Physics: half life

Physics: half life

Why are some nuclei radioactive?

• Radioactivity- the property possessed by unstable elements of spontaneously emitting energetic particles (such as beta or alpha particles) by the decay of their atomic nuclei in order to become stable.

RADIOACTIVITY

•The activity of a radioactive source is the number of unstable atoms in that source that decay per second.

•It is measured in Becquerel (Bq)

EXAMPLE

• •Uranium 234 has an activity of 231.3 MBq.

• •However as it decays this activity decreases as there are less unstable nuclei with in it’s nucleus.

• As the number of parent nuclei decrease the activity of the sample decreases .

HALF LIFE: There are two definitions of half-life.

• 1.The time it takes for the number of nuclei of the isotope in a sample to halve

• 2.The time it takes for the count rate from a sample containing the isotope to fall to half its starting level

Open Ended

The half-life of a material is 8 hours.

How long will it take for the activity to fall below 25%?

ANSWER

24 hours 3 half lives

Open Ended

The half-life of a material is 10 hours.

How long will it take for the activity to fall below 6%?

ANSWER

6 half lives 60 hours

Using the half lives backwards

Carbon 14 has a HL of 5730 and is used in to calculate the age of objects. A skull is approximately 28,000 years old. What proportion of C-14 remains?

Open Ended

Carbon 14 has a HL of 5730 and is used in to calculate the age of objects. A skull is approximately 10,000 years old. What proportion of C-14 remains?

ANSWER

10000/5730 = 1.74=

2 half lives

Open Ended

Carbon 14 has a HL of 5730 and is used in to calculate the age of objects. A skull is approximately 50,000 years old. What proportion of C-14 remains?

ANSWER

8.7= 9 half lives

Open Ended

The activity of a substance is 8000Bq, it’s half life is 5hrs. What is the activity after 40 hours?

ANSWER

40/5 = 8

8000/8 = 1000

Open Ended

The activity of a substance is 2000Bq, it’s half life is 1hr. What is the activity after 5 hours?

ANSWER

5/1 = 5

2000/5 = 400

Open Ended

A radioisotope has a half life of 20 hours. A sealed container contains 10 mgs of this isotope. What ratio and mass remains: -

a) After 10 hours b) after 30 hours

ANSWER

a) 1:1 10mg

b) 1:2 5mg

Open Ended

A radioisotope has a half life of 5 hours. A sealed container contains 100 mgs of this isotope. What ratio and mass remains: -

a) After 20 hours b) after 30 hours

ANSWER:

a) 1:16 12.5mg

b)1:152 0.78mg