I feel like I'm missing something really fundamental regarding the cross section of the photoelectric effect. I'm looking at this chart, and it seems that the lower the energy of the incident photon, the more likely a photoelectric interaction is to happen. Higher energy interactions get less likely, until a cut-off point, at which they no longer occur.   If the energy of the ejected electron is equal to the incident photon energy minus the binding energy, I understand why a lower limit exists (since there must be enough energy to eject the e-), but why should there be an upper limit? Why does the cross section reduce with energy at all?
Consider the diagram below:  (Author: Joshua Hykes )  From this diagram we can see that the absorption coefficient for the photoelectric effect generically decreases with the increase in energy of the photon. What is the physical reason for this decrease. (i.e. why does the probability of photoelectric absorption decreases with increasing photon energy).  Note: I am aware that the large spin is due to more inner electrons been able to be lifted.  Edit For Clarity  I will here rephrase the question: in the above diagram there is a general decrease in the absorption coefficient due to the photoelectric effect (i.e. the absorption of a photon by an electron and subsequent emission of that electron) with energy (ignoring the occasional absorption edge). This is of course linked to a decrease in the probability of the photoelectric effect occurring (and a corresponding decrease in the cross section). My question is: what is causing this decrease with increasing energy. (For the purpose of this question I am ignoring the contribution due to the Compton effect and pair production and simply focusing on the cross section due to the photoelectric effect.)
I have some  gained from survey questions.  In my case they are  responses (Strongly Disagree-Disagree-Neutral-Agree-Strongly Agree).  In my data they are coded as 1-5.  I  don't think means would mean much here, so what basic summary statistics are considered usefull? Encopresis, I'm at my wits end? Does anybody have suggestions? What is JavaScript's highest integer value that a number can go to without losing precision? Is this defined by the language? Is there a defined maximum? Is it different in different browsers? What UML (Unified Modelling Language) tools are available? Why Bobbi and Hunter can no longer work for S.H.I.E.L.D.? Is it true that if $A$ is a nonnegative primitive matrix, then $AA^T$ is also primitive? Obviously $A^T$ is primitive but in general product of primitive matrices is not primitive.  Any hint? While my code is compiling, I usually like to go through some reviewing and especially the late answers.  What I do see a lot in answers to old questions is the self-defense mechanism     ""   As if there is something fundamentally wrong with answering old questions. This leads me to two questions/thoughts:   Should we edit phrases like that out of the answer? (Given it's a good/valid answer to begin with, without any of the likely "old answer" problems. I realize the example linked to has its flaws.) Should we perhaps explicitly encourage answering old unanswered questions somewhere in the FAQ? (I'm unaware of any such recommendation, but might have read over it. Let me know if I did) What material fills the synaptic cleft? Is it water? The synaptic cleft is the gap between the pre-synaptic and post-synaptic neurons, and neurotransmitters are transferred between the neurons within this region. What substance exits in this space, is it water? If so, what force makes the neurotransmitters move toward the receptors despite colliding with water molecules? True Ubuntu on Chromebook ARM Reclassifying raster layer with certain colors for value ranges in QGIS? I'm using QGIS. I want a raster layer with 6 different colors each color representing a certain value range (e.g. -1 to 0 red, 0 to 0.2 orange).  In the layer style menu I can only approximate colors. Making layer or group private/hidden/invisible in Layers Panel of QGIS I have a number of layers in the Layers Panel inside a group but I don't want them shown (there's quite a lot so it looks untidy). These layers are also joined to several others. Is it possible to make these layers invisible to the user (i.e. not shown in the Layers Panel, it is not about setting the visibility of the layer) but can still be joined to other layers, or perhaps to make the group invisible? 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Using 32642 and the national one, the length is quite similar (6442m vs. 6428m)  But using 4326 (unprojected WGS84), I get 8057m  In all cases, I just used the Field Calculator and introduced $length.  I was already positively surprised that the result using 4326 was in meters and not in degrees.  To calculate the distance on the dataset in 4326, I assume that Qgis does some kind of re-projection in the background. Does anyone know which one?   Shapefiles here if anyone wants to play around:   EDIT - adding more info after more tests  As you can see, now both my shapefile and the project are in EPSG:4326, so unprojected:    Here the rough measurement of my shapefile, with the Qgis measure tool:   When I use the field calculator, the preview gives me a value in degrees, while the result seems to be in meters:      This would mean that the function $length, like the measurement tool, always gives a result in meters, but using a very coarse approximation of the degrees => meters transformation:    Is this a bug? or should it really behave like this? I find it at best counter-intuitive (but maybe it's just me) and, at worse, completely misleading (it makes very little sense to use, everywhere on earth, the length of 1 degree at the Equator). Any input or opinion here? Should I open a bug report (at least for the inconsistency between the preview and the actual result)? At 12 o'clock, the hour hand and minute hand of the clock can be swapped, and the clock still gives the same time, but at 6 o'clock, it can not be swapped. So in what cases when we swap the hour and the minute hand position does a clock still give a valid time? Did spacetime start with the Big bang? Proving that if $G/Z(G)$ is cyclic, then $G$ is abelian Possible Duplicate:             If $G$ is a group and $Z(G)$ the center of $G$, show that if $G/Z(G)$ is cyclic, then $G$ is abelian.  This is what I have so far:  We know that all cyclic groups are abelian. This means $G/Z(G)$ is abelian.  $Z(G)= \{z \in G \mid zx=xz \text{ for all } x \in G \}$.  So $Z(G)$ is abelian.    Is it sufficient to say that since $G/Z(G)$ and $Z(G)$ are both abelian, $G$ must be abelian? I am installing Ubuntu 16.04 in an Intel NUC computer with UEFI enabled. I am using an SSD from another computer. During the installation I chose Erase the disk and make a new full install  At the end of the process I see:  grub-efi-amd64-signed failed installation /target/ Ubuntu 16.04    And the system does not boot.  I tried to use boot-repair but apparently it does not solve the problem. Prove, in this figure, that $EFGH$ is a parallelogram How to modify the system default background image I am about to ship a new 10.04 installed system to a customer. This is the 1st system that we are shipping with our preinstalled software and an additional hardware test platform, so I thought it would be a nice idea to have the desktop wallpaper include our company's logo.  What I want to do is just to add the logo to the default background image. the questions are:   Where are the set of backgrounds stored? What are the rules for creating a custom background from scratch? 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