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1. http://www.ocr.org.uk/Images/171728-...-materials.pdf
For question 23a of the paper above why taking the background radiation into account improve the quality of results. Won't it affect every result by the same amount Mark scheme at bottom of document
2. (Original post by runny4)
http://www.ocr.org.uk/Images/171728-...-materials.pdf
For question 23a of the paper above why taking the background radiation into account improve the quality of results. Won't it affect every result by the same amount Mark scheme at bottom of document
well the obvious method would be to increase the thickness of a lead barrier between source and detector until the count rate due to the photons from the source was halfed... but how are you going to know when that happens unless you subtract the background rate?

e.g.
count rate with no barrier 25
count rate with a 6mm barrier 15

has that barrier halved the count rate due to the source?
if we don't know the background rate we can't tell.

if the background rate is 5 the answer is yes
(15-5)/(25-5) = 1/2

if the background rate is 10 the answer is no
(15-10)/(25-10) = 1/3

if you don't know the background rate, you can't tell what the ratio is.
3. (Original post by Joinedup)
well the obvious method would be to increase the thickness of a lead barrier between source and detector until the count rate due to the photons from the source was halfed... but how are you going to know when that happens unless you subtract the background rate?

e.g.
count rate with no barrier 25
count rate with a 6mm barrier 15

has that barrier halved the count rate due to the source?
if we don't know the background rate we can't tell.

if the background rate is 5 the answer is yes
(15-5)/(25-5) = 1/2

if the background rate is 10 the answer is no
(15-10)/(25-10) = 1/3

if you don't know the background rate, you can't tell what the ratio is.
but won't the background count rate be the same everywhere so every result will be affected by the same amount
4. (Original post by runny4)
but won't the background count rate be the same everywhere so every result will be affected by the same amount
Yes, but it's not a linear relationship you're looking for between the photon energy and half-thickness, so although each result will be translated by the same amount, that changes the relationship. You get different answers depending on whether you include it or not, so it is necessary for accurate results.
5. (Original post by runny4)
but won't the background count rate be the same everywhere so every result will be affected by the same amount
Did you even read the reply? It is entirely dependant on the size of the effect of background radiation.
6. (Original post by runny4)
but won't the background count rate be the same everywhere so every result will be affected by the same amount
Afaict this question requires you to work out a ratio between the count rate with one thickness of lead shielding and another shielding thickness of lead shielding. one of those thicknesses might be zero.

ratio means doing a division and adding the same number to both sides of the division sign changes the answer.

e.g. 3÷5 ≠ (3+1)÷(5+1)

so not subtracting the background rate will affect the accuracy of the answer
7. (Original post by Joinedup)
Afaict this question requires you to work out a ratio between the count rate with one thickness of lead shielding and another shielding thickness of lead shielding. one of those thicknesses might be zero.

ratio means doing a division and adding the same number to both sides of the division sign changes the answer.

e.g. 3÷5 ≠ (3+1)÷(5+1)

so not subtracting the background rate will affect the accuracy of the answer
ok thank you

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