Chapter 8: Nuclear Radiation and Its uses Flashcards
I) In order to select the appropriate nuclide for a particular purpose it is necessary to know the following 4 factors:
The types of radiation the nuclide emits.
The ability of the radiation to penetrate matter.
The effect of the radiation on specific materials.
The rate of decay (activity) of the radiation source.
I) How is radiation used in the treatment of cancer?
Radiation is targeted at the cancerous area.
The DNA of the malignant cells is severely damaged so they can’t reproduce and eventually they die.
Normal tissue that is irradiated also gets damaged but these cells are able to rep are themselves the the treatment stops.
I) Explain how certain isotopes are used in medicine for their radioactivity. Give an example.
Isotopes may be stable or radioactive depending on the number of its neutrons. However, the chemical behaviour of the isotopes is identical so radioactive nuclides can be inserted into the body as pills or injections as a replacement of the stable isotopes.
An example of this is iodine-131 in the form of iodine chloride which is used in the treatment of thyroid cancer.
I) Describe briefly what happens in radioactive decay.
The unstable nucleus (parent) decays spontaneously into a more stable nucleus (daughter).
Energy is released in the process.
I) Describe briefly the 3 main types of radiation.
Alpha - an alpha particle is a helium nucleus with a charge of +2e.
Beta - a beta particle is a fast moving electron with a charge of -e. This emission is accompanied by the emission of an electron neutrino.
Gamma - gamma radiation is a photon of EM radiation. This usually occurs following the release of an alpha or beta particle.
Some artificially produced radio-nuclides may also decay by the emission of a positron and a neutrino.
I) How is the energy from nuclear decay released?
The energy released by the nucleus appears as kinetic energy of the ejected particles or as the energy of an emitted gamma-ray photon. In addition, some of the energy becomes kinetic energy of the daughter nucleus as it recoils to conserve momentum.
I) Describe how each of the radiation types would behave when a magnetic field is applied perpendicular to its direction of motion.
Alpha and Beta particle more in opposite directions which is consistent with the signs of their charges. However, since the Alpha particle is more massive, the radius of its path is much larger than that of a Beta particle. Gamma-ray photons are unaffected by a magnetic field.
I) Describe the process of alpha decay.
When an Alpha Particle is emitted by a nucleus the proton number decreases by 2 and the mass number decreases by 4.
I) Describe the process of beta decay.
When a beta particle is emitted a neutron effectively becomes a proton so the proton number increases by 1 and the mass number remains the same. An electron anti-neutrino is emitted at the same time as the beta particle.
The energy emitted is the same in each decay but the beta particle have a range of energies. The remaining energy is taken up by the anti-neutrino.
I) Describe the process of gamma decay.
A gamma-ray photon is emitted when the nucleus is in an excited state following the emission of an alpha or beta particle.
I) Name the 4 decay series’.
Thorium series, Neptunium series, Uranium series and the Actinium series.
I) Compare the relative ionising properties of Alpha, Beta and Gamma radiation
Alpha is the Most ionising.
Beta it slightly ionising.
Gamma is the Least ionising.
I) Explain what happens to each radiation as it encounters an obstacle.
Alpha - Alpha Particles have a range of about 5cm in air.
Alpha can be stopped by a thin sheet of paper.
(A 5MeV alpha particle can produce 180,000 ions as it travels through air. When all of its kinetic energy is dissipated, the alpha particle finally combines with stray electrons in the air to form helium.)
Beta - Beta Particles have a range of up to 0.5m in air.
Beta can be stopped by 2-4mm of aluminium.
(When beta particles interact with atoms, some of the energy becomes gamma radiation. Therefore, the intensity of radiation from a beta source doesn’t drop to zero when the beta particles have been stopped.)
Gamma - Gamma-ray Photons have an infinite range.
The number of Gamma-ray Photons is halved by 12mm of lead.
(Gamma rays may lose their energy by ionising atoms or by other processes. The absorption process results in the intensity fall exponentially with distance travelled though a medium. The half-thickness of a medium is the distance travelled by the radiation before it’s intensity halves. Dense materials like lead has a small half-thickness and higher energy photons lead to greater half-thicknesses.)
Side note: The intensity of a beam of Positrons falls more rapidly than beta sure to the fact that the positrons can also be annihilated when they intreat with the electrons of regular atoms.
I) List the possible sources of background radiation and give a rough estimate to the relative number of units emitted per year by the source.
Natural Radioactivity in the air - 800 Medical Applications - 500 Ground and Buildings - 380 Food and Drink - 370 Cosmic Rays - 310 Nuclear Weapons Testing - 10 Air Travel - 5 Nuclear Power - 3
I) Talk about how Gamma radiation obeys the inverse square law.
Over short distances in air, the total number of gamma-ray photons from a source doesn’t change significantly meaning that the intensity varies approximately according to an inverse-square law.
A radioactive source is a small ‘point’ source and will emit N gamma-ray photons per second uniformly in all directions. The gamma ray intensity is the number of gamma-ray photons per second per square meter.
The count rate C is proportional to the number of photons passing through the detector.
(Look in the Textbook for more details on equations…)
For section 8.3 use textbook as it is very equation based
Use Textbook.
I) List the ways in which isotopes are made in hospitals.
Nuclear reactors and Particle accelerators which are then transported to the hospitals. If the distance travelled is far, the isotopes may loose significant amounts of their activity before they are delivered which has to be accounted for when determining the patients dose.
Some isotopes are produced in generators on site.
Larger hospitals may have their own small cyclotron.
I) What changes do radioactive isotopes use in cancer treatment undergo when they are bonded to other atoms or molecules in the body?
None - the type of radiation emitted and the decay constant remain unchanged.
I) What does PET Stand for?
Positron Emission Tomography
I) Which isotopes are used as positron emitter? What are their half lives?
Mainly Fluorine-18 with a half life of 110 mins
Carbon-11 and Nitrogen-13 are also used but since they have a half life of about 10 mins, they have to be manufactured on site.
I) How does the Fluorine-18 used in Positron Emission Tomography attach to the body?
It bonds to glucose
I) How do PET scanners work?
PET imaging depends on the annihilation of the emitted positrons with electrons which gives off a pair of gamma-ray photons. By analysing the gamma rays that are emitted the detectors can identify the points of origin of the gamma ray pairs and build up a 3D image of the body.
I) How is Fluorine-18 produced?
Fluorine-18 is produced in cyclotrons that accelerate particles to between 10-20MeV. Protons are fired at Oxygen-18 water which produces the Fluorine-18 water solution.
Because of their short half-life, the cyclotrons have to be on sites close to the hospitals.
I) How is the change in activity of a radioactive source (Fluorine-18) accounted for and how is it measured?
The activity is measured in activity per unit volume.
If a specific volume of fluorine-18 is needed at a particular time. If the scan is delayed by 110 mins (the half-life of fluorine-18) then twice the volume would have to be administered to give the same initial activity.
These calculations are important to ensure that the dose given is suitable to form the images and to ensure that the patient is not given more radiation the is necessary.
I) What is the most commonly used radioactive isotope in medical applications?
Technetium-99m
I) What is the Parent of Tc-99m?
Molybdenum-99 which has a half-life of about 67 hours.
I) How is Molybdenum-99 produced?
Molybdenum-99 is produced in the fission of uranium-235 in nuclear reactors.
After processing it is transported to hospitals where it is stored and the Tc-99m is extracted from the ‘Tc-99m generators’.
Since the half-life of Molybdenum-99 is almost 3 days, its activity falls to about 25% of its original activity within a week, after which it has to be renewed.
I) How is Technetium-99m extracted?
In the generators, the Molybdenum-99 is in the form of MoO4 ions and as the molybdenum nuclei decay TcO4 ions are formed. The technetium-99m oxide is removed from the generator by passing salt solution through it.
I) What type of radiation does Tc-99m emit and why is it useful in medicine?
Tc-99m emits 140keV gamma radiation which is a relatively low energy for gamma. The radiation is less likely to ionise the atoms in the patient’s cells than higher energy gamma radiation but it can still be monitored by detectors from outside the body.
Tc-99m is used for gamma ray scanning, producing images of the body.
It is also used as a tracer for investigating the function of different organs in the body (the brain, bone marrow and heart).
I) What does the ‘m’ signify in Technetium-99m?
The ‘m’ signifies that the isotope is metastable. This means that the nucleus remains in an excited state of a longer period than is usual for gamma emitters (6 hours rather than a few seconds).
I) What happens after Tc-99m stops emitting gamma radiation?
It becomes Tc-99 which emits low energy beta radiation with a very long half life (2 x 10^5 years). This means that a patient only receives a low dose during investigations.
