Radioactivity

Question 1

What are the different kinds of radioactive emission?

Answer 1

Alpha → emission of an He nucleus ie. 2 protons and 2 neutrons

Beta → high-speed beta particles (electrons or positrons) emitted from the nucleus

β^- emission: neutron → proton + electron

β⁺ emission: proton → neutron + positron

EM radiation occurs with alpha and beta decay:

Gamma rays → emitted following nuclear transitions

X-rays → emitted when orbital electrons change orbit

Question 2

What is the rate of radioactive decay related to, and what order process is it?

Answer 2

The rate of radioactive decay is related to the number of radioactive atoms present.

Radioactive decay is a first order process.

Question 3

How do you convert from Bq into Ci?

Answer 3

1 Bq = 1 dps

1 Ci = 2.22 x 10¹² dpm

1 Ci = 3.7 x 10¹⁰ dps

1 Ci = 37 G Bq (37 x 10⁹ Bq)

Question 4

What is specific radioactivity?

Answer 4

It relates the quantity of radioactive substance to the total amount of substance present.

It is measured in dpm mol ^-1 , μCi mol^-1 etc

Only the amount of cold (non-radioactive) substance is included when calculating specific radioactivity, because there is usually a very small amount of hot substance.

Question 5

How does a Geiger-Müller tube (Geiger counter) work?

Answer 5

A gas in held in a chamber between a pair of electrodes, across which a voltage is applied. When radiation causes the ionisation of the gas, this generates an electric current.

Question 6

What is a quenching agent and how can its effects be compensated?

Answer 6

A scintillator is a material the is luminescent when struck by ionising radiation. The light intensity can then be measured to determine the quantity of radioactive material present.

If a quenching agent is also present in the sample, it will decrease the intensity of fluorescence. Heat is given off instead of light, which is much harder to detect.

Quenching means the recorded count rate, cpm, is often much lower than the true disintegration rate, dpm. The scintillation counter has to be calibrated to determine the efficiency of counting an isotope in a given sample.

cpm x 100 = dpm x % efficiency of counting

Question 7

What safety precautions must be taken when working with radioisotopes?

Answer 7

Distance yourself appropriately from sources of radiation

Use appropriate shielding

Contain radioactive materials in a defined working area

Wear appropriate protective clothing

Monitor your work area and your person during the experiment and when finished

Minimise accumulation of radioactive waste