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SPRING SEMESTER 2017
02/07/2017
Cherie and I had a brief meeting with Professor Belloni. He gave us a run down of what the problem is and what we will be doing to help further this research.
Basically, in the detector there are these brass slabs with clear, plastic scintillators in between them that allow us to see the particles that go though them. Scintillators do this by ionizing radiation and transforming the energy into light energy, produce a small spark of light that an be measured. However, after a long time the radiation breaks down the plastic, turning it opaque, almost into a cardboard brown color, which does not allow us to be able to measure the light since not much light can get through. It is was too expensive to replace these plastic scintillators every time that they wear down (and time consuming), so research is being done in order to find a way to make it so that the plastic does not wear down as quickly. The goal is to find a plastic that will be resistant to temporary and permanent damage. Cherie and I will be measuring the effects of radiation on plastic and collecting data from samples done in the lab here at the University of Maryland, and some samples that were in the particle accelerator at CERN, being sent to us.
In order to become familiar with the physics that we will need to know to conduct this research, Professor Belloni sent us some links to modules and papers that give us some more information on the detectors, scintillators, and radiation.
Links can be found at the bottom of the page, under "REFERENCES".
02/10/2017
We had a meeting today at 3:30 with those who are also working on this research. We discussed the types of measurements we will be making. There are three different measurement types that we will be doing. The first is Absorption. The second is Emission. The third is Alpha Source. For Alpha Source, we need to have radiation training so that we handle the samples and the machinery properly, so that we don't break them. Professor Belloni also presented some graphs that he wishes to put in a paper about this research to be published. He went over some things about the test beam that they use at CERN for the radiation research.
Some interesting facts about the beam:
- Protons are shot, which hit something and knock off pions, which then decay into muons
- Leftover pions are stopped by a thin material so that the beam is solely muons. Muons don't get stopped because they are a heavier particle
- The direction of the muons are tracked by frames with a grid of thin wires running across them. When the muons pass through the grid, it creates a small voltage with the electrons in the wire, which is then tracked to show where the muons are moving.
- The beam is focused by quadrupole magnets. Which look like this:
The beam is flattened in one direction and then stretched in the other. However, the beam then goes through another quadrupole magnet that is rotated 90 degrees, which then focuses the beam.
02/17/2017
Today's meeting, Professor Sarah Eno went over what exactly happens to the scintillators.
There are 3 components of the plastic scintillators: Polystyrene, Primary Dopants, and Secondary Dopants. The Polystyrene is the component that is most affected by the radiation. When the large string molecules are hit by the radiation particles, the molecules are split. The split parts of the molecules are called radicals. The radicals move around in the substance and reconnect with other radicals when they happen upon them. However, sometimes when they connect with other radicals, they don't make the same molecule as before, changing the molecular structure of the material, causing the scintillator to change color as the molecular structure changes. Sometimes the molecules don't reform back into Polystyrene, they form Polyene. When these radicals don't form back into the original plastic, they create color centers, which are parts of the plastic where the light is absorbed and not emitted, turning the plastic a sort of yellow color in that area. These color centers cause a reduction in photon yield over time.
But what do the scintillators do exactly? Scintillators create a spark of light when a particle interacts with it. The light is then absorbed by the scintillator and emitted at a longer wavelength. The difference in the peaks of the average wavelength absorbed and emitted is called Stoke's Shift. However, with just the base plastic absorbing and emitting the light, the shift isn't large enough, and a large chunk of the absorption overlaps with the emission, causing some of the emitted light to be reabsorbed into the plastic. This causes a reduction in the possible photon yield. Since the largest photon yield possible is prefered, fluors (dopants) are added. The fluors take the emitted light from the plastic, absorb it, and then re-emit that light at a longer wavelength again. There are primary fluors which are about 1% of the scintillators and secondary fluors which are about 0.05% of the fluors. The fluors create a big enough Stokes Shift so that no emitted light is being reabsorbed into the plastic, creating the biggest photon yield possible.
02/23/2017
I completed the Radiation Training to do. All there is left to do is complete the take home test, and then I will be certified to work with radiation. In the training course, we learned about all the different types of radiation, but mostly ionizing radiation, which consists of a tiny bit of UV light, and then x-ray and gamma waves. We also touched on alpha and beta radiation.
Alpha radiation can do quite a lot of damage, however, it is very unlikely because it only travels about a centimeter, and can not penetrate the skin. Alpha radiation is easily stopped by something as thin as a piece of paper.
Beta radiation is the emission of a beta particle when a neutron changes in to a proton. A beta particle can also be called an electron. (I'm not exactly sure how an electron is emitted in this type of decay because a neutron is made of two down quarks and one up quark, and when shifting to a proton, the quarks change to two up quarks and one down quarks. I don't really know where the electron comes from.)
I've done a little bit of research in to the previous speculation. I believe that when a neutron changes into a proton, the change of a down quark to an up quark released a W boson, which then decays into an electron and an electron neutrino. Oppositely, if a proton is decaying into a neutron, the change of an up quark to a down quark released a W boson which decays into a positron and an electron neutrino.
Any person is not allowed to exceed 5rads/year, and if they do reach that level, they are not allowed to continue any radiation work for the rest of the year. The level is lower for people under the age of 18 and pregnant women.
In order to track how much radiation is coming from a substance or material, we use a Geiger-Muller detector, which measures the radiation as counts per minute, which would be decays per minute. To track how much radiation we individually get, we use a dosimeter, which we will have to order. However, we're only working with an element that produces alpha radiation, which won't penetrate the skin, so it's not really dangerous.
Radiation can be determined by the inverse square law. If you are standing near a source and double your distance between you and the source, the radiation that you receive will be decreased by a factor of 4. Oppositely, if you stand near a source and then half your distance between you and the source, you quadruple your radiation.
There is a unit called Banana Equivalent Unit (BED) which is a unit that equivalent to the amount of radiation found in a banana, which is an incredibly small amount of radiation.
02/24/2017
- Weekly Friday meeting. We were told to get the Wunderlist app and sign up so that we could be added. Wunderlist is an app which helps us keep a checklist of the measurements that we need to do and the measurements that have been done.
- Emission and Alpha Source measurements are done in labs in PSC while the Absorption measurements are done in the Chemical and Nuclear Engineering building.
- There are two main sizes of the plastic samples: Finger tiles(1x1x5cm) and sigma tiles. Each sample gets three different measurements (Emission, Absorption, and Alpha Source) at three different temperatures (-30C, -20C, -10C).
- Measuring two different plastics: EJ200 and EJ260.
- EJ200: Color centers are created which absorb light. Blue light is absorbed and therefore the plastic turns yellow. It has a higher light output and goes bad faster.
- EJ260: (green range) this plastic doesn't create color centers because it has a different wavelength. It has a lower light output and goes bad slower.
- ELJEN are commercial scintillators (ELJEN Technologies).
NOTATION/TERMS:
- PS: Polystyrene
- Fluors (fluoropolymer):
1) Two fluors: shift wavelength to longer wavelength to match PMT (photomultiplier tube) to where the PMT is most sensitive
2) First fluor transfers to 350nm
3) Second shifts to 450nm (24%) more sensitive
- PVT: polyvinyl toluene (structured like a ball of string, not like a lattice)
- SCSN81: polystyrene based material
- EJ200-SP: specially made; NOT the same as PS
- 3 components of the scintillator (percentage in weight):
1) Base (100%)
2) Primary fluors (1%) = P
3) Secondary fluors (0.01%) = X
- ”1X1P” coefficient is the percent; so 1X2P would be double the concentration of the primary
- Thickness labels (variations T#)
- Effects of geometry and dimensions
- Nominal doping- 1X1P (nominal= standard/control)
- CRF: near the beam pipe; high energy gamma source
- We will be working with beta and gamma category of radiation
03/03/2017
Weekly Friday meeting. We went over what people have been doing over the past week. Some people did measurements and some people have been learning C++ or learning to use GEANT.
After the meeting I went down to the lab and was trained in Emission measurements. We must use gloves to handle the samples and only take one sample out at a time because the samples themselves are not labeled, only the paper that they are wrapped in. We place them into the machine and use a program to put a wavelength through the plastic and measure the emitted wave, with which we take the peak measurements in a notebook that is kept in the lab.
03/08/2017
Doing some reading on the subject.
Important Notations:
- Ec - Critical Energy for electrons (MeV)
- Eµc - Critical Energy for Muons (GeV)
- X0 - radiation length (g cm-2)
- k - Bremsstrahlung photon energy (MeV)
- δ(βγ) - density effect correction to ionization energy loss
- W - energy transfer to an electron in a single collision (MeV)
- A - atomic mass of absorber
- Z - atomic number of absorber
- z - charge number of incident particles
- E - incident particle energy γMc2 (MeV)
- T - kinetic energy, (γ − 1)Mc2 (MeV)
- M - incident particle mass (Mev/c2)
- mec2 - electron mass * c2 (0.510998928 MeV)
- re - classical electron radius e2/4πǫ0mec2 (2.8179403267 fm) (where ǫ is epsilon)
- NA - Avogrado's number (6.022 141 29 × 1023 mol−1)
- Ne - electron density ((units of re)-3)
Equation for maximum energy transfer in a single collision:
Wmax = (2mec2β2γ2)/[1 + 2γme/M + (me/M)2)
03/10/2017
Weekly Friday meeting. Went through the progress that everyone has.
03/13/2017
Met with Professor Belloni with a bunch of questions about particles and scintillators. Here are a couple questions and their answers:
1. What are binary and ternary solutions of Fluors?
- They are just the second and third levels of the scintillator. The binary fluor is about 1% of the scintillator and the ternary is about 0.05% of the scintillator.
2. What are geometry effects?
- The geometry effects are the differences between measurements of 1x1x5 pieces of plastic and a 10x10x.4 piece of plastic with a scintillating fiber.
Notes on Plastic Scintillators:
- 3 types of scintillators: crystalline, liquid, plastic.
- Scintillators use the ionization of particles to generate photons (in a blue to green wavelength region).
- Plastic scintillators are the most common and densities for plastic scintillators range between 1.03 and 1.20 g cm^-3. Their yields are about 1 photon per 100 eV of energy deposit
- Transport efficiency affects the photo-electron signal
- Scintillators do not respond linearly to ionization density. Denser ones emit less light.
- Birk's formula:
- Plastic scintillators are sensitive to proton recoils from neutrons.
- Plastic scintillators are common because of their low cost and they're easy to manipulate into different shapes and are used in tracking and calorimetry.
- Polystyrene (PS) and Polyvinyltoluene (PVT) have aromatic rings which are good for scintillating.
- The charged particle moves through the matter and releases energy as optical photons. The scintillators get excited by absorbing the photon and the de-excited by emitting a longer wavelength photon.
- Stokes shift is the shift between the wavelength of the absorbed wave and the emitted wave. A larger stokes shift is desirable because it means less self absorption.
- Fluors are used as "waveshifters" to shift the scintillation light to a more convenient wavelength.
- There is a small overlap between the distributions of absorption and emission and therefore some of the emitted light can be reabsorbed. This is called "self-absorption".
- Plastic scintillators used in high energy physics are binary or ternary solution of selected fluors in a plastic base containing aromatic rings.
- Most scintillators are Polystrene or Polyvinyltoluene which can be p to 5% brighter.
- The ionization in the plastic base produces UV photons with a shorter attenuation length
- Can get longer attenuation lengths by dissolving a primary fluor into the base, which re-radiates absorbed energy at wavelengths where the base is more transparent.
- A primary fluor can decrease decay time in pure polystyrene. which increases total light yield.
- The average distance between a fluor molecule and an excited base unit is around 100 A, much less than a wavelength of light. At these distances the predominant mode of energy transfer from base to fluor is not the radiation of a photon, but a resonant dipole-dipole interaction.
- This is coupling between the base and the fluor
- Strong coupling greatly increases speed and light yield of the plastic scintillators.
- Scintillation light is captured by a lightpipe comprising a wave-shifting fluor dissolved in a nonscintillating base. ???????????????
- Craze: developing microcracks on the surface of the plastic that can interfere with the light by internally reflecting light.
- Crazing is likely from oils, solvents, or fingerprints coming in contact with the surface.
- Decrease light yield with increased pressure of oxygen
- Light yield may or may not be changed by a magnetic field.
- Radiation with the scintillators causes color centers which absorb light strongly in UV and blue wavelengths
- Damage depends on dose, dose rate, atmosphere, temperature, and material
- Color centers are less disruptive at longer wavelengths
03/18/2017
I feel that following up on the types of radiation would be beneficial, since they can be kind of confusing, especially since a couple of them are pretty similar.
Alpha Decay Radiation: An unstable element will try to become more stable by emitting alpha radiation. This is known as alpha decay. An alpha particle consist of 2 protons and 2 neutrons, the equivalent of a helium nucleus. The unstable element will continue to emit alpha particles until it becomes stable. Alpha radiation is incredibly weak, and therefore, is not very dangerous. It cannot penetrate the skin, and can be completely blocked by a piece of paper.
Beta Decay Radiation: In the nucleus of an atom, a neutron will change into a proton. In other words, the neutron, which is made up of 2 down quarks, and 1 up quark, will have ones of its down quarks change into an up quark, creating a proton, which is made up of 2 up quarks and 1 down quark. The change from a neutron to a proton emits a beta particle and an electron neutrino. A beta particle is just an electron (or positron), however, it is called a beta particle to differentiate it from the electrons orbiting the nucleus. Beta particles can travel a few meters in air, but can be stopped by a small piece of plastic or metal. It is mostly dangerous if it is ingested.
Bremsstrahlung Radiation: This radiation is caused by an acceleration of a
particle. Generally. an electron will pass by a nucleus, causing it to change the direction of its path. Because of this change in direction, there is a negative acceleration in the direction that the particle was initially traveling in. Because of conservation of energy, this energy is then given off as radiation. That, my friends, is Bremsstrahlung.
Gamma Radiation: This type of radiation occurs when a high energy, unstable nucleus tries to become stable. To do this, it will give off energy as photons, or gamma rays. Gamma rays have no mass and therefore can travel much farther than the other radiations. It is much harder to stop gamma rays but can be done so with a thick piece of lead.
X-Rays: This type of radiation is given off if an electron occupies an orbit shell that is greater than the ground state, putting the atom into an excited state. Ground state is reached by the electron moving to the inner orbital shell, stabilizing itself and giving off a photon.
Both gamma radiation and x-rays are ionizing radiation. This means that the radiation has enough energy to knock an electron out of orbit, therefore ionizing it.
04/01/2017
I talked to Lorraine DeSalvo on the first floor about getting a key to give me access to the measurement rooms and the undergraduate office. Thankfully he had a key left to give me.
04/03/2017
I practiced emission measurements for a few hours with Yongbin. He had me pull up the program by myself and measure the reference samples so that they were consistent on all the sides.
04/04/2017
I did emission measurements again with Yongbin. Same thing as the day prior to this one.
04/05/2017
Emission measurements again with Yongbin. He certified me to do measurements by myself now.
04/11/2017
Today I did my first measurements of emission. I had someone from the group warm up the machine a couple hours prior to when I would be taking measurements.
Measurements are take in spectrofluorometer. This machine shoots intensive light at the samples so that it excites the greatest amount of
particles inside the samples at once. The sample then emits light, and the emitted light is measured 90 degrees from the incident light. The angle is 90 degrees so that none of the incident light is measured as emitted light. There is a photomultiplier that measures the incident light photo yield, and a photomultiplier that measures the emission photon yield. For these measurements, we mainly take a look at the ratio between these two measurements.
For these measurements, we have sets that were irradiated and stored at different temperatures that are the main measurements. This is to see how temperature may affect the healing of the plastics. Before the irradiated samples can be measured, a reference sample and then the sample that is the same as the irradiated one, but unirradiated, must be measured. This way we can ensure consistency of measurements, and see how the radiation and affected the emission light of the scintillator. In the photo on the right, the sample called EJ200-2X03 is the reference sample for the EJ200 scintillators. The next sample, EJ200PS_1x2P_1, is the scintillator with a base of polystyrene that is unirradiated. The EJ200PS_1X2P_10 is the EJ200 scintillator made of polystyrene that has been irradiated.
Each of these plastics are 1x1x5cm. 2 sides of each sample is measured to ensure consistency. Generally, a good rule of thumb is that the measurements between the sides are approximately within 3% of each other. If they aren't the measurements should be repeated.
Also, the plastics should be handled with gloves only so that no oils from the skin get on the samples and affect their emission.
04/17/2017
Today I took another set of emission measurements. Here is a photo of what the EJ200 scintillator samples look like, as well as an image of the graphs that the program on the computer produces from the information given by the spectrofluorometer.
04/20/2017
Another day, another measurement! I've also attached a photo of an EJ260 type scintillator, which looks green. In addition, attached is a photo from the top of the irradiated EJ200 scintillator. If looking carefully, a yellowish tint can be seen. Before these measurements
SUMMER SEMESTER 2017
05/11/2017
Today I had a meeting with Professor Drew Baden and two other undergraduates. We will be working with Professor Baden on testing circuit boards and building a cosmic ray telescope.
REFERENCES:
1. http://www.particleadventure.org/ - This first link is to a website with some modules. It gives a good, more simple, explanation of the Standard Model, Accelerators and Particle Detectors, the Higgs Boson, and Particle Decay and Annihilation.
2. http://pdg.lbl.gov/2012/reviews/rpp2012-rev-passage-particles-matter.pdf - A published paper on accelerators and detectors.
3. http://iopscience.iop.org/article/10.1088/1748-0221/3/08/S08004/pdf - A paper on the CMS detector.
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