Chapter 25: Radioactivity

Question 1

What are the defining features of radioactive decay?

Answer 1

Spontaneous and random - cannot predict when an individual nucleus will decay

Question 2

What features of a nucleus may cause radioactive decay?

Answer 2

• Too many or too few neutrons
• Too heavy overall
• Too much energy

Question 3

Name 4 types of radiation

Answer 3

• Alpha
• Beta (plus and minus)
• Gamma

Question 4

What is the ionising order from most to least

Answer 4

Alpha
Beta
Gamma

Question 5

What is an alpha particle

Answer 5

A particle which contains two protons and two neutrons, the same as a helium nucleus

Question 6

What type of radiation can be stopped by lead or concrete

Answer 6

Gamma

Question 7

How far does a beta particle typically penetrate the air?

Answer 7

50cm to 1m

Question 8

What is the activity of a source?

Answer 8

The number of radioactive decays per second (Becquerels)

Question 9

What is the half-life of an isotope?

Answer 9

The average time taken for the activity of the sample to half

Question 10

What occurs when a particle and antiparticle meet?

Answer 10

• Annihilation
  Release two gamma rays, in order to conserve momentum
  Mass will transform into the energy equivalent

Question 11

What is pair production

Answer 11

When a gamma ray has enough energy to produce a particle and its antiparticle

Question 12

Why does beta plus decay have a very low penetration

Answer 12

Because it will annihilate with matter almost immediately

Question 13

What isotope is commonly used to find the age of artifacts

Answer 13

Carbon-14 (radiocarbon dating)
Half life of around 5700 years

Question 14

Techniques and Procedures used to investigate the absorption of alpha, beta and gamma

Answer 14

• Use GM tube to record background count rate
• Place radioactive source into room and measure count rate after 5 mins
• Place few sheets of paper in front of the source and repeat, if count rate drops to 0, emit alpha only
• Repeat with aluminium foil and cm of lead to detect beta

Question 15

How do you find the activity of a source?

Answer 15

activity = decay constant x number of particles

Question 16

What equation do you use for graphical methods and spreadsheet modelling of radioactive decay?

Answer 16

ΔN = (λΔt) x N

Question 17

Worked Example: Power from a radioactive source
The activity of an alpha-emitting source is 5 x 10^12 Bq. The kinetic energy of each alpha particle is 4MeV. Calculate the power emitted by this source

Answer 17

Step 1: Energy of alpha particle
= 6.4 x 10^-13J
Step 2: Activity means that there are 5 x 10^12 alpha particles emitted per second
5 x 10^12 x 6.4 x 10^-13 = 3.2 W