Fabio Bellini “Sapienza” Università di Roma & INFN Roma

DBD 2011 Osaka 17/11/2011

LUCIFER: A Scintillating Bolometers Array for the Search of

Double Beta Decay

Bolometers for DBD searchWell established technology

‣ DBD source embedded in a crystal cooled down at few mK

‣ (Only) energy measured via temperature variation ΔT =E/C induced by particle energy release

‣ Need very low heat capacity (dielectric, diamagnetic): TeO2: ΔT ~0.1 mK/MeV

‣ TeO2: excellent energy resolution (~0.3% @ 2-3 MeV) and massive detector

‣ low background ~few⋅10-2 cts/keV/kg/y

Need ~10-3 cts/keV/kg/y to access inverted hierarchy

2

Heat bath

Weak thermal coupling

Thermometer

Absorber Crystal= DBD source

Energy release

130Te76Ge 100Mo116Cd

Environmental

“underground”Background:238Uand232Thtrace

contamina<ons

82Se

The isotope choiceThe possibility to use different candidates depends on:

‣ capability to grow large radio-pure crystals with good mechanical and thermal properties

‣ isotopic abundance and cost/easiness enrichment

All isotopes tested as bolometer in crystalline form with the exception of 136Xe and 150Nd

Gain ~ 100 if Qββ > 2615 keV common highest γ line (208Tl) with BR ~36% in Th chain3

The α problemBolometers are fully sensitive, up to detector surface ⇒ no dead layer

Surface contamination of the bolometers themselves or of the materials surrounding them emitting α particles gives a continuum background in the Region of Interest

Very difficult to reduce this background below 0.05 cts/keV/kg/y below and above 2615 keV

‣ need α rejection >98% to reach 10-3 cts/keV/kg/y4

The solutionScintillating bolometers: use different α/γ light emission for background discrimination

The light detector: a thin opaque bolometer facing a polished side of the main bolometer

The experimental basis of this technique was the R&D activity performed by S.Pirro at LNGS in the framework of the Bolux(INFN), ILIAS-IDEA (EC WP2-P2) program

5

Thermistor

BolometerEnergy Release

Light detector

LUCIFERLow‐background Underground Cryogenics Installation For Elusive Rates

ERC‐2009‐AdG 247115

6

Lucifer is a Latin word (from the words lucem ferre), literally meaning "light-bearer", which in that language is used as a name for the dawn appearance of the planet Venus, heralding daylight.

Principal Inves.gator: F. Ferroni

Co-­‐ Inves.gator: A.Giuliani

Coordinator: S.Pirro

Bringing light underground

The candidates: CdWO4

Pro:

‣ ~0.5 kg crystal successfully tested

‣ very good crystal quality

‣ high light yield

Cons:

‣ only 32% of useful material

‣ 113Cd (huge neutron cross section) ⇒(n,γ) reaction ⇒ possible continuum γ background7

Qββ (keV)

Useful material(% weight)

LY (keV/MeV) QF

CdWO4 2809 32 ~17 ~0.16

Astropart.Phys. 34:143 ,2010

W180U238 U234

210Po

Tl208

K40

Ligh

t [k

eV]

0

20

40

60

80

400 1000 2000 3000 4000 5000Energy (Heat) [keV]

Qββ

The candidates: ZnMO4

Pro: good pulse shape discrimination on main (heat) bolometer

Cons: poor light yield ,only small crystals (~30 g) up to now

8

Qββ (keV)

Useful material(% weight)

LY (keV/MeV) QF

ZnMO4 3034 44 ~1 ~0.2

JINST 5:P11007,2010.

Astropart.Phys. 34:797 ,2011

Qββ

The candidates: ZnSe

Pro:

‣ ~340 g crystal successfully tested

‣ good light yield and radio-purity

‣ pulse shape discrimination on light detector

‣ the most mass effective

Cons:

‣ inverted Quenching Factor!

‣ crystal production: not yet solid protocols and reproducibility issue9

Qββ (keV)

Useful material (% weight)

LY (keV/MeV) QF

ZnSe 2995 56 ~7 ~4

Light Detector(Ge)

ZnSe bolometer

Astropart.Phys. 34:344 ,2011

The candidates: ZnSeNo explanation for the inverted Quenching Factor.

Discarded hypotheses:

‣ ZnSe self-absorption

‣ Light collection efficiency

‣ Light detector transparent to certain wavelengths

10Heat Energy (keV)

0 1000 2000 3000 4000 5000 6000 7000 8000

Ligh

t Ene

rgy

(a.u

.)

0

100

200

300

400

500

600

700

800

Light Energy (a.u.)0 100 200 300 400 500 600 700 800

Shap

e In

dica

tor

0.05

0.055

0.06

0.065

0.07

0.075

0.08

0.085

Time (s)0.15 0.2 0.25 0.3 0.35 0.4 0.45

Arb

itrar

y U

nits

0

0.2

0.4

0.6

0.8

1

αα’s

from 23

8 U e 23

4 U sourc

es

β,γ

γ’s (n, γ) from AmBe source

Light Detector(Ge)

ZnSe bolometer

Light detector

The scintillating candidates

Baseline crystal for LUCIFER: ZnSe

11

Qββ (keV)

Useful material (% weight)

LY (keV/MeV) QF

CdWO4 2809 32 ~17 ~0.16

ZnMO4 3034 44 ~1 ~0.2

ZnSe 2995 56 ~7 ~4

ZnSeLuminescence properties well known

Crystal growth known:

‣ Bridgman technique at 1525° C

‣ high twining tendency

‣ high volatility: stoichiometry control

Effort focused on:

‣ enrichment

‣ ZnSe synthesis

‣ efficient crystal growth

12

SCINT09_P1-16

3

content of as-grown crystal ingot along the growth axis, a consequence of the higher evaporation coefficient of Zn with respect to Se. The Se excess concentration may vary from 0.001% to 0.05% between the top and the bottom of the as grown ingot. Further differences between sample composition and structure are obtained through post-growth thermal treatment, made in Se or Zn vapor-enriched atmosphere. Fig.1 provides a picture of the eight cubic samples belonging to the eight sets of samples studied in this work.

Fig. 1 ZnSe cubic samples (10x10x10 mm3) prepared in different growth and post-growth conditions for this work.

III. EXPERIMENTAL TECHNIQUES AND RESULT Optical transmission measurements at room temperature

were performed on the optically polished ZnSe slabs (10x10x1 mm3) belonging to the eight sets of samples, using a Perkin Elmer Lambda900 spectrophotometer. As Figure 2 shows, the absorption spectra of the eight samples are different in the region near the fundamental absorption edge. The differences extend up to 2.15 eV, corresponding to the 475 - 575 nm which explains the different colour from pale green to dark red of the samples (see Figure 1).

Fig. 2 Optical absoption spectra measured at room temperature on ZnSe slabs of 1mm width, prepared in the same conditions as samples 1 to 8 given in Figure 1.

Steady state X-ray excited radio-luminescence and thermo-luminescence measurements were performed using a home made measuring set-up consisting of a vacuum chamber (10-8 Torr) and a cryostat allowing working temperatures in the range from 8K to 320K. In situ X-ray irradiation with an X-ray tube (Philips 2274) was performed through a Beryllium window. The emitted light (both for radio and thermo-

luminescence) was measured by a CCD (Jobin Yvon Spectrum One 3000) coupled to a monochromator (Jobin Yvon Triax 180). Radio-luminescence spectra presented in this work were made in identical conditions on crystal samples of identical geometry (10x10x1 mm3) and mechanical quality (optically polished); therefore it is possible to perform a quantitative analysis of the luminescence yield for different preparation conditions of the samples.

As Figure 3 shows, the X-Ray excited spectra observed at room temperature (T = 20°C) have the same structure for all samples studied though the relative intensity of different emission bands may differ from one sample to another. Besides the excitonic emission band at ~460 nm (2.7 eV) clearly seen in the inset of Figure 3 (logarithmic scale), three main emission bands are evidenced by a detailed analysis of each spectrum:

ε1 = 1.28 eV ; λ1 = 970 nm ε2 = 1.92 eV ;; λ2 = 645 nm ε3 = 2.03 eV ;; λ3 = 610 nm ε4 = 2.70 eV ;; λ4 = 460 nm Studying the precise microscopic origin of each band is not

the subject of the present work. Nevertheless it is to be stressed that the 645 nm band (enhanced by Te doping in commonly produced ZnSe:Te scintillators [7]) is to be attributed only to VZn centres (Zn vacancies). As mentioned above the Te contamination is to be excluded in our case.

Fig. 3 X-ray excited luminescence measured at room temperature on ZnSe slabs of 1mm width, prepared in the same conditions as samples 1 to 8 given in Figure 1. Emission peaks are better evidenced in the inset.

Preliminary low temperature X-ray excited luminescence spectra were measured on sample No. 1 considered as representative, since at room temperature it clearly features all emission bands. The corresponding spectra represented in Figure 4, have fundamentally the same composition for the entire temperature range (10K to room temperature), there is no supplementary emission band, i.e. the nature of luminescent centres remains the same. As expected from other studies [7] the emission efficiency in the red region of the spectrum (610 nm band) is considerably increased at low temperatures while the infrared emission band at 970 nm remains practically unchanged. This behavior is further

5 cm

ZnSe productionNeed radio-chemical pure Se

‣ ICPMS measurements

Enrichment (URENCO)>95%

‣ Chemical problems in Se conversion (mainly reagent contamination)

Beads (powder not good for crystal)

‣ require dedicated instruments (HPGe gamma spectroscopy)

‣ Purification (99.999%)⇒zone refining

Synthesis of ZnSe

‣ High or low temperature method (yield optimization)

Growth of ZnSe crystal

‣ Avoid twining and reach reproducibility

13

raw$(elemental)$Se$1$

2$ cer0fica0on$SeF6$synthesis$

SeF6$enrichment$

Se$conversion$5$

4$

3$

Zn$elemental$ enriched$Se$elemental$

7$ 8$

Se$beads$produc0on$$6$

purifica0on$9$

10$ cer0fica0on$

ZnSe$synthesis$11$

ZnSe$crystal$growth$ recovery$and$

recycling$

13$15$

12$

mechanical$processing$

package$and$shipment$

14$

16$

cer0fica0on$

26#

Light detectorsLight Detectors are generally pure Germanium disks (thickness 0,3-1 mm)

Performances are evaluated on the 55Fe doublet: 5.9 & 6.5 keV x-Ray

‣ Good energy resolution: σ~130 eV theoretical resolution σ~80 eV

14

LD test: TeO2 Cerenkov lightTeO2 bolometers don’t scintillate: detection of Cerenkov light

Cerenkov threshold: 50 keV for β, α below threshold ⇒ particle discrimination

15

TeO2:Sm (30 ppb natSm)3.0x2.4x2.8 cm3

116.65 gVM2002

reflecting foil

Light detector of pure Ge66 mm diameter, 1mm thick.

One side coated with SiO2 to increase absorption of μm wavelengths.

147Sm: α decay at 2310 keV

Energy [keV]-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

coun

ts / 0

.05

0

20

4060

80100

120

140160

180

200L1L2_Sm147Entries 851

Mean 0.01123RMS 0.08898Integral 851

/ ndf 2r 13.55 / 11const 8.6± 192 mean 0.003042± 0.008343

m 0.00249± 0.08702

L1L2_Sm147

[keV

]LE

-0.3-0.2-0.1

00.10.20.30.40.5 background

[keV

]LE

-0.3-0.2-0.1

00.10.20.30.40.5 Th calibration232

Energy[keV]0 1000 2000 3000 4000 5000 6000

> [k

eV]

L<E

-0.050.000.050.100.150.20

Results

16

73 eV/MeV 184 eV @2.527 MeV

arXiv:1106.6286 submitted to Astropart. Phys.

Energy [keV]-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

coun

ts / 0

.05

0

5

10

15

20

25

30

35

40

45L1L2_Tl208 Entries 185

Mean 0.1969RMS 0.07263Integral 185

/ ndf 2r 10.73 / 6const 3.09± 40.77 mean 0.0058± 0.1926

m 0.00000± 0.08702

L1L2_Tl208

~2 σ separation R&D on going on 5x5x5 cm3 TeO2 crystal: light collection optimization

LUCIFER DetectorSingle module: 4ZnSe -1light detector Tower: 12 single modules

Hosted @ Laboratori Nazionali del Gran Sasso

‣ Equivalent vertical depth relative to a flat overburden: ~ 3.1 ± 0.2 km.w.e

‣ Gamma flux:~0.73 γ/(s cm2)

‣ Neutron flux: ~4·10-6 n/(s cm2) below 10 MeV

‣ Muon flux: (2.58±0.3)·10-8 μ/(s cm2)17

Preliminary

ConclusionsThe main challenge for a 0νDBD next generation bolometer experiment is the α background rejection to ~10-3 cts/keV/kg/y

The scintillating bolometer is a promising technique

‣ the LUCIFER goal is to demonstrate the feasibility of this technique on a reasonable large scale

‣ but has a remarkable physics reach by itself

assuming ΔE ~ 10keV, live time ~ 5 y, bkgd~10-3 cts/keV/kg/y

Data taking foreseen in 2014

18

ZnSe 82Se weight(kg) Half life(1026 y) mββ (meV)

baseline 17.6 2.3 51-65

J.Mendez et al. arXiv:0801.3760; F.Simkovic et al. Phys.Rev. C77 045503,(2008);

J.Suhonen et al. Int.J.Mod.Phys E17 1 (2008)