A Low-Temperature Energy Calibration System for the CUORE Bolometric Double Beta-Decay Experiment

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1 A Low-Temperature Energy Calibration System for the CUORE Bolometric Double Beta-Decay Experiment Karsten M. Heeger University of Wisconsin on behalf of the CUORE Collaboration 1

2 CUORE Double Beta Decay Experiment CUORE: Cryogenic Underground Observatory for Rare Events will be a tightly packed array of 988 bolometers with mass of ~ 200 kg of 130 Te 80 cm 19 Cuoricino-like towers with 13 planes of 4 crystals each Operated at Gran Sasso laboratory Special cryostat built w/ selected materials Cryogen-free dilution refrigerator operated at ~ 10mK Shielded by several lead shields 2

3 TeO 2 Bolometers 5 cm 790g per crystal TeO 2 Bolometer: Source = Detector deposited energy Heat sink: Cu structure (8-10 mk) Thermal coupling: Teflon (G = 4 pw/mk) Thermometer: NTD Ge-thermistor (dr/dt 100 kω/µk) Absorber: TeO 2 crystal (C 2 nj/k 1 MeV / 0.1 mk) Single pulse example For E = 1 MeV: ΔT = E/C 0.1 mk Signal size: 1 mv voltage signal energy deposited Time constant: τ = C/G = 0.5 s Amplitude (a.u.) Energy resolution: ~ 5-10 kev at 2.5 MeV Time (ms) 3

4 Search for 0νββ in 130 Te cartoon of 2νββ and 0νββ spectra Experimental Signature of 0νββ - peak at the transition Q-value - enlarged by detector resolution over unavoidable background due to 0νββ Q( 130 Te)= ± 2 kev energy is the key event signature of candidate events Cuoricino summed spectrum individual energy calibration of all 988 bolometers critical for summing energy spectra 4

5 Calibration of Cuoricino/CUORE Bolometers Gain Stabilization For each bolometer an energy pulse generated by a Si resistor is used to correct pulse amplitudes for gain instabilities ( every 5 min). Voltage-Energy Conversion Fit of a calibration measurement with a gamma source (e.g. 232 Th). Energy calibration performed regularly. (~ monthly). Cuoricino calibration Cuoricino summed spectrum from all detectors 5

6 CUORE Detector Calibration System need to place sources next to crystals to allow calibration of all bolometers Physics Objectives uniform energy calibration for all 988 CUORE bolometers in energy region from kev calibration point near Q-value (2530 kev) Key Issues heat load requirements of cryostat calibration rate of < 100mHz for each bolometer to avoid pile-up safe and reliable operation of source system during livetime of experiment (> 5 years) ability to retract sources completely out of cryostat during normal data taking ability to exchange source strings and use different types of γ sources minimize radioactive background from calibration system reasonable calibration time (< 1 week), minimize loss in detector livetime 6

7 CUORE Detector Calibration System need to place sources next to crystals to allow calibration of all bolometers vertical cross section of the cryostat 300K 40K 4K 0.7K 80mK 10mK Pb shield ~10mK Pb shield motion system: insertion and extraction of sources in and out of cryostat guide tubes: no straight vertical access source strings: move under own weight in guide tubes source locations top view of detector array with source positions 7

8 CUORE Detector Calibration System ~1m motion box Motion Box and Drive Spool System guide pulley vacuum flange electrical feedthrough guide tubes load cell motor narrow spool for spiral winding vacuum feedthrough with shaft detectors drive spools can be removed for individual source exchange 8

9 CUORE Detector Calibration System ~1m Source String flexible, moves in guide tube under own weight small mass: < 5 grams vertical distribution of source activity can be adjusted Kevlar string source wire ~10mm Cu crimp PTFE heat shrink Guide Tubes stainless and/or machined from solid, low-background copper radioactive source wire 56 Co: proton activated Fe wire 232 Th: Thoriated Tungsten wire 9

10 Calibration Source Simulations source positions external sources internal sources Optimization of Source Strength, Position, and Distribution achieve uniform illumination of all crystals with internal/external sources determine max source activity, minimize calibration time event rate in crystals Max hit rate of 100mHz per crystal to avoid pile-up, based on Cuoricino experience Activity per discrete source: internal/external sources: 87 mbq/430 mbq internal/external sources edges: 126 mbq/1010 mbq layer 5 10

11 Calibration Source Simulations radioactive sources: 56 Co and/or 232 Th 56Co: proton activated Fe wire; 232Th: Thoriated Tungsten wire both have been used in Cuoricino Calibration time vs counts in peak ~100 events over background per peak are required for successful calibration 11

12 Cryogenic Challenges Calibration system must be integrated with complex detector cryostat Must meet available cooling power requirements at all thermal stages Stage T [K] Calibration system cooling power budget [W] 300K 40K 4K 0.7K 80mK 10mK guide tubes lead shield 40K ~ 1 IVC STILL m HEX µ MC µ lead shield bolometric detectors TSP 0.01 < 1µ Cooldown of source string Friction heat during insertion/extraction Static heat load of guide tubes: thermal conductance&radiation emission 12

13 Cryogenic Challenges Cool Down of Source String - source strings need to be cooled below 4K to meet allowable thermal loads of cryostat and detectors - sources thermalized and parked in region around 4K flange Friction from Source String Motion 300K 40K 4K 0.7K 80mK 10mK sliding friction + friction of string in guide tube bends bolometric detectors T 2 T 1 = e µ kβ 13

14 Cryogenic Challenges Cool Down of Source String - source strings need to be cooled below 4K to meet allowable thermal loads of cryostat and detectors - sources thermalized and parked in region around 4K flange Friction from Source String Motion 300K 40K 4K 0.7K 80mK 10mK Power dissipated [W] velocity 0.1mm/sec bolometric detectors Distance traveled by source [m] it will take several hours to extract source string 14

15 Cryogenic Challenges Static Heat Load thermal conductance + radiation emission from guide tubes & source string Design Approach lower conductivity material for the guide tubes where possible stainless steel in upper region of cryostat backgrounds require ultra-pure copper to be used in lower-region of cryostat thermal decoupling of guide tubes at HEX stage Thermal Modeling Dissipated on 40K 4K STILL HEX (int) HEX (ext) HEX (tot) MC perfect thermal coupling weak thermal coupling stainless steel (low background) copper internal guide tubes 300K 40K 4K STILL HEX MC vibration decoupling TSP BSP external guide tubes this section runs along HEX vessel detectors cryostat guide tubes 15

16 Conclusions Energy is the key event signature for 0νββ candidate events in CUORE and for discriminating backgrounds. Energy calibration is critical for summing the spectra from the 988 individual CUORE detectors. We have developed a design for a movable, low-temperature calibration source insertion system for source insertion/extraction. Key components are being prototyped. Motion tests through guide tubes have been performed at 300K to demonstrate feasibility. We are preparing for cold motion tests at <4K to verify assumptions of our thermal modeling. Schedule 2008/2009: cryogenic tests of small-scale fully-functional prototype at Wisconsin 2009: installation of 2 full-scale prototype guide tube routings in the CUORE cryostat at LNGS and tests at 300K and < 4K. 2010: installation and commissioning of the calibration system in the CUORE cryostat 16

17 CUORE Collaboration University of California at Berkeley A. Bryant 2, M.P. Decowski 2, M.J. Dolinski 3, S.J. Freedman 2, E.E. Haller 2, L. Kogler 2, Yu.G. Kolomensky 2 University of South Carolina F.T. Avignone III, I. Bandac, R. J. Creswick, H.A. Farach, C. Martinez, L. Mizouni, C. Rosenfeld Lawrence Berkeley National Laboratory J. Beeman, E. Guardincerri, R.W. Kadel, A.R. Smith, N. Xu Lawrence Livermore National Laboratory K. Kazkaz, E.B. Norman 4, N. Scielzo University of California, Los Angeles H. Z. Huang, S. Trentalange, C. Whitten Jr. University of Wisconsin, Madison L.M. Ejzak, K.M. Heeger, R.H. Maruyama, S. Sangiorgio California Polytechnic State University T.D. Gutierrez Universita di Milano-Bicocca 5 C. Arnaboldi, C. Brofferio, S. Capelli, M. Carrettoni, M. Clemenza, E. Fiorini, S. Kraft, C. Maiano, C. Nones, A. Nucciotti, M. Pavan, D. Schaeffer, M. Sisti, L. Zanotti Sezione di Milano dell INFN F. Alessandria, L. Carbone, O. Cremonesi, L. Gironi, G. Pessina, S. Pirro, E. Previtali Politecnico di Milano R. Ardito, G. Maier Laboratori Nazionali del Gran Sasso M. Balata, C. Bucci, P. Gorla, S. Nisi, E. L. Tatananni, C. Tomei, C. Zarra Universita di Firenze and Sezione di Firenze dell INFN M. Barucci, L. Risegari, G. Ventura Universita dell Insubria 5 E. Andreotti, L. Foggetta, A. Giuliani, M. Pedretti, C. Salvioni Universita di Genova S. Didomizio 6, A. Giachero 7, P. Ottonello 6, M. Pallavicini 6 Laboratori Nazionali di Legnaro G. Keppel, P. Menegatti, V. Palmieri, V. Rampazzo Universita di Roma La Sapienza and Sezione di Roma dell INFN F. Bellini, C. Cosmelli, I. Dafinei, R. Faccini, F. Ferroni, C. Gargiulo, E. Longo, S. Morganti, M. Olcese, M. Vignati Universita di Bologna and Sezione di Bologna dell INFN M. M. Deninno, N. Moggi, F. Rimondi, S. Zucchelli University of Zaragoza M. Martinez Kammerling Onnes Laboratory, Leiden University A. de Waard, G. Frossati Shanghai Institute of Applied Physics (Chinese Academy of Sciences) X. Cai, D. Fang, Y. Ma, W. Tian, H. Wang special thanks to Samuele Sangiorgio, 2also LBNL Larissa Ejzak, Tom Wise, and Reina Maruyama 3also LLNL for slides and figures 4also UC Berkeley 5also Sezione di Milano dellʼinfn 6also Sezione di Genova dellʼinfn 7also LNGS 17

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