Properties of radiation

Question 1

How does an ionising chamber work?

Answer 1

• The chamber contains air at atmospheric pressure.
• Ions created in the chamber are attracted to the oppositely charged electrode where they are discharged.
• Electrons pass through the picoammeter as a result of the ionisation chamber.
• The current is proportional to the number of ions per second in the chamber.

Question 2

Ionisation of radiation.

Answer 2

• α radiation causes strong ionisation.
  
  • 10⁴ ion pairs per mm in air at standard pressure.
• β radiation has a much weaker ionising effect than α radiation.
  
  • 100 ion pairs per mm in air at standard pressure.
• γ radiation has a much weaker ionising effect than α or β. This is because photons carry no charge so they have less effect.
  
  • 1 ion pair per mm in air at standard pressure.

Question 3

How does a cloud chamber work?

Answer 3

• A cloud chamber contains air saturated with a very low temeperature vapour.
• Ionisation through the chamber causes a track of condensed vapour droplets.
• This is because the supersaturated vapour in the chamber forms droplets when ionising particles pass through.

Question 4

Describe the alpha radiation track.

Answer 4

• Produce visible straight tracks that radiate from the source.
• The tracks from a given isotope have the same length, the alpha particles have the same range.

Question 5

Describe the beta radiation track.

Answer 5

• Produce wispy tracks that are easily deflected by air molecules.
• Tracks are less visible than alpha particles as they are less ionising.

Question 6

What is the count rate?

Answer 6

• The number of counts per unit time detected by a Geiger Müller Tube.
• The background count rate should be measured.
• IE the count rate without the source present.

Question 7

Describe a method to determine absorpion strength of different types of radiation.

Answer 7

• Measure background count (count for 5 minutes and calculate per minute) at a fixed distance.
• Count for at least 60 seconds, possibly 100 seconds for each thickness.
• Calculate the count rate (number of counts / time)
• Measure count rate with no absorber.
• Measure sheet thickness using a micrometer screw gauge.
• Measure count rate with different thicknesses of absorber.
• Subtract the background count rate from all readings.
• Plot a graph of count rate / absorber thickness.

Question 8

Absorption strengths of α, Β, and γ

Answer 8

• α: stopped by sheet of paper or dead skin cells.
• Β: stopped by few (approx 5) mm of aluminium
• γ: stopped or reduced in intensity by several cm lead, couple of m of concrete

Question 9

How does a geiger tube work?

Answer 9

• The thin mica windows at the end of the tubes allows α or β particles enter the tube.
• γ can enter the tube through the walls as well.
• A metal rod down in the middle of the tube is at a positive terminal.
• The tube wall is connected to the negative terminal of the power supply and is earthed.

Question 10

What is the dead time of the tube?

Answer 10

• The time taken to regain its non-conducting state after an ionising particle enters. usually .2ms.
• Another particle that enters the tube in this time will not cause a voltage pulse.

Question 11

When a particle of ionisation radiation enters the tube the particle enters the argon atoms along the track:

Answer 11

• The negative ions are attracted to the rod and the positive ones to the walls walls.
• The ions accelerate and collide with other gas atoms, producing more ions.
• These ions create more ions in the same way so within a short time, many ions are created and discharged in at the electrodes.
• A pulse of charge passes round the circuit through resistor R, causing a voltage pulse across R which is recorded as a single count by the pulse counter.

Question 12

Range of radiation

Answer 12

• α: fixed range depends on energy and source, up to 100mm
• Β: up to 1 m
• γ: has unliminted range, count decreases as radiation travels in all directions, the proportion of radiation from the source decreases to the inverse square law.

Question 13

Energy for radiation

Answer 13

• α: Alpha particles from a given isotope are always emitted with same KE.
• Β: Beta particles emitted with a range of energies as an antineutrino is also emitted.
• γ: Energy depends on frequency (E = hf) but is constant for a given source