You are Here: Home

# Working out intensities of X-rays using I=Ioe^(-ux)

Announcements Posted on
Talking about ISA/EMPA specifics is against our guidelines - read more here 28-04-2016
1. An X-ray beam of initial intensity 50 Wm−2 is incident on soft tissue of attenuation coefficient 1.2 cm−1. Calculate its intensity after it has passed through a 5.0cm thickness of tissue.

A hint for this question is:
The attenuation coefficient is given in cm-1, hence you can leave the thickness in cm.

So according the hint I get the answer:
I=Ioe^(-ux)
50 x e^(-1.2x5)= 0.12Wm-2, which is the correct answer according to the textbook.

But why should I leave the attenuation coefficient and the thickness in cm, when the intensity is measured in Wm-2? I thought that the calculation should have been:
50 x e^(-0.0125x0.05)= 49.97Wm-2

2. (Original post by whotosee)
An X-ray beam of initial intensity 50 Wm−2 is incident on soft tissue of attenuation coefficient 1.2 cm−1. Calculate its intensity after it has passed through a 5.0cm thickness of tissue.

A hint for this question is:
The attenuation coefficient is given in cm-1, hence you can leave the thickness in cm.

So according the hint I get the answer:
I=Ioe^(-ux)
50 x e^(-1.2x5)= 0.12Wm-2, which is the correct answer according to the textbook.

But why should I leave the attenuation coefficient and the thickness in cm, when the intensity is measured in Wm-2? I thought that the calculation should have been:
50 x e^(-0.0125x0.05)= 49.97Wm-2

It seems odd but it's ok.
The units on the LHS are Wm-2 and the units on the RHS are Wm-2 x e(number)
e(number) has no units (it's a pure number) so it doesn't affect the units of that side of the equation. The number in the bracket is also a pure number because its units cancel out.
cm.cm-1 gives no unit.
So as long as the coefficient and the length are in the same length unit it works!
3. (Original post by Stonebridge)
It seems odd but it's ok.
The units on the LHS are Wm-2 and the units on the RHS are Wm-2 x e(number)
e(number) has no units (it's a pure number) so it doesn't affect the units of that side of the equation. The number in the bracket is also a pure number because its units cancel out.
cm.cm-1 gives no unit.
So as long as the coefficient and the length are in the same length unit it works!
Thank you, the units cancelling out makes sense, but it means that I'll get a different answer every time I use a different unit?
(Like when I got 49.97Wm-2 when I converted everything to metres instead of 0.12Wm-2 when using cms)
In that case what units am I supposed to be using? Thanks
4. (Original post by whotosee)
Thank you, the units cancelling out makes sense, but it means that I'll get a different answer every time I use a different unit?
(Like when I got 49.97Wm-2 when I converted everything to metres instead of 0.12Wm-2 when using cms)
In that case what units am I supposed to be using? Thanks
You correctly converted cm to m
5cm = 0.05m
You divide by 100
But you don't do this when you convert cm-1 to m-1
Then you multiply by 100

For example, if you have 5 items in a line per cm, then you would have 500 in a line 1m long. That is 500 per m
5. (Original post by Stonebridge)
You correctly converted cm to m
5cm = 0.05m
You divide by 100
But you don't do this when you convert cm-1 to m-1
Then you multiply by 100

For example, if you have 5 items in a line per cm, then you would have 500 in a line 1m long. That is 500 per m
Aah, I see!! Thank you very much

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
your full birthday is required
1. Oops, you need to agree to our Ts&Cs to register

Updated: April 5, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How to predict exam questions

No crystal ball required

### Stop saying "uni"

Poll
Study resources
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.