Physics AQA Year 10: Radioactivity

What is the radius of an atom?

about 1 x 10 to the power of -10 m

What is the basic structure of the atom?

a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons

How big is the nucleus compared to the atom?

Less than 1/10,000

Where is the mass of the atom concentrated?

in the nucleus

What is the mass number?

Number of protons and neutrons

What is the atomic number?

number of protons

What is an isotope?

atoms with the same number of protons but different number of neutrons

What was the first ever model of the atom?

. Ancient Greeks

. Millions of tiny, uncuttable pieces of matter

What was the plum pudding model?

. Thomsom

.Ball of positive charge with with negative electrons embedded in it

What was the alpha particle scattering experiment?

. Done by Rutherford

. Beam of alpha particles directed at very thin sheet of gold in a vacuum surrounded by a scintillation screen

What were the results of the alpha particle scattering experiment?

. Most of the alpha particles did pass straight through the foil

. A small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil

. A very small number of alpha particles came straight back off the foil

What did Rutherford conclude from his experiment?

. Most alpha particles went straight through the foil so the atom must be mostly empty space

. A small number of alpha particles were deflected at large angles so there's a concentration of positive charge in the atom - like charges repel, so the positive alpha particles were being repelled by positive charges

. The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - the tiny number doing this means the chance of being on that exact collision course was very small, and so the 'target' being aimed at had to be equally tiny

What was discovered after the nuclear model?

. Bohr discovered electrons orbiting at specific distances (energy levels)

. His theoretical calculations agreed with experimental observations

What was discovered after energy levels?

protons

What was discovered after protons and when?

. Chadwick discovered neutrons

. Around 20 years after the nucleus became an accepted scientific idea

What is radioactive decay?

. some atomic nuclei are unstable

. the nucleus gives out radiation as it changes to become more stable

. this is a random process

What is activity?

the rate at which a source of unstable nuclei decays

What is activity measured in?

Becquerels (Bq)

What is count-rate?

The number of decays recorded each second by a detector (e.g Geiger-Muller tube and counter)

What may nuclear radiation be emitted as?

. alpha particle

. beta particle

. gamma rays

. neutron

Symbol for alpha (Still need write both numbers next to it for alpha particle)

α

Symbol for beta (Still need write both numbers to the left of it for beta particle)

β

Symbol for gamma

γ

What is an alpha particle? (nature)

. 2 protons and 2 neutrons

. same as helium nucleus

What is a beta particle? (nature)

a high speed electron ejected from the nucleus as a neutron turns into a proton

What are gamma rays? (nature)

electromagnetic radiation from the nucleus

What does the top number next to a symbol mean when talking about radioactivity?

mass number

What does the bottom number next to a symbol mean when talking about radioactivity?

charge of particle

What is the charge of an alpha particle?

+2

What is the charge of a beta particle?

-1

What is the charge of a gamma ray?

no charge

What is the mass of an alpha particle?

4

What is the mass of a beta particle?

very small

What is the mass of a gamma ray?

0

What is the ionising power of an alpha particle?

high

What is the ionising power of a beta particle?

small

What is the ionising power of a gamma ray?

tiny

What is ionising radiation?

. can expel electrons from atoms near their path

. can lead to cell damage or mutations leading to cancer

What is the penetrating power of alpha particles?

. skin

. a sheet of paper

. <5cm of air

What is the penetrating power of beta particles?

. a few m of air

. a few mm of aluminium

. a few mm of lead

What is the penetrating power of gamma rays?

. a few m of concrete

. 3-5 cm of lead

. many km of air

What is the half-life of a radioactive isotope?

. The time it takes for the number of nuclei of the isotope in a sample to halve

OR

. the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level

How can we calculate half life if radioactive decay is random?

. It is impossible to predict which nucleus will decay next or the exact time when a specific nucleus will decay

. Predicts when about half the nuclei will have decayed if the sample is large enough

How would you calculate the count rate after 3 half lives?

original count rate / (2 ^ 3)

How would you find isotopes in order of increasing stability of their nuclei?

. Substance with longer half-life has more stable nuclei

. So answers are in order of increasing half-life

What is radioactive contamination?

the unwanted presence of materials containing radioactive atoms on other materials

What is irradiation?

. the process of exposing an object to nuclear radiation

. irradiated object does not become radioactive

Why is irradiation hazardous?

. radiation is ionising

. could give cell damage, radiation sickness, or cancer

How can people protect themselves from irradiation?

. stay 2 m away

. wear lead apron

. use long tongs when handling to increase distance

Why is contamination hazardous?

. due to the decay of contaminating atoms

. type of radiation emitted affects level of hazard

. you become radioactive

. you irradiate people around you

. you receive a much higher dose of radiation

How can people protect themselves from contamination?

. Wear gloves

. Keep sources locked away when not being used

Where does background radiation come from?

. natural sources such as rocks and cosmic rays from space

. man-made sources such as the fallout from nuclear weapons testing and nuclear accidents

What is radiation dose measured in?

Sieverts (Sv)

How many mSv in a Sv?

1,000

What is the aim of radiotherapy (gamma knife)?

. Kill tumour / cancer

. Remove unwanted things in the body

How does radiotherapy work (gamma knife)?

. Multiple gamma rays are aimed at the tumour

from different directions

. Patient is held in place so they can't move

What type of radiation is radiotherapy (gamma knife) and why?

. gamma

. it needs to penetrate into the body until the

tumour

Does radiotherapy (gamma knife) have a long or short half life and why?

. Long half life

. Because the source is in the machine, which then

works for many years

What are the risks of radiotherapy (gamma knife)?

Radiation damages healthy cells in the patient

How are the risks of radiotherapy (gamma knife) minimised?

Rays are sent from different directions, so the healthy cells receive low radiation dose but the tumour receives high radiation dose

What is the aim of radiotherapy (brachytherapy / beta implant)?

Kill tumour/cancer

How does radiotherapy (brachytherapy / beta implant) work?

. A radioactive source is implanted in the tumour

. The source usually emits beta radiation

What type of radiation is radiotherapy (brachytherapy / beta implant) and why?

. Beta

. it needs to be quite ionising and not very penetrating so the radiation is absorbed locally by the tumour

Does radiotherapy (brachytherapy / beta implant) have a long or short half life and why?

. short

. just for the duration of the treatment

What are the risks of radiotherapy (brachytherapy / beta implant)?

Ionising radiation damages healthy cells and can

cause cancer

How are the risks of radiotherapy (brachytherapy / beta implant) minimised?

. Have a short half life, so the source is radioactive for not too long

. Use a small amount

What is the aim of Radioactive tracers / PET scans?

. Find out where the tumour or other problem is

. Medical imaging

How do Radioactive tracers / PET scans work?

. Inject the patient with a radioactive tracer (a

chemical used by body with a radioisotope)

. We use a special camera to detect where the

radiation comes from

What type of radiation is Radioactive tracers / PET scans and why?

. Gamma

. so the radiation can travel through the

body (high penetrating power)

Do Radioactive tracers / PET scans have a long or short half life and why?

. Short half life

. so the radioactive source is only in the body for a short time

What are the risks of Radioactive tracers / PET scans?

Ionising radiation damages healthy cells and can

cause cancer

How are the risks of Radioactive tracers / PET scans minimised?

. Don't use this if pregnant

. Use a short half-life so the radioactive source is

only inside the body for a short time

What is the aim of Sterilisation by irradiation?

Kill bacteria and micro-organisms on food and

medical equipment

How does Sterilisation by irradiation work?

. Gamma rays are projected on the food/medical

equipment

. The food/equipment is already in its package

What type of radiation is Sterilisation by irradiation and why?

. Gamma

. Very penetrating so bacteria everywhere is killed

Does Sterilisation by irradiation have a long or short half life and why?

Long half life so the machine works for a long time

What are the risks of Sterilisation by irradiation?

. THE FOOD/EQUIPMENT DOES NOT BECOME

RADIOACTIVE

. Risk is for the workers only: radiation exposure

How are the risks of Sterilisation by irradiation minimised?

. Workers use protective equipment like lead

aprons, they stay away/in another room

. Their radiation dose is monitored and they will be moved to another activity if too high

What is the aim of a smoke alarm?

Detect smoke before there's a big fire

How does a smoke alarm work?

. The radioactive substance causes the air inside

the smoke alarm to be ionised and creates a

current which stops an alarm from ringing

. When smoke enters the alarm, it absorbs the

radiation, stopping the current and ringing the alarm

What type of radiation is Smoke alarm and why?

. Alpha

. because it is the most ionising radiation

Does a smoke alarm have a long or short half life and why?

. Long half-life

. to make sure the smoke alarm keeps working for a long time

What are the risks of a smoke alarm?

No risks unless you eat your smoke alarm

How are the risks of Smoke alarm minimised?

Alpha radiation is absorbed by the packaging as it is least penetrating

What is nuclear fission?

the splitting of a large and unstable nucleus e.g uranium or plutonium

What generally causes fission to occur?

the unstable nucleus absorbs a neutron

What happens when fission occurs?

. Nucleus splits into 2 smaller nuclei, roughly equal in size

. Emits 2 or 3 neutrons plus gamma rays

. Energy is released by the fission reaction

What do all the fission products have in common?

All have kinetic energy

What fuel is generally used in nuclear fission?

. Uranium

. Plutonium

What can happen after fission has occurred?

The neutrons may go on to start a chain reaction

Why might we want and not want to control a chain reaction?

. Want in a nuclear reactor to control energy released

. Don't want in a nuclear weapon as explosion is caused by uncontrolled chain reaction

What does a moderator do in a nuclear reactor?

material that slows down neutrons to ensure they're absorbed

What do control rods do in a nuclear reactor?

absorbs the neutrons to control the rate of reaction

What is nuclear fusion?

. The joining of 2 light nuclei to form a heavier nucleus

. In this process some of the mass may be converted into the energy of radiation

What conditions are needed for fusion to occur?

. very high temps

. very high pressure

. mostly happens in stars

Why is fusion better than fission?

. Can't melt down

. Doesn't produce nuclear waste

. Higher energy density