The XENONnT Time Projection Chamber

Arianna Rocchetti - Albert-Ludwigs-Universität Freiburg 17.06.2018 The XENON Collaboration: Phases of the XENON Project:

Target mass: 14 kg Target mass: 62 kg Target mass: 2t Target mass: 6 tons Drift length: 15 cm Drift length: 30 cm Drift length: 97 cm Drift length: 148 cm XENONnT : Larger target mass and lower background.

We aim at keeping the lead in the search for WIMPs with LXe detectors. 3 Signals in the XENON Detector:

NUCLEAR RECOIL: WIMP and neutron scatter off the atomic nucleus.

ELECTRONIC RECOIL: photons and scatter off atomic electrons.

4 Backgrounds:

Science Run 1, Calibration Data ER ● Intrinsic contaminants ER (222Rn daughters, 85Kr and 136Xe) ● Radioactive Contamination from materials

● Elastic Scattering of solar NR off electrons Corrected S2 [PE] Corrected S2 [pe] NR ** ● Radiogenic neutrons: ○ Spontaneous fission Corrected S1 [pe] ○ (α,n) reactions from the U and Th ● Muon inducted neutrons ● Coherent scattering of neutrinos off Xe-nuclei 5 XENONnT: a background reduced XENON1T

● Larger TPC fitting in the XENON1T outer vessel : (use the same holding structure and leveling mechanism).

● Screening campaign for detector materials.

● Cleanliness procedure strengthened. ● Strict installation scheme.

● Rn reduction: introduce less, pump faster and distillation.

● Neutron Veto

● Design drift : 200V/cm -> optimised also for

lower values. 6 Backgrounds:

Science Run 1, Calibration Data ER ● Intrinsic contaminants ER (222Rn daughters, 85Kr and 136Xe) ● Radioactive Contamination from materials

● Elastic Scattering of solar NR neutrinos off electrons Corrected S2 [PE] Corrected S2 [pe] NR ** ● Radiogenic neutrons: ○ Spontaneous fission Corrected S1 [pe] ○ (α,n) reactions from the U and Th ● Muon inducted neutrons ● Coherent scattering of neutrinos off Xe-nuclei 7 The Monte Carlo Simulation

Input: Cryostat

● Geant4 toolkit -> Detector Geometry and Gaseous Xenon Physical processes; PMTs ● Activities for each radioactive isotope

and masses of the components; TPC sensitive region ● Neutron yield and spectrum.

Liquid Xenon Output: TPC below ● Energy deposit in LXe PMTs ○ from each isotope ○ from each material ○ for each interaction vertex

8 The Monte Carlo Simulation ER 238U,235U,226Ra,228Th,232Th NR 60Co,40K,37Cs,238U,235U,226Ra,228Th,232Th

Energy deposit in LXe : ○ from each isotope in each material ○ for each interaction vertex ER spectrum of SS interaction

Rate (ev/kg*day*keV) z [mm]

PRELIMINARY

r [mm] keV 9 The Monte Carlo Simulation Analysis: Energy deposit -> Signal S1 and S2

1. Convert the energy Taken into account: the efficiencies in collecting deposit into signal S1 and the signal.

S2 . Capacity of distinguish two scatters depends on: 1. Width of S2 2. Apply selection CUTS to 2. Peak separation efficiency of the S2 peak identify an event as finding algorithm background S2 1 S2 S2 1 2 Amplitude (pe/bin)

Snapshot of waveforms for simulated multiple scattering: 10 time (us) time (us) The Monte Carlo Simulation Analysis: Selection CUTS are:

1. Convert the energy 1. Single Scatter (SS) in TPC deposit into signal S1 and WIMPs are not expected to give multiple scatters. S2.

2. Interaction in the Fiducial Volume 2. Apply selection CUTS to identify an event as 3. Energy Range background [4, 50] keVr for NR that corresponds to [2, 12] keVee for ER Energy region for WIMP search is [3, 70] pe . Results: Science Run 1, Calibration Data

ER

NR Corrected S2 [pe]

Corrected S1 [pe]

Source Background (ev/4t/y) Background (ev/4t/y)

No discrimination (99.75% ER Discri., 40% NR Acc.)

ER from materials < 35 < 0.09

PRELIMINARY NR from radiogenic neutrons < 1.8 < 0.45 PRELIMINARY 12 Minimum spin-independent WIMP- of σ =1.6 * 10-48 cm2 at mχ=50 GeV/c2 for 20 t * y exposure*

*arXiv:1512.07501 For further questions: [email protected] Back-up slides: Purification System:

Remove electro-negative impurities to concentrations below 1ppb.

- Replace circulation pump; - new liquid purification system - Gaseous recirculation/purification through a hot getter

Drifting lifetime has a plateau. Radon Mitigation Strategies:

Rn is part of the 238U chain, long half life τ=3.8 days. It diffuses in LXe. Background reduction of a factor 10 in XENONnT, For XENON1T: Green: in contact with GXe Cryogenic distillation. We’ll Red: sources that emanate use the 85Kr removal column directly in LXe. Only change of in reverse mode. purification is extraction on a time scale shorter than 222Rn mean life. High flux on an online cryogenic distillation column.

Purple: 222Rn emanated by plastic materials within the QDrive recirculation pumps

Liquid Scintillator NEUTRON VETO:

NEUTRON VETO: detect radiogenic neutrons coming from the TPC materials. Neutrons are thermalized in the liquid scintillator and captured on Gadolinium, releasing a gamma ray of ~8MeV.

Gd-loaded Liquid Scintillator Contained inside transparent acrylic vessels

Rejection factor >80%, neutron background ∼5%.

Total neutron background in the 4 t fiducial volume below 1 event in the total exposure of 20 t×y.