Moriah College Year 11 Term 4
Radioisotopes Assignment
Chemistry HSC Assessment 1
Aidan Baron
November
2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
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)
131
Equation: 53

I     10 e131
54 Xe
131
[Daughter Isotope Gamma Radiation: 54
Xe  131
54 Xe   ]
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)
Aidan Baron Chemistry November 2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
[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.
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 Tellurium130 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 Iodine131 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.
131
0
Te 01n131
Te

I

52
53
1 e
130
52
Aidan Baron Chemistry November 2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
Evaluation:
Benefits:

Radiopharmaceutical therapy with Iodine131 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.
Problems/ Detractors

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.
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
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.
241
95
4
Am 237
93 Np  2 He
or
241
95
4
Am 237
93 Np  2 
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:
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:
Figure 1.1
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.
Aidan Baron Chemistry November 2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
Figure 1.4
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
241
95
4
Am  Np 237

93 2 He
He 2  Z  He  Z  2
Aidan Baron Chemistry November 2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
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:
240
94

241
Pt  01n 241
Pt


94
95 Am
Aidan Baron Chemistry November 2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
Evaluation:
Benefits:

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
Problems/ Detractors

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.
Aidan Baron Chemistry November 2012
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Chemistry Radioactive Isotopes Assignment – Aidan Baron Nov 2012
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Thank you.
Aidan Baron Chemistry November 2012