2.8 - Half-life

Question 1

What does working out the half life of a radioactive substance calculate?

Answer 1

When a radioactive element will become stable.

Question 2

When an unstable nucleus gives out an alpha or beta particle, the nucleus turns into the nucleus of a new element. This process is called ______ _____. Although ______ _____ is a random process, statistically, over a time called the half-life, half of the parent radioactive nuclei will have decayed.

Answer 2

radioactive decay

Question 3

Define the half-life of a radioactive isotope

Answer 3

The time it takes for the number of unstable nuclei in a substance to halve

The time it takes for the count rate from a sample to fall to half its initial level

Question 4

Why are isotopes with long half-lives particularly harmful?

Answer 4

They remain radioactive for much longer periods of time

They must be stored in specific ways to avoid humans and the environment from being exposed to radiation for too long.

Question 5

An isotope has an activity of 6400Bq. The half-life is 8 hours. What is the activity after 32 hours?

Answer 5

32 hours is 4 half-lives so we need to half it 4 times

6400Bq–>3200Bq–>1600Bq–>800Bq–>400Bq

Question 6

What is activity? What is it measured in?

Answer 6

Activity is the measure of the number of radioactive decays per second. It is measured in becquerels (Bq). Activity depends on the number of unstable atoms present and the half-life

Question 7

How can radioactive decay be modelled?

Answer 7

By:
Throwing dice
Throwing cubes
Flipping coins

Question 8

State two uses of nuclear radiation in the field of medicine.

Answer 8

1. Examining of internal organs

2. Controlling and destroying unwanted tissue

Question 9

Describe the nature of radioactive decay?

Answer 9

Random
Which nuclei decays and when is determined only by chance
It is impossible to predict which nuclei will decay and when

Question 10

Explain how to determine and verify half-life from a decay curve

Answer 10

Read off the time axis the time it takes for the number of unstable nuclei to half.
To verify, check that the time it takes for that number to half again is the same

Question 11

The half-life of a radioactive isotope is 27 years. How long will its mass take to fall from 2 g to 0.25 g?

Answer 11

2 g → 1 g → 0.5 g → 0.25 g. Three arrows represent three half-lives. 3 × 27 years = 81 years

Question 12

An isotope has a half-life of 30 years. Estimate how long it will take for the number of nuclei to decay to below 200 if the starting number is 8,000?

Answer 12

8,000 → 4,000 → 2,000 → 1,000 → 500 → 250 → 125. It takes five half-lives to fall to 250 nuclei, and six half-lives to 125 nuclei. So somewhere between 5 × 30 = 150 years and 6 × 30 = 180 years.

Question 13

Explain the basic idea behind carbon dating.

Answer 13

All living things absorb Carbon-14 daily.
When they die, we no longer absorb carbon-14
By measuring the amount of carbon remaining, the age can be calculated.

Question 14

Explain how a medical radioactive tracer (camera to monitor internal organs) is used.

Answer 14

1) The tracer is inserted in the body (often through the bloodstream)
2) The tracer is radioactive so decays.
3) A detector outside the body can detect this radiation and locate the tracer

Question 15

What can be said about the half-life of a given radioactive isotope?

Answer 15

It is constant

Question 16

What is the unit of radioactive activity?

Answer 16

Becquerel (Bq)

Question 17

In a paper rolling mill, the thickness of the paper is monitored by how much beta radiation is received at the detector. Why use beta radiation?

Answer 17

Alpha wouldn’t penetrate the paper. Gamma is so penetrating that there would be little difference in the reading detected if the paper became thick. The count rate reduces with increasing thickness if a beta source is used.

Question 18

Why is gamma radiation used in the medical industry with respect to medical instruments?

Answer 18

Gamma radiation kills microbes and can be used to sterilize medical instruments

Question 19

What does ionising mean?

Answer 19

That it interacts with cells in the body and damages DNA which causes cancer

Question 20

An isotope of americium which emits alpha particles is used in smoke alarms. Alpha radiation ionises the air and this allows a small current to flow between two electrodes. Alpha is weakly ______ so smoke stops it, the current drops and the alarm goes off.

Answer 20

penetrating

Question 21

How is Gamma radiation used in cancer treatment externally?

Should the Gamma radiation have a long or short half-life?

Answer 21

Externally gamma radiation can be beamed at cancer cells to kill them.
The gamma source used should have a long half-life to maintain the dose of radioactivity delivered to the tumour. A long half-life also means that the source in the radiotherapy equipment will not need changing regularly.

Question 22

How is Alpha radiation used in cancer treatment internally?

Should the Alpha radiation have a long or short half-life?

Answer 22

Internally an alpha source with a short half-life can be injected directly into the tumour. This is called targeted alpha therapy (TAT). Alpha is strongly ionising – so will kill the cancer cells. It is also weakly penetrating so stays within the tumour and doesn’t harm the healthy cells outside the tumour