Radioactivity Decay And Radiation Biology Essay

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The alpha particle was discovered and named in 1899 by an English scientist, Ernest Rutherford while working with uranium. Rutherford and his co-workers used alpha particles in experiments to probe the structure of atoms in thin metallic foil. From 1909 to 1911 this work achieved the first concept of the atom: a tiny 'planetary system' with negative charged particles, electron, orbiting around a positively charged nucleus, through the Rutherford-Bohr planetary model of the atom (2).

Alpha particles, denoted by the Greek alphabet 'α', is made up of two protons and two neutrons bound together into a particle (which is identical to a helium nucleus) (3). The alpha particle is a type of ionizing radiation and is one of the most widespread forms of radiation. An alpha particle is essentially a helium nucleus, which consists of two neutrons and two protons, without electrons, giving it a net positive charge (4). It is a relatively heavy, high-energy particle and has a velocity in air of approximately one-twentieth the speed of light, depending upon the individual particle's energy (5).

Due to alpha particles relatively high mass, they are the most destructive form of ionizing radiation, but the trade-off is that their penetration is low.(6). Alpha particles have such a low penetration force that they are able to be stopped by few centimeters of air, by the skin or even by a piece of paper, presenting little danger unless swallowed (7).

b) Beta Particles

Henri Becquerel discovered the beta particle in the late 19th century (8). In 1900, Becquerel showed that the beta particles were identical to electrons, which had been recently discovered by Joseph John Thompson (9)

The beta particle is denoted as 'β+' while the ordinary beta particle is 'β−'. It is a high-speed electron or position released from a degenerating radioactive nucleus. Beta particles have medium-energy and low mass therefore they are one of the least damaging forms of radiation but still a very significant health concern (10). Even though beta particles have the same properties as electrons, they have much higher energies than typical electrons orbiting the nucleus. Beta particles are not naturally radioactive but they can cause damage, break chemical bonds and create ions and damage tissue (11).

Bata particles have an electrical charge of -1 and their speed depends on how much energy they have. The beta particle's excess energy, in the form of speed, causes harm to living cells and when transferred, this energy can break chemical bonds and form ions (12).

c) Gamma Rays

In 1900 Paul Villard, a French chemist and physicist, discovered gamma radiation while studying radiation emitted from radium (13).

A gamma ray is a 'packet' of electromagnetic energy called a 'photon'. Gamma rays (gamma photons) are the most energetic photons in the electromagnetic spectrum. They are emitted from the nucleus of some unstable (radioactive) atoms. Gamma rays have about 10,000 times as much energy as the photons in the visible range of the electromagnetic spectrum Gamma rays have no mass and no electrical charge - they are pure electromagnetic energy. Gamma radiation is very high-energy ionizing radiation (14).

The gamma rays, denoted as 'γ', are electromagnetic radiation of high frequency and have high energy, therefore they can travel at the speed of light and can cover hundreds to thousands of meters in air before their energy has subsided. Gamma rays, unlike alpha particles, can pass through many kinds of materials, including human tissue. Very dense materials, such as lead, are commonly used as shielding to slow or stop gamma rays (15). Gamma rays can become a health hazards when being absorbed by living tissue because they are a form of ionizing radiation (16).

Question 2 - Why do isotopes of elements spontaneously undergo radioactive decay?

In the elements, their nuclei undergo experience some of the "stress" in the atom and undergo radioactive decay order to release some of the "stress". At a certain point, the nucleus of an atom gets too large to sustain all of those protons and neutrons. The atom then relieves the "stress' and radioactive decay occurs (17).

Question 3 - What element do substances become once they have undergone radioactive decay?

When radioactive elements undergo various types of radioactive decay, until a stable atomic nucleus arises as a daughter product (18), and also when the sequence of alpha and beta decay until they finally become stable isotopes, they form lead.

As seen in the example provided in the Table, all of the natural series terminate with isotopes of lead, i.e., lead-206, lead-207, and lead-208. Secondly, all of these are stable isotopes of the element and no further radioactive emission therefore takes place (19).

Showing of Radioactive Decay of Elements

Question 4 - Explanation of the term 'Half-life'

'Half-life' is the period of time it takes for a substance undergoing decay to decrease by half, or the time it takes for one-half of the sample to decay (20). It is the time required to convert one half of a reactant to product. The term is commonly applied to radioactive decay where the reactant is the parent isotope and the product is a daughter isotope (21). The original name given to 'half-life' was 'half-life period' in the 1950's (22). To prevent the chemical from staying permanently within the norm you can set a decay rate. This sets a period of time over which the chemical amount will slowly drop down towards zero (23).

'Half-lives' are usually used to describe undergoing exponential decay, for example radioactive decay, where the 'half-life' remains constant during the whole life of the decay. The 'half-life' can also be defined for non-exponential decay processes then in these cases the half-life varies throughout the decay process. The converse of half-life is doubling time (24).

The chemical half-life gene defines the decay rate of all the chemicals inside their norm. Each chemical has a value ranging from 0 through to 255, representing the amount of substance within the norm (25).

Table 2. Half-life of the some substances

Question 5 - Carbon Dating

Radiocarbon dating was developed in 1949 by Willard Libby and his colleagues at the University of Chicago. He was then awarded the Nobel Prize in chemistry in 1960 for his discovery. His first demonstration to prove radiocarbon dating was done when Libby accurately estimated the age of wood from an ancient Egyptian royal barge for which the age was known from historical documents. Consequently carbon dating was developed by a team led by Willard Libby. He introduced the Libby half-life which was the carbon-14 half-life of 5568±30 years. Later a more accurate figure was introduced, the Cambridge half-life, of 5730±40 years (28).

Radiocarbon dating, or carbon dating, is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 (14C) to determine the age of carbonaceous materials up to about 58,000 to 62,000 years of age (29). Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis (30). The low activity of the carbon-14 limits age determinations to the order of 50,000 years by counting techniques. That can be extended to perhaps 100,000 years by accelerator techniques for counting the carbon-14 concentration.(31). Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere. This neutron bombardment produces the radioactive isotope carbon-14. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things (32).

The most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. When plants, during photosynthesis, add carbon dioxide into organic material, they incorporate a quantity of 14C which matches the level of isotopes in the atmosphere. Then as plants die or are consumed by organisms i.e. humans, the 14C fraction of the organic material decreases at a fixed exponential rate due to the radioactive decay of 14C. The remaining 14C fraction of the decayed plant is compared to that expected from atmospheric 14C. This comparison allows the age of the sample to be estimated (33).

The carbon-14 forms at a constant rate, therefore by measuring the radioactive emissions from the once-living matter and comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made (34).

Question 6 - What is a Geiger Counter?

A Geiger Counter, also known as Geiger-Müller Counter, is a particle detector that measures ionizing radiation and is also used to detect if objects emit nuclear radiation.

It is an inert gas-filled tube which briefly conducts electricity when a particle or photon of radiation makes the gas conductive. Usually helium, neon or argon with halogens are added. Then the tube amplifies this conduction by a cascade effect and outputs a current pulse, which is then often displayed by a needle or lamp and/or audible clicks.

Some Geiger Counters can be used to detect gamma radiation because the density of the gas in the apparatus is usually low, allowing most high energy gamma photons to pass through undetected. The sensitivity can be lower for high energy gamma radiation because lower energy photons are easier to detect, and are better absorbed by the detector. Gamma photons interact with the walls producing high-energy electrons which are then detected. Even though a sodium iodide scintillation counter is a better device for detecting gamma rays, Geiger detectors are still more favourable due to their low cost and robustness.

A variation of the Geiger tube is used to measure neutrons. This is when the boron trifluoride and a plastic moderator is used to slow the neutrons inside the detector. This creates an alpha particle inside the detector and thus neutrons can be counted (35).

Question 7 - Nuclear Fission

Nuclear fission is the splitting up of atom (36) into two or more smaller nuclei plus some by-products (37), including free neutrons and lighter nuclei, which may eventually produce photons (in the form of gamma rays) (38). The strong nuclear force binding energy causes the fission to release substantial amounts of energy (39).

This nuclear fission can occur when a nucleus of a heavy atom captures a neutron or can even happen spontaneously. The nuclear fission splits into several smaller fragments, these fragments, or also called fission products, are equal to half the original mass. The remaining mass has been converted into energy according, to Einstein's equation.(40).

Nuclear fission products are radioactive and take a long time to disintegrate, causing nuclear waste problems. This has lead to debates on whether to use nuclear fission as an energy source due to the nuclear waste accumulation. Nuclear fission produces energy for nuclear power stations and nuclear bombs because certain substances called nuclear fuels undergo fission when struck by free neutrons and in turn generate neutrons when they break apart. This causes a self-sustaining sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon (41).

Question 8 - Critical Mass

Critical mass is the smallest mass of a fissionable material that will sustain a nuclear chain reaction at a constant level (42) or support a self-sustaining nuclear chain.(43). It is also seen as the amount of a substance that is necessary for a nuclear chain reaction to start (44).

Critical mass could also be seen as when fission events occur in the mass of fissile material, neutrons are released. Some neutrons are captured and lead to fission, while others escape the mass or are absorbed by some other kind of nucleus. If the expected number of neutrons , which triggers new fissions, is less than one, a nuclear chain reaction may occur causing it's size to decrease. This is known as criticality. Though, if the expected number of neutrons is greater than one, the chain reaction will increase exponentially and the configuration is called a critical mass (45).

Question 9 - Difference between Nuclear Fusion and Nuclear Fission

Nuclear 'fission' is the splitting of a massive atom into two or more smaller ones, while nuclear 'fusion' is the fusing of two or more lighter atoms into a larger one (46). In simple terms, nuclear fission splits a massive element into fragments, releasing energy in the process. While nuclear fusion joins two light elements, forming a more massive element, and releasing energy in the process (47) Nuclear fission reaction don't normally occur in nature unlike nuclear fusion which occurs in stars, such as the sun.

A further difference is that for nuclear fission to occur, the critical mass of the substance and high-speed neutrons are required, while for nuclear fusion, high density and temperature environment is required. The nuclear fusion produces few radioactive particles compared to nuclear fission where many highly radioactive particles are produced.

The energy ratios between fusion and fission is also different because the energy released by nuclear fusion is three to four times greater than the energy released by fission. Although, the energy released by nuclear fission is a million times greater than that released in chemical reactions. This is due because nuclear fission requires little energy to split two atoms, compared to nuclear fusion where extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion (48).

Question 10 - Why are fusion reactions not used in nuclear reactors?

Nuclear reactors are large machines that control nuclear chain reactions while releasing heat at a controlled rate. If an uncontrolled nuclear reaction is in a nuclear reactor, it can potentially result in widespread contamination of air and water with radioactivity for hundreds of kilometres around the perimeter (49). Nuclear fusion is much more difficult to achieve than nuclear fission, which is what current nuclear power technology is based on. Secondly, it is also difficult to create a sustainable and stable nuclear fusion reaction and create a reaction that has a greater output than input. If this was possible, it would give the earth unlimited technology as it would have an energy output similar to that of a star (50).

Thirdly, even though fusion reactors don't have the fission product waste disposal problem of fission reactors, fusion reactors generate large number of fast neutrons, leading to large quantities of radioactive by-products (51).

In summary nuclear fission is more controllable than nuclear fusion; fission is easier to create and sustain than fusion and fusion generates large quantities of radioactive by-products despite not having the fission product waste disposal issues (52).

Question 20 - How is the fission reaction of the atom bomb controlled?

The atomic bomb is usually heavier than the critical mass. The radioactive materials in the bomb are kept apart and are less than the critical mass until they collide into each other (53). When nuclear fusion begins, only one neutron is released and is free to strike an atom of U-238, this then gives rise to three more neutrons. These three neutrons strike another three atoms of U-238 and this releases three more neutrons from each atom, making a total of nine neutrons. This process is slowly initiated and it explains why the atom bomb does not blast off immediately (54).

Another method could be to collect enough plutonium inside the bomb, and if thick enough, the neutrons can not escape without hitting another nucleus, and the chain reaction will start. Then by inserting special neutron-absorbing material in between portions of the plutonium the rate at which the chain reaction proceeds can be controlled, resulting in a 'slow burn' instead of an explosion (55).

To control the fission reaction, rods made of cadmium or boron are suspended between the fuel rods. These rods can be raised or lowered and thus check the reaction by absorbing the right number of neutrons. Hence they are called 'control rods'. When the control rods are completely inserted into the fuel, i.e., inside the reactor, all the neutrons are absorbed and the reaction does not start.

These rods are then slowly raised till they absorb the right number of neutrons and thus leaving behind just enough neutrons to sustain the chain reaction. That is, when the control rods absorb two neutrons leaving behind one neutron to bring about further fission reaction we say that the reactor has 'attained criticality'. At this stage, the number of atoms getting fissioned in one second is constant which means that energy is generated at a constant rate (56).

Nuclear Fission

Question 21 - Two Japanese cities bombed

The two Japanese cities which were bombed during World War II were Hiroshima and Nagasaki. On the 6th August 1945, the United States dropped an atomic bomb on Hiroshima, Japan. This atomic bomb was equivalent of 20,000 tons of TNT, killing tens of thousands of civilians (57). Then on the 15th August 1945, three days later to the previous bombing, the United States dropped another bomb on Nagasaki, Japan. Eventually ending World War II after Japan surrendered in 1945


Question 22 - Death Rate

When these atomic bombs exploded they caused short and long term effects. Within the first two to four months of the bombings, the acute effects killed 90,000-166,000 people in Hiroshima and 60,000-80,000 in Nagasaki. Most of these deaths occurred on the first day. Hiroshima's Health Department suspected that of the people who died on the day of the explosion 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months after the bombs 20-30% died from flash burns or effect of burns, 15-20% died from radiation sickness, and 50-60% died from other injuries and illnesses. Since the bombings 231 people, who were under observation, died from leukaemia and 334 people died from cancer due to the exposure to radiation released by the bombs. In both cities, most of the dead were civilians (60).

Question 23 - How the Hydrogen Bomb works and how it differs from the Atom Bomb

In an atomic bomb, the energy source is a mass of radioactive material such as uranium or plutonium. This material is very unstable due to the atom's nuclear being ready to fall apart or

'decay' at the slightest nudge, and explode by releasing unneeded energy and extra neutrons. In the following diagram, the plutonium (B in Diagram 1) is given that nudge by the outer casing of TNT (A in Diagram 1), which explodes all around it.(1)

Diagram 1: Atomic Bomb Diagram 2: Hydrogen Bomb

In comparison, the hydrogen bomb not only releases much more energy, using a process called 'nuclear fusion', but it is not triggered by TNT, rather it is 'triggered' by an atomic bomb (61).

The central core (B in Diagram 2) is a mass made up of atoms which are both isotopes of hydrogen, called deuterium and tritium. These are both hydrogen bombs made up of extra neutrons in each nucleus. Small atomic bombs are placed around the outside, this causes the deuterium and tritium to be squeezed into a very dense mass. This process is called nuclear fusion, and it releases great quantities of energy. As the core of the bomb explodes, it causes the bomb casing (C in Diagram 2), which is made from uranium, to undergo fission, creating even more energy. Overall, an atomic bomb sets off a fusion bomb, which also triggers another atomic bomb creating a hydrogen bomb (62)

In summary, the differences between the atomic bomb and the hydrogen bomb are that the atom bombs work on the principle of 'atomic fission' (splitting the atomic nucleus), while hydrogen bombs work on the principle of 'atomic fusion' (combining atomic nuclei). Secondly, the hydrogen bomb is hundreds or thousands of times more powerful than the atom bomb and the hydrogen bomb uses an atom bomb as a trigger (63). Thirdly, the hydrogen bomb leaves behind no radiation and the activation energy is tremendous. Whilst in comparison the atomic bomb leaves behind massive after effects and requires a significant less amount of energy to occur (64).

Radiation Sickness

Question 24 - Health Hazards Associated with Radiation

There are many health hazards caused by radiation. In the atomic bomb the radioactive materials which decay spontaneously produce ionising radiation which remove electrons from atoms or break chemical bonds. Any living tissues in the human body can be damaged by ionising radiation in a unique manner. Then the body attempts to repair the damaged tissue, sometimes the damage cannot be repaired or is too severe or widespread to be repaired. Also, mistakes made in the natural repair process can lead to cancerous cells.

Being exposed to radiation causes different kinds of health effects. In general, the amount and duration of radiation exposure affects the severity or type of health effects. There are two broad categories of health effects: stochastic and non-stochastic.

Stochastic effects are associated with long-term, low-level (chronic) exposure to radiation. The increased levels of exposure to radiation cause these health effects to be more likely to occur but do not influence the type or severity of the effect. Cancer is considered the primary health effect from radiation exposure because cancer is the uncontrolled growth of cells. This occurs when there is damage occurring at cellular or molecular level allowing for the uncontrolled growth of cells i.e. cancer.

Other effects occur because radiation can change the DNA that ensures cell repair and replacement produces a perfect copy of the original cell. This is called 'mutation' and can be teratogenic or genetic. Teratogenic mutations are caused by exposure of the fetus in the uterus and affects only the individual who was exposed. Genetic mutations are passed on to the offspring.

While non-stochastic effects occur when there is exposure to high levels of radiation and effects become more severe as the exposure increases. These short-term, high-level exposure is referred to as 'acute' exposure. Unlike cancer, health effects from 'acute' exposure to radiation usually appear quickly. These effects include burns and radiation sickness. Radiation sickness is also called 'radiation poisoning', which causes premature aging or even death within two months if over exposed. The symptoms of radiation sickness include: nausea, weakness, hair loss, skin burns or diminished organ function (65).

Question 25 - Medical Applications for Cancer Treatment, X-rays and Radioactive Tracers

a) Cancer Treatment

Radiation is used in cancer treatment to cure cancer whilst maintaining acceptable function and cosmetics. Radiation can also be used alone or combined with chemotherapy and/or surgery. Radiation is recommended when the patient needs it to alleviate pain after the tumor has spread or until it will be most beneficial for the patient's comfort. Radiation therapy can also be used in the treatment for various skin cancers, cancer of the mouth, nasal cavity, pharynx and larynx; brain tumors and many gynaecological, lung cancers, and prostate cancers. Radiation therapy plays a leading role in conjunction with surgery and/or chemotherapy in breast cancer, bowel cancer, bladder cancer, Hodgkin's disease, leukemia and lymphomas, thyroid cancer and childhood cancers (66).

b) X-rays.

Radiation therapy works by destroying cells, either directly or by interfering with cell reproduction using high-energy X-rays, electron beams or radioactive isotopes. This is when a radiated cell attempts to divide and reproduce itself, it fails to do so and dies in the attempt. In other cases, normal cells are able to repair the effects of radiation better than when there are malignant and other abnormal cells, therefore normal cells are able to recover from exposure to radiation and survive better than malignant cells. If radiation is applied in the correct region and in the correct amounts then the cancer dies and the patient is well again because the normal tissue survives. If not all cancer cells are killed then the cancer may reproduce and further radiation may needed but in lesser quantities because the normal tissues are less able to withstand the effects of further radiation.

Even though there is evidence of radiation exposure during medical x-ray procedures, the benefits of having an x-ray for a patient presenting some medical symptom outweigh the risk of the radiation affecting the person. The x-rays are known as 'ionizing radiation' which means that the radiation is of a high enough energy to break chemical bonds and therefore has the potential to be harmful to living organisms. When these bonds are broken there is, unfortunately, the potential to cause damage to cell nuclei.

However, this same situation makes x-rays as effective as cancer treatment in the form of radiation therapy. Another benefit is that x-rays allow people to see the structure of internal organs and bones and evaluate without the need for surgery. The radiation know to occur during an x-ray is believed to be low and harmful effects are directly proportional to the amount or dose the patient endures (67).

c) Radioactive Tracers

A radioactive tracer, also called a 'radioactive label', is a substance containing a radioisotope and a radioactive atom to allow easier detection and measurement.. A radioisotope is an isotope that has an unstable nucleus and that stabilizes itself by spontaneously emitting energy and particles. They were initially invented by Quinn Hanson and later developed by George de Hevesy and are used to measure the speed of chemical processes and to track the movement of a substance through a natural system such as a cell or a tissue. (68)

The simplest radioactive tracer studies show the tagging of a biological entity with a radioactive isotope (radioisotope). The entity is then tracked by following the radiation from the isotope. Mostly in biological tracer experiments, the radio-isotope is placed into the system and it's radiation is measured with Geiger-Müller Counters or scintillation detectors. Extremely soft (low-intensity) radiations can be detected by the use of photographic film.(69)

The radioactive tracer is relied on heavily by the medical profession for direct radiation fields and radioactive isotopes for identifying and treating disease and are used extensively to test new drugs and conduct research into cures for disease.(70)

Radioactive materials consist of radionuclides and radioactive isotopes. A radionuclide is any type of radioactive material, including elements and the isotopes of elements. An isotope of an element is a particular atomic "version" of it. Radionuclides are used in more than 11 million nuclear medicine procedures every year in the United States. Examples of radioactive tracers in medical procedures are bone scans that can detect the spread of cancer six to eighteen months sooner than X-rays and kidney scans are much more sensitive than X-rays or ultrasound in fully evaluating kidney function (70). They also are used in 100 million laboratory tests on body fluid and tissue specimens. Isotopes are atoms of the same element with different atomic structures.

Radioactive tracers are used mainly for bone scans, kidney scans. Radioactive isotopes and radioactively labelled molecules are used as tracers to identify abnormal bodily processes. When a patient is injected with a radioactive element, a special camera can take pictures of the internal workings of the organ.