Optimization of Light Detectors for the CRESST Experiment Anja Tanzke Max-Planck-Institute for Physics Technische Universit¨ at M¨ unchen March 19th 2012
Direct Dark Matter Search with the CRESST Experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) Direct detection of WIMPs via elastic scattering off nuclei Scintillating CaWO 4 crystals as target material Interactions excite phonons and produce scintillation light Scintillation light is used to distinguish different kinds of interactions A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 2 / 15
CRESST Detector Modules Phonons signal and light signal are measured Phonon Channel Light Channel Signals measured with Transition Edge Sensors (TES) operated at mK temperatures A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 3 / 15
Light-Phonon-Plane Coincident light phonon measurement enables event identification Typical energy resolution A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 4 / 15
Light-Phonon-Plane Coincident light phonon measurement enables event identification Typical energy resolution Light channel energy resolution improved by a factor of 5 Width of the bands is determined by the light channel energy resolution Goal E Improvement of the light channel’s signal-to-noise ratio ∆ E A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 4 / 15
Light Channel’s Signal-to-Noise Ratio E ∆ E depends on many parameters Measured Energy E = c · ∂ U ∂ Φ 0 I T r ∂ I · R S + R T · m · C T · pq · E dep ∂ Φ 0 Noise (@ E = 0) � � � � 4 k B ( T C R T + T S R S ) + c 2 + f 2 R T I 2 J + I 2 SQ + I 2 ∆ E = 1 / f = ( R T + R S ) 2 R n A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 5 / 15
Light Channel’s Signal-to-Noise Ratio E ∆ E depends on many parameters Measured Energy E = c · ∂ U ∂ Φ 0 I T r ∂ I · R S + R T · m · C T · pq · E dep ∂ Φ 0 Noise (@ E = 0) � � � � 4 k B ( T C R T + T S R S ) + c 2 + f 2 R T I 2 J + I 2 SQ + I 2 ∆ E = 1 / f = ( R T + R S ) 2 R n A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 5 / 15
Energy Fraction Absorbed by the Light Detector pq E ∆ E ∼ pq (@ E = 0) Data of CRESST Light Detectors A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 6 / 15
Energy Fraction Absorbed by the Light Detector pq E ∆ E ∼ pq (@ E = 0) Data of CRESST Light Detectors Increased sensitivity for improved light-production, -transport or -absorption A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 6 / 15
Energy Fraction Absorbed by the Light Detector pq E ∆ E ∼ pq (@ E = 0) Data of CRESST Light Detectors Increased sensitivity for improved light-production, -transport or -absorption → Black silicon A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 6 / 15
Transition Temperature T C 1 1 E ∆ E ∼ C T · f ( T C ) ∼ T C (1+ ε ) Data of CRESST Light Detectors A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 7 / 15
Transition Temperature T C 1 1 E ∆ E ∼ C T · f ( T C ) ∼ T C (1+ ε ) Data of CRESST Light Detectors Sensitivity improves for lower transition temperature A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 7 / 15
Transition Slope m ∆ E ∼ m = ∆ R E ∆ T A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 8 / 15
Transition Slope m ∆ E ∼ m = ∆ R E ∆ T Transition slope m depends on read-out current Self-heating effect Critical current effect A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 8 / 15
Transition Slope m ∆ E ∼ m = ∆ R E ∆ T Transition slope m depends on read-out current P T Self-heating effect → ∆ T Shift = G BT Critical current effect → expected in the lower part of the transition A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 8 / 15
Calculation of the Self-Heating Effect A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 9 / 15
Calculation of the Self-Heating Effect A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 9 / 15
Calculation of the Self-Heating Effect Upper part of the transition can be explained with the self-heating effect Difference in the lower part of the transition due to the critical current effect A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 9 / 15
Reduction of the Normal Conducting Resistance R n large R n small R n Reduced critical current effect for small R n A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 10 / 15
Reduction of the Normal Conducting Resistance R n large R n small R n Reduced critical current effect for small R n small R n opens the possibility for large transition slopes A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 10 / 15
Thermometer Current I T E ∆ E ∼ I T 9 different light detectors are optimized for best signal-to-noise Limiting factors of thermometer current? A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 11 / 15
Thermometer Current I T E ∆ E ∼ I T 9 different light detectors are optimized for best signal-to-noise Limiting factors of thermometer current? A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 11 / 15
Thermometer Current I T E ∆ E ∼ I T 9 different light detectors are optimized for best signal-to-noise Limiting factors of thermometer current? Thermometer current is mostly limited by cooling power A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 11 / 15
Thermal Coupling between Bath and Thermometer G BT Increase of I T requires increase of cooling power P BT = G BT ( T C − T B ) Only possible with larger thermal coupling → Increase of thermal coupling A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 12 / 15
Thermal Coupling between Bath and Thermometer G BT Increase of I T requires Temperature rise for given increase of cooling power energy deposition reduced P BT = G BT ( T C − T B ) with larger thermal coupling Only possible with larger thermal coupling → Increase of thermal coupling → Reduction of thermal coupling A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 12 / 15
Thermal Coupling between Bath and Thermometer G BT Increase of I T requires Temperature rise for given increase of cooling power energy deposition reduced P BT = G BT ( T C − T B ) with larger thermal coupling Only possible with larger thermal coupling → Increase of thermal coupling → Reduction of thermal coupling → Optimization of thermal coupling A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 12 / 15
Optimization of the Thermal Coupling G BT Signal height calculated for different thermal couplings Largest signal expected for about ten times larger coupling A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 13 / 15
Summary and Outlook Motivation Light channnel energy resolution is the crucial factor for CRESST Results Increase of light-production, -transport or -absorption → Black Silicon Lower transition temperature → Transition temperature of about 15 mK Larger transition slope → Reduction of critical current effect with smaller normal-conducting resistance Increased read-out current with optimized thermal coupling A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 14 / 15
Thank you for your attention A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 15 / 15
Backup A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 15 / 15
Self-Heating Effect R 2 P T = R T I 2 T = R T I 2 S ( R S + R T ) 2 Tot cooling power: P BT = ( T T − T B ) G BT P T → measured ∆ T Shift = T T − T B = G BT A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 15 / 15
Critical Current Effect the thermometer current I T influences the measured resistance the change of the current dI T influences the observed transition → critical current effect is expected to appear for R T � R S A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 15 / 15
Read-out current R 2 I T = I Tot S R S + R T A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 15 / 15
Temperature Rise in the Thermometer − t − t − t − t � � � � τ + − e τ − − e ∆ T T = A + e + A − e (1) τ N τ N = A + ( t ) + A − ( t ) (2) � 1 1 1 � − G AT + G BT � + (1 − ε ) · G AT � · E LD A ± = · · ε · 1 − τ N 1 1 C T C A C T τ ∓ − τ ∓ τ ± τ ± (3) 2 τ ± = √ (4) a 2 − 4 b a ± a = G AT + G BT + G AT + G AB (5) C T C A b = G AT G BT + G AT G AB + G BT G AB (6) C T C A A. Tanzke (MPP/TUM) Light Detector Optimization for CRESST March 19th 2012 15 / 15
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