Undergrad Why is this unit used in graphs; keV/amu ?

[cmb]

I see various graphs in papers about atomic collisions in units showing "keV/amu".

Why is this unit used, why not just "keV" when, presumably, it is the overall keV of an atomic collision that is important?

 

[vanhees71]

I can't say this without knowing the context. I'm a nuclear physicists and in part of our community they use the unit MeV/u, which makes no sense. Looking at the context you realize however soon that what they in reality mean is collision energy per nucleon when colliding two nuclei. The correct writing (used in the major part of the community): "The collision energy is $2.76 \; A \text{TeV}$ or $\sqrt{s_{NN}}=2.67 \; \text{TeV}$".

 

[Vanadium 50]

Let me get this straight. You aren't going to tell us where it is used (beyond "various graphs") but expect us to tell you why it used there? How do you expect us to do that?

 

[cmb]

Let me get this straight. You aren't going to tell us where it is used (beyond "various graphs") but expect us to tell you why it used there? How do you expect us to do that?

Well, TBH, I see it all the time in topics related to atomic collisions, so excuse me for presuming that anyone who knows the answer would already know this is a norm in published papers on the subject.

Seeing as you've asked so nicely, look at the cross section plots for ion collisions here;
[Moderator's note: Link removed due to unclear copyright situation. The fact that the source lists a fee in an old currency does not mean it is free now.]

 

[Vanadium 50]

keV/amu is energy per mass. That's not impossible to be a relevant quantity. And I am sure somewhere in this 195 page document there is the plot you are thinking about, I don't think it is my responsibility to find it.

 

[cmb]

You've missed the question.

I asked why keV/amu is a significant unit when I thought that the particle collision energy as a whole, the gross energy, was the significant factor.

I was hoping someone who actually knows the answer, or who generates plots like this, might have said something like "typically, events are proportional to specific energy rather than absolute energy" or maybe "yes, you're right, really, it's just the way people do it".

PS Every one of the few dozen ion collision plots in that attachment uses this unit.

 

[vanhees71]

And it's too much for you to tell us the page number? Well, you seem not to be too interested in an answer...

 

[Vanadium 50]

PS Every one of the few dozen ion collision plots in that attachment uses this unit.

The figure on Page 3 is cm^2 vs. eV.
The figure on Page 5 is cm^2 vs. eV.
The figure on Page 7 is cm^2 vs. eV.
The figure on Page 9 is cm^2 vs. eV.
The figure on Page 11is cm^2 vs. eV.
The figure on Page 13 is cm^2 vs. eV.
The figure on Page 15 is cm^2 vs. eV.
The figure on Page 17 is cm^2 vs. eV.
The figure on Page 19 is cm^2 vs. eV.

Every one? That's not what I see. Vanhees is right - if it's too much trouble for you to even give us a page number, how can we possibly answer you?

I was hoping someone ... who generates plots like this

Plots like what? You still haven't provided an example.

 

[fresh_42]

I was hoping someone who actually knows the answer, or who generates plots like this, might have said something like "typically, events are proportional to specific energy rather than absolute energy" or maybe "yes, you're right, really, it's just the way people do it".

This thread is running in circles. I do not understand what it makes so difficult to talk about a specific graphic rather than hoping someone will provide the answer you apparently are waiting for. Furthermore, some condescending attitudes towards people who do actually know is hardly the tune we want to perform scientific debates.

As I had to remove the link due to an undecidable copyright issue, this thread is closed.