extrapolation of nemo technique to future generation of 2β-decay experiments

Extrapolation of NEMO technique to future generationof 2β-decay experiments ∗)

A.S. Barabash

Institute of Theoretical and Experimental Physics, Russian Acad. Sci.,B. Cheremushkinskaya 25, 117259 Moscow, Russia

NEMO Collaboration

Received 31 January 2002

Possibilities of NEMO technique for future neutrinoless double-beta-decay experimentsare discussed. Main idea is to have a realistic project with planned sensitivity for half-lifeon the level ≈ (1–2) ×1026 y (sensitivity to neutrino mass ≈ (0.05–0.1) eV). It is demon-strated that this can be achieved using improved NEMO technique to investigate 100 kgof 82Se. Possible improvements of NEMO technique and background conditions are dis-cussed. Scheme of future SUPERNEMO detector and main characteristics of experimentare presented. Such detector can be used also to investigate 0νββ decay in 100Mo, 130Teand 116Cd with sensitivity up to ≈ (5–10) × 1025 y (or with sensitivity to neutrino mass≈ 0.1 eV).

PACS : 23.40.-s, 14.60.PqKey words: double-beta decay, massive neutrinos

1 Introduction

The main goal of NEMO experiment is to study the neutrinoless double-beta(0νββ) decay in 100Mo with sensitivity ≈ 1025 y, which corresponds to the sen-sitivity for effective neutrino mass 〈mν〉 of the order of ≈ (0.1–0.3) eV. In 1988the NEMO Collaboration started a R&D program in order to construct a detectorfor studying (0νββ) decay with such sensitivity. Two prototypes, NEMO-1 [1] andNEMO-2 [2] have proved the feasibility of such project and have contributed tothe background studies for the NEMO-3 project [3]. Besides, the NEMO-2 detec-tor performances made it possible to measure half-lives of the allowed double-beta(2νββ) decay of 100Mo [4], 116Cd [5], 82Se [6] and 96Zr [7]. The NEMO-3 detector isnow under construction at the Frejus Underground Laboratory (4800m w.e.1)) andwill be ready to take data by the end of 2001. In this paper we investigate possibil-ities of NEMO technique for future, more sensitive neutrinoless double-beta-decayexperiments.

2 Brief description of NEMO-2 and NEMO-3

First of all we would like to present here a brief description of NEMO-2 andNEMO-3 detectors, because for future SUPERNEMO detector we will use practi-cally the same technique.

∗) Presented by the author at the Workshop on calculation of double-beta-decay matrix elements(MEDEX’01), Prague, Czech Republic, June 11–15, 2001.

1) w.e. – water equivalent

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2.1 NEMO-2

NEMO-2 [2] consists of a 1m3 tracking volume filled with helium gas and 4% ethylalcohol (Fig. 1). Vertically bisecting the detector is the plane of the source foil(1× 1m2). The tracking portion of the detector is made of open Geiger cells withoctagonal cross sections defined by 100µm nickel wires.

On each side of the source there are 10 planes of 32 cells with alternating verticaland horizontal orientations. The cells provide three-dimensional tracking of chargedparticles by recording the drift time and two plasma propagation times in each cell.

A calorimeter made of scintillators covers two opposing, vertical sides of thetracking volume. Two configurations of the calorimeter have been implemented.The first one consisted of 2 planes of 64 scintillators (12×12×2.25cm3) associatedwith “standard” photomultiplier tubes (PMTs). This configuration was used inthe experiment with 100Mo. The next configuration has included 2 planes of 25scintillators (19 × 19 × 10 cm3) with PMTs made of low radioactive glass. Thetracking volume and scintillators are surrounded by a lead (5 cm) and iron (20 cm)shield.

21

2

3

Fig. 1. The NEMO-2 detector. (1) Central frame with the metallic foil. (2) Trackingdevice of 10 frames with 2× 32 Geiger cells each. (3) Scintillator array. (The shielding is

not shown.)

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Extrapolation of NEMO technique to future generation . . .

The performance and operating parameters are as follows. The threshold forthe scintillators is set at 50 keV, the energy resolution (FWHM) is 18% at 1MeVand the time resolution is 275 ps for a 1MeV electron (550ps at 0.2MeV).

NEMO-2 detector was operated in the Frejus Underground Laboratory (4800mw.e.) from 1991 to 1997. During this period ββ-decay processes in 100Mo, 116Cd,82Se, 96Zr and 94Zr were investigated. Half-life values for 2νββ decay and half- lifelimits on 0νββ, 0νββχ0, and the 0νββ transition to the 2+ and 0+ excited stateshave been extracted from the data [4–8].

2.2 NEMO-3

The NEMO experiments [3] use tracking detector which is not only able to measurethe full energy released, but other parameters of the process such as the singleelectron energy, the angle between electrons, the coordinates of events, etc. Theoptimal operating parameters of the detector were investigated with the prototypeNEMO-2 [2, 4–7]. Currently the NEMO-3 detector is under construction and will beable to accommodate up to 10kg of various double-beta-decay candidates (100Mo,116Cd, 82Se, 130Te, 96Zr, 150Nd, etc). The sensitivity after 5 years of measurementswill be at the level 1025 y for 0νββ decay with 〈mν〉 ≈ (0.1–0.3) eV, and ≈ 1023 yfor 0νββχ0 decay (〈gee〉 ≈ 10−5), and finally ≈ 1022 y for 2νββ decay.

A general view of the detector’s cylindrically symmetric geometry is shown inFig. 2. The detector consists of a tracking volume filled with helium gas, a thin

Fig. 2. Schematic view of the NEMO-3 detector.

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(≈ 50µm) source foil divides the tracking volume vertically into two concentriccylinders with a calorimeter at the inner and outer walls. The tracking system con-sists of 6180 Geiger cells 2.7m long which are parallel to the detector’s verticalaxis. Accuracy of vertex reconstruction will be better then 1 cm (σ). Energy andtime-of-flight measurements are performed by the plastic scintillators covering thetwo concentric surfaces discussed above and their associated end caps. The totalnumber of low radioactive photomultipliers will be 1940. At 1MeV, the energy reso-lution which depends on the scintillator shape and the associated PMT ranges from11% to 15% (FWHM) and the time resolution is 250ps (σ). The detection thresh-old is 30 keV. A magnetic field (≈ 30 Gauss) will be used to reject backgroundsconnected with pair creation and incoming electrons. An external shielding, madeof 20 cm thick, low radioactivity iron, covers the detector in order to reduce γ-rayand thermal neutron external backgrounds coming from the LMS laboratory cave.To thermalize fast neutrons present in the laboratory, an additional outer shieldingis considered, too. It will be made of 10 water tanks fixed on flank sides of thedetector and providing 35 cm water layer, as well as of polyethylene plates of 20 cmthickness covering the top and the bottom of the detector. All 20 sectors are alreadybuilt. The placement of completed sectors on the frame in Frejus laboratory startedin December 1998. This final stage of the construction is expected to continue untilthe end of 2001. Presently, it is planned to start operating with 7 kg of 100Mo, 1 kgof 82Se, 0.5 kg of 116Cd, 0.5 kg of 130Te, 50 g of 150Nd, 10 g of 96Zr, 10 g of 48Ca andwith some sectors filled with foils especially designed to check background.

3 SUPERNEMO – next generation of NEMO technique

3.1 Main ideas

a) The main idea is to propose a realistic project, which can be truly achieved withina reasonable time. This is why we propose 1) to use the very well known NEMOtechnique and 2) to only investigate 100 kg source (such a quantity of enrichedisotope can be produced during a few years in Russia).

b) The next idea is to select the isotope for which maximal sensitivity can bereached. We propose to investigate 100kg of 82Se, because of high enough energyof the 0νββ transition (E2β = 3MeV) and a rather low probability of 2ν decay (itgives a small contribution to the 0ν region). And, in addition, 82Se can be producedusing the centrifugal method — this is why we hope to produce 100kg quite easilyand with a reasonable cost.

c) Next, we propose a modular scheme for the detector — four identical moduleswith 25kg enriched source in each. It gives a possibility to start data taking quitesoon (before finishing the entire construction) and to use the experience of theproduction of the first module for the following ones. The scheme of one moduleof the detector is shown in Fig. 3. The module consists of two plastic scintillatorcounter walls and the source between them. On each side of the source there are afew layers of Geiger cells. As in NEMO-2 and NEMO-3, the energy of electrons will

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Se: 20 x 2.2 m , 60 mg/cm82 22

Plastic scintillator walls: 20 x 3 x 0.1 m3

Fig. 3. The scheme of one SUPERNEMO module.

be measured by plastic scintillator counters and tracks will be reconstructed usinginformation from Geiger cells. The new installation can be located at the FrejusUnderground Laboratory (4800m w.e.).

3.2 Main parameters of the installation

The main parameters of the installation are the following:– 100kg source of 82Se (or other nuclei);– planar geometry (4 modules);– weight of plastic scintillator — 50 tons;– ≈ 5,000 low-background PMTs (for 30× 30× 10 cm3 plastic scintillators);– ≈ 30,000 Geiger cells;– passive shielding by 20 cm of Fe and 20 cm of borated polyethylene.The planar geometry simplifies the construction and gives the possibility to

use standard blocs and components. Notice that the number of PMTs is only 2.5times higher than in NEMO-3. The number of Geiger cells is only 5 times higher,in comparison with NEMO-3. It means that one module of the new detector (SU-PERNEMO) will be even simpler than the NEMO-3 detector.

3.3 Main characteristics of the tracking detector

Main characteristics of tracking detector are the following:– energy resolution is (10–12)% (FWHM) at 1MeV;– time resolution is 250ps at 1MeV;– vertex resolution is 1 cm (1σ);– efficiency (0ν decay) is ≈ 20%;– purity of 82Se is < 0.05mBq/kg for 214Bi and < 0.005mBq/kg for 208Tl.One can see that the main characteristics of SUPERNEMO are approximately

the same as for NEMO-3. However, here we hope to obtain a better energy resolu-

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tion (10–12% instead of 11–15% in NEMO-3) and a higher efficiency (20% insteadof 12%). And we believe that these requirements can be realized. The reasons forthis view are the following: 1) during production of the plastic scintillator countersfor NEMO-3 we had many counters with a resolution ≈ (10–12)%; 2) a better effi-ciency can be reached as a result of some improvements (no magnetic field, bettergeometry, decreasing the number of the wires in the tracking volume, decreasingthe diameter of wires, and improved selection of useful events).

3.4 Sensitivity of the experiment

3.4.1 External background

In [9], the external background in the NEMO-3 detector was estimated. Onecan extrapolate these results to SUPERNEMO and demonstrate that the externalbackground in the energy interval (2.8–3.2)MeV can be smaller than one.

3.4.2 Internal background

There are two contributions to the internal background: 1) the radioactive impu-rities inside of the source, and 2) the tail from 2ν decay. On the basis of experiencewhich we now have, we believe that it is possible to reach SUPERNEMO require-ments for the purity of the source and reach zero contribution to background. Thus,the main internal background is connected with the tail from 2ν decay. This con-tribution was estimated using expected parameters of SUPERNEMO for the fourmost prospective isotopes, 82Se, 100Mo, 116Cd, and 130Te. The results are ≈ (1–2),≈ 20, ≈ 5, and ≈ 0 background events, respectively.

3.4.3 Expected sensitivity

Using the background and efficiency estimations, one can obtain the expectedsensitivity of experiments with the mentioned isotopes — see Table 1. The expected

Table 1. Best present limits at 90% CL on 0ν decay for 82Se, 100Mo, 116Cd, 130Teand expected sensitivity of NEMO-3 and SUPERNEMO. The last line shows the bestpresent limits on effective neutrino mass 〈mν〉 and the sensitivity to 〈mν〉 of NEMO-3 and

SUPERNEMO experiments.

Best present limits NEMO-3 SUPERNEMO

82Se > 1.4× 1022 y [10] ≈ 1× 1025 y ≈ (1−2)× 1026 y100Mo > 5.5× 1022 y [11] ≈ (0.5−1) × 1025 y ≈ 5× 1025 y116Cd > 7× 1022 y [12] ≈ (0.5−1) × 1025 y ≈ 7× 1025 y130Te > 1.4× 1023 y [13] ≈ (0.5−1) × 1025 y ≈ 1× 1026 y

〈mν〉 (eV) < 2.5–8 ≈ 0.1–0.5 ≈ 0.05–0.2

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sensitivity is obtained for a 100 kg enriched source and for 5 years of measurement.Notice that all above mentioned isotopes can be produced in such quantities in Rus-sia over a reasonable time using the centrifugal method. In the case of 130Te, natu-ral Te ( ≈ 34% of 130Te) can be used. The estimated sensitivity of SUPERNEMOwith a natural Te source is ≈ 5 × 1025 y (the sensitivity to neutrino mass is ≈(0.1–0.15)eV). Of course, other prospective isotopes (for example, 150Nd, 96Zr and48Ca) can be investigated with SUPERNEMO, too. The main problem here is theimpossibility to produce 100kg of such isotopes at the present time. But, if in thefuture these isotopes are produced, they can also be investigated by SUPERNEMO.

4 Conclusions

It is shown that the NEMO technique can be extrapolated to a larger SU-PERNEMO detector with 100kg of 82Se. The expected sensitivity for 5 years ofmeasurement is estimated to be ≈ (1–2) ×1026 y, which corresponds to a sensi-tivity in 〈mν〉 on the level of ≈ (0.05–0.1) eV. The same detector can be used toinvestigate 0νββ decay in other prospective nuclei (100Mo, 116Cd, and 130Te) witha sensitivity of ≈ (0.5–1) ×1026 y.

The information from NEMO-3 will give us the possibility to improve our knowl-edge about external and internal backgrounds, the efficiency for 0ν and 2ν decays,the effect of magnetic field, etc. Then it will be possible to prepare a real proposalfor SUPERNEMO.

References

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[2] R. Arnold et al.: Nucl. Instr. Meth. A 354 (1995) 338.

[3] NEMO Collaboration: Preprint LAL 94-29, 1994.

[4] D. Dassie et al.: Phys. Rev. D 51 (1995) 2090.

[5] R. Arnold et al.: Z. Phys. C 72 (1996) 239.

[6] R. Arnold et al.: Nucl. Phys. A 636 (1998) 209.



[9] Ch. Marquet et al.: Nucl. Instr. Meth. A 457 (2001) 487.

[10] S.R. Elliot et al.: Phys. Rev. C 46 (1992) 1535.

[11] H. Ejiri et al.: Phys. Rev. C 63 (2001) 065501.

[12] F.A. Danevich et al.: Phys. Rev. C 62 (2000) 045501.

[13] A. Alessandrello et al.: Phys. Lett. B 486 (2000) 13.

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