GCSE Physics: Radioactivity

Which number is the proton (atomic) number

Which number is the proton (atomic) number

Bottom, smaller number

Which number is the nucleon (mass) number

top, bigger number

What does the nucleon (mass) number tell you about protons & neutrons

total number of protons + neutrons

What does the proton (atomic) number tell you about protons & neutrons

number of protons

Isotope

the same number of protons and electrons but a different number of neutrons.

it is the same element but chemically slightly different

Relative mass of an electron

1/1836

Relative mass of proton and neutron

1

Rutherford’s experiment

• Rutherford fired positively charged alpha particles at gold foil and recorded their paths.

• According to the plum pudding model that was believed at the time, particles should refract in various directions due to scattered electrons.

• Most particles went straight through; some refracted slightly, and a few bounced back.

• Conclusion: Atoms are mostly empty space with a small, positively charged nucleus that repels some particles.

How are neon lights created

• Atoms absorb energy and some of its electrons jump to another shell.

• When they return to their old shell the atom emits energy as visible light of a particular wavelength (colour).

• Electrons can make different orbit changes (jump to different places) and each change produces a different wavelength of light

Emission spectrum of light

• each line is one colour (wavelength)

• you only get specific colours, because the electrons can only jump between a finite amount of shells.

• this one is the one for neon but each gas has a different one

Ionising radiation

• radiation that causes electrons to escape from atoms, forming positively charged ions (cations)

• the electron has so much energy that it completely escapes from the atom

• if there isn’t enough energy, there won’t be any ionisation

2 Ways of measuring radiation

1. Photographic Film: absorbs radiation and changes colour

2. Geiger-Muller Tube (GM tube): counts the amount of radiation particles that enter a tube

Main sources of background radiation

• cosmic radiation (stuff in space)

• food and drink

• radon (gas seeping out of the ground)

• rocks and buildings

• x-rays

• power stations

• artificial radiation

Unit for radiation dosage

millisievert (mSv)

Radioactive decay

• radioactive atoms have unstable nuclei

• they decay be emitting radiation - it makes them lose energy and become more stable

• random process, but half-lives enable it to be predicted with a large number of nuclei

Alpha particles (4 things)

• α

• helium nucleus (2 protons, 2 neutrons)

• stopped by sheet of paper

• most ionising

• least penetrating

Beta minus particles (4 things)

• β^-

• same as positrons just with negative charge

• stopped by 3mm aluminium

• electron emitted from a nucleus

Positrons (4 things)

• β⁺

• same as beta minus just with positive charge

• stopped by 3mm aluminium

• electron emitted from a nucleus

Gamma rays (5 things)

• γ

• electromagnetic wave with no charge

• stopped by thick lead / several meters of concrete

• least ionising

• most penetrating

How does ionising & penetrating correlate

Strongly ionising radiation can easily attract and remove electrons from atoms, causing it to lose energy rapidly, so it has low penetration power.

Weakly ionising radiation struggles to remove electrons, retains more energy over a longer distance, and so has high penetration power.

Every time a particle attracts an electron from another atom, it looses power, which is why the ones that attract easily loose power the quickest

Alpha decay

A radium nucleus splits into an alpha particle and a radon nucleus

table for radioactive decay

Half-life

the average time taken for the number of unstable nuclei to halve. Also the average time taken for the activity to halve.

Activity

number of nuclear decays each second

Unit for measuring radioactive activity

Bq - becquerel
one nuclear decay per second

Formula for activity after n half-lives

2ⁿ

Uses of radioactivity: smoke detectors

• An americium-241 source gives off alpha particles

• Alpha particles ionise the air, and these charged particles move across the gap, producing a current

• A detector senses the number of ionised air particles as a current

• Smoke in the machine slows down the ions and so the detector current falls

• The siren sounds when the detector current falls below a certain level

Uses of radioactivity: paper makers

• Beta particles are emitted and sent towards paper

• Some of the beta particles are absorbed by the paper

• Detector counts the number of beta particles that get through the paper

• When the paper is too thick, the computer senses that the number of beta particles getting through the paper has fallen

• The computer increases the pressure applied to the rollers to make the paper thinner

Why are gamma rays used as tracers in medicine

they are the most penetrating - they will travel further and through most things

Radiotherapy

using radiation to treat cancer

How do PET scanners work

1. patient is injected with tracer (a radioactive substance)

2. tracer is attached to a molecule that collects in the tumour - it can by any molecule

3. tracer is placed in the tumour

4. when the nuclei in the tracer decay, they emit positrons

5. when a positron meets an electron they are both destroyed and two gamma rays are emitted in opposite directions

6. the PET scanner moves around the patient and detects the gamma rays and calculates where the radiation came from

7. A computer builds a 3D image of the inside of the patient

Why are the half-lives of radioactive isotopes used in medical tracers short

so that any radioactivity in the body will decay quickly leaving less time for it to damage the body

Internal radiotherapy

radioactive ‘seeds’ that are implanted into the body near the tumour

External radiotherapy

sending beams of gamma rays/x-rays/protons into a patient to kill cells in the tumour

Why are the half-lives of radioactive isotopes used in external radiotherapy long

if the radioactive isotope used has a short half-life, it will need to be replaced sooner

Irradiation

when something is exposed to radiation, but the sources aren’t involved and the object won’t become radioactive. it’s less harmful than contamination

Contamination

when radioactivity has escaped containment, and the sources could go onto the object/body. It is more dangerous to the object/person involved, as they could become radioactive

How do scientists/doctors protect themselves from radiation

• leaving the room

• wearing protective clothing

• using equipment when handling sources/materials

• not standing too close

2 main effects of ionising radiation on the body

1. Radiation burns: tissue damage e.g. reddened skin

2. Mutations: damage to cells, changing the DNA - this can cause diseases such as cancer

Why are nuclear disaster areas long-term exclusion zones

the radioactive sources there have very long half-lives meaning they still aren’t decayed enough to be safe

Nuclear fusion

two light nuclei join together to produce a heavier nucleus

Nuclear fission

one heavy nucleus splits into two lighter nuclei (the opposite of fusion)

Uranium-235 nuclear fission equation

Uranium-235 + neutron → two daughter nuclei (the products) + some neutrons + energy

number of neutrons produced depends on the daughter nuclei produced, as they have to add up to 236 (number of neutrons in the uranium + the one neutron that was absorbed)
e.g.
Uranium-235 + neutron → Krypton-91 + Barium-142 + 3 neutrons

Nuclear fission process

1. neutron travelling fast collides with a Uranium-235 nucleus

2. The Uranium-235 absorbs the neutron

3. The Uranium-235 + nucleus splits, creating two daughter nuclei and an amount of neutrons

4. The neutrons created can then go on to collide with other Uranium-235 nuclei and repeat the process, creating a chain reaction

Nuclear fission power stations (what are the uses for these things)

Moderator: slows down neutrons so that they are more likely to collide with the uranium
Fuel rods: contain the uranium-235
Control rods: absorb neutrons to control the rate of the reaction
Steam from the heat exchanger: makes the turbines spin
Turbines: turn the generator, steam condenses in here and then goes to be cooled
Generator: produces electricity
Coolant: runs through the reactor core, turns water into steam
River, sea or lake: steam needs to be cooled before going back to the heat exchanger so is pumped through a river, sea, or lake to cool it down

What happens if a power stations needs to produce more energy

some control rods are taken out to increase the rate of reaction

Nuclear fusion process

• Nuclei are positively charged, so normally repel each other.

• To combine in nuclear fusion they need to be at really high temperatures and high pressure, which makes them travel at really high speeds.

• This makes them ignore the fact that they repel each other and combine.

How is energy released from nuclear reactions

When nuclei under go fission/fusion, their total mass decreases and the ‘missing’ mass is released as energy

Atom definition

• positively charged nucleus (protons and neutrons)

• surrounded by negatively charged electrons

• tiny nucleus compared to the whole thing

• almost all mass in nucleus

Background radiation

low-level, natural radiation present in the environment, that everyone absorbs

β^- decay

neutron becomes a proton + an electron

β⁺ decay

proton becomes a neutron + a positron

Uses of radioactivity: irradiating food

• food can be irradiated with gamma rays to kill bacteria

• makes it safer to eat

• means it can be stored for longer before going off

• doesn’t make the food more radioactive/dangerous

Uses of radioactivity: sterilising equipment

• surgical instruments need to be sterilised to kill microorganisms

• usual way is to heat them but some instruments cant be heated - so radiation is used

• they are sealed in bags and irradiated with gamma rays (which penetrate the bags)

Why do tracers used in PET scanners have to be produced nearby

they have short half-lives, so will decay if they have to be transported for a long time

What is nuclear fusion the source of

energy for stars