Moriah College Year 11 Term 4 November2012 Radioisotopes Assignment

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Moriah College Year 11 Term 4
November2012

Radioisotopes Assignment

Chemistry HSC Assessment 1

Aidan Baron

Medical: Iodine-131

Iodine-131 is a radioactive isotope of Iodine. Radioiodine as it is commonly called is comprised of 53 protons, 53 electrons and 78 neutrons. Thus it is ¹³¹I. It is iodine because of its 53 protons, but has an atomic mass of 131 due to the total of protons and neutrons in the nucleus.

Iodine-127 is the naturally occurring stable, non-radioactive form of Iodine.

Nuclear Decay:

The Neutron rich nucleus of Iodine -131 decays primarily by Beta (β^-) decay.
(After Beta decay, there is also some Gamma decay, though less than 9.1% of that of Beta)

Equation:

[Daughter Isotope Gamma Radiation:]

In the above equation, Iodine-131 undergoes beta decay and so emits a beta particle; which is essentially a neutron that turns into a proton and high energy electron. The atom loses a neutron but gains a proton; this changes the atom’s identity from iodine to Xenon. The Atomic mass stays the same however, because there are still a total of 131 protons and neutrons in the nucleus. (After this process, the resulting isotope then emits Gamma radiation as Gamma waves to further decrease its destabilising energy.)

Instability:

Iodine-131 is unstable and classified as Neutron rich due to it having more than a 1:≥1 ratio of protons to neutrons. Because the isotope is neutron rich, making it unstable, it seeks to become more stable by decreasing its number of neutrons. In this way, the nucleus releases a neutron in the form of a beta minus particle, which is a proton and high energy electron.

Half-Life:

Iodine-131 has a half life of 8.0197 days. This is the time it takes for 50% of the beta decay (and some Gamma) to have occurred.

Use:

Iodine-131 is used as a radiopharmaceutical, which is a radioactive isotope administered to humans or living things for therapeutic purposes. Iodine-131 is used in medicine primarily for the treatment and diagnosis of thyroid tumour/ cancer. The thyroid [Figure 1.1] is an endocrine gland located in the neck behind the thyroid cartilage (Adam’s apple). The thyroid has follicles which selectively absorb iodine as iodide ions from the blood to make thyroid hormones vital for human growth.

The iodine is administered as an iodide salt (sodium iodide)
[see figure 1.2] in a capsule or liquid that is swallowed orally. The iodine is quickly absorbed by the gastrointestinal tract into the bloodstream where it will filter past the thyroid, be absorbed by the thyroid and begin destroying thyroid cells.
[See Figure 1.3]
Thyroid cell/cancer destruction occurs through the Ionising radiation of Beta Decay. Occasionally a gamma camera is also used to view the absorption of Iodine-131 during therapy to confirm its affect. soidum iodide.jpg

Use and Properties:

Iodine-131 as a beta emitting radioisotope (whose daughter isotope also emits gamma) has a moderate ionising ability and moderate-high penetrating power. This is due to the size of the Beta particle which affects its penetrating power and the properties of the beta particle which affect its ionising ability. The proton emitted is able to penetrate through the space between nuclei and electron clouds and around the outside of the atom, but not through the nuclei or cloud, thus penetrating more than an alpha particle but less than a gamma wave. The proton emitted has a positive charge and so is able to ionise electrons from nearby atoms. Unlike alpha particles, only one electron can be extracted and so its overall ionising ability is less than alpha but greater than gamma. These properties, as well as the isotope’s half life of ~9 days mean that it is suitable for in vivo destruction and imaging of Thyroid tumours. Being used in humans, the half life cannot be overly long otherwise adverse radiation will occur, also, the ionising ability must not be too powerful, otherwise the isotope will do too much damage to surrounding areas and the penetrating power must be high enough to penetrate the first layers of tissue and get to the actual thyroid gland, but low enough that the radiation does not pass out of the local area. Also, the small amount of gamma radiation generated by the daughter isotope of iodine, being Xenon-131 helps to image with a gamma camera the progress of the treatment which is an added benefit. For these reasons, a Beta emitting isotope is needed, and Iodine-131 is suitable as it has a half life long enough to affect a treatment but short enough not to remain in the body after thyroid destruction has occurred.

Production:

A target (sample) of Tellurium Oxide TeO₂ (from natural deposits) is enriched to Tellurium-130 and then irradiated in a nuclear reactor using neutron bombardment to make Tellurium-131. Te-131 is highly unstable and emits beta^-radiation and decays into Iodine-131 within approximately 25 minutes. In essence, tellurium is concentrated, and an extra neutron added to the nucleus making it unstable, this instability causes the nucleus to emit a beta particle to stabilise itself. The emission of the beta particle (a proton and high energy electron) turns the tellurium into iodine, thus resulting in Iodine-131.

Evaluation:

Benefits:

Problems/ Detractors

Radiopharmaceutical therapy with Iodine-131 is less invasive than surgery and recovery is arguably faster with no scar tissue, meaning that the patient experiences greater quality of life during and after treatment.
Iodine-131 therapy is far Less expensive than the cost of surgery and hospitalisation for recovery from surgery. (cost to patient is between 1-5 thousand dollars compared to surgery which can be up to 60 thousand dollars)
Therapy may be administered by one or two trained personnel and supervised by a physician in nuclear medicine. This is far more efficient than surgery which requires the total attention of an anaesthetist, anaesthetic nurse, surgeon, registrar surgeon, scrub nurses and theatre nurses for a lengthy period of time.
I-131 therapy means that imaging can take place during treatment to confirm effects due to Gamma Decay of Daughter Xe isotope.
I-131 therapy significantly reduces the risk of procedure-associated infection by not requiring invasive procedures like surgery.

Patient is ‘radioactive’ for a period of a week and so their lifestyle during this period is limited. E.g. no/ very restricted physical contact to prevent radiation of others, especially young children.
Patient is exposed to radiation which in rare cases may cause adverse affects / radiation of nearby tissues .e.g. mutation and damage to neck structures and vocal chords.
I-131 Therapy is relatively expensive ($1-5K) because a nuclear reactor is needed to produce the isotope so this therapy is not yet an option for developing countries even though this therapy is a much better alternative to more expensive surgery.

Conclusively, radioiodine therapy is a valuable and effective tool in treating thyroid cancers as it is cost effective and less invasive than other alternatives, namely surgery. Although there are risks associated with treatment, they are very low and the benefit of treatment, (eradicating potentially life threatening cancers quickly and non-invasively) far outweighs the small risk of inadvertent overexposure to radiation.

Industrial: Americium-241

Nuclear Decay:

Americium-241 is an artificially produced (not naturally occurring) Radioisotope.

It is known as Americium-241 because it has an atomic mass of 241. This includes 95 protons in the nucleus, which make the element Americium, 95 electrons in the electron cloud and 146 neutrons in the nucleus.
Americium-241 is an alpha decaying radioisotope.
Am-241 undergoes alpha decay by releasing an alpha particle (a helium nucleus containing 2 protons and 2 neutrons). After emitting the alpha particle, the americium becomes Neptunium-237 (Np-237) with 93 protons and 144 neutrons, its most stable isotope.

Instability of nucleus:

Americium-241 has an atomic number of 95 (95 protons). An element with greater than 83 protons is generally unstable and therefore in most cases radioactive. Because americium-241 is a heavy, unstable element with a surplus of neutrons but also protons in the nucleus, it undergoes alpha decay as this is the most efficient way to get rid of the most mass quickest. This is because unlike beta and gamma decay, alpha decay is the emission of an alpha particle which is comprised of 2 protons and neutrons, the largest radioactive particle. Stable Americium has 243 particles in the nucleus; that is 148 neutrons as opposed to the 146 of Am-241. This imbalance of P:N ratio can also cause nuclear instability.

Half Life:

Figure 1.1
Americium-241 has a Half Life of 432.6 years. This means it takes 432.6 years for 50% of the amount of radioactive Americium in a given quantity to emit an alpha particle and become stable Neptunium.

Use:

The traditional and primary use of americium-241 in industry is for use in Ionisation Smoke Detectors. Ionisation smoke detectors are instruments used constantly in the household and industry as a warning device in the event of a fire or heavy smoke.

The device uses a very small amount (90 µCi) of Americium-241 as a radioactive source to emit alpha particles [see figure 1.1]. The alpha particles are emitted from the radioisotope and ionise the air as they come into contact with it.

Figure 1.4

Figure 1.2

Figure 1.2

Ionisation of the air occurs when the alpha particle which has 2 protons and 2 neutrons in the nucleus, therefore giving it a +2 charge, comes into contact with other gas elements in the ambient air. The alpha particle rips the electrons off these elements to balance its own charge, leaving the particles in the air ionised. The ionised air allows an electric current to pass through it from one electrode to another on opposite sides of the ionisation chamber in which this occurs.

[Figure 1.2 & 1.4]
Smoke is detected and the alarm is triggered when smoke enters the chamber and interrupts the electrical current flowing through the ions. The circuit detects a decrease in current and triggers an alarm. [Figure 1.3]
In the equation below, Am-241 releases an alpha particle (helium nucleus). This helium nucleus then comes into contact with an unspecified element. This element is then ionised, the electrons being ripped off it and transferred to the cloud of the Helium Nucleus, making the helium stable and the previous element and ion.

Figure 1.3

Accounting for use:

Americium-241, as an alpha decaying radioisotope, has a very high ionising ability and very low penetrating power.

Its high ionising ability is due to the electro-negativity of the helium nucleus which needs 2 electrons to balance it.
The low penetrating power of the particle is due to its very large size. The alpha particle has a whole nucleus which it needs to pass through the space between other particles. For this reason alpha particles can be stopped by a few centimetres of air, liquids or solid materials.
Am-241 is used in the household environment and so the radiation exposure of people to it must be extremely low. For this reason, americium is a suitable radioisotope as the thin plastic walls of the ionising chamber and then the external mould of the smoke detector itself are more than sufficient to stop any radiation from reaching the outside of the detector let alone nearby people.
This low penetrating power also becomes useful as it means that a lot of air can be ionised within the small ionising chamber because there is such a high occurrence of ionisation due to the alpha particle’s size. This is especially advantageous over other forms of decay from other radioisotopes such as beta and gamma decay where it would take a much greater distance and there would be a much lower incidence of ionisation, meaning that an electrical current could not necessarily be sustained between the electrodes.
The high ionising ability of Am-241 becomes very useful in this context because it means that within a relatively short distance and with a relatively high frequency, the alpha particles emitted from the Americium-241 will ionise the air in the ionising chamber and provide enough ionised particles to sustain an electrical current between the electrodes.

Americium-241 has a relatively long (by human standards) half life of 432.6 years.

This long half life means that Am-241 is economic for use in households and as a product for general purchase because the radioactive source will never need to be replaced in a person’s lifetime, and by the time the radioactive source runs out, the technology will have been redundant for 100 years or so already.

Production:

Am-241 is produced when Plutonium-240 is neutron bombarded in a Nuclear reactor and captures 1 neutron to become Plutonium-241. Plutonium-241 then decays with beta minus decay (losing a neutron which then becomes a proton and a high energy electron) to Americium-241 (the plutonium becomes americium because although a neutron is lost from the nucleus, a proton is gained, changing the element’s identity and retaining the same atomic mass.). Plutonium-240 is an isotope of plutonium and can be produced with through various parent routes but in all cases, parent isotopes are mined and then undergo either decay or neutron capture. Equation below:

Evaluation:

Benefits:

Problems/ Detractors

Safe for use near humans due to low penetrative power in Smoke detectors.
Cost effective compared to optical smoke detectors which are larger and more expensive to make
Low power consumption, a 9 volt battery can sustain operation for months to years.
Because of low cost, small concentrations and long life it can be used in developing countries
Am-241 in smoke detectors can and is proven to save human life: incredible benefit to very low risk

Disposal of devices can potentially lead to poisoning of the environment and radiation leakage.
Radiation source will outlive need for technology, meaning that the unit and radiation source will be disposed of in the future which may cuase pollution and waste if not recycled or disposed of properly. This is likely as the general population does not know that smoke detectors contain Americium-241.
Production and use of Am-241 smoke detectors in developing countries, while having the potential to save hundreds of lives may also put many at risk if proper safety regulations are not in place and adhered to, as is common at present in these regions. The effect of this may be inadvertent ionisation of human tissue in people which may cause a variety of serious illnesses.

Conclusively, Americium-241 is a commonly used and effective radioisotope for use in smoke detectors throughout the world. Although there is the potential for great risk and harm to occur to humans and the environment if not treated carefully in production and disposal, the benefit of smoke detectors in households is a proven lifesaving tool. As long as care is taken in accordance with international nuclear authorities, the use of Americium-241 can greatly benefit humanity and so should be valued, but also respected as a useful radioisotope.

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Thank you.

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