SP6 - Radioactivity

Question 1

What is particle theory (kinetic theory)?

Answer 1

The model that explains the properties of different states of matter in terms of movement of particles.

Question 2

What is the absorption spectrum?

Answer 2

A spectrum of light (or other EM radiation) that includes black lines. These are caused by some wavelengths being absorbed by the materials that the light (or radiation) passes through.

Question 3

What is the emission spectrum?

Answer 3

A set of wavelengths of light or other EM radiation showing which wavelengths have been emitted by a substance.

Question 4

What is ionising radiation?

Answer 4

Radiation that can cause charged particles (ions) to form. It can cause tissue damage and DNA mutations.

Question 5

What was Dalton’s atomic model?

Answer 5

• atoms are indivisible
• nothing smaller than an atom
• atoms are tiny hard spheres

Question 6

What was Thompson’s Plum Pudding model?

Answer 6

• negatively charged electrons ‘raisins’ in a positively charged soup ‘cake’
• atoms are spherical
• only electrons can move (explaining ionic bonding)

Question 7

What was Rutherford’s gold foil experiment?

Answer 7

Rutherford set out to prove the plum pudding model. To do this he got a vacuum chamber and put gold foil in the middle. A radioactive source of alpha particles was placed in it and a detector was used which could be moved around. Some alpha particles bounced back, but most went straight through the gold foil, disproving the plum pudding model.

Question 8

What is the Bohr model?

Answer 8

The Bohr model is the atomic model which is generally accepted today. It contains a nucleus, where most of the mass is concentrated, which has protons and neutrons in it and electrons which orbit the nucleus in shells.

Question 9

What is an isotope?

Answer 9

A form of an element with the same number of protons but a different number of neutrons, resulting in a different mass number.

Question 10

How can electrons move between shells?

Answer 10

If an atom absorbs energy, an electron can move to a ‘higher’ orbit (shell). When an electron returns to a lower orbit, the atom emits energy as visible light of a particular wavelength.

Question 11

What is background radiation?

Answer 11

Ionising radiation that is around us all the time from a number of sources. Some background radiation is naturally occurring, but some comes from human activities.

Question 12

What are some sources of background radiation?

Answer 12

• radon gas
• medical industry (X-rays, PET scans, radiotherapy etc)
• soil
• cosmic rays
• food and drink
• nuclear power stations

Question 13

How is radioactivity measured?

Answer 13

Geiger-Müller (GM) tube - contains gas that is ionised upon contact with radiation. This creates a short pulse of electrical current to flow, which makes a noise. The count rate is the number of alpha particles, beta particles or gamma rays detected in a certain time (this is the reading recorded minus the background radiation)
Dosimeter - photographic film gets darker as more radiation reaches it, which can be seen when it is developed. Newer dosimeters have materials which change colour without needing to be developed.

Question 14

What are the properties of alpha particles?

Answer 14

• helium nucleus (2 protons, 2 neutrons)
• charge +2
• mass 4
• highly ionising
• weakly penetrating (only travels a few cm’s in air)
• can be stopped by paper

Question 15

What are the properties of beta particles?

Answer 15

• high speed electron
• charge -1
• mass 1/1836
• moderately ionising
• moderately penetrating (travels a few m’s in air)
  -can be stopped by 3mm thick aluminium

Question 16

What are the properties of gamma rays?

Answer 16

• wave/ray
• EM radiation
• no charge
• no mass
• weakly ionising
• highly penetrating (travels several km’s in air)
• stopped by lead (a few cm’s thick) or concrete (a few m’s thick)

Question 17

What is radioactive decay?

Answer 17

When an unstable nucleus changes by giving out ionising radiation to become more stable. Radioactive decay is spontaneous and unpredictable.

Question 18

What is alpha decay?

Answer 18

When the nucleus emits alpha particles.
Mass number goes down 4
Atomic number goes down 2

Question 19

What is beta minus decay?

Answer 19

When a neutron decays to form a proton plus an electron (which is ejected).
Mass number doesn’t change
Atomic number increases by one

Question 20

What is beta plus decay?

Answer 20

When a proton decays to form a neutron plus a positron.
Mass number doesn’t change
Atomic number decreases by one

Question 21

What is neutron decay?

Answer 21

When the nucleus ejects a neutron.
Mass number decreases by one
Atomic number doesn’t change

Question 22

What is half life?

Answer 22

The average time taken for half of the radioactive nuclei in a sample to have decayed (the activity halves). Half life is predictable.

Question 23

What is the unit for radioactivity?

Answer 23

Becquerels (Bq)
Counts per minute

Question 24

What is irradiation?

Answer 24

Irradiation is exposing something to ionising radiation (e.g. in order to sterilise food or medical equipment with gamma rays). The radiation does not remain in the object.

Question 25

What is contamination?

Answer 25

An unwanted addition that makes something unsuitable or impure (e.g. a person may be contaminated with a radioactive substance). The radiation stays in the object.

Question 26

How can radiation be used to detect the thickness of paper?

Answer 26

Paper is made by squeezing wood pulp between rollers. Paper can be made different thicknesses (due to different squeezing forces from the rollers). A detector is used to count the rate at which beta particles (from a radiation source) get through the paper. When the paper is too thin, more beta particles penetrate the paper and the detector will record a higher count rate. A computer senses this and reduces the force of the rollers to make the paper thicker. If it is too thick, the opposite happens.

Question 27

How can radiation be used to detect leaking pipes?

Answer 27

A gamma source added to water is used to detect leaks in water pipes buried underground. Where there is a leak, water flows into the surrounding earth. A Geiger-Müller tube following the path of the pipe will detect higher levels of radiation where there is a leak.

Question 28

How can radiation be used to irradiate food?

Answer 28

All foods contain microorganisms that will cause them to decompose. Gamma rays can irradiate the food to kill bacteria. This makes it safer to eat and means it can be stored for longer without going off.

Question 29

How can radiation be used in smoke detectors?

Answer 29

A smoke alarm contains a source of alpha particles (americium-241). This ionises air and causes ions to be attracted to electrically charged plates, creating a current. When smoke gets into the smoke detector, it slows down the ions and so the current dips. If the current gets below a certain level, an alarm sounds.

Question 30

How can radiation be used to detect cancer?

Answer 30

In a PET scanner, a radioactive tracer undergoing beta plus decay is injected into a patient. It emits positrons, which when interacting with an electron, causes annihilation. This causes two gamma rays to travel in opposite directions. These gamma rays are detected by a gamma camera. The active areas take up more tracer, which the computer shows up as brighter colours.

Question 31

What are the features of the two types of radiotherapy?

Answer 31

Internal:
- beta emitter
-inside the body
-close to the tumour

External:
-most common
-beam of gamma rays/X rays/proton
-directed at tumour from outside the body

Question 32

What is nuclear fission?

Answer 32

Nuclear fission is the reaction in which the nucleus of a large atom, such as uranium, splits into two smaller nuclei. It releases large amounts of energy and is the process used in nuclear power stations.

Question 33

Describe the chain reaction of fission.

Answer 33

A neutron hits a uranium or plutonium nucleus. The nucleus splits into two smaller (daughter) nuclei, which are radioactive and some neutrons, which are released. Energy is also released. This process continues.

Question 34

What are the components of a nuclear reactor core and why are they necessary?

Answer 34

Fuel rods - rods containing the fuel for the nuclear reactor (uranium or plutonium)
Control rods - rods that can be lowered to absorb neutrons and slow down (or stop) the nuclear chain reaction
Moderator - usually heavy water (H30), which slows down neutrons, so that they are more likely to hit unstable nuclei
Concrete shielding around the outside - to prevent radiation and stray neutrons from escaping the core

Question 35

What is nuclear fusion?

Answer 35

Nuclear fusion is a nuclear reaction, in which lighter nuclei are combined to form heavier product nuclei with the release of enormous amounts of energy.

Question 36

What are the conditions required for fusion?

Answer 36

• high temperature (millions of degrees)
• high pressure
• high density
• strong gravitational field

Question 37

Why does there need to be a high temperature for fusion to occur?

Answer 37

If the temperature is not high enough, the particles will simply collide and rebound, due to electrostatic repulsion (both positive). If the temperature is high enough, the particles have enough energy to overcome their repulsion and fuse.