P7 (not mine)

What is a radioactive substance?

What is a radioactive substance?

A radioactive substance contains unstable nuclei that become stable by emitting radiation.

What are the three main types of radiation given out?

alpha , beta, gamma

alpha particles

Positively charged particles

Is radioactive decay random or systematic?

Random - you can't predict which nucleus in a sample will decay next

Write two differences between radiation from uranium and radiation from a lamp

radiation from U consists = particles, radiation from lamp = electromagnetic waves,

radiation from U is ionising, radiation from lamp is non-ionising

Write two differences between radioactive atoms compared with the atoms in a filament lamp

radioactive atoms have unstable nuclei whereas atoms in lamp filament do not,

decay of radioactive atom cannot be stopped whereas atoms in lamp filament stop emitting radiation when

filament current switched off

Write four features of every nucleus of every atom

nucleus much smaller than atom,

nucleus positively charged,

mass of atom concentrated in nucleus

all positive charge of atom concentrated in nucleus

How can you measure radiation levels?

Geiger counter

History of the atom

Before the discovery of the electron, atoms were thought to be tiny

spheres that could not be divided.

Plum Pudding Model

The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

Alpha particle experiment

The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model

replaced the plum pudding model.

Results of alpha scattering experiment

• most passed straight through the metal foil

• some were deflected back towards the source

Why was the experiment carried out in a vacuum chamber

To prevent air molecules absorbing the alpha particles

Bohr Model

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.

Protons

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge.

The name proton was given to these particles.

Chadwick

The experimental work of James Chadwick provided the evidence

to show the existence of neutrons within the nucleus.

Nuclear Model of the Atom

A conceptual model of the atom in which a

small positively - charged nucleus is

surrounded by planetary electrons.

Differences between nuclear model and plum pudding model

• nuclear: all positive charge concentrated in nucleus much smaller than atom, plum pudding: positive charge spread out throughout atom,

• nuclear: most mass concentrated in nucleus, plum pudding: mass spread out throughout atom

• nuclear: most atom empty space, plum pudding: no empty space

Why did the alpha scattering experiment lead to the rejection of the plum pudding model

nuclear model explains why some alpha particles scattered through large angles,

in plum pudding model such large-angle scattering should not be observed

How big are atoms?

Atoms are very small, having a radius of about 1 × 10-10 metres

What is activity and what is it measured in?

Activity is the rate at which a source of unstable nuclei decays.

Activity is measured in becquerel (Bq)

What is count rate?

Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).

Describe the three types of radiation?

• an alpha particle (α) - this consists of two neutrons and two protons, it is the same as a helium nucleus

stopped by paper

• a beta particle (β) - a high speed electron ejected from the nucleus as a neutron turns into a proton

stopped by a thin sheet of metal

• a gamma ray (γ) - electromagnetic radiation from the nucleus stopped by thick led

Which part of an atom emits a beta particle?

nucleus / neutron

How does the composition of an atom change when it emits an alpha particle?

its atomic number goes down by two

its mass number goes down by four

How does the composition of an atom change when it emits a beta particle?

neutron changes to a proton or number of neutrons goes down 1

and the number of protons goes up by 1

its atomic number goes up by one

mass number is unchanged

Describe the structure of alpha particles.

two protons plus two neutrons

What are beta particles?

(fast moving) electrons (from the nucleus)

Ionisation

Any process in which atoms become charged

The radiation from a radioactive substance can knock electrons out of the atoms

Irradiated

exposed to radiation, not radioactive

Peer review

A process by which the procedures and results of an experiment are evaluated by other scientists who are in the same field or who are conducting similar research.

What is a half-life?

The half-life of a radioactive isotope is 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.

What is meant by the term 'half-life'?

(average) time taken for number of nuclei to halve

What is the activity of a radioactive source

The number of unstable atoms in the source that decay per second

Explain the risk to the patient of using a radioactive substance as a medical tracer

radiation causes ionisation that may then harm / kill healthy cells

Even though there may be a risk, doctors frequently use radioactive substances for medical diagnosis and treatments.

Suggest why.

benefit (of diagnosis / treatment) greater than risk (of radiation)

Radioactive tracers

Radioactive tracer graph

Why is radioactive iodine used for this test

Gamma cameras

Gamma radiation - cancer

Radioactive implants

What type of radiation would be used to obtain an image of an internal organ?

beta or gamma,

can be detected outside body

What type of radiation would be used to find out weather a kidney is blocked

gamma,

radioactive source injected into patient to enter organ to be imaged so needs to do least damage whilst in body,

gamma radiation passes through body tissue and detected using gamma camera

Two sources of background radiation

any two from:

• food,

• drink,

• radon

Describe how nuclear radiation is used to destroy a tumor using a radioactive implant

small 'seeds' of radioactive isotope placed in tumour,

radiation from isotope destroys cancer cells,

use isotope with half-lives not long enough to damage normal cells surrounding tumour,

half-life not too short or unstable nuclei decay before radiation destroys tumour

Importance of using the right isotope

too short: radioactive isotope decays too much before scan completed,

too long: patient exposed to ionising radiation unnecessarily

Which type of radiation has the greatest range in air?

gamma

Which of the three types of radiation is the most strongly ionising?

alpha

What is radioactive contamination?

Radioactive contamination is the unwanted presence of materials

containing radioactive atoms on other materials. The hazard from

contamination is due to the decay of the contaminating atoms. The

type of radiation emitted affects the level of hazard.

What is irradiation?

Irradiation is the process of exposing an object to nuclear radiation.

The irradiated object does not become radioactive.

Where does background radiation come from?

Background radiation is around us all of the time. It comes 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.

The level of background radiation and radiation dose may be

affected by occupation and/or location.

What are nuclear radiations used in medicine for?

Nuclear radiations are used in medicine for the: