XENON Dark Matter Project
Karen Chen
Boston College
Nevis Labs, Columbia REU 2009
1
Outline

I: Xenon Detector Concepts


II: Previous Work


ER and NR Discrimination
XENON100
III: Current Work

XENON100 Upgrade
2
Xenon Detector Concept
Xe Dual phase TPC
anode



Evidence of non-baryonic
dark matter
WIMPs
Elastic collisions
cathode
3
Xenon Detector Concept
1.
anode
2.
3.
4.
cathode
5.
Interaction
(S1) Light
e- drift
Proportional
scintillation
(S2) Light
4
ER and NR Sources

Nuclear recoils (NR)

Neutrons




Created
from cosmic
cathode
muons
(alpha,n)
WIMPs
Electronic recoils (ER)

anode
Gamma
and beta rays

construction materials
5
ER and NR Sources

Maximize WIMP events


Large detector size
Minimize Background



Low radioactivity materials
Shielding
Underground laboratory
(LNGS)
6
XENON Detectors
Past, Present, and Future

XENON10 (2005-2007)


XENON100 (2006-2009)



Demonstrated dual phase xenon TPC for WIMP search
50kg fiducial volume (FV) mass
Simulation and Experimental results
XENON100+ (2009-2012)


100kg FV
Under current R&D (that’s me!)


Detector Geometry
Background simulations


Based on XENON100 data
XENON1T (2013-2015)


1 ton FV
Early R&D
7
XENON100

Screen materials for radioactivity

238U, 232Th, 40K,

Ge detector
Vary by manufacturer and thickness

and 60Co
Measured radiation rates for materials in XENON100 (plus QUPIDS)
8
XENON100

How many bananas is that?






PMTs and bases – 4.2 Bq
Stainless steel – 4.1 Bq
PTFE – 0.1 Bq
Total – 8.4 Bq
Banana* ~ 20 Bq, ~2x
Human** ~ 4000 Bq, ~476x
9
**Wikipedia *http://www.radlab.nl/radsafe/archives/9503/msg00074.html
XENON100

Background rate for different materials
MC Simulation by Alex
10
PMTs and QUPIDs
Primary BG source



Photomultiplier tubes (PMTs)
Lowest radioactivity on the market!
Need lower radioactivity PMTs!


Quartz Photon Intensifying Detector
(QUPID)

Developed by Hamamatsu Photonics and
Prof. Arisaka (UCLA)
11
PMTs and QUPIDs
XENON10 and XENON100

98 top PMTs with ~24% QE

80 bottom PMTs with ~34% QE
XENON100+ and XENON1T

Same top array

19 bottom QUPIDs

$$$
QUPIDs created for
XENON100 Upgrade
12
XENON100 Upgrade
Improvements on XENON100
 Reducing Background

Lower radioactivity materials


QUPIDs
More Xe, less material


Cryostat - Domed for stability
Steel thickness from 1.5mm to 0.1mm


Need to test
Exception:

More can be better
Shielding
Scaling Up
Add radius or height?
13
XENON100 Upgrade

Steel vs Copper Cryostat

Copper




Stainless Steel



MC Simulation by Alex
High thermal
conductivity
Soft Metal
Low BG
Medium thermal
conductivity
Sturdy
High BG
14
XENON100 Upgrade

Copper


LXe
170K


High thermal
conductivity
Soft Metal
Low BG
Stainless Steel



Medium thermal
conductivity
Sturdy
High BG
15
Shielding

Separate the xenon


Cooling tower moved for Xenon100
The trade off:

Less external BG


neutrons, muons
More intrinsic BG


Shielding for Xenon100 Upgrade
radioactive decay
Cutting Costs:

XENON1T Shield
16
XENON100 Upgrade

Detector Geometry

Double the mass





Height or radius?
Radius limited by QUPIDs
Increase the height
height  drift length
QUPIDs
Drift Length Concerns


High voltage
Pileup Problem
17
Pileup Problem

What is pileup?

Events recorded by trigger


Record length



Noise or signal?
Time for electron to drift from one end to the other
S1 and S2 signal in one event
Multiple events -> Uncertainty
18
Pileup Problem
1.
2.
3.
4.
5.
6.

Event A S1 Signal
Electron A drifts
Event B S1 Signal
Electrons drift
Event B S2 Signal
Event A S2 Signal
Which signal corresponds to
which event?
Detector
19
Pileup Problem
Estimate likelihood of pileup
n = true interaction rate
m = recorded count rate
τ = dead time (record length)
m = ne-nτ*
Percent Loss = 1 - e-nτ

But what is the trigger rate?
20
*Radiation Detection and Measurement by Knoll pgs 120-123
XENON100 Upgrade

Ideas into Monte Carlo Simulations


If I use this geometry, what BG can I expect?
Geant4

Create the detector geometry


XENON100
Simplified: Bell, Cryostat, PMTs, Teflon panel
Simulate the decay chains
238U, 232Th, 40K, 60Co
Scale by radioactivity of each material
Analyze - Make appropriate cuts
Multiple scatters, energy
Fiducial volume
21
XENON100 Upgrade
Steel Cryostat (Inner)
Bell
PMTs
Steel Cryostat (Inner)
Teflon
Teflon
Panel
QUPIDs
TPC/Target
Xe Veto
22
XENON100 Upgrade

Ideas into Monte Carlo Simulations


If I use this geometry, what BG can I expect?
Geant4

Create the detector geometry



XENON100
Simplified: Bell, Cryostat, PMTs, Teflon panel
Simulate the decay chains

238U, 232Th, 40K, 60Co

Scale by radioactivity of each material
Analyze - Make appropriate cuts
Multiple scatters, energy
Fiducial volume
23
XENON100 Upgrade

Simulation check:

Rates scale with mass
XENON100 (Alex)
XENON100 Upgrade (Karen)
24
XENON100 Upgrade

Side note:

Manipulating energy spectrum with thickness
K40
U238
Th232
Co60
K40
U238
Th232
Co60
25
XENON100 Upgrade

Side note:

Material thickness and K-40 spectrum
26
XENON100 Upgrade

Event Rate and Energy
All Materials
PMTs
Steel
Teflon
Copper
Trigger rate estimate:
~0.05Hz
27
XENON100 Upgrade

Ideas into Monte Carlo Simulations


If I use this geometry, what BG can I expect?
Geant4

Create the detector geometry




XENON100
Simplified: Bell, Cryostat, PMTs, Teflon panel
Simulate the decay chains

238U, 232Th, 40K, 60Co

Scale by radioactivity of each material
Analyze - Make appropriate cuts


Multiple scatters, energy
Fiducial volume
28
XENON100 Upgrade

Number of scatters

Detector Resolution


Single scatter events in the target volume


~3mm
Good efficiency from PMTs
Events in the xenon veto


Low efficiency of veto PMTs
need >50keVee of energy
29
XENON100 Upgrade

Different energy in veto cuts
30
XENON100 Upgrade

Different energy in veto cuts

Histogram of events in the best volume cut
31
XENON100 Upgrade
Xenon100 Event distribution
(Alex)

Event Distribution

Fiducial Volume Cut


Low background core
Radial vs Height cuts
32
XENON100 Upgrade

Event Distribution: PMTs
33
XENON100 Upgrade

Event Distribution: Steel
34
XENON100 Upgrade

Event Distribution: Teflon
35
XENON100 Upgrade

Event Distribution: Copper
36
XENON100 Upgrade

Event Distribution: All
37
XENON100 Upgrade

Event Distribution Patterns


Top Heavy: Steel and PMTs
Radial: Teflon, Steel (somewhat)
Radial cut - - - - - - - - - - - - > Height cut
38
XENON100 Upgrade

Added Top Xenon Veto
Xe Top Veto
Xe Veto
39
XENON100 Upgrade

Current Design and BG rate

Steel contribution is lowered!

Looks promising!


Reached low background rates in proposal
Doubled FV
40
Summary

XENON100


BG contribution from different materials
XENON100 upgrade





Steel vs Copper cryostat
Doubling the mass -> height ->drift length
Pileup – not an issue
Ideas for detector geometry
Analyzed MC simulation results




Effect of veto energy cut
Background levels, trigger rate
Re-simulated with top LXe veto -> Steel BG
BG levels within design levels in NSF proposal
41
Acknowledgements

XENON Group





Rafael
Elena Aprile
Guillame, Bin, Kyungeun (Elizabeth), Luke
Emily
Nevis REU


Mike Shaevitz, John Parsons
All my fellow REU students
42
Questions?

XENON100


BG contribution from different materials
XENON100 upgrade




Steel vs Copper cryostat
Doubling the mass -> height ->drift length
Pileup – not an issue
Analyzed MC simulation results




Effect of veto energy cut
Background levels, trigger rate
Re-simulated with top LXe veto
BG levels within design levels in NSF proposal
43