COBRA CZT detectors for neutrinoless double beta decay

COBRA – CZT detectors for neutrinoless double beta decay measurements Martin Freer, University of Birmingham

COBRA collaboration University of Sussex University of Warwick University of Liverpool Rutherford Appleton Laboratory University of Birmingham University of York University of Dortmund Material Research Centre Freiburg Laboratori Nazionali del Gran Sasso University of Bratislava University of Jyvaskyla University of La Plata Technical University Prague Washington University at St. Louis University of Surrey (UK), University of Hamburg (Germany), Jagellonian University (Poland), Louisianna State University (USA), Technical University Dresden (Germany)

Fundamental neutrino properties Are neutrinos their own antiparticles? What is the absolute neutrino mass? β- β- β- Energy

Sensitivity 0 : Peak at Q-value of nuclear transition Half-life factors: Half-life and neutrino mass Critical factors (factor of 2 in neutrino mass requires factor of 16 improvement): • Background • Energy resolution • Mass • time

2 - decay 2 is the ultimate irreducible background Energy resolution important semiconductor Fraction of 2 in 0 peak: S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci. 2002 Signal/Background:

How big does a detector need to be? ( Background free) 50 me. V implies half-life measurements of 1026 -27 yrs 1 event/yr - need 1026 -27 source atoms This is about 1000 moles of isotope, implying 100 kg Need to get backgrounds to below the above level! (currently achieved levels about 0. 1 ct/ke. V/kg/yr)

COBRA (Source = Detector) • Cd. Zn. Te (room temp semiconductor) nat. ab. (%) Q (ke. V) Decay mode

Burst Alert Telescope (BAT) Gamma-ray applications Resolution < 1% @ 662 ke. V 32, 768 pieces of 4 x 2 mm (CZT) form a 1. 2 x 0. 6 m sensitive area

CZT for COBRA To probe neutrino masses below 50 me. V • • • Isotopic abundance (90%, i. e. enrichment) Detection efficiency (65%) Mass (420 kg Cd. Zn. Te, about 180 kg Cd) Energy resolution (1 -2% at 2. 8 Me. V) Background at peak position (10 -3 -10 -4 cts/kg/ke. V/yr) Two strands: (i) large array of CPG CZT detectors at Gran Sasso (Current) (ii) Development of Pixel CZT detector (Future – PPRP)

Gran Sasso (GPG –CZT)

Measurement of 4 -fold forbidden decay of 113 Cd Half-life around 1015 yrs. Only three isotopes of this type are known, 50 V, 113 Cd and 115 In.

Sensitivity (background free) (background limited) Crucial items (for every experimemt): 50 me. V - mass (enrichment) - energy resolution - background

Pixel Detectors 3. 2 mm 2800 ke. V electrons 3 Me. V 2800 ke. V electron pairs 0

Alpha-decay Compton Scattering Acceptable Pixel Response Event not vetoed Unacceptable pixel response Event Vetoed

What are the main natural backgrounds? Total U-238 Th-232 K-40 • U, Th chains, 40 K and 137 Cs. • Remove events where more than 1 crystal has E>10 ke. V. Cs-137

130 Te For 200 m 130 Te and 116 Cd would hit >7 pixels 1 β 70% 0 2β 1

Identification of alpha-particles

Pixelisation - Running 256 pixel det with ASIC, 1. 6 mm pixel size crystal ASIC readout Additional 16 pixel detector with conventional readout running Single pixel 57 Co spectrum 122 ke. V 136 ke. V

Compton photo E 2 60 Co 154 Eu E 1 Compton

Anode Cathode Anode

Background measurements 218 Po T 1/2=3. 10 mins Cathode 214 Po 218 Pb (7. 68 Me. V) (6. 00 Me. V) Anode Cosmic rays Uranium decay chain 238 U

Scanning the 16 pixel - Liverpool Homogeneity scan 30 m precision 1 GBq 57 Co source Sliced scanning at side with Flash ADC z-information in risetime

Pixel detectors - next step 64 pixel detectors 2 x 2 x 0. 5 cm 3 Pixel electrodes will be replaced by 200 m pixels, mask in hand

Summary COBRA collaboration is presently completing the installation of a 64 element CZT array in Gran Sasso. This will further improve half-life limits. Ultimate tool for double beta search would be a high resolution Pixel CZT detector – presently engaged in design of a 200 micon pixel detector TIMESCALE: prototype Pixel detector and readout by 2011 running experiment by 2013

Enrichment Isotopical enrichment is crucial to keep experimental size under control and target mass high Crystal growing requires 7 N input material quality Enriched material has to be purified for that, this has to be explored Goal: To produce an enriched Cd. Zn. Te detector, setup logistic Crystal growing will be done at Material Research Centre Freiburg (Germany)

Background 116 Cd (Q=2809 ke. V) • Alphas, Betas, Gammas • Cosmogenics Measurement: E. Porras et al. , NIM B 111, 325 (1996) • neutrons 113 Cd (nth, )114 Cd • 2 • muon induced neutrons

Pixel pads 50 m pixel detector, produced at Washington Univ. at St. Louis Will require new readout electronics , ie ASIC

Pixel-detectors and readout • Develop prototype pixel detector • Full testing, simulation, optimisation • Develop and test prototype ASIC for pixel readout • Library of well defined events (ion implantation) • Evaluate responses, data rates and performance • Running pixel detector underground • Demonstrate radiopurity

The 64 detector array Crucial to identify background components long term stability, …energy measurement only Installed at LNGS in April 2006 Mass factor 16 higher, about 0. 42 kg Cd. Zn. Te Worldwide largest experiment of 1 cm 3 CPG detectors Remaining 48 will be installed in two weeks

Underground at Boulby Pixel detectors will be installed in Boulby, readout totally different from LNGS setup. Link with Boulby already established

The strategy • We believe that pixelated CZT detectors are THE step forward in background reduction with respect to pure passive methods • Experimentally confirm the large background reduction of pixelated detectors, includes development of high resolution detectors and readout electronics • Ultimate prove that such a system running underground is clean enough • Setup logistic for enriched detector production and produce a running detector with good parameters We have a clear strategy and but we need manpower

New Results PRELIMINARY

Additional issues • LNGS very supportive, offering more space at a different location • New funding from German Research Society (DFG): 100 k. Euro plus two 2 year Ph. D positions • US grant submitted (DUSEL R&D opportunities): 600 k$

Shielding and Veto (WP 4) • Simulated LNGS neutron flux • <1 neutron per year! (in 64000 detectors) detectors D. Stewart et al. , accepted by Nucl. Inst. Meth. A • Development of light sensors, optical simulation, test module construction • Mechanical design , experimental test of shielding • Ge-facility for material selection • Full background model COBRA rather compact experiment

Monte Carlo and Physics (WP 6) Sophisticated MC based on GEANT 4 exists • Upgrade of simulation framework • Development of detector response framework • Development of data analysis framework

Latest Limits Based on 4 detectors only First COBRA Double beta results T. Bloxham et al. , submitted Rare 113 Cd Beta decay C. Goessling et al. , Phys. Rev. C 72, 064328 (2005) world best

Double beta decay Neutrino mass sensitivity QRPA: F. Simkovic, ILIAS DBD meeting, Valencia 2006 Latest shell model calculations make Cd-116 most promising I isotope to search for ! E. Caurier et al. , ar. Xiv: 0709. 0277

Advantages • Source = detector • Semiconductor (Good energy resolution, clean) • Room temperature (exp. handling) • Modular design (Coincidences) • Industrial development of Cd. Zn. Te detectors (medical physics, homeland security) • 116 Cd Q-value above 2. 614 Me. V (no gamma-BG from natural decay chains) • Tracking („Solid state TPC“)

Detector studies (WP 1+WP 5) Paint contribution at 2. 8 Me. V: about 0. 2 counts/ke. V/kg/yr New larger space at LNGS • N 2 atmosphere running • 3 D scanning • Depth information via pulse shape • New grid design • Suitable passivation, contacting • Photoluminescense • Thermal neutron capture 113 Cd (n , )114 Cd th • Ion implantation (in-situ, ex-situ calibration) • N 2 atmosphere running • Installation of a sophisticated slow control system (temperature, humiditiy, vibrations. . . ) • Long term behaviour study • Improve shielding • Coincidence studies • Publish physics results based on 64 array • Calibration All activities focus on implications for a large scale experiment

Underground location Best location determined by high energy neutron background due to muon interactions in the rock

Examples: 16 pixel 4. 4 Me. V from 12 C

0 Any ∆L=2 process can contribute to 0 Rp violating SUSY V+A interactions Leptoquarks Double charged Higgs bosons Compositeness Heavy Majorana neutrino exchange Light Majorana neutrino exchange. . . 1 / T 1/2 = PS * NME 2 * 2

The standard lore Light Majorana neutrino exchange Measured quantity Quantity of interest Effective Majorana neutrino mass 1 / T 1/2 = PS * NME 2 * (<m > / me)2 Phase space integral calculable Nuclear transition matrix element