Nuclear Material HalfLife is the required for of a radioisotopes nuclei to decay into its products For any radioisotope of ½ lives Remaining ID: 599326
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Slide1
Uses for Nuclear MaterialSlide2
Half-Life
is the required for of a radioisotope’s nuclei to decay into its products.
For any radioisotope,
# of ½ lives
% Remaining0100%150%225%312.5%46.25%53.125%61.5625%
Half-life
time
halfSlide3
Half-LifeSlide4
Half-Life
For example, suppose you have 10.0 grams of strontium – 90, which has a half life of 29 years. How much will be remaining after x number of years?
You can use a table:
# of ½
livesTime (Years)Amount Remaining (g)001012952582.53871.254116
0.625Slide5
Half-Life
Or an equation!
m
t
= m0 x (0.5)
n
mass remaining
initial mass
# of half-livesSlide6
Half-Life
Example 1: If gallium – 68 has a half-life of 68.3 minutes, how much of a 160.0 mg sample is left after 1 half life? 2 half lives? 3 half lives?
m
t
= m0* (.5)nmt = 160mg * (.5)1mt = 160mg * (.5)2mt = 160mg * (.5)3mt = 80mgm
t = 40mgmt = 20mgSlide7
Half-Life
Example 2: Cobalt – 60, with a half-life of 5 years, is used in cancer radiation treatments. If a hospital purchases a supply of 30.0 g, how much would be left after 15 years?
n = total time/half-life
n = 15/5
= 3mt = m0* (.5)nmt = 30g * (.5)3mt = 3.75gSlide8
Half-Life
Example 3: The half-life of polonium-218 is 3.0 minutes. If you start with 20.0 g, how long will it take before only 1.25 g remains?
m
t
= m0* (.5)n1.25 = 20g * (.5)n0.0625 = (.5)nln(0.0625) = ln((.5)n)ln(0.0625) = n*ln(.5)4= n
4*3mins=12 minsSlide9
Half-Life
Example 4: A sample initially contains 150.0 mg of radon-222. After 11.4 days, the sample contains 22.75 mg of radon-222. Calculate the half-life.
n = total time/half-life
m
t = m0* (.5)n22.75 = 150g * (.5)n0.152 = (.5)nln(0.152) = ln((.5)n)ln(0.152
) = n*ln(.5)2.72= n2.72 = 11.4/half-life
half-life = 11.4/2.72 half-life = 4.19 daysSlide10
Uses of Radiation
Medical applications
Radiation of cancer cells
Radioactive tracers to detect disease
Sterilization of equipmentCommercial products (smoke alarms)Radioactive datingSlide11
Radiation therapy
High doses of radiation can causes the normal functioning of living cells to mutate and leads to abnormal growth and eventually cancer.
VERY HIGH doses will kill cells – especially fast-growing ones like cancer cells
Gamma ray treatmentSlide12
Radioactive
Tracers in Diagnosis
Used to follow the flow of a substance through the body.
Pattern of
colors/locations can tell doctors how well particular organs are functioning.Technetium-99 is one of the most common – used extensively in imagingIodine-131 for thyroid functionThalium-201 for cardiac problemsFlourine-18 for PET scansSlide13
Sterilisation
S
terilisation
- Killing microorganisms on medical instruments using a strongly ionising source of radiation. Used on medical instruments while they are still within their packaging.Food can also be irradiated to increase shelf-life.
Sterile syringe within its packagingSlide14
Commercial products: Smoke
detectors
A radioactive source inside the alarm ionises an air gap so that it conducts electricity – americium-241, an alpha emitter
Very long half-life
In a fire, smoke prevents the radiation and therefore a drop in electric current which sets off the alarm.Slide15
Radioactive Dating
Radiocarbon dating:
the ages of specimens of organic origin can be estimated by measuring the amount of cabon-14 in a sample.Slide16
Radiocarbon dating
Living material (for example a plant) contains a known tiny proportion of radioactive carbon-14. This isotope is produced when high speed neutrons (part of cosmic radiation) collide with nitrogen gas in our
atmosphere.
C
14
6
p
1
1
+
N
14
7
n
1
0
+
When organisms die, they no longer have a constant proportion of carbon-14. It decays
by beta emission back to the stable nitrogen-14 with a half-life of about 5600 years.
C
14
6
N
14
7
β
-
0
-1
+Slide17
Calculating ages
Example: A piece of wood taken from a cave dwelling in New Mexico is found to have a carbon-14 activity (per gram of carbon) only 0.636 times that of wood today. Estimate the age of the wood. (The half-life of carbon-14 is 5730 years.)
m
t
= m0* (.5)n0.636 = 1 * (.5)n0.636 = (.5)nln(0.636) = ln((.5)n)ln(0.636) = n*ln(.5)0.65= n
0.65*5730= 3741.1 yrsSlide18
Limitations of radiocarbon dating
The dating process assumes that the level of
cosmic radiation
reaching the Earth is
constant – corrected by using known ages of objects, esp trees (tree rings)Radiocarbon dating is limited to reasonably young samples no older than ~50,000 years because the amount of carbon-14 becomes to small to measure accuratelyRocks and other very old objects are dated using isotopes with significantly longer half-lives.Potassium-40 decays to argon-40: half-life = 1.25 billion yearsUranium-238 decays to lead-206: half-life =
4.47 billion years