Radioactivity and particles

Question 1

What is the electron?

Answer 1

A very little particle with very little mass. It has negative electric charge.

Question 2

Give the relative mass and relative charge of electrons, protons and neutrons.

Answer 2

Electrons relative mass is 1, it’s relative charge is -1
Protons relative mass is 2000 (1840 to be precise). It’s relative charge is +1.
Neutrons relative mass is 2000 (1840 to be precise). It’s relative charge is 0.

Question 3

What is the relative charge of an atom? Why?

Answer 3

Neutral because the number of positive charges carried by the protons in its nucleus is balanced by the number of negative charges on the electrons in the electron ‘cloud’ around the nucleus.

Question 4

What does the atomic number tell us? What is the symbol used for it?

Answer 4

How many protons each of its atoms contain. The symbol used for it is Z.

Question 5

What does the atomic mass tell us? What is the symbol used for it?

Answer 5

The total number of protons and neutrons in the nucleus of an atom. The mass of the electrons that make up an atom is so small it is not counted. The symbol used for it is A.

Question 6

What is an atomic notation?

Answer 6

Each particular type of atom will have its own atomic number and mass number. An atom of element X has an atomic notation where the chemical symbol is shown and the mass number is at the top left (shown by A) and the atomic number is the bottom left (shown by Z).

Question 7

What is an isotope?

Answer 7

Atoms of the same element (i.e. The same number of protons) with a different number of neutrons and therefore a different mass number.

Question 8

How do isotopes of an element change their physical properties and why?

Answer 8

Their mass is obviously different, as well as the stability of their nucleus. The protons are held together with a nuclear force. This force is very strong and acts over a small distance to hold the nucleus together. Too many neutrons will make the Nucleus unstable.

Question 9

What happens to an unstable nucleus of an isotope?

Answer 9

An unstable nucleus will eventually decay. When the nucleus of an atom decays it gives out energy and may also give out alpha or beta particles.

Question 10

What happens when unstable nuclei decay?

Answer 10

They give out ionising radiation (alpha or beta particles or gamma rays) which causes atoms to gain or lose electric charge, forming ions. They do this at random.

Question 11

What are the three types of radiation?

Answer 11

Alpha, beta and gamma radiation.

Question 12

What does alpha radiation consist of?

Answer 12

Fast-moving particles called alpha particles that are thrown out of unstable nuclei when they decay. They are helium nuclei - helium atoms without their orbiting electrons.

Question 13

Describe alpha particles (i.e. ionising power, penetrating power)

Answer 13

Alpha particles have a short range. They can once travel a few cm in air and cannot penetrate more than a few mm of paper. However they have high ionising power.

Question 14

Why do alpha particles have such a short range?

Answer 14

They interact with atoms along their paths, causing ions to form. This means they rapidly give up the energy that they had when they were ejected from the unstable nucleus.

Question 15

What does beta radiation consist of?

Answer 15

Very fast-moving electrons called beta particles that are ejected by a decaying nucleus - when a neutron decays into a proton and an electron (the beta particle)

Question 16

If beta electrons come from a decaying nucleus, and the nucleus of an atoms is made up of just protons and neutrons, how is this possible?

Answer 16

The stability of a nucleus depends on the proportion of protons and neutrons it contains. The result of radioactive decay is to change the balance of protons and neutrons in the nucleus to make it more stable. Beta decay involves a neutron (of relative charge 0) splitting into a proton (+1) and an electron (-1). The proton remains in the nucleus and the electron is ejected at high speed as a beta particle.

Question 17

Describe beta particles and explain it in comparison to alpha (i.e. ionising power, penetrating power)

Answer 17

Beta particles have a greater range than alpha particles as they interact with matter in their paths less frequently than alpha particles. They have medium ionising power and can travel long distances in air, pass through paper easily and are only absorbed by denser materials such as aluminium; you only need 1-2mm.

Question 18

Describe gamma rays (i.e. ionising power, penetrating power)

Answer 18

Gamma rays are electromagnetic waves with very short wavelengths. They have no mass and no charge so are weakly ionising and interact only occasionally with atoms in their paths. They are extremely penetrating and can pass through all but the densest materials with ease. It take several cm thick lead or a meter or so of concrete.

Question 19

How is gamma radiation emitted?

Answer 19

In ‘packets’ of energy called photons.

Question 20

What is a nuclear transformation and why does it occur?

Answer 20

An unstable atom will decay by emitting radiation. If this involves the nucleus ejecting either an alpha or beta particle the atomic number will change. This means that the alpha or beta decay causes the original element to transform into a different element.

Question 21

Show and explain the general formula of the alpha decay equation?

Answer 21

A A-4 4
Y –> W + He+energy
Z Z-2 2
This is because as an alpha particle is formed (the helium nucleus) it carried away four nucleons (a word for protons and neutrons), reducing the mass number of the unknown element by 4. Two of these nucleons are protons so the atomic number gets reduced by 2. The total number of nucleons and protons on each side is balanced.

Question 22

Show and explain the general formula of beta decay.

Answer 22

A A 0
X –> Y + e + energy
Z Z+1 -1
The beta particle is an electron so it has practically no mass so changes the mass number by 0. As it has a charge of -1 the atomic number is increased by 1.

Question 23

When does gamma radiation actually occur?

Answer 23

When an unstable nucleus has just emitted an alpha or beta particle it sometimes has surplus enemy which is emits as gamma radiation.

Question 24

What are atoms made up of? But what takes up most of the space?

Answer 24

Electrons, protons and neutrons. Most of the atom is actually empty space. The nucleus is about 10,000 times smaller than the atom itself.

Question 25

What is the unit of radioactivity? And what is it a measure of?

Answer 25

Becquerels, how many nuclei are disintegrating per second. A more practical unit is kBq (an average of 1000 disintegrations per second).

Question 26

What is a Geiger muller tube?

Answer 26

A glass tube with electrically conducting coating on the inside surface. The tube contain a special mixture of gases at very low pressure. There is an electrically insulated electrode in the middle.

Question 27

What happens in a Geiger muller tube?

Answer 27

When ionising radiation enters the tube is causes the low pressure gas inside to form ions. This allows a pulse of current to flow from the electrode to the conducting layer which is detected by an electronic circuit. It is usually connecting to a counting circuit, keeping track of how many ionising particles or gamma rays have entered the tube.

Question 28

What is background radiation?

Answer 28

Low-level ionising radiation that is produced all the time.

Question 29

Sources of background radiation and percentage.

Answer 29

Radon gas - 50%
Ground and buildings - 14%
Medical - 14%
Food and drink - 11.5% 
Cosmic rays - 10%
Nuclear power - 0.3%
Other - 0.2%

Question 30

What should you always do when measuring radiation of an object?

Answer 30

Use the Geiger muller tube to detect the background radiation first and then subtract.

Question 31

Is radioactive decay entirely random?

Answer 31

Radioactive decay is a random process, just like tossing a coin as we cannot predict whether it will be heads or tails. However if we flip a coin 1000 times it would be surprising if the number of heads was not around 500.

Question 32

How would we demonstrate nuclear decay in an experiment?

Answer 32

If you had the time, you could take 1000 coins and toss them. Remove all the ones that land on heads and note the remaining coins. Repeat this process for around 6 trials and you will begin to see a trend. (Obviously each time around 50% are heads)

Question 33

Why is the graph of nuclear decay going to be a curve? What is this kind of decrease called?

Answer 33

As the number of nuclei decreases by half it will go down. It will decrease by half in the same about of time (roughly) but only go down by half as much as the first time as there are only half left. The smaller the quantity, the more slowly the quantity decreases. The decrease is proportional to the size, it is called exponential decay.

Question 34

What is the half life of a radioactive sample?

Answer 34

The average time taken for half of the original mass of the sample to decay into another isotope. The half life is different for different radioactive isotopes.

Question 35

How would you measure the half life of a radioactive material?

Answer 35

Using a Geiger muller tube first measure local background radiation and then measure the rate of decay at regular time intervals.

Question 36

What is the rate of decay proportional to?

Answer 36

The amount of radioactive isotope present.

Question 37

How is radioactivity used as medical tracers in diagnosis?

Answer 37

Radioactive isotopes are used as tracers to help doctors identify diseased organs. The gamma ray camera can show where a patient has absorbed a tiny amount of the radioactive substance. Doctors can tell from the image obtained how well particular organs are functioning.

Question 38

How does radiotherapy work?

Answer 38

High doses or radiation can cause normal cell function to change. This can lead to abnormal growth and cancer however very high doses will kill cells. Cancer cells can be targeted with gamma radiation the reduce a tumour. Multiple low energy beams focus and meet at a target (the tumour) where they are strong enough to have an effect. The individual beams are not strong enough to kill living cells. To find out the location of the cancer cells alpha or beta emitting chemicals are used.

Question 39

What properties must particles used in medical tracers have?

Answer 39

They must emit gamma radiation as alpha and beta would not be able to pass out the body to the special camera.
It must have a long enough half life to remain reasonably active but not too long they it does not irradiate the patients body for longer than needed.
It must not be toxic nor decay into a substance that is toxic or radioactive.

Question 40

Why is ionising radiation good for sterilisation and what does this process entail?

Answer 40

Ionising radiation can kill living cells so it is used to kill micro-organisms on surgical instruments and all other medical equipment. They are packaged in air tight bags before being placed close to strongly ionising radiation sources so as not to be re-contaminated before use. The radiation will penetrate the packaging and destroy bacteria without damaging the bags. This is also sometimes done with food.

Question 41

How can a radiative source be used to show underground leaks in pipes?

Answer 41

A radioactive tracer that emits beta radiation can be added to a fluid. Where a leak occurs will be shown by an increase in count rate detected in that area. High level of radioactivity means presence of leakage. The radiation is detected with a Geiger Muller tube.

Question 42

How is radiation used with Automatic thickness monitoring?

Answer 42

The amount of radiation received by a detector depends on the thickness of the foil that is placed in between the emitter and the detector. If the thickness increases the reading falls. The detector is attached to a computer which brings the rollers closer together and so decreases the foil thickness.

Question 43

Properties of the source with automatic thickness monitoring.

Answer 43

Beta must be emitted as alpha would not pass through the aluminium and gamma would not be affected by such small changes.
A long half life source would be used as the computer would mistake it for he foil getting thicker if it was just the source getting weaker.

Question 44

What is radiocarbon dating?

Answer 44

It measures the level of an isotope of carbon called carbon-14 (C-14). It is produced when high speed neutrons (part of cosmic radiation) collide with nitrogen gas in our atmosphere.

Question 45

How does radioactive carbon dating work?

Answer 45

Radioactive C-14 is produced when high speed neutrons (part of cosmic radiation) collide with nitrogen gas in our atmosphere.
C-14 is chemically identical to the non-radioactive C-12 so reacts which the oxygen in our atmosphere to form CO2. This is then absorbed by plants during photosynthesis and so all living plants contain a certain proportion of C-14, as will any animal that eats the plant.
They are continuously replaced by new ones even after they decay in living matter.
When an organism dies in no longer absorbs any more C-14 and therefore the amount will gradually decrease.
The age of once living material can be estimated by comparing the radioactive carbon content to that of living material.

Question 46

Limitations to radiocarbon dating?

Answer 46

The dating process assumes that the level of cosmic radiation reaching earth is constant.
It is limited to samples no longer than about 60,000 years ago (about 10 half lives) beyond which the amount of C-14 in the source becomes too small to measure accurately.

Question 47

How can you date rocks?

Answer 47

Inorganic, nonliving matter does not absorb carbon-14, so different techniques must be used for finding out the age of rocks and minerals.
When formed, the rock may contain a radioactive isotope such as you uranium-238. This is called the parent nuclide. Overtime this decays to thorium-234 which itself decays by various stages until the stable isotope lead-206 is formed. These are called daughter nuclides.
The older the rock sample, the greater the lead 206 sample will be.

Question 48

Describe Thomson’s model of the atom

Answer 48

“Plum pudding model”, spherical cloud of positive charge with electrons in it

Question 49

Describe Geiger and Marsden’s experiments with gold foil and alpha particles

Answer 49

A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil but something else also happened.
Some of the alpha particles emerged from the foil at different angles and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

Question 50

How did the results of the Geiger and Marsden’s experiment?

Answer 50

Small number of deflections indicates that most of the atom is empty space with only a very small nucleus
Backwards deflections can only occur if the charge and mass were concentrated in a small part of the atom - the nucleus
The ‘plum pudding’ model would not produce backwards deflections
It must positive to repel the positively charged alpha particles

Question 51

What are the 3 characteristics of the nuclear model of the atom?

Answer 51

Small, massive, positive nucleus
Orbited by negative electrons
Most of the atom is completely empty space

Question 52

What are the rules for particle movement?

Answer 52

The faster an alpha particle’s travelling the less it will be deflected by a nucleus
The more positively charged a nucleus is (i.e. the higher the atomic number) the more an alpha particle will be deflected.
The closer an alpha particle passes to the nucleus, the more it will be deflected

Question 53

What is the most commonly used fissile material?

Answer 53

Uranium-235

Question 54

How can the nucleus of U-235 be split?

Answer 54

A neutron is fired at the nucleus
The neutron is temporarily absorbed, making the nucleus unstable, causing it to split
The products product are 2 daughter nuclei - which are often radioactive because they have a different number of neutrons and lighter than the uranium - a small number of neutrons and lots of kinetic energy

Question 55

What is the product you want from nuclear fission?

Answer 55

The kinetic energy of the products

Question 56

What happens in a chain nuclear fission?

Answer 56

The neutrons (2-4) produced in the first fission reaction hit other fissiles (the uranium nucleus), releasing daughter nuclei, neutrons and energy again. This repeats in a chain reaction. With every generation more kinetic energy is released. This energy can be converted into heat energy in the nuclear reactor.

Question 57

What is the big problem with nuclear power?

Answer 57

It produces huge amounts of radioactive waste which is very difficult and expensive to dispose of safely

Question 58

Describe what happens inside a gas-cooled nuclear reactor?

Answer 58

Free neutrons in the reactor ‘kick start’ the fission process
The atoms produced then collide with other atoms, causing the temperature to rise
The moderator, usually graphite or water, slows neutrons so that they can successfully collide with uranium nuclei and sustain the chain reaction
Control rods limit the rate of fission by absorbing excess neutrons
A gas (CO2) is pumped through the reactor to carry away the heat generated
The gas is then passed through a heat exchanger where it gives its energy to water. This water is heated which turns into steam, which turns a turbine, which turns a generator.

Question 59

What is the function of the moderator?

Answer 59

The moderator, usually graphite or water, slows neutrons so that they can successfully collide with uranium nuclei and sustain the chain reaction

Question 60

What is the function of the control rods?

Answer 60

Control rods limit the rate of fission by absorbing excess neutrons

Question 61

How can ionising radiations be detected using a photographic film?

Answer 61

Radioactivity will darken photographic film, and this effect can be used to measure how much radiation has struck the film.

Question 62

What is the activity of a radioactive source equal to?

Answer 62

The number of decays per second

Question 63

What is activity measured in?

Answer 63

Becquerels (Bq)

1 becquerel = 1 decay per second

Question 64

Properties required of the source in tracing underground leaks in pipes

Answer 64

Must emit beta radiation as alpha is not able to pass through the ground above the pipe and gamma would give the same count rate whether or not a leak was present
Must have a long enough half life to remain reasonably active over the period of investigation but not too long so that it doesn’t remain a hazard to the environment.

Question 65

What are the dangers of ionising radiation?

Answer 65

• radiation can cause mutations in living organisms
• radiation can damage cells and tissue
• the problems arising in the disposal of radioactive waste.

Question 66

Why does the activity of a radioactive sample reduce with time?

Answer 66

Because with each half-life, the mass of the sample decreases, so if with each half-life the mass decreases to half, it is emitting less and less radiation with each half life so the radiation also decreases

Question 67

How can the risks of working with radioactive sources be reduced?

Answer 67

Wearing protective clothing, reduce the exposure time, handle material with tongs

Question 68

What is a nucleon?

Answer 68

A proton or a neutron (so both if they ask, basically the mass number)