Poster presented at 13th Pisa Meeting on Advanced Detectors - La Biodola, Isola d'Elba (Italy) - May 25 - 30, 2015
S8 - Detector Techniques for Cosmology, Astroparticle and General Physics
A scintillating bolometer array for double beta decay studies:
the LUCIFER experiment
Luca Gironi on behalf of the LUCIFER collaboration
2νDBD of 100Mo
The Low‐background Underground Cryogenics Installation For Elusive Rates (LUCIFER)
ZnMoO4
Reconstruction of the
background spectra. The best fit
line (solid blue) is shown. The
components are 2νDBD (gray
region), 65Zn (light blue),
internal and external 210Pb
(yellow), internal and external 40K (green), 208Tl (pink), and flat
α background (orange).
Neutrinoless double beta decay
Importance of the process:
- It can happen only if ν is a Majorana particle
- From the decay half-life we can measure the effective
Majorana mass mββ:
The LUCIFER setup will consist of an array of 30 individual module detectors, arranged in a tower-like structure installed underground in the Laboratori Nazionali del Gran Sasso. Each module will be
composed by a ~0.5 kg enriched (95%) ZnSe crystal equipped with a Ge-crystal light detector operated as bolometers at ~10 mK. The goal of Lucifer is to reach sensitivity of few 1025 y and to be a
demonstrator for a background free experiment.
ZnSe crystal
Light Detectors
The large statistics collected during the operation of a ZnMoO4 array, for a total exposure of 1.3 kg day of 100Mo, allowed the first bolometric observation of the 2νDBD of 100Mo. The analysis of coincidences
between the crystals allowed the assignment of constraints to the intensity of the different background sources, resulting in a reconstruction of the measured spectrum down to an energy of ∼300 keV.
Crystal mass (g) Anticoincidence Coincidence
247 509 ± 26 4.4 ± 0.2
235 472 ± 24 5.4 ± 0.3
329 661 ± 34 6.2 ± 0.3
Number of events from the 2νDBD of 100Mo in the anticoincidence
spectrum and in the coincidence spectrum for each crystal.
Experimental approach: the scintillating bolometers
Nuclear decay:
G0ν = phase space factor
M0ν = nuclear matrix element
Sensitivity (S): the process half-life corresponding to the maximum signal that could be observed (over the background fluctuations) at a given
statistical Confidence Level (C.L.).
Looking for a monochromatic peak
at the Q-value of the decay.
Successful R&D pursued within LUCIFER with Mo-based compound
before choosing ZnSe.
Our light detector is a Ge thin crystal operated as bolometer. This device is calibrated through an ionizing 55Fe source.
Ø=44.5 mm, h=0.175 mm
They demonstrate the very
appealing possibility to
discriminate particle
through pulse shape analysis
on the heat channel.
Smeared α source
β/γ events
Uei = PMNS matrix elements
mi = mass eigenvalues
In order to achieve a high sensitivity we need: excellent energy resolution, large source mass, very low background.
The leading idea: to combine the two available
information (heat and scintillation light) to
distinguish the nature of the interacting particles,
exploiting the different scintillation yield of β/γ, α
and neutrons.
Scintillating bolometer: a bolometer coupled to a light detector. The first consists of a scintillating absorber thermally linked to a phonon
sensor while the latter can be any device able to measure the emitted photons.
Example of scatter plot obtained with scintillating
crystals.
Isotope Q-value i.a.
100Mo 3034 keV 9.6 %
Isotope Q-value i.a.
82Se 2996 keV 8.7 %
n = confidence level
Nββ = isotope number
T = live time
M = detector mass
B = background
∆ = energy resolution
Lig
ht
shap
e p
ar.
[a
.u.]
Energy [keVee]
β/γ events
smeared α source
82Se Qββ
DP(Qββ) = 11
σ = 96 eV
55Fe
ieiββ
mUm2
200
2
2
0
21
1
MG
m
m
Te
/
eZAZA 22,,
ysyststatT181002
2110660370157 )(.)(..)Mo(
/
n
TNS
2ln
BM
TNS
2ln
B
822
.ln
0B
TNS
)C.L. %( . 6882n
‘Zero’ background experiments
Experiments with background
)C.L. %( 68 BTMn
Performances as a function of the
sensor polarization current.
Det
ecte
d l
igh
t [k
eV]
Energy [keVee]
82Se Qββ
ZnSe peculiarity: α
particles scintillate
more than β/γ’s:
Q.F.≈4
431g crystal
)()(
)()(
)(
/
/
EE
EE
EDP22
Discrimination Potential
The light emitted and the shape of the same light pulses are reported as a function of
the energy released in the ZnSe bolometer. A 𝛾-source was used to produce events (blue)
up to 2615 keV (208Tl) while a smeared 𝛼 source was placed under the crystal to provide
a continuum of 𝛼’s extending to lower energies (red). FWHM(2615keV)=13.4±1.3 keV
FWHM(2615keV)=6.3±0.5 keV
DP(Qββ) = 20
without light detection
2νDBD 0νDBD
Energy [keV]
Co
un
ts
FW
HM
(eV
) R
ise
Tim
e (m
s)
Ibol (pA)
Main advantages of the bolometric technique:
- Possibility to choose different 0νDBD candidate
- Excellent energy resolution (~0.1%)
- Source = Detector
Background reduction
without a double readout.