1. Serge Nagorny – GSSI-INFN
The current status of LUCIFER experiment
Serge Nagorny – GSSI-INFN
on behalf of LUCIFER collaboration
Gran Sasso – May 7th, 2015

3. Which nuclide is the best for such search?
from experimental point of view
Environmental gamma background of 238U and 232Th chains
2615 keV
48Ca
82Se
96Zr
100Mo
116Cd
150Nd

4. LUCIFER Low-background Underground Cryogenics Installation for Elusive Rate
Demonstrator array of enriched scintillating bolometers
Total enriched 82Se isotope mass is 15 kg
Expect background < 10-3 c/(keVkgy)
Q, keV
Useful material
LY/, keV/MeV
QF
Zn82Se
2996
56%
6.5
4.2

5. Working principle of scintillating bolometers
First measurement of light and heat in a bolometer made in 1992 at INFN Milano
A. Alessandrello et al., “Development of a thermal scintillating detector for double beta decay of 48Ca ” Nucl. Phys. B - Proceedings Supplements 1992 Vol: 28.

6. Does ZnSe works as a scintillating bolometer?
ZnSe, 430 g, 524 hours
Excellent particle discrimination
FWHM = keV
Background free ROI
Natural radioactivity (U/Th/40K)
on level Bq/kg

7. Enriched 82Se isotope Produced by URENCO company (Netherland)
natural SeF6
spectroscopy on
232Th chain < 0.3 mBq/kg
235U chain < 0.2 mBq/kg
238U chain < 7 mBq/kg
40K < 1.1 mBq/kg
75Se 0.20±0.06 mBq/kg
cascade of centrifuges
chemical conversion gas to metal
ICP-MS measurements
S ppm
Te ppm
others < 0.5 ppm
82Se metal
distillation
15 kg of 82Se (95.5% enrichment level) delivered to LNGS

8. spectroscopy on GeMPI4@LNGS
High purity Zinc
produced by National Science Center KIPT (Ukraine)
12 kg of HP Zn, 828 hours
spectroscopy on
232Th chain < 0.1 mBq/kg
235U chain < 0.1 mBq/kg
238U chain < 6 mBq/kg
40K < 0.4 mBq/kg
65Zn 5.2±0.6 mBq/kg
only cosmogenic activation
No U/Th lines over 34 days !
ICP-MS measurements
Cd ppm
others < 0.2 ppm
15 kg of HP Zinc (99,9995%) delivered to LNGS

9. Mechanical processing
Crystal production
ZnSe is well known compound…
…but high quality big volume crystals are not available on the market
and presently could be grown only at Institute for Scintillating Materials (Ukraine)
crucial points should be solved in…
Zn82Se powder synthesis
non-stoichiometry due to high volatility of Se
Bridgman growth
low yield of crystal production
nonrepeatability of crystal quality
not fully controllable growth process
Mechanical processing
minimization of losses of enriched material
recycling of crystals scraps

10. bkg @ ROI < 10-3 c/(keVkgy) Half-life sensitivity, 1026 y
Experiment sensitivity
460 g
ZnSe
30-36 crystals
17 kg of Zn82Se
ROI = 10 keV
ROI < 10-3 c/(keVkgy)
Live time, y
Half-life sensitivity, 1026 y
m, meV 5
0.6
65-194 10
1.2
46-138

11. First experimental results
from low background measurements on HPGe detector
Transition
Decay channel
Level of daughter nucleus
E, keV
Experimental limit T1/2, y at 68% C.L.
HP Zn
82Se
Previous results
64Zn64Ni
+(0+2)
g.s.
511
 1.51021
 9.41020 for 2 [1]
 8.51020 for 0 [1]
2K0
1079.0
 1.01022
 3.21020 [1]
KL0
1086.4
2L0
1093.7
82Se82Kr
2-
21+
776.5
 2.41022
 2.81021 [2]
 1.41021 [3]
22+
1474.9
 2.01022
 1.61021 [3]
01+
1487.6
 2.81022
 3.01021 [3]
[1] P. Belli et al., arXiv: v1 [nucl-ex] 18 Oct 2011.
[2] Arnold et al., NP A 636 (1998) 209.
[3] J. Suhonen, et al., Z. Phys. A 358 (1997) 297.

12. Summary
Chemical and Radiopurity of HPZn & 82Se metals comply with all requirements for high quality low background ZnSe crystal production
Cosmogenic activation of detector materials currently became an dominant component of internal detector background (can be minimized by using materials with well known history, by storage deep underground, by land transportation in shielded container, by minimization of production time, by depletion Zn metal on 64Zn isotope, etc.)
First enriched Zn82Se crystal is planned to be grown end of June, 2015
One year is required for whole amount of Zn82Se crystal production

16. Typical mounting & scatter plot for ZnSe crystal
QF > 1

17. Bolometric light detectors
The light detector is a Ge thin crystal
Ø=44.5 mm, h=0.175 mm
1 face is coated with 60 nm layer
of SiO2 to increase light absorption
These devices are calibrated through an
Ionizing 55Fe source placed close to them;
55Fe shows two X-lines at 5.9 and 6.5 keV
The 55Fe energy spectra evaluated on three different UMICORE Ge-crystals. σ represents the evaluated energy resolution on the peaks, while σbase represents the baseline resolution.
150 Ge Wafers are presently in our hands

18. Hall C – Cryostat, a very crowd place
It has to be pointed out that all the measurements described are performed in the same, unique place: the
Hall C CUORE R&D cryostat
The setup was -slowly- changed in the last two years in order to increase by 70 % the experimental space.
Main Dewar elongation
New “buffles”
Vacuum chamber elongation
4 5x5x5 cm3 BGO scintillating crystal (CUORE)
8 CUORE crystals (background study)
2 ZnSe + 2 CdWO4 (Lucifer)
21 different detectors

22. Requirements to raw material for ZnSe crystal production
High performance of ZnSe crystals as scintillating bolometer strongly depends on:
High perfection of crystalline structure (FWHM & Signal amplitude in Heat channel)
High Light Yield (FWHM & Signal amplitude in Light channel, particle discrimination)
Chemical purity
K, Ti, V, Cr, Mn, Fe, Co, Ni < 1×10-6 g/g
Mg, Bi, Pb, Sn, Al, Cu, Cd < 5×10-6 g/g
U, Th << 1×10-9 g/g
Radiopurity
A(40K, U/Th& daughters) < mBq/kg
no 137Cs, 90Sr, …
minimization of cosmogenic nuclides activity