6 Radioactivity Flashcards
What does a radioactive substance contain?
an unstable nuclei that becomes stable after emitting radiation
What are the three main types of radiation from radioactive substances?
- alpha
- beta
- gamma
What is radioactive decay known as?
a random event
we cannot predict it or influence when it will happen
What can measure background radiation and where does it come from?
a Geiger counter…
- in the environment (e.g. in the air or the ground or in building materials)
- from space (cosmic rays)
- from devices such as x-ray tubes
What was the ‘plum pudding model’?
some scientists thought that an atom was like a plum pudding with:
- the positively charged matter in the atom evenly spread out
- electrons buried inside
What was the set up of Ernest Rutherford’s radioactivity investigation?
- used a gold leaf (1 layer of gold atoms) and passed alpha particles through it
- known as scattering experiment to see if the alpha particles would bounce back
What did Ernest Rutherford find in his investigation?
- most of the alpha particles passed straight through the metal foil
- the number of alpha particles deflected per minute decreased as the angle of deflection increased
- about 1 in 10 000 alpha particles were deflected by more than 90degrees
What did Rutherford deduce from his scattering experiment results?
he knew that alpha particles are positively charged so he deduced that there is a nucleus at the centre of every atom that is…
- positively charged because it repels alpha particles (like charges attract/unlike charges repel)
- much smaller than the atom because most alpha particles pass through without deflection
- where most of the mass of the atom is located
Why did Rutherford’s nuclear model of the atom become accepted?
- it agreed exactly with the measurements made in the experiment (found the diameter of the nucleus was about 100 000 times smaller than atom)
- it explained radioactivity in terms of changes that happen to an unstable nucleus when it emits radiation
- predicted the existence of the neutron which was later discovered
Why was the plum pudding model abandoned?
could not explain why some alpha particles were scattered through large angles
What is the atomic (proton) number?
- the number of protons in the nucleus
- symbol: Z
What are isotopes?
- atoms of the same element with different numbers of neutrons
- (same proton number, different neutron number)
What is the mass number?
- the number of protons and neutrons in a nucleus
- symbol: A
What is radioactive decay?
when an unstable nucleus becomes more stable by emitting an alpha or beta particle or by emitting gamma radiation
What does an alpha particle consist of?
2 protons
2 neutrons
+2 relative charge
( a helium nucleus)
What happens when an unstable nucleus emits an alpha particle?
- atomic number goes down by 2 and mass number by 4
- equation:
A A-4 4
Z X –> Z-2 Y + 2 alpha particle
What does a beta particle consist of?
an electron (high speed) (emitted by nucleus with too many neutrons compared to protons- a neutron in the nucleus changes into a proton and a beta particle which is instantly emitted at high speed from the nucleus)
What happens when an unstable nucleus emits a beta particle?
- atomic number of nucleus goes up by 1 but its mass number stays the same (because neutron changes into proton)
- equation:
A A 0
Z X –> Z+1 Y + -1 beta particle
How is gamma radiation emitted by some unstable nuclei?
(high energy EM wave)
- after an alpha or beta particle has been emitted
- gamma radiation is uncharged and has no mass
- does not change the number of protons or neutrons in a nucleus
What is the penetration power of alpha particle radiation?
- can be absorbed by a thin sheet of paper
- has a range of around 5 cm in air
What is the penetration of beta particle radiation?
- can be absorbed by an aluminium sheet about 5 mm thick and a lead sheet about 2-3 mm thick
- has a range of around 1 m in air
What is the penetration power of gamma radiation?
- can be absorbed by a thick lead sheet about several cm thick and concrete more than 1 m thick
- has an unlimited range in air
What can be used to separate a beam of alpha, beta and gamma radiation?
a magnetic or electric field
shows properties as the charges influence it…
What is ionisation?
when atoms become charged because they lose or gain electrons
Which radiation types can ionise and how strongly?
They ionise substances they pass through which in living cells can damage or kill them…
- alpha: yes, very strongly
- beta: yes, weaker
- gamma: yes, very weakly
What is the half-life of a radioactive isotope?
the average time it takes:
- for the number of nuclei of the isotope in a sample (and therefore the mass of parent atoms) to halve
- for the count rate of the isotope in a sample to fall to half its initial value
What is the activity of a radioactive source?
the number of atoms/nuclei that decay per second
What can we use the Geiger counter for?
- to monitor the activity of a radioactive sample
- can measure a count rate
What does the use we can make of a radioactive isotope depend on?
- its half life
- the type of radiation it gives out
Give an example of how a radioactive substance can be useful for monitoring?
Kidney monitoring (radioactive iodine water substance goes into kidney- if functioning reading will go up then down where it flows straight through, if blocked then reading on detector stays up)
What makes a radioactive substance good at monitoring?
- if it has a long enough half-life but will then decay and be gone
- if it emits gamma radiation as it can be detected outside the body
- if it decays into a stable product
What are the 2 types of radioactive dating?
- carbon dating: used to find the age of organic material (half-life around 5600 years)
- uranium dating: used to find age of igneous rocks (half-life around 4500 million years)
What do we need for a radioactive dating sample?
a radioactive isotope that is present in the sample which has the same half life as the age of the sample
