Development of Hermetic Liquid Xenon Detector for the DARWIN Experiment
Ryuta Miyata （Nagoya University, ISEE）
Abstract

The DARWIN experiment is a future direct dark matter search with 50 tons of LXe. For the DARWIN experiment, it is necessary to reduce the !!!𝑅𝑛 concentration in the detector to 10% of XENONnT!!!
!!!
!!!
to reduce the𝑅𝑛 background. To achieve this𝑅𝑛 concentration, hermetic xenon detector, which seals the center of LXe volume with high purity quartz and prevents contamination of𝑅𝑛emanating from other detector components, has been proposed.
In this poster presentation, I will report on the development of the hermetic chamber made of quartz and PTFE, and the development of separated circulation systems for inside and outside thehermetic chamber to evaluate its sealing performance.

1. DARWIN experiment

5. Optimization of gasket- DARWIN : A future direct dark matter(DM) search
after XENONnT or LZ with 50 ton liquid xenon(LXe).
- By detecting both the primary scintillation light (S1)and the electroluminescence scintillation light (S2)produced from the drifting electrons, WIMPs/BGevents can be reconstructed.

< Optimization of gasket >- In general, the greater the surface pressure applied to the gasket, the better the sealing is.
- Based on our experiences, however, the quartz is likely to break around 7.5(𝑁 + 𝑚) of torque.
- Using He leak detector, I tested the sealing performance with multiple shapes of gaskets
and torques.
3mm

2mm

66mm

The Xenon Road to Direct Detection of Dark Matter at LNGS: The XENON Project /Pietro Di Gangi on behalf of the XENON Collaboration- Our main goal is to search WIMPs down to theneutrino fog, which is a region made difficult
to explore by the neutrino background(BG).
- the major background to achieve thissensitivity is radon-induced one.

2.

!!!

𝑅𝑛-induced BG!!!

𝑅𝑛 concentration in various direct DM search- Based on the result of the test, I estimated the !!!𝑅𝑛 concentration in DARWIN.
Assumption 1 : the leak rate depends only on the difference of partial pressure.
!!!

𝑅𝑛 concentration in GXe outside the hermetic chamber is 10(𝜇𝐵𝑞/𝑘𝑔).

- Under these assumptions, !!!𝑅𝑛 concentration inside the hermetic chamber inDARWIN(𝐶&'()'*)+, ) follows

𝐹-, : He leak rate
𝐶&'(./0*)+, : !!!𝑅𝑛 concentration in GXe outside the hermetic chamber in DARWIN
𝐶-,(1)2 : He concentration in the air
𝑅 : Ratio of the diameter of this hermetic chamber and the hermetic chamber in DARWIN
𝑀 : Xenon mass in the sensitive volume
𝜏 : !!!𝑅𝑛 lifetime!!!

𝑅𝑛 emanates from the detector components(PMTcable, cryostat, etc.).
- β-rays is distinguished from DM by the S1/S2 ratio,
however, it is not possible to identify β-rays completely.
(In the DARWIN experiment, the probability ofdistinguishing them is expected to be 99.9%.)d6- In the DARWIN experiment, the !!!𝑅𝑛 concentration
in LXe need to be reduced to 10% of that in theXENONnT experiment.

Rate (𝑡/𝑦/𝑘𝑒𝑉)- In XENONnT,
𝑅𝑛-induced BG was reduced by xenon
purification system, but we need a new way to further
reduce the !!!𝑅𝑛 concentration to achieve our goal.

4< Results >3- Leak rates decreased as the torque applied
to bolts was increased.
- We achieved the target leak rate at room
temperature. 2 mm wide gasket is better
than 3mm.
- I also measured the leak rate periodically for
34 days at 7(𝑁 + 𝑚) and confirmed that theleak rate remained constant.
→ The next step is test in GXe/LXe.

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2

4

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10Energy (𝑘𝑒𝑉)1214

3. Hermetic liquid xenon detector
Conventional xenon detector!!!

Hermetic xenon detector

𝑅𝑛 concentration (𝜇𝐵𝑞/𝑘𝑔)

Target value considering """𝑅𝑛derived from xenon circulators

Torque per bolt(𝑁 # 𝑚)

Left axis: He leak rate measured
Right axis: corresponding !!!𝑅𝑛 concentration

stainless steel vessel

Hermetic xenon detector

GXe

Seal the sensitive volume to prevent !!!𝑅𝑛contamination.

6. Xenon circulation systems

PMT

PTRquartz< Requirements >sensitive
volume!!!

- Achieve
𝑅𝑛 concentration inside the hermetic
detector to 1% of the outside.
- The top and bottom flanges must be transparent toxenon scintillation light (175 nm).
- The material must be low radioactive.

PTFE

quartzLXe

Material: Quartz and PTFE (already used for PMT
windows and reflective materials in XENONnT)- Two circulation systems
separated for inside and outside
the hermetic chamber isrequired to evaluate sealing
performance in GXe/LXe.
- Inject !!!𝑅𝑛 source outside
the hermetic chamber, andcompare the concentration
inside and outside.

4. Development of a hermetic chamber< Key points in development >- To prevent the quartz from breaking, I need totighten the flange with as little force as possible.
- Gasket material has to be softer than quartz andPTFE, and available at low temperatures(≈100°C) plus high pressure(≈2atm). Expanded
PTFE(ePTFE) fits these ctiterion.

"""

He leak rate(𝑃𝑎 # 𝑚! /𝑠)

5!!!

𝑅𝑛 emanates from detector components(stainless steel vessel, PMT cables, etc.), and it
contaminates the sensitive volume.

< Estimation of 𝟐𝟐𝟐𝑹𝒏 concentration at DARWIN scale >𝐶&'(./0*)+,1
𝐶&'()'*)+, = 𝐹-, ××𝑅× ×𝜏
𝐶-,(1)2
𝑀- The β decay of !$%𝑃𝑏, the daughter of !!!𝑅𝑛, causes
electronic recoil background.
-← multiple shapes of gaskets
I tested with gaskets of 2 mm and 3 mmwidths.

66mm

Assumption 2 : theDARWIN: towards the ultimate dark matter detector / DARWIN collaboration

22mmHeater

Outer vessel

External Xenon
Circulation System

Internal Xenon
Circulation SystemHermeticchamber!!!

𝑅𝑛The outdoor !!!𝑅𝑛sourse
concentration ismeasured with a!!!
𝑅𝑛 meter.

The indoor !!!𝑅𝑛concentration ismeasured with a!!!
!!!
𝑅𝑛
𝑅𝑛 meter anddetector
PMTs.

!!!

𝑅𝑛detector

PMT

heater andcooling section!!!

𝑅𝑛 detector

Glass flangeePTFEgasket

M6 × 12PTFE body5cm< Road map >1. Design the hermetic detector with 10cm diameter flange.
2. Optimize gaskets and investigate the relationship between
the torque applied to the bolts and sealing performance.
3. Evaluate the sealing performance at room temperature.
4. Evaluate the sealing performance in gaseous
xenon(GXe)/LXe

10cm

7. Conclusion- For the DARWIN experiment, the !!!𝑅𝑛 concentration needs to be reduced to 10% of that of
the XENONnT experiment. We are proposing the technique of hermetic detector.
- I developed a hermetic chamber with a PTFE body and optimized the width of gaskets. I
achieved the target leak rate at room temperature.
- Now, I’m preparing separated circulation systems for inside and outside the hermetic detector
to evaluate sealing performance in GXe/LXe.