An Atom Trap Trace Analysis Device to Measure Kr in Xe
A. Loose, L. W. Goetzke, E. Aprile, and T. Zelevinsky
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The Xe gas sample is injected into the vacuum system,
and the Kr atoms are excited to the lowest metastable
state by a high-Q helical resonator [4]. The 84Kr* beam is optically collimated by 2D optical
molasses, slowed by the Zeeman Slower (ZS), and trapped in a magneto-optical trap (MOT). A
6 MHz red-detuned laser is used for both the transverse cooling and MOT, and a 310 MHz
red-detuned laser is used for the ZS.
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About 27% of our universe is made of non-baryonic, cold dark matter. Whereas the existence
of dark matter is well established, its nature remains unknown, despite an intense experimental
program using a variety of approaches. Among these, the measurement of nuclear recoils induced
by the the elastic scatter of dark matter particles with normal matter would provide the most direct
evidence. Sensitivity to such a rare event demands ultra-low background and massive detectors,
operated deep underground. Large volume liquid xenon (LXe) detectors, such as the LUX and
XENON100/XENON1T experiments [1], are leading the field of dark matter direct detection.
An all-semiconductor laser system is used to probe the
closed 53P2 – 53D3 transition of metastable 84Kr* (γ ≈
2π x 6 MHz) at 811.5 nm. It is locked to a gas reference
cell using saturated absorption spectroscopy.
fluorescence [kHz]
The Hunt For Dark Matter
atta.phys.columbia.edu
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84Kr* single atom counts for an ultra-pure Xe sample.
System Efficiency
The system performance is equivalent to reaching ppt-level sensitivity of 84Kr detection in ∼ 1.5
hours of measurement time, with further improvements underway [6].
Sketch of the XENON1T detector at LNGS, Italy.
Trace amounts of Kr in the detector medium Xe lead to an intrinsic background from the decay of
radioactive 85Kr, a long-lived (t1/2 = 10.8 years) β-emitter. This background from 85Kr limits
the detection sensitivity. The sensitivity reach of next-generation detectors such as XENON1T
requires a Kr/Xe contamination below 0.2 ppt. Practical and fast measurements at such extremely
low levels of contamination are our goal.
gas consumption rate: 6 × 1016 (Ar) atoms/s
2.5 × 1016 (Xe) atoms/s
metastable atom flux: 6 × 1011 atoms/s (Ar)
angular flux density: 9 × 1013 atoms/s/sr (Ar)
source efficiency: 10−5 (Ar*/Ar)
10−4 (Kr*/Kr in Xe)
MOT loading rate: 1.8 × 108 atoms/s (Ar)
overall efficiency: 3 × 10−9 (Ar, trapping mode)
1.2 × 10−8 (Kr, detection mode)
flux enhancement: TC1: 20×; TC2: 2×;
TC3 (horizontal): 1.2×
RF Discharge Source
The helical resonator consists of a shielded Cu coil surrounding an aluminum nitride (AIN) tube.
An amplified RF signal at 180 MHz creates a sustained plasma discharge, which is converting
the ground state Kr atoms to metastable Kr*, as required for slowing and trapping. To avoid
contaminating the system with Kr, the setup is optimized with 40Ar∗. At the ppt level, crosssample contamination due to Kr implanted into the walls of the RF discharge and released at a
later time becomes important.
Xe Purification and Sampling
To determine the system efficiency for Kr in Xe gas, we admixed well-defined (12 and 0.8 ppm)
amounts of Kr to 10 ppb level pure Xe. We then compared the known loading rate for Ar in
trapping mode to the measured Kr loading rate in detection mode. Using the 84Kr abundance
of 0.57, we estimate an 35-fold increase of trapping efficiency for Kr in Xe in detection mode.
Taking the lower consumption rate of the Xe discharge into account, the overall system efficiency
is increased by a factor of 84.
We validated the ATTA calibration by direct comparison to a rare gas mass spectrometer operating at the Max Planck Institute for Nuclear Physics in Heidelberg. We are continuing the
characterization of the system using ultra-pure Xe samples.
All-metal sample pipette with 1 l reservoir.
4.5 m distillation column for XENON1T.
The sample gas is cooled to ≈160 K by a Cu rod coupled to a pulse tube refrigerator (PTR)
as it flows into the RF source chamber. Since the capture velocity of our ZS for Kr is 245 m/s,
cooling increases the capture fractions from 18% for Kr (at 400 K) to 59%. The PTR (Iwatani,
PDC08) can be operated continuously for years with little to no adjustment or servicing.
Cryogenic distillation is an established technology for Xe purification from Kr at and below the
part per trillion (ppt) level. A high throughput column has been developed for XENON1T by the
Weinheimer group at WWU in M¨unster. We constructed custom bakeable all-metal pipettes to
transport ultrahigh-purity Xe samples from the detector to our ATTA setup.
To the best of our knowledge, the role of Xe as carrier gas in the production of metastable Kr
atoms in a RF discharge has not yet been investigated. A systematic study using Ne, Kr and
Xe as carrier gases for Ar can be found in Ref. [5], where Xe yielded the optimal fractional
metastable atom population of 2 × 10−4.
The ATTA Technique
Single Atom Detection
The Atom Trap Trace Analysis (ATTA) technique, pioneered by Z.-T. Lu et al. at ANL [2,3]
is used to quantify the amount of 85Kr present in the XENON1T experiment. We detect 84Kr*,
the most abundant (≈57%), stable isotope, which will be at the ppt level in Xe. The amount of
85Kr is inferred by using its relative abundance.
For ppt Kr in Xe, only one 84Kr* atom is expected to be in the MOT at a time. A single atom
trapped in the MOT scatters photons at a rate of 107/s. We use an avalanche photodiode (APD)
to detect this fluorescence.
References
[1] E. Aprile et al. (XENON100), ‘Limits on spin-dependent WIMP-nucleon cross sections from 225 live days of
XENON100 data’, Phys. Rev. Lett. 111, 021301 (2013); lux.brown.edu; xenon1t.org
[2] C. Y. Chen et al., ‘Ultrasensitive Isotope Trace Analyses with a Magneto-optical Trap’, Science 286, 1139 (1999).
[3] X. Du et al., ‘An atom trap system for practical 81-Kr dating’, Rev. Sci. Instrum. 75 (10), (2004).
[4] W. W. Macalpine, ‘Coaxial Resonators with Helical Inner Conductor’, Proc. of the IRE, 2099 (1959).
[5] K. Rudinger et al.,‘The role of carrier gases in the production of metastable argon atoms in a rf discharge’, Rev.
Sci. Instrum. 80, 036105 (2009).
[6] E. Aprile et al., ‘An atom trap trace analysis system for measuring krypton contamination in xenon dark matter
detectors’, Rev. Sci. Instrum. 84, 093105, (2013).
The ATTA experiment is supported by the NSF MRI program
and Columbia University. We would like to thank the NSF for
their continued funding of the XENON experiments.