Liquid noble-gas detectors (LNGD) are widely used or are planned to be used in many different particle physics applications, including dark matter (DARWIN [1] ), neutrino tracking (DUNE [2] ), and neutrinoless double beta decay (0νββ) (GERDA [3, 4] , LEGEND [5] ). When radiation interacts in a LNGD, it produces vacuum ultraviolet (VUV) scintillation light (Fig. 1 ) as well as ionization electrons. These two energy deposition processes, combined with an electric field, enable TPC detectors to achieve energy resolution of a few percent and full three-dimensional track reconstruction. As WIMP dark matter detectors, they can discriminate between nuclear recoils signals and electronic recoil backgrounds with a rejection power of at least 10 8 [6, 7] . Without an applied electric field, the scintillation light yield is enhanced which still allows for pulse shape discrimination to effectively discriminate against backgrounds [8] .
Although He and Ne have been considered [9] , Ar and Xe monolithic LNGDs have advantages due to the large scintillation yields, longer scintillation wavelengths, and fast scintillation time constants. For the liquid Ar (LAr) case, the boiling temperature is near the preferred operating temperature of Ge detectors and Ar can provide substantial radiation shielding. These features led to the GERDA concept of operating Ge detectors bare within a LAr environment [3] . By deploying bare Ge crystals, the mounting and readout infrastructure could be minimized reducing backgrounds associated with that hardware. An added benefit is the response of LAr to radiation. Since 0νββ is a single-site energy deposit process, only background can produce a coincident response between the Ge and LAr. GERDA used this anti-coincident technique to great advantage achieving the lowest background index of any 0νββ experiment [4] . If the light detection of the LAr in GERDA could be significantly enhanced, it would result in improved background rejection. Doping the LAr with Xe is one technique to improve the light detection and is being considered for LEGEND-1000. However, a Xe-doped LAr (XeDLAr) volume of sufficient size to demonstrate its use for LEGEND has not yet been demonstrated. It is important to demonstrate that the doping can be done at a large scale in a controlled manner, and to determine if XeDLAr mixture is stable.
Doping LAr with 0.1-1000 ppm of Xe [11] , (10-1000ppm) [12, 13, 14, 15, 16] shifts the wavelength of emitted scintillation light from 128 nm, where LAr scintillates, to 175 nm where Xe scintillates. This wavelength shift is advantageous for several reasons. Argon has a long attenuation length at 175 nm [15] but only a 50-60 cm attenuation length at 128 nm [17, 16] . Additionally, devices sensitive to detect both 128 nm and 175 nm are more sensitive at 175 nm. In order to detect the 128 nm light with conventional photo-multipliers (PMT), the light must first be wavelength shifted. The coupling of light detection devices to wavelength shifting materials (e.g. TPB [18] ) typically occurs at the surface of the device, and therefore nearly half the light does not directly enter the PMT because the secondary re-emission process is isotropic. In contrast, the XeDLAr wavelength shifting occurs Figure 1 : Scintillation spectrum for all liquid noble gases, excluding radon. Dotted lines represent opacity for several window materials used to observed this light. Credit: [10] at the interaction point. Pulse shape discrimination with XeDLAr is still possible, though LAr is still superior. If all photons are collected, pulse shape discrimination becomes worse as a function of Xe concentration [13, 11] . If a fused silica filter is used to remove wavelengths below 160 nm, pulse shape discrimination improves with Xe concentration above 250 ppm [12] . At concentrations of 250 ppm Xe, a fast component emerges in the Xe spectrum that helps recover the pulse shape discrimination.
Obviously there are many advantages to a XeDLAr LNGD but in order to accurately quantify them, XeDLAr needs to be demonstrated on a large volume. The largest active volume previously used was 13.6 l [13] , with most experiments using volumes of order 1 l or less. If XeDLAr is to replace pure LAr in any future experiment, Xe doping needs to be demonstrated on a large scale. One concern is the possibility of clumping: reference [16] claims to measure clumping at Xe concentrations of 3%, while a chemical analysis shows that Xe is 16% soluble in LAr at 87 K [19] . Another concern is the stability of the mixture, since Xe has a much higher freezing point than Ar (161.40 K versus 83.81 K).
Xenon doping was performed in previous smaller experiments using one of two methods. Either a warm argon-xenon mixture was injected into the liquid volume, or the mixture was prepared in the gaseous phase and then condensed into liquid. The warm gas injection technique will not work on large scale experiments because xenon homogeneity will take a long time to achieve, while a pre-mixed gas is not feasible due to the large gas storage volumes required. Additionally, these doping techniques result in large totalconcentration uncertainties making reported scintillation time constants difficult to compare. The smallest doping concentration uncertainty reported to date is 23.5% [13] . New injection techniques need to be considering if xenon doping is to be pursued on a large scale.
A large Xe doping test stand is also critical for understanding the effects of the long attenuation length at 175 nm in XeDLAr. This long attenuation length can result in an increase in light yield as observed previously by Ref. [13] (×1.4 increase at 300±80 ppm) and Ref. [14] (×1.25 increase at 1093±547 ppm). The discrepancies in these increased light yield measurements can be attributed to different detectors sizes, with the larger increase being measured in the larger detector. Although the attenuation length was measured by Ref. [16] , it was predicted by Ref. [20] to be at least an order of magnitude longer. Additionally, the two attenuation length measurements for a Xe concentration of 3% made by Ref. [16] , 170±23 cm and 118±10 cm, were not consistent with each other. Finally, some of these experiments [13] do not purify the Ar but rather rely upon the base purity of research grade Ar (99.9999%) provided by the vendor. Research grade Ar will produce less scintillation light than pure Ar and thus measuring the increase due to Xe doping is complicated due to the presence of unknown contaminates. Given the uncertainties in the previous work, a novel liquid argon purification system that can precisely dope large volumes is needed before XeDLAr can be assessed for a large experiment like LEGEND.
When ionizing radiation interacts with a noble gas, dimers form which, when relaxing back to the ground state, produce scintillation light. Scintillation light is emitted through the transition from one of the two lowest molecular excited states, 3 Σ + u (triplet) or 1 Σ + u (singlet), to the ground state 1 Σ + g . These states cannot be easily distinguished spectroscopically but do have different relaxation times. The 1 Σ + u state for liquid argon has a half life of around 4-7 ns while the 3 Σ + u state half lives for pure liquid argon have been measured to be between 1.2-1.6 µs ( [21] , [22] , [23] ). This large difference in measured 3 Σ + u state lifetimes for "pure" argon is likely due to small con-centrations (∼ 1 ppm) of residual atmospheric xenon 1 . The 1 Σ + u state for liquid xenon has a half life of 2-4 ns and the 3 Σ + u state 21-28 ns [25] . The main feature of the argon and xenon scintillation spectra is the Gaussian feature centered at 128 nm and 175 nm for argon and xenon respectively. The summed probability distribution function for the 1 Σ + u and 3 Σ + u components can be written as:
where τ s and τ t are the time constants for the 1 Σ + u and 3 Σ + u state respectively and A s and A t are the relative intensities of each components. The integral of I(t) over all time is normalized to unity and therefore A s + A t = 1.
When Xe is inserted into LAr, it quenches the Ar dimers through collisional de-excitation. Unlike other contaminates (e.g. N), this collision allows for effective energy transfer between the Ar dimer and Xe, allowing the Xe atom to create a dimer of its own, as:
Here collision refers to a molecular collision between a Xe atom and a Ar * 2 or ArXe * dimer. The VUV spectroscopy of this reaction as a function of Xe concentration can be seen in Fig. 2 . As the Xe concentration increases, the Ar dimer energy is more efficiently converted to the Xe dimer. Xenon and Ar have very different scintillation time profiles with the biggest difference being their 3 Σ + u state lifetimes (∼1 µs versus ∼30 ns). When Xe is injected into Ar at low concentrations, the 3 Σ + u state of the Ar is quenched by collisions, thus the time distribution can be described [12] :
where τ f and τ s are the decay times of the fast and slow components respectively, τ d is a time characterizing the conversion of Ar * 2 to Xe * 2 . A 1 , A 2 and Figure 2 : The VUV emission of electron-beam excited LAr doped with 0.1, 1, 10, 100, 1000 ppm Xe is shown. Note that at higher Xe concentrations, another peak at 147 nm appears [11] A 3 are the intensities of these three terms. In order to normalize Equation 3, the following relation is defined:
As the Xe concentration increases, the average collision rate between a Xe atom and an Ar dimer in the 1 Σ + u state approaches the Ar 1 Σ + u decay time (∼ 5 ns). To model this effect, an additional term must be added to Eqn. 3:
where τ df and τ ds characterize conversion times for the fast and slow components separately [12] . Additionally the following relation, A 1 +A 2 −A 3 −A 4 = 1, is needed to normalize Equation 4. These time constants have been measured by several groups and the summary of these results can be found in Ref. [12] . These measurements have unresolved tension, which is likely due to the Xe doping techniques and/or residual Xe or contaminants in the Ar.
Our stainless steel cryostat consists of two nested cylinders (inner (IV) and outer (OV) vessels) (Fig. 3) . The IV can hold approximately 100 l of liquid. The space between IV and OV is held near vacuum to prevent convective heating. An aluminized infrared-reflective wrapping (Fig. 3 , right) surrounds the inner vessel to reduce radiative heating from the warm outer can. The inner vessel is attached to the lid of the outer vessel with four G-10 [26] rods.
The vessel is drip cooled using an AL60 Cryomech cold head [27] providing approximately 60 W of cooling power at 80 K. The cold head is mounted on top of the OV and is connected to the IV through a thin walled, 2.75-in ConFlat bellows. The cold head drips condensed Ar down a thin stainless steel rod directly to the bottom of the inner vessel. The cold head is surrounded by a Teflon funnel, which is used to separate the warm and cold Ar gas. As Ar evaporates, it flows up the funnel toward the cold head, recondenses, and drips back into the liquid. Warm Ar gas is inserted above the cold head and falls as it cools. The cold head is instrumented with a 50 W heater and a thermocouple, which maintains the cold head at LAr temperature.
The inner vessel was filled from the ullage of a research grade LAr cylinder (99.999% pure). Both inner and outer vessels were leak checked prior to each fill. No thorough vessel pump and bake cycling was found necessary as a result of our re-circulation system described below. Gas was flowed through a SAES PS4-MT3/15-R getter, which can purify noble gasses to less than 1 ppb of all common contaminates, but not noble gases [28] . The quantity of Ar in the inner vessel is measured using a shipping scale accurate to ±0.5 lbs. The mass of LAr in the inner vessel is measured as the vessel filled at a liquification rate of 0.58 kg/hr. This implies a net cooling power of about 44 W, assuming the Ar gas started at a temperature of 300 K and is liquified at 87.15 K.
The gas doping system consists of a small measured cylindrical volume, 19±1 ml, connected to a supply gas bottle on one side and the recirculation system on the other (Fig. 4) . The doping gas is inserted at ∼1500 Torr. The gas pressure is measured using a high pressure Baratron gauge with a precision of 1 Torr. The pressure is chosen to be larger than the recirculation system pressure to ensure the gases mix adequately. The measured temperature of the lab room, after the doping system has reached equilibrium, is taken as the doping gas temperature. This temperature typically fluctuates about 2 kelvin around the set point of the room thermostat. This doping volume is evacuated before each doping using a dry scroll pump down to a pressure of 10 mTorr. By ensuring the use of high purity gases, molar concentrations of the dopant could be calculated using the ideal gas law. Each doping step has approximately the same statistical uncertainty due to insertion of gas at the same pressure and temperature. Hence, as the total number of doping cycles increased, the relative statistical error on the total concentration decreases, causing the total error to be dominated by the systematic error of the doping volume.
The inner vessel is instrumented with two 12-stage Hamamatsu, 3-in PMTs (R11065) [29] which are box&linear-focused, with Synthetic Silica 3-in windows and Bialkali photo-cathodes designed to operate at LAr temperatures. The PMT windows have a mounted tetraphenyl butadiene (TPB) coated acrylic disk to wavelength shift the Ar scintillation light into the sensitive region of the PMTs. The average quantum efficiency over the TPB emission spectrum is ∼29%, making it well suited for measuring VUV scintillation [? ] . TPB is vapor deposited to a thickness of about 2 µm. To prevent the PMTs from floating in the LAr, the TPB disks are tied down to the Al holders with stainless steel wire.
After completing an initial test run, it was found that the TPB on the acrylic disks had dislodged and redeposited on the wall of the cryostat. Though TPB has been shown to be stable in liquid argon [31] , it is speculated that the volatile boundary between the gas and liquid phases of the Ar can be abrasive to the vapor deposited TPB. It is not known what effect this has on the light yield. To ensure maximum light for each successive run, new TPB coated disks are used.
The PMT cathode ground scheme is chosen to allow signals and high voltage to share a cable, reducing the heat load from the wires. Each PMT is instrumented with a fiber optic cable and a blue LED is used for low light calibration of the PMTs. The conflat feed-throughs are mounted on the warm side of the cryostat to ensure thermal cycling did not damage the ceramic/steel interface. The PMT signals are amplified using a Phillips Scientific Octal Variable gain amplifier Nim Model 777 [32] . These signals are digitized using a Tetronix TDS 3052B digital oscilloscope and read out to a laptop using custom Python software. The scope has a slow read-out rate of ∼1 Hz though it can digitize at 5 GS/s allowing for 1 ns bins over 10 µs.
A pulse finding method was adapted from Ref. [33] , which uses a smoothed derivative method to identify signal pulses as large (3.5σ) fluctuations in the noise. Four threshold crossings of the derivative define a pulse. An example Figure 5 : A single photon pulse found using the pulse finding algorithm [33] . The blue line is the background noise while red line is the highlighted found pulse. The thick dashed green line is the derivative of the blue plus red lines (signal plus background). The two yellow lines are the 3.5 σ rms derivative thresholds for the crossings, with two crossings per threshold required for a pulse.
of this pulse finding can be seen in Fig. 5 . Local minima are identified using the derivative allowing individual pulses to be distinguished in the event of pulse pileup. This feature proved essential for our measurement of the triplet life of Ar.
Prior to measuring the triplet lifetime for pure argon, a distribution of the number of dark pulses is generated in order to evaluate the single photon response of the PMT. A double Gaussian distribution is fit to the noise peak and single photon peak. The Gaussian mean and sigma of the single photoelectron (SPE) peak characterize the single photon response of the PMT. From this fit, a gain of (4.64 ± 1.45) × 10 6 is estimated after accounting for the ×2.86 amplification of the pre-amp circuit. The pulse charge versus peak voltage can be seen in Fig. 6 
The 3 Σ + u lifetime can be measured by fitting equation 1 to a histogram of pulse arrival times weighted by their integrated charge. Pulse arrival time histograms were created for 40 runs containing 5,000 events each and triplet lifetime was measured for each run. After-pulsing was noticed in the data between 1 µs and 2.5µs. While effects of after-pulsing in PMTs are well characterized [34] , it cannot be reduced without decreasing the single photon resolution. Therefore, we investigated mitigating the after-pulsing effect on the LAr triplet lifetime using two fitting methods. In the first method, the triplet lifetime was found using a single exponential plus a constant fit. A start time of 2.5 µs was chosen in order to avoid the after-pulsing. A triplet lifetime of 1456 ± 38 ns was measured with this method. Alternatively, we fit a Landau shape to the first after-pulse and a Gaussian shape to the second after-pulse. These functions were added to the first function and the sum was fit starting from 1.75 µs. This method yields a systematically larger value of the triplet lifetime of 1502 ± 30ns. An example of these two different fitting methods can be seen in Fig. 7 . It is not obvious why the two methods yield different results and thus an average was taken as the true triplet lifetime, measured to be 1479 ± 38(stat) ± 23(sys) ns where the statistical uncertainty was taken to be the larger of the two different fitting method results and the systematic uncertainty was taken to be difference in the two fitted triplet lifetimes. Figure 7 : Two fits to the triplet scintillation distribution in pure LAr. The top plot is the fit with a single exponential and a constant from 2.5 µs to 10 µs and gives a value of 1456 ± 38 ns. The bottom plot shows the fit with a Landau shape for the first after-pulse and a Gaussian shape for the second after-pulse with an exponential and a constant fit the tail. The fit to the after-pulsing region is shown in the insert. This second fit gives a value of 1502 ± 30ns. We take the average value of 1479 ± 38(stat) ± 23(sys) ns where the statistical uncertainty was taken to be the larger of the two different fitting method results and the systematic uncertainty was taken to be difference in the two fit results.
In order to validate the gas doping and recirculation system, a small measured quantity of N 2 was injected using the methods described in Section 3. 12
Following the injection the N 2 was filtered out using the getter. Past Xe doping experiments [14] had large uncertainties (∼50%) on the injected quantity. The effects of N on Ar scintillation light are well known and have been characterized in Refs. [23] and [35] . The relationship between the measured triplet lifetime (τ ) and the true triplet life time (τ ) follows Birk's law:
where k is Birk's constant and C is the molar concentration of impurities measured in parts per million. Birk's constant has been measured to be 0.11 ppm −1 µs −1 and 0.13 ppm −1 µs −1 for N in LAr by the two previously mentioned authors. They obtained τ = 1.26 µs compared to our value of 1.48 µs. Reference [23] was aware that their τ was smaller than that typically measured ( [22] , 1.46 µs) and speculated that if they used the fitting method of Ref. [22] and extrapolated their τ using Eqn. 5 to pure LAr, their value becomes τ = 1.45 µs, which is in good agreement with our measurement. Before doping, the recirculation and purification system was run and τ measured frequently. Once the τ measurement was stable, it was assumed that the Ar was pure. The nitrogen contaminant was then inserted and τ decreased according Eqn. 5. As the Ar was purified, it is assumed that the contamination concentration decreased as:
where C 0 is the initial concentration measured in molar parts per million and τ f ilter is a fitted time constant characterizing the filtration process.
We performed one N 2 doping run injecting 1.06±0.07 ppm of N 2 . The triplet lifetime dropped from 1.479 µs to 1.239 µs as seen in Fig. 8 . From this reduced lifetime a Birk's constant of 0.12±0.02 ppm −1 µs −1 was found, consistent with the previous measurements. Using this measured value of Birk's constant, Eqn. 5 was fit to the triplet purification curve. Initial measurements of the filtration time constant found it to be 9.5±0.6 d (left fitted curve in Fig. 9 ). To increase the filtration rate, the turbo pump was valved off from the guard vacuum while leaving the roughing pump on thereby decreasing the insulating vacuum from 5.0 × 10 −7 Torr to 1.4 × 10 −3 Torr. Additionally the cold-head set point was raised from -190 C to -187 C thus increasing the internal pressure of the cryostat from 550 Torr to 640 Torr. A modification to Eqn. 5 was made to account the for non-zero start time to obtain:
where t 0 was the non zero start time of the fit. The filtration time constant measured by fitting Eqn. 7 to the triplet purification curve after these changes was measured to be 4.5±0.3 d (right fitted curve in Fig. 9 ). From the first fit, using the measured Birk's constant, an initial concentration of 1.13±0.02 ppm was obtained and the second fit yielded an initial concentration of 1.1±0.1 ppm. Both values are consistent with the initial concentration measurement of 1.06±0.07 ppm.
By monitoring the cosmic ray muon peak before and after doping (see Section 5), it was found that the most probable value (MPV) shifted from 334±7(syst)±1(stat) PE to 283±7(syst)±1(stat) PE, which corresponds to a 15.3±3.1% drop. This drop is compatible with Ref. [23] who measured the total quenching of the waveform using gammas from a 60 Co source.
Having validated our doping procedure with nitrogen, we then used xenon. Xe doping was done in four steps of 1.00±0.06 ppm, 2.0±0.1 ppm, 5.0±0.3 ppm, and 10.0±0.5 ppm. Xenon was injected in 0.5 ppm increments until the de- The left fitted red line shows the fit obtained when the heat leak for the system was minimized, yielding a filtration rate of 9.5±0.6 d.
The right fitted red line shows the fit obtained when a heat leak was added to the system, yielding a filtration rate of 4.5±0.3 d.
sired molar concentration was reached. Each 0.5 ppm injection took about 30 min. High concentrations of Xe (∼100 ppm) were not considered. Once the concentration was stable and fully mixed, 50,000 random background events were collected at each concentration to create the scintillation distributions (waveforms) seen in Fig. 10 . The waveforms were fit from 1.05 µs to 10 µs to use as much of the waveform as possible while avoiding the singlet contribution. The start time of the fit was varied by ±25 ns in order to estimate the systematic uncertainty associated with the fit window.
A modified version of Eqn. 3 was used to measure the time constants of the mixture:
where τ s is the slow component, τ d is the energy transfer time between the Xe and triplet state of the Ar, A 1 , A 2 and A 3 are the intensities, and τ long is an additional time constant to account for the exponential tail at late times. This artifact is consistent with light coming from the cold gas in the ullage [36] . The slow component, τ s , represents the time constant for the combination of unquenched Ar dimer light and Xe dimer light created through the mechanism outline in Section 2.1. Though Xe decays rather quickly compared to Ar, the population of Xe dimers is pumped by the Ar-Xe mixed state, thus is able to have a much longer decay time compared to pure liquid Xe. The total concentration of Xe dimers and Ar dimers in the triplet state is A 1 e −t/τs − A 2 e −t/τ d . The fitted waveforms are seen in Fig. 11 and the results from these fits are summarized in Table 1 .
Previous measurements [12, 14] of τ s for 1 ppm Xe reported values between 1 µs and 2.5 µs, though the uncertainty on the measured concentration is also 1 ppm. At 10 ppm Xe, τ s has been measured as 280 ns and 750 ns, with the concentration uncertainty at least 3 ppm. τ d has not been measured for 1 ppm Xe, but for 10 ppm the value ranges from about 250 ns to about 700 ns. Past measurements of these time constants have a much larger Xeconcentration uncertainty making them difficult to compare with our measurements. Disagreements between the previous measurements is likely due the concentration uncertainty and the unknown initial xenon concentration in what is assumed to be pure liquid argon.
The software package Geant4 [37] was used to simulate the light collection efficiencies of scintillation events in our apparatus. In order to accurately model optical physics in Geant4, it is required to input the optical properties, including surface reflectivity, attenuation length, scintillation yield, absorp- tion and emission spectra, and Rayleigh scattering lengths. Finally, optical surface properties for every material must be defined in order to accurately model the angular distributions of reflections [38] .
Simulated light yield for liquid argon detectors requires running radioactive decay simulation (i.e. K 42 , Uranium, or Thorium) and optical simulations simultaneously. This requires an extensive amount of computing resources due to the many discrete steps an optical photon can take. For efficiency, the simulations are broken into two separate steps. Since scintillators emit light isotropically and have a linear yield over a wide range of energies, an optical "heat" map can be created mapping the photon detection probability as a function of initial position throughout the scintillator volume. Once these maps are created, they provide the average light yield as a function of position without having to run an optical simulation in conjunction with the radioactive decay. Though generating an optical map is quite computationally intensive, it only needs to be done once.
The average photon yield, N , is calculated as:
where Y scint is the scintillator light yield, E dep (r) is the energy deposition in the scintillator as a function of position, P detection (r) is the detection probability as a function of position, and detector is the light detector efficiency. The average yield is calculated per step for each particle track and summed at the end of the event in order to estimate the average light yield of the event.
Liquid argon optical heat maps are created by generating VUV photons with a mean wavelength of 128 nm and Gaussian sigma of 2.93 nm and propagating them through the volume. Photon detection probability is calculated per (5 mm) 3 voxel as the number of photons detected divided by the total number of VUV photons generated in that voxel. Random vertices in the liquid argon are chosen and the photon direction is generated isotropically.
The optical volumes include gaseous Ar, LAr, stainless steel, TPB, acrylic and the PMT windows. 10 ppm of Xe was also simulated. The PMTs are modeled as stainless steel tubes. The 3-in cathode window, embedded on the top, is simulated as the sensitive optical element with known quantum efficiency. An 1 ⁄8-in thick acrylic disk covers the cathode, while a 2-µm thick TPB disk covers the acrylic. The generated LAr and XeDLAr optical maps can be seen in Fig. 12 . In order to project this three dimensional map into two dimensions, the third dimension is averaged. The XeDLAr attenuation length was extended from 60 cm, appropriate to pure argom, to 1000 m, a value much larger than the cyrostat dimension, since the XeDLAr attenuation length has not been measured at these concentrations.The scintillation central wavelength was shifted from 128 nm to 175 nm and the sigma has been shifted from 2.93 nm to 8.60 nm. The low vapor pressure of Xe in cold gaseous Ar results in a neglibible concentration of Xe in the cold gas of the ullage (+180 mm < Z < +300 mm). We note that XeDLAr allows for improved light collection farther from the PMTs, which can be attributed to the increased attenuation length.
We used cosmic ray muons to calibrate the optical maps. Cosmic muons are triggered on by placing two 0.35×17.0×8.50 in 3 scintillating panels above and below the cryostat rotated so that their long axes are perpendicular. This double coincidence requirement selects muons that are preferentially vertical thus limiting the variance in the traversal of the Ar (Fig. 13) . Additionally, muons are minimum ionizing particles with a roughly constant dE/dx of about 2.11 MeV/cm for a 266 MeV muon [39] . This energy is deposited uniformly along the muon path, thus allowing for the effects of longer attenuation lengths to be explored. The long path length also mitigates low light collection and non-uniform light response.
Muon kinetic energies were generated according to a power law, with downward momentum chosen from a cos 2 (θ) distribution and uniform horizontal momentum. The predicted photo-electron (PE) spectrum for Ar and XeDLAr can be seen in Fig. 14 . These spectra have been convolved with the PMT single photon resolution (σ SP E = 0.3125 PE). Both spectra were fit to Landau functions and have a most probable value (MPV) at 4235±46.5 PE and 2138±24 PE for XeDLAr and pure LAr respectively, predicting an increased light yield of 1.98±0.03 in XeDLAr.
The MPV of the distribution depends on the PMT quantum efficiency and the TPB scintillation efficiency and the LAr scintillation light yield (which in turn depends on the purity). The shape of the distribution depends on the LAr attenuation length. PMT, TPB efficiencies and scintillator yield affect the total light collection linearly, while the attenuation length has an exponential dependence. The total light collection efficiency can be calculated as:
where x is the distance of the interaction in the liquid argon to the detector, λ atten is the LAr attenuation length, T P B is the TPB quantum efficiency, P M T is the PMT quantum efficiency, and Y scint is the scintillator yield. The attenuation length in XeDLAr is not known at these concentrations. In order to understand the effect that the attenuation length has on the shape of the distribution, several XeDLAr optical maps were made with varying attenuation lengths. The shape of the distribution is defined as the "sigma" of the Landau distribution divided by the MPV, where sigma is the scale parameter reported by the CERN ROOT Landau fit. The shape of the distribution (sigma/MPV) is measured to be 0.293 ± 0.008. This ratio as a function of XeDLAr attenuation is shown in Table 2 . It can be seen that below an attenuation length of 5.0 m, the shape begins to significantly Table 2 : Summary of muon peak shape versus attenuation length obtained from the simulation. Each muon peak was fit with a Landau function and the MPV and sigma were obtained. The errors are the statistical errors from the fits.
change. We use the shape parameter to give a lower limit on the attenuation length in XeDLAr from the data in Section 7 below.
We used two scintillator panels, placed above and below our cryostat, to collect muon cosmic ray data. The double-coincident triggered data in 10 ppm XeDLAr is shown in Fig. 15 (left). Our trigger coincidence allowed a significant number of accidental events to be collected as well. Data from separately collected accidental triggers was subtracted from our spectrum to obtain corrected spectrum Fig. 15 (right). The data was fit to a Landau distribution with a most probable value (MPV) of 604.8±6.4 PE. The shape of the distribution, defined as the ratio of sigma to MPV is measured to be 0.293 ± 0.008.
To extract a lower limit on the attenuation length, we require the simulated shape parameter (sigma/MPV) to be within one standard deviation of the measured value. A plot of attenuation length versus muon peak shape can be seen in Fig. 16 . It can be seen from the figure that the simulated 5.0 m attenuation length is the lowest value consistent with the data, and we use this value in subsequent simulations. We take the value of 3.0 m as the lower bound on the attenuation length.
The simulation predicts a MPV 3911 ± 18 PE, which is 6.47 ± 0.08 times larger than what is measured in 10 ppm XeDLAr. When the simulated muon spectrum is scaled by the ratio of the measured and simulated MPVs (×6.47) and fit with a Landau function, then a MPV of 595±3 PE is found with a shape parameter of 0.306±0.003 (top right plot in Fig. 17 ). The shape of the simulation does not quite match the measured value after scaling, suggesting either that 5.0 m may be too small for the attenuation length or that the low PE tail of the data is distorting the fit. Nevertheless, comparison of the two spectra overlapped shows good agreement (bottom plot of Fig. 17 ). The MPV is consistent with the measured 604.8 PE for the peak seen in 10 ppm XeDLAr. Differences between these two values arises from the low PE tail seen in data, which is likely residual background not fully subtracted.
The large factor of 6.47 between the predicted PE yield and the measured yield for cosmic ray muon events is plausibly due to several factors. First, the degradation of the TPB. During an initial trial run, TPB was found coating the walls of the cryostat and PMT bases. We believe this happen in this run as well. 2 Second, the nominal value of 40 photons/keV light yield, may not be a valid. The authors of Ref. [25] state that they measure a yield of about 20 photons/keV even though they have pure Ar. The absolute light yield of XeDLAr remains to be measured. Third, the quantum efficiency of the PMTs may have been reduced after contamination with He. Though it is hard to quantify how much these effects are reducing the light yield, by selecting the correct attenuation length and accounting for this factor of 6.47, the simulation reproduces the data. These results validate the optical heat map technique described in Section 6.
The total increase in light yield resulting from Xe doping was measured by a direct comparison of the muon peak seen in pure Ar data to the 10 ppm XeDLAr data.
The cosmic muon peak should be visible in the pure LAr random trigger spectrum, even without the double coincidence requirement. Although the peak is dominated by cosmic muons, there is a contribution from unknown radioactive backgrounds. Additionally, cosmic muons without the scintillator trigger have different paths than with the trigger. Figure 18 shows the random trigger spectrum in pure (un-doped)liquid (red histogram) and the simulated double coincident muon spectrum (blue markers). The simulated spectrum has been scaled by the factor ×6.47. The two solid curves in the figures are the Landau fits. The MPV and sigma from the simulated fit (red curve) are found to be 338±2 and 123±1 respectively. Though the peak seen in data is dominated by the Landau component, the underlying shape due to ambient radioactivity adds to the uncertainty. The data were fit in two different ranges in order to estimate this uncertainty. The low boundary of the fit range was fixed at 150 PE while the upper boundary was varied from 450 to 600 PE. A MPV of 334±7(syst)±1(stat) was found with a sigma of 85±4(syst)±1(stat). The data MPV value is consistent with the estimated simulation MPV. It is not surprising that the shape of the two spectra do not match as the muon paths are different. Figure 18 : A comparison of simulated and measured muon peaks in pure (undoped) LAr. The red histogram is data, while the blue markers are simulation. The two curves are Landau fits with the red curve fit to data and the blue curve fit to the simulation. The data was taken using the random trigger which accounts for the difference in shape with the simulation (see text).
The same scale factor (×6.47) used for both simulated pure LAr and 10 ppm XeDLAr, were able to reproduce the data. This is consistent with our understanding of the origin of this factor which is independent of the scintillation source. The simulation predicts an increased light yield of 1.83±0.02 for 10 ppm XeDLAr relative to pure LAr, consistent with our measured increase of 1.81±0.04. Muons produce light throughout the entire volume thus taking advantage of the longer attenuation length. This comparison between the simulated double coincidence muon spectrum for pure and XeDLAr further validates the optical heat map simulation technique.
We expect that contaminants will quench the total light output less in XeDLAr than in pure LAr. The total scintillation distribution is shifted to shorter times, and so one expects that the probability of colliding with contaminants (via collisional damping) is less likely. To evaluate this conjecture, the shift of the muon peak from a double coincidence was measured. This quenching effect cannot be modeled using Birk's law because the energy transfer between the Xe and Ar dimer creates a mixed state that does not emit light but still can be quenched. This energy transfer process is also quenched, thus a comparison of quenching constants between pure LAr and XeDLAr cannot be made. We considered two molar concentrations of nitrogen: 1.00 ± 0.06 ppm and 9.95 ± 0.5 ppm N 2 . These nitrogen concentrations were chosen because commercially available research grade LAr can be obtained at 99.999% (10 ppm) or 99.9999% (1 ppm) purity. The N 2 contaminated XeDLAr scintillation time constants were measured and summarized in Table 3 . The scintillation distributions for all three N 2 concentrations (0 ppm, 1 ppm, and 10 ppm) can be seen in Fig. 19 . All three distributions are normalized and then scaled by the maximum value of the 0 ppm N 2 measurement. It can be seen that at 10 ppm of N 2 , significant afterpulsing is distorting the scintillation time distribution. The total light yield in 10 ppm of N 2 is much lower (see below) thus after-pulsing contributes significantly more, especially around 1.5 µs, to the total light seen in the distribution. At 1 ppm of N 2 and 10 ppm of Xe in LAr, the muon peak from a double coincidence trigger was found to shift from 604.8±6.4 PE to 542±4 PE, which corresponds to 10.4±1.0% reduction in light. This drop in un-doped LAr was measured to be 15.3±3.1%. In 10 ppm N 2 and 10 ppm XeDLAr the muon peak shifts to 294±3 PE, a 51.4±0.7% decrease in light yield. A similar drop in light yield is seen in pure LAr when 10 ppm of N 2 is injected [23] .
These doping measurements suggest that XeDLAr is less sensitive to the total contamination concentration of N 2 . Unlike N 2 , Xe will quench the Ar triplet in some characteristic time T d , and will then re-emit light at a longer wavelength. This Xe state is less likely to be quenched because, at 10 ppm Xe, T d is measured to be 213±6 ns which is much shorter than the Ar triplet life time of 1479 ns. Additionally, XeDLAr, at 10 ppm N 2 and 10 ppm Xe, has nearly the same total light yield from cosmic muons as pure LAr (294 PE versus 338 PE). Thus, if one were to buy five 9's LAr from a Figure 19 : Time distribution intensities for 10 ppm of Xe and 0 (blue), 1 (red), and 10 ppm N 2 (green) in LAr. The area of the blue curve is fixed to 1, the red curve contains 1 ± 0.04 ppm N 2 and has an area of 0.908, and the green curve contains 9.95 ± 0.13 N 2 and has an area of 0.770. commercial vendor and dope it with 10 ppm of Xe, the total light output would be similar to pure LAr. It is also worth noting that five 9's commercial grade Ar has other contaminates like oxygen, air, Co and CO 2 , all of which have different quenching factors. We speculate that it still might be possible to see less quenching at higher Xe concentrations because T d will be shorter.
It has been demonstrated that a small concentration of gas can be reliably and precisely injected into a cryostat containing ∼120 kg of LAr. Nitrogen at a concentration of 1.06±0.07 ppm was inserted and the quenching factor (Birk's constant) was measured to be 0.12±0.02 ppm −1 µs −1 , consistent with past literature. This demonstrates that small quantities of gas can be precisely injected. The N 2 contaminant was then removed using a ullage recirculation and purification system with a filtration time constant as low as 3.6 d.
We investigated small XeDLAr concentrations between 1 and 10 ppm. It was shown that up to a factor of ∼ 1.8 increase in light yield can be obtained for XeDLAr when compared to pure LAr. We have also shown that the attenuation length of 175 nm light in XeDLAr is much longer than the attenuation length at 128 nm. This longer attenuation length is particularly important for very large active volumes of XeDLAr.
The light increase from XeDLAr was first modeled using the software package MaGe [40] , which is built using Geant4. These simulations use a novel simulation technique called optical heat maps as an alternative to photon tracking. These optical heat maps contain the probability of photon detection as a function of position inside the simulated detector. Using these maps, a simulated double coincidence muon spectrum was created for pure LAr and XeDLAr. The data was consistently modeled once the attenuation length was determined and an overall light yield correction factor (6.47) was applied. This demonstrates that the optical heat map technique can accurately model the detector response.