Liquid Xe purification in a large-scale
dark matter search experiment XENONnTYoshino Kaminaga (the Univ. of Tokyo) for the XENON collaboration

II. XENONnT experiment

I. What is Dark Matter (DM)?

The large direct DM search experiment

What we know about DM⇒ An unknown masscomponent is required!
It could be a key
to reveal the nature ofthe universe.

Observed and predictedrotation curves of the M33 galaxy

The matter
we understand5%

Created by M. Leo, using E. Corbelli and P. Salucci, astro-ph/9909252, 1999

Darkmatter27%DM properties
Dark energy
u Gravitationally interacting
68%
u It does not absorb, reflect, or emit lightu Stable for the age of the universe

Underground laboratory in Gran Sasso, Italyu Using 8.5t of xenon (Xe)uXENON10 XENON100 XENON1T XENONnT2005-2007 2008-2016 2012-2018 201925kg
161kg
3200kg
8500kgXeuCompositions of our universe

III. Detector and Signals of XENONnTDual-phase Xe Time Projection Chamber
Liquid Xe; an excellent scintillatoru Online purification availableu S1 : Scintillation & S2 : Ionization signals
u3D positions of interaction points :(x, y : S2 position, z : timeS2−timeS1)

uParticle identification : (S2/S1)NR<(S2/S1)ERu

Electric recoil byβ/γ-ray
Xe

Main target: WIMPs

・Weakly Interacting Massive Particles
・A good candidate of DM, mass ~x100 of proton
・Very rare event (expected: ≦1 event/year)

494 photomultiplier
Challenges of operating a dual-phase TPC
tubes arraysu Large mass & low background are required
u Reduce radioactive background・Rn, Kr radioisotopes make background events⇒ Removed by distillations
u Purification of large detector
・H2O absorbs light (attenuates S1 and S2)⇒ Removed by gas phase purification
・O2 catches electrons (attenuates S2)⇒ Removed by gas & liquid phase purification
12 3 = 12 0 4 !%/'! (5( [s]: electron lifetime in Xe)

16%&
257 µs = ppb
An indicator of==$!! 0!! $!! 7!! 8'%&7!!
S2 sensitivity$!! ~10"" [L/mol/s]: attachment rate,
0!! [mol/L]: concentration

1# =Electrons loss
in the LXeat $! =467μs

Nuclear recoil by
WIMPs/neutron

The European PhysicalJournal C. 77, 881 (2017)ｃ

IV. New LXe Purification System in XENONnTNew LXe purification system and its performance
Remove impurities from LXe by either Q5 or St707 (see below)u High mass throughput thanks to liquid phase purification (500x GXe)
⇒ 1000 slpm (~12x GXe purification)
LXeu

purification
filters

Impacts of the Xe purity on physics
Electron lifetime reaches >~5x the maximal drift time・High S2 sensitivity
・Improved electron recoil rejection⇒ Greater WIMPs sensitivity※Since this is a simulation,uuPurity
monitor
Q5（BASF “Cu-0226 S”）

u

Cu-based, for initial purification
High efficiency ~100% & High RI

SAES St707

Getter-based, for science run
Middle efficiency ~56% & Low RI
uAchieved：
2.2 msElectronlifetime

0.66 ms>10ms

O2 eq.
purity~1 ppb ~0.02 ppb~0.01 ppb

Also: Rn distillation in
XENONnT reduced 222Rn
Upper limit on WIMP-nucleon cross-section
to < 1/7 of XENON1T
vs. electron lifetime(20 t*yr, WIMP: 50 GeV/c2) Kato, 2019

V. Summary
XENONnT is searching for dark matter by using Xe.
u Purifying Xe is important for such a rare-event search.
u XENONnT contains the purest xenon on the Earth!
XENONnT has the lowest
background environment!
u XENONnT reached theupper limit given by in
XENON1T in ~1/3 of
exposure time (in SR0.)u Data taking is ongoing!
u~0.02 ppb

τe=0.66 ms

Electrons loss in the LXe at $! =15ms

Electron survival rate:>80% in XENONnT(was 30% in XENON1T)

it differs in part from actual
conditions.

uXENON1T XENONnTFull TPC
drift time 0.67 ms

XENON collaboration

Evolution of the electron lifetime during XENONnT Science Run 0 (SR0)Upper limit on WIMP-nucleon cross-section at 90% CL

Phys. Rev. Lett. 131, 041003