms, causing the temperature in the reactor to rise
  3. The moderator, usually graphite or water, slows neutrons so that they can successfully collide with uranium nuclei and sustain the chain reaction
  4. Control rods, often made of boron, limit the rate of fission by absorbing excess neutrons
  5. A gas, typically carbon dioxide (CO2), is pumped through the reactor to carry away the heat generated
  6. The gas is then passed through a heat exchanger, where it gives its energy to water, this water is heated and turned into steam, which turns a turbine, which turns a generator, generating electricity

Question 37

What is the penetrating power of different types of radiation?

Answer 37

1. Alpha particles are blocked by paper, skin or a few centimetres of air
2. Beta particles are stopped by thin metal
3. Gamma rays are blocked by thick lead or very thick concrete

Question 38

What is an alpha particle?

Answer 38

A helium nuclei ejected from unstable nuclei, they are heavily ionising and have only a short range, travelling only around 10cm in air, They are stopped by thin card

Question 39

What is a beta particle?

Answer 39

Are fast moving electrons ejected from an unstable nuclei. They are less ionising and travel long distances in air. They are stopped by 1-2 mm of aluminium

Question 40

What are gamma rays?

Answer 40

Gamma rays are photons of high energy EM ways. They are extremely penetrating and interact with atoms which may then emit ionising radiation. They are only stopped by tens of cm of lead

Question 41

How do you detect ionising radiation using photographic film?

Answer 41

• This becomes fogged when exposed to ionising radiation and is used in badges worn by workers at risk from continuous exposure
• The badges are checked regularly to ensure that safety limits have not been exceeded

Question 42

How do you detect ionising radiation using the Geiger- Müller tube?

Answer 42

• When ionising radiation enters the GM tube it ionises the gases within allowing a pulse of current to pass between the electrodes
• This is then fed to either a counter or a rate meter
• Often the current pulses are made to produce audible ‘clicks’

Question 43

What is the application of radioactivity in sterilisation?

Answer 43

• Ionising radiation is used to kill of microorganisms and bacteria on surgical equipment and on some foodstuffs.
• The items to be sterilised can be sealed in airtight packaging and sterilised through the packaging

Question 44

What is the use of non-medical tracers?

Answer 44

-The flow of liquids and gases through industrial processes can be mapped using radioactive tracers and detectors or gamma cameras

Question 45

How can you change the control rods in reactor?

Answer 45

The control rods can be raised out of the reactor core allowing the chain reaction to speed up, or lowered completely to shut down the chain reaction

Question 46

What does the graphite moderator do?

Answer 46

Is used to absorb some of the energy of fast neutrons so they are readily absorbed by nuclei of U-235 sustaining the chain reaction

Question 47

Is the half-life different for different radioactive isotopes?

Answer 47

Yes

Question 48

What does the fission of U-235 produce?

Answer 48

Two daughter nuclei and a small number of neutrons

Question 49

How can a chain reaction be set up?

Answer 49

A chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei

Question 50

What is the role played by the control rods and moderator when the fission process is used as an energy source to generate electricity?

Answer 50

1. The moderator, usually graphite or water, slows neutrons, by absorbing some of the kinetic energy of the neutrons to slow them down, so that they can successfully collide with uranium nuclei and sustain the chain reaction
2. Control rods, often made of boron or cadmium, limit the rate of fission by absorbing excess neutrons and taking them out of the fission process completely

Question 51

What happens to the activity of a radioactive source?

Answer 51

• The activity of a radioactive source decreases over a period of time and is measured in becquerels
• The activity of a radioactive source is equal to the number of decays per second
• Activity is measured in becquerels (Bq)
• 1 becquerel = 1 decay per second

Question 52

Which two types of radiation would be deflected by an electric field?

Answer 52

Alpha and Beta (as both have a charge and Gamma does not)

Question 53

How can scientists use this radioactivity to find the age of a piece of granite?

Answer 53

1. There is a known activity when rocks formed
2. Measure the activity now
3. Compare the activity now to the original activity
4. Hence determine the number of half-lives elapsed
5. Hence calculate age from reference to half-life

Question 54

What is the atomic (proton) number of an element?

Answer 54

• The atomic number of an element tells us how many protons each of its atoms contains
• The symbol we use for atomic number is Z

Question 55

What is the mass (nucleon) number of an element?

Answer 55

• The number is the total number of protons and neutrons in the atom
• The mass number of an element is given the symbol A