INFN Sezione di Roma, ITALY Production of radiopure TeO2 crystals for neutrinoless double beta decay application I.Dafinei INFN Sezione di Roma, ITALY

preamble Though TeO2 is not a scintillator, the technological aspects related to its production for DBD applications are similar to those requested for for radiopure scintillators needed for EURECA

content introduction TeO2 crystal general properties crystal growth specific requirements for DBD use large scale production problems TeO2 chemistry crystal growth and processing transport and storage production management conclusion

ββ emitters of experimental interest introduction why DBD ? neutrino oscillation experiments can give only: values of mixing angles mass differences (predict range of effective mass) DBD measurements may: give the scale of neutrino mass demonstrate the Maiorana nature of neutrino why TeO2 ? 130Te largest isotopic abundance existing industrial production of an “easy to find” compound accessible price ββ emitters of experimental interest CUORE module: 4 x 760g 2b - 2.530MeV TeO2 crystal 5x5x5 cm3 in CUORE experiment, the crystals of TeO2 are detector and source

TeO2 general properties TeO2 (paratellurite) short:: 1.88 Å long:: 2.12 Å a = 4.8088 Å c = 7.6038 Å relatively low melting point distorted rutile (TiO2) structure anisotropy of expansion coefficient

as grown TeO2 crystal ingot TeO2 crystal growth TeO2 crystal is particularly repellent to impurities most of radioactive isotopes have ionic characteristics incompatible with substitutional incorporation in TeO2 seed grown Xtal molten TeO2 heating Czochralski Bridgman Bridgman grown crystals are more stressed than Czochralski ones annealing at about 550°C helps in removing the residual stresses as grown TeO2 crystal ingot Pt “crucibles” filled with TeO2 powder

Acousto-optic devices TeO2 usual application Acousto-optic devices Frequency shifter Fiber coupled output devices Q switch

TeO2 quality requirements DBD research application: large size (50×50×50 mm3) very high (radio)purity acousto-optic tunable filter (AOTF): large size (typically > 500g/piece) very good optical quality U、Th < 10-12 g/g

DBD requirements radio-purity natural radioactivity activation products crucible material main radioactive series

large scale production problems TeO2 crystals currently produced at industrial scale for acousto-optics applications have mechanical and optical characteristics much above DBD application requirements major production problems come from (radio)purity constraints contamination: cannot be avoided by shielding vetoes do not always work bulk surface internal external rough estimation of impurity levels impurity allowed (g/g): T ≥ 1024 yr Ti < 1010 yr close to or below detection limit of most sensitive techniques used for quantitative elemental analysis (NAA, ICP-MS) radio-purity quality control during production is impossible freeze production conditions found in the R&D phase sampling radio-purity check of raw materials and consumables special precaution during operation and handling solution “

large scale production problems bulk surface powder synthesis, crystal growth mechanical processing, handling, package, transport, storage contamination raw materials and reactants Te metal contamination 238U < 2*10-10 g/g 232Th < 2*10-10 g/g 210Pb < 10-4 Bq/kg 40K < 10-3 Bq/kg 60Co < 10-5 Bq/kg acids contamination 238U < 5*10-12 g/g 232Th < 5*10-12 g/g water contamination 238U < 4*10-12 g/g 232Th < 4*10-12 g/g contamination of other liquids intermediary products TeO2 powder (after densification by calcination) 238U < 2*10-10 g/g 232Th < 2*10-10 g/g 210Pb < 10-4 Bq/kg 40K < 10-3 Bq/kg 60Co < 4*10-5 Bq/kg Pt contamination in as grown crystals Pt (element) < 10-10 g/g 190Pt (isotope) < 3*10-6 Bq/kg final product (TeO2 crystal ready-to-use) 238U < 3*10-13 g/g 232Th < 3*10-13 g/g 210Pb < 10-5 Bq/kg 60Co < 10-6 Bq/kg

TeO2 chemistry Te calcination 1st Xtal growth calcination refining TeO2 at SICCAS TeO2+HCl→TeCl4+H2O TeO2 2Te+9HNO3 → Te2O3(OH)NO3+8NO2+4H2O Te2O3(OH)NO3→2 TeO2+HNO3 TeCl4+4NH4OH→Te(OH)4+4NH4Cl Te(OH)4→TeO2+H2O HNO3 HCl TeCl4 NH4OH TeO2 99.999% Te washing filtering washing, drying calcination 1st Xtal growth 1/3 ingot selected grinded calcination 2nd Xtal growth

Pt crucibles preparation TeO2 growth CUORE crystals growth unit at SICCAS CUORE crystals production at SICCAS crystal growth room growth control room place on duty storage room Pt crucibles preparation Technological space Annealing specially dedicated technological area separated raw material circuit separated consumables and ancillaries circuit production flow survey by parallel sampling radio-contamination control of raw materials, consumables and ancillaries (SINAP)

CUORE crystals growth unit at SICCAS TeO2 growth CUORE crystals growth unit at SICCAS improvements needed for large size crystals Furnace Heating Crucibles Redesigned furnace and its temperature field

limited free atmosphere exposure TeO2 growth CUORE crystals growth unit at SICCAS reduced contamination risk double windows double doors limited free atmosphere exposure

raw mechanical processing CUORE dedicated mechanical processing area new cutting machines (inner and outer cut) new grinding machine new lapping bench

raw mechanical processing CUORE dedicated mechanical processing area new cutting machines (inner and outer cut) new grinding machine new lapping bench

final processing SICCAS / INFN clean room class 1000 qualified general standard respected easy maintenance guarantee ultrapure water 300 l/h P/B 1 m air shower passage final shaping and lapping chemical etching and polishing packaging dressing room 2 dressing room 1 storage area outer ultra-pure water generator (2) clean surrounding ultra-pure water generator (1) Xray difractometer lapping bench ultrasound washer microwave oven chemical etching bench polishing bench packaging bench

final processing

package, shipment and storage vacuum boxes triple vacuum bags

package, shipment and storage Measurements of activation cross sections in proton beams cosmogenic contamination CERN PS and ISOLDE Energy (GeV) Fluence (p cm-2 ) TeO2 mass (g) Te mass (g) Time from exp. (h) Meas. time (s) 24 0 9.41 1010 0.424 0.339 841 50400 24 a 6.1 1010 51.11 40.9 2224 1097 24 b 2590.5 24 c 24895 8661128 1.4 a 1.65 1010 0.457 0.365 1196 297022 1.4 b 4827 251358 total exposure time to cosmic rays from metallic Te synthesis to LNGS delivery must be less than 6 months sea transport from China to Italy, in order to minimize cosmic rays exposure Energy (GeV) CERN σ (mb) LBL Theory 0.8 <0.5 0.22 1.4 0.40±0.06 0.77 1.85 0.63±0.15 0.85 23 1.0±.04 1.15 Ettore Fiorini, CUORE meeting Como, Feb.21-23, 2007

package, shipment and storage visual inspection at delivery LNGS permanent storage area

production management Crystal Identity Sheet (side 1) Production flow peculiarities: outside the Clean Room -dedicated boxes for crystals transport Inside the Clean Room components of polishing machines not used in the Clean Room taken away dedicated storage place for each consumable, and equipment furniture adapted to operator size “clean” method for the preparation of SO-E5 polishing slurry commissioned anti-splashing faucets of UP water wash-basins conditioning of teflon basins, crystal holders, etc by ultrasound washing (slightly acid, using SINAP ultrapure HNO3) Crystal Identity Sheet associated to each crystal “

production management Crystal Identity Sheet (side 2) -crystal dimensions after cutting and shaping (before entering the clean room): (50.030mm ≤ a ≤ 50.090mm) -purpose: (15 - 20)µ exported by chemical etching (10 - 15)µ exported by polishing Total exported in Clean Room : (25 – 35)µ/face i.e. (50 - 70)µ /edge -for the sake of simplicity, operators register only the grey-marked cells -detailed calculation of final dimension and filling of corresponding cells are made “offline” by the production coordinator as well as the storage in the database of the whole set of data -certification of planarity is a key problem -geometric tolerances measured before entering the clean room are taken as final tolerances “

production management production quality ceck The production process is subject to a complex validation protocol which includes radio-purity certification procedures to be applied in each production phase. Immediate actions are foreseen in case of failure, including halt of crystal production till the problem is solved “

production management CUORE crystals production database -LabView GUIs generating .txt files -crystal production data are further stored in a global CUORE database “

production results total crystals: 94 soft directions: 188 hard directions: 94 “

production results atomic layers soft face: 1470/µ hard face: 1320/µ atomic layers taken away by etching: soft face: >17900 layers hard face: >16100 layers “

production results atomic layers soft face: 1470/µ hard face: 1320/µ atomic layers taken away by polishing: soft face: >11300 layers hard face: > 9150 layers “

production results crystal validation runs (LNGS, hall C) bolometers performance “

production results crystal validation runs (LNGS, hall C) surface contamination “

production results crystal validation runs (LNGS, hall C) bulk contamination “

crystal validation runs (LNGS, hall C) 1. bulk contamination limits These are the limits (@95%) on CCVR2 bulk contaminations. new crystals old crystals Crystal Channel 238U (low) 238U (high) 232Th (low) 232Th (high) H76 2 3.12E-13 4.14E-13 2.26E-13 2.92E-13 H96 5 2.20E-13 3.47E-13 8.25E-14 6.94E-13 sum 2+5 1.64E-13 2.69E-13 1.19E-13 4.24E-13 H11 3 1.99E-13 4.18E-13 7.47E-14 1.18E-13 H7 7 2.00E-13 5.15E-13 1.57E-13 2.62E-13 3+7 9.97E-14 3.49E-13 7.85E-14 1.47E-13

cosmogenic activation ??? possible developments enriched TeO2 ? first tests, bad experience scintillating TeO2 ? Sm doping? calibration and possible discrimination of α events in the region of interest of 130Te DBD0 (2-528 MeV) Nucl.Instr.Meth.in Phys.Res. A 554, Issues 1-3 , 1 December 2005, 195-200 SCINT2005 Proc. 8th Int. Conf., Alushta, Crimea, Ukraine, Sept. 19-23, 2005, pp. 106-108 phys. stat. sol. (a) 204, No. 5, 1567–1570 (2007) / DOI 10.1002/pssa.200622458 isotope abundance half life decay energy product 147Sm 14.99 % 1.06×1011 y α 2.310 MeV 143Nd 146Sm 1.03×108 y 2.529 MeV 142Nd cosmogenic activation ???

TeO2 crystal is among the best choices for DBD experiments conclusion TeO2 crystal is among the best choices for DBD experiments growth and processing of TeO2 crystals having a radio-purity compatible with foreseen CUORE total background of 0.001 counts/keV/kg/y is a very challenging task TeO2 crystals produced at SICCAS (China) till present have bolometric and radio-purity characteristics within tolerance limits imposed by CUORE collaboration TeO2 is a very lucky case: production already existed there is a possible market for high quality crystals New crystals needed for EURECA may not be so lucky and dedicated activity (financing effort) will possibly be needed

post scriptum •TeO2 crystal was a very lucky case: industrial scale production already existed the improvements imposed by DBD application were relatively easy to be implemented the market for high quality crystals will exist also after the end of relatively large scale production for CUORE • New (radio-pure) crystals for EURECA may not be so lucky and dedicated efforts will be needed for their production: -final users to define very clearly their requests -crystals growers to accept interferences and possible constraints to their activity -funding agencies to understand that unusual challenges need unusual support