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9.2 Production of materials: 5. Nuclear materials
| Syllabus reference (October 2002 version) | ||
|---|---|---|
| 5. Nuclear chemistry provides a range of materials |
Students learn to: |
Students: |
Extract from Chemistry Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
[Edit: 1 June 10]
Prior learning: Preliminary module 8.2.3.
Background: Atoms contain protons and
neutrons in a nucleus surrounded by electrons in energy level
shells. Isotopes of an element are atoms of that element
containing the same number of protons but different numbers of
neutrons.
If the nucleus of an atom contains excess energy the nucleus is
unstable and can emit radiation. The radiation emitted is
characteristic of the nucleus and can be in the form of particles or radiation. The emitted radiation can be
used in many ways in industry and medicine.
distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable
- An isotope of an element, E, is
represented by
,
where
A represents the mass number (the number of protons + neutrons)
Z represents the atomic number (the number of protons).
- Isotopes of the same element have the same atomic number
(Z).
- Only 255 of the more than 3100 known isotopes are stable. In a
stable isotope nucleus, the protons and neutrons are in a low
energy level and are unable to emit radioactivity.
- Radioactive isotopes are unstable. They emit radiation as
they spontaneously release energy. This is called
radioactive decay. An unstable isotope can be called
a radioisotope, an abbreviation of the term
radioactive isotope.
- The time for the radioactivity level from a given amount
of radioactive isotope to be halved is called its half-life.
Each radioactive isotope has a characteristic half-life. In one half-life half of the atoms in a sample
of that isotope have undergone a radioactive decay.
- Radioactive isotopes can emit three types of radiation:
-
Radiation
Symbol
Radiation emitted in this type of decay alpha
42He+2 particle
beta
e-1 particle
gamma
high frequency electromagnetic radiation
- Essentially there are the two main factors that affect the stability of a nucleus. These are its size and the proton to neutron ratio in the nucleus. The nucleus is held together by an attraction caused by the strong nuclear force. This attraction acts between all adjacent particles in the nucleus and opposes the electrostatic repulsion between protons. The graph below illustrates the importance of the proton to neutron ratio. The blue data points are the most stable nuclei for each element up to and including bismuth. Once the atomic number increases above 20 it is clear that there is no longer a one to one ratio of protons to neutrons in the most stable nuclei and extra neutrons are needed to make a stable nucleus.
describe how commercial radioisotopes are produced
- One process invovled in the commercial production of radioisotopes is nuclear fission. This is basically the splitting of a large nuclei into smaller nuclei and is typically initiated by the absorption of a neutron by the larger nuclei.
- The following example is only one of many possible results of nuclear fission.
-
- When the uranium nucleus breaks up into two nuclei, many
different possible isotopes can form.
- Differences in chemical properties of the elements
produced can be used to chemically separate the different
radioisotopes. Any U-235 that has not undergone fission can
be separated and recycled into new fuel rods.
- The high-speed neutrons emitted can be used to bombard atoms of various elements to produce useful neutron rich isotopes.
describe how
transuranic elements are produced
Background
Transuranic elements are elements with an atomic number above that of uranium with atomic number Z= 92.
- Twenty transuranic elements have been made and studied sufficiently. The current periodic table for the HSC in 2011 lists 112 elements as the properties of the elements 113 and above have not been fully authenticated. The claim to
production of element 118 has been withdrawn by the originating
laboratory as no other laboratory anywhere in the world has
been able to replicate this production.
- The process of changing one element into another is called transmutation. The two main ways that a transuranic nuclei can be produced are by bombarding a nucleus with ions or neutrons. These ions or neutrons can be captured by the target nucleus and produce heavier nuclei.
- Only three of the transuranic elements, those with atomic
numbers 93, 94 and 95, have been produced in nuclear reactors by neutron bombardment.
- When U-238 is bombarded with neutrons it can be converted to U-239 that undergoes beta decays to produce neptunium and plutonium.
- Pu-239 is changed to americium-241 by neutron bombardment.
- Americium-241 is used in most house smoke alarms.
- Transuranic elements from atomic number 96 and up are all made by accelerating a small charged nucleus (such as He, B or C) in a charged particle accelerator to collide with a heavy nucleus (often of a previously made transuranic element) target.
identify instruments and processes
that can be used to detect radiation
| Background High energy radiation that causes ionisation of atoms is called ionising radiation and is potentially harmful to living things. |
- Most radioactive emissions are ionising radiation and are
usually detected by a Geiger-Muller tube connected to a
counter. The Geiger-Muller tube contains gas that ionises and
produces a small pulse of electricity each time it is ionised
by radiation. The counter counts the number of pulses.
- Low energy radiation that is too weak to ionise atoms is called non-ionising radiation and can be detected by a scintillation counter. Scientists investigating reactions in living things often prefer to use non-ionising radioisotopes because ionisation could cause unwanted chemical changes in living things. The non-ionising radiation emitted transfers energy to a solvent molecule and then to a fluorescent molecule that emits light. A photomultiplier produces an amplified electrical pulse from the light. Electrical circutry called a counter detects the amplified electrical signal and counts the pulses.
process information from secondary sources to describe recent discoveries of elements
- The nineteen transuranic elements with the atomic numbers above 95 (Z between 96 and 116, leaving out undiscovered 113 and 115) require high-energy particle accelerators to be produced. Use an Internet search engine and recent references to find out how particle accelerators are used to discover new transuranic elements. To process the sources you find, assess their reliability by comparing the information provided. Look for consistency of information.
use available evidence to analyse benefits and problems associated with the use of radioactive isotopes in identified industries and medicine
- Gather information to complete a table like the one following on how gamma sources, such as Cobalt-60 (Co-60) and Technetium-99m (Tc-99m), are used in industries and medicine and use available evidence to analyse the benefits and problems associated with the use of radioactive isotopes. Note that the alternative to gamma sources, X-rays, are not as penetrating and require high voltage equipment that uses a lot of electrical energy. However, the more expensive X-ray equipment is more easily disposed of and does need to be locked away in secure locations like potentially harmful gamma ray sources.
|
Gamma source |
Use |
Benefits |
Problems |
|---|---|---|---|
| Co-60 for checking defects in metal wings | |||
| Tc-99m for imaging an internal organ |
identify one use of a named radioisotope:
- in industry
- in medicine
In industry
- Cobalt-60 (Co-60) is used in a process called industrial radiography, to inspect metal parts and welds for defects.
In medicine
- Technetium-99m (Tc-99m) is used in a wide range of medical applications, such as pinpointing brain tumours.
describe the way in which the above named radioisotopes are used and explain their use in terms of their properties
In industry
- Cobalt-60 is used in industrial radiography to inspect metal parts and welds for defects. Beams of radiation are directed at the object to be checked from a sealed source of Co-60. Radiographic film on the opposite side of the source is exposed when it is struck by radiation passing through the objects being tested. More radiation will pass through if there are cracks, breaks, or other flaws in the metal parts and will be recorded on the film. By studying the film, structural problems can be detected.
- Co-60 is used because it is an emitter of gamma rays which will penetrate metal parts. Co-60 has a half-life of 5.3 years and can be used in a chemically inert form held inside a sealed container. This enables the equipment to have a long lifetime and not require regular maintenance.
In medicine
- Technetium-99m (Tc-99m) is used in over half of the current nuclear medicine procedures, such as pinpointing brain tumours. Tc-99m can be changed to a number of oxidation states. This enables production of a wide range of biologically active chemicals. The Tc-99m is attached to a biological molecule that concentrates in the organ to be investigated.
- Tc-99m is used because:
-
- it has a very short half-life of 6 hours
- it emits low energy gamma radiation that minimises damage to tissues but can still be detected in a person's body by a gamma ray sensitive camera
- it is quickly eliminated from the body
- technetium is reasonably reactive; it can be reacted to form a compound with chemical properties that leads to concentration in the organ of interest such as the heart, liver, lungs, bones or thyroid.
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