Transcript "Double beta decay searches with enriched and scintillating bolometers"

Double beta decay searches with enriched and scintillating bolometers
Stefano
Pirro
- Milano - Bicocca
The Future of Neutrino Mass Measurements - NuMass 2010 INT Seattle February 8-11, 2010
Stefano Pirro – NuMass 2010
Background Issues (1)



Stefano Pirro – NuMass 2010
Background Issues (2)
XX
Internal trace contaminations (238U and 232Th)
Surface  contaminations
Surrounding trace contaminations (238U and 232Th)
Neutron induced background: direct and through (n,n’)
Long living - Emitters
106Ru 106Rh
110mAg
(Q=3.5 MeV) (Fission)
(Q=2.9 MeV) (fast n activation)
Stefano Pirro – NuMass 2010
Surface and Bulk contaminations
-region
-region
76Ge
130Te
116Cd
100Mo
82Se
136
Xe
CUORICINO  Background
Environmental “underground” Background:
238U and 232Th trace contaminations
Furthermore a not negligible part of the background can arise from high
energy neutrons from -spallation
Stefano Pirro – NuMass 2010
Some History
The first measurement of light and heat in a bolometer was performed in 1992 by the Milano group
But this technique, using a silicon PD at low temperatures showed several difficulties
 Radioactivity induced by the PD itself
 Cold stage charge preamplifier inside the cryostat
 Relatively small surface area of the PD
For these reasons the technique was abandoned
In 2004 we started to develop bolometer as LD (as CRESST and Rosebud Experiments)
The activity was then funded by INFN through the BoLux (R&D) Experiment* 2007-2009 and by EC
The CUPIDO R&D* project (INFN) funded for 2010 will go on with R&D on this technique.
* Responsible: Stefano Pirro
Stefano Pirro – NuMass 2010
Principles of operation
=C/G
T=E/C
Stefano Pirro – NuMass 2010
Light Detectors - Performances
Our Light detectors are generally Pure Germanium disks (thickness 0.3-1 mm) . The Performances of a LD are normally
evaluated through the Energy resolution on the 55Fe doublet (5.9 & 6.5 keV X-Ray)
Ge (Ø = 66 mm)
FWHM=550 eV
Ge (Ø = 35 mm)
FWHM=250 eV
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Summary of (almost) all the measured crystals
Good Scintillation light
Poor Scintillation light
No Scintillation light
PbMoO4
ZrO2
MgMoO4
ZnSe
Li2MoO4
TeO2
CdMoO4
SrMoO4
CdWO4
CaF2
CaMoO4
ZnMoO4
Stefano Pirro – NuMass 2010
Undoped
48CaF
2
In 2007 we tested a CaF2 crystal. The light output was “rather poor” but definitively enough to
discriminate alpha’s
Calibration (232Th) on a 3x3x3 cm3 PURE CaF2 crystal
There was a lack of an actual calibration due to the “lightness” of the compound; nevertheless the
Signal/noise ratio of the CaF2 was excellent.
Stefano Pirro – NuMass 2010
Results on the first array of
116CdWO
4
crystals (1)
4 3x3x3 cm3 (215 g each) CdWO4
1 common LD facing the 4 crystals
CdWO4 – 3x3x6
Stefano Pirro – NuMass 2010
Results on the first array of
116CdWO
4
crystals (2)
The data on the single 420 g 3x3x6 cm3 crystal is presented here.
The obtained scatter plot is shown it corresponds to 1066 hours of background measurement
44 days background
2615 keV 208Tl 
The MC simulation predicts a background level of 10-4 c/keV/kg/y in the region of interest
Stefano Pirro – NuMass 2010
Zn100MoO4 – A promising Molibdate
A 22 g ZnMoO4 crystal was grown by Institute for Scintillation Materials (Kharkov, Ukraine) In
collaboration with by Institute for Nuclear Research (Kiev, Ukraine)
226Ra, 222Rn, 218Po, 214Bi-214Po
(56 mBq/kg)
210Pb
(360 mBq/kg)
Stefano Pirro – NuMass 2010
Zn82Se – an extremely Puzzling compound (1)

Ionizing particles
/
Ln (Light)
Looking at the coincidences between Heat in ZnSe and “Light” in the light detector, three
population appears
Stefano Pirro – NuMass 2010
Zn82Se – an extremely promising compound (2)
First Results on a 4 cm  5 cm height 337 g ZnSe Crystal
337 g “new” ZnSe Crystal
210Po
/
Light

Calibration with 232Th and a smeared  source
Stefano Pirro – NuMass 2010
“Beyond” Scintillating bolometers
Scintillating bolometers are extremely powerful in order to discriminate background.
Nevertheless, they require a double readout that implies a dedicated technical and construction effort.
This is the “price” one has to pay in order to really “knock down” background.
It would be nice to obtain the same w/o this effort…
Stefano Pirro – NuMass 2010
Results on “large” crystals –
48Ca100MoO
4
CaMoO4 is not a “perfect candidate” for future DBD Experiment since it contains 48Ca
But this compound did show an extreme interesting feature
[ms]
The results are obtained on a ~160 g crystal
147Sm-
2310 keV
Rise time
Decay
timeofofthe
theCaMoO
CaMoO44crystal
crystal––no
nolight
lightdetection
detection
[au]
CaMoO4 Bolometers permits alpha discrimination (99,8%) without Light detection
Stefano Pirro – NuMass 2010
Zn82Se – an extremely promising compound (3)
This compound shows another very interesting feature: ’s show different thermal pulse development

/
 rejection > 97 % without light detection
Stefano Pirro – NuMass 2010
Zn100MoO4 – A promising Molibdate (2)
Light- Heat scatter plot
No Light information - PSA
This seems a characteristic of Molibdate crystals (also observed on other molibdates)Stefano Pirro – NuMass 2010
Conclusions - I
 We tested several types of scintillating crystals with interesting  emitters (100Mo,116Cd, 82Se, 48Ca)
 Within them CdWO4 is “ready to use”
 The “outsider” ZnSe is now, probably, the best candidate , even if some more tests are needed
 Molibdates need more R&D both for radioactivity and scintillation light
 This technique is the only one that can be used for several interesting DBD emitters with excellent energy
resolution (0.3  1 % FWHM)
 Simulations show that a background level of 10-4 c/keV/kg/y can be “easily” reached without
too much “restrictions” on internal radioactivity.
 Within few months we plan to test 2 small arrays of CdWO4 and ZnSe crystals ( O(kg) ) in order to
completely test the technique (unexpected “surprises” can always happen)
 Particle Identification through Pulse Shape Analysis could result in a new, unexpected, ally
Stefano Pirro – NuMass 2010
Conclusions-2
0 -1
2
2
1/ 2 nuclei HAVE to be investigated by DBD experiments
Different
(T )  G(Q, Z ) M  m 
CUORE
76Ge
116Cd
DBD Detectors with Q  >2615 keV can reach background
levels  2 orders
100Mo
of magnitude smaller with respect to the others
130Te
82Se
Gerda-Majorana
If we think about possible “ III generation experiments” the key point will be
(if we consider solved the background issue….) Energy Resolution
( 1 %)
SNO+
136
Xe
EXO
Super-Nemo
Scintillating Bolometers are the only detectors that can fulfill all these 3 requirements
Stefano Pirro – NuMass 2010