dagg3r

A Radioactive source produces an intensity of radiation of 6000 Bq/M^2 at a distance of 5 meters from a source. Determine the intensity of the radiation at a distance of

a)10 metres

* Well i think it will be 3000 Bq/m^2 since it is further distance and is halved, can someone verify?

b) 2.5 meters

* I reckon it is 12000 Bq/M^2 since it is a closer distance from the source. someone verify?

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Tom Mattson

Staff Emeritus
Gold Member
Sorry, you got them both wrong. As can be determined from the units, intensity is measured in bq/m2. The denominator should tell you that the intensity is an activity (in bq) per unit area (in m2).

Intensity thus varies inversely with the square of the distance from the source.

dagg3r

QUOTE:
Sorry, you got them both wrong. As can be determined from the units, intensity is measured in bq/m2. The denominator should tell you that the intensity is an activity (in bq) per unit area (in m2).

Intensity thus varies inversely with the square of the distance from the source.
-------------------------------------------------------------------

ok i'm not sure what you mean, but if it is the square of the distance from the source, so if the intensity is 6000bq/(m^2) and the distance is 10 metres, so does that mean the distance is 100 metres? can you please answer the first question, and i will try to answer the next or guide me through what to do because i am confused with

"Intensity thus varies inversely with the square of the distance from the source"

Tom Mattson

Staff Emeritus
Gold Member
Originally posted by dagg3r
ok i'm not sure what you mean, but if it is the square of the distance from the source, so if the intensity is 6000bq/(m^2) and the distance is 10 metres, so does that mean the distance is 100 metres?
No, the distance is still 10m. What I am saying is that the intensity varies as:

I=k/r2

where k is some constant (not important for this problem). Now if you, say, double the distance, you actually decrease the intensity by a factor of 22=4.

See it now?

dagg3r

Oh I get it, ok take a look at this

so if it was 6000 bq/M^2, and the distance was 10 m,

it will be 6000/(10^2) = 6000/ 100 = 60bq/m^2
is that right?

Tom Mattson

Staff Emeritus
Gold Member
Originally posted by dagg3r
so if it was 6000 bq/M^2, and the distance was 10 m,

it will be 6000/(10^2) = 6000/ 100 = 60bq/m^2
is that right?
No, because the 6000 bq/m2 is the intensity at r=5m. In your analysis, you set k=6000 bq/m2, which is incorrect.

You can do it with a ratio, if you want.

6000=k/52
x=k/102

then divide and solve. Alternatively, you could simply note that the distance doubles and do what I did in my last post.

dagg3r

ok this is what i have understood, (Your prob think i'm an idiot) well sorry

1. if i increase the distance, i decrease the intensity
2. if i decrease the distance, i increase the intensity.

ok, the first thing i learnt was
6000=k/(5^2)

ok the problem i am having is you said
x=k/(10^2) ok i know that 10^2 comes from the distance squared, what i do not understand is how to obtain the intensity with I=k/r^2

right now i cant solve for x because i do not know what k is, and does 6000=k/(5^2) have anything to do with it? i know that the answer should decrease the intensity, so it should be lower than 6000 bq/m^2 so how do i do it? try to answer it but dont do the homework for me, i want to try and solve it, but i dont understand how to get I

dagg3r

wait i think i know how. check htis answer

if it is 10 metres.

i did 150000/100 = 1500 bq/m^2

if it is 2.5 metres

i did 150000/5 = 30000 bq/M^2 is that right?

Tom Mattson

Staff Emeritus
Gold Member
Originally posted by dagg3r
if it is 10 metres.

i did 150000/100 = 1500 bq/m^2
Yes; when the distance doubles, the intensity decreases by a factor of 4.

if it is 2.5 metres

i did 150000/5 = 30000 bq/M^2 is that right?
No; 2.52 does not equal 5.

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