Present status of CUORE / CUORICINO Andrea Giuliani Università dell’Insubria and INFN Milano 3rd IDEA meeting, Orsay, April 14 – 15, 2005

The CUORE collaboration 14 institutions – 4 countries - ~ 60 physicists

The (source  detector) technique This is the most sensitive experimental technique up to now With high energy resolution, it can be realized in two ways: Ge diodes + 76 Ge  high energy resolution 3.5 keV FWHM reasonable candidate (Q=2039 keV) This technique has been dominating the field for decades and is still one of the most promising for the future E. Fiorini – 60s  Bolometers Te,.... high energy resolution 4 keV FWHM exceptionally 7 keV FWHM routinely good candidate (Q=2528 keV) other good or excellent candidates can be studied The bolometric technique for the study of DBD was proposed by E. Fiorini and T.O. Niinikoski in 1983

sensitivity F sensitivity F lifetime corresponding to the minimum detectable number of events over background at a given (1  ) confidence level b: specific background coefficient [counts/(keV kg y)] importance of the nuclide choice (but large uncertainty due to nuclear physics) sensitivity to m ee  (F/Q |M nucl | 2 ) 1/2  1 b  E MT Q 1/2 1/4 |M nucl | F  (MT / b  E) 1/2 energy resolution live time source mass b  0 F  MT b = 0 Experimental sensitivity to 0 -DBD

130 Te presents several nice features:  high natural isotopic abundance (I.A. = %)  high transition energy ( Q = ± 1.3 keV )  encouraging theoretical calculations for 0  DBD lifetime excellent feature for future reasonable-cost expansion of Double Beta Decay experiments large phase space, lower background (clean window between full energy and Compton edge of 208 Tl photons) m ee  0.1 eV   y Isotopic abundance (%) 48 Ca 76 Ge 82 Se 96 Zr 100 Mo 116 Cd 130 Te 136 Xe 150 Nd  DBD half-life (y) for m ee = 0.1 eV (different calculations) 48 Ca 76 Ge 82 Se 96 Zr 100 Mo 116 Cd 130 Te 136 Xe 150 Nd Comparison with other candidates: Transition energy (MeV) 48 Ca 76 Ge 82 Se 96 Zr 100 Mo 116 Cd 130 Te 136 Xe 150 Nd C E Properties of 130 Te as a DBD emitter

Some basic concepts on bolometers Signal:  T = E/C Time constant = C/G LOW TEMPERATURES  Wide material choice  (Phase 2 or 3?) Very good energy resolution  (no 2 background) SOURCE = DETECTOR technique  (Source mass optimization) The detector is FULLY SENSITIVE  (no dead layer) All the energy deposited is measured  (bulk and surface bkg are  )

The TeO 2 bolometers history: Moore’s Law CUORE Cuoricino Mi-DBD 4 detectors array 340 g 73 g MASS (Kg) Year

CUORE / CUORICINO in Gran Sasso Labs Cuoricino (Hall A) CUORE R&D (Hall C) CUORE location (Hall A)

M = ~ 40.7 kg ~ 5  Te nuclei The CUORICINO set-up CUORICINO = tower of 11 modules, 4 detector (790 g) each 2 modules, 9 detector (330 g) each I run :29 5x5x5 15 3x3x6 TOTAL 130 Te MASS 59 moles II run :40 5x5x5 17 3x3x6 TOTAL 130 Te MASS 83 moles This detector is completely surrounded by active materials. Useful for BKG origin models

CUORICINO shieldings CUORICINO Tower Cold finger Roman lead shield Coldest point

330gCalibration (U + Th) sum spectrum of all the detectors CUORICINO results (1) 790g average 2.6 MeV (during calibrations) 7.5  2.9 keV (790g) – 9.6  3.5 keV (330g) The best energy resolution ( keV is 3.9 keV

Background sum spectrum of all the big detectors in the DBD region T 1/2 0 ( 130 Te) > 1.8 x y (90% c.l.) MT = 10.8 kg y (big + small, natural) BKG = 0.18 ± 0.01 counts/ (kev kg y) CUORICINO results (2) m ee < 0.2 – 1.1 eV Updated to 6 th Dec ’04 FWHM (790g) 7.8 keV (330g) 12.3 keV

Is CUORICINO able to scrutinize the HM experiment claim? m ee = 50 meV – half life for different nuclei and models [10 26 y] T 1/2 ( 76 Ge)/T ½ ( 130 Te) expected T ½ ( 130 Te) (units: y) limit: > 1.8 CUORICINO prospects (1) Elliot Vogel 2002 Staudt et al.

(to be compared with events of the HM claim, with a BKG level which is 0.11 / 0.19 = 0.6 lower in HM and with an energy resolution which is 2.5 x better in HM) good chance to have a positive indication BUT: cannot falsify HM if no signal is seen CUORICINO prospects (2) Staudt et al. Expected event number in 3 y in a 16 keV energy window (2 FWHM) 1  BKG fluctuation = (0.19 * 16 * 40.7 * 3) 0.5 = S/N ratio (  )

Special dilution refrigerator CUORE is a closely packed array of 988 detectors (cylindrical option) M = 741 kg Each tower is a CUORICINO-like detector CUORE 19 towers with 13 planes of 4 crystals each

F 0  = 2.1  y 10 y sensitivity with pessimistic b = 0.01 counts/(keV kg y)  = 10 keV m ee < 0.02 – 0.1 eV m ee < 7 – 38 meV enriched CUORE CUORE background and sensitivity Montecarlo simulations of the background show that b ~ counts / (keV kg y) can be reached with the present bulk contamination of det. materials  The problem is the surface background (beta - alpha, energy-degraded): it MUST be reduced by a factor  40  10 y sensitivity with optimistic b = counts/(keV kg y)  = 5 keV m ee < 0.01 – 0.05 eV F 0  = 9.2  y

We have identified 4 possible sources for the residual BKG in the DBD region:  Neutrons  208 Tl multi-compton events   and  from TeO 2 surface   and  from Cu (or other mat.) surfaces facing the crystals Excluded since adding B-polyethilene shield had no effect The alpha continuum extends down to the DBD region CUORICINO background model (1) CUORICINO ~ 0.2 counts/ keV kg y PRELIMINARY !

208 Tl multi-compton in 0 -DBD region 214 Bi 60 Co S. E. 208 Tl To understand our background we NEED surface contaminations

CUORICINO background model (2) Surface contaminations determine peaks at the  energy, with tails (shape depending on contamination depth) Crystal bulk contaminations determine gaussian peaks at the Q-value of the decay In the ANTICOINCIDENCE bkg spectrum In the COINCIDENCE spectrum only CRYSTAL SURFACE contam. contribute Fix the U and Th crystal cont. levels and depth through MC reconstruction of the COINCIDENCE spectrum in the spectral region 2.5 – 6.5 MeV  Contamination depth in crystals  1  m

problem in this region Reconstruct the ANTICOINC. spectrum in the spectral region 2.5 – 6.5 MeV  INGREDIENTS:  210 Po bulk contamination of the crystals (5.4 MeV gauss. Peak, decaying)  210 Pb surface contamination of the Cu + crystal ( MeV constant peak)  U + Th crystal surface contam. (fixed through the coincidence spectrum) CUORICINO background model (3)

surface contamination level: ~ 1 ng/g vs bulk c.l. : < 1 (0.1) pg/g for Cu (TeO 2 ) contamination depth: ~ 5  m in agreement with direct measurement on Cu CUORICINO background model (4) bulk crystal cont. Introduce 238 U or 232 Th surface contamination level and depth profile due to the Cu structure facing the detectors

Dangerous events Rejectable events (by anticoincidence) CUORICINO background origin TeO 2 crystal

Full Montecarlo simulation on the basis of the CUORICINO and Mi DBD background analysis  Bulk contamination of Cu and TeO 2  < counts / kev kg y  Contamination in the cryostat shields  can be made negligible by the granular structure and more Pb  Surface contamination as it is  0.04 counts / kev kg y (reduction due to decrease of Cu area and different geometry, but not enough) A reduction by a factor 10 in Cu surface contamination and by a factor 4 in TeO 2 surface contamination would correspond to a FULL success of CUORE Crystals and Copper cleaning procedure by chemical etching and surface passivation under development The CUORE background

Surface Contamination Reduction New Cleaning procedure Crystal etching (Nitric acid) Lapping with clean powder (2μ SiO 2 ) New assembling procedure with selected clean materials Copper Crystal Radio-clean materials Etching Electro polishing Passivation procedure

Use a thin Ge (or TeO 2 ) crystal to make a composite bolometer fast high saturated pulse “classical” pulse Energy deposited in the TeO 2 crystal (DBD-like event) “classical” pulse Energy deposited in the Ge crystal (degraded alpha event) Development of surface-sensitive bolometers

CUORE for multi-isotope search Calorimetric technique is powerful, but provides limited information In case of discovery or hints for discovery, cross checks are mandatory  remove doubts about unexplained lines of other origin  test nuclear models  reduce systematic uncertainty on the relevant parameters, like m ee Already tested bolometrically As good as TeO 2 CaF 2, Ge, PbMoO 4, CdWO 4 Other compounds under test Tests are in progress Suitable compounds to be searched for

Conclusions  Cuoricino experiment may confirm the HM claim soon, provided the nuclear matrix elements are reasonably favourable  An intense R&D work is going on to reduce the BKG, in order to permit to CUORE experiment to investigate the inverse hierarchy region of the neutrino mass pattern  A full Montecarlo simulation for CUORE has been developed, on the basis of the CUORICINO and Mi DBD background analysis