Introduction
Types of Radiation  Radiation Safety 
Nuclear Reactions HalfLife
Nuclear Fission & Fusion  Nuclear
Stability & Binding Energy  Practice Exam
The rate of decay for a radioactive isotope varies
considerably. The halflife of a radioisotope is constant and independent
of the sample size. The time that it takes for half of any sample to
decay is called the halflife.
Halflives of Several Radioisotopes
Isotope: 





Halflife: 
5 years 
14 days 
5730 years 
8 days 
4.5 billion years 
Use: 
chemotherapy 
detect tumors 
archeological dating 
hyperthyroidism 
geological dating 
NOTE: has
a halflife of 4.5 billion years. Very old rock samples can be dated based
on their content.
The oldest rock found on earth (in Greenland) was dated 3.7 billion years. The solar system has an estimated age of 4.6 billion years based on dating meteorites.
EXAMPLE C
Barium122 has a halflife of 2 minutes. A fresh
sample weighing 80 g was obtained. If it takes 10 minutes to set up an
experiment using barium122, how much barium122 will be left when the
experiment begins?
Every halflife, 2 minutes, half of the original amount will undergo nuclear decay:
Time: 
start 
2 min 
4 min 
6 min 
8 min 
10 min 
Mass: 
80 g 
40 g 
20 g 
10 g 
5 g 
2.5 g 
At the end of 10 minutes (5 halflives) only 2.5 g are left, the rest has decayed.
EXAMPLE D
If 10 mg of iodine 131 is given to a patient, how much is left after
24 days? The halflife of iodine131 is 8 days.
Since the halflife is 8 days, 24 days corresponds
to 3 halflives. After one halflife 5 mg are left; after two halflives,
2.5 mg; and after 3 halflives 1.25 mg remain.
EXAMPLE E
Carbon14 has a halflife of 5730 years and is used to date archaeological objects.
All living organisms have a constant carbon12/carbon14
ratio. When the organism dies carbon12 levels remain constant but carbon14
decays. The changing carbon12/carbon14 ratio can be used to determine
the date of the artifact. For example, fresh charcoal made from a tree
contains carbon14 which will give a radioactive count of 13.60
disintegrations per minute per gram of carbon. Prehistoric cave paintings
were found in Spain. A piece of charcoal found in the ancient cave in Altamira,
Spain gave 1.70 disintegrations per minute per gram of carbon. From
this information, determine the age of the cave paintings. After one halflife
the number of disintegrations will go from 13.60 to 6.80; after two halflives
it is 3.40 and after three halflives 1.70. Therefore 3 halflives have
elapsed since the paintings were done. Since the halflife of carbon14
is 5730 years the paintings are about 5730x3=17,190 years old.
PROBLEM 4:
Perform the following problems
a. Iodine131 is used to destroy thyroid tissue in the
treatment of an overactive thyroid. The halflife of iodine131 is 8 days.
If a hospital receives a shipment of 200 g of iodine131, how much I131
would remain after 32 days?
b. The halflife of carbon14 is 5730
years. A piece of linen found today contains carbon14 and gives an activity
of 15 counts per minute per gram of carbon. If an anthropologist found
an ancient piece of linen believed to date back to the Neolithic period
which gave only 7.5 counts per minute per gram of carbon, how old is the
ancient linen ?
c. Technetium99m is used for brain scans. If a laboratory receives a shipment of 200 g of this isotope and after 24 hours only 12.5 g of this isotope remain, what is the halflife of technetium99m?
d. Mercury 197 is used for kidney scans and has a halflife of 3 days. If the amount of mercury197 needed for a study is 1.0 gram and the time allowed for shipment is 15 days, how much mercury197 will need to be ordered?
e. The halflife of strontium90 is 25 years. How much strontium90 will remain after 100 years if the initial amount is 4.0 g?
f. If the halflife of uranium232 is 70 years, how many halflives will it take for 10 g of it to be reduced to 1.25 g?
Introduction
 Types of Radiation  Radiation Safety 
Nuclear Reactions HalfLife
Nuclear Fission & Fusion  Nuclear
Stability & Binding Energy  Practice Quiz
a. 12.5 g Click
here to return to problems
b. 5730 years Click
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c. 6 hours Click
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d. 32 g Click
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e. 0.25 g Click
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f. 3 halflives Click
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