 So once more, good afternoon for everyone. This is the last talk for the exercise. How do you throw in calibration and some words of that? This is some words on the symmetry equipment, the calibration procedure, correction factors, the regression quality of the correction factor Q, the elimination calculation, reference depth, and I think I will... Thank you, thank you. So the symmetry equipment, we have these two types of chambers, cylindrical chambers and the plane-parallel chambers. So these are recommended to be used for for electron beams with energies of 10 MeV and above. Parallel chambers with respect to electrons for electron beams with energies below 10 MeV. And of course also in the built-up regions and for depth-dose measurements. We had some discussion on that. So depth-dose measurements are recommended for photons and electrons to use these plane-parallel chambers. These are some examples which you have already seen. These are electrometers, which you are all familiar with. And this is one type of an electrometer again from the company PDW and the radiative check source. And this is what I want to show to you. This is offered by PDW, offering from a catalog each type of connector and each type. So it's sometimes useful. But as I told you already, you can do some things that you can have the high voltage on the outer wall. Therefore, if you ask PDW, can I offer that? It's on your own risk. We don't guarantee for nothing, but they will give you the connectors. These are water phantoms to be used. And now there is one remark which is important. It is said in the TRS protocol, water is always recommended as the phantom material for the calibration of mega-voltage photons and electrons, water and not solid. Here is an example of solid phantom. This is our so-called water equivalent material. They are never, never, never water equivalent. They are very close to that, but they are not really water equivalent. I would recommend you to do an experiment using if you have a plane pearl chamber, if you have the Marcus chamber and you can take away the cap, so measurement free in air. Then you can use photons or electrons. You can use two types of materials. Say you take PMMA, a small piece of PMMA, say 2 mm thick, and another one that maybe you can use is a material we just call polystyrol, which includes some chlorine. So the point is if you two put these two sheets together and you change it, the situation will be the same. So you expect the same signal? No. Because of what? This is an interesting thing to really understand, because of what? Effective? No, no. It's the same material. The radiation is going to each material, though the effects are the same, whether it's first and the second one or the other. No, it's not the reason. Production of seconders will be the same, because if it's photon irradiation, there's almost the same, there's no change of the fluence. And if you produce them here or here, it's the same production. The production itself is not changed. But if you think on the interaction of electrons, how electrons are interacting? They are interacting by energy loss, as described by the stopping power, but they also have some scattering. And the scattering is different from the material. Quite different. If you use polystyrene, you have chlorine in that, and chlorine is scattering the electrons much more than water or something. So our fluence of the secondary electrons is quite different because you have a polystyrene just below or below that. So it is due to the scattering, the different scattering properties of the materials, which is now changing the fluence in the detector. And by changing the fluence, you have different signals. And this is the case if you use a solid material. It may introduce a change of the fluence that enters the water. You have a different fluence. And therefore a dosimetry performed in a solid material requires, of course, first a scaling factor. If you use PMMA, it's in a different attuation. You have a scaling factor, and this scaling factor is referred to as CPL in the document. And the second factor is the so-called fluence. This is the depth scaling factor, and this is the fluence scaling factor. The fluence is changed by the material. And if you use another material than water, you change the fluence, and therefore you have to introduce a fluence correction factor. It's called HPL. This, by the way, refers to any verification method. So it's very popular that you do for INRT. There are many phantoms now available. You can put in your chamber into the phantom, and you do the measurements. It is not water. So it has some influence. Here are some numbers of this HPL. It's 1% here. So it's 1% changing in solid water. So even for clear polystyrene, we have almost 3%. The problem is that the uncertainty of this HPL factor is quite huge. So the messages introducing or performing a calibration method in solid introduces an uncertainty which is unnecessary. You could even have, say, changes of 1% something like that. So it's a strong recommendation to perform, especially for electrons, to perform a calibration measurement only in water. And the physical reason is the change of the fluence, which is easy to understand, but normally you forget it. We think it's always an attenuation. We don't think in the scattering interactions, but this is also relevant for the symmetry. Our formula is the same as for photons, but now we have a KQ value, of course, which refers to the electrons. Again, the problem of positioning. Now this refers to the calibration of electrons. Beam calibration for cylindrical chambers. This is for high-energy photons here. For high-energy electrons, 0.5 times r-deeper than the measuring depth. And also for depth-dose measurement for electrons, 0.5 times r-deeper than the measuring depth. And this is for plane-parallel chambers. We have already seen this. So for high-energy electrons, we have this positioning for plane-parallel chambers and for cylindrical chambers. Correction factors. We have the same, which I have already discussed, and I can go through this very quickly. So we again have a table of quality correction factors now again as a function of beam quality or beam quality index for electrons. And please remember that I told you that this quality, this combination of quality, is different according to the radiation type. For the photons, it's TPR, not TPR, TPR-2010. And here, the quality parameter used for magnet-voltage electron beam is commonly based upon, that is, this old-fashioned, this based on the half-value depth in the old-fashioned, on the so-called r50 value. So we have here a measured depth-dose, and we have here our maximum dose, 100%, and we go to the 50%, and this is our r50. So we have, before we can get the KQ value, we have to determine the r50. It's the prescription how it has to be measured. Now I go to this number more quickly. There's one problem with the measurement that is the following. If you do a measurement with ionization chamber, you measure the dose in air. And you have to translate the dose to water, and you see the translations given here. This is our well-known translation formula, the Spencer ethics, the stopping power ratio and the perturbation factor. This stopping power ratio between water and air is dependent on energy, and since the electron is changing the energy, this stopping power ratio is continuously changing. So we have here a factor which depends on depth, and therefore we have to apply a correction for that. The blue curve is measured depth-dose, and we call it the depth ionization curve, and this is the depth-dose curve. And for this, there is a solution which simply shows if we have r50 obtained by our measurement, we can translate it into that using this formula for two different energies. And I think there's also a formula... No, the formula is not shown here. One word about the calculation of KQ, which has to be used. We have here this W values for our user quality and for our calibration quality. Normally this cancels out because they are more or less the same. We have our stopping power ratio for Cobalt-60, for the calibration quality and for our user quality, the high-energy electrons, and we have to take into account this patubation factor for which values are given in the document of the TRS. One further condition for high-energy electrodes is that the KQ value is valid only if the calibration measurement is performed at the reference depth. And the reference depth is energy-dependent and is obtained again from this parameter r50. So if you first measure the r50, then you know at which depth you have to perform the calibration. And again, I refer to this mistake. You cannot do this on the maximum depth. So if you want to do that, you have to translate your measurement from the reference depth to the depth of the maximum. And for this, the formula is also given, which seems to be a little complicated, but it can be easily implemented in an Excel file and then can be used. And this translates. This is giving the stopping power ratio as a function of depth, and then you can correct for that. I think I will stop here now. Cross-calibration is something which is not so easy to understand and it takes some time. If someone is interested, I can give a private lecture. I'll be here at the end of the week, so I will go home at Saturday in the morning. So we have time. If someone is performing a cross-calibration, what simply means that you have one ionization chamber, which is calibrated and you have your own chamber and you want to do a cross-calibration, there is a clear prescription how you can do this with electrons. It's a little bit tricky, because if you read the text, you have to read it ten times before you really understand it. Therefore, an exercise would be quite nice, but I will stop here and we will go to the computer lab and we simply will perform an exercise with a calibration of 15 MD photons, which seems to be quite simple, but normally it takes some time to get through. So my recommendation is this computer lab is quite huge. I think there are more than 50 computers available. I ask you that always two are sitting together on one computer, so 25 computers are prepared, because using this needs always interaction and question and discussion, and it's always quite nicer to do it together and to help each other. So my recommendation is please use two together, one computer, and I will explain then a little bit more what has to be done, what is the equipment and how it is done. Okay, now lectures are done. We go to the exercise.