Properties of radiation Flashcards Preview

Nuclear > Properties of radiation > Flashcards

Flashcards in Properties of radiation Deck (13):

How does an ionising chamber work?

Q image thumb

  • 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.


Ionisation of radiation.

  • 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.


How does a cloud chamber work?

  • 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.


Describe the alpha radiation track.

  • 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.


Describe the beta radiation track.

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


What is the count rate?

  • 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.



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

Q image thumb

  • 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.


Absorption strengths of α, Β, and γ

  • 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


How does a geiger tube work?

  • 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.


What is the dead time of the tube?

  • 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.


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

Q image thumb

  • 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.


Range of radiation

  • 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.


Energy for radiation

  • 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