Radiation is any form of energy originating from a source, including both waves and particles. Radiation doesn't have to be ionising - light waves and sound waves are both forms of radiation.
The intensity of radiation decreases the further way you get from a source. The medium it is travelling through will affect the intensity- the denser the medium, the more radiation will be absorbed and the intensity decreases.
There is an equation for this:
Intensity = Power/Area
The unit for intensity is Watts/metres squared.
The intensity is inversely proportional to the square of the distance from the source. This is known as the 'inverse sqaure law' - If the distance is doubled, the intensity is reduced by four times (two squared).
Dangers of ionising radiation
Ionisation can be caused by radiation from radioactive isotopes. Ionisation can kill living cells completely, damage them so they can't divide or alter the genetic material in a cell.
Damaging the genetic material in a cell is particulary dangerous because it can cause the cell to mutate and divide uncontrollably, causing cancerous tumours.
We are exposed to levels of background radiation all the time; radioactive radon gas released from rocks, cosmic waves, food and drink and medical remenants. We are also exposed to radiation from artificial sources; X-rays and gamma rays used in medical technology expose us to levels of ionising radiation.
Limiting the effects of ionising radiation
Medical personnel limit their and the patient's exposure to ionising radiation in a number of ways. This includes:
- Medical staff standing a distance away from the source, normally in a remote control room. This is because the intensity of radiation decreases with distance.
- Limiting exposure of staff and patients to the source - this might mean only prescribing a scan if the benefits outweigh the risks, and monitoring how much radiation personnel are recieving. There are laws in place to make sure staff do not recieve more than 5mSv/year averaged over five years.
- Limiting the dosage of ionising radiation. Lower doses of X-rays and gamma rays can be used to reduce cell tissue damage.
- Shielding from physical barriers. Gamma rays cannot penetrate thick lead, so personnel and patients may wear lead aprons or other shielding to stop the ionising radiation reaching their bodies. The remote control room which the radiograoher operates from is lead and steel lined.
A nucleus will be unstable if it has:
- Too many neutrons
- Too few neutons
- Too many protons and neutrons together (the mass is too large)
- Too much energy
If nuclei are unstable, then they will emit radiation; B- decay, B+ decay, alpha decay or gamma radiation.
The curve of stability
If you plot the number of neutrons (N) against the number of protons (Z) for stable isotopes you get a curve of stability:
Any isotope which doesn't lie on the blue curve is unstable. This means it is radioactive, and will emit radiation or particles. Isotopes above the line of stability have too many neutrons to be stable, and isotopes beneath the curve have too few neutrons to be stable.
Any isotope above the curve of stability with too many neutrons will undergo B-minus decay.
When B-minus decay occurs, a neutron is changed into a proton, and an electron is emmited from the nucleus.
The proton (atomic) number increases by one, but the mass number stays the same:
When B-decay occurs, one down quark changes into an up quark. The particle on the end has a charge of -1, showing the emmision of an electron.
B-plus decay occurs when isotopes are below the curve of stability, so have too many protons (or too few neutrons).
In beta-plus emmision, a proton is changed into a neutron, and a positron is released.
The proton number decreases by one, and the mass number remains the same:
When B-plus decay occurs, one up quark changes into a down quark. The positive charge on the emmited particle shows a positron has been emmited.
Positron radiation is positively charged beta radiation.
The positron is the antiparticle of the electron. This means it is very similar to an electron - they have the same mass (O) but their relative charge is +1:
They also share the same propeties as electrons - they are light and fast moving, moderately ionising and stopped by a sheet of thin metal.
When electrons and positrons meet, they annihiliate and produce two gamma rays travelling in opposite directions.
Alpha decay only happens in very heavy atoms with more than 82 protons. This includes isotopes of uranium and radium.
As previously explained, atoms with too large a mass will always be unstable and give off radiation.
As alpha radiation is equivalent to a helium nuclei, the proton number decreases by two, and the mass (nucleon) number decreases by four. This is because a helium nucleus contains two protons and two neutrons, so a charge of +2 and a mass of 4 is lost.
After alpha or beta decay, a nucleus often has excess energy. Gamma is always emmited alongside beta or alpha decay- gamma rays are never emmited on their own.
As gamma is energy, there is no change to the mass or atomic number.
Gamma can also be emmited when two antiparticles meet- when electrons and positrons meet, for example, they annihiliate to produce two gamma rays in opposite directions.
Neutrons are more penetrating than alpha and beta and even more penetrating than gamma.
Neutrons are not directly ionising, but can be absorbed by the nuclei of the atoms within the substances they travel through. Absorbing a neutron can make a nucleus radioactive. These radiocative nuclei will then emit ionising radiation (alpha, beta or gamma) which is why they are referred to as indirectly ionising.
Neutrons are absorbed most by light nuclei. This includes hydrogen, and all substances which contain a lot of hydrogen, including water and polythene. These are used to make neutron shielding, to absorb neutrons.
Neutron absorption can make nuclei emit gamma radiation, so lead is often added to neutron radiation shielding in nuclear reactors.
Protons and neutrons are made up of smaller particles called quarks. It takes three quarks to make a proton or a neutron.
Up (U) quarks have a charge of +2/3 and a mass of 1/3.
Down (D) quarks have a charge of -1/3 and a mass of 1/3.
Up and down quarks combine to form the relative mass and charges of protons and neutrons; protons contain UUD quarks and neutrons contain UDD quarks.
Radiation in medicine
Some radioactive isotopes are used as tracers to diagnose some medical conditions. The tracer is injected into the patient or swallowed. An external detector follows its progress as it moves around the body. A computer uses these readings to create an image showing where the strongest areas of radiation are.
One of the radioactive isotopes used in medicine is iodine-131, which is absorbed by the thyroid gland. The amount of radiation it gives off indicates if the thyroid is functioning correctly. Another isotope commonly used is flourodeoxyglucose (FDG), which is taken up by the body and treated as glucose. As cancer tumours take in a lot of glucose to grow quickly, tumour cells glow brightly on a PET scan as the radiation from the isotope is all coming from one place. This helps the diagnosis of cancer.
Radioactive isotopes emit beta or gamma, which can pass out the body without damaging too many internal cells. They have short half lives so they stop being dangerously radioactive soon after the scan. Isotopes for medicines are usually made in cyclotrons near the hospital for this reason.
Positron-emission tomography (PET) scanners is a medical technique used to show tissue or organ function.
PET scans show areas of damaged tissue in the heart by detecting areas which have a lower blood flow than usual. This is useful for diagnosing coronary artery disease, or damage caused by a heart attack. They can also look at how blood flows through the brain, diagnosing conditions such as alzheimers and epilepsy.
PET scans can identify cancerous tumours by showing metabolic activity in tissue - cancer cells are growing quickly so take up a lot of glucose, which means glucose-based tracers such as FDG gather in the affected area and emit gamma radiation, which is detected by the computer screen.
PET and annihilation
PET scanners work on the principle of annihilation. When a particle meets its antiparticle, the two particles annihilate and produce two gamma rays in oppsoite directions:
This is true for electrons and positrons. When a positron-emmiting radio isotope is injected into a patient, the emmited positron collide with electrons in the organs, causing them to annihiliate and produce a pair of gamma rays travelling in opposite directions. There will be a higher uptake of the radio isotope in cancer cells, so more gamma radiation comes from these areas. Detectors around the body detect each pair of radiowaves. Three pairs of gamma radiation give an accurate loactation of the tumour by using triangluation.
Conservation of momentum, mass and charge in PET s
Momentum is always conserved in the annihilation. The particles have the same mass and opposite velocities, so the total momentum before the collision is zero. When the collision occurs, the two gamma rays have the same energy but are travelling at exactly opposite velocities, meaning their overall momentum is also zero.
In any particle reaction, the charge before the reaction equals the charge after the reaction. The total charge before the positron-electron annihiliation is zero, as the charge on the positron of +1 is balanced by the -1 charge on the electron. Gamma rays have no charge so the charge of the reaction at the end is still zero.
Lastly, mass energy is always conserved in a positron/electron annihilation. Einstein said that mass is a form of energy - mass can be converted to other forms of energy. Mass energy is conserved because all the mass of the electron and positron have been converted into energy.
Energy (joules, J) = Mass (kilograms, kg) x Speed of light (3x10*8 m/s)*2