# I Proton through solid object

1. Mar 28, 2017

### pranj5

When protons or nuclei of light elements like Helium passes through gas, it ionises the molecules on it path. But that's not the case when proton or nuclei of light elements pass through solid objects like metal. What's the reason behind that?

2. Mar 28, 2017

### Staff: Mentor

Why do you think it wouldn't happen in solid objects?

3. Mar 28, 2017

### Khashishi

A piece of metal is essentially one big molecule. So, if a piece of metal loses or gains some charge, it's essentially an ion.

4. Mar 28, 2017

### pranj5

Can you give any example that it happens in solid objects too?

5. Mar 29, 2017

### Staff: Mentor

All semiconductor detectors use the energy transfer to electrons, for example.

6. Mar 29, 2017

### ZapperZ

Staff Emeritus
You are approaching this the wrong way. You made a statement that really isn't obvious (i.e. protron and ions going through solids without causing such ionization) without citing any valid evidence for it, and then asking someone else to show why you're wrong. This is not how you make an argument on this forum, or in a typical scientific discussion. It is you who is responsible for backing up your claim.

Please note that it is well-known that proton beams can cause significant changes to solids. It is why we have proton therapy. Proton accelerators have significant residual radiation because the protons can activate the beam pipe walls or beam dump when they hit them. In fact, if you do a search of the list of ionizing radiation, proton beams are certainly a very effective source! And this is true for solids.

So there is no "reason" for it as asked in your first post that I quoted, because the premise that you presented isn't true.

Zz.

7. Mar 29, 2017

### pranj5

Semicondustors don't use protons and/or nuclei of light materials.
I don't know much examples of protons or light nuclei passing through soild. What I know is the famous Geiger-Marsden experiment, in which a thin foil of gold is bombarded with α particles. There is no mention that the gold foil has been ionised by the bombardment. Do you know any experiment where protons and/or light nuclei is used to bombard a solid object?

8. Mar 29, 2017

### ZapperZ

Staff Emeritus
http://iopscience.iop.org/article/10.1088/0953-8984/13/40/315

You need to remember that in many cases, these solids are often grounded, so any net electrical charge has been neutralized.

Again, read what I had written above. Considering the proton therapy is a known medical procedure, you could have easily looked that up. If you think a beam of protons will just "pass through" your skin without inducing anything, then I have a particle accelerator for you to meet.

Zz.

9. Mar 29, 2017

### pranj5

Semiconductors have some unique electrical properties and it can't be said that when they are bombarded by protons, the same result would be obtained in case of a solid metal. And for proton therapy, we should keep in mind that our body (or any kind of living tissue) is much more complex than a simple metal block/plate. Our body contains liquids with dissolved gases in them.
In the description of the experiment, it isn't mentioned anywhere that the gold foil is grounded.

10. Mar 29, 2017

### ZapperZ

Staff Emeritus
Remember that in your ORIGINAL POST, you said ".... pass through solid objects like metal..." I assume that you care more about the object being a "solid" rather than being specific to a metal.

There is nothing here to prevent ionization of metals. The difference being that a metal (i) can easily be grounded and (ii) have huge amount of free electrons that can quickly neutralize the ionized atoms. Still there's nothing here that says that positive ions or protons can't cause such ionization. In fact, it is well-known that proton or positive ion bombardments can cause secondary electron emission in solids, INCLUDING metals.

http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1103&context=ameslab_iscreports

So where do you think these electrons came from?

Zz.

11. Mar 29, 2017

### Staff: Mentor

You asked about "solid objects like metals", not about solid objects made out of hydrogen or other light elements. Semiconductors are not metals, but that difference doesn't matter here (and they are certainly solid): They have electrons that can gain energy if ionizing radiation passes through the object.

12. Mar 31, 2017

### pranj5

A very good source and thanks for the information. As per the paper, one proton of 23 kilovolts of energy can knock off 3.59 electrons maximum and the energy of the electrons knocked can be just 30 volts maximum. In the paper, 23 kilovolt is the lowest energy level for protons and that means the lowest energy proton with the maximum number of stray electron generation can loose only 107.7 volt out of the 23 kilovolt.
In contrast, when a proton passes through a gas, it ionises all the molecules it can until it's drained off with its energy. In short, even for the sake of logic, we can consider that protons can knock of electrons from metals but the amount is so little in comparison to gas, it's simply negligible.

13. Mar 31, 2017

### pranj5

Can you give any example where semicoductors were bombarded by protons?

14. Mar 31, 2017

### Vagn

il has been ionised by the bombardment. Do you know any experiment where protons and/or light nuclei is used to bombard a solid object?[/QUOTE]
Proton detectors certainly are bombarded by protons.

For instance:
http://www.sciencedirect.com/science/article/pii/S1567173905001951
http://iopscience.iop.org/article/10.1088/1748-0221/11/09/P09005/meta

A proton passing through a solid will excite a huge number of electrons, but the energy distribution of the electrons is given by
$$\frac{dn}{dE}\propto \frac{1}{E^2}$$
So most of the electrons excited in a solid will only gain a small amount of energy, and won't be enough to cause ionisation, see p26 in the pdf here. Such small energy processes aren't possible in gases as gases don't have a continuous density of states like solids do, as such the only processes that are allowed are those that result in transitions between the discrete energy levels and those that cause ionisation.

Note also that the mean free path of a free electron in a solid is also extremely short, a few nanometres usually, so any ionised electrons would likely not make it out of the solid to be detected.

Last edited: Mar 31, 2017
15. Mar 31, 2017

### Staff: Mentor

That is the energy of the few electrons that completely leave the material. There are also electrons in the material gaining energy, these are not counted in that study.
Every detector looking for protons. Detectors for particle physics get hit by protons frequently, for example.

16. Mar 31, 2017

### pranj5

It's so far the most valuable input here. Whatsoever, kindly tell whether that also means that proton also losses less energy while passing through solids in comparison to passing through gases?

17. Mar 31, 2017

### Staff: Mentor

To a good approximation, the energy loss depends only on the electron density, which is roughly proportional to the density of the material (apart from hydrogen (as target material!)).
Bethe formula.

18. Jun 25, 2017

### pranj5

As per the formula above; higher the energy of the proton, lower is the amount of energy transferred to the electrons. And from your post, it seems that energy transfer from a high velocity Proton is small in case of solids, especially metals in comparison to gases. Am I right?

19. Jun 25, 2017

### Staff: Mentor

At very high energies that trend reverses, but it is true over a large energy range.
I don't see how you got that impression.
Per distance the energy loss is much larger due to the higher density, per amount of material crossed the energy loss is similar.

20. Jun 25, 2017

### pranj5

You haven't mentioned, at what range the trend will reverse. Whatsoever, thanks for the information.
What you have said can be easily understood as solids have much higher density in comparison to gas. What I want to mean is that the energy loss of the proton for a specific number of atoms/molecules is lower in solid/metal in comparison to gases. In the solid/metal, most of the electrons will gain a small amount of energy while in case of gases the energy gain will be so high that the atom/molecule will be ionised. Am I right?