Ongoing Work at RPI

Radon Injection Chamber

Currently, the Brown group's primary laboratory project is the construction of a test stand to study and develop the calibration strategy for the nEXO experiment.

To be able to resolve neutrinoless double beta decay events, the detector requires very fine energy resolution- less than 1% at the Q-value for double beta decay of xenon-136, or the theorized characteristic energy for neutrinoless double beta decay of xenon-136. In addition, it is necessary to quantify the spatial non-uniformity of the detector's signal collection efficiency, which is achieved through the construction of light and charge maps. Previous generations of neutrinoless double beta decay detectors could be calibrated externally, but nEXO, due to its immensely large target, cannot be effectively calibrated this way due to self-shielding; the outer edges of the detector itself shield the innermost regions from external radiation, so it is not possible to develop light and charge maps of the entire detector using this strategy. For this reason, novel calibration techniques are required for nEXO to be able to meet its research goals.

One promising method is the use of internal calibration sources. Injected sources would be able to mix throughout the xenon and eventually penetrate the detector center making them a nice complement to external sources as long as radioactive considerations are made (ie. the source does not impair data taking by contributing to background near the experimental Q-value). Our group focuses on radon-220 - an ideal source choice as radon and its daughters produce clear signals that will characterize light response thus allowing for the improvement of energy resolution of the detector. Additionally, it ensures minimal possible background contamination as it reaches its final, stable state of lead-208 within several half-lives of the 10hr lead-212. 

Through the development of a small-scale radon injection chamber our work aims to understand how the full radon-220 decay chain moves in xenon by monitoring alpha activity within a single phase detector. These high energy alphas yield well-defined signatures due to their dominant ratio of light to charge - a feature that will allow for the construction of a light map of the detector. Complementary to the experimental work, a Monte Carlo model is being developed by to parameterize the motion of radon and its daughters in gaseous and liquid xenon. Using the data from our test stand, isotope-dependent diffusion and plateout constants will be constrained and the impacts of turbulent and laminar flow will be addressed. A custom xenon condenser line will allow for the liquid xenon levels, flow rate, and pipe diameter to be optimized. Using radon injection pulses and long term mixing both early and late alphas will be able to be tagged and delayed Bi-Po coincidence will be employed to distinguish events. Upon completion the results of this work will be able to inform the design of nEXO and its calibration scheme through recommending the use of dissolved calibration sources to create a light map with less than 1% error as required for nEXO's energy resolution and detailing the length of calibration runs needed to obtain a useful number of alpha events.

The test stand at RPI follows the schematic shown below. For the purpose of this work, a solid thorium-228 generator that emanates gaseous radon-220 is used as a radon source. Using an on/off control the xenon can be routed through the mixing chamber or bypass it completely. During an experimental run, doped gaseous xenon enters a custom copper condenser line that is thermally linked to a heat exchanger that is held steady at cryogenic temperatures. As the xenon passes through the condenser line, it liquefies and then enters the detector vessel. Inside the detector vessel is a single Hamamatsu  VUV-sensitive photodiode that detects scintillation light and sends signals back to our data acquisition system. After passing through the detector chamber, the xenon then continues on through our gas system where it can recirculate or be recollected into the gas bottle. 

Paleodetector

The use of paleodetectors in the search for dark matter has been introduced as a way to reach a large exposure to rare collisions between dark matter and normal matter by looking at a relatively small amount of material that has been present over the order billion year timescale of the earth. The nuclear recoils predicted in these interactions are assumed to leave permanent damage in ancient minerals. 

In our group we aim to test this theoretically proposed detection method for experimental feasibility. To do this, we look for observations of nuclear recoil induced lattice damage in archeological samples of olivine using transmission electron microscopy. Calibration samples are exposed to a neutron beam, and the images are compared to a non-irradiated sample. We hope our results will motivate the development of paleodetectors for dark matter searches, and provide a feasible experimental method for imaging the features expected from dark matter interactions in ancient minerals.

NEST

The Brown group is also involved with simulation work through the Noble Element Simulation Technique (NEST) collaboration. NEST is a publicly available C++ package that models the scintillation and ionization signals in LXe and LAr detectors using a combination of empirical and first-principles methods. A python version of the code (aptly named NESTpy) is also available for public use. The collaboration website and access to the code can be found at: https://nest.physics.ucdavis.edu/. 

The NEST group at RPI is specifically interested in creating a more adaptable version of NEST that is effective for a wider array of experiments including those that observe differing W-values (the amount of energy needed to create a quantum of charge or light) and those interested in higher field regimes such as nEXO. To do this, our group is working to implement the modified Thomas-Imel box model (TIB) into NEST while incorporating various real world datasets including EXO-200, LUX, XENON1T, and others.