P2 - Atoms & Radiation Flashcards
What are the masses and charges of protons, neutrons and electrons?
Protons have a mass of 1 and a charge of +1
Neutrons have a mass of 1 and no charge (0)
Electrons have a mass of 1/2000 and a charge of -1
Describe how the theory of atoms developed over time
Dalton thought that each element was made of a different type of atom and that atoms couldn’t be broken up
100 years later, Thomson discovered that electrons could be removed from atoms and suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them (plum pudding theory)
10 years later, Rutherford and Marsden fired a beam of alpha particles at thin gold foil and expected that the positively charged alpha particles would be deflected by the electrons in the plum pudding model
However, most of the particles went straight through but a few came straight back at them
From this, they suggested the atomic model we use today
What are isotopes?
Isotopes are variants of atoms that have the same number of protons and electrons but a different number of neutrons
What do isotopes tend to do?
They are usually radioactive which means that they decay into other elements and give out radiation
What is interesting about radioactivity?
It is completely random when radiation is emitted and it isn’t affected by any physical conditions like temperature etc.
Describe the relative proportions of background radiation
51% radon gas
14% rocks and building materials
12% food
12% medical X-rays
10% cosmic rays
1% nuclear industry
What does it mean if something is ionising and what affects this?
It is a measure of how much they can bash into atoms and knock electrons off them creating ions
Particles are more ionising if they are larger because they are more likely to hit atoms and ionise them
What does it mean if something has high penetrating power and what affects this?
It can penetrate further through materials
Smaller particles penetrate further because they don’t hit as many atoms etc.
What are the 3 types of radiation?
Alpha particles
Beta particles
Gamma radiation
What is an alpha particle?
2 protons and 2 neutrons which is the same as a helium nucleus
+2 charge
What are the properties of alpha radiation?
Relatively large and heavy and slow moving
Low penetration power and high ionising ability
What are beta particles?
Simply an electron with virtually no mass and a -1 charge
What are the properties of beta radiation?
Quite fast and quite small so they penetrate and ionise moderately
What happens when beta particles are emitted?
A neutron turns into a proton in the nucleus
What are gamma rays?
They are high frequency electromagnetic waves that have no charge or mass
What are the properties of gamma radiation?
Weakly ionising and a high penetration power
How do you do calculations for alpha decay?
Example:
238 234 4
U –> Th + He
92 90 2
How do you do calculations for beta decay?
Example:
14 14 0
C –> N + e
6 7 -1
How do you do calculations for gamma radiation?
Nothing changes because gamma rays have no mass or charge
How are the 3 types of radiation deflected by electric and magnetic fields?
Alpha and beta particles are deflected in opposite directions because alpha is positive and beta is negative
Gamma doesn’t have a charge so the fields don’t affect it
What are the 5 locations / occupations that give a larger radiation dose?
Underground rocks can cause higher levels at the surface especially if they release radon gas which gets trapped in houses
Nuclear industry workers need to wear protective clothing and face masks to stop them touching or inhaling radioactive material
Radiographers in hospitals wear lead aprons and stand behind lead screens
At high altitudes, you are at higher risk of cancer because of cosmic rays (problem for pilots)
Miners have an increased risk because of the rocks underground that are all around them
How does the number of decays per second change as a radioactive sample gets older?
It decreases
Why do we use half-life to measure how quickly the activity drops?
Because the activity never actually reaches zero
What is half-life?
The average time is takes for the number of nuclei in a radioactive sample to halve
What is the activity of a radioactive sample measured in?
Bequerels
Counts per minute
Etc.
How do you calculate the half-life of a sample in the exam?
Example:
The activity of a sample has gone from 640 to 80 cpm in 2 hours. Calculate the half life
Initial - 640
After one - 320
After two - 160
After three - 80
So 2 hours is 3 half-lives meaning that a half-life is 40 minutes
How do you calculate the half-life of a sample from an activity / time graph?
Look at the initial activity
Half it
Go across from the “half activity” until you touch the curve
Then go down and the time that you get to is the half-life
What are the 4 main uses of radiation?
Smoke detectors
Tracers in medicine
Radiotherapy
Sterilisation of food and medical instruments
How do smoke detectors use radiation?
A weak source of alpha radiation is placed in the detector close to 2 electrodes
The source causes ionisation which causes a current between the electrodes
If smoke absorbs the radiation, the current stops and the alarm sounds
How is radiation used for tracers in medicine?
Some isotopes can be injected into the body and their progress can be monitored by an external detector
The injected sample must be beta or gamma so that the radiation passes out of the body and the half-life should be short so that the patient isn’t exposed to radiation for too long
How does radiotherapy work?
Gamma rays kill at living cells so they can be used to kill cancerous cells
The rays have to be directed carefully and at just the right dosage to kill the cancer without damaging the normal cells too much
How is sterilisation of food and medical equipment done with radiation?
Gamma rays can be used to kill microbes on food and medical instruments
What is the advantage of using gamma rays to sterilise medical equipment?
The high temperatures of boiling are not needed
Describe how radiation causes cancer
All types of radiation enter living cells and ionises or destroys them
This causes mutated cells to divide uncontrollably and cause cancer
Which are the most dangerous types of radiation that cause cancer?
Beta and gamma are the most dangerous outside the body because they are the most likely to penetrate the body
Alpha is the most dangerous inside the body because the radiation doesn’t penetrate out of the body
What are the 5 safety measures to do with radioactivity?
Use radioactive sources for a short time as possible to keep exposure time down
Use tongs (never allow skin contact with a source)
Hold source at arm’s length to keep down the amount of radiation that hits you
Keep source pointing away from you and avoid looking directly at it
For larger operations, use lead aprons, screens etc. as a protective measure because it stops all types of radiation although a lot is needed to stop gamma
