Measurement of the absolute
reflectance of PTFE immersed in
liquid xenon
F. Neves, A. Lindote, A. Morozov, V. Solovov,
C. Silva and I. Lopes
IDM2016, 18-22 July
Outline
●
Context & method;
●
Experimental setup;
●
Experimental results;
●
Reflectivity models & MC simulations:
●
–
Fitting the exp. results using a diffuse-only reflectivity model;
–
Fitting the exp. results using a diffuse+specular reflectivity model;
Summary & Conclusions.
IDM 2016, 18-23 July
2
Context
Several Liquid Xenon (LXe) detectors used in direct DM searches (ZEPLIN-II, LUX,
XENON, PANDA-X) and other rare event searches (EXO, MEG) used PTFE
reflectors to improve the light collection from xenon scintillation (178 nm);
PTFE reflectance has been measured to be 50-70% for xenon scintillation light in
vacuum and at room temperature and it is known to be much higher when
immersed in LXe;
Nevertheless, extrapolations for the reflectance of the PTFE immersed in LXe are
systematically and significantly lower than the values � (>95%) obtained by fitting the
measured light collection to detailed MC simulations of light propagation;
Temperature (≈-100 ºC), light absorption in the liquid by impurities and Rayleigh
scattering put severe constrains on using traditional methods (e.g. goniometers) to
measure directly the reflectance of PTFE immersed in LXe;
Even in a dedicated setup, detailed MC simulations of light propagation in the liquid
are necessary to take into consideration the effect of Rayleigh scattering and light
absorption, with the former depending strongly on the experimental conditions.
Measure candidate samples to choose the best PTFE for LZ;
IDM 2016, 18-23 July
3
Context
Several Liquid Xenon (LXe) detectors used in direct DM searches (ZEPLIN-II, LUX,
XENON, PANDA-X) and other rare event searches (EXO, MEG) used PTFE
reflectors to improve the light collection from xenon scintillation (178 nm);
PTFE reflectance has been measured to be 50-70% for xenon scintillation light in
vacuum and at room temperature and it is known to be much higher when
immersed in LXe;
Nevertheless, extrapolations for the reflectance of the PTFE immersed in LXe are
systematically and significantly lower than the values � (>95%) obtained by fitting the
measured light collection to detailed MC simulations of light propagation;
Temperature (≈-100 ºC), light absorption in the liquid by impurities and Rayleigh
scattering put severe constrains on using traditional methods (e.g. goniometers) to
measure directly the reflectance of PTFE immersed in LXe;
Even in a dedicated setup, detailed MC simulations of light propagation in the liquid
are necessary to take into consideration the effect of Rayleigh scattering and light
absorption, with the former depending strongly on the experimental conditions.
Measure candidate samples to choose the best PTFE for LZ;
IDM 2016, 18-23 July
4
Context
Several Liquid Xenon (LXe) detectors used in direct DM searches (ZEPLIN-II, LUX,
XENON, PANDA-X) and other rare event searches (EXO, MEG) used PTFE
reflectors to improve the light collection from xenon scintillation (178 nm);
PTFE reflectance has been measured to be 50-70% for xenon scintillation light in
vacuum and at room temperature and it is known to be much higher when
immersed in LXe;
Nevertheless, extrapolations for the reflectance of the PTFE immersed in LXe are
systematically and significantly lower than the values � (>95%) obtained by fitting the
measured light collection to detailed MC simulations of light propagation;
Temperature (≈-100 ºC), light absorption in the liquid by impurities and Rayleigh
scattering put severe constrains on using traditional methods (e.g. goniometers) to
measure directly the reflectance of PTFE immersed in LXe;
Even in a dedicated setup, detailed MC simulations of light propagation in the liquid
are necessary to take into consideration the effect of Rayleigh scattering and light
absorption, with the former depending strongly on the experimental conditions.
Measure candidate samples to choose the best PTFE for LZ;
IDM 2016, 18-23 July
5
Experimental setup
The method:
Measure the light
collection efficiency
as a function of h
(Changes the LXe
volume and PTFE
area) and compare with
a detailed MC model of
the setup.
The setup:
Allows an easy and fast
replacement of the
PTFE samples;
sub-mm precision in
determining the
thickness (h) of the LXe
layer between the 241Am
source and the PMT;
IDM 2016, 18-23 July
6
Experimental setup
Level
sensor
Moving
PTFE wall
7.6 mm
241
Am source
Fixed
PTFE walls
PMT
(R1668)
Optical
Surface
(fixed walls)
Moving
PTFE wall
PMT HV divider
IDM 2016, 18-23 July
7
Experimental results
Measured 3 PTFE samples
● 807NX, NXT85 (APT);
● 8764 (Technetics) – LUX;
Cool down to ≈-100 ºC
and let the setup stabilize
for ~12 hours prior to
condensing Xe;
Multiple runs per sample
(cool down / warm up cycle)
and multiple sweeps per run
to test reproducibility;
Xenon purity monitored (elivetime): > 40 us (sens.
max).
Absolute light collection
(�) indicates that the LUX
PTFE has the highest
reflectance followed by the
NXT85 and 807NX.
IDM 2016, 18-23 July
8
Optical Simulation
Moving PTFE
wall
MC simulation using the ANTS2 package
(http://coimbra.lip.pt/ants/ants2.html)
Simulated only the inner chamber;
241
Am source
Photons
(not detected)
Fixed PTFE
walls
Photon
(detected)
PMT
window
Optical processes/boundaries considered:
PMT quartz window: n=1.59 (Gupta, 1998);
SS (source): n=1.07, k=0.6 (Karlsson, 1982);
Rayleigh scatt. (LXe): 29 cm (Seidel, 2002);
Absorption (LXe): � LXe=[0.1; 5] m;
PTFE reflectivity models:
Diffuse-only (D): APTFE=[0.6;1];
Diffuse+Specular (DS): APTFE=[0.6;1],
nPTFE=[1.3;2.0]; nLXe=1.69 (Solovov, 2003)
3D/4D � template grid on the free optical
parameters ( � LXe,APTFE[,nPTFE])+h with linear
interpolation for each of the PTFE models (D/DS).
(shown: 1 event with 20 photons)
IDM 2016, 18-23 July
9
Optical simulation:
241
Am source
The reflectance of the SS (241Am source) is
unknown, but this is the 1st surface seen by
~50% of the light.
Tested three values of the reflectance
(Karlsson, 1982 [SS1, SS2], Solovov
2003 [SS3]) for different SS:
Significantly different values of the
absolute light collection (⧍�~30%)!
A=0.95
nPTFE=1.7
� =1900 mm
But the same relative light
collection (scale factor).
IDM 2016, 18-23 July
10
Analysis (fitting) procedure
Cannot fit the absolute light collection (�) due to the uncertainties on the
Stainless Steel (SS) reflectance and also on the PMT QE. Instead we fit the
relative light collection by scaling the simulated light collection (�𝓜 ):
scale
factor
exp. light
collection
exp. stat.
errors
sim. stat.
errors
sim. light
collection
re-positioning
of the source
𝓜: Model specification: Diffuse-only (D); Diffuse+Specular (DS);
�: Model-specific parameters: ( � LXe,APTFE)D or ( � LXe,APTFE,nPTFE)DS;
I𝓜: Simulated light collection from 100k photons per α particle, for a total of 300 αs
distributed uniformly over the 241Am source surface;
� src: Measured to be <0.1 �m when re-positioning the source (room temperature);
IDM 2016, 18-23 July
11
PTFE reflectivity: diffuse model
The Albedo (A) sets the probability of the light being reflected
(not absorbed) from the PTFE;
The reflected light follows a Lambert law: dI/dΩ=Acos(�);
No surface description (e.g. roughness);
In spite of its simplicity, this is the model assumed by most
experiments using reflector PTFE panels (e.g. LUX);
IDM 2016, 18-23 July
12
PTFE reflectivity: diffuse model
Can only set a lower limit for the reflectance
(R) due to A and � being degenerate
Fails to describe
the excess of light
for h<40 mm
PTFE
(Fitting h>40 mm where the Lambert
component dominates and for λ ≤ 5 m)
IDM 2016, 18-23 July
R (reflectivity)
Best fit
95% CL
807NX
0.972
>0.97
NXT85
0.986
>0.984
LUX
0.987
>0.985
13
PTFE reflectivity: diffuse+specular model
Probability of specular reflection: Rspec(nPTFE/nLXe;�)
(Snell’s equations for unpolarized light);
Probability of diffuse reflection: Rdif=A[1-Rspec(nPTFE/nLXe;�)];
No surface description (e.g. roughness);
The reflectance now depends on the incident light distribution.
IDM 2016, 18-23 July
14
PTFE reflectivity: diffuse+specular model
The reflectance is calculated for a white sky
illumination (Bi-Hemispherical Reflectance);
Can only set a lower limit on the BHR due to
the degeneracy between A and �;
Good agreement with LUX data (PRD, in
preparation)
IDM 2016, 18-23 July
PTFE
BHR
Best Fit
95% CL
807NX
0.961
>0.955
NXT85
0.975
>0.973
LUX
0.978
>0.975
15
Summary & conclusions
We designed & built an experimental setup for measuring the absolute reflectance
of PTFE immersed in LXe;
We fitted the experimental results from three different samples of PTFE against a
detailed MC simulation using two different optical models for PTFE reflectivity:
diffuse-only and diffuse+specular reflection;
The implemented method is sensitive to small differences, � (0.1%), in the absolute
reflectivity of the PTFE samples;
Our results confirms that the reflectance of the PTFE immersed in LXe is very high
� (>97%) and also strongly support the existence of a specular reflection component,
which is usually not considered in the simulation of LXe DM detectors:
The correct description of this specular component may be very important
(depending on the geometry) for the correct reconstruction of events near the PTFE
walls (e.g. LZ, LUX, XENON);
… the ability to choose the PTFE with the highest reflectivity for the xenon scintillation
light plays a very important role in obtaining a low energy threshold in these detectors!
IDM 2016, 18-23 July
16
Summary & conclusions
We designed & built an experimental setup for measuring the absolute reflectance
of PTFE immersed in LXe;
We fitted the experimental results from three different samples of PTFE against a
detailed MC simulation using two different optical models for PTFE reflectivity:
diffuse-only and diffuse+specular reflection;
The implemented method is sensitive to small differences, � (0.1%), in the absolute
reflectivity of the PTFE samples;
Our results confirms that the reflectance of the PTFE immersed in LXe is very high
� (>97%) and also strongly support the existence of a specular reflection component,
which is usually not considered in the simulation of LXe DM detectors:
The correct description of this specular component may be very important
(depending on the geometry) for the correct reconstruction of events near the PTFE
walls (e.g. LZ, LUX, XENON);
… the ability to choose the PTFE with the highest reflectivity for the xenon scintillation
light plays a very important role in obtaining a low energy threshold in these detectors!
IDM 2016, 18-23 July
17
Extra plots
IDM 2016, 18-23 July
18
LUX PTFE: 95% CL band
� (mm)
DS Model
Best Fit
A
IDM 2016, 18-23 July
19
LUX PTFE: 95% CL band
� (mm)
DS Model
Best Fit
nPTFE
IDM 2016, 18-23 July
20
LUX Results
IDM 2016, 18-23 July
21