Module 6 Radioactivity

Question 2

What is radioactivity?

Answer 2

Emission of ionising radiation from an unstable nucleus.

Question 3

What are the three most common types of radiation?

Answer 3

Alpha, beta minus, and gamma rays.

Question 4

How can radioactivity be detected?

Answer 4

Using a Geiger muller tube.

Question 5

What is the usual unit for count rate?

Answer 5

Becquerels.

Question 6

What are sources of background radiation?

Answer 6

Cosmic radiation, radioactive elements in rocks of Earth’s crust.

Question 7

What will background radiation result in when measuring activity of samples?

Answer 7

Increased measure of activity by the same amount and therefore a systematic error.

Question 8

How to account for background radiation when measuring count rate?

Answer 8

Measure count rate with no source present and subtract count rate from future experiments.

Question 9

What can be changed to make a more suitable container apart from changing the thickness of an absorber?

Answer 9

Use a different type of absorber.

Question 10

What is the range of alpha particles in air?

Answer 10

A few cm (5-10).

Question 11

What is the range of beta particles in air?

Answer 11

A few meters.

Question 12

What is the range of gamma radiation in air?

Answer 12

A few kilometres.

Question 13

What can alpha particles be absorbed/blocked by?

Answer 13

Paper or skin.

Question 14

What can beta particles be absorbed/blocked by?

Answer 14

A few mm of aluminium.

Question 15

What can gamma radiation be absorbed/blocked by?

Answer 15

A few cm of lead or a few m of concrete.

Question 16

Why do radioactive particles have limited range?

Answer 16

They lose energy by ionising atoms in their path as they pass through a material or medium.

Question 17

Why do alpha particles have the shortest range?

Answer 17

Large size and charge, therefore cause a lot of ionisation and lose energy quickly.

Question 18

Why does gamma radiation have a very long range?

Answer 18

No charge, negligible size, high energy, and low ionising power - loses energy very slowly as it passes through matter.

Question 19

What is the nucleus left after decay has taken place?

Answer 19

Daughter.

Question 20

Why is gamma radiation emitted?

Answer 20

Emitted along with alpha or beta particles to remove excess energy.

Question 21

What does spontaneous mean?

Answer 21

Not affected by external factors such as pressure, temperature, or chemical reactions.

Question 22

What does random mean?

Answer 22

Cannot predict when a certain nucleus will decay or which will decay in a given time.

Question 23

What does A mean in the decay equation?

Answer 23

Activity. Number of nuclei decaying per unit time.

Question 24

What is the decay constant?

Answer 24

Probability of a nucleus decaying per unit time.

Question 25

What is N in the decay equation?

Answer 25

Number of undecayed nuclei.

Question 26

Why is the decay constant a number between 0 and 1?

Answer 26

It's a probability.

Question 27

What does the value of A actually mean?

Answer 27

How much N will decrease by in a second.

Question 28

What is the equation to find the number of daughter nuclei using the exponential formula?

Answer 28

N= N_0*e^(-λt).

Question 29

What does the -λN symbol mean in the spreadsheet equation for decay?

Answer 29

-A.

Question 30

If λ is given in years^-1, what will the value of t be given in the exponential equation?

Answer 30

Years.

Question 31

How to find N from a mass and molar mass M?

Answer 31

N = (mass / M) * Avogadro's number.

Question 32

What value of t must be taken for accurate results in the spreadsheet model?

Answer 32

Much less than half life so N does not vary too much.

Question 33

What is the approximate half life of carbon 14?

Answer 33

5700 years.

Question 34

How is carbon-14 made?

Answer 34

In the atmosphere by cosmic rays (cause nitrogen to undergo beta minus decay).

Question 35

Why does the ratio of carbon-14 to carbon-12 remain the same over time?

Answer 35

Carbon-14 is constantly replenished by cosmic rays and incorporated into living matter by photosynthesis.

Question 36

Given that the ratio of C-14 to C-12 is 1.3x10^-12, show that the activity of carbon is 0.25Bq per g.

Answer 36

1 gram of C12 has 1/12xNa number of nuclei. Multiply by ratio to find number of C14 nuclei per g. Multiply by decay constant (find using half life of 5700 years).

Question 37

Describe and explain how carbon dating can be used to find the age of a biological sample.

Answer 37

Measure the count rate of the relic to find A. Measure the count rate of a living sample with a similar mass of carbon, this is A_0. C-14 decays exponentially so the exponential decay equation can be used.

Question 38

What are the limitations of carbon dating?

Answer 38

Cannot be used for modern samples as activity will barely change. Cannot be used for samples over 50,000 years old as the activity will be lower than background rate. May not be accurate for older samples as there may have been changes in Earth's magnetic field which change the ratio of C-14/C-12 by influencing cosmic rays.

Question 39

Why is uranium used to date old rocks?

Answer 39

Has an extremely long half life.

Question 40

What is radioactive power?

Answer 40

Activity x energy released per decay.

Question 41

What device is used to convert radioactive power to electricity?

Answer 41

Using an RTG.

Question 42

Give examples of where radioactive materials are used as nuclear batteries.

Answer 42

Spacecrafts or pacemakers.