Physics
Double Award
Double Award
Cambridge IGCSE
TOPIC 5: NUCLEAR PHYSICS
Radioactivity is commonly regarded as a dangerous effect, and it took some time for scientists to establish that invisible alpha, beta and gamma rays were causing damage to the body. Now we understand more about radioactivity and how to use it safely, applications have been invented in a range of fields, particularly in medicine.
As discussed in section 5.2a, all radioactivity can cause ionisation. This can disrupt the chemical bonds in molecules. The chemistry of the human body is extremely complicated, and exposure to a high level of nuclear radiation can cause significant medical issues, damaging cells and tissues.
You may have learnt that our cells rely on the molecule DNA to reproduce successfully. If the DNA is damaged by radiation, it may cause mutations in the cell. This can happen to all living organisms. Cancer is caused by cell mutation, and exposure to nuclear radiation can therefore lead to cancer.
Alpha radiation is blocked by our skin, but is extremely ionising and therefore very dangerous if an alpha emitter enters our blood stream or is inhaled. Gamma is less ionising, but can penetrate deep into tissues and damage vital organs.
There are two commonly used words to describe radiation hazards, with different meanings:
For example, apples in a supermarket can be irradiated to kill micro-organisms to make them last longer, but not contaminated as this would be a health hazard to consumers.
To make radioactive isotopes safer, there are some simple techniques used to minimise human exposure and lower the risk to users:
1. Storage. Always store isotopes in containers with a thick, dense metal lining, such as lead. This will block all alpha, beta and gamma radiation. Store all isotopes in locked metal cupboards, and restrict access to a few necessary users!
2. Distance. Always use tongs when handling isotopes. This keeps the isotope away from our hands and other living tissues, minimising short range exposure, especially for alpha emitters. Keep a large distance between living tissues and source as much as possible.
4. Shielding. Always stand behind concrete, or brick walls, when ionising radiation is being used regularly. You may have noticed your dentist doing this during a tooth X-ray. You might only do one of these a year, but dentists are exposed several times a day so need to minimise their exposure!
5. Movement. If isotopes are being moved, they must be in lead cases, and stored in a locked vehicle if transport is involved.
6. Time. Reduce exposure time by handling the isotopes quickly and efficiently, then standing at a distance or putting them back into safe storage.
You will need to be able to describe the uses of radiation in medicine and industry. Here we will look at some of the most common examples that come up in questions:
As well as causing cancer, it seems bizarre that radiation can also cure it. However, cancer cells are still living organisms, and high levels of radiation will kill these cells. Radiotherapy involves giving the cancer tumour a high dose of radiation, enough to eventually kill all the cancer cells and prevent further spreading. There is bound to be some exposure of healthy tissues around the cancer tumour, but non-cancerous cells have been found to be a bit more resilient to high doses of radiation.
In addition, if the radiation is focused at the tumour as shown in figure 2, then the surrounding tissues receive a much lower dose.
Figure 2: Radiotherapy
Gamma rays directed at a tumour
Blood flows all around our body, taking oxygen, sugar and other nutrients to cells and tissues. If we inject a low dose of radioactive substance into the blood (e.g. technetium-99), it will spread throughout the body. If the substance decays through gamma radiation, we can detect this outside the body, and therefore trace the flow of the radioisotope.
A gamma ray camera can build up a picture of where in the body the radioactive tracer substance is going, and identify medical problems without the need for surgery.
The radioisotope must have a half life of a few hours. This allows enough time to take an image (see figure 3), but after a day or two, the radiation emitted has reduced to negligible levels. The patient will have not been exposed to dangerous levels of radiation.
Figure 3: Medical tracers used to identify a tumour
By Jens Maus
We know that ionising radiation can kill cells and microbes. Using ionisation to sterilise medical equipment is a safe and cost effective way of ensuring that, for example, all surgical tools are completely free of microbes. One common application is to sterilise already packaged items such as syringes, shown here in figure 4. Beta or gamma radiation needs to be used to penetrate the plastic packaging.
Figure 4: A sterile plastic syringe
In the same way that ionising radiation can be used to kill microbes in medical apparatus, the technique can be used on food. The irradiated food is sterile, but completely safe to eat as it has not been contaminated by any radioactive isotopes. Fresh fruit and vegetables will last much longer on a supermarket shelf and hence have a higher value to the suppliers if sterilised in this way.
Aluminium is rolled into thin sheets by heavy rollers. But how do the engineers make sure the expensive aluminium is the correct thickness? A physical measurement will require the rollers to stop moving whilst readings are taken.
Instead, a radioactive source is used to send radiation through the foil to a Geiger counter. (Figure 5).
Figure 5: Detecting foil thickness
In this example, a beta emitter is required, as the level of beta particles reaching the detector varies greatly with the foil thickness. (Alpha particles would all be stopped, and gamma rays would all pass through the foil).
If the foil becomes too thick, the activity falls suddenly, and the rollers can be automatically adjusted to make the foil thinner again without the need to stop the process.
Did you know that the smoke alarms - that are fitted in hotel rooms and on the ceiling of many homes - contain a radioactive isotope? A tiny quantity of an alpha particle source (typically the radioactive isotope americium) emits alpha particles. These ionise the air, which can produce a tiny current between electrodes, just like in an electrolysis experiment with charged ions conducting between the anode and cathode. A circuit in the detector checks to see if this tiny current is present.
However, smoke in the detector disrupts this process, blocking the radiation and carrying away or dissipating charged ions. The circuit then detects the change in current and triggers the alarm.
Figure 5: An ionisation smoke detector
OpenStax, CC BY 4.0
The source needs to emit alpha radiation, and also have a long half life so that the device does not need replacing often.
In addition, the very small quantity of alpha radiation cannot escape into the room as it is blocked by the casing. It also has a very short range in air, so is entirely safe to use in a home environment.
Questions:
1. A small quantity of a radioactive isotope is injected into a patients body during a medical examination. A special camera is used to detect nuclear radiation emitted from the isotope as it travels around the body in the blood stream.
a) Gamma is detected - it is the only one of the 3 that can pass through thick body tissues and bones.
b) If the half life is 6 months, the patient will be exposed to this gamma radiation for a long time, increasing the total exposure and risking illness or death. (Damage to tissues and cells, cell mutations, cancer).
2. Radon gas decays by emitting alpha particles. Alpha particles have a short range in air and are blocked by thin layers of paper or clothing.
a) The gas can still be inhaled, damaging lung tissue at close range, or entering our blood stream through the lungs. this would lead to internal damage to organs.
b) Damage to tissues and cells / cell mutations /cancer.
3. A factory is producing sheets of paper for wrapping products. A radioactive isotope that can partially pass through the paper is used to constantly monitor the thickness of the paper.
a) Beta particles are used as they can pass through thin paper but are stopped by thicker aluminium. Alpha particles are stopped completely by thin paper and will not reach the detector. Gamma will all pass through regardless of the paper thickness.
b) This is a complicated question. If the half life is too low, the count rate will drop, and the detector and processor will assume the paper is too thick. The rollers and machinery will then start to produce thinner paper.
Have a go at this end of unit test on the whole of topic 5, nuclear physics:
