Measuring radioactivity using low-cost silicon sensors
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Computeranimation
Transkript: Englisch(automatisch erzeugt)
00:03
Hello creatures. To be honest, I never thought that I would be introducing a talk on measuring
00:23
radioactivity like ever in my life. But then again, considering the world's current state at large, it might be not such a bad idea to be prepared for these things, right? And gladly, our next speaker, Oliver Keller, is an expert in detecting radioactive stuff. Oliver is a physicist and works at one of the most prominent nerd-happy places,
00:47
the CERN, since 2013. He is also doing a PhD project about novel instruments and experiments on natural radioactivity at the University of Geneva. And to add even more RC3 pizzazz,
01:03
Oliver is active in the Open Science community and passionate about everything open source. All that sounds really cool to me. So without further ado, let's give a warm virtual welcome to Oliver and let's hear what he has to say about measuring radioactivity with using low-cost silicon sensors. Oliver, the stream is yours.
01:28
Thanks. That was a very nice introduction. I'm really happy to have this chance to present here. I'm a member since quite some years, and this is my first CCC talk, so I'm quite excited.
01:42
Yeah, you can follow me on Twitter or I'm also on Mastodon, not so active, and most of my stuff is on GitHub. Okay, so what will we talk about in this talk? I'll give you a short overview also about radioactivity because it's a topic with many different
02:01
details. And then we will look at the detector more in detail and how that works in terms of the physics behind it and electronics. And then finally we'll look at things that can be measured, how the measurement actually works, what are interesting objects to check, and how
02:21
this relates to silicon detectors being used at CERN. So the project is on GitHub called BIY Particle Detector. It's an electronic design which is open hardware. There's a wiki with lots of further details for building and for troubleshooting. There's a little web browser tool
02:43
I will show later briefly, and there are scripts to record and nicely plot the measurements. Those scripts are BSD licensed and this is written in Python. There are two variants of this detector. One is called electron detector, the other one alpha spectrometer. They use the same circuit
03:03
board, but one is using four diodes, the other one one photodiode. There's a small difference between them, but in general it's pretty similar. But the electron detector is much easier to build and much easier to get started using. Then you have complete part lists and even a complete
03:24
kit can be bought on kitspace.org, which is an open hardware community repository. I really recommend you to check it out. It's a great community platform and everyone can register their own GitHub project quite easily. Now this is a particle detector in a tin box,
03:46
so you can use the famous Altoids tin box or something for Swiss chocolate for example. You can see it's a rather small board, about the size of a nine volt block battery, and then you need in addition about 20 resistors, capacitors, and these
04:08
silicon diodes, plus an operational amplifier, which is this little chip here, this little black chip here on the right side. You can see it's all old school, large components. This is on purpose, so it's easy to solder for complete electronic beginners.
04:24
By the way, this picture is already one user of this project who posted their own build on Twitter. Okay, so natural radioactivity. I would say it's a story of many misconceptions. Let's imagine we had this little stick figure here on the ground.
04:44
Below us we have uranium and thorium. We also have potassium 40 in the ground, and potassium 40 is pretty specific and peculiar. It actually makes all of us a little bit radioactive. Every human has about 4,000 to 5,000 radioactive decays every second because of
05:11
radioactive isotopes, which is just everywhere. It's in bananas, but it's also in us because we need it for our body chemistry. It's really important. Even some of those decays are even
05:23
producing antimatter, so how cool is that? Okay, so what would we be measuring on the on ground? Well, there could be some gamma rays or electrons. Those are from beta decays. Or from the uranium, there is one radionuclide appearing in the decay chain, which is called
05:45
the radon. The radon is actually a gas. From the ground, the radon can diffuse upwards and travel with air and spread around. It's a bit like a vehicle for radioactivity
06:00
from the ground to spread to other places. That radon would decay with alpha particles, producing electrons in beta decays and also gamma radiation further down in the decay chain. Just to recapitulate, I've said it already twice, alpha particles
06:24
are actually helium nuclei, so it's just two protons and two neutrons, and the electrons are missing. In a beta decay, basically one neutron is transformed into a proton and an
06:40
electron, and there's also an electron anti-neutrino generated. But this is super hard to measure, so we're not measuring those. Mostly we will be measuring electrons from beta decays. That's why you see all these little e's indicating beta decays. Okay, if you would go to the hospital here on the left side,
07:02
we would probably find some x-rays from checking our bones or something like this, or even gamma rays or alpha particles being used in treatments, or very modern even proton beams are sometimes generated for medical applications. Now here on the right side,
07:24
if you go close to a nuclear power plant, we probably measure nothing, unless there's a problem. In this case, most likely we would find some gamma radiation, but only if there's a problem. Okay, and then actually, that's not the whole story. This is terrestrial radiation,
07:44
but we also have radiation coming from upwards, showering down on us every minute, and there's actually nothing we can do against it. So protons are accelerated in the universe, basically the biggest particle accelerator nature has, and once they hit our atmosphere,
08:04
they break apart into less energetic particles, and it's many of them. So in the first stage, there's lots of pions generated and also neutrons, but neutrons are really hard to measure, so I'll ignore them for most of the talk. Then those pions can decay into gamma rays
08:25
and then trigger a whole chain of positron electron decays, which again create gamma rays and so forth, and this goes actually the whole way down to the Earth. We will have a little bit of that on the sea level. And the other more known part of
08:43
atmospheric radiation is actually muons, so some pions decay into muons, which is kind of a heavy electron, and also neutrinos, but neutrinos are again very hard to measure, so I'll ignore them for most of this talk. And if you look here on the right side, on this altitude scale,
09:03
you'll see an airplane would be basically traveling where most of the atmospheric radiation is produced, and this is why if you go on such an airplane, you have actually several times more radiation in there than here on Earth. And of course on the ground, it also depends where you
09:23
are. There are different amounts of uranium and thorium in the ground, and this is just naturally there, but it depends on the geology of course. Okay, so I've talked quite a bit about radiation, and I'm saying I want to use silicon to detect it. So what radiation exactly? Maybe let's take a step back and think about
09:47
what we know maybe from school. So we have this rainbow for visible light, right? This is in terms of wavelength. We have 800 to 400 nanometers spanning from
10:01
the infrared red area to over green to blue and into the violet. And lower than those wavelengths, or let's say bigger, millimeter waves, meter waves, and even kilometer, that would be radio waves, radio frequencies for our digital communication
10:20
systems, Wi-Fi, mobile devices, and so forth. But I want to look actually more towards the right, because that's what we are measuring with these detectors. It's a shorter wavelength, which actually means higher energy. So on the right side, we would be having ultraviolet radiation, which is kind of at the border to what we can measure. And these 800 to 400 nanometers
10:48
translate into 1.5 to 3 electron volts, which is a unit that particle physicists really prefer, because it basically relates the energy of an electron after it has been accelerated by 1 volt
11:05
and makes it much easier to work with nuclear or particle physics, because everything, all the energy is always related to an electron. And this formula here is just a reminder that the
11:21
wavelengths can be always converted into energy, and it's inversely proportional. So wavelength increases to the left and the energy to the right. And if you increase energy more from the visible range, so let's say thousands of electron volts, then we arrive here, millions, mega electron volts, even giga electron volts. And there's now a pretty important
11:46
distinction between those two areas. And that is the right one is ionizing radiation, and the left one is non-ionizing radiation. UV is a little bit in the middle of that, so some parts of the UV spectrum can be ionizing. It also depends a lot on the material that the
12:06
radiation is interacting with. For these detectors I'm talking about today, and alpha, beta, gamma radiation, this is all ionizing. So some examples, lowest energy on the lower spectrum would be X-rays, then electrons, gammas from radioactive radionuclides that I already
12:28
talked about in the previous slide, alpha particles, and then muons from the atmosphere would be more in the electron volts range, and so forth. And for these higher energies, of course, you need something like the LHC to accelerate particles to really high energies.
12:46
And then you can even access the tera electron volts regime. Okay, silicon diodes. What kind of silicon diodes? I'm using in this project low-cost silicon pin diodes. One is called BPW34. It's manufactured from Vishai or Osram,
13:07
cost about 50 cents, so that's what I mean with low cost. There's another one called BPX61 from Osram. It's quite a bit more expensive. This is the lower one here on the right. It has a metal case, which is the main reason why it's more expensive, but it's quite
13:23
interesting because that one we can use for the alpha detector. If you look closely, there is glass on top, but we can remove that. We have a sensitive area, so this chip is roughly seven square millimeters large, and it has a sensitive thickness of about 50 micrometer,
13:44
which is not a lot. So it's basically the half of the width of a human hair, and in total it's a really small sensitive volume, but it's enough to measure something. Just as a reminder, how much of gammas or x-rays we would detect with this? Not a lot, because
14:05
this high energetic photon radiation kind doesn't interact very well in any kind of matter, and because the sensitive area is so thin, it will basically permeate through it and most of the times not interact and doesn't make a signal. Okay, what's really important,
14:27
since we don't want to measure light, we have to shield light away. We need to block all of the light. That means the easiest way to do it is to put it in a metal case. There it's electromagnetically shielded and completely protected from light as well. Electromagnetic
14:44
radiation or radio waves can also influence these detectors because they are super sensitive. So it should be a complete faraday cage, a complete metal structure around it. There's lots of hints and tips how to achieve that on the wiki on the github of this project.
15:07
Okay, let's think about one of those pin diodes. Normally there's one part in the silicon which is n-doped, negatively doped, and there's another part usually which is positively doped.
15:24
And then you arrive at a simple so-called p-n junction, which is a regular semiconducting diode. Now pin diodes add another layer, a so-called intrinsic layer, here shown with the eye, and that actually is the main advantage why this kind of detector works quite well.
15:45
It has a relatively large sensitive signals. So if you think about, let's say, a photon from an x-ray or a gamma decay or an electron hitting the sensor. By the way, this is a cross-section
16:03
view from the side. So it doesn't really matter, but let's say they come here from the top into the diode. And we're looking at the side. Then we have actually ionization, because this is ionizing radiation. So we get free charges in the form of electron-hole pairs.
16:24
So electrons would be here, the blue ball, and the red circle would be the holes. And depending on the radiation kind, how this ionization takes place is quite different. But the result is if you get a signal, it means there was ionization. Now if just
16:43
this would happen, we could not measure anything. Those charges would quickly recombine, and on the outside of the diode, there would be a little signal. But what we can do is we can actually apply a voltage from the outside. So here we just put a battery.
17:05
So we have a positive voltage here, a couple of volts. And then what happens is that the electrons will be attracted by the positive voltage, and the holes will travel to a negative potential. And we end up with a little net current or a small bunch of charges that can
17:27
be measured across the diode as a tiny, tiny current peak. The sensitive volume is actually proportional to the voltage. So the more voltage we put, the bigger is our volume,
17:42
and the more we can actually measure with certain limits, of course, because the structure of the pin diode has a maximum thickness just according to how it is manufactured. And these properties can be estimated with CV measurements. So here you see an example of
18:01
a couple of diodes, a few of the same type, the two that I've mentioned. There are different versions. One has a transparent plastic case, one has a black plastic case. It doesn't really matter. You see basically in all the cases more or less the same curve. And as you increase the voltage, the capacitance goes down. This is great and basically shows us that those silicon
18:23
chips are very similar, if not exactly the same chip. Those differences are easily explained by manufacturing variances. And then because this actually, if you think about it, it looks a bit like a parallel plate capacitor, and actually you can treat it as one. And if you
18:43
know the capacitance and the size, the area, you can actually calculate the distance of these two plates or basically the width or the thickness of the diode. And then we arrive at about 50 micrometer if you put something like 8 or 10 volts.
19:05
Okay, now we have a tiny charge current. Now we need to amplify it. So we have here a couple of diodes. I'm explaining now the electron detector because it's easier. We have four diodes at the input. And this is the symbol for an operational amplifier. There are two of those
19:21
in the circuit. The first stage is really the special one. So if you have a particle striking the diode, we get a little charge current hitting the amplifier. And then we have here this important feedback circuit. So the output is fed back into the input, which in this case makes a negative amplification. And the amplification is defined actually by this capacitance
19:43
here. The resistor has a secondary role. The small capacitance is what makes the output large or smaller. The larger the output and it's inverted. Then in the next amplifier step, we just increase the voltage again to a level that
20:02
is useful for using it later. But all of the signal quality that has been achieved in the first stage will stay like that. So signal to noise is defined by the first stage. The second one is just to better adapt it to the input of the measurement device that's connected.
20:23
So here this is a classical inverting amplifier with just these two resistors defined amplification factor. It's very simple. It's just a factor of 100 in this case. So if you think again about the charge pulse and this circuit here is sensitive, starting from about 1000 liberated charges in those diets as a result from ionization.
20:49
We get something like 320 microvolt at this first output. And this is a spike that quickly decreases. Basically these capacitors are charged and quickly discharged with this resistor. And
21:04
that is amplified again by a factor of 100. And then we arrive at something like at least 32 millivolts, which is conveniently a voltage that is compatible with most microphone or headset inputs of computers or mobile phones. So a regular headset here has these four connectors
21:25
and the last ring actually connects the microphone. The other is ground and left-right for the earbuds. Okay, how do we record those pulses? This is an example of a thousand pulses overlaid
21:44
measured on an oscilloscope here. So it's a bit more accurate. You see the pulse is a bit better. This is kind of like the persistence mode of an oscilloscope. And the size of the pulse is proportional to energy that was absorbed. And the circuit is made in such a way that the
22:02
width of the pulse is big enough such that a regular sampling frequency of a sound card can actually catch it and measure it. Yeah, this is potassium salt. So this is cut here. This is called low salt in the UK. There's also German variants. You can also just buy it in the pharmacy
22:21
or in certain organic food stores as a replacement salt. On the right side is an example from this small columbite stone which has traces of uranium on it. And this is measured with the alpha spectrometer. And you see those pulses are quite a bit bigger. Here we have
22:41
50 microseconds and here we have more like one milliseconds of pulse width. Now there's a software on a browser. This is something I wrote using the web audio API and it works on most browsers. Best is Chrome. On iOS of course you have to use Safari.
23:05
And that records once you plug the detector it records from the input at 48 or 44.1 kilohertz the pulses. Here's an example with the alpha spectrometer circuit. You get these nice large pulses. In case of the electron detector the pulse is much shorter. And you see the
23:22
noise much more amplified. This red line is kind of the minimum level that the pulse needs to trigger. It needs to be bigger than that. Like the trigger level of an oscilloscope. You can set that with those buttons in the browser. You need to find a good value. Of course if you change your input volume settings for example this will change. So you have
23:45
to remember with which settings it works well. This pulse for example is even oscillating here. For electron detector it's basically nice to count particles. For the alpha detector
24:01
it's really the case where the size of the pulse can be nicely evaluated and we can actually do energy measurements. These energy measurements can be also called spectrometry. If you look closer at these many pulses that have been recorded and we find that there is really
24:22
much more intensity which means many more same pulses were detected. We can relate it to radium and radon if we use a reference alpha source. And I have done this. I have measured the whole circuit with reference sources and provide the calibration on GitHub. And you can reuse the
24:41
GitHub calibration if you use exactly the same sound settings that I have used for recording. For example these two very weak lines here are from two very distinctive polonium isotopes from the uranium decay chain. The top part here which is really dark corresponds basically
25:05
in the histogram view to this side on the left which is electrons. Most of these electrons they will actually enter a chip and leave it out without being completely absorbed by it. But alpha particles interact so strongly that they are completely absorbed within the 50 micrometers
25:24
of sensitive volume of these diets. And okay here's a bit difficult to see peaks but far end of the high energy spectrum you see two really clear peaks and those stem can only stem from polonium actually. I mean we know it's uranium and that can only
25:43
be polonium which is that isotope that produces the most energetic alpha particles which is natural. I said if you use the same setting like me you can use it. So the best is if you use actually the same sound card because there if you put it to 100
26:03
input sensitivity you will have exactly the same result like in my calibration case. And this sound card is pretty cheap but also pretty good. It costs just two dollars and has a pretty range and resolves quite well 16 bits. And think oh you can do that with an Arduino as well.
26:20
It's actually a bit hard to do a really well defined 16-bit measurement even at 48 kilohertz it's not so easy and this keeps it cheap and kind of straightforward and you can have just some python scripts on the computer to read it out. And this is as a reminder in order to measure alpha particles we have to remove the glass here on top of the diet. So I'm doing it
26:44
just with cutting into the metal frame and then the glass breaks away easily. That's not the problem there's more on that on the wiki. Now we can kind of compare alpha and gamma spectrometry. Here's an example this is a uranium glazed ceramics the red part is
27:04
uranium oxide that was used to create this nice red color in the 50s 60s 70s. And in the spectrum we have two very distinctive peaks and nothing in the high energy regime only this low energy range has a signal. And this corresponds actually to uranium 238
27:22
and 234 because they use actually purified uranium. So all of the high energy progeny or daughters of uranium they're not present here because that was purified uranium. And this measurement doesn't even need vacuum I put it just like this in a regular box. Of course if you
27:42
would have vacuum you would improve these peaks by a lot so this widening here to the left basically that this peak is almost below the other one that is due to the natural air at regular air pressure which already interacts a lot with the particles and absorbs a lot of
28:01
energy before the particles hit the sensor. So in terms of pros and cons I would say the small sensor is quite interesting here in alpha spectrometry because it's enough to have a small sensor so it's cheap and you can measure very precisely on specific spots.
28:22
And on the other hand of course the conditions of the object influence the measurement a lot so for example if there's some additional paint on top the alpha particles might not make it through but in most of these kind of samples alpha radiation actually makes it through the top
28:41
a transparent paint layer. In terms of gamma spectrometry you would usually have these huge and really expensive sensors and then the advantage of course is that you can measure regardless of your object you don't really need to prepare the object a lot you might want some lead shielding around it and that's again expensive but okay at least you can do it you
29:09
basically costly because the sensor is quite expensive while versus in this setup for 15 to 30 euro you have everything you need and here you're looking at several hundred to
29:23
several thousand euros. Okay now measuring I have to be a bit quicker now I notice so I talked already about the potassium salt there's also fertilizer based on potassium baking powder uranium glass is quite nice you can find that easily on flu markets
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often also old radium watches here's another example of a uranium glazed kitchen tile in this case this was actually in the kitchen so the chances are that you at home find actually some of those things in the cupboards of your parents or your grandparents this is an example of
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torated glass which has this distinctive brownish color which actually is from the radiation and a nice little experiment that I can really recommend you to look up is called radioactive balloon experiment here you charge the balloon electrostatically and then it will catch polonium from the air and that's really great you basically get a radioactive balloon
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after it was just left for 15 minutes in a normal regular room. Okay now as a last kind of context of all of this to end this presentation I want to quickly remind how important these silicon detectors are for places like CERN this is a cross-section through the
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atlas detector and here you have basically the area where the collisions happen in the atlas detector so this is just a fraction of a meter and you have today 50 to 100 head-on collisions of two protons happening every 25 nanoseconds not right now but soon again
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machines will be started again next year and you also can by the way build a similar project which has a slightly different name it's called build your own particle detector this is atlas made out of lego and on this
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website you find a nice plan how to build or ideas how to build it from lego to better visualize the size and yeah interact more with particle physics in case of the cms detector this is the second biggest detector at CERN
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here you see nicely that in the middle at the core of the collision you have many many pixel and microstrip detectors which are made of silicon and these are actually 16 square meters of silicon pixel detectors and 200 square meters of microstrip detectors also made of silicon
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so without basically that silicon technology and moron detectors wouldn't work because this fine segmentation is really required to distinguish all of these newly created particles as a result of the collision so to summarize the website some github there's really this big wiki which
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you should have a look at and there's a gallery of pictures from users there's some simulation software that i used as well and i didn't develop it but i wrote how to use it because the spectra can sometimes be difficult to interpret and there's a new discussions forum
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that i would really appreciate if some of you had some discussions there on github and most of the things i show today are actually written in detail in a scientific article which is open access of course and i want to highlight two related citizen science projects on the one hand it's a safecast which is about a large nice sensitive geiger-müller
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based detector that has a gps and people upload their measurements there this is quite nice and also open geiger is another website mostly german content but also some of it is english that also uses diode detectors showed many nice places he he calls it geiger caching
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um so places around the world where you can measure something some old minds things like this and yeah if you want updates i would propose to follow me on twitter i'm right now writing up to other articles with more ideas for measurements and some of the things you have seen
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today thanks a lot well uh thanks a lot uh oliver i hope everyone can hear me now again yes thanks for mentioning the citizen science projects as well it's really cool i think
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we do have a few minutes for the q and a and also a lot of questions coming up in our instance at the IRC so the first question uh was uh can you talk a bit more about the snr of the system did you pick particular resistor values and op amps to optimize for noise
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was it a problem yeah so noise is a big issue here um basically the the amplifier is is one i found that this around four four euros i'm trying to find the slide um
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yeah you have to look it up on github the amplifier type but this is the most important one and then actually the the resistors are here the resistors in the first stage sorry the capacitors is the second important thing they should be really small since i'm limited here with with hand solderable capacitors um basically i choose the one that were just
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still available let's say and this is basically what is available is basically a 10 picofarad capacitor you should put two of them one after another you have the capacitance so you get five and this by the way is also then the capacitor so i kind of tried to keep same same um
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same resistor values as much as possible um and here the output for example this is to adjust the output signal for a microphone input in the alpha spectrometer i i changed the values quite a bit to make a large pulse um but yeah it's basically playing with the time constants of
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this network and this network all right i hope that answers the question for the person yeah but people can get a contact do right after the shock maybe as well um so there's
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another question um have you considered using an i2s codec with a raspberry pi pressure marks radiation h at um should be almost no setup i'm completely repeatable so last ones are up for comment uh i don't know that component but um yeah as i said um
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using a sound card is actually quite straightforward but of course there's many ways to um to yeah get fancy and this is really this should actually attract the teachers and high school students as well this project so this is one of the main reasons why certain
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technologies have been chosen rather simple than um let's say fancy yeah so it should work with a lot of people i guess and uh one another question was how consistent are the sound cards um did you find the same calibration worked uh the same
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several of them so yeah so if you want to use my calibration um you should really buy this two dollar card from ebay cm 108 um i've i've haven't seen a big difference from card to card and in this one but of course like from one computer to the mobile phone it's a huge
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difference in input sensitivity and noise and it's very difficult to reuse a calibration in this case but you still can count particles um and the electron detector is is anyway um mostly it actually just makes sense for counting because the electrons are not
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completely absorbed so you get an energy information but it's not the complete energy of the electron so on yeah you could use it for x-rays but then you need an x-ray machine so yeah who doesn't need an x-ray machine right yeah so maybe one question i have because
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i'm not very familiar with the tech stuff but what actually can be done with it right in the field so you mentioned some working with teachers with with these detectors um what have you done with that in the wild so to say um so what's quite nice is you can
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characterize stones with it for example so since you can connect it to a smartphone it's completely mobile um and it it goes quite well in combination with a geiger counter in this case so with the geiger counter you just look around where where's some hot spots and then you can go closer with the alpha spectrometer and actually be sure that there
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is some traces of thorium or rhenium on the stone for example or in this type of ceramic um these old ceramics you can go to the flu market and just look for these very bright red ceramics and measure them on the spot and then decide which one to buy okay so that's what i'm
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going to do with it right but thanks for for highlighting a bit the practical so i think it's really cool to educate people about some scientific things as well um another um
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question from the rsc didn't you have problems with common mode rejection while connecting your device to the sound card at best have you tried to do a ad conversion digitization on the board itself already transfer transfer via sp diff question mark yeah so of course i mean
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this is the thing to do if you want to make a like a super stable rock solid um measurement device um but it is really expensive i mean that's we are looking here at 15 euros and um yeah that's the reason to have this separate sound card just to enable um with very
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low resources to do this but um i'm looking for these pulses yeah so this common mode rejection is a problem and also this is kind of um i'm missing the english term yeah it is kind of oscillations here um if you design a specific analog to a digital conversion
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of course you would take all of that into account and it wouldn't happen but here this happens because the circuit can never be exactly optimal for a certain sound card input it will always be some mismatch of impedances and yeah all right so um maybe these special technical
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issues and details this could be something you could discuss with oliver on twitter or baby oliver you want to join the IRC room for your talk as well people were very engaged
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during your talk so this is always a good sign in um in that sense i'd say um thank you for for being part of this first remote chaos experience thanks again for uh for your talk and for taking the time and yeah the best for you and enjoy the rest of the conference
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i say of the congress and a warm round of virtual plus and big thank you to you oliver thanks i'll try in the chat room right now