Embedded Files
31 January 2018
Today I met with professors Sarah Eno and Alberto Belloni. At my request, I will be working on a hardware based project. From what was said, I believe I will be continuing the research of Cat Cassell and Cherie Landa, whose logs can be found in the previous year.
Professor Eno recommended the following readings:
http://pdg.lbl.gov/2011/reviews/rpp2011-rev-passage-particles-matter.pdf, section 27.2.1, 27.2.2, 27.2.7, 27.2.8
https://docs.google.com/file/d/0B_Ce2ncoxFYka2V5US1MR2xvbXM/, chapter 8, section I.
https://www.sciencedirect.com/science/article/pii/0969806X93900402
I meet again in two days, this time with some different people, and I will learn more specifically about what I will be doing this semester.
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7 February 2018
Now I know exactly what this project entails, so I will explain in layman's terms.
When a particle passes through a plastic scintillator, it excites the molecules of the scintillator, and these excited molecules emit light as they fall back to the ground state. This light is then picked up by a photodetector, and that is how the system knows that a particle has passed through the scintillator tile. At the LHC, these plastic scintillators are used in many different detectors as they are versatile and can be used to detect many different types of particles. However, due to the nature of the experiments conducted at the LHC, these scintillator tiles are bombarded with lots of radiation. Unfortunately, plastic scintillator is rad soft. This means radiation damages the material. A normal plastic scintillator tile is clear. After irradiation, that same tile will appear black. This is of course problematic, because now the light will be absorbed and not reach the photodetector, so the tile effectively cannot function. It takes some time for this darkening process to completely compromise the scintillator, but eventually the tiles need to get replaced. Below is a graph of the transmittance (how much light gets through) of a tile after increasing doses of radiation.
As you can see, a larger dose equates to a lower performance of the scintillator.
We are looking for an alternative material that will last longer under the radiation conditions in the LHC so that the tiles do not need to get replaced as often.
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18 February 2018
This entry will explain the phenomenon known as the Stoke's Shift. Recall from the previous entry that a scintillator works by absorbing energy and then re-emitting it as light. Well, light is a form of energy, so a scintillator can and does absorb some of the light that it emits. But how does any of the light get through the scintillator? The light that gets absorbed then gets re-emitted, but at a longer wavelength. This shift in wavelength is called the Stoke's shift. Below is an experimental observation of the Stoke's shift produced by PVC.
Very little of the light that is emitted overlaps with the light that can be absorbed, so almost all of the emitted light can pass through the scintillator.
Notice from the graph above that the emitted light has wavelengths around 300-370 nm. This is not ideal because the photodetectors that are commercially available and accurate best detect light around 500 nm. So in actual practice, the scintillators are doped with primary fluor that absorbs in the range that the base emits, and a secondary fluor that absorbs in the range that the primary emits, all shifting the light longer and longer to around 500 nm. The graphs look more like this:
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26 February 2018
I have made a lot of progress since my last entry.
First of all, I earned my radiation safety training certification. There was a small online portion and a 3 hour classroom part.
Throughout the training, I learned:
The different types of radiation (Alpha, beta, gamma, x-ray)
How to shield from the different types of radiation
Radiation doses allowed for different parts of the body
The specific dangers of open radiation sources (liquids)
How to use a Geiger-Muller counter to find contaminated areas
How to conduct a wipe down test
How to dispose of radioactive waste
There are three types of measurements that we do on the scintillators both pre and post irradiation, and the difference between them is the data that interests us. The three measurements are emission/absorption spectrum, cosmic ray analysis, and alpha source measurements (I need the radiation safety training to handle the alpha sources).
So far I have done emission spectra and cosmic ray on tiles that have not been irradiated, and I will now explain both of these.
The emission/absorption spectrum analysis is performed using this machine, called a spectrofluorometer:
The scintillator tile is placed in the blue part of the spectrofluorometer, a laser excites the scintillator, the scintillator emits light, and the spectrofluorometer measures the intensity of this light as function of wavelength. Each tile is measured 12 times: on each of the 4 sides and with 3 different wavelengths of light.
The 3 wavelengths of the laser used to excite the scintillator are different for each type of scintillator because they are selected to produce a specific reaction. I measured EJ200 tiles, and the wavelengths used are 400, 350, and 285 nm. Recall from the previous entry that a scintillator tile contains the base and two fluors. For EJ200 tiles, the secondary fluor absorbs around 400 nm, the primary absorbs around 350 nm, and the base absorbs around 285 nm. So when we use these specific wavelengths, we catch the scintillator at important parts of its chain of Stoke's shifts. We do this so that when we compare the before and after of irradiation, we can see if any of the three components gets affected more or less than the others.
The second type of measurement I have done is the cosmic ray analysis. Unlike the spectrofluorometer that needs a laser source to excite the scintillator, the source for the cosmic ray analysis is atmospheric muons. In an oversimplified explanation, all we do for cosmic ray is put the tile in a box and leave it alone for a day.
This box is sealed against any kind of light penetration. When an atmospheric muon passes through the scintillator, it emits light. The light emitted from the tile gets directed by an optical fiber to a Silicon Photomultiplier (SiPM), which is just a very expensive photodetector. The measurement takes an entire day because we need enough muons to get enough data to produce a useful histogram.
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3 March 2018
Here is a picture of the cosmic ray setup box.
The two black cylinders on the right are photomultiplier tubes. The red box in the back is the power supply for the Silicon Photomultiplier (SiPM), the tiny silver box in the middle of the green circle. As mentioned above, this whole setup lies in a light-tight box. The whole thing is connected to an oscilloscope that actually reads out the signals from the muons and measures their relative intensity.
This is what an experimental tile looks like:
The scintillator is inside the white tyvek, with an optical fiber wrapping through the tile and coming out. The tyvek is in place to reflect the photons back in and contain them, because the ultimate goal is to have the photons hit the optical fiber and be transported to the end of the fiber. The fiber gets placed against the SiPM, so every photon that the scintillator produces reaches and gets read by the SiPM. At least that is the intent. In reality, the tile produces about 10,000 photons per muon, depending on the muon's energy, and only about 6-7 photons actually reach the SiPM.
Here is a picture of the setup with the scintillator in place.
The SiPM is what actually reads out the signal from the scintillator we are testing, so why do we need those other two photomultiplier tubes on the right? We want to be sure that when the SiPM receives a signal, the scintillator tile actually had a muon pass through it. Often the SiPM registers random noise that would throw off our data. So we introduced a way to eliminate this noise. The two shiny tiles above and below the white tile are referred to as "trigger tiles." If a muon passes through both of the trigger tiles, then it will have also passed through the white tile, and all three photomultipliers will see a signal at the same time. We can then be certain that this signal was caused by a muon event, so we record that signal.
I started a run today, and they take about a day so tomorrow I will finish the run and analyze the data.
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4 March 2018
I finished the run today. Below are the results.
The Black, Red, and blue lines are three runs taken by Francesca Ricci-Tam, a post-doc that works in the lab. The Magenta is the run that I just took. As you can see, Francesca's three runs were all self-consistent, and my run is significantly different. This is very bad, because it means that my data is not useful. I need to figure out why my run was different and how to fix it.
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5 March 2018
We found out why my data was so far off. When Francesca was setting up the oscilloscope for my run, she used the wrong y-scale for the data collection. It was too small, so some of the higher energy muons had their peaks cut off, and the oscilloscope registered less energy than the event actually had. In the graph, everything is flipped (the x-axis is mostly negative). This is due to some oddity in the oscilloscope but in reality everything should be flipped around the 0. So my data actually did register less energy than Francesca's runs.
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10 March 2018
Everything our lab does internally is carried out through CERN. All the data, all the tutorials, everything. So for me to have remote access to it, I got my own CERN account.
Now I have access to the entire lab's twiki, and I can upload my slides during our weekly meetings on Fridays.
I feel all big and important now.
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13 March 2018
Below is the same plot from March 4th, but flipped about the x-axis and with one more run that I took (in cyan).
You should notice that my runs are still not consistent with Francesca's, but now my runs are self-consistent with each other. This presents a large problem, because now neither of us know exactly who was right. It's fine that I messed up twice, but for me to mess up twice in exactly the same way is what gives us all concern. Maybe I did mine right and Francesca messed up three times, or maybe all 5 of our runs were wrong.
This effectively brings our research to a halt until Francesca and me are able to reproduce each other's results. Results that can't be reproduced are not scientifically viable.
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15 March 2018
We have a plan. The EJ200_4 tile that we were testing is no longer useful, since it has already been irradiated and is currently annealing, meaning the light output will be changing. If Francesca and I were to use this tile to try and produce the same results, it couldn't be done. Instead, we are going to use the EJ200_8 tile. This tile is called the reference tile because we use it as the reference for all other measurements (and it will never be irradiated). EJ200_8 has been measured tons of times so there is a lot of data already out there that we can access and compare with what we measure for EJ200_8. We'll both be present each time one of us starts or stops a run so that we can figure out what we're doing differently.
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16 March 2018
Francesca is leaving for Fermilab, because I guess you're allowed to go places when you're smart. Anyway, I can't really do anything until she gets back, which should hopefully be in about a week. Stay tuned.
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25 March 2018
Francesca got back yesterday, so I started a run today (of EJ200_8) while she observed.
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26 March 2018
I stopped the run that I started yesterday. Francesca was in the lab observing.
Since I got bored, I asked Francesca to teach me how to create the histograms of the cosmic measurements myself. I will have updates about that later on.
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2 April 2018
Francesca started her run today, so I went to the lab to watch what she did. So far neither of us have seen anything that we've been doing differently.
I have learned a bit about how we make the histograms. When the oscilloscope detects a signal, it records the time and the integrated area of the signal itself, and outputs these to a txt file. The area is what gets plotted in the histogram, with number of events on the y-scale. Below is a small sample of one such txt file. As you can see, this is about how many events we expect to see in 200 seconds. However, the runs last a day, so we typically get about 20,000-30,000 entries in total, all in a single txt file. We use a python script to parse through all this data and plot it in root (a free data analysis framework, that CERN uses). These root plots are the histograms that we analyze for trends.
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3 April 2018
I went into the lab today to watch as Francesca stopped her run. I still haven't seen anything we're doing differently.
She will analyze the data tomorrow, since I still don't know enough to do it myself.
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4 April 2018
Below is our two runs side by side, mine in black and Francesca's in red. They may seem quite similar, but there is still about a 7% discrepancy, which is not good enough.
Geng-Yuan, a more experienced post-doc, gave us an idea as to why we're getting different values. Apparently, the SiPM has differing quantum efficiencies along its entire area. I'm not sure why, or what exactly this means in a physical sense, but he told us that it performs the best when the fiber is pointed to the top left. So Francesca and I are going to redo our runs, this time making sure to point the fiber at the same spot on the SiPM.
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9 April 2018
Francesca began her run today, so I went in to observe. She made sure to get the fiber as close to the top left of the SiPM as possible without having any of it hanging off the edge.
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10 April 2018
Today Francesca stopped her run, and since I was already at the lab we decided I might as well start mine too. In order to test my ability to place the fiber, Francesca moved the stand. I repositioned the fiber to point it at the target spot, and tightened it in place. Francesca approved my positioning, and I started my run.
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11 April 2018
We went into the lab today and I stopped my run. Francesca will analyze the data and get back to me tomorrow on the results.
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12 April 2018
Below are the results of our two runs. This time, my run is in red and Francesca's is in black. Also, this time we only have about a 0.7% discrepancy. Yay! This means our issue has been resolved!
As Geng-Yuan suggested, the problem was with our positioning of the fiber against the SiPM. Below is the appropriate location of the fiber (keep in mind that the square is only about half a centimeter along one side).
Now that our issue has been resolved, we are going back to testing EJ200_4. Recall that EJ200_4 has already been irradiated, and that irradiated scintillators go through an annealing process, where their reduced light output increases over time. So we will be taking measurements of EJ200_4 every few days to observe this phenomenon.
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14 April 2018
It has come to my attention that I should explain more about the annealing process and what exactly we're trying to accomplish in this lab.
During irradiation of a scintillator, some of its molecular chains are broken (reducing light yield). Some of those chains will reform over time (increasing light yield). This process is called annealing, but it should be noted that once irradiated, a scintillator will never return to its original light yield.
The mathematical relationship between the light output of a scintillator before and after being irradiated is given by
L = (L0)e(-d/D)
where:
L = light yield after irradiation
L0 = light yield before irradiation
d = radiation dose (known value)
D = dose constant
We want to find the dose constant each time we measure the scintillator, so that we can plot D as function of time. What do we do with this plot?
Well, our purpose here in the lab is to improve the scintillators that are used in the LHC. Right now, they are susceptible to radiation damage, which decreases their light yield and slowly compromises their usefulness. For the time being, researchers can just adjust the data to account for the light loss, but eventually they will be so dark that no light will be able to get through. When this happens, the scintillators become completely useless, and will have to be replaced. That shouldn't be for about another six years, so in the meantime our lab is trying to find a better type of material for the scintillators so that they can withstand more radiation damage.
So back to the plot of D over time. Basically, we are looking for the scintillator that exhibits the best relationship of dose constant over time, because this is a measure of how well/quickly it anneals. This is only one variable that goes into selecting the best type of scintillator to replace those in the LHC. Other groups in the lab are testing other variables such as geometry of the scintillator tile (currently they are thin tiles but could a different shape perform better?), and light output measurements using an alpha source instead of cosmic rays.
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19 April 2018
Something really weird happened in the lab today. I went in to start a measurement run of EJ200_4, and when I started the program it gave an error message. Neither I nor Francesca knew what it meant, so she got Geng-Yuan to help figure it out. After some trial and error, he found out that the computer had no internet. Apparently all the ethernet ports in the lab stopped working.
Long story short, no one can do anything until PhysHelp (the IT guys) come in and fix the problem. They should be in sometime later today.
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20 April 2018
Good news: the internet in the lab is fixed.
Bad news: apparently the oscilloscope was the victim of a malware attack. The oscilloscope is a $60,000 piece of equipment that reads all of the signals from the scintillators. Without it, nothing can be done, and we can't really afford to buy a new one. So now we are waiting yet again for this new problem to be fixed, but this one is supposedly much more complicated and will take a lot longer.
This royally sucks because the end of the semester is almost here, and thanks to the reproducible results issue we were having, I have done basically nothing that was actually significant to our research. Oh well. Sometimes that happens. For now I'll just continue learning how to make my own histograms. Francesca will be helping me a lot, because she also can't do very much at the time.
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24 April 2018
I can make my own plots now! I had to download root locally and set it up and all that stuff (actually not as hard or as long as it seems), and had to clone Geng-Yuan's bitbucket repository. Francesca gave me two sample txt files of cosmic data so I could play around with everything myself. Below is the root histogram of those two runs plotted on the same axes. For some reason, the code automatically plots everything in the negative x direction (technically it should all be flipped about the zero, because negative energies don't really make sense). I need to figure out how to do that manually.
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2 May 2018
The oscilloscope has finally been fixed! So that burned two weeks. Oh well.
I went into the lab today and started a run of EJ200_4, and I'll go in tomorrow to stop it. Francesca said she'll try to get me some Dose Constants before everything ends.
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3 May 2018
More bad news. I went in today to stop the run and the oscilloscope stopped working during the run. We don't know exactly what was wrong, maybe the OS automatically ran some updates and stopped the data collection process (it's running Windows). Anyway, for now Francesca and Geng-Yuan are going to attempt to sort out what's wrong. That could be tomorrow or it could be next week, I have no idea. So for now I'm just waiting around, and working on making my research poster.
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