no longer an exclusively vicarious one.

Sunday, March 06, 2005

Chemistry: Production of Materials: 5

5. Nuclear chemistry provides a range of materials.

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 AEZ, where A represents the mass number (the number of protons + neutrons), and Z represents the atomic number (the number of protons). Isotopes of the same element have the same atomic number (Z). What differs is the number of neutrons. The number of neutrons a nucleus may contain can be too many, not enough or a “stable number”. The overfilled or under filled nuclei tend to be unstable, and as the nuclei become larger, the stable nuclei need more neutrons than protons (1:1.5 as opposed to 1:1 in smaller nuclei). Only 279 of about 2000 known isotopes are stable.Radioactive isotopes are unstable. They emit radiation as they spontaneously release energy. This is called radioactive decay. 3 types of radiation can be released:
Radiation
Type
Charge
Deflection in magnetic field
Deflection in electric field
Symbol
Penetrating ability
Alpha
4He2 particle
+
Deflected
Towards –ve plate
Low (paper)
Beta
0e-1 particle
-
Large deflection
Towards +ve plate
Medium (alfoil)
Gamma
high frequency electromagnetic radiation
Nil
Nil
Nil
Very high (lead/concrete)

Describe how transuranic elements are produced
Transuranic elements are elements with an atomic number above that of uranium with atomic number Z= 92.
Twenty-two transuranic elements have been made. Only three of the transuranic elements, those with atomic numbers 93, 94 and 95, have been produced in nuclear reactors (the others all in cyclotrons).
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 by neutron bombardment.

Transuranic elements from atomic number 96 and up are all made by accelerating a small nucleus (such as He, B or C) in a charged particle accelerator (cyclotron) to collide with a heavy nucleus (often of a previously made transuranic element) target. However, cyclotrons are not able to produce large quantities of radioisotopes. In the case of nuclear reactors, the nucleus to be used is placed inside the reactor and then it is bombarded with neutrons.

Describe how commercial radioisotopes are produced
Most commercially used radioisotopes are created in nuclear reactors. Nuclear reactors produce lots of neutrons from fission reactions. Some of these neutrons are captured by heavy nuclei, such as uranium-238. This makes a new nucleus with the same atomic number but a higher atomic mass. Some of these new heavier nuclei can then convert to nuclei of other elements by beta decay. 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

Identify instruments and processes that can be used to detect radiation
- Photographic film: Becquerel showed in 1896 that uranium emission could darken photographic film. It is used in radiation badges for workers working in radioactive environments.
- Geiger-Muller counter: This is a tube with a thin mica window (for alpha particles to enter) containing argon at a pressure of 10kPa. The radiation ionises the Ar atoms, forming Ar+ cations and electrons. The electrons are accelerated towards the axial anode, ionising more Ar atoms and a cascade of electrons reaches the anode. The electric pulse is amplified and detected as clicks on an audio amplifier.
- Scintillation counter: (non-ionising radiation) several substances emit flashes of light when struck by alpha, beta or gamma rays. The 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. A counter counts the pulses. Phosphor used could be ZnS or NaI.
- Cloud chambers: These consist of cold, supersaturated vapour (ethanol kept cold with dry ice). It condenses on the ionised track left by radioactive emissions.

Identify one use of a named radioisotope:
- in industry
- in medicine

Cobalt-60
Caesium-137
Chemical symbol
1. Use
In medicine: gamma rays released are used in radiotherapy to treat cancer.
In industry: in levelling gauges.

Describe the way in which the above named industrial and medical radioisotopes are used and explain their use in terms of their chemical properties


Cobalt-60
Caesium-137
Chemical symbol
1. Use
In medicine: gamma rays released are used in radiotherapy to treat cancer.
In industry: in levelling gauges.
2. Production
Produced as a by-product of nuclear reactor operations, when structural materials, such as steel, are exposed to neutron radiation
Produced when uranium and plutonium absorb neutrons and undergo fission, eg. in nuclear reactors and nuclear weapons. The splitting of uranium and plutonium in fission creates numerous fission products, including Cs-137
3. How used
Co-60 releases gamma rays because it has an unstable nucleus. Unstable nuclei spontaneously emit radiation as they are transformed back to a stable state (ie. As Co-60 decays to Ni-60). High intensity gamma radiation will kill cells. It is used in a technique called radiotherapy to treat cancer by targeting the cancer cells with a beam of radiation and then rotating the source of the beam. The normal cells receive a lower dose of gamma radiation than the cancer cells, where all the rays meet. Radiotherapy aims to kill the cancer cells while doing as little damage as possible to healthy normal cells.
Cs-137 releases gamma rays because it has an unstable nucleus. Unstable nuclei spontaneously emit radiation as they are transformed back to a stable state. Radiation emitted from Cs-137 will be reduced in intensity by matter between the radioisotope and a detector. The amount of this reduction can be used to gauge the presence or absence of the material, or even to measure the quantity of material between the source and the detector (or the thickness or thinness).
4. a) Benefits
The half-life of cobalt-60 is 5.27 years. This is short enough to make isolation a useful treatment strategy for contaminated areas. In some cases, simply waiting 10 to 20 years allows for sufficient decay to make the site acceptable for use again.
In gauging, there is no contact with the material being measured, therefore no radioactive contamination. Because it has a half-life of 30 years, Cs-137 can be used many times without needing to be replaced, thus reducing costs in industry.
4. b) Problems
All ionising radiation, including that of cobalt-60, is known to cause cancer. Therefore, exposures to gamma radiation from cobalt-60 result in an increased risk of cancer. The magnitude of the health risk depends on the quantity of cobalt-60 involved and on exposure conditions (length of exposure, distance from the source, whether the cobalt-60 was ingested or inhaled). Because of their metallic housings, sources of Co-60 can get mixed in with scrap metal and pass undetected into scrap metal recycling facilities. If melted in a mill, they can contaminate the entire batch of metal and the larger facility, costing millions of dollars in lost productivity and cleanup costs.
Caesium-137 can be mistaken for potassium by living organisms and taken up as part of the fluid electrolytes. This means that it is passed on up the food chain and reconcentrated from the environment by that process. This makes the cleanup of caesium-137 difficult, as it moves easily through the environment. The half-life of caesium-137 is 30.17 years, which is relatively long, especially if it enters a living organism, because like all radionuclides, exposure to radiation from caesium-137 results in increased risk of cancer. Caesium-137 is an inorganic salt, and is highly soluble in water. If a leak were to develop in a storage facility, the radioactive material could easily contaminate surrounding water.

Other examples:
Radioactive isotope
Use
Benefits
Problems
Iodine-131
(a by-product from nuclear fission in nuclear reactors)
For a number of medical procedures, including to monitor and trace the flow of thyroxin from the thyroid.

With its short half-life of 8 days, it is essentially gone from a body in less than three months.
It emits fairly high-energy beta particles and a number of gamma rays. The gamma rays are of sufficient energy to be measured outside the body if deposited in tissue such as the thyroid. Because iodine selectively deposits in the thyroid, the primary health hazard for iodine is thyroid tumours resulting from ionising radiation emitted.
Strontium-90
(a by-product from nuclear fission in nuclear reactors)
In thickness gauges (for paper, cardboard) because of its release of beta particles – the penetration of these particles indicates the thickness of a material.
Has a half-life long enough for repeated use (28 years).
Strontium is radioactive, and decays very slowly, so if exposed, it will take 28 years to decay. Strontium-90 mimics the properties of calcium and is taken up by living organisms and made a part of their electrolytes as well as deposited in bones. As a part of the bones, it is not subsequently excreted like caesium-137 would be. It has the potential for causing cancer or damaging the rapidly reproducing bone marrow cells.

Process information from secondary sources to describe recent discoveries of elements
Twenty-two transuranic elements have been made. 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. The majority of the transuranium elements were produced by two groups:
- A group at the University of California, Berkeley, under three different leaders: Edwin Mattison McMillan, first to produce a transuranium element, Glenn T. Seaborg, and Albert Ghiorso, who had been on Seaborg's team.
- A group at the Gesellschaft für Schwerionenforschung (Society for Heavy Ion Research, GSI) in Darmstadt, Hessen, Germany, under Peter Armbruster.
Recent discoveries of elements:
- Meitnerium (109): first synthesized on August 29, 1982 by a German research team led by Peter Armbruster and Gottfried Münzenberg at the Institute for Heavy Ion Research at Darmstadt. It was done by bombing a target of bismuth-209 with accelerated nuclei of iron-58. The creation of this element demonstrated that nuclear fusion techniques could be used to make new, heavy nuclei.
- Darmstadtium (110): first created on November 9, 1994 at the GSI in Germany. It has never been seen and only a few atoms of it have been created by the nuclear fusion of isotopes of lead and nickel in a heavy ion accelerator (nickel atoms are the ones accelerated and bombarded into the lead).
- Roentgenium (111): first created at the GSI on December 8, 1994. Only three atoms of it have been created (all 272Rg), by the fusion of bismuth-209 and nickel-64 in a linear accelerator. (Nickel was bombarded onto the target.)
- Scientists from the Dubna facility in Russia and from Berkeley have claimed to have created elements 112-116 but so far these claims have not been verified.
- In 1999, the team at Berkeley claimed to have created element 118 by bombarding lead targets with an intense beam of krypton in a synchrotron, however their statement was retracted in 2001. No one else has been able to verify their claim.
Resource: http://en.wikipedia.org/wiki/Transuranium_elements

Use available evidence to analyse benefits and problems associated with the use of radioactive isotopes in identified industries and medicine
Radioactive isotopes of various elements can be used for a range of purposes, and have the potential to be very efficient and useful, but their use must be balanced against the dangers that are inherent in handling any sort of radioactive material.
In medicine the isotope cobalt-60 is used to treat cancer. Its high intensity gamma radiation is directed at the malignant cells and is used to kill these cells, while trying to minimise damage to healthy cells. As with any radioisotope, exposure to gamma radiation may also damage healthy tissue, and this is one of the risks to be considered when undergoing radiotherapy for cancer. Another medical isotope is iodine-131, used to monitor the efficiency of the human thyroid. It is used to trace the flow of thyroxine to the gland, and is of great help when trying to diagnose thyroid-related problems. It has a short half-life (8 days) and this means that any exposure to radiation from this source will only be minimal and over a short period of time. However, the gamma rays emitted from iodine-131 are strong enough to be measured outside the body once in the thyroid, and this means that the radiation is still strong enough to cause damage to cells. Because iodine-131 is still used in diagnosing thyroid problems, it can be seen that the usefulness and efficiency of this method of testing outweighs the potential dangers in the eyes of the medical industry.
In industry, the gamma radiation produced by the radioisotope caesium-137 is used in levelling gauges to ensure that materials are level. The amount of gamma radiation that penetrates the material depends on the amount of material there is between the source of the radiation (in this case caesium-137) and the detector. In much the same way, strontium-90 is used as a thickness gauge for other, thinner materials. Here, instead of measuring the amount of gamma rays penetrating the material, beta particles are measured. Both of these industrial isotopes are created as by-products from the nuclear fission of elements such as uranium or plutonium, in nuclear reactors or from nuclear weapons. These types of nuclear reactions can be quite risky in themselves, and although many safety measures are in place at nuclear reactors around the world, it only takes one bad accident to have far-reaching and long-lasting effects (eg. Chernobyl 1986). In particular, both strontium-90 and caesium-137 can be mistaken by the body as calcium and potassium respectively, and are taken into the body as part of fluid electrolytes. In this manner, the radiation from the isotopes directly enters the body and can begin to harm living tissue. The half-lives of these two radioisotopes are about 30 years, which is a dangerously long time for a living organism to be exposed to radiation. However, for use in industry, the lengths of these half-lives is cost effective, because the isotopes only need to be replaced about every 30 years.
Besides the risks involved in producing all of these radioactive isotopes, and the health hazards from direct contact with the isotopes, there is also an environmental issue. Co-60, I-131 and Cs-137 are all soluble in water, one of the reasons for their use in medicine. Because of this, any leaks in storage facilities could result in the contamination of surrounding water, and thus the surrounding environment. Excessive radiation exposure is harmful to all living organisms, and if radioactive sources are ingested or absorbed, radiation build-up could occur within food chains and webs.
These examples of radioisotopes used in medicine and industry are a small cross section of the all the radioactive isotopes used. Each has its merits and makes the process of diagnosis and treatment more efficient in medicine, and the process of mass production more efficient in industry. However, the benefits from using these isotopes must be balanced with careful and strict safety precautions, due to the danger involved in handling radioactive substances.

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