Physics 7 - Radioactivity and Particles

Question 1

7.1 List the units and unit symbols of: activity of a radioactive sample

Answer 1

Becquerel (Bq)

Question 2

7.2 what is the relative mass of an electron

Answer 2

1/2000

Question 3

7.4 Where are alpha (α) particles, beta (β−) particles, and gamma (γ) rays emitted from and which ones are ionising

Answer 3

All are ionising and are emitted from an unstable nucleus

Question 4

7.5 Describe the nature of alpha (α) particles, beta (β−) particles, and gamma (γ) rays **and give their nuclear symbol

Answer 4

Alpha ((_2^4)α) - slow moving helium nuclei (2p + 2n).

Beta ((_−1^0)e) - fast moving electrons.

Gamma ((_0^0)γ) - high energy electromagnetic radiation. Emitted after either alpha or beta

A + B have charge so are deflected by magnetic and electric fields. Gamma doesn’t.

Question 5

7.5 Describe the penetrating power of alpha (α) particles, beta (β−) particles, and gamma (γ) rays

Answer 5

Alpha - poor penetration in materials because they have a large charge and mass. Stopped by paper and a few cm of air

Beta - moderately penetrating - few mm of aluminium

Gamma - very penetrating - stopped by a few cm of lead or a few metres of concrete

Question 6

7.5 Describe and explain the ionising ability of alpha (α) particles, beta (β−) particles, and gamma (γ) rays

Answer 6

Alpha - strongly ionising - large charge and mass so easily knock off electrons

Beta - moderately ionising - have charge but small mass

Gamma - weakly ionising - no charge or mass and tend to pass through atoms. When they do collide they can cause damage.

Question 7

7.6 practical: Design an experiment to investigate the penetration powers of different types of radiation

Answer 7

Set up a Geiger-Muller counter, and measure the background count for 30s (stopwatch). Divide the count by 30 and repeat and take an average.

Place the source in front of the detector and measure the count over 30s and determine the rate per second - then subtract background count. Repeat and take an average

Insert different materials between the source and the detector (paper, a few mm of aluminium, lead) and repeat the procedure.

If the count remains unchanged then it can penetrate the material; if it drops considerably then it can’t.

Question 8

7.7 describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the four main types of nuclear radiation (alpha, beta, gamma and neutron radiation)

Answer 8

Alpha - mass number drops 4, atomic number drops 2
Beta - mass number is unchanged, atomic number adds 1
Gamma - no change
Neutron - mass number decreases by 1, atomic number is unchanged

Question 9

7.8 Explain how to balance nuclear equations in terms of mass and charge and give examples of the four main types of nuclear radiation

Answer 9

Mass and charge must remain equal before and after the decay.

The charge of a nucleus = atomic number. Mass of a nucleus = mass number

Question 10

7.9 Give examples of how to detect ionising radiation

Answer 10

Photographic film or Geiger-Muller detectors

Question 11

7.10 List the sources of background (ionising) radiation

Answer 11

Substances on Earth: air, food, building materials, soil, rocks
Cosmic rays: from the sun and outside our solar system (possibly supernovae or possibly other galaxies)
Living things: isotopes in plants and animals can be radioactive
Human activity: fallout from nuclear explosions, nuclear waste (tiny proportion)

Question 12

7.11 Define the activity of a radioactive sample

Answer 12

The rate of decay of an unstable nucleus

N.B. detectors only pick up a fraction of activity - small aperture size

Question 13

7.11 Describe how the activity of a radioactive source changes over time

Answer 13

Activity decreases exponentially over time (decreases by half every half-life)

Question 14

7.11 Give the unit for activity and describe what it means

Answer 14

Becquerels - number of detections per second

Question 15

7.12 Define half-life

Answer 15

Time taken for half of the nucleus to decay OR time taken for the activity to drop by half

Question 16

7.12 How can a radioactive isotope be determined from a sample

Answer 16

Every radioactive sample has a different half life so you can use this to determine the isotope

Question 17

7.12 Give the range of half-lives and state what determines the half-life of an isotope

Answer 17

The more unstable the nucleus, the shorter the half life.

Half lives vary from billions of years to thousandths of a second

Question 18

7.13 Describe how to determine the half-life of an isotope given the initial activity, the final activity and the time taken

Answer 18

Remove the background radiation count from the initial and final activity.

Determine how many times it takes to divide the initial by two to get to the final (# of half lives)

Divide time taken by # of half lives

Question 19

7.13 Describe how to determine the half-life of an isotope given an activity-time graph

Answer 19

Draw a line from half of the activity to the curve. Draw down.

To check, do the same for the next half and the time taken should be the same.

Question 20

7.14 Give three uses of radioactivity in medicine and describe how the radioactive isotopes are used, including the properties of the radioactive isotope.

Answer 20

Medical tracers
• Use beta or gamma sources so they can penetrate the body and disappear quickly (short half life)
• Source is injected or swallowed and can be detected externally. Used to check whether organs use the elements they expect

Treat Cancer
• Use a gamma source to penetrate the patient, with a long half life so activity remains constant.
• Gamma kills cancer cells - and is carefully directed at the tumour to not harm the rest of the body.

Food and equipment sterilised
• Use strong gamma sources with a long half life
• Irradiate food or medical equipment to kill microbes
• Doesn’t damage equipment or food like high temperatures or chemicals

Question 21

7.14 Give two uses of radioactivity in industry and describe how the radioactive isotopes are used, including the properties of the radioactive isotope.

Answer 21

Industrial tracers
• Short half life so its isn’t a long term hazard and gamma so it can be detected through pipes
• Detect leaks in pipes by placing the source in it, and where there is increased count above ground, the source is leaking.

Smoke detector
• Alpha, because it is stopped by air
• Ionize air so complete circuit is made, long half life so lasts a long time
• If smoke gets between detector and source, the alpha particles are absorbed and the circuit is broken so the alarm goes off.

Question 22

7.15 Describe the difference between contamination and irradiation

Answer 22

Contamination - radioactive atoms get onto or into an object - then decay releasing radiation and causing harm

Irradiation - objects near a radioactive source are irradiated by it but do not become radioactive

Question 23

7.16 describe the dangers of ionising radiations

Answer 23

Can cause mutations in living organisms, causing them to rapidly multiply - cancer
Can kill cells or tissues in higher doses.

Question 24

7.16 Describe how you can reduce risks from contamination and irradiation

Answer 24

Irradiation
• Keep sources in clearly labelled lead boxes
• School - Use tongs to increase distance to source
• Industry - Stand behind barriers, be in a different room, use a remote-controlled arm
• Limit exposure time

Contamination
• School - wear gloves and use tongs
• Industry - wear protective suits and masks to stop breathing it in

Question 25

7.16 Give examples of low-level radioactive waste

Answer 25

Clothing and syringes from hospitals and nuclear power stations

Question 26

7.16 Give examples of high-level radioactive waste

Answer 26

Spent fuel rods, material from the processing of radioactive material

Question 27

7.16 Describe the disposal of different levels of radioactive waste and the problems arising

Answer 27

Low-level waste - bury in a secure landfill site

High level waste - seal into glass blocks, then into metal canisters, then bury deep underground - can stay highly radioactive for thousands of years so is very dangerous

Issues - difficult to find a place to bury: must be geologically stable so it doesn’t leak (which could contaminate soil, plants or even drinking water)