Radioactivity

Question 1

Explain why the count rate shown at graph A is not due to alpha particles, no matter what kinds of radiation are emitted by the source [1]

Answer 1

Alpha particles have very short range in air

Question 2

State and explain what type of radiation(s) is/are emitted by the source based on graphs B and C. [3]

Answer 2

Beta and gamma rays
Gamma rays are present and they travel undeflected and are picked up by he GM tube at position P.
Graph shows a drop in value from 1000cpm to 450cpm at Q.

Beta particles are present as they have been deflected downwards by the magnetic field and are picked up by the GM tube at Q.

Question 3

Explain why the sum of the average count rate obtained at B and C is greater than that at A. [1]

Answer 3

Sum does not add up as background radiation is recorded at both positions P and Q

Question 4

half life

Answer 4

Time taken for half the number of radioactive atoms in a sample to decay

Question 5

Proton or atomic number

Answer 5

Number of protons in an atom

Question 6

Nucleon or mass number

Answer 6

Total number of neutrons and protons in the nucleus of an atom

Question 7

Nuclides

Answer 7

Atoms whose nuclei contain a particular number of protons and neutrons

Question 8

Isotopes

Answer 8

Atoms of the same element that have the same number of protons but different numbers of neutrons

Question 9

Radioactivity/nuclear decay/radioactive decay

Answer 9

Process by which an unstable atomic nucleus loses its energy by emission of electromagnetic radiation or particles in order to form a more stable nuclei of the same element or other elements

Question 10

Spontaneous

Answer 10

Happens by itself without any external influence

Question 11

Random

Answer 11

Impossible to predict exactly where and when an emission, the disintegration of a nucleus occurs

Question 12

Ionisation

Answer 12

Ability to eject electrons from atoms to form ions

Question 13

Ionising radiation

Answer 13

Radiation with high energy that can knock off electrons from atoms to form ions

Question 14

Background radiation

Answer 14

Nuclear radiation in an environment where no radioactive source has been deliberately introduced

Question 15

Nuclear fission

Answer 15

Process in which the nucleus of an atom splits and releases a huge amount of energy

Question 16

Nuclear fusion

Answer 16

Process in which 2 light atomic nuclei combine to form one heavier atomic nucleus and releases a huge amount of energy

Question 17

1 becquerel

Answer 17

1 disintegration per second

Question 18

Alpha particle radiation and their properties

Answer 18

Nature: An alpha particle is actually the nucleus of a helium atom.

Mass: heavy

Charge: positive

Speed: Up to 10 –20 % speed of light

Ionising power: strong

Penetrating power: have a range of about 5cm in air and are easily stopped by paper

Effect of electric field: slight deflection towards the negative plate

Question 19

Beta particles and their properties

Answer 19

Nature: A beta particle is actually a fast moving electron ejected from a radioactive nucleus

Mass: light

Charge: negative

Speed: Up to 80 % speed of light

Ionising power: moderate

Penetrating power: have a range of about few metres in air and are stopped by aluminium

Effect of electric field: strong deflection towards the positive plate

Question 20

Gamma rays and their properties

Answer 20

Nature: high energy electromagnetic waves emitted by a radioactive nucleus

Mass: zero

Charge: no

Speed: speed of light

Ionising power: very weak

Penetrating power: have a range of about few hundred metres in air and are stopped by lead

Effect of electric field: not deflected

Question 21

Radioactive detectors

Answer 21

1. Photographic plates
2. GM tube (Geiger-mueller tube)

Question 22

How to measure background radiation

Answer 22

1. Remove all known radioactive sources.
2. Start the counter and the stopwatch.
3. Stop the counter after 10 minutes and record the number of counts.
   Number of counts per minute = (number of counts in 10 minutes)/10
4. Repeat your measurement at least once and calculate an average value.

Question 23

Uses of radioactivity

Answer 23

1. Radiotherapy (Cobalt-60 emits strong gamma radiation to kill cancerous cells)
2. Sterilisation (gamma rays are
   used to sterilise food and hospital equipment.)
3. Radioactive Tracers
4. Thickness Gauge**
5. Carbon dating

Question 24

Safety aspects of radiation

Answer 24

(I) All radioactive sources should be kept in thick lead boxes.
(ii) Special clothing such as lead-lined suits should be worn by persons doing
radioactive experiments. Dosimeters should be worn also.
(iii) Food and drinks are strictly prohibited when a person is doing radioactive
experiments.
(iv) The international radiation symbol should be displayed in risk areas.

Question 25

Dangers of nuclear radiation

Answer 25

Nausea, vomiting, hair loss, diarrhoea, haemorrhage, destruction of intestinal lining, central nervous system damage and death
CAUSE CANCER

Question 26

Explain why Californium-241 is not suitable for use in a smoke detector

Answer 26

It has a short half-life of X minutes and thus needs to be frequently replaced.

Question 27

Explain how a radiation emitted could be used in thickness gauging.

Answer 27

When the sheets are of unequal thickness, count rate of gamma radiation shown on the detector will fluctuate.

Question 28

Advantage of using the gamma radiation source with a long half-life

Answer 28

Only a small amount of gamma radiation is needed per use due to its long half-life
Do not need to be constantly replaced

Question 29

Describe and explain how readings of detector would be affected by the random nature of the gamma ray emission

Answer 29

Due to random nature of gamma radiation, we cannot predict when and which gamma particles nucleus will decay and reading is inconsistent

Question 30

Why gamma radiation is dangerous

Answer 30

Highly penetrative power, easily pass thru skin cells, highly Ionising thus can cause cancer

Question 31

Explain why there is background radiation

Answer 31

Radiation can be emitted by natural sources and man-made sources