Cavity Ring-Down Absorption Spectrometer

Research Objectives The team is involved in designing, building, and commissioning a trace gas analyzer based on Cavity Ring-Down Spectroscopy (CRDS) technology to measure ultra-low levels of trace impurities in noble gases. The BHSU analyzer will improve on the sensitivity of any commercial apparatus by at least a factor of ten in order to meet the requirements of several dark matter and neutrino experiments proposed for DUSEL, including DArCSide, LUX, and others.

How it works The BHSU CRDS system will measure the decay rate of a laser pulse trapped in a highly reflective cavity. The laser pulse will reflect back and forth thousands of times at the speed of light between two super mirrors giving the instrument an effective path length of greater than 100km, which makes the instrument extremely sensitive to very small concentrations of impurities.
The greater the concentration of impurities, the faster the beam will decay in intensity. The concept is similar to car headlights in fog vs a clear night. The more fog particles that there are in the air, the more the light is absorbed. As the laser particles are absorbed by the impurities, less light will make it to the detector each pass. The decreasing amout of light for each pass is measured as decay rate.
The laser will be tuned using an acousto-optic modulator (AOM) to an infra-red wavelength that will be absorbed by a specific impurity. The AOM uses soundwaves pushed through a piece of glass or plastic to block and release the laser light at a specific frequency to create the desired wavelength of light. This concept is similar to talking into a house fan, as the blades turn, they create pressure waves that block and release the sound of your voice to produce a chopped up signal on the other side of the fan.
The impurities we will be looking for are; oxygen gas, nitrogen gas, and water vapor. Each of these reduce the light produced from collisions of astroparticles with the argon. The impurities will be measured at the parts-per-trillion (ppt) level. Nitrogen gas does not have near infrared absorption bands which means that the nitrogen gas may need to be converted to Ammonia gas for detection.

**DArCSide project at DUSEL**

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