Hi there, another female physicist here! I was wondering what your research focuses on? I didn't get to attend cuwip this year, so I missed out on hearing from all the women in the field.
My research group is working on constructing a liquid organic scintillator fast neutron spectrometer for the purpose of creating a detector that can filter out thermalized neutron signals from an actual dark matter detector. I’m too early on in physics to have the proper nuclear/particle physics background, but uncharged Weakly Interacting Massive Particles (WIMPs) are a supposed candidate for dark matter. Thus, one thing of great importance for our detector is that we are ruling out false positive signals from uncharged neutrons that can enter the detector.
My part of the research specifically focuses on the chemistry of the detector (for now; I will be moving onto software soon!).
First of all, what even is a scintillator? It’s a mixture of an aromatic solvent, wave shifters, and other chemicals that allow us to detect the presence of particles through light signals produced within the scintillating solvent. In our case, a fast neutron enters the scintillator and collides with nuclei (any nucleus, but most likely the solvent nuclei). This collision, based on classical kinematics, kicks off a proton of similar mass that travels through the scintillator and interacts with pi-bonding orbitals in the aromatic solvent, exciting then de-exciting those electrons, which gives off a pulse of light that can be detected by the electronics of the detector via PhotoMultiplier Tubes (PMTs).
So we get a light pulse signal from this nuclear recoil reaction of the solvent. But that signal can be ambiguous; other types of particles can produce similar signals which could lead to false positives. So we enrich the scintillator with an element like Li-6, because in the nuclear recoil reaction that occurs with Li-6 a very specific pulse in the MeV range is given off. When we have these two pulses happen in coincidence (one after the other) we can say that the particle we detected was indeed a fast neutron.
I spent most of the summer working on synthesizing three different chain lengths of the lithium carboxylates I wanted to use, only because they cannot be bought. From there I confirmed my structures via IR Spectroscopy, making sure there was no water in my samples. That’s incredibly important because in the case of a completely hydrophobic scintillator cocktail, any additional water would form an emulsion with the surfactants in the scintillator which would lead to reduced optical clarity of the scintillator.
The surfactants in the scintillator I was using were only there because it is more commonplace to use something like LiCl to dissolve the lithium into the scintillator. Another guy in my research group has done this. It works okay, but there are issues with proper dispersion of the LiCl in the tube (due to solubility, even with the surfactants) and discoloration from quenching of the Cl.
Optical clarity (i.e., how clear the scintillator is) is an important aspect of our detector because the signal used to detect the presence of the neutrons we’re looking for is light. If the scintillator is foggy (not optically clear) then the light can’t travel through the scintillator and get picked up by the PMTs. Having perfect clarity isn’t as important for a small detector like we’re making (it’s much more important in detectors that are meters across) but we want the best we can get so long as we can also have good light output and pulse shape discrimination. Because if we can’t get enough light from the scintillator and it doesn’t create clear enough pulses, our signals will be lost in the background noise.
An additional problem that optical clarity poses is in relation to our PMTs. The ones we have are refurbished from another project, so we got what we got. Their sensitive range is between 290-650 nm, which might seem fine if you look at the UV-Vis spectra below. But the highest sensitivity of our PMTs is a bell curved peaking around 420 nm, where those big drop offs on the UV-Vis plot occur. So even if, in principle, it’s not important for a small detector to be perfectly clear, given the specific sensitivity of our PMTs, we need better optical clarity to successfully detect a signal in the first place.
I’ve made some solutions of the various lithium carboxylates in the scintillator fluid Ultima Gold AB (which is diisopropyl naphthalene-rich) and so far the only one that dissolves clearly is lithium dodecanoate (12C chain), with the highest transmittance of light given by a 0.1M sample. We need a lithium-loading of closer to 1M for this to even compare to the LiCl scintillator. So for the spring semester my PI ordered some new scintillator that’s similar to Ultima Gold AB but without the surfactants. Which is a great idea because I don’t even need the surfactants. That’ll hopefully allow me to dissolve more lithium into the scintillator given the freed up space (which is especially important because my compound is kinda big) and allow for more optical clarity.
We’ll see how it goes this semester!