Question 22M.2.SL.TZ2.5
| Date | May 2022 | Marks available | [Maximum mark: 9] | Reference code | 22M.2.SL.TZ2.5 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Comment, Estimate, Outline, Write down | Question number | 5 | Adapted from | N/A |
An experiment is carried out to determine the count rate, corrected for background radiation, when different thicknesses of copper are placed between a radioactive source and a detector. The graph shows the variation of corrected count rate with copper thickness.
Outline how the count rate was corrected for background radiation.
[1]
background count rate is subtracted «from each reading» ✓
OWTTE
a) A majority of candidates were able to say that background radiation count was subtracted from all readings.
b) A fairly easy question with most candidates being able to take readings from the graph to get a final count rate of approximately 380 counts per second. Many did not seem to have used a ruler to help their reading.
c) This was a bit chaotic with candidates showing all sorts of misconceptions. The first marking point was the one most commonly awarded. The 2 big misconceptions were that the copper and lead were radioactive themselves and produced the radiation, or that the higher the figures the better absorbers they were. Far too many candidates thought that the question was only about the radiation passing through the 3.5 mm of lead and copper. Most of these candidates realised that there must be some gamma radiation in the radiation detected. Far fewer stated that there could not be any alpha. Opinions varied as to whether there was beta, but any sensible answers were given credit.
d) This question was generally well answered, with most candidates getting
When a single piece of thin copper foil is placed between the source and detector, the count rate is 810 count minute−1. The foil is replaced with one that has three times the thickness. Estimate the new count rate.
[2]
thickness is 0.25 «mm» ✓
380 «count min−1» ✓
MP1 and MP2 can be shown on the graph
Allow a range of 0.23 to 0.27 mm for MP1
Allow ECF from MP1.
Accept a final answer in the range 350 – 420
Further results were obtained in this experiment with copper and lead absorbers.
Comment on the radiation detected from this radioactive source.
[4]
lead better absorber than copper ✓
not alpha ✓
as it does not go through the foil / it is easily stopped / it is stopped by paper ✓
there is gamma ✓
as it goes through lead ✓
ALTERNATIVE 1
can be beta ✓
as it is attenuated by «thin» metal / can go through «thin» metal ✓
ALTERNATIVE 2
not beta ✓
it is stopped by «thin» metal ✓
Another radioactive source consists of a nuclide of caesium that decays to barium .
Write down the reaction for this decay.
[2]
✓
✓
Accept β or e in MP1.
Do not penalize if proton / nucleon numbers or electron subscript in antineutrino are missing.