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  1. L

    Direction of Photo Electron Emission

    Well, that "any direction" answers seems to conflict with experiments from 1920-30's, I wonder why did they find a preferential angle...
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    Relative biological effectiveness (RBE) of photon and electron

    Thanks for the reply. I slightly forgot, what we were tought again and again, that photon interaction with matter can only occur via 3 processes (in which electrons occur anyway...). So in some sense, it's not the photon, it's the electron that's the culprit (alhthough it is the photon who...
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    Direction of Photo Electron Emission

    I was looking for information on how the photo electrons are emitted when under X-ray radiation. In this ancient review paper here http://journals.aps.org/pr/pdf/10.1103/PhysRev.30.488 [Broken] they state that the most common angles for non polarized X-ray beams (of various energies) range...
  4. L

    Relative biological effectiveness (RBE) of photon and electron

    mathman, could you give examples where a same energy photon and electron would give different results in terms of biological damage ?
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    Relative biological effectiveness (RBE) of photon and electron

    The title was too long, original question was: Why is the relative biological effectiveness (RBE) of photon and electron the same ? I was suspecting the charge of electron to possibly have some effect... Also, in biological matter, would there be a difference between photon damage and...
  6. L

    Why only delayed neutrons matter ?

    Ah, it's more clear now ! I don't really like the oversimplifications made. It makes sense why one has reflectors then. So basically, it is crucial how you make the pellets, how many of them are in a rod and how many rods per assembly. I think when I get too looking properly at cross sections...
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    Why only delayed neutrons matter ?

    Thanks for the analogy, I think I get it why the delayed ones do the work (because every text booork website tells you the same, but I am yet to find an answer about what happens to prompt neutrons). So IGNORE delayed neutrons (I think the question title is a bit misleading now...) My...
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    Why only delayed neutrons matter ?

    Thanks for the answer, but it is still not clear to me, how these prompt neutrons are controlled in the reactor ? I mean, maybe I am missing something - the prompt neutrons come from fission of your fuel and delayed come from fission from your FP. But how do you control the prompt neutrinos, to...
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    Why only delayed neutrons matter ?

    Hi all, I've been seeing this again and again, that delayed neutrons are the ones that are allowing thermal nuclear reactors to work etc. etc. What I do not understand is how are prompt neutrons controlled and how delayed ones are allowed to "do their thing" no problem ? Wiki says...
  10. L

    Method of characteristics and shock waves

    I did not notice I deleted that part. I got x = 1 - Ct (I did it by differentiating, and equating the logs. Are we still talking about the same problem as in the picture ? Because it says the shock forms at t=1, x=1. Anyway, never mind. I understand nothing. Thanks for your help anyway!
  11. L

    Method of characteristics and shock waves

    So x vanishes for t = 1/C... So the x intercept. If C is the "sigma" this confuses me even further...
  12. L

    Method of characteristics and shock waves

    It should be x = K (1+t) So a straight line, for some constant K. If I use the range x is in I get that -1<t<\frac{1}{K}+1. I think I am wasting your time by now. I think it could be high school maths that's my problem. I have no idea where I get K from and no idea how this leads to shock time...
  13. L

    Method of characteristics and shock waves

    Maybe you could tell me about how you think (your thought process) when you are drawing the characteristics (especially the sloping ones) ? I have circled the ones that maybe will make life easier for both of us. Because, however I approach it, I get nonsensical line. Let's say for the first...
  14. L

    Method of characteristics and shock waves

    I am still not sure if this all helped (not that your explaining is bad, but me being daft). I am now looking at shocks and weak solutions. I (think I) know how to mechanically get weak solutions, but when it comes to drawing the characteristics for the weak solution - I'm in trouble. For...
  15. L

    Method of characteristics and shock waves

    So where was I wrong ? (I have not really seen x being set to something before (except when playing around with limits)) Did you mean it like sigma (I take it these can be interchanged for t=0) ?
  16. L

    Method of characteristics and shock waves

    I assume you mean the \sigma = 1 which will give the characteristic x = \alpha t + 1, where \alpha = u and in range from 0 to 1. \alpha = x-1/t so u = x-1/t
  17. L

    Method of characteristics and shock waves

    I assume you are asking for this: u(0,\sigma) = \sigma ? Otherwise, maybe this u(\sigma,t) = F(\sigma) = u(x,t) = F(x/(1+t)) ?
  18. L

    Method of characteristics and shock waves

    OK \sigma = x / (1+t)
  19. L

    Method of characteristics and shock waves

    Following the same idea, would it not just be x = \sigma t + \sigma ? (And I expanded the solution in my previous post). Sorry, I thought I solved the problem in the previous post.
  20. L

    Method of characteristics and shock waves

    I assume you are talking about the most recent problem, so u(0,\sigma) = \sigma and that will let us solve for x i.e. x = \sigma t + \sigma for the given limits (with now 0 \leq \sigma < 1 ). I then rearrange for \sigma = x / 1+t now look at limits 0 \leq x/1+t < 1 and then consider the...
  21. L

    Method of characteristics and shock waves

    Thank you for your help. However, I think I am just to daft to understand these. I am not sure what I do not understand. I bet it has to do with understanding the surfaces. The Monge equations, as far as I understand, are about the tangent vector ? Sort of like streamlines in fluid mechanics ...
  22. L

    Method of characteristics and shock waves

    Not really sure how to answer that, but whatever u(t=0,x) is equal to, will stay the same along the characteristic (note, that I am answering using the definition, not understanding). Is that what you are asking about ?
  23. L

    Method of characteristics and shock waves

    I really do not know, my notes said: "Note that we may choose t_{0} = 0 without loss of generality since the Monge equations are invariant under the change of variable. In other examples it may be more appropriate to choose x_{0} = 0 instead, using the same argument." I thought that the...
  24. L

    Method of characteristics and shock waves

    Solving your given Monge equations: t = s + t_{0} ; x = ut + x_{0} ; u=F({\sigma}) Then choose t_{0} = 0 (Could have equally done that to x_{0} I assume because it does not matter where we start, might as well chose 0) Then u(x,t) = F(x-us) s=t
  25. L

    Method of characteristics and shock waves

    Not sure what you mean there, but I will pretend to understand :) I'm having trouble with this question: The "shock location is obvious" for me it is only obvious if I draw the picture, maybe I should look at the limits ? But the real problem to me is understanding how were the limits...
  26. L

    Method of characteristics and shock waves

    Yes, I know how to algebraically get the expressions in the picture of the solutions I have attached. What I do not understand is why are we allowed to compare x and t ?
  27. L

    Method of characteristics and shock waves

    I think I get that (and I thought that was the whole point of characteristics, that u is the same on a characteristic). What I do not get is how we compare time t and space x...
  28. L

    Method of characteristics and shock waves

    Sorry, I have not realized that did not fit. It's u_{t}+uu_{x}=0 and u(x,0) = -1 for x<0 and u(x,0) = 1
  29. L

    Method of characteristics and shock waves

    A very simple problem that is probably the key problem for me is the fact that x and t are easily compared and interchanged. I do not understand how one can compare time and distance. For example saying u = 1 for x<-t. Should I somehow think in 3D ? This is a very simple problem with the t...
  30. L

    The concept of parity and it's significance

    Thanks for the reply ! I just read about how parity violation in weak interactions, in particular, example of β-decay of polarised 60 Co. Although I still could not imagine parity as some trait, I think I might see the importance, I almost feel ashamed for my ignorance...
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