Radioactivity Flashcards
Atom structure
Atoms are the building blocks of all matter
Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
They consist of small dense positively charged nuclei, surrounded by negatively charged electrons
Rutherford experiment
A beam of alpha particles (He2+ ions) were directed at a thin gold foil
they discovered that :
- Most of the alpha particles passed straight through the foil
- Some of the alpha particles changed direction but continued through the foil (<180)
- A few of the alpha particles bounced back off the gold foil (<90)
This was the first evidence of the structure of the atom
- This happened because the atom is mainly empty space so most of the particles go in
- very few are deflected back (this is because the nucleus is very small) - Some are deflected through small angles (B)
This happens because the positive α-particles are repelled by the positive nucleus
Atoms vs ions
An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
An atom will lose or gain electrons to become more stable this is because they want to be more stable
Positive ions are therefore formed when atoms lose electrons (more protons than electrons)
Negative ions are therefore formed when atoms gain electrons
(More electrons than protons)
Describing the nucleus
The number of protons in an atom is called its proton number (it can also be called the atomic number)
The atomic number is also equal to the number of electrons in an atom
Elements in the periodic table are ordered by their atomic number
The total number of particles in the nucleus of an atom is called its nucleon number (or mass number)
So to find the neutrons its Nucleon Number - Proton Number
Isotopes
Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
Isotopes tend to be more unstable due to their imbalance of protons and neutrons
This means they are more likely to decay
Isotopes occur naturally, but some are more rare than others
Relative charge and relative mass
Relative charge
Proton = +1
Neutron = 0
Electron = -1
The proton number decides the relative charge
Relative mass
Proton = 1
Neutron = 1
Electron = 1/1836
Nuclear fission
The splitting of a large, unstable nucleus into two smaller nuclei to create energy
Isotopes of uranium and plutonium both undergo fission and are used as fuels in nuclear power stations
During fission, when a neutron collides with an unstable nucleus, the nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons
Gamma rays are also emitted
Energy transferred is from nuclear potential energy to kinetic energy
The mass of the products (daughter nuclei and neutrons) is less than the mass of the original nucleus
When they go under nuclear fission the amount of neutrons and protons are the same
Nuclear fusion
When two light nuclei join to form a heavier nucleus to release energy
This process requires extremely high temperatures to maintain
This is why nuclear fusion has proven very hard to reproduce on Earth
The mass of the product (fused nucleus) is less than the mass of the two original nuclei
because the remaining mass has been converted into energy which is released when the nuclei fuse
When they go under nuclear fission the amount of neutrons and protons are the same
Background radiation
The radiation that exists around us all the time
Types
- Natural sources
- Man-made sources
Natural sources
- rocks (15%)
- cosmic rays from space (10%)
- foods (11%)
- radon gas (50%)
Sources of background radiation
Radon gas (in the air)
- Airborne radon comes from the ground
- This is from the natural decay of uranium in rocks and soil
Rocks and building
- heavy radioactive elements occur naturally in rocks and therefore in building blocks
Cosmic rays
- The sun emits an enormous number of protons every second
Some of these enter the Earth’s atmosphere at high speeds
When they collide with molecules in the air, this leads to the production of gamma radiation
Radioactive material in food and drink
- Naturally occurring radioactive elements can get into food and water since they are in contact with rocks and soil containing these elements
Dectecting radiation
Ionising nuclear radiation is measured using a detector connected to a counter
Count rate is the number of decays per second recorded by a detector and recorded by the counter
It is measured in counts/s or counts/min
The count rate decreases the further the detector is from the source
This is because the radiation becomes more spread out the further away it is from the source
If it counts 16,000 decays in 1 hour what is the count rate
1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds
Time period = 1 × 60 × 60 = 3600 seconds
Counts ÷ Time period = 16 000 ÷ 3600 = 4.5
Therefore, there are 4.5 decays per second
Radiation decay
Some atomic nuclei are unstable
This is because of an imbalance in the forces within the nucleus (the nucleus is too heavy)
Unstable nuclei can emit radiation to become more stable
Radiation can be in the form of a high energy particle or wave
As the radiation moves away from the nucleus, it takes some energy with it
This reduces the overall energy of the nucleus
This makes the nucleus more stable
Radioactive decay is a random process
This means it is not possible to know exactly when a particular nucleus will decay
It is spontaneous and random in direction
Types of radioactive decay
Alpha (α) particles
Beta (β-) particles
Gamma (γ) radiation
Alpha Particles
- The symbol for alpha is α
- An alpha particle is the same as a helium nucleus
- This is because they consist of two neutrons and two protons
- Alpha particles have a charge of +2
- range in air a few cm
- stopped by paper
- ionisation is high
Beta Particles
- The symbol for beta is β-
- Beta particles are fast-moving electrons
- They are produced in nuclei when a neutron changes into a proton and an electron
- Beta particles have a charge of -1 or +1
- 4 - 10m in air
- stopped by a few mm aluminium
- medium ionisation
Gamma Rays
- The symbol for gamma is γ
- Gamma rays are electromagnetic waves
- They have the highest energy of the different types of electromagnetic waves
- Gamma rays have no charge
- range of air is infinite
- low ionisation
Ionising effect of radiation.
All nuclear radiation is capable of ionising atoms that it hits
When an atom is ionised, the number of electrons it has changes
This is mostly done by knocking out an electron so the atom loses a negative charge and is left overall positive
Therefore When radiation passes close to atoms it can knock out electrons, ionising the atom
Alpha is by far the most ionising form of radiation
Alpha particles leave a dense trail of ions behind them, affecting virtually every atom they meet
Therefore it can be very dangerous when entered the skin
Beta particles are moderately ionising
The particles create a less dense trail of ions than alpha, and consequently have a longer range and it can penetrate skin causing significant damage
Gamma is the least ionising form of radiation (although it is still dangerous)
Because Gamma rays don’t produce as many ions as alpha or beta, they are more penetrating and have a greater range
Therefore the ionising effects depend on the kinetic energy and charge of the type of radiation
The higher the kinetic energy and charge the more ionising it is
Deflection in electric and magnetic fields
Using Flemings right hand rule to figure out the deflection in magnetic fields (alpha particles as they act as a current)
Beta particles are heavier than alpha so they deflect MORE than alpha particles
Gamma rays go straight through this is because there is NO charge
During radiation decay
During α-decay or β-decay, the nucleus changes to a different element
During alpha decay an alpha particle is emitted from an unstable nucleus
When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes
The mass number decreases by 4
The atomic number decreases by 2
The charge on the nucleus also decreases by 2
This is because protons have a charge of +1 each
So the daughter nucleus is a new element because it has a different proton and/or nucleon number to the original parent nucleus
Same with beta
During beta decay, a neutron changes into a proton and an electron
The electron is emitted and the proton remains in the nuclei
A completely new element is formed because the atomic number changes and the mass number remains the same
Gamma decay
During gamma decay, a gamma ray is emitted from an unstable nucleus
The process that makes the nucleus less energetic as it emits a lot of energy but does not change its structure
Decay equations
During decay equations the sum of the mass and atomic numbers before the reaction must be the same as the sum of the mass and atomic numbers after the reaction
the big unstable atom —> radiation (eg: alpha) + not so unstable atom
Remember
Beta has a atomic mass of -1 and a nucleon mass of zero
Alpha has a nucleon mass of 4 the atomic mass of 2
Gamma is an energy so it has a nucleon mass of zero and an atomic mass of zero
Half life
It is impossible to know when a particular unstable nucleus will decay
But the rate at which the activity of a sample decreases can be known
This is known as the half-life
The time taken for half the nuclei of that isotope in any sample to decay
Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length
So if 1/2 of the isotope remains the number of half life is 1 and if 1/4 of the isotopes remain the number of half life is 2
To determine what the half-life is from a graph you must look at the middle of the y axis and once you touch the curve go down to the x axis and that’s how much time to decay 1 half life is
Remember: When measuring radioactive emissions, some of the detected radiation will be background
Uses of radiation
Radiation is used in a number of different ways:
- Medical procedures including diagnosis and treatment of cancer
- Sterilising food (irradiating food to kill bacteria)
- Sterilising medical equipment (using gamma rays)
- Determining the age of ancient artefacts
- Checking the thickness of materials
- Smoke detectors (alarms)
Smoke detectors
- Alpha particles are used in smoke detectors
- The alpha radiation will normally ionise the air within the detector, creating a current
- The alpha emitter is blocked when smoke enters the detector
- The alarm is triggered by a microchip when the sensor no longer detects alpha
Measuring thickness of materials
- As a material moves above a beta source, the particles that are able to penetrate it can be monitored using a detector
- If the material gets thicker, more particles will be absorbed, meaning that less will get through
- If the material gets thinner the opposite happens
- This allows the machine to make adjustments to keep the thickness of the material constant
Diagnosis and Treatment of Cancer
- Although radiation can cause cancer, it is also highly effective at treating it
- Beams of gamma rays are directed at the cancerous tumour
- Gamma rays are used because they are able to penetrate the body, reaching the tumour
- The beams are moved around to minimise harm to healthy tissue whilst still being aimed at the tumour
Sterilising Food and Medical Equipment
- Gamma radiation is widely used to sterilise medical equipment
Gamma is most suited to this because:
- It is the most penetrating out of all the types of radiation
- It is penetrating enough to irradiate all sides of the instruments
- Instruments can be sterilised without removing the packaging
Food can be irradiated in order to kill any microorganisms that are present on it
This makes the food last longer, and reduces the risk of food-borne infections
Dangers of radioactivity
Ionising radiation can damage human cells and tissues at high doses:
This could be in terms of:
Cell death
Tissue damage
Mutations
Safety precautions of radioactive waste
Keep the source in a lead lined container until the time it is needed
Use tongs to move the source, rather than handling it directly
The source should be kept at as far a distance from the student as possible during the experiment
The time that the source is being used should be minimised
After the experiment the student should wash their hands
The date and the time that the radiation has been used for should be recorded
Gloves, protective clothing
Disposing radioactive waste
If an isotope has a long half-life then a sample of it will decay slowly, meaning it will remain radioactive for a long time
Therefore risk of contamination
Radioactive waste with a long half-life is buried underground to prevent it from being released into the environment
