# Linear enercy transform (LET) and momentum

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• rudyb
In summary, Rudy has some understanding of LET but when he compares it with other terminologies such as momentum, then it gets little contradicting.
rudyb
I have some understanding of Liner Energy Transfrom (LET), but when I compare it with other terminologies such as momentum, then it gets little contradicting.
For example, I know that if talk about particles and ions, then a gold ion (AU) has much higher LET than an Iron particle (Fe). And therefore AU has a much shorter penetration distance than Fe. AU gives up much of its energy in a short distance.
But what is confusing to me is that "AU" is a much heavier particle than "Fe", and when I think about this subject in terms of momentum, then AU has a higher momentum than Fe, since it is heavier.
So, in terms of classical physics and the subject of momentum, then AU, should travel a higher distance than Fe, since it has higher momentum!
Can someone please explain the flaw in my explanation, and what is it that I am not understanding correctly?

Thanks,
--Rudy

Hello Rudy,

rudyb said:
I have some understanding of Liner Energy Transfrom (LET)
Can't find Liner Energy Transfrom and can't find Linear enercy Transform either. Can find linear energy transfer and it has to do with "energy loss of the charged particle due to electronic collisions".
So it applies to Au ions and to Fe ions (not to Fe particles -- !?).
If you have a reference we might be able to comment. There are more factors than just the atomic mass that influence interaction cross section.

berkeman
Hi,
I am sorry, I had misspelled it. Yes, I was talking about Linear Energy Transfer. Please look at the table below that I have included.

I am not sure but I believe that "Range" is inversely proportional to the square of atomic number (Z^2), correct me if I am wrong please.
I understand that an ion with higher LET will have a shorter range, since it will give off more energy per unit of length. But the part that doesn't make much intuitive sense is when I start thinking about this in terms of classical physics 101 (e.g. momentum).
If we have two objects at the same speed, but one weighs more, then it will travel a longer distance. And this make intuitive sense, because it is heavier.
Now, isn't it that Au ion is heavier than Fe ion, so shouldn't we expect for Au to travel a longer distance?! Because to me Au has higher momentum.
This is the part that I get confused. I would appreciate if I get a clarification to my confusion.

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What about the second column in your (?) table ? Is that MeV/nucleon ? Or per neutron ? Is there a reference for the table ?

I really think my question is very standard question. The specific really doesn't matter...
Look at this for example:

You can find a similar plot in almost any reference material. So, the general rule is that the heavier the ion, then its LET is higher...
So, I don't understand why this is the case?! Can you explain please?
Because as I said, to my understanding, I think that the heavier the ion is then the more momentum it has, therefore I am expecting for it to travel farther, before coming to stop, which would imply that it should have a lower LET...!
But I know this is not the case, and in reality it is the opposite... The heavier the ION, then higher the LET... Why?

## 1. What is Linear Energy Transfer (LET)?

Linear Energy Transfer (LET) is a measure of the amount of energy deposited per unit length of a particle's track as it travels through a medium. It is often used in radiation physics to describe the ability of different types of radiation to transfer energy to the materials they pass through.

## 2. How is LET related to momentum?

LET and momentum are related because both are measures of a particle's ability to transfer energy to a material. The higher the momentum of a particle, the larger its LET will be as it travels through a medium. This is because particles with higher momentum have more energy and are more likely to interact with the atoms in the material, causing them to deposit more energy per unit length.

## 3. Why is LET important in radiation therapy?

In radiation therapy, the goal is to deliver a precise dose of radiation to a specific area in the body in order to kill cancer cells. LET plays a crucial role in this process, as it determines the amount of energy deposited in the tissues and the effectiveness of the radiation treatment. High LET radiation is more effective in killing cancer cells, while low LET radiation is more likely to cause damage to healthy tissues.

## 4. How does LET affect the biological effects of radiation?

LET is an important factor in determining the biological effects of radiation exposure. High LET radiation, such as alpha particles, have a higher probability of causing damage to biological tissues compared to low LET radiation, such as x-rays. This is because high LET radiation deposits more energy in a smaller area, resulting in more concentrated damage to cells.

## 5. What are the different types of LET and how do they differ?

There are two main types of LET: high LET and low LET. High LET radiation, such as alpha particles and heavy ions, have a larger mass and charge and therefore deposit more energy per unit length compared to low LET radiation, such as x-rays and gamma rays. This difference in energy deposition leads to different biological effects and is an important factor to consider in radiation protection and therapy.

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