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Nemo-3 double beta decay experiment: Present status
Nuclear Physics B (Proc. Suppl.) 138 (2005) 207–209 www.elsevierphysics.com
Nemo-3 double beta decay experiment: Present status A.S. Barabash and the NEMO Collaboration∗a a
Institute of Theoretical and Experimental Physics, B. Cheremushkinskaya 25, 117259 Moscow, Russia
The objective of the NEMO Collaboration is to search for neutrinoless double beta decay with sensitivity for the effective neutrino mass on the order of (0.1-0.3) eV. The NEMO-3 detector has been operating in the Fr´ejus Underground Laboratory since June of 2002. In this report, the design of the detector is reviewed and some characteristics are presented. Finally, the first preliminary results for double beta decay of 100 Mo, 82 Se, 116 Cd and 150 Nd are reported.
1. INTRODUCTION The recent results of neutrino oscillation experiments [1–3] strongly suggest the existence of massive neutrinos. However, these experiments cannot provide the absolute value of the neutrino mass and answer the question of the nature of neutrinos. The discovery of neutrinoless double beta decay (ββ0ν) would be a signature that the neutrinos are Majorana particles. Moreover, the observation of neutrinoless double beta decay would constrain the mass spectrum of neutrinos, their absolute masses and, under specific conditions, will give the information of the CPviolation in the lepton sector (see review [4]). 2. THE NEMO-3 EXPERIMENT 2.1. The NEMO-3 detector The main goal of the NEMO-3 experiment [5] is to study neutrinoless double beta decay to a halflife of around 1025 years. The NEMO-3 detector (Fig. 1) is located in the Fr´ejus Underground Laboratory(LSM) under 4800 m equivalent water to reduce the flux of cosmic rays. For the search of ββ0ν, the isotopes present in the form of foils in NEMO-3 are 100 Mo (6.9 kg), 82 Se (0.9 kg), 116Cd (0.4 kg), and 130 Te (0.5 kg). Other isotopes, specifically 150 Nd (37 g), 96 Zr (9 g) and 48 Ca (7 g), are used to study two-neutrino double beta decay (ββ2ν). Other sources (0.6 kg ∗ The
authors would like to thank the Fr´ejus Underground Laboratory staff for their technical assistance in building and running the experiment.
0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.11.049
Figure 1. Schematic view of the NEMO-3 detector : (1) source foil, (2) 1940 plastic scintillators coupled to (3) low-activity photomultiplier tubes, (4) tracking volume with 6180 drift cells operating in Geiger mode
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A.S. Barabash / Nuclear Physics B (Proc. Suppl.) 138 (2005) 207–209
of natural tellurium oxide and 0.6 kg of copper) are to study the background. The thickness of the sources has been optimized to reduce energy loss in the foils. The particle detection portion of the NEMO-3 detector is 6 m in diameter and made of two parts. The tracking detector is a wire chamber made of 6180 drift cells in a mixture of helium, 4% alcohol and 1% argon, and is operating in Geiger mode. The calorimeter consists of 1940 plastic scintillators coupled to low-radioactivity photomultiplier tubes. The detector is surrounded by a solenoid, which produces a 25 Gauss magnetic field. Next there is an 18 cm thick iron shield for gammas and, finally, an anti-neutron shield made of tanks of water and wood 35 cm thick. The NEMO-3 calorimeter can detect a 1 MeV photon with an efficiency of 50%. The tracking allows one to make a distinction between an electron and a photon and to detect a delayed track, which can correspond to a delayed α desintegration, up to 700 µs after the first (β,γ) desintegration. The magnetic field makes it possible to identify electrons from positrons. After a period of “test runs” from June through December 2002, the detector was completed and has started to take data under more stable conditions since February 2003. 2.2. Performances of the detector The performance of the detector has been checked with test runs. The reconstruction of vertex was tested with calibration runs involving 60 207 Bi sources. The 207 Bi sources can emit one electron. In the one electron channel at 1 MeV, the transverse and longitudinal resolutions are 0.2 cm and 0.8 cm, respectively. The energy calibration is checked using 207 Bi and 90 Sr calibration runs. The 207 Bi sources can produce 0.5 MeV or 1 MeV conversion electrons, whereas the 90 Sr source can be used to test the end-point of the β − desintegration, at around 2.3 MeV. The typical energy RMS found for 1 MeV electron is ∼ (6 − 7)%. The performance on timing measurements by the PMTs was tested with 60 Co (which simultaneously emits two photons) and 207 Bi (which can emit two electrons) sources.
3. FIRST PRELIMINARY PHYSICS RESULTS The runs taken in June-December 2002 and February-August 2003 have been used. They correspond to ∼ 1500 h and ∼ 3000 h of data collection, respectively. Different portions of the data were analyzed in each individual case. 3.1. Double beta decay of 100 Mo The event selection criterion requires that two electron tracks be associated with active PMTs, and that the two tracks originate from the same vertex on the foil. Using time-of-flight criteria, the decay can be confirmed as occurring in the source. To reject α particle, no delayed hits should be present near the vertex.
Figure 2. Distribution of the sum of the two electron energies, for 100 Mo ββ2ν events (650 h). The crosses represent the data where the background contribution (green line) has been subtracted. The blue line represents the simulation of 2β2ν events.
For the 100 Mo sources, 13,750 events were selected, with a signal-to-background ratio of 40. The distribution of the summed energies of the electrons is shown in Fig 2, after substraction of the background. The angular distribution of the
A.S. Barabash / Nuclear Physics B (Proc. Suppl.) 138 (2005) 207–209
two electrons has also been compared with simulations (Fig 3).
209
decay were obtained: T1/2 (82 Se) = [9.1 ± 0.4(stat) ± 0.9(syst)] · 1019y, T1/2 (116 Cd) = [3.1 ± 0.15(stat) ± 0.3(syst] · 1019 y, and T1/2 (150 Nd) = [7.7 ± 0.7(stat) ± 0.8(syst)] · 1018 y. For ββ0ν decay of 82 Se, 116 Cd and 150 Nd, only limits (at 90% C.L.) were obtained: > 4.7 · 1022 , > 1.6 · 1022 and > 1.4 · 1021, respectively. 4. CONCLUSION
Figure 3. Distribution of the cosine of the angle between the two electrons, for 100 Mo ββ2ν events (650 h). The crosses represent the data with the background contribution subtracted. The blue line is the simulated ββ2ν data.
The preliminary value of the ββ2ν half-time decay for 100Mo is : T1/2 = [7.8 ± 0.09(stat) ± 0.8(syst)] · 1018y. A conservative limit for the neutrinoless double beta decay is estimated as T1/2 > 6 · 1022 y at 90% C.L. To investigate ββ2ν decay to 0+ excited state of 100Ru, 2920 h of data were analyzed. In this case, two electrons and a gamma emitted at the same time from the same point in the foil were studied and a positive 4 sigma effect was obtained. It gives the possibility of estimating the 20 half-life value for such transition as (9+3 y. −2 ) · 10 3.2. Double beta decay of other isotopes Here 1850 h of data were analyzed. A total of 400 useful events (in ββ2ν energy region) were obtained for 82 Se, 336 for 116 Cd and 147 for 150 Nd. Signal-to-background ratio was greater then 3 in all cases. The following half-life values for ββ2ν
The NEMO-3 detector is now fully operational and data collection is under stable conditions. The tracking detector and the calorimeter performance are as they were expected. The first portion of data has been analyzed and the preliminary results for ββ decay of 100 Mo, 82 Se, 116Cd and 150 Nd are presented in this report. REFERENCES 1. Y. Fukada et al. (SuperKamiokande Collaboration), Phys. Rev. Lett. 81 (1998) 1562. 2. Q. R. Ahmad et al. (SNO Collaboration), Phys. Rev. Lett. 89 (2001) 11301. 3. K. Eguchi et al. (KamLand Collaboration), Phys. Rev. Lett. 90 (2003) 21802. 4. S.M. Bilenky et al., Phys. Rep. 379 (2003) 69. 5. NEMO Collaboration, Preprint LAL 94-29, LAL Orsay, 1994
Nemo-3 double beta decay experiment: Present status A.S. Barabash and the NEMO Collaboration∗a a
Institute of Theoretical and Experimental Physics, B. Cheremushkinskaya 25, 117259 Moscow, Russia
The objective of the NEMO Collaboration is to search for neutrinoless double beta decay with sensitivity for the effective neutrino mass on the order of (0.1-0.3) eV. The NEMO-3 detector has been operating in the Fr´ejus Underground Laboratory since June of 2002. In this report, the design of the detector is reviewed and some characteristics are presented. Finally, the first preliminary results for double beta decay of 100 Mo, 82 Se, 116 Cd and 150 Nd are reported.
1. INTRODUCTION The recent results of neutrino oscillation experiments [1–3] strongly suggest the existence of massive neutrinos. However, these experiments cannot provide the absolute value of the neutrino mass and answer the question of the nature of neutrinos. The discovery of neutrinoless double beta decay (ββ0ν) would be a signature that the neutrinos are Majorana particles. Moreover, the observation of neutrinoless double beta decay would constrain the mass spectrum of neutrinos, their absolute masses and, under specific conditions, will give the information of the CPviolation in the lepton sector (see review [4]). 2. THE NEMO-3 EXPERIMENT 2.1. The NEMO-3 detector The main goal of the NEMO-3 experiment [5] is to study neutrinoless double beta decay to a halflife of around 1025 years. The NEMO-3 detector (Fig. 1) is located in the Fr´ejus Underground Laboratory(LSM) under 4800 m equivalent water to reduce the flux of cosmic rays. For the search of ββ0ν, the isotopes present in the form of foils in NEMO-3 are 100 Mo (6.9 kg), 82 Se (0.9 kg), 116Cd (0.4 kg), and 130 Te (0.5 kg). Other isotopes, specifically 150 Nd (37 g), 96 Zr (9 g) and 48 Ca (7 g), are used to study two-neutrino double beta decay (ββ2ν). Other sources (0.6 kg ∗ The
authors would like to thank the Fr´ejus Underground Laboratory staff for their technical assistance in building and running the experiment.
0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.11.049
Figure 1. Schematic view of the NEMO-3 detector : (1) source foil, (2) 1940 plastic scintillators coupled to (3) low-activity photomultiplier tubes, (4) tracking volume with 6180 drift cells operating in Geiger mode
208
A.S. Barabash / Nuclear Physics B (Proc. Suppl.) 138 (2005) 207–209
of natural tellurium oxide and 0.6 kg of copper) are to study the background. The thickness of the sources has been optimized to reduce energy loss in the foils. The particle detection portion of the NEMO-3 detector is 6 m in diameter and made of two parts. The tracking detector is a wire chamber made of 6180 drift cells in a mixture of helium, 4% alcohol and 1% argon, and is operating in Geiger mode. The calorimeter consists of 1940 plastic scintillators coupled to low-radioactivity photomultiplier tubes. The detector is surrounded by a solenoid, which produces a 25 Gauss magnetic field. Next there is an 18 cm thick iron shield for gammas and, finally, an anti-neutron shield made of tanks of water and wood 35 cm thick. The NEMO-3 calorimeter can detect a 1 MeV photon with an efficiency of 50%. The tracking allows one to make a distinction between an electron and a photon and to detect a delayed track, which can correspond to a delayed α desintegration, up to 700 µs after the first (β,γ) desintegration. The magnetic field makes it possible to identify electrons from positrons. After a period of “test runs” from June through December 2002, the detector was completed and has started to take data under more stable conditions since February 2003. 2.2. Performances of the detector The performance of the detector has been checked with test runs. The reconstruction of vertex was tested with calibration runs involving 60 207 Bi sources. The 207 Bi sources can emit one electron. In the one electron channel at 1 MeV, the transverse and longitudinal resolutions are 0.2 cm and 0.8 cm, respectively. The energy calibration is checked using 207 Bi and 90 Sr calibration runs. The 207 Bi sources can produce 0.5 MeV or 1 MeV conversion electrons, whereas the 90 Sr source can be used to test the end-point of the β − desintegration, at around 2.3 MeV. The typical energy RMS found for 1 MeV electron is ∼ (6 − 7)%. The performance on timing measurements by the PMTs was tested with 60 Co (which simultaneously emits two photons) and 207 Bi (which can emit two electrons) sources.
3. FIRST PRELIMINARY PHYSICS RESULTS The runs taken in June-December 2002 and February-August 2003 have been used. They correspond to ∼ 1500 h and ∼ 3000 h of data collection, respectively. Different portions of the data were analyzed in each individual case. 3.1. Double beta decay of 100 Mo The event selection criterion requires that two electron tracks be associated with active PMTs, and that the two tracks originate from the same vertex on the foil. Using time-of-flight criteria, the decay can be confirmed as occurring in the source. To reject α particle, no delayed hits should be present near the vertex.
Figure 2. Distribution of the sum of the two electron energies, for 100 Mo ββ2ν events (650 h). The crosses represent the data where the background contribution (green line) has been subtracted. The blue line represents the simulation of 2β2ν events.
For the 100 Mo sources, 13,750 events were selected, with a signal-to-background ratio of 40. The distribution of the summed energies of the electrons is shown in Fig 2, after substraction of the background. The angular distribution of the
A.S. Barabash / Nuclear Physics B (Proc. Suppl.) 138 (2005) 207–209
two electrons has also been compared with simulations (Fig 3).
209
decay were obtained: T1/2 (82 Se) = [9.1 ± 0.4(stat) ± 0.9(syst)] · 1019y, T1/2 (116 Cd) = [3.1 ± 0.15(stat) ± 0.3(syst] · 1019 y, and T1/2 (150 Nd) = [7.7 ± 0.7(stat) ± 0.8(syst)] · 1018 y. For ββ0ν decay of 82 Se, 116 Cd and 150 Nd, only limits (at 90% C.L.) were obtained: > 4.7 · 1022 , > 1.6 · 1022 and > 1.4 · 1021, respectively. 4. CONCLUSION
Figure 3. Distribution of the cosine of the angle between the two electrons, for 100 Mo ββ2ν events (650 h). The crosses represent the data with the background contribution subtracted. The blue line is the simulated ββ2ν data.
The preliminary value of the ββ2ν half-time decay for 100Mo is : T1/2 = [7.8 ± 0.09(stat) ± 0.8(syst)] · 1018y. A conservative limit for the neutrinoless double beta decay is estimated as T1/2 > 6 · 1022 y at 90% C.L. To investigate ββ2ν decay to 0+ excited state of 100Ru, 2920 h of data were analyzed. In this case, two electrons and a gamma emitted at the same time from the same point in the foil were studied and a positive 4 sigma effect was obtained. It gives the possibility of estimating the 20 half-life value for such transition as (9+3 y. −2 ) · 10 3.2. Double beta decay of other isotopes Here 1850 h of data were analyzed. A total of 400 useful events (in ββ2ν energy region) were obtained for 82 Se, 336 for 116 Cd and 147 for 150 Nd. Signal-to-background ratio was greater then 3 in all cases. The following half-life values for ββ2ν
The NEMO-3 detector is now fully operational and data collection is under stable conditions. The tracking detector and the calorimeter performance are as they were expected. The first portion of data has been analyzed and the preliminary results for ββ decay of 100 Mo, 82 Se, 116Cd and 150 Nd are presented in this report. REFERENCES 1. Y. Fukada et al. (SuperKamiokande Collaboration), Phys. Rev. Lett. 81 (1998) 1562. 2. Q. R. Ahmad et al. (SNO Collaboration), Phys. Rev. Lett. 89 (2001) 11301. 3. K. Eguchi et al. (KamLand Collaboration), Phys. Rev. Lett. 90 (2003) 21802. 4. S.M. Bilenky et al., Phys. Rep. 379 (2003) 69. 5. NEMO Collaboration, Preprint LAL 94-29, LAL Orsay, 1994