# X-ray tube used for cancer therapy

Heres the problem:
An x-ray tube used for cancer therapy operates at 4.0 MV, with a beam current of 25 mA striking the metal target.Nearly all of this power is transferred to a stream of water flowing through the holes drilled in the target. What rate of flow, in Kg/sec, is needed if the temperature rise of the water is not to exceed 50 degrees celsius.

I/m thinking power = 4000 V * .025 A = 100 watts

then the specific heat of water = 4186 j but this problem doesent give an initial temperature of the water.. Am I right so far?

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Staff Emeritus
Gold Member
If it doesn't give you a starting condition, assume a worst case scenario and design from that.

Also, is it MegaVolts or milliVolts? You've got the equation set up for kiloVolts.

im confused about worst case scenario. I don't understand what to do...

Homework Helper
Gold Member

Originally posted by jpnnngtn
Heres the problem:
(snip)temperature rise of the water is not to exceed 50 degrees celsius.

(snip)this problem doesent give an initial temperature of the water..

How do you define "temperature rise?"

I guess I define tempwerature rise as a positive change in temperature. ...right?

FZ+
Precisely - it's the change in temperature, and hence energy of the water that is significant. So you don't need to know the initial temperature of the water.

Now that that is established, I am stil confused on exactly what I need to do... How to find the flow of water in Kg/sec that is..Where would I start. The electricity part is not a problem. it is just implementing it.

Sonty
50*4186*rate (kg/s)=100KW, hence
rate=0.477783