Wednesday, November 16, 2016

Working on the High-Temperature Electromagnetic Calorimeter at Jefferson Labs

            As the end of the fall semester comes closer, something else draws near: the deadlines to apply for internships for this coming summer. You might think summer is still far away, but you need to start now if you want to make the most of it. There are so many options for doing research over the summer, whether to buff up your resume or just start getting involved in your field. You can apply for a Science Undergraduate Laboratory Internship (SULI), or for a Research Experience for Undergraduates (REU) program. If you are curious whether doing an internship over the summer is worth it, I have to tell you that it truly is.
Me with parts for lead glass electromagnetic calorimeter.
            I applied for the SULI program in late 2015, I found out that I had been accepted into the program during the early spring of 2016. I was allowed to work at Jefferson Labs during the summer, helping Bogdan Wojtsekhowski to construct a new calorimeter.
            Calorimeters are a key instrument for studying particles, and a variety of different calorimeters are used in institutions all over the world. Different calorimeters have their advantages and disadvantages, based on the materials used, the geometry or design of the calorimeter, etc. At Jefferson Labs, work is being done to mitigate the disadvantages of a specific type of calorimeter: a homogeneous electromagnetic calorimeter, constructed from lead glass.
            A homogenous electromagnetic calorimeter constructed from lead glass is needed for investigating the nucleon structure via the study of a proton elastic electric form factor. However, there is a problem with using lead glass in calorimeters. Over time, as the lead glass is exposed to radiation, it becomes discolored. This discoloration negatively impacts the performance of the calorimeter. When the lead glass is heated up, though, it becomes clear once again.
            Bogdan Wojtsekhowski of Jefferson Labs came up with the novel idea of operating the calorimeter at a high temperature in order to prevent discoloration to the lead glass. The new, high temperature EM calorimeter is still being tested, with small prototypes being made first to make sure the idea works. Each prototype is constructed larger than the previous, to ensure that the designs can be scaled up without any undesirable effects.
            My role at Jefferson Labs was to resolve the two main problems being encountered while constructing the calorimeter. First, the light guides which were glued to the lead-glass were developing strain and snapping off. This led to the second problem: when the light guides broke off from the lead-glass, the residual epoxy was extremely resilient and difficult to remove.

            The epoxy removal was the easier issue to resolve. The adhesive, Eccobond F202 Bipax, had a max service temperature of approximately 240°C. Since the lead-glass and light guides do not soften until reaching temperatures in excess of 500°C, I placed the materials with residual epoxy in an oven set to 340°C for 6 hours. This caused the adhesive to become discolored and brittle, making it easy to chip off using a razor blade and blunt object. This was done after applying alcohol to the epoxy to prevent the chips from flying, and the razor blade was kept at a shallow angle to prevent damage to the glass.
            Preventing strain in the light guides required more work. Strain develops in glass as a result of improper cooling. If the outer part of the glass is allowed to cool too quickly, it solidifies around the still soft middle. Then, when the middle cools, it expands into to already solid out part. This is what causes the glass to develop strain. In order to fix this, the light guides had to be annealed, a process which involves reheating the glass to relieve stress within the glass and then cooling it down uniformly. The light guides, being made of Borosilicate +33, needed to be annealed according to the parameters in the table to the left while using a ramp rate of 10°C/min. In order to heat large quantities of light guides at these temperatures, I had to design an apparatus to hold the light guides inside the oven, and get permission for them to be put through an ultra-high vacuum, or UHV, oven. In order to prevent contamination of the oven, I had to wipe down every light guide that would be heated, as well as the apparatus that they would be stored in.
            As part of my internship, I was required to create a poster regarding my work, and write a more in-depth paper detailing what I had done. More in depth information on the work regarding the high-temp. EM calorimeter, can be found in Bogdan Wojtsekhowski’s February 11, 2016 report. The calorimeter is part of Jefferson Lab’s Super Bigbite Spectrometer, and you can find more information on it at their website.
            Through the SULI program, I got hands-on experience in an actual lab, learned a lot from professional physicists, and met many other undergraduates from around the world who shared a passion for physics.