1. Scintillating Bolometers for Double Beta Decay
Stefano Pirro
- Milano
Background Limitation for Simple bolometers
Principles of operation of scintillating bolometers
Some “history”
Cd-Mo-Ca based crystals
ZnSe crystals
Conclusions
CSNII, Napoli 1 Dicembre 2009
Stefano Pirro - CSNII - 1 Dic 2009

2. Surface and environmental contamination
-region
-region
CUORICINO  Background
Environmental “underground” HPGE Background:
238U and 232Th trace contaminations
76Ge
116Cd
130Te
100Mo
82Se
Furthermore a not negligible part of the background can arise from high
energy neutrons from -spallation
Stefano Pirro - CSNII - 1 Dic 2009

3. A serious problem : Surface contaminations
CUORICINO TO CUORE
Bkg in DBD region
Continuum
210Po α line
Sum energy:
2527 keV
3600 keV
5407 keV
Stefano Pirro - CSNII - 1 Dic 2009

4. Background identification : Bulk contaminations
BI-Po
Thanks to Bi-Po’s and Bi-Tl’s internal contaminations do not play a significant role
Does not hold for 234Pa (Q=2195 keV)
Stefano Pirro - CSNII - 1 Dic 2009

5. Stefano Pirro - CSNII - 1 Dic 2009
Some “Old” History
The first measurement of light and heat in a bolometer was performed in 1992 by the Milano group
that demonstrated the possibility to discriminate -particles in a 1 cm3 CaF2 .
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 (and other…) the technique was abandoned
In 2003 we started to develop thin absorbers detectors in order to measure  surface radioactivity
Since the early beginning it was clear that these devices were extremely sensitive (FWHM hundreds of eV)
allowing, therefore, the measure of scintillating light
Stefano Pirro - CSNII - 1 Dic 2009

6. Principles of operation
=C/G
T=E/C
A Bolometric Light Detector is fully active a particle detector
The time response of a BLD is the same of a standard bolometer O (ms)
The QE of a BLD is, probably, comparable with that one of PD’s but it is not easy to measure it
Normally PURE (undoped) Ge or Si crystal are used as absorbers
Stefano Pirro - CSNII - 1 Dic 2009

7. Light Detectors - Performances
Our Light detectors are generally Pure Germanium disks (thickness 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
Stefano Pirro - CSNII - 1 Dic 2009

8. Stefano Pirro - CSNII - 1 Dic 2009
First Detector
The first Scintillating detector was mounted in March 2004
It was constituted by a 1 cm3 PbMoO4 crystal, simply glued on a copper frame, facing a 36 mm diam
Ge Crystal, acting a LD
Thermistor
Heater
Reflecting cavity
Stefano Pirro - CSNII - 1 Dic 2009

9. First Detector – Results (1)
Light On Ge
210Pb  210Bi (47 keV) +(15keV) +  (61 keV)
210Bi  210Po (1.17 MeV)
210Po 206Pb (5,3 MeV)
Heat in PbMoO4
S. Pirro - Cryoscint - Oxford, 7 Dec 2004
Stefano Pirro - CSNII - 1 Dic 2009

10. First Detector – Results (2)
Light signal on Ge detector
47 keV on PbMoO4
Our “simple” Light detector setup is extremely sensitive
S. Pirro CryoScint - Oxford, 7 Dec 2004
Stefano Pirro - CSNII - 1 Dic 2009

11. First “LARGE” mass high RESOLUTION scintillating bolometer for DBD
A very impressive result was indeed obtained as a by-product.
The idea was to measure the thermal neutrons flux inside the CUORE R&D cryostat using
a CdWO4 crystal, via the (113Cd,n) reaction.
6.3 cm dia 1 mm thick Ge
3x3x2 cm3 CdWO4
(140g)
Qbb(116Cd)=2805 keV
S. Pirro - Scintillating Double Beta bolometers NANP05 Dubna 23 June 2005
Stefano Pirro - CSNII - 1 Dic 2009

12. Stefano Pirro - CSNII - 1 Dic 2009
Setup
Mixing Chamber
Stainless steel wire
Light Detectors
First damping mass
(Roman Lead)
Very “un-clean” setup”
Cuore Radioctivity
test run
(II Damping stage)
S. Pirro - Scintillating Double Beta bolometers NANP05 Dubna 23 June 2005
Stefano Pirro - CSNII - 1 Dic 2009

13. Background measurement
During April 2005 a background measurement was performed with the RAD detector
417 h live time measurement
S. Pirro - Scintillating Double Beta bolometers NANP05 Dubna 23 June 2005
Stefano Pirro - CSNII - 1 Dic 2009

14. Scintillating bolometers and ”official” funding
The activity was officially funded by INFN through the BoLux (R&D) Experiment* and by EC
Ilias (FP6)
The Bolux group (1.5 –1.8 FTE) was originally formed by INFN-Mi-Bicocca
In 2007 I asked for an EC Starting grant to perform a small Experiment based on CdWO4 but the project
was badly rejected - “essential weaknesses in the proposal “
BoLux was “extended” for 2009 with the join of Roma 1 (total 2.6 FTE)
A “PRIN” was presented to MIUR in 2009 and is still under evaluation
The CUPIDO R&D* project (INFN) was accepted by CSN5 for (total 3.4 FTE) but was hardly cut by referees.
The activity of Scintillating bolometers is inserted in ILIAS-NEXT (FP7) proposal with a request of money
of 300 k€
……. Lucifer was approved by ERC (Ferroni)
* Responsible: Stefano Pirro
Stefano Pirro - CSNII - 1 Dic 2009

15. Summary of (almost) all the measured crystals
Good Scintillation light
Poor Scintillation light
No Scintillation light
ZnSe
PbMoO4
SrMoO4
CdMoO4
CdWO4
CaF2
CaMoO4
ZnMoO4
Li2MoO4
ZrO2
MgMoO4
TeO2
Stefano Pirro - CSNII - 1 Dic 2009

16. Results on the first array of CdWO4 crystals (1)
4 3x3x3 cm3 (215 g each) CdWO4
1 common LD facing the 4 crystals
CdWO4 – 3x3x6
Stefano Pirro - CSNII - 1 Dic 2009

17. Results on the first array of CdWO4 crystals (2)
CUORE test crystal
Large light detector
Internal Roman lead shield
4 CdWO4 3x3x3 cm3 crystal array
CdWO4 3x3x6 cm3
Small light detector
Stefano Pirro - CSNII - 1 Dic 2009

18. Results on the first array of CdWO4 crystals (3)
The data on the single 420 g 3x3x6 cm3 crystal are presented.
The obtained scatter plot is shown it corresponds to 1066 hours of background measurement
44 days background
Background-Free area
2615 keV 208Tl 
The MC simulation predicts a background level of 10-4 c/keV/kg/y in the region of interest
Stefano Pirro - CSNII - 1 Dic 2009

19. Results on “large” crystals – CaMoO4
CaMoO4 is not a “perfect candidate” for future DBD Experiment since it contains 48Ca
But this compound did show an extreme interesting feature
Rise time of the CaMoO4 crystal – no light detection
[ms]
[au]
Decay time of the CaMoO4 crystal – no light detection
147Sm keV
CaMoO4 Bolometers permits alpha discrimination (99,7%) without Light detection !!!!
Stefano Pirro - CSNII - 1 Dic 2009

20. ZnMoO4 – 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 - CSNII - 1 Dic 2009

21. ZnSe – an extremely interesting compound
ZnSe crystal was/is an extremely puzzling and interesting material.
ZnSe crystal has a huge scintillation output but the emission spectra is close to the absorption spectra (1 mm)
It normally doped with Te in order to increase the overall light output (we tested UNDOPED crystals)
 4 cm dia, 1.7 cm height, 120 g
2 cm dia 3 cm height, 39 g
Stefano Pirro - CSNII - 1 Dic 2009

22. ZnSe – a puzzling scintillation
In the first run (2007) we observed a “very” strange scatter plot
Stefano Pirro - CSNII - 1 Dic 2009

23. Stefano Pirro - CSNII - 1 Dic 2009
ZnSe – Light Pulse
Moreover a “good” surprise arise from the time development of the Scintillation Signal
The scintillation signal has a difference in the O(10ms) range. This effect is absolutely
unexpected, even at very low temperatures
Stefano Pirro - CSNII - 1 Dic 2009

24. ZnSe – Light Detector PulseShapeAnalysis (1)
Ionizing particles
/
Looking at the coincidences between Heat in ZnSe and “Light” in the light detector, three
population appears
Stefano Pirro - CSNII - 1 Dic 2009

25. ZnSe – Light PSA (2) /  337 g “new” ZnSe Crystal
First Results on a 4 cm  5 cm height 337 g ZnSe Crystal
337 g “new” ZnSe Crystal
Light
Calibration with 232Th and a smeared  source
210Po
/
Smeared  source 
Beta region
Rejection > 99.5 %
Stefano Pirro - CSNII - 1 Dic 2009

26.  rejection > 97 % without light detection
ZnSe – Heat PSA
This compound shows another very interesting feature: ’s show different thermal pulse development
/ 
 rejection > 97 % without light detection
Stefano Pirro - CSNII - 1 Dic 2009

27. Stefano Pirro - CSNII - 1 Dic 2009
Crystal Summary
Reproducibility
Radiopurity
Availability
/n rejection
116Cd
CdWO4
82Se
ZnSe
100Mo
…. ZnMoO4
Stefano Pirro - CSNII - 1 Dic 2009

28. Stefano Pirro - CSNII - 1 Dic 2009
Conclusions 1
Developed a new powerful technique to be used for future DBD Experiments
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 needs more R&D both for radioactivity and scintillation light
Stefano Pirro - CSNII - 1 Dic 2009

29. Stefano Pirro - CSNII - 1 Dic 2009
Conclusions 2
Different nuclei HAVE to be investigated by DBD experiments
DBD Detectors with Q  >2615 keV can reach background levels  2 orders of magnitude smaller with respect to the present (funded) experiments
130Te
76Ge
100Mo
116Cd
Gerda
CUORE
If we think about possible “ III generation experiments” the key point will be
(if we consider solved the background issue….) Energy Resolution ( 1 %)
Scintillating Bolometers are the only detectors that can fulfill all these 3 requirements
Stefano Pirro - CSNII - 1 Dic 2009