8.) Topic 8 Half Life

Question 1

What is half-life

Answer 1

The time it takes for the number of unstable nuclei to halve in an isotope

Question 2

How long would elements with very long half lives remain radioactive
(long or short amount of time)

Answer 2

Very long time

Question 3

How long would elements with very short half lives remain radioactive
(long or short amount of time)

Answer 3

Short amount of time

Fractions of seconds

Question 4

Is it possible to determine which radioactive nuclei will decay next and why

Answer 4

No because the process is random

Question 5

Is it possible to determine when the radioactive nuclei will decay next and why

Answer 5

No because the process is spontaneous

Question 6

State 2 techniques that can be used when carrying out experimental work, to combat the random nature of decay

Answer 6

Take repeat readings Carry the experiment out over a long period of time

Question 7

What does it mean by activity?

What is it measured in?

Answer 7

The number of radioactive decays per second

Becquerels

Question 8

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 8

3 arrows represent 3 half lives. 23 x 7 = 81

81 years

Question 9

The activity of an isotope falls from 600 Bq (becquerel) to 150 Bq in 10 days. What is its half-life?

Answer 9

2 arrows represent 2 half lives

If 2 half lives is 10 days therefore 1 half life is 5 days

Question 10

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 10

Somewhere between 150 and 180 years

Question 11

How can we model radioactive decay

Answer 11

Step one. Collect the coins and count them. This is the starting number of parent radioactive atoms. Record this number. Between 60 and 100 coins is a good starting number.

Step two. Put the coins into a container, shake them, and then throw them into a tray.

Step three. Remove coins showing heads. These represent atoms that have decayed.

Step four. Count the remaining coins and record the number in a table against the throw number.

Step five. Repeat steps two to four until only two or three coins remain.

Step six. Plot a graph of number of coins remaining (-axis) against throw number (-axis).

Question 12

What is carbon dating?

Answer 12

Carbon dating is a scientific technique for determining the approximate age of an artefact that
contains once-living matter, using the half-life of the naturally occurring carbon-14 isotope.

Question 13

What is carbon-14 half life?

Answer 13

5730 years

Question 14

Name some uses of radioactive materials

Answer 14

Carbon dating, sterilising, medical equipment, killing cancer cells, smoke alarms and controlling the thickness of aluminum foil

Question 15

In a paper rolling mill

(a) why use beta radiation?
(b) Would the beta source have a long or short half-life?
(c) which isotope would we use?

Answer 15

(a) 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.
(b) Long. You don’t want to keep replacing the source every few days, and a short half-life would mean that the count rate detected might be reducing if the paper stayed the same thickness.

(c) Strontium-90 would be a good source for a thickness detector as it is a beta emitter with a
suitable half-life and the intensity of the beta particle beam would not change significantly from
day to day. The source would be extended underneath and across the full width of the paper so
that the detector sampled the full width.

Question 16

What type of radiation would be used for sterilisation and why

Answer 16

Gamma radiation because it kills microbes and can be used to sterilize medical instruments and kill the bacteria on fruit and vegetables so they stay fresh longer.

Question 17

What type of radiation would be used for smoke alarms and why

(b) why would amercium-241 be a good choice for the radiostope in a smoke detector
(c) Why doesn’t a smoke alarm need a lead shield around it?

Answer 17

Alpha radiation. The radiation ionises the air allowing an electric current to flow between 2 electrodes. If smoke is present, it will block this current, break the circuit and set off the alarm

(b) Because it’s an alpha emitter with a long half life so it doesn’t need replacing often and the beam intensity does not drop significantly day to day
(c) Alpha particles are absorbed inside the detector by the walls by the radioactive source holder and plastic cover

Question 18

State 2 uses of nuclear radiation in the field of medicine

Answer 18

Examining of internal organs

Controlling and destroying unwanted tissue

Question 19

What type of radiation would we use in medical tracers.

(b) What isotope would we use and why

Answer 19

Gamma radiation because it is weakly ionising so cannot damage the cells in our body

We would use Technetium-99 because it has a half life of only 6 hours. This means that it will not remain in the body for a long time so won’t continue to emit radiation. Also, it’s strongly penetrating so that it can be detected outside the body. It passes out the body easily

Question 20

What is a tracer

Explain how a medical tracer is used?

Answer 20

Something which shows how an object moves

Tracer is inserted into the body (often through the bloodstream)

Tracer is radioactive so it decays

A detector outside the body can detect this radiation and locate the tracer
e.g. arterial blockages and internal bleeds cause a build-up of radioactivity that can be easily detected outside the body

Question 21

What type of radiation would be used externally for cancer treatment

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

What type of radiation would be used internally for cancer treatment

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. The short life also reduces the risk

Question 23

For each of the following forms of radio therapy suggest and explain which radiostope you would choose

(a) External beam therapy
(b) Brachytherapy
(c) Unsealed-source radiotherapy

Answer 23

(a) A gamma emitter with a half-life measured in hundreds of years would be needed so that the
beam of gamma rays remained as constant as possible – europium-152 or barium-133.

(b) A beta emitter with a half-life of tens of days would be suitable – iridium-192.
(c) A beta emitter with a short half-life – iodine-131

Question 24

Exam question

What is an alpha particle

Answer 24

A subatomic particle consisting of 2 protons and 2 neutrons

Question 25

Exam question

Why would a smoke detector not work if the radioactive source emitted gamma rays only

Answer 25

Because gamma is so penetrating it wouldn’t be stopped by the smoke and it is weakly ionsing

Question 26

Exam question

Explain why the radioactive source in a smoke detector is not a risk to human health

Answer 26

Because the distance is quite large from the detector and humans and alpha particle are weakly penetrating so can be stopped by the air

Question 27

Exam question

Why is amerium-241 more suitable for use in smoke detector than curium-242

Answer 27

Longer half life so detector is active for longer and doesn’t need replacing much