Expression of Interest in the Super-NEMO double beta decay experiment

June 11, 2004

Ph. Adamson ¹⁷, R. Arnold ⁶, C. Augier ¹², J. Baker ⁵, A. Barabash ⁸, V. Brudanin ⁹, S. Egorov ⁹,

F. Hubert ¹, Ph. Hubert ¹, A. Gniady ⁶, B. Grinyov ⁷, N. Ishihara ¹¹, L. Jenner ¹⁷, S. Jullian ¹²,

Y. Kato ¹¹,O. Kochetov ⁹, V. Kovalenko ⁹, D. Lalanne ¹², K. Lang ¹⁶, F. Leccia ¹, C. Longuemare ¹³,

G. Lutter ¹, V. Lyubunskiy ⁷, Ch. McGrath ⁵, Ch. Marquet ¹, F. Mauger ¹³, I. Nemchonok ⁹,

H. Ohsumi ¹⁵, V. Senchyshyn ⁷, F. Piquemal ¹, J.-L. Reyss ², R. Saakyan ¹⁷, X. Sarazin ¹², Y. Shitov ⁹, L. Simard ¹², F. Simkovic ³, A. Smolnikov ⁹, I. Stekl ⁴, J. Suhonen ¹⁰, S. Sutton ¹⁴, G. Szklarz ¹²,

J. Thomas ¹⁷, V. Timkin ⁹, V. Tretyak ⁹, L. Vala ⁴, I. Vanushin ⁸, V. Vasiliev ⁸, V. Vorobel ⁴,

Ts. Vylov ⁹, V. Yumatov ⁸

1- CENBG, IN2P3-CNRS and Bordeaux University, France

2- CFR, CNRS Gif sur Yvette, France

3- Cornelius University, Slovakia

4- CTU, Prague University, Czech Republic

5- INEEL, Idaho Falls, USA

6- IReS, IN2P3-CNRS and Strasbourg University, France

7- ISMA, Kharkov, Ukraine

8- ITEP, Moscow, Russia

9- JINR, Dubna, Russia

10- Jyvaskyla University, Finland

11- KEK, Japan

12-LAL, IN2P3-CNRS and Paris-Sud University, France

13- LPC, IN2P3-CNRS and Caen University, France

14- Mount Holyoke College, USA

15- Saga University, Japan

16- Texas University, USA

17- UCL, London, UK

1. Abstract

The observation of neutrino oscillations has demonstrated that neutrinos have mass, and so this proves that there exists physics beyond the "Standard Model." A direct consequence of this new physics is the renewed interest in Double Beta Decay (DBD) experiments, which provide the only way to determine the fundamental nature of the neutrino (Majorana or Dirac). In addition, DBD experiments offer the possibility of discovery of the mass scale and verification of the mass hierarchy.

Given that the rate of zero neutrino DBD events is very low, an experiment which detects and can characterize particles through tracks and energy measurements can effectively reject backgrounds and select "gold events." For such an experiment, an effective zero level background is not only expected, but required.

The purpose of this Expression of Interest (EOI) is to start planning this experiment, with the expertise acquired from the earlier NEMO detectors. The Collaboration currently plans to proceed with the next generation detector, Super-NEMO, which will be designed to house 100 kg of enriched isotopes and achieve a sensitivity of 30 meV.

2. Introduction

One of the most exciting results of the last decade has been the growing evidence that neutrinos have a non- vanishing mass. In the Standard Model, neutrinos have only left-handed chiral components and therefore in the context of the Standard Model are exactly massless.

From Davis’ experiment [1] to more recent ones (SAGE, GALLEX, SNO, SK, MACRO, K2K, CHOOZ KamLAND, among others) there has been a growing interest in neutrino physics. There are presently three neutrino measurements, which have started to revolutionize the thinking behind neutrinos. From studies of atmospheric and solar neutrinos from Super-Kamiokande [2] and SNO [3], and supporting evidence from KamLAND [4], it follows that there exist discrepancies in the theoretical predictions and experimental results. These results can be explained by invoking neutrino oscillations as the mechanism for the discrepancy. This in turn implies that neutrinos have a non-zero mass. All the fermions in the Standard Model, with the exception of neutrinos, are Dirac particles with their antiparticles being the charge conjugates of the particles and with equal but opposite quantum numbers. Neutrinos, however, have an extra possibility due to their neutral charge. Neutrinos may have a Majorana mass, which would imply that the neutrino and antineutrino are effectively the same particle and cannot be distinguished. This possibility seems to be favoured by the theoretical community. Indeed, it is a requirement in some GUTs such as SO(10) or strings.

Currently there are experiments searching for neutrinoless double-beta decay (0nbb) in a number of selected nuclei. Many of these experiments have published results and have plans for a next generation experiment that will start data collection sometime in the next decade.

This next generation of neutrinoless double-beta decay experiments will be sensitive to a mass region associated with the inverted hierarchical mass spectrum [5] given the natural and plausible assumptions of three neutrino mixing and the scales given by the solar,atmospheric and reactor data, if the neutrinos are of the Majorana type.

Neutrinoless double beta decay occurs at a rate which depends on the values of nuclear matrix elements and the <mu > element of the mass matrix of Majorana neutrinos (the effective Majorana mass parameter). The latter encompasses all the dependence on the neutrino sector and can be expressed in terms of the oscillation parameters. These parameters are two mixing angles theta12 and theta13 of the lepton unitary mixing matrix U, two mass squared differences (SNO plus KamLAND and SuperKamiokande), and the non-oscillation parameters which are the lightest neutrino mass and the two Majorana CP-violating phases. The predictions for the effective Majorana mass obtained with the present data on the oscillation parameters depends strongly on the type of neutrino mass spectrum, which is of either the normal hierarchical, inverted hierarchical or quasi-degenerate type. In particular, since the solar mixing angle is found to exclude maximal mixing, significant lower bounds can be placed on the effective Majorana mass for an inverted hierarchical (larger than 10 meV) or quasi-degenerate mass spectrum (larger than 50 meV), such that an experimental limit or measurement of 30 meV is an interesting probe of the hierarchical type.

The 30 meV measurement is in the range of the future neutrinoless double beta decay experimentsif nature corresponds to the inverted hierarchical or quasi-degenerate spectrum. Alternatively, a measurement of a strong upper bound on the effective Majorana mass provides critical information on the type of neutrino mass spectrum. Furthermore, if the absolute neutrino mass scale and the effective Majorana mass are defined with sufficient precision, it is in principle possible to obtain information on CP-violation in the lepton sector due to Majorana CP-violating phases. Again, if neutrinoless double beta decay is observed, it will be a major achievement, establishing that lepton number is violated and the neutrino is a massive Majorana particle, and possibly the type of hierarchy and absolute neutrino mass scale. If only limits are measured it will improve our understanding of the mass spectrum and the parameter space of Grand Unified Theories.

It is important to keep in mind that the rate of 0nbb depends on the nuclear matrix element, which is presently known for some nuclei to within a factor of three. These uncertainties make it difficult to select the "best" nuclei and the tail of the 2nbb decay must be considered carefully so that it does not interfere with the ability of the experiment to positively identify neutrinoless double beta decay events. If the events occur at a rate that is high enough and it is a zero background experiment it should be possible to detect so-called "gold events." Such events have a distinctive signature, where the summed kinetic energy of the two emitted electrons equals the energy of the transition between parent and daughter nuclei. The nuclear matrix elements come into play in the translation of the theoretical prediction of the effective neutrino mass.

3. The Super-NEMO experiment

As with the earlier NEMO experiments, the intended mode of operation is the identification of the vertex and measurement of the energies of the emitted electrons in the source. The ability to distinguish electrons from positrons with the help of a magnetic field, and identification of gammas and alpha particles so as to reject backgrounds is essential.

The experiment will again use a tracking device and energy measurements will be performed with a calorimeter or by the tracking device itself. There are several different isotopes that have been proposed. These include ¹⁰⁰Mo, ⁸²Se, ¹³⁰Te, ¹³⁶Xe, and ¹¹⁶Cd where the total weight of double beta decay isotopes will be at least 100 kg, which is up by a factor 10 from the current NEMO 3 detector [6,7].

The present limits on the neutrino mass which are extracted from DBD experiments are a few hundred meV, which is also expected from NEMO 3. After some improvements in the current techniques it is expected that a limit can be placed on after five years in the region of a few tens of meV. The Super-NEMO experiment, as has been the case with NEMO 3, will have the capability of studying different isotopes so that it remains flexible to future nuclear matrix element calculations which may identify the most promising isotope. The distinctive signatures of different particles provide a clear identification of DBD and background events. Thus it is possible to measure backgrounds from external gammas or neutrons as well as backgrounds from internal radioactivity. This is necessary to prove the hypothetical zero level background in the 2-electron channel. Consequently, with a few "gold events", the expected sensitivity can be reached.

4. Design Layout of the Detector

It is expected that the detector will be modular in design, promoting a proposal to distribute the fabrication over several of the collaboration’s sites. The large mass of enriched isotope(s) will be produced in Russia. The required level of purity is a few microBequerel per kilogram in the two isotopes ²¹⁴Bi (10 mBq/kg) and ²⁰⁸Tl (1 mBq/kg). It is considered feasible to reach this level of purification through efforts in Russia and in the US. This level of purification corresponds to a factor of ~20 improvement over the design parameters for NEMO 3. The sub-module design should be very flexible to changing the height, length and width proportions, thus keeping the detector shape flexible with respect to the particular layout of the underground laboratory. The sub-modules will be constructed above ground for accessibility and later moved to the underground laboratory to form super-modules. Their design will be closely based on the technology of their NEMO predecessors. Though it is thought that the thin source foils should be split into several thinner foils of 10 mg/cm² thickness for better energy resolution. Surrounding the foils will again be a tracking volume and calorimeter walls, but also between these thinner foils there will be active detection. There are currently plans to study the calorimeter as it is essential that there be an improved and perhaps separated electron and gamma-ray detection. Ideally one wants to optimise the electron energy resolution while increasing the gamma ray detection efficiency. For tagging gamma-rays, a calorimeter of large scintillators is proposed to efficiently record the decays of trace amounts of background radiation, such as ²⁰⁸Tl and ²¹⁴Bi. Thus, this detector measures its own impurities and all the external backgrounds so as to provide the zero background level performance in the region of interest for the 2-electron channel of neutrinoless double beta decay.

On each side of the source foil there will be a tracking detector. The electron tracking can be done either by Geiger-drift cells as in NEMO 3 or by a TPC. This also provides a tag on alpha particles to reject events coming from ²¹²Bi or ²¹⁴Bi while giving a handle on the abundance of these two isotopes. These two options for the tracking detector are under consideration, and a period of R&D of 18 months is planned. The choice involves a comparison of the energy resolution with plastic scintillator and/or Si detectors when compared with the energy resolution of a TPC. The detector and its shield will again have to be made of ultra-low radioactivity materials, such as in NEMO 3. The constraints in the case of a zero-background experiment lower the permissible contributions from the glass of the PMTs and so the use of a new type of photosensor is being researched. All the components of electronics will need to be outside the shielding to minimize radioactivity near the source foil. Additionally, a very careful selection of all the detector’s material will be required to achieve the desired level. The shielding will need to be effective at stopping gammas and neutrons from natural radioactivity in the lab. Thus a borated water shield is proposed. The possibility of an active shield for detection of secondaries from high energy muons is also of interest.

The experience gained with NEMO 3 suggests that these new challenges are reasonable with limited R&D. The most difficult task is the improvement by at least a factor of two in the energy resolution for electrons.

In terms of instrumentation several sub-modules will form a super-module. The current thinking is to have five super-modules each on the order of the size of NEMO 3 with the approximately 20 kg of double beta isotopes in each. The choice of the isotope is not yet finalized but ⁸²Se appears to be very promising. It will also be possible to measure the level of radiopurity of the first 20 kg of ⁸²Se in NEMO 3.

The highly modular structure of the detector will permit installation and operation of the first super-module while the others are being commissioned. So it is not necessary to wait for the completion of the construction of all five supermodules to start collecting data. On the contrary, it is advantageous to start data collection immediately after installing the first supermodule, provided that the double beta decay source meets the required of ultra-low radioactivity. This practice is now rather common in planning the start up of future experiments and provides an early handle on gaining experience on improving the construction of subsequent super-modules.

5. Production of Isotopes

The isotopes will be produced in Russia by an enrichment process (~ 95% enrichment). The debate on the choice of which isotopes to study depends strongly on the theoretical nuclear matrix element calculations and a significant theoretical effort here would help. As stated earlier the possible isotopes are ⁸²Se, ¹³⁰Te, ¹³⁶Xe, ¹¹⁶Cd and ¹⁰⁰Mo. In the case of ¹³⁶Xe the issues of a gaseous source would have to be addressed. This might be possible with an inner volume that is isolated from the tracking detector gas. Other isotopes can be used either as "metallic" foil, as in the case of ¹⁰⁰Mo, or as a powder deposited on a film with an organic binder.

One of the advantages of ⁸²Se is that it can be produced in sufficiently large quantities via a well known gas centrifuge process. Present facilities in Russia can produce a total of 30-50 kg/year.

Apart from the enrichment of the isotopes there will most likely have to be a rigorous process of physical or chemical purification. This purification program will greatly benefit from the more than 15 years of R&D for NEMO 3 as similar techniques will be used for isotope purification. Based on this experience one can estimate that the purified source production can be completed in 2 years for the first batch of 20 kg and if the level of purification is sufficient the whole production could follow within 4 years.

6. R&D

Given this next generation detector relies heavily on the very well known technology developed for NEMO 3 the amount of R&D needed is expected to be minimal. However, it is essential that such a project must have an improved energy resolution. Depending on the choice of the tracking detector, the resolution must be addressed in the following ways.

If the choice is for scintillators for measuring the electrons energy (as done in NEMO 3) there needs to be an improvement of the resolution DE/E (FWHM). It can be achieved 8% and the goal is a resolution better than 6% at 1 MeV. The granularity of the large detection’s surface (400 m² or more) will require calorimeter units with a typical size 10*10*2 cm³. These values are for double beta emitters with a half-life greater than 10²⁰years to handle the contribution of the 2-neutrino DBD. An alternative method could be silicon detectors with 5% at 1 MeV which currently exist but needs to be tested in this low-radioactivity environment.

The second choice is a TPC. There are two possibilities here, a high or low pressure TPC. The low pressure TPC, a DCBA type, yields momentum measurements with a magnetic field. More exotic techniques cannot be excluded if a high pressure TPC is selected that also works as a calorimeter for electrons. The energy resolution to be reached here is again 5% at 1 MeV.

The R&D effort on plastic scintillators and PMTs will be directed at the improvement of the energy resolution. Our previous experience shows that this task is achievable either with plastic scintillators (as in NEMO 3), liquid scintillators or a hybrid detector combining plastic and non-organic scintillators. Studies of improvements in light yield and light collection are planned. The efficiency and uniformity of photocathodes in PMTs is also a concern to be addressed with the manufacturing firms.

Some R&D must be performed to make the ultra-low radioactivity source foils thinner. If the thickness is lowered from 50 to 10 mg/cm², there may be an issue with mechanical strength.

7. Possible time scale

A possible time scale for developing the experiment in outlined below. The project can be subdivided into four phases. If a TPC is selected the time scales will most likely be longer.

Phase 1 (2004-mid 2006), R&D feasibility studies :

Scintillator technology for improved light yield

Trade off study between plastic and hybrid scintillators

Silicon detector technology feasibility

TPC tests with DCBA chamber

System design of super-module: mechanical, electrical, support equipment

Production concept definition

Preliminary design review

Phase 2 (mid 2006-end 2007), EDA (Engineering, Design, Acceptance) :
WBS (Work Breakdown Structure) and "Schedule and Costing" and "Project Management Plan".

Development and characterisation of the first sub-module

Detailed design of the first super-module, critical design review

Procurement of components and assembly of the first sub-module

Laboratory test of first sub-module

Underground test of first sub-module

Preparation for production and pre-production review

Phase 3 (2008 - end 2010), construction :

Production and commissioning of the first super-module

Phase 4 (2011), operations :

Start physics running with the first super-module.

Production and commissioning of the other three super-modules

In the plan laid out above it is assumed that the sources will be produced in parallel with the modules.

8. Conclusion

There is an exciting opportunity to head up the search for neutrinoless double beta decay with the Super-NEMO detector over the next 5-7 years. The range available with 100 kg covers much of the value of neutrino mass predicted by the oscillation experiments and proposed mass spectrum. So there is a real opportunity to discover neutrinoless double beta decay.

The NEMO collaboration has already built reliable and low background detectors and this next generation detector is based on well-known techniques. Consequently, the R&D phase can be short.

Finally, if there is some strong theoretical argument, the choice of the DBD isotope can be changed because the design of the detector is independent of the source, as long as the source is in the form of foils.

References

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[2] Y. Fukada et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett 81 (1998) 1562.

[3] Q.R. Ahmad et al., (SNO Collaboration)., Phys. Rev. Lett. 89 (2002) 11301.

[4] K Eguchi et al., (KamLAND Collaboration) Phys. Rev. Lett 90 (2003) 21802.

[5] S. Pascoli and S.T. Petcov, hep-ph/0310xxx.

[6] NEMO Collaboration, Preprint LAL 94-29, LAL Orsay, 1994.

[ 7] D. Dassié et al., Two-neutrino double-beta decay measurement of Mo100, Phys. Rev. D 51 (1995) 2090

and R. Arnold, et al, Nucl. Instr. Meth. A (2004)