The Light Channel of the CRESST Experiment Anja Tanzke - - PowerPoint PPT Presentation

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The Light Channel of the CRESST Experiment Anja Tanzke Max-Planck-Institute for Physics Technische Universit at M unchen May 9th 2014 Table of Contents Introduction to the CRESST Experiment 1 Light Channel 2 Foil 3 Reflectivity of

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SLIDE 1

The Light Channel of the CRESST Experiment

Anja Tanzke

Max-Planck-Institute for Physics Technische Universit¨ at M¨ unchen

May 9th 2014

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SLIDE 2

Table of Contents

1

Introduction to the CRESST Experiment

2

Light Channel

3

Foil Reflectivity of the Foil Scintillation of the Foil

4

Results

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 2 / 23

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SLIDE 3

Direct Dark Matter Search with the CRESST Experiment

CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) Direct detection of Dark Matter in the form of WIMPs (Weakly Interacting Massive Particles) via elastic scattering off nuclei located at the LNGS (Laboratori Nazionali del Gran Sasso) in Italy Scintillating CaWO4 crystals as target material

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 3 / 23

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SLIDE 4

CRESST Detection principle

Energy depositions in the crystal mainly excite phonons

temperature rise in the crystal (O(µK)) → detectors operated at mK temperatures

small fraction of deposited energy produces scintillation light → separate light detector both signals measured with Transition Edge Sensors (TES) made of a W film change of resistance in the film measured with a SQUID based readout

heat bath heat bath thermal coupling light detector (with TES) target crystal reflective and scintillating housing TES

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 4 / 23

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SLIDE 5

Detector module

Detector module: Phonon detector + Light detector surrounded by a reflective and scintillating housing simultaneous measurement of

Phonon Channel: deposited energy in the crystal (independent

  • f type of particle)

Light Channel: scintillation light → allows discrimination of different types of particles

heat bath heat bath thermal coupling light detector (with TES) target crystal reflective and scintillating housing TES

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 5 / 23

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SLIDE 6
  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 6 / 23

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SLIDE 7

Event discrimination

Phonon signal = Energy deposited in the crystal Light signal used to discriminate different types of interactions Light Yield = light signal/phonon signal Light Yield characteristic for each event type excellent discrimination between dominant background (e−-recoils) and potential signal events (nuclear recoils)

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 7 / 23

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SLIDE 8

Event discrimination

Phonon signal = Energy deposited in the crystal Light signal used to discriminate different types of interactions Light Yield = light signal/phonon signal Light Yield characteristic for each event type WIMP search region (ROI) including O, Ca and W bands below 40keV excellent discrimination between dominant background (e−-recoils) and potential signal events (nuclear recoils)

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 7 / 23

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SLIDE 9

Light Channel Energy Resolution

Width of the bands is mainly determined by the light channel energy resolution

Energy resolution of a typical CRESST detector module

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 8 / 23

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SLIDE 10

Light Channel Energy Resolution

Width of the bands is mainly determined by the light channel energy resolution

Energy resolution of a typical CRESST detector module Light channel energy resolution improved by a factor of 5

Improved light channel’s energy resolution increases discrimination power

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 8 / 23

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SLIDE 11

Light Detection

Energy resolution of the light detector ∆E depends on the fraction of recoil energy that is absorbed by the light detector pq Energy fraction absorbed by the light detector pq

energy fraction transformed into scintillation light p fraction of scintillation light absorbed by the light detector q p and q are difficult to distinguish → only pq can be determined absolute calibration of the light detector with an X-ray (55Fe) to determine pq

Energy resolution of the light detector ∆E

determined for small energies also depends on other parameters (e.g. the transition of the TES) than pq, but can be corrected for these

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 9 / 23

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SLIDE 12

Energy Fraction absorbed by the Light Detector pq

Energy fraction absorbed by the light detector pq for different modules currently running in CRESST (Run33) larger fraction of absorbed light pq → better energy resolution ∆E

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 10 / 23

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SLIDE 13

How to increase the amount of absorbed Light?

produced scintillation light = p · Erec

❄

crystal

increase light

  • utput

material with higher light

  • utput
  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 11 / 23

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SLIDE 14

How to increase the amount of absorbed Light?

produced scintillation light = p · Erec absorbed scintillation light = q · p · Erec

❄ ❅ ❅ ❅ ❘

crystal

increase light

  • utput

material with higher light

  • utput

light detector

better light absorber improve detector design

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 11 / 23

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SLIDE 15

How to increase the amount of absorbed Light?

produced scintillation light = p · Erec absorbed scintillation light = q · p · Erec

❄

  • ✠

❅ ❅ ❅ ❘

crystal

increase light

  • utput

material with higher light

  • utput

foil

larger reflectivity

light detector

better light absorber improve detector design

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 11 / 23

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SLIDE 16

Foil VM2002

Reflective and scintillating multi-layer polymeric foil VM2002 Reflectivity measurement at 300K cut-off wavelength at 375nm Emission spectrum of CaWO4 (at 300K) Absorption of SOS (silicon on sapphire) Light Detector (at 300K)

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 12 / 23

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SLIDE 17

Foil Lumirror

Reflective Foil Lumirror Reflectivity measurement at 300K cut-off wavelength at 325nm fluorescence contribution between 320 and 420 nm

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 13 / 23

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SLIDE 18

Comparison of VM2002 and Lumirror

Reflectivity can change when foil is cooled down to mK temperatures Compare the reflectivity at mK temperatures:

2 cryogenic measurements with the same detector module (one with each foil) everything except the foil stays the same

Result

VM2002: pq = 1.58% Lumirror: pq = 1.42% Lumirror foil reflects 10% less light of CaWO4 at mK temperatures

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 14 / 23

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SLIDE 19

Background from α contamination on surfaces

alpha contamination on surfaces inside the detector module can induce background main source is 222Rn from ambient air which deposits on the detector and the housing

222Rn decays to 210Po, which induces a background by its decay 210Po →206 Pb(103keV ) + α(5.3MeV )

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 15 / 23

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SLIDE 20

Scintillation as veto for surface α decays

alpha hitting the foil → additional scintillation light Foil events can be cut due to a different pulse shape Improvement possible with a material scintillating better than the foil VM2002

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 16 / 23

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SLIDE 21

Parylene C

Parylene C is a good scinillator at room temperature clean raw material available Foil can be coated via polmerization (commercial process) additional cleaning during production process additional advantage: Reset of the “radon-history”of the foil

Exposure of foil to radon contaminated air cannot be controlled (comercial product) Coat the foil with a homogeneus Parylene layer

→ Measurement of scintillation of Parylene C at mK temperatures

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 17 / 23

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SLIDE 22

Scintillation of Parylene C

Setup to measure the scintillation of Parylene C at mK temperatures Energy calibration with an X-ray source (55Fe) Sapphire disk to prevent alphas hitting the light detector directly

Result

a 5.6MeV alpha produces 4.7keV scintillation light in 12µm Parylene comparison: a 5.6MeV alpha produces 2keV scintillation light in the foil VM2002 → Parylene C scintillates more than twice as well as the foil VM2002

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 18 / 23

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SLIDE 23

Parylene coated Foil in CRESST

due to low count rates foil events can only be measured in CRESST 7 modules in the current CRESST Run (Run33) were equipped with Parylene coated foil Comparison of uncoated foil and Parylene coated foil

2 modules with the same module design both are equipped with an X-ray source for the absolute calibration of the light detector module 1: equipped with uncoated VM2002 foil module 2: equipped with a VM2002 foil coated with 10µm Parylene

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 19 / 23

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SLIDE 24

Module with VM2002 Foil (uncoated)

a foil event with 100keV recoil energy produces 0.78keV detected light

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 20 / 23

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SLIDE 25

Module with Parylene coated Foil

a foil event with 100keV recoil energy produces 1.45keV detected light

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 21 / 23

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SLIDE 26

Comparison

foil event with 100keV recoil energy produces 0.78keV detected light foil event with 100keV recoil energy produces 1.45keV detected light

Result

Parylene coated foil produces twice as much light → foil events are higher in the light yield-energy plane

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 22 / 23

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SLIDE 27

Results

Resolution of the light channel is improved with more detected light VM2002 foil is (up to now) the most reflective foil for CaWO4 Parylene C is a good scintillator at mK temperatures Scintillation of the foil can be improved with a Parylene coating

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 23 / 23

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SLIDE 28

Backup

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 23 / 23

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SLIDE 29

Radon decay chain

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 23 / 23

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SLIDE 30

Module Designs

reflective and scintillating housing light detector (with TES) target crystal TES holding clamps light detector (with TES) absorber crystal carrier crystal (with TES) glue scintillating holding clamps

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 23 / 23

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SLIDE 31

Parylene Coating

  • A. Tanzke (MPP/TUM)

Light Channel of CRESST May 9th 2014 23 / 23