Increase voltage = increase in intensity of electrons - X-ray tube

In summary, when you increase the voltage on an x-ray tube, you increase the intensity of the emitted electrons.
  • #1
nousername
31
1
Hey,

In x-ray tubes, how come an increase in the voltage results in an increase in the intensity of the electrons. i.e. the no. of electrons emitted per unit time per unit second.

Thanks.
 
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  • #2
kirollos said:
Hey,

In x-ray tubes, how come an increase in the voltage results in an increase in the intensity of the electrons. i.e. the no. of electrons emitted per unit time per unit second.

Thanks.

There are two different answers to your question, depending on the range of voltage that is being applied.

In the beginning, as you are increasing the voltage from zero, you are starting to collect more and more of the electrons that have been emitted from the thermionic cathode. Electrons that would have gone off into a different direction and not hitting the anode would now be collected at a higher potential. At some point, you'll reach a "plateau" where all the electrons emitted would be collected.

However, this is not a flat plateau. As you continue to increase the potential even higher, you now get into a "Schottky" effect. This is where the applied potential is large enough that it is starting to lower the effective work function of the surface. The larger the applied potential, the lower the effective work function. The lower the effective work function, the more electrons emitted from the cathode.

Thus, you see a higher electron intensity at a larger voltage.

BTW, this effect is not "classical".

Zz.
 
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  • #3
Hey ZapperZ,

Thanks for your explanation mate. it was awesomne! With the second point about the increased potential difference resulting in a decrease in effective work function of the metal surface... is that because the electrons are now being attracted by the positive potential (anode) and so less energy is required to remove them?

Thanks!
 
  • #4
kirollos said:
Hey ZapperZ,

Thanks for your explanation mate. it was awesomne! With the second point about the increased potential difference resulting in a decrease in effective work function of the metal surface... is that because the electrons are now being attracted by the positive potential (anode) and so less energy is required to remove them?

Thanks!

Sort of. Remember that for metals (which is the normal material for these thermionic cathodes), the external field can only penetrate up to the skin depth. So here, the external field changes the field, and thus the surface barrier potential also changes. All of these affect the work function.

So yes, one can sort of say that the electrons are more "attracted" to the anode, but a more accurate description should be in terms of the barrier potential.

Maybe this figure might help. This is a viewgraph from one of my talks that I've used frequently. In terms of the image potential that can be used to represent the work function, one can see that the potential barrier is lowered when an external field (-eEz) is applied on the surface, thus lowering the effective work function.

schottky1.jpg


Zz.
 
  • #5


Hello,

Thank you for your question. The increase in voltage in an X-ray tube results in an increase in the intensity of electrons because of the relationship between voltage and energy. When a higher voltage is applied, the electrons in the tube are given more energy, which allows them to move with greater speed and intensity. This increased energy allows them to collide with the metal target in the X-ray tube with more force, resulting in a higher number of electrons being emitted per unit time.

Additionally, the increase in voltage also increases the electric field strength within the tube, which helps to accelerate the electrons towards the target. This results in a higher intensity of electrons being emitted from the tube.

I hope this helps to clarify the relationship between voltage and electron intensity in an X-ray tube. Let me know if you have any further questions. Thank you.
 

1. What is the relationship between voltage and intensity of electrons in an X-ray tube?

The intensity of electrons emitted from an X-ray tube is directly proportional to the voltage applied to the tube. This means that increasing the voltage will result in an increase in the number of electrons emitted, thus increasing the intensity of the X-rays produced.

2. How does increasing voltage affect the quality of X-rays produced?

Increasing the voltage in an X-ray tube also increases the energy of the electrons, which in turn affects the quality of the X-rays produced. Higher energy electrons can penetrate deeper into the body, resulting in higher quality X-ray images.

3. Is there a limit to how much voltage can be applied to an X-ray tube?

Yes, there is a limit to the amount of voltage that can be applied to an X-ray tube. This limit is determined by the tube's design and the strength of its components. Applying too much voltage can damage the tube and affect the quality of the X-rays produced.

4. What other factors besides voltage can affect the intensity of X-rays in an X-ray tube?

In addition to voltage, the current, target material, and tube current (mA) can also affect the intensity of X-rays produced in an X-ray tube. These factors work together to determine the overall power of the X-ray beam.

5. How does the increase in intensity of electrons affect the exposure time in an X-ray image?

An increase in the intensity of electrons in an X-ray tube can result in a shorter exposure time for producing an image. This is because there are more electrons available to create the X-ray beam, which can shorten the time needed to create a sufficient image.

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