Double beta decay searches with enriched and scintillating bolometers

Stefano Pirro – NuMass 2010

Stefano Pirro

Double beta decay searches with enriched and scintillating bolometers

- Milano - Bicocca

The Future of Neutrino Mass Measurements - NuMass 2010 INT Seattle February 8-11, 2010


Background Issues (1)Background Issues (1)


Background Issues (2)Background Issues (2)

Internal trace contaminations (238U and 232Th)

Surrounding trace contaminations (238U and 232Th)

Neutron induced background: direct and through (n,n’)

Surface contaminations

Long living - Emitters 106Ru 106Rh (Q=3.5 MeV) (Fission)

110mAg (Q=2.9 MeV) (fast n activation)

X X


Surface and Bulk contaminationsSurface and Bulk contaminations

CUORICINO Background

Furthermore a not negligible part of the background can arise from high energy neutrons from -spallation

-region -region

Environmental “underground” Background:238U and 232Th trace contaminations

130Te76Ge

100Mo116Cd

82Se

136 Xe


Some History 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

* Responsible: Stefano Pirro

The CUPIDO R&D* project (INFN) funded for 2010 will go on with R&D on this technique.


Principles of operationPrinciples of operation

=C/GT=E/C


Light Detectors - PerformancesLight Detectors - PerformancesOur 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


Li2MoO4

ZrO2

Good Scintillation light Poor Scintillation light No Scintillation light

MgMoO4

TeO2

Summary of (almost) all the measured Summary of (almost) all the measured crystalscrystals

ZnSe

PbMoO4

SrMoO4

CdMoO4

CdWO4

CaF2

CaMoO4

ZnMoO4


UndopedUndoped 4848CaCaFF22

There was a lack of an actual calibration due to the “lightness” of the compound; nevertheless theSignal/noise ratio of the CaF2 was excellent.

Calibration (232Th) on a 3x3x3 cm3 PURE CaF2 crystal

In 2007 we tested a CaF2 crystal. The light output was “rather poor” but definitively enough to discriminate alpha’s


Results on the first array of Results on the first array of 116116CdCdWOWO44 crystals crystals (1)(1)

CdWO4 – 3x3x6

4 3x3x3 cm3 (215 g each) CdWO4

1 common LD facing the 4 crystals


Results on the first array of Results on the first array of 116116CdCdWOWO44 crystals crystals (2)(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

Backg

roun

d-Free

area

2615 keV 208Tl

The MC simulation predicts a background level of 10-4 c/keV/kg/y in the region of interest

44 days background


ZnZn100100MoMoOO4 4 –– A promising MolibdateA 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)


Ln (Light)

ZnZn8282SeSe – – an extremely Puzzling an extremely Puzzling compound (1)compound (1)

Looking at the coincidences between Heat in ZnSe and “Light” in the light detector, threepopulation appears

/

Ionizing particles

Stefano Pirro – NuMass 2010Calibration with 232Th and a smeared source

210Po

Beta regionRejection > 99.5 %Smeared so

urce

Lig

htZnZn8282SeSe – – an extremely an extremely promisingpromising compound compound

(2)(2)First Results on a 4 cm 5 cm height 337 g ZnSe Crystal

337 g “new” ZnSe Crystal

/


““Beyond” Scintillating Beyond” Scintillating bolometersbolometers

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…


Decay time of the CaMoO4 crystal – no light detectionRise time of the CaMoO4 crystal – no light detection

[ms]

[au]

Results on “large” crystals – Results on “large” crystals – 4848CaCa100100MoMoOO44

CaMoO4 is not a “perfect candidate” for future DBD Experiment since it contains 48CaBut this compound did show an extreme interesting feature

CaMoO4 Bolometers permits alpha discrimination (99,8%) without Light detection without Light detection

147Sm- 2310 keV

The results are obtained on a ~160 g crystal


ZnZn8282SeSe – – an extremely an extremely promisingpromising compound compound (3)(3)

This compound shows another very interesting feature: ’s show different thermal pulse development

/

rejection > 97 % without light detection


ZnZn100100MoMoOO4 4 –– A A promising Molibdatepromising Molibdate (2) (2)

Light- Heat scatter plot

No Light information - PSA

This seems a characteristic of Molibdate crystals (also observed on other molibdates)


Conclusions - IConclusions - 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)

Particle Identification through Pulse Shape Analysis could result in a new, unexpected, ally

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)


Conclusions-2 Conclusions-2

221-02/1 ),()( mMZQGT

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 others

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

130Te76Ge

100Mo116Cd

82Se

136 Xe

Gerda-Majorana

CUORE

EXOSuper-Nemo

SNO+

Double beta decay searches with enriched and scintillating bolometers

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