Atoms and radioactivity

The structure of the atom:

- Proton

- Neutron

- Electron

Relative mass & charge of proton:

- relative mass = 1

- relative charge = +1

Relative mass & charge of neutron:

- relative mass = 1

- relative charge = 0

Relative mass & charge of electron:

- relative mass = approx 0

- relative charge = -1

The Atomic number of an element is:

- number of protons/electrons

The Atomic mass/ mass number of an element is:

- Number of protons + neutrons

An atom becomes an ion when:

- It loses or gains an electron (or more).

loss = positive charge

gains = negative charge

What are isotopes?

Atoms with same numbers of protons but different numbers of neutrons

What are radioisotopes?

Unstable isotope that emits radiation; to become more stable.

What does more abundance mean?

much higher % on the earth than the others.

Electron shells:

- Every atom has the same no. of protons & electrons; makes them electrically neutral.

- The electrons are arranged in shells/energy levels.

- An electron can sometimes absorb an electromagnetic ray and be temporarily to a high energy level which is described as being in an Excited state.

- The electron will soon drop/fall back to the ground state emitting another electromagnetic ray in the process. The higher the energy of the EM ray, the bigger the jump in energy levels.

What does ionising radiation mean?

The radiation can cause atoms that it interacts with to become charged, turns the atoms into an ion.

Plum pudding model:

discovered electrons in 1897

- atoms= sphere of + charge with - charge inside it.

When was electrons discovered?

1897

Rutherford's experiment:

- alpha particles through thin layer of gold

- alpha particles deflected & bounced back

- showed most of mass concentrated in nucleus in atom

Energy levels:

- niels bohr in 1912, showed electrons in atom orbit nucleus at specific distances.

When was the protons discovered?

- 1919= charge of nucleus can be divided into smaller charges called protons

When was neutrons discovered?

- 1932= chadwick

Which observations made Rutherford realise that the Plum Pudding model did not work?

- Alpha particles bounced back/deflected at 90 degrees.

- can only happen if something was repelling the positive alpha particles.

Types of Background Radiation sources?

- Medicine

- cosmic

- radon gas

- food + drink

- nuclear industry

Background radiation:

- half of the background radiation is due to radon gas; emitted during the radioactive decay of small amounts of uranium found in rocks.

- 1/10 of background radiation is due to rays from space.

What is ionising radiation?

- radiation (small fast moving particles/EM rays) which have enough energy to remove all electrons from its atom; the atom is ionised, causing ions to be formed.

What is activity?

- the rate at which a source of unstable nuclei decay.

- activity is measured in bequerel (Bq).

What is count-rate?

- the number of decays recorded each second by a detector.

Alpha: a

- 2 protons & 2 neutrons

- relative charge = +2

- relative mass = 4

- penetrating ability is low; stopped by paper & few cm of air.

- ionising ability is strong

- is deflected by an electric field

- is deflected by a magnetic field

- used in smoke detectors.

Beta: B

- electron

- relative charge = -1

- relative mass = approx 0

- penetrating ability is moderate; stopped by aluminium & 50cm of air.

- ionising ability is strong

- is deflected by an electric field

- is deflected by a magnetic field

- used in thickness gauging

Gamma: Y

- Electromagnetic radiation

- relative charge = 0

- relative mass = 0

- penetrating ability is high; reduced by thick lead.

- ionising ability is weak

- used in radiotherapy, tracing and sterilisation

Alpha & beta particles are deflected in

Electric & magnetic fields bc they are charged.

Deflected in opposite directions as Alpha is + charged & Beta is - charged.

In an electric field:

- Alpha deflected towards negative plate

- Beta deflected towards positive plate

- Opposites attract

In an electric and magnetic field:

- Beta is deflected more than alpha as it is half the charged, but 8000x less mass.

- Gamma isn't deflected as it's not charged

What is nuclear decay?

- When an unstable nucleus emits alpha/beta particles, the no. of protons in the nucleus changes. This means the atom has changed to that of a different element. This is known as nuclear decay and the nucleus will become more stable.

Nuclear decay: Alpha particles

- Alpha particle is a helium nucleus, made up of 2 protons & 2 neutrons. This means the atomic mass will decrease by 4 and the atomic number will decrease by 2.

4

.....He

2

Nuclear decay: Beta particles

- Beta particle is emitted when a neutron turns into a proton and emits a fast moving electron (which is the beta particle). This means the atomic number will increase by 1 but the atomic mass will stay the same.

0

......e

-1

Nuclear decay: Gamma particles

- When a gamma ray is released, there is no change to the nucleus, bc no particles are released, only energy.

Smoke detectors: Applications of ionising radiation

- Uses Americium-241 which is an alpha emitter

- Beta/gamma radiation is not suited as would penetrate the smoke and still cause some ionisations; but enough anyways

Tracing: Applications of ionising radiation

- Gamma from leak penetrates through the ground to the detector

Thickness gauging: Applications of ionising radiation

- Alpha is too easily absorbed, gamma won't be stopped.

- Depending on thickness, beta will pass through

Sterilising food: Applications of ionising radiation

- Gamma passes through vacuum sealed packaging or through fruit/veg; killing microbes

Radioactive decay is:

- Random process; can't make it emit radiation faster or slower.

- Only way to measure a radioactive source is to record it's activity/count rate, and find how activity changes with time.

What is half life?

- time taken for no. of unstable nuclei present to fall to half the original numbers.

- time taken for activity of radioactive sample to fall to half the original value.

Carbon dating: application of a long half life

- all living things have fixed amounts of carbon

- when they die, c-12 stays the same but c-14 starts to decay.

- decays by beta emission to nitrogen gas

- proportion of c-14 falls

- compare to modern day organism proportion

- c-14 has a half life of 5700 yrs so can date back to old stuff

What does contamination mean?

- radioactive materials/nuclei are on skin or inside the body; unwanted presence

What does irradiated mean?

- in the presence of ionising radiation which may pass through your body from outside/reach you as waves.

Radioactive tracers

- Tracer is ingested/injected. Radiation from gamma emitter inside body detected from outside; organ function tracked.

- Kidney test details; patient drinks water containing isotopes builds up in kidney if it's blocked; passes out if not blocked, count rate over time shows this.

- Iodine half life is 8 days.

Gamma cameras

- patient injected with solution of radioactive isotope; emits gamma radiation; isotopes absorbed by organ; gamma camera detected radiation leaving body; creating image of organ.

- gamma radiation can be detected outside body; half life long to make image.

Gamma radiation to destroy tumours

- gamma rays destroy cancerous cells, narrow beam minimises healthy cell damage.

- isotopes emits gamma, penetrates deeper into tumour than alpha/beta. half life of 5 years.

Radioactive implants:

- tiny radioactive implants, emit beta/gamma radiation; implanted into cancerous tumours.

- Half life is long enough to irradiate tumour over desired time, short enough that most radioactive nuclei will have decayed soon afterwards.