Many accelerator-based particle and nuclear physics experiments make use of apparatuses called spectrometers to measure the momenta of particles coming out of collisions. Spectrometers function much like a prism, but instead of bending the trajectories of light based on wavelength, they use powerful superconducting electromagnets to bend the trajectories of charged particles based on their momenta.

Physicists use tracking detectors to reconstruct particles’ trajectories. Then, using measurements and models of the strengths of the magnetic fields that the particles pass through, they calculate their momenta.

As a graduate student, I designed, assembled, tested, and installed tracking detectors called gas electron multipliers (GEMs). GEMs are typically thin planes, a few centimeters thick and covering hundreds or thousands of square centimeters, filled with a noble gas like argon. When a charged particle passes through the detector, it ionizes some of the gas molecules. Then, electric fields generated by a cathode plane and a few thin kapton foils coated on both sides with copper accelerates the ionized electrons to the back plane of the detector. As they make their way to the back, these electrons ionize more of the gas, createing a cascade of further ionizations. The resulting narrow cloud of electrons is collected by a lattice of thin copper strips on the back readout board. The strips are connected to analog-to-digital converters that record which strips collected electrons and how much charge they collected.

Analysis software then analyzes the distribution of the collected charges to estimate the location of the original particle that passed through the GEM plane to a precision of approximately 70 micrometers. With two or more GEMs placed back-to-back, software can reconstruct a particle’s three-dimensional trajectory.

Working with Huong Nguyen, I assembled and tested a dozen GEM detectors for the upcoming set of experiments at JLab that will utilize the Super BigBite Spectrometer.

I helped develop a diagnostic GUI that displays raw digitized data from sets of GEMs. It displays signal amplitudes as a function of strip number and time sample. The layout likely looks esoteric to the layperson, but I swear it’s informative to GEM experts!

Based on my experience assembling GEMs for the Super BigBite Spectrometer, I worked with Kondo Gnanvo to design prototype GEMs for spectrometers still in the design phase at JLab and BNL. We built and tested one such prototype in 2018 at Fermilab’s test beam facility.