 We're actually going to use this periodic table later, so I hope you still have one handy. One benefit of speaking toward the end of the week last is that you already know much of the basics. I've already been covered earlier in the week, so I'm going to use those skills you already know and not really cover the basics. So I'm John Sealy, and there's my email address. If you want to email me with questions or whatever, I'll be happy to answer if you put in your email that you are at the school. So what we're going to cover is experimental x-ray spectroscopy. So you've been hearing mostly computational lectures except for the very first ones on Monday by Professor Kunze. So this will be a continuation of the experimental aspects of x-ray spectroscopy. So this is just a brief outline. I'll talk about spectrometers, x-ray spectrometers in particular. When you do computational simulations of spectra, you really need to know what kind of spectrometer recorded those x-ray spectra so that you can properly interpret the x-ray spectra. And there are two types, reflection crystals or Bragg crystals, transmission crystals, allowing crystals, and the spectra I'll be showing will be mostly from transmission crystal type spectrometers. And then in particular, I'll be showing you experimental spectra, and most of those will be from large laser facilities or other large facilities like pulse power facilities that produce bright, hard x-ray spectra. And the science motivation I think you already know by now, it's to study the atomic physics of atoms and highly charged ions, in particular, intershell transitions, and high atomic number atoms, copper, higher tungsten, Z equals 72 from your chart, is tungsten, gold. There's also x-ray transitions in those highly charged ions. You know by now that the highly charged ions, intershell excitations, helium like transitions, these are really of great interest now. We'll be talking about the types of sources where those hard x-ray spectra are produced. And in particular, laser produced plasmas, they tend to be hot, killable, temperatures, and they tend to be dense. From solid density, 10 to the 23 per centimeter cubed, and if the plasma expands down to 10 to the 20, 10 to the 19, that's still high density compared to Tokamax or solar or astrophysical plasmas, which are much lower density. So this area of research is called high energy density, or HED, physics and plasma diagnostics. And then of course the codes, you've been hearing about all the nice codes that are available and have been made free software in many cases on websites. So all of that is a really nice service by the community of the more senior people you've been hearing, such as Jung Jung and Yuri Rochenko and Howard Scott and all the others. So one aspect of experimental x-ray spectroscopy is to validate those codes. Of course we hope they're perfect, but the codes are not always perfect. So when experimental spectra are recorded and the codes are used to simulate those spectra, sometimes they don't agree. The experiment might be wrong, no, that's not a possibility. So we usually look to the code to see if we can look to suggest some improvements in the code. And there's always this back and forth between the code developers and the experimentalists. So we're always going back and forth. And that's really part of the fun aspect of this type of research. So the codes you're all familiar with are not discussed those. So one of the biggest motivations is to use the x-ray spectra, interpret the x-ray spectra using codes to determine the properties of the x-ray source, namely the plasma. So bound bound transitions, you know about, measurement of transition energies. And this is part of the atomic physics code validation process. And ionization balance, that's part of the kinetics code validation where the populations are calculated and the line strengths and the line ratios. And then to measure plasma properties such as the electron density, temperature, opacity, opacity, you've heard a lot from Howard Scott. Continuum is always a big interest. There is the continuum. What's the real continuum? What processes contribute to the continuum? And part of that is a super thermal electron energy component. In many plasmas, there's a thermal component that will be lower temperature, one EV or one hundred EV or one kilovolt temperature in the case of the hotter laser produced plasmas. But there might be a super thermal component that has much higher energy, 30 kilovolts, one MEV, electron energy. So one has to be aware of the different electron energy components in your x-ray source. And then line widths, line shapes, energy shifts. For this, you need higher spectral resolution and we can now achieve this in the hard x-ray region where the photon energies are 10 kilovolts and above. So measuring Doppler broadening, which is a measurement of the ion temperature in the plasma. Stark broadening, which Ed Yenny talked about very well earlier in the week, is a measurement of the density in the plasma and opacity. And then the x-ray source might be a short time duration. Nanosecond, femtosecond in some cases. So and it also the spectrometer might produce spectra with spatial resolution so that you can see the spectral lines from the dense hot plasma core from the lower temperature, lower density plasma surrounding that core. So we have temporal resolution. You can study the fast dynamics of how the plasmas formed and how the temperature and density are changing. And then finally, absolute spectral line intensities. This also is part of the code validation process. Typically we work with relative line intensities, line ratios. But in some cases you want to measure the absolute fluids from the plasma and that's a strong function of the density, for example. So how well can those absolute intensities be calculated? So there's this back and forth between the experimental spectra and the simulated spectra. Okay, here, of course there are many sources of x-rays in the universe. Astrophysical, solar flares, low density laboratory plasmas like Tokamax, electron beam ion traps, these are low density x-ray sources. Higher density laboratory sources, laser produced plasmas are really of great interest now because there's some really very powerful lasers around the world in different laboratories and they can produce very hot, dense and bright x-ray spectra. So there are long pulse lasers long being relative term where the pulse duration is nanoseconds, 10 to the minus 9 seconds, that's considered long for a laser pulse. Short pulse lasers would be picoseconds or femtoseconds, 10 to the minus 12 and 10 to the minus 15 seconds. In the case of free electron lasers, for example, femtosecond type pulse duration. And then pulse power generators, that's a high density laboratory source, but a longer duration like 50 nanoseconds or 100 nanoseconds. But these can be extremely bright x-ray sources. X-ray free electron laser, XFEL, the first really good operating laser was in Germany called FLASH. And it started out as a ultraviolet, extreme ultraviolet source with wavelengths around 50 angstroms or 100 angstroms and then it has become much more energetic photons. And then in the U.S., there's the LCLS free electron laser. I'm trying to remember what the acronym is, linear coherent light source I think. And this is located in California at the Stanford Research Laboratory. And it can produce x-ray laser beams, coherent pencil type, x-ray laser beams up to, soon, it'll be up to a 25 kilovolt photon energy. Europeans are now extending the FLASH facility up to higher energies. And European XFEL will leapfrog LCLS and become the most produced, the highest energy photons soon. And then laboratory x-ray sources are typically used for testing your x-ray spectrometer before you take it to one of these large facilities because you want it to work on the first attempt. You don't want to be embarrassed. You want to test it first so it works. And also, calibrations are done using laboratory sources. And I'll be talking in particular about an electron bombarded anode. So you have a source in the laboratory with a cathode that produces the high energy electrons. Those are accelerated by a large potential, like 300 kilovolts in the case of this source. And those 300 kilovolt electron bombarded anode, which is typically a metal, like tungsten. And then that produces energetic photons from the anode material, which might be tungsten or copper, whatever you choose. And then there are very high energy radioactive sources, x-rays and gamma rays. So here's an example of a spectrum of iron from a solar flare produced by a satellite spectrometer. So this was in orbit above the atmosphere. The x-rays do not reach the surface of the Earth. They're absorbed in the atmosphere, which is good for us. So we're not getting an x-ray dose continuously. So this spectrometer is on a satellite facing the sun. And it's just a simple, flat crystal, which rocks back and forth and scans the photon energy, which is diffracted to a detector. So this is the only spectrum I'll show using wavelength of guinea. All the other spectra will be versus photon energy. And I do recommend that you use versus photon energy. But in the solar community, it's always wavelength. So here's the spectrum diffracted from one set of crystal planes, which are the germanium 220 planes. And this is a crystal made of germanium. It's a perfect crystal in reflection. And the lattice planes, which are diffracting, the crystal was cut so that the 220 planes are parallel to the surface of this reflection crystal. So this spectrum is from the 220 Miller indices plane. And then there's a higher resolution spectrum here displaced from the 13 minus 1 planes, which have a smaller lattice spacing, so higher dispersion, better resolution. This was completely unexpected, this higher resolution spectrum. And we wrote several papers on this completely unexpected spectrum. So here's a blow up, and the quality is not very good, because I blown it up. But you can see the spectrum. So when you look at a spectrum for the first time, you look for the strongest, most intense spectral line. So that's this line. It's a solar flare. It's a hot plasma. We know it's iron from the wavelength scale. And the strongest line you know by now in a hot plasma is the helium-like resonance line. So this is the helium-like resonance line, which we call W. So here's the transition from the singlet P1 level to the ground state. It's an E1 transition, so it's very strong. Typically, practically always the strongest line in your spectrum. So now let's look for the other helium-like transitions. We know, you know these are weaker from what you've learned this week. And there's the M2 transition from the triplet P2 level called X and then Y and Z. So we have a wavelength scale based on the transition energy of the W line. And then from the spectrometer, which we've tested in the laboratory, we know the dispersion. We know how the wavelength changes with rocking angle or position. So we have this line W for the energy, fiducial. Then we can calculate the energy scale. And now we can look for the X transition because we know it's calculated transition energy and Y and Z. But there are all these other lines, which by now you know are the transitions in the lithium-like iron charge state. From the doubly excited levels, the so-called di-electronic satellite transitions in lithium-like ion, which are satellites to the strong helium-like resonance line. So here are the identifications of the lithium-like di-electronic satellite transitions in the lithium-like iron with an N equal 2 spectator. And then here are the same, similar type transitions with an N equal 3 spectator, higher levels. So the screening is smaller, so these transitions are closer to the resonance line. And all of these transition energies can be calculated and the intensities, which are shown by the stick spectra. And these letter designations I'll cover later, but this transition called J is formed primarily by di-electronic recombination. And the ones with dash like Q and R are formed mostly by electron collisional excitations. So you can, by calculating these line intensities using codes and assuming a temperature and density, you can actually determine the temperature and density of this solar flare spectrum. So that's the power of X-ray spectroscopy. Interpret the spectrum, determine the properties of the X-ray source. So there are two types of X-ray spectrometers in reflection. It's called the Bragg case. So the crystal planes are parallel to the surface and the Bragg condition is satisfied in lambda equals 2D sine theta, where N is the diffraction order, lambda is the wavelength. And we use wavelength because the lattice spacing D is in units of nanometers, for example, a length unit. So we're using wavelength. And theta is the angle of incidence. And the diffraction occurs over a very narrow angle range. So the angle of reflection is also theta within a very, very tiny deviation in angle. And that's given by the Bragg condition. In transmission, this is called the LOWE case. Bragg and LOWE were X-ray physicists, crystal logarithms, more than 100, roughly 100 years ago, I guess. And in the case of transmission, the diffracting planes are perpendicular to the surface. In reflection parallel to the surface in transition, diffracting planes are perpendicular to the surface. Incident ray still has the angle theta with respect to the diffracting planes. And the diffracted ray is also theta, so the same Bragg condition. In the Bragg case, at higher energies, shorter wavelengths, the angle theta becomes very small. So this is a very small grazing angle. And it's difficult to work at extremely small grazing angles, like one degree or half a degree or even several degrees sometimes. So Bragg case type diffraction in reflection is typically angles, say, 10 degrees or larger. Those are easy to work with, which means the photon energy is about 10 kilovolts or smaller. In the transmission case, you're working with angles with respect to the crystal surface. And it's much easier to work in transmission than at small grazing angles in reflection. So the Bragg angles can be much smaller. And the photon energies can be much larger, greater than 10 kilovolts. So 10 degrees, 10 kilovolts is sort of the place you switch from the Bragg reflection type spectrometer to the Lowy transmission type spectrometer. These crystals can be bent, they can be bent concave facing the X-ray source. And or convex, there can be single bending like cylindrical bending or doubly bent such as spherical or conical bending. So the type of spectrometer I'll be talking about mostly is transmission type spectrometer in particular where the crystal is convex facing the X-ray source. So the rays are diffracted as they pass through the crystal and they are registered by a detector where the detector is on the so-called rolling circle. And the rolling circle has a diameter equal to the radius of the crystal bending. So if this is the crystal, here's the X-ray source, then the rolling circle has a diameter equal to the radius of the crystal bending. And it turns out in this type of spectrometer that was developed by Kochwa in 1932 that the spectral lines are focused on the rolling circle. So if your detector is on the rolling circle or even a flat detector tangent to the rolling circle, the spectral lines are focused so you get very, very high resolution spectra. So here's a sketch of this type of spectrometer and they can be small. So this is 107 millimeters, you know, four inches so they can be small and compact. You can carry them, carry them around very easily. This is a bending form and the crystal is bent onto that mandrel so it has the proper cylindrical shape with the radius that you want. The X-ray source is over here so the rays are come through this window, pass through the crystal or diffracted and then they go to a detector. And then the rays above that are registered down here and then other rays can come through this other window and go to the top of the detector. And it turns out that the higher energy photons are diffracted with small angles as you know and so those photons are hitting the detector close to the spectrometer axis. So these are high energy photons in this case 80 kilovolts and then the lower energy photons have a larger brag angle so they are registered farther from the spectrometer axis. So we get two spectra on the top of the detector and a second mirror image spectrum on the bottom of the detector and here is a spectral image. So here is the spectrum on one side of the detector and the same spectrum on the other side. Energy is going from high energies to low energies on this side and from high energies to low energies on this side. And this spectrum happened to be from that tungsten laboratory source at NIST which I mentioned. So these are the tungsten characteristic K lines from that tungsten anode. And here is a line out of the spectrum so this is the K alpha 1, K alpha 2. I think the battery in this is dying. And then K beta and I'll tell you what transitions those are in just a moment. But for now just keep in mind the type of spectrometer that we're using. So here is a sketch of how those K transitions are produced. So here is a energy level diagram. Well it seems to be weak but it's working. Thank you. Can you see the laser pointer or is it too weak? Too weak. So this is a simple level diagram of an atom. So here is the 1SK level and the unequal two levels are called L. So K and L. And the L1 refers to the 2S level. And the L2 and 3 refer to the 2P levels. So L2 is 2P 1 half levels. L3 is 2P 3 halves levels. So if an energetic electron or a photon collides with this neutral atom it can knock out a 1S electron. So we have a hole, a 1S electron hole. And then a higher level electron can decay from a L level to the K level and fill that hole. When that happens we get a so-called K photon. So these are the characteristic K transitions from this neutral tungsten atom which the spectrum I just showed on the previous slide. And when the L electron decays to the ground state it leaves behind a hole in the L shell. Oh that's much better. Now I can stand back here. So now we have a hole in the L level. Well that can be filled from an electron from the N level for example and that produces a so-called L transition. So K lines terminate on the K levels and L transitions terminate on the L levels. Pretty simple. And there are many more L transitions because of the multitude of states. So here's a diagram that shows you what all of these designations are. So the K alpha 1 and this terminology is historical. Developed by a physicist Siegbaum you know 50 years ago or so. But this is something you need to know if you're going to be simulating X-ray spectra. So there are these K-L type designations but like I said the lowest level K is simply a 1S level. So if you know the hydrogen atom you can understand this very well. L1 refers to the 2S, L2 to the 2P 1.5, 2P 3.5 and so forth. There's another type of designation K alpha 1 or it can be referred to as a K to L3 transition. So K is the lower level, L3 is the upper level. It's really quite simple if you know the hydrogen atom which I'm sure you all know. So here's a big table with lots of information but I think it's simplest to look at this type of designation. So here are all the strong L lines from tungsten. Here are the designations like I referred to. L2 to M4 levels. But here are the hydrogenic type designations of these transitions. So the strongest line is L alpha 1. It's a L3 to M5 transition. L3 is 2P 3.5. M5 is 3P 5.5. So this you can really understand very easily. Okay, those are radiative transitions but if the characteristic X-ray lines come from the higher line levels they can not radiative decay but a faster process is OJ. And you've heard this before. So OJ type transition, a M electron fills a hole in the L level for example and then rather than emitting a photon it ejects an M electron. So it's a radiation less transition. There's no photon coming out. But these levels can be broadened by these very fast reactions. OJ process leaves two holes behind in the same N type or M level, same group of levels. If the two holes are left behind in two different levels L and M for example is called Koster-Kernig type transition. So transitions from higher levels are not necessarily radiative but they are OJ or Koster-Kernig and those fast OJ type transitions can broaden the spectral line. So we see that the higher line transitions tend to be broad. So here's an experimental measurement of the width of the two types of transitions from the O level, N equal 5 level, very, very high lying. So we measure the widths and then we can calculate what are the Koster-Kernig rates that produced that broadening. So this is an experimental x-ray diagnostic that can be used to measure the OJ lifetimes or the Koster-Kernig lifetimes. I'm going to speed up a little bit now because I've been going quite slow. So now I'll move to a type of x-ray source which is called a pulse power generator. So this is similar to the laboratory tungsten source I described where you have a cathode and anode. Fast electrons are accelerated and crash into the anode, which in this case is tungsten, and produce a hard x-ray spectra. The accelerating voltage here is about 2 MeV and the current is very large, about 0.6 megaamps. This type of generator would fill a room several times this size. So the voltage is high, the current is high, and the x-ray flux is extremely high. And it's a somewhat larger duration because capacitors are used to store this energy. So the x-ray pulse duration is about 50 nanoseconds. Here's a spectrum from that type of source. So here are the L lines which you're learning about. So here's the L-1, L-beta-1 transition, which is a 2p1-1-1-2, 3d3-1-2 type transition. Here's a L-beta-2 transition from the 4p5-1-2 level. And the vertical lines are the characteristic x-ray transition energies from the neutral or first ionized tungsten atom. So what do you notice here? Well, these calculated and experimental transition energies match very well. But look, this experimental transition has shifted to higher energy. Well, why is that? What do you think? 3p3d3p4d integral 4 level from a higher level. So this is from a somewhat slightly ionized plasma where the ionization is down to just about the 4d level. So this 4d5 level is perturbed by ionization, but the lower levels, 3d3p levels are more tightly bound, so they're less perturbed. So this transition has shifted to higher energy by ionization. So whenever you're looking at x-ray spectra, first of all, look at the strong lines, look at what you know, and then notice things that are different or unexpected do not match what you expect. So this is what we look for in experimental x-ray spectroscopy. What's unusual? We actually wrote a paper with very raw chinko on this, just that. And during that data analysis, we needed to calculate the shift in the transition energy with ionization. So that was done using the grant code or grasp code which you've heard about. And here is the energy shift with ionization of the lower level of the L-beta one transition from the three, integral three levels. So it's not shifted very much with ionization. But here is the shift of the L-beta two line from the 4d level, which is shifted with ionization. And here is the derivative of the energy shift. So you see when the ionization is low, like 6s being ionized, 5d, it's a small shift in energy, increasing but small. But when the ionization is now in the 4f, which is approaching the 4d level, then the energy shift becomes much, much larger. So this is an example of using the grasp code to interpret spectral line shift in the experimental x-ray spectrum. Okay, now moving to laser produced plasmas. Very energetic. The electrons can be produced. Positrons can be produced if the focus intensity is high. Large currents, large magnetic fields. This is what a large laser facility looks like. There's a person. This is the laser lab at Rochester in the USA. Here's our spectrometer in a vacuum chamber which attaches to the larger laser target chamber. This is the NIF laser at Livermore. There's a person inside the vacuum chamber, the target chamber, so you can see this is huge. It's about six or seven meters in radius. So it's like a mountain climber inside the target chamber. These are spectra from the Rochester Omega laser from a Krypton-filled gas bag type target. This is the experimental Krypton spectrum. So we see K-shell transitions and two-to-one type transitions, three-to-one type transitions, four-to-one type transitions, and then Heung-Kun did some very nice calculations using fly chuck, simulating these spectra. So from these simulations, we can say that the electron temperature was 2.6 kilovolts. The electron density was 2 times 10 to the 18. Now I'm going to move to another type of x-ray source, the free electron laser. Free electron lasers are very bright. There's flash, LCLS, the European free electron laser. So this is actually a type of laser, similar in principle to this green laser, but it uses what's called an undulator, which is a linear series of strong magnets that are alternating south pole, north pole, south pole. So that causes the electron beam to oscillate in phase, and photons from the front end of the undulator can be amplified as they pass through these oscillating electron bunches. So the beam of photons is amplified, just like in this device, but now it's an x-ray beam, much higher energy. So we did an experiment at LCLS. Here's the experimental group, and every good experiment needs a good computational physicist to plan the experiment and to help interpret the data. So here's the free electron beam. It was 1 to 2 kilovolt photon energy, very bright, very short duration, high repetition rate. So we were taking data almost continuously from this rotating target. We also had a smaller optical laser, which preformed a plasma on this aluminum target. So we shot the optical laser that preformed a fairly hot aluminum plasma with charged states up to healing like aluminum, preformed, and then we probed that plasma with a very short free electron laser. And we tuned the energy of the free electron laser to photoboxite transitions in that preformed aluminum plasma. So with the optical laser alone, for example, we saw a helium-like aluminum resonance line W. And then when we tuned the photon energy down in energy, we could excite lower energy transitions in lithium-like aluminum, brilium-like, boron, carbon-like, and so forth. So this is a way to tune your probing laser so that you can photopump specific transitions in specific charged states. So it's a very powerful technique to probe selected transitions, and it all depends on having this pump laser and the probe laser, pump probe type experiment. So here are the transitions from the optical laser. So on a large scale, so this is helium-like resonance line W, Y, dielectronic satellites. And the satellite J is formed by dielectronic recombination primarily, and the other satellites are from electron-collisional excitation. So you see the dielectronic satellite J is rather strong in the optical laser-only plasma because this is a short... a hot, dense, but short-duration laser, so it's recombining primarily. Here are the designations of all those satellites, and this is in the literature, but just call your attention that they are all from doubly excited levels in the lithium-like charged state, and some are primarily from dielectronic recombination like J. Many of the others are primarily electron-collisional excitation. So here's a spectrum with the pump laser applied to that preformed plasma. So we tune the free electron laser photon energy to resonantly photopump the helium-like Y transition, so this transition is normally weaker than the resonance line. It would be down here someplace, but when you photopump it, it becomes quite intense. And also the satellite transitions, which can be excited by electron collisions, they have a large oscillator strength, and they can be photopumped because of that large oscillator strength. So these satellite transitions, which are easily excited by electron collisions, can also be easily photopumped. So there's a huge enhancement here. The dielectronic satellite J is formed mainly by dielectronic recombination, and it is not photopumped at all. So here's a composite of the photopumped spectra, which are the upper spectra and the optical laser-only spectrum without photopumping. And you see the big enhancement of the satellite transitions, which can be photopumped, and the J transition, dielectronic type transition, is only slightly enhanced on this long scale. Here's another composite of photopumping, the lower charge states of aluminum. So here's the lithium-like transitions being photopumped by tuning the free electron laser photon energy to this position. And then when we tune the free electron laser down in energy, we can photopump boron-like, carbon-like, nitrogen-like. This free electron photon energy, for example, photopumps the carbon-like. So this is a very powerful way of photopumping selected transitions in selected charge states. URI did a very nice plot calculation of what transitions are pumped when you tune the free electron laser in photon energy. And then here is the axis for the aluminum transition energies. So here are the transition energies from the aluminum ions, and here is the tuned free electron photon energy. So when you're tuning to high energies, you're producing mostly helium-like. There's basically nothing in here from the lower charge states when you're tuning in high energy. When you tune the free electron energy down, then you excite the lithium-like transitions, tune lower, brilium-like, and so forth. Okay, now I'll move to another subject, which is absolute calibration of x-ray spectrometers. So typically you've seen, we look at the relative line strengths and you can learn a lot about the plasma from the relative line strengths, but sometimes you want to know the absolute fluence from your plasma. So for that, we need to absolutely calibrate the sensitivity of the spectrometer. And this we do at NIST. There are other places you can calibrate spectrometers, at NIST there's an absolutely calibrated x-ray source. So here's the x-ray source, here's an ionization chamber detector, and then we can place our spectrometer here in front of this x-ray source. We know the absolutely calibrated fluence from that x-ray source. We can measure a signal with our spectrometer, and then we can relate the two. And then when you take that calibrated spectrometer to a laser facility, for example, and you measure a signal on that spectrometer, based on the absolute sensitivity calibration, you can take your signal from the laser-produced plasma, for example, and you can measure the fluence, absolute calibrated fluence from that x-ray source. So here's a signals from several different absolutely calibrated sensors where we are measuring the fluence from the NIST x-ray source. So this NS-15 is a calibrated fluence from the NIST x-ray source, that's this curve. And then the other curves are spectra recorded by several different types of x-ray sensors. So these two curves are measurements from two different transmission crystal spectrometers that have high resolution. So these high resolution tungsten L lines from the x-ray source were measured by these two high resolution transmission crystal spectrometers. And then this darker curve is from a silicon-photon counting sensor. It's got a silicon chip. The photon comes in, creates charge in the silicon chip. You measure the size of that charge and that determines the photon energy. So that's what this darker curve is. It's lower resolution than the crystal spectrometers, but the curves pretty much lie on the absolute fluence curve from the x-ray source. So that means we've calibrated these three different sensors to sort of this type of accuracy, which is about 10%, 15% in some cases. So the absolute instrument calibrations can be done. It takes a lot of effort, but it can be done to about 10% or 15% accuracy. Okay, I think I'm just about finished, so I just want to give you an introduction. When we come back, we'll have experimental x-ray spectroscopy Part 2. And during that 50-minute time period, we're going to use the skills you've learned this week to analyze this spectrum. So this is an x-ray spectrum. At this point, we don't know what it is. What are the spectral lines? Can we determine the plasma temperature and density from when this x-ray spectrum came? Can we determine temperature density? Can we determine other properties of that plasma, like the plasma expansion velocity? Will there be some unexpected discoveries? And the answer is yes. So when you come back, we will go through the analysis, and I hope you'll help and make suggestions as we go through. I'll stick around during the break.