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

2. 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

3. 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

4. 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

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

6. 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

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

8. 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 < g/g

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

10. 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 “

11. 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) < 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

12. 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
TeO % Te
washing
filtering
washing, drying
calcination
1st Xtal growth
1/3 ingot selected
grinded
calcination
2nd Xtal growth

13. 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)

14. 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

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

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

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

18. 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

19. final processing

20. package, shipment and storage
vacuum boxes
triple vacuum bags

21. 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)
0.424
0.339
841
50400 a
51.11
40.9
2224
1097 b
2590.5 c
24895 a
0.457
0.365
1196
297022 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

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

23. 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 “

24. production management
Crystal Identity Sheet (side 2)
-crystal dimensions after cutting and shaping (before entering the clean room):
(50.030mm ≤ a ≤ mm)
-purpose:
( )µ exported by chemical etching
( )µ exported by polishing
Total exported in Clean Room :
(25 – 35)µ/face i.e. ( )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 “

25. 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 “

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

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

28. 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 “

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

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

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

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

33. crystal validation runs (LNGS, hall C)
1. bulk contamination limits
These are the limits 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

34. 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,
SCINT2005 Proc. 8th Int. Conf., Alushta, Crimea, Ukraine, Sept , 2005, pp
phys. stat. sol. (a) 204, No. 5, 1567–1570 (2007) / DOI /pssa
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 ???

35. 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 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

36. 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