17. Ion-Nuclear Interactions I - Scattering and Stopping Power Derivation, Ion Range
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- čas přidán 19. 09. 2019
- MIT 22.01 Introduction to Nuclear Engineering and Ionizing Radiation, Fall 2016
Instructor: Michael Short
View the complete course: ocw.mit.edu/22-01F16
CZcams Playlist: • MIT 22.01 Introduction...
After a brief review of photon interactions to prepare for the problem set, the ways in which ions (charged particles) interact with nuclei are introduced. The formula for stopping power (energy loss per distance traveled through matter) is derived from a physical model and compared with the full stopping power formula. The two are strikingly similar. Integrating the inverse of stopping power also gives the range of ions in matter - unlike photons, ions stop at nearly fixed distances in matter.
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Michael is fantastic, friendly, smart, and really passionate about teaching
Those students are a dream following the lecture so closely they can detect the slightest mistake immediately. Outstanding as always .
this goes both ways. I wish my professors cared even what my name is
@@CowboyRocksteady To be fair, the class size appears to be no more than ~15-20 undergrads. At my school, the typical undergrad lecture has ~50-90 students. But yea, you really do get the sense that he cares about the students and making them honestly engage with the content.
In other previous material they fare a little worse, but it is usually true that as any course progresses they improve. This is pretty similar to Rutherford scattering, and I guess they'll use this result there too.
Also dx/dt = v is an example of a big mistake you as a teacher wish you have never done. I could feel Short's embarrassment there - and it is a big one. It is the first thing you learn in physics, and you see he can feel the units are wrong immediately so he's confused why it isn't working. Since he decided to give classes without many notes, those mistakes happen sometimes, and when they are simple it is ok - like here. When they are complicated, tho, it might take the whole time the class takes place to figure it out, which is annoying when you are trying to understand what somebody is doing and they go back a million expressions just because they refused to take notes to teach. Of course that is a style difference, but I only recommend Short's style for people with gifted memory tbh.
I'm watching this from Morroco and it really helped me to understand todays class , thank you
This was an incredibly well done lecture. Thank you Michael and MIT!
Great lecture material. So much better than my own university. Michael Short explains the subject so well.
Absolutely well done and definitely keep it up!!! 👍👍👍👍👍👍
It was great ! Thank you so much.
The lectures are actually not too bad to follow along with, but those problem sets look brutal
Thank you!
Michael is so slick!
The ln(T)/T graph is inverted. Logarithms tend to infinity very fast close to 0 and wins from 1/x really easily.
You can see that ln(T)/T has a global maximum by taking the second derivative and seeing it is negative for the point of maximum and noticing that the "curvature" is only positive after e^(3/2).
That minus sign would fix that, so I think that is the real mistake in that graph.
Those blackboards are nice. They must keep pretty good care of them considering how much they get used. Also, nice to know I live on MIT Standard time lol
Can stopping power in any particle in any matter be negative value in nuclear physics? (referring to the minute of 30.22 for instance.) I think you said around 34.15 min that if the stopping power is negative, this means particle picks up an energy as it goes along the material. Thus, the conclusion is that it is not possible at all. Probably, this also means there is a threshold energy for every particle to start to loose energy inside any material. You later on said that the stopping power at zero T energy should be zero, and that might not be correct. For instance if you calculate proton stopping power in a silicon, below 0.075MeV, the stopping power goes to a negative value.
De Broglie Uncertainty Principle? Serious shit.
I've never seen the uncertainty principle with de Broglie being associated with it. I think he mixed up the uncertainty principle with the de Broglie's wave-particle duality but nomenclature mistakes in physics really aren't viewed as important. Physically this isn't a big deal since the idea he talks about afterwards is correct (as in, there's always some uncertainty for position and momentum).
This comment of mine is just in case you searched this in the internet and couldn't find it.
31:42 how did that dT at the end go to dx?
It should be R=int dx= int (dx/dT)dT= int (dT/dx)^{-1} dT. The integral is from T_0 to 0, where T_0 is the initial kinetic energy of the ion. In the next line we ignore ln(T) since T is much larger than ln(T).
He starts the integral in the wrong units if you do a dimensional analysis, so his initial expression for the integral is wrong. Stopping power, as he himself said, is a distance. So Alexey above gives you the right answer, which is dimensionally sound.
By the way his ln(T)/T graph is inverted too, but if he drops a minus sign he fixes it (-ln(T)/T is more like his drawn curve), so that is likely a sign mistake.
That is the problem of going to class without notes. For people with perfect memory it works. For all others it just generates compounding mistakes. Thankfully he does memorize a few other parts of the lecture, but I knew coming in we'd have those problems here. It isn't a big deal, but it is also pretty easy to avoid those mistakes, so... Just bring notes.
Thankfully his physics is better than his math.
13:15 I fail to see why the net force would be zero: if the particle is deflected, it's further away from the electron on the second half of its journey compared to the first half so the force it experiences should be smaller.
He mentions that they're assuming the e- isn't deflected, this is obviously not quite true but its a reasonable simplification.
He ignores the net x force and only considers the net y force which gives the deflection. This is a standard physics slight of hand. This is how you wind up with black holes , dark matter, and other non physical ideas in modern physics.
@@SlowRiderDucati ????
This is spectacularly wrong and demonstrates that you simply do not understand any of those concepts.
If the charge particle doesn't move (which is a big assumption that I can see working for nuclear particles, but for electrons would depend on the material), then the problem is symmetric relative to that mid point in the cylinder he drew.
That is all there is to it.
Also, you could just do the integral of the x force and see that such integral would involve the sine of theta. When integrated, this will be an odd function relative to the point in the center of his cylinder (the point that defines theta) under the static charge assumption, and the integral is 0. But this is, of course, just a mathematical way of expressing the symmetry I commented previously on.
Huh
Haha. Banana data. Amazing.