Radioactivity experiments part 2

Question 1

what does this bit of kit show

Answer 1

shows how a Geiger tube and a counter may be used to invesitgate absorbtion by different materials

each particle of radiation that enters the tube is registered by the counter as a single count

Question 2

What is the count rate? Why does it need to be measured before the experiment?

Answer 2

• the number of counts in a given time is measured and used to work out the count rate
  
  • the number of counts divided by the time taken
• the count rate must be tested beforehand due to background radioactivity
  
  • the count rate without a sournce present

Question 3

How is the experiment for absorbtion done?

Answer 3

• the count rate is then measured with the source at a fixed distance from the tube without any absorber present
  
  • the background count is subtracted from the count rate with the source present to give the corrected count rate from the source
• the count rate is then measured with the absorber in a fixed position between the source and the tube
  
  • the corrected count rates with and without the absorber present can then be compared

Question 4

What does a Geiger tube contrain?

Answer 4

• sealed metal tuube that contains argon gas at low pressure
• • the metal rod down the middle of the tube is at a positive potential
• the tube wall is connected to the negative terminal of the power supply and is earthed

Question 5

How does the Geiger tube work?

Answer 5

• the thin mica window at the end of the tube allows radiation to enter the tube
• when a particle of ionising radiation enter the tube, the particle ionised the gas atoms along its track
• the negative ions are attracted to the road and the positive ions to the wall
• ions accelerate and collide with other gas atoms, producing more ions - this effect continues
• within a very short time, many ions are created and discharged at the electrode
• a pulse of charge passes round the circuit through the resistor, causing a voltage pulse across it, which is recored as a single count by the pulse converter

Question 6

How is range in air shown?

Answer 6

• same kit as teh abosrbtion test, but without the absorbers
• the count rate is measured for different distances ebtween the source and the tube, starting with the source close to the tube
• background count rate must also be measured, so the count rate can be corrected

Question 7

Describe the range in air of an alpha particle?

Answer 7

• short range - few cms
• count rate decreases sharply once the tube is beyond the range of the alpha particles
• the range differs from one source to another
  
  • indicating that the initial KE differs from one source to another
• KE is the same of particles from the same source

Question 8

Describe the range of beta radiation

Answer 8

• range upto 1m
• count rate gradually decreases with increasing distance until it is the same as the background count rate at a distance of about 1m
• the reason for the gradual decrease of count rate as distance increases is that the beta particles fom any given source have a range of initial kinetic energies up to a maximum
  
  • faster beta particles travel further in air than the slower beta particles as they have greater intial kinetic energy

Question 9

Describe the range of gamma radiation?

Answer 9

• has unlimited range in air
• count rate gradually decreases with increasing distance because the radiation spread out in all directions
• the propottion of the gamma photons from the source entering the tube decreases according the invese square law