physics folio form 5

Maktab Rendah Sains MARA, Taiping

PHYSICS

RADIOISOTOPES

MUHAMMAD ARIFF IDLAN BIN ISMAIL

11289

503

EN. ABDUL HAMID BIN MOHAMAD

Table of Contents

Radioisotopes

How to produce radioisotopes

1. Separation

2. Synthessis

How to use radioisotopes as tracers

1. Agriculture

2. Medicine

3. Industry

How to dispose radioactive wastes

RADIOISOTOPES

Radioisotopes are isotopes which are unstable and undergo radioactive decay. As a result, radioactive emission is also given out. A radioisotope is so-named because it is a radioactive isotope, an isotope being an alternate version of a chemical element that has a different atomic mass. The nucleus of a radioisotope usually becomes stable by emitting an alpha and/or beta particle. These particles may be accompanied by the emission of energy in the form of electromagnetic radiation known as gamma rays. This process is known as radioactive decay. Radioisotopes have very useful properties: radioactive emissions are easily detected and can be tracked until they disappear leaving no trace. Alpha, beta and gamma radiation, like x-rays, can penetrate seemingly solid objects, but are gradually absorbed by them. The extent of penetration depends upon several factors including the energy of the radiation, the mass of the particle and the density of the solid. These properties lead to many applications for radioisotopes in the scientific, medical, forensic and industrial fields.

Some radioisotopes exist naturally such as , , and .

Others are produced artificially in nuclear laboratories. Examples of artificial

radioisotopes are , , and .

Naturally-occurring industrial radioisotopes:1. Carbon-14: Used to measure the age of water (up to 50,000 years)2. Chlorine-36: Used to measure sources of chloride and the age of water (up to 2 million

years)3. Lead-210: Used to date layers of sand and soil up to 80 years4. Tritium (H-3): Used to measure 'young' groundwater (up to 30 years)

Artificially-produced industrial radioisotopes:1. Americium-241:

Used in backscatter gauges, smoke detectors, fill height detectors and in measuring ash content of coal.

2. Caesium-137:Used for radiotracer technique for identification of sources of soil erosion and deposition, in density and fill height level switches.

3. Chromium 57:Used to label sand to study coastal erosion.

4. Cobalt-60, Lanthanum-140, Scandium-46, Silver-110m, Gold-198:Used together in blast furnaces to determine resident times and to quantify yields to measure the furnace performance.

5. Cobalt-60: Used for gamma sterilisation, industrial radiography, density and fill height switches.

6. Gold-198 & Technetium-99m:Used to study sewage and liquid waste movements, as well as tracing factory waste causing ocean pollution, and to trace sand movement in river beds and ocean floors.

7. Gold-198:Used to label sand to study coastal erosion.

8. Hydrogen-3 (Tritiated Water): Used as a tracer to study sewage and liquid wastes

9. Iridium-192Used in gamma radiography to locate flaws in metal components.

10. Krypton-85:Used for industrial gauging.

11. Manganese-54:Used to predict the behaviour of heavy metal components in effluents from mining waste water.

12. Nickel-63Used in light sensors in cameras and plasma display, also electronic discharge prevention and in electron capture detectors for thickness gauges.

13. Selenium-75:Used in gamma radiography and non-destructive testing.

14. Strontium-90:Used for industrial gauging.

15. Thallium-204:Used for industrial gauging.

16. Ytterbium-169:Used in gamma radiography and non-destructive testing.

17. Zinc-65:Used to predict the behaviour of heavy metal components in effluents from mining waste water.

How to produce radioisotopes

1. Separation

Some isotopes occur in nature. If radioactive, these usually are radioisotopes with very long half-lives. Uranium 235, for example, makes up about 0.7 percent of the naturally occurring uranium on the earth. The challenge is to separate this very small amount from the much larger bulk of other forms of uranium. The difficulty is that all these forms of uranium, because they all have the same number of electrons, will have identical chemical behavior: they will bind in identical fashion to other atoms. Chemical separation, developing a chemical reaction that will bind only uranium atoms, will separate out uranium atoms, but not distinguish among different isotopes of uranium. The only difference among the uranium isotopes is their atomic weight. A method had to be developed that would sort atoms according to weight.

One initial proposal was to use a centrifuge. The basic idea is simple: spin the uranium atoms as if they were on a very fast merry-go-round. The heavier ones will drift toward the outside faster and can be drawn off. In practice the technique was an enormous challenge: the goal was to draw off that very small portion of uranium atoms that were lighter than their brethren. The difficulties were so enormous the plan was abandoned in 1942. Instead, the technique of gaseous diffusion was developed. Again, the basic idea was very simple: the rate at which gas passed (diffused) through a filter depended on the weight of the gas molecules: lighter molecules diffused more quickly. Gas molecules that contained U-235 would diffuse slightly faster than gas molecules containing the more common but also heavier U-238. This method also presented formidable technical challenges, but was eventually implemented in the gigantic gas diffusion plant at Oak Ridge, Tennessee. In this process, the uranium was chemically combined with fluorine to form a hexafluoride gas prior to separation by diffusion. This is not a practical method for extracting radioisotopes for scientific and medical use. It was extremely expensive and could only supply naturally occurring isotopes.

2. Synthesis

A more efficient approach is to artificially manufacture radioisotopes. This can be done by firing high-speed particles into the nucleus of an atom. When struck, the nucleus may absorb the particle or become unstable and emit a particle. In either case, the number of particles in the nucleus would be altered, creating an isotope. One source of high-speed particles could be a cyclotron. A cyclotron accelerates particles around a circular race track with periodic pushes of an electric field. The particles gather speed with each push, just as a child swings higher with each push on a swing. When traveling fast enough, the particles are directed off the race track and into the target.

A cyclotron works only with charged particles, however. Another source of bullets are the neutrons already shooting about inside a nuclear reactor. The neutrons normally strike the nuclei of the fuel, making them unstable and causing the nuclei to split (fission) into two large fragments and two to three "free" neutrons. These free neutrons in turn make additional nuclei unstable, causing further fission. The result is a chain reaction. Too many neutrons can lead to an uncontrolled chain reaction, releasing too much heat and perhaps causing a "meltdown." Therefore, "surplus" neutrons are usually absorbed by "control rods." However, these surplus neutrons can also be absorbed by targets of carefully selected material placed in the reactor. In this way the surplus neutrons are used to create radioactive isotopes of the materials placed in the targets.

With practice, scientists using both cyclotrons and reactors have learned the proper mix of target atoms and shooting particles to "cook up" a wide variety of useful radioisotopes.

How to use radioisotopes as tracer

Radioactive isotopes have many useful applications in a wide variety of situations, for example, they can be used within a plant or animal to follow the movement of certain chemicals. In medicine, they have many uses, such as imaging, being used as tracers to identify abnormal bodily processes, testing of new drugs and conducting research into cures for disease.

Agriculture

Phosphorus uptake by plantsPlants take up phosphorus-containing compounds from the soil through their roots. By adding a small amount of radioactive phosphorus-32 to fertiliser and then measuring the rate at which radioactivity appears in the leaves, it is possible to calculate the rate of uptake of phosphorus from the soil. The information gathered could help plant biologists to identify plant types that can absorb phosphorus quickly. These plants may give better yields resulting in more food or fibre at less expense.

Pesticide levelsTo measure pesticide levels, a pesticide can be tagged with a radioisotope such as chlorine-36, and this is applied to a field of test plants. Over a period of time, radioactivity measurements are made. Estimates can then be made about how much accumulates in the soil, how much is taken up by the plant and how much is carried off in run-off surface water.

Industry

Gamma RadiographyGamma Radiography works in much the same way as x-rays screen luggage at airports. Instead of the bulky machine needed to produce x-rays, all that is needed to produce effective gamma rays is a small pellet of radioactive material in a sealed titanium capsule.

The capsule is placed on one side of the object being screened, and some photographic film is placed on the other side. The gamma rays, like x-rays, pass through the object and create an image on the film. Just as x-rays show a break in a bone, gamma rays show flaws in metal castings or welded joints. The technique allows critical components to be inspected for internal defects without damage.

Gamma sources are normally more portable than x-ray equipment so have a clear advantage in certain applications, such as in remote areas. Also while x-ray sources emit a broad band of radiation, gamma sources emit at most a few discrete wavelengths. Gamma sources may also be much higher energy than all but the most expensive x-ray equipment, and hence have an advantage for much radiography. Where a weld has been made in an oil or gas pipeline, special film is taped over the weld around the outside of the pipe. A machine called a "pipe crawler" carries a shielded radioactive source down the inside of the pipe to the position of the weld. There, the radioactive source is remotely exposed and a radiographic image of the weld is produced on the film. This film is later developed and examined for signs of flaws in the weld.

X-ray sets can be used when electric power is available and the object to be x-rayed can be taken to the x-ray source and radiographed. Radioisotopes have the supreme advantage in that they can be taken to the site when an examination is required - and no power is needed. However, they cannot be simply turned off, and so must be properly shielded both when in use and at other times.

Non-destructive testing is an extension of gamma radiography, used on a variety of products and materials. For instance, ytterbium-169 tests steel up to 15 mm thick and light alloys to 45 mm, while iridium-192 is used on steel 12 to 60 mm thick and light alloys to 190 mm.

Gamma SterilisationGamma irradiation is widely used for sterilising medical products, for other products such as wool, and for food. Cobalt-60 is the main isotope used, since it is an energetic gamma emitter. It is produced in nuclear reactors, sometimes as a by-product of power generation.

Large-scale irradiation facilities for gamma sterilisation are used for disposable medical supplies such as syringes, gloves, clothing and instruments, many of which would be damaged by heat sterilisation. Such facilities also process bulk products such as raw wool for export from Australia, archival documents and even wood, to kill parasites. Currently ANSTO in Australia sterilises up to 25 million Queensland fruit fly pupae per week for NSW Agriculture by gamma irradiation. See also The Peaceful Atom.

Smaller gamma irradiators are used for treating blood for transfusions and for other medical applications.

Food preservation is an increasingly important application, and has been used since the 1960s. In 1997 the irradiation of red meat was approved in USA. Some 41 countries have approved irradiation of more than 220 different foods, to extend shelf life and to reduce the risk of food-borne diseases.

Tracing/Mixing UsesEven very small quantities of radioactive material can be detected easily. This property can be used to trace the progress of some radioactive material through a complex path, or through events which greatly dilute the original material. In all these tracing investigations, the half-life of the tracer radioisotope is chosen to be just long enough to obtain the information required. No long-term residual radioactivity remains after the process.

Sewage from ocean outfalls can be traced in order to study its dispersion. Small leaks can be detected in complex systems such as power station heat exchangers. Flow rates of liquids and gasses in pipelines can be measured accurately, as can the flow rates of large rivers.

Mixing efficiency of industrial blenders can be measured and the internal flow of materials in a blast furnace examined. The extent of termite infestation in a structure can be found by feeding the insects radioactive wood substitute, then measuring the extent of the radioactivity spread by the insects. This measurement can be made without damaging any structure as the radiation is easily detected through building materials.

Medical

Medical tracersRadioactive isotopes and radioactively labelled molecules are used as tracers to identify abnormal bodily processes. This is possible because some elements tend to concentrate (in compound form) in certain parts of the body – iodine in the thyroid, phosphorus in the bones and potassium in the muscles. When a patient is injected with a compound doped with a radioactive element, a special camera can take pictures of the internal workings of the organ. Analysis of these pictures by a specialist doctor allows a diagnosis to be made.

The thyroid gland, situated in the neck, produces a hormone called thyroxine, which regulates the rate of oxygen use by cells and the generation of body heat. Within each molecule of thyroxine, there are 4 iodine atoms. If a patient is made to drink a solution of sodium iodide that has been doped with radioactive iodine-131, most of it will end up in the thyroid gland. A special camera can capture the radiation emitted by the iodine-131, and an image of the gland can be constructed. An assessment can then be made about the shape, size and functioning of the gland.

Positron emission tomography (PET)

A positron emission tomography (PET) scan measures important body functions, such as blood flow, oxygen use and glucose use. The information gathered helps doctors find out how well organs and tissues are functioning.

Radionuclides used in PET scanning are isotopes with short half–lives, such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min) and fluorine-18 (~110 min). These radionuclides are added into compounds normally used by the body such as glucose (or variations of glucose), water or ammonia. Such labelled compounds are known as radiotracers. In some situations, the patient is required to breath oxygen gas labelled with oxygen-15.

The radionuclides used in PET decay by a process called positron emission. A positron is the antimatter version of the electron. When a positron meets an electron, an annihilation event occurs, resulting in the production of two gamma rays. The two emitted gamma rays travel in opposite directions.

The scanning instrument picks up the location of these gamma rays and, with the aid of a powerful computer, generates a map of where these events are occurring. By combining the PET scan with a CT scan, a more complete picture of how well an organ is functioning can be made.

Due to the short half-lives of most radioisotopes, the radiotracers must be produced using a cyclotron (a type of particle accelerator) and radiochemistry laboratory that are close to the PET imaging facility. The half-life of fluorine-18 is long enough such that fluorine-18 labelled radiotracers can be manufactured commercially at an off-site location.

How to dispose radioactive wastes

How radioactive waste is disposed depends on the material's half-life and its level of radioactivity.

Nuclear waste is categorized by its origin, not by its level of radioactivity. Low-level waste includes that which was made radioactive by neutron exposure or became contaminated by high-level waste. Its level of radioactivity can therefore vary widely.

Low-level waste is usually disposed of as ordinary trash after decaying to safe levels. It may also be transported to facilities that accept low-level waste for burial.

Fission waste is generally left more accessible than low level waste, not due to lower level of radiation but instead to monitor for leaking because of its long-term danger, and to keep options open when more permanent disposal solutions arise.

Storage and disposal of low-level waste

1. Low-level waste is usually stored on the site of its creation, by licensees who meet regulatory standards in handling the material.

2. During the time of on-site storage, the radioactivity level decreases, in some cases to levels at which the waste can be thrown out as regular trash.

The more radioactive a material, the faster it decays into atoms that are not radioactive. So the care needed to store highly radioactive low-level waste tends to be a temporary situation.

Low-level waste can range from weakly radioactive items such as contaminated protective shoe covers to more highly radioactive material, such as retired reactor walls made radioactive by neutron exposure. Therefore, storage methods and length of time for decay varies.

3. Once enough waste has accumulated on-site, it may be transported to disposal facilities where the waste is buried in vaults, boreholes or geologic repositories.

Storage and disposal of intermediate-level waste

1. Intermediate- level wastes come from sources such as nuclear power stations, industries and research laboratories.

2. Few examples are components in nuclear reactors and chemical sludges.

3. Intermediate- level wastes have long half-life and high radiation level.

4. Liquid wastes are solidified in concrete before disposal. Other radioactive wastes are placed in concrete block and then burried underground.

Storage and disposal of high-level waste

1. As a normal effect of fission, nuclear reactors produce transuranic waste, that is, atoms having atomic number greater than uranium's.

Most transuranic waste does not emit high levels of penetrating radiation, but is harmful if ingested and does have a long half-life. For example, the half-life of plutonium-239 is 24,000 years.

Most spent nuclear reactor fuel is stored at the same site as the reactor, in water pools. So the first step of storage is to place the waste in a water pool, to serve as a radiation shield to protect workers.

2. The fuel is later dissolved in chemicals to recover unfissioned uranium, leaving the waste suspended in liquid.

3. The next step is often to reprocess the waste for use in nuclear weapons or for reuse in new fuel.

Even then, the rest of the steps need to be followed, because reprocessing can be accomplished only to a certain degree due to the difficulty of separating isotopes and finding uses for each of them.

4. To reduce storage volume, the waste is then concentrated by removing water, through a process called calcination.

5. The waste is then bonded into glass (through vitrification with fragmented glass) or concrete and then burried underground or is transported for storage at surface sites in stainless steel silos.

6. The strategy of surface storage is akin to the difference between storing a residential oil tank above ground instead of burying it. In the former case, leaks can be seen as soon as they start. The aim is to keep the waste from leaking and entering the biosphere, especially entering ground water used for drinking.