Mainz LXe TPC
Design of a two-phase Xenon
TPC for the study of fast
scintillation in LXe dark matter
detectors
Bastian Beskers
HAP DarkMatter detector workshop
November 2012
1
Outline
MainzTPC design and photo-sensors
principle of a dual-phase TPC
● motivation
● technical design of the MainzTPC
● photosensors/measurements
● summary
●
2
principle dual-phase TPC
S1: primary scintillation
interaction
excitation
Xe*
ionization
Xe+
dimers
Xe*2
excimers
Xe*2
scintillation light
S1
ground state
2 Xe
free
e-
+
secondary scintillation
S2
3
principle dual-phase TPC
S2: proportional scintillation
interaction
excitation
Xe*
ionization
Xe+
dimers
Xe*2
excimers
Xe*2
scintillation light
S1
ground state
2 Xe
free
e-
+
secondary scintillation
S2
4
3D-position reconstruction
z-position
time difference between S1
and S2
x/y-position
position of S2 detected by a
photosensor array
electronic recoil
e-,γ
∆t
nuclear recoil
neutron, WIMP
5
background discrimination
background
discrimination:
S2/S1
measurement of charge and light yield
Compton scatter experiment
neutron scatter experiment
6
TPC design
high light yield
● minimize non-reflective and not photo
sensitive areas
● high-transparency meshes
● minimize number of meshes
good energy resolution
● x/y position resolution for
discrimination, fiducialization
and light/charge corrections
● high light yield
TPC design
optimized for
uniform electrostatic driftfield
● fine meshes
● fieldcage with low pitch
compton experiment
● small diameter (5 cm)
● low amount of passive
scattering material around
(< 5mm LXe)
7
TPC design
top PMT
x/y-position-resolution
using 8 large area APDs
x/y-resolution <=1.3 mm
d = 52 mm
high transparency meshes
pitch: 268 µm
wire-diameter: 14 µm
transparency: 88 % @ 90 °
bottom-PMT
8
TPC design
100 MΩ
1 mm
electrostatic
drift field
fieldcage: pitch of wires: 1mm
|E| = 3.05
kV/cm
Er / |E| < 2%
electron traces
|E| = |EZ|
9
Instrumentation
photo-sensors
2 PMTs (Hamamatsu R6041):
QE > 30% @ 178 nm
● compact design
● 2 inch diameter
● 32 mm heigth
● cathode active diameter: 45 mm
●
8 APDs:
active area: 14x14 mm²
● QE ~ 30% @ 178nm
● no housing - little passive material
●
10
PMT: R6041
PMT single photo electron pulseshape
Datasheet figures (800V)
gain: 1 x 106
anode pulse rise time: 2.3 ns
electron transit time: 16 ns
transit time spread: 0.75 ns
supply voltage
volts
rise time
ns
fall time
ns
pulse width
ns
gain
10 e- per p.e.
750
2.249
14.16
33.96
-
800
2.229
14.29
33.73
2.3
850
2.132
16.09
33.56
4.9
900
1.997
18.34
33.39
9.4
6
11
APD: RMD S1315
measuring gain and QE (next weeks)
rotatable disc (45° precise positioning)
with Am241-source and optical fibre
mounted on bottom side (not visible
here)
avalanche photo diodes
setting up the cooling and temperature
control system and preparing to fill with
Xenon these days
12
APD gain
measured gain of APDs @ 165 K
measurement using a LED
measured gain ≤ 1500
less than expected - saturation?! => will be tested
13
Instrumentation
electronics
S1 pulse shape measurement
could possibly be used for background
discrimination (complementary to S1/S2)
● can help understand LXe scintillation/ionization
and influence of drift field
●
5 GS/s FADC
(Struck SIS3305)
10 bit
● 2/4/8 channels
● 5/2.5/1.25 GS/s
● 1.5 GHz bandwidth
●
E = 0 kV/cm
14
Status and next steps
Status:
● parts are beeing manufactured at the moment
● Slow-Control System with online monitoring and alarms running
● ongoing tests with the photosensors
● simulations done on driftfield, position reconstruction and
systematic effects
Next steps:
● design the cryo-system (heat exchange, cooling, Xe-purification)
● tests with PMTs in E-fields
● amplification of APDs
● preparing the readout electronics
15
Bastian Beskers
beskers@uni-mainz.de
HAP DarkMatter detector workshop
Bad Liebenzell
November 2012
Questions?
Thanks to all collaborators:
Uwe
Oberlack
Rainer
othegraven
Cyril
Grignon
Elvar
Kjartansson
Matthias
Morbitzer
Pierre
Sissol
Thomas
Jennewein
16
http://xenon.physik.uni-mainz.de/
17
Mainz LXe TPC: simulation
Geant4: position reconstruction:
reconstruction error
using 8 APDs - 14x14 mm²
reconstruction error
< 0.5 - 1.3 mm
● only 8 readout channels
required
● no strong φ dependency
●
18
Mainz LXe TPC: simulation
COMSOL: electrostatic field
simulated different potential distributions with same resulting
potential between cathode and anode
no topshield-mesh
=
higher
transparency
19
Mainz LXe TPC: Phase 2
- Compton Scattering -
requirements, systematic errors and optimization studied in diploma
thesis by Pierre Sissol
●
doppler-broadening: influence on energy-resolution
●
source-strength - distances - angular resolution
in Compton-scatter-experiment
20
LXe scintillation pulse shape
Can PS be used to improve
background discrimination in
future LXe DM searches?
two excimer states
triplet 3Σ+: τT ~ 20 - 27 ns
singlet 1Σ+: τS ~ 2 - 4 ns
singlet-to-triplet ratio As/At
is dependent on:
● nuclear/electronic recoil
● electrical drift field
Physical Review B - Vol.27 - No. 9, May 1983
A. Hitachi, T. Takahashi
21
G. Plante, Phys. Rev. C 84, 045805 (2011)
normalized
measured
energy
drift field quenching
factor
22
Mainz LXe TPC
scintillation pulse shape:
comparing electron, alpha and fission
fragment induced scintillation:
electron:
●
E = 0 kV/cm
τ = 45 ns
alpha:
●
τS = 4.2 ns
●
τT = 22 ns
fission fragments:
●
τS = 4.1 ns
●
τ = 21 ns
PhysicalTReview B - Vol.27 - No. 9, May 1983
A. Hitachi, T. Takahashi: “ Effect of ionization density on the time
dependence of luminescence from liquid argon and xenon”
23
Mainz LXe TPC
scintillation pulse shape:
comparing different electrical driftfields:
E = 0 kV/cm
●
τ = 34±2 ns
E = 4 kV/cm
●
τS = 2.2±0.3 ns
●
τT = 27±1 ns
J . Phys. C : Solid State Phys., Vol. 11, 1978
Shinzou Kubota, Masahiko Hishida and Jian-zhi Raun:
“Evidence for a triplet state of the self-trapped exciton states
in liquid argon, krypton and xenon”
24