Low-energy neutrino physics with KamLAND

Nuclear Physics B (Proc. Suppl.) 217 (2011) 89–94 www.elsevier.com/locate/npbps

Low-energy neutrino physics with KamLAND Tadao Mitsuia , for the KamLAND collaboration a

Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan

Recent results from KamLAND, including reactor neutrino and preliminary geoneutrino data, are reviewed. The re-purification of the scintillator performed between 2007 and 2009 has been found quite effective for the reduction of geoneutrino background. KamLAND-Zen (KamLAND Zero-Neutrino double-beta decay search) is the next plan of KamLAND utilizing 400 kg of 136 Xe dissolved into the liquid scintillator. The R&D and construction status is reviewed.

KamLAND (Kamioka Liquid scintillator Antineutrino Detector) [1] is a 1-kton ultra pure liquid scintillator (LS) detector located at Kamioka, Japan, with 2700 m.w.e. overburden. It identifies electron antineutrinos (¯ νe ’s) via inverse beta decay ν¯e +p → e+ +n, where e+ provides a “prompt signal” while n is thermalized, diffused and captured on a proton producing a 2.2-MeV γ as a “delayed signal”. With a delayed coincidence between those signals with a mean interval of 207 μs, only ν¯e flavor is tagged, while the background, including other flavor neutrinos with larger flux, is strongly suppressed. With this advantage, small fluxes of ν¯e ’s from distant (∼ 180 km) nuclear reactors (reactor neutrinos) and those from the interior of the Earth (“geoneutrinos”) are observed. By measuring reactor ν¯e oscillation, KamLAND “visualizes” the neutrino oscillation phenomenon most impressively (Figure 1). KamLAND was originally planned to solve the solar neutrino problem. When four solutions were allowed from the flux data only (solar neutrino deficit), “independent signals” were carefully studied to determine the solution. Energy spectral distortions of 8 B flux are expected for the SMA and Just-So solutions. SuperK limited such distortion stringently, resulting in those solutions excluded. Zenith angle dependences of 8 B flux are expected for the LMA and LOW solutions. With the precise measurements by SNO and Su0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.075

perK, the LOW was found almost inconsistent, with the LMA being the only surviving solution, together with the evidence of the flavor transformation of 8 B neutrinos, established by SNO.

Data - BG - Geo νe Expectation based on osci. parameters determined by KamLAND

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1. KamLAND for the Neutrino Oscillation Workshop

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Figure 1. ν¯e surviving ratio as a function of (distance) / (neutrino energy), L0 /Eν , where L0 = 180 km is a typical distance between reactors and KamLAND. The plot is from [2].

The “most independent” signal for the LMA is the vacuum oscillation (disappearance) of reactor ν¯e ’s with the corresponding parameters Δm221 and θ12 assuming CPT invariance. The first result of KamLAND [1] showed it unambigu-

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Spectral difference


ously. This consistent result between matter and vacuum oscillations demonstrated the MikheyevSmirnov-Wolfenstein (MSW) effect. Furthermore it contributed to a robust understanding of the neutrino-matter interactions [3] including nonstandard interactions (NSI) or spin-flavor precession (SFP). By measuring artificial neutrinos, KamLAND has provided indispensable data for the complete solution to the solar neutrino problem.

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Figure 2. Upper panel: Relative difference of the expected reactor ν¯e spectra in arbitrary unit. The upper line (upper around 5 MeV) is the difference between expected spectra of Δm221 = 7.58 × 10−5 eV2 (best fit) and 7.71 × 10−5 eV2 (1σ of statistical uncertainty larger than the best fit) [2]. The lower line is the difference between the spectra with best energy scale and 1σ (1.373 %) deviated scale. Lower panel: Energy calibration (see text).

The reactor neutrino oscillation data has been improved in 2005 [4], and in 2008 [2] (Figure 1), with which KamLAND has been and will have to contribute to neutrino physics and Neutrino Oscillation Workshop. 2. Reactor neutrino As described above, measuring the reactor neutrino oscillation is the most important task of KamLAND. When CPT invariance is assumed, solar and KamLAND combined analyses are performed [5,6], in which KamLAND data mostly contribute to determining Δm221 . Since Δm221 corresponds to the “wavelength” in the energy spectral distortion (Figure 1), we have to check possible sources of fake distortions in order to estimate the systematic uncertainties of Δm221 . The possible sources are uncertainties of reactor source spectrum, energy scale of the detector, energy dependent efficiency of the detector, among which we found the energy scale uncertainty is the largest source of the uncertainty of Δm221 . The upper panel of Figure 2 shows how the energy scale error fakes the spectral distortion by different Δm221 . As seen, the spectral difference by shifted energy scale and shifted Δm221 are unfortunately very similar. To minimize this uncertainty, we are carefully performing the energy calibration using γ, e+ and e− sources as shown in the lower panel of Figure 2. LS shows a nonlinearity between deposited energy (Ereal ) and light yield (Evis ) because of quenching effect and Cherenkov light contribution, which are represented by “Cherenkov-Birks” model (curves in the figure) fitted with the calibration data of (from lower energies) 68 Ge, 65 Zn, 60 Co, n-p, and n-12 C for γ, 68 Ge, 11 C, and 10 C for e+ , and 12 B for e− . As a result, the energy scale uncertainty (linear term) is 1.4 %, contributing to Δm221 uncertainty of 1.9 % [2], being comparable to the statistical uncertainty. Recently, it was pointed out that KamLAND data also contribute to limiting θ13 [5,6]. With stimulated by those authors, the KamLAND collaboration is also performing three generation analysis, which has been submitted [7] just after this workshop.

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Geoneutrinos are ν¯e ’s from 238 U, 232 Th, and K decay chains in the Earth’s interior. Those radioactive elements in the crust and mantle (maybe in the core for 40 K) are considered to contribute about 40 % to the Earth’s heat source. Measuring geoneutrinos and determining the amounts of these radiogenic heat sources are important to understand Earth’s heat budget and the power of the plate tectonics. In 2005, the first experimental investigation of geoneutrinos was reported [8], in which the significance of finite geoneutrino signal was ∼ 2σ confidence level (C.L.). KamLAND is only sensitive to 238 U and 232 Th geoneutrinos. In [2], the C.L. was improved to ∼ 2.7σ. Since KamLAND suffers background of 13 C(α, n)16 O interaction and reactor neutrinos (lower panels of Figure 3), C.L. is carefully calculated including systematic uncertainties of background. The whole spectrum of the prompt energy (Ep ) from 0.9 to 8.5 MeV is fitted with floating Δm221 , θ12 , U and Th geoneutrino intensities at the same time, also with floating other marginalized parameters, i.e., reactor, detector, and background uncertainties, of course, including 13 C(α, n)16 O. Careful treatment is important not only to convince ourselves of the “discovery” with higher C.L. in the near future, but also for a correct treatment of the data. After Borexino also reported the geoneutrino observation [9] at 4.2σ C.L., global analyses [10] are possible, in which correct statistical information is essentially important. To reduce the 13 C(α, n)16 O background, KamLAND LS was re-purified by distillation from 2007 to 2009 (upper panels of Figure 3). The original and main purpose of this purification is to observe solar 7 Be neutrinos by reducing 210 Bi and 85 Kr background, and this analysis is also ongoing. For geoneutrino analysis, the reduction of 210 Po is quite effective because α from it is the main source of 13 C(α, n)16 O [4]. Including the data after this purification, a preliminary result was reported [11], in which finite geoneutrino signal is more than 4σ C.L. As seen in the figure, the higher-energy part around Ep = 2.3 MeV is slightly significant in the preliminary result. This

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Figure 3. Upper panels: Single rates of 210 Po, 210 Bi, and 85 Kr from 2003 to 2009. The rates decreased remarkably through twice of purification performed in 2007 and 2008-2009. Lower panels: Preliminary result of geoneutrino analysis [11] (Data-2010) compared with the previous result [2].

part is only the contribution from 238 U, so the determination of Th/U ratio by geoneutrinos is expected with further improved data in the near future. 4. KamLAND-Zen For the next plan of KamLAND, a search for neutrinoless double beta decay (0νββ) is scheduled to start in 2011 (KamLAND Zero Neutrino ββ search, “KamLAND-Zen”). We plan to install a smaller balloon (“mini-balloon”) in the KamLAND (Figure 4), in which 400 kg 90 % enriched 136 Xe will be dissolved. With a background simu-

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ter 3.16 m contains 17.7 m3 XeLS, which dissolves 400 kg 90 % enriched 136 Xe at 3 wt% concentration. This choice of the smallest balloon mostly reduces the cosmogenic background such as 10 C (Figure 5). It is also good for the background from the balloon film such as 214 Bi because of the smallest area of the film, assuming that the film is clean enough so that no fiducial cut is necessary.

An inner balloon of 3.16 m φ, containing 17.17 m3 LS, 400 kg 90%-enriched 136Xe

Figure 4. Schematic view of KamLAND-Zen detector.

lation (Figure 5), we aim at search for neutrino effective mass mν  down to 60 meV with 2.5 years of exposure. Although not described in this report, the 2nd phase is planned with up to 1000 kg of 136 Xe searching for mν  ∼ 25 meV. Liquid scintillator (LS) and 136 Xe are good combination for 0νββ experiment [12], because (i) xenon is a noble gas, (ii) 136 Xe 2νββ is slow, requiring not very high energy resolution, and (iii) 136 Xe enrichment has been established. As a noble gas, xenon can be dissolved into LS up to 3 wt%, without serious damage on LS. Actually by dissolving 3 wt% of xenon, light yield of LS decreases by 15.5 ± 5.3 %. However the light yield recovers by changing LS components, i.e., KamLAND LS component is pseudocumene (PC) = 20 %, dodecane (N12) = 80 %, PPO = 1.36 g/l, while KamLAND-Zen xenon loaded LS (XeLS) is PC=17.7 %, decane (N10)=82.3 %, PPO 2.7 g/l. Light yield of KamLAND LS and XeLS agree well. By using N10 instead of N12, the densities of XeLS (inside of the mini-balloon) and KamLAND LS (outside) also agree within 0.04 %. As seen in Figure 4, mini-balloon with a diame-

Figure 5. Simulated energy spectra of background and signal (136 Xe 0νββ with an effective mass mν  = 150 meV). The assumed balloon film is nylon-6 with a thickness 15 μm, density 1.14 g/cm3 , 238 U, 232 Th, and 40 K concentrations 10−12 , 10−12 , and 10−11 g/g respectively. For 214 Bi and 10 C, two spectra for each correspond to spectra before and after background rejection, by tagging and dead-time free electronics respectively for 214 Bi and 10 C (see text).

On top of purchasing enriched xenon and developing XeLS (already done), the following modification and construction are needed for KamLAND-Zen. (1) Develop, fabricate, and install the miniballoon, and fill it with XeLS. (2) Construct a xenon gas handling system that stores xenon, dissolves it into LS and extracts it from LS.

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(3) Develop and install a dead time free electronics to measure almost all of spallation neutrons after cosmic-ray muon events. For (1), details are in the next section. For (2), we have finished to design it, and construction will start in January 2011. For (3), the electronics is needed to reduce 10 C background. Most of 10 C production is accompanied by neutrons, then triple coincidence μ-n-10 C can tag 10 C in the same way as 11 C [13]. The obstacle of this tagging is dead time of the electronics after large μ signal. To overcome it, we developed a dead time free electronics MoGURA [14] that digitizes and buffers all the wave forms and records them when a trigger is issued. By adjusting trigger conditions, all the wave forms after μ can be recorded. MoGURA has been installed, and now a test run is ongoing with almost expected performance. 5. KamLAND-Zen mini-balloon The mini-balloon should have the following characters: (i) strong, tough, and compatible with LS, (ii) clean (low concentration of radioactive impurities) and thin, and (iii) optically transparent. We have been looking for plastic films that fullfil the requirements. Nylon is the best candidate currently. As described in the previous section, 214 Bi in 238 U chain is the most serious background. On top of choosing low 238 U nylon, tagging of 214 Bi should be considered, i.e., tag1: 214 Bi → 214 Po (half life 164.3 μs) → (α) → 210 Pb, and tag-2: 214 Pb → (β + γ) → 214 Bi (half life 19.9 m) → 214 Po. The advantage of tag-1 is a short coincidence time, although 214 Po α is easily stopped in the balloon film. The tag-1 efficiencies simulated by Geant4 are 90, 80, and 60 % for film thicknesses 15, 25, and 50 μm respectively. With long coincidence time of tag-2, a challenge is reduction of background of 214 Pb β +γ (0.5 ∼ 1 MeV), such as 40 K in the film and 210 Bi (daughter of 210 Pb resulted from 222 Rn) in LS. After making some smaller test balloons, a fullscale balloon was fabricated in February 2010, which was inflated with helium in a gymnasium (Figure 6). This test balloon is thick (80 μm) and made of polyethylene that is not compatible with LS. This balloon is mainly for insertion test de-

Figure 6. Full-scale 80-μm thick polyethylene balloon inflated with helium.

scribed later. As seen in the figure, the balloon is tear-drop shaped. By this shape, the sphere and straight tube part, which is necessary for access from the top chimney, are connected without using any other materials. In this tear-drop shape, the cone shaped neck is the most weak part to break. In this gymnasium test, we found that, by filling the balloon with slightly smaller volume than its full-size volume, the neck part seems no tension and very safe, while the sphere part becomes full spherical shape, because of pressure gradient. We have found this “slightly smaller volume” should be the target operation condition. Using this test balloon, insertion test was performed using a water pool (Figure 7). The miniballoon was tightly covered with thicker films and tied by ropes. After inserted into the pool, the cover was removed by untying ropes pulled from the top, after that the water was filled from the top. All the operations were successfully done from the top chimney, with monitoring by cameras installed in the water. In the next step, we fabricated a full-scale 25μm thick nylon balloon (Figure 8). In this fabrication, we tried almost the same procedure as the real one, i.e., welding, leak test, repairing, folding and shipping. We will perform a water pool test with this balloon in December 2010.

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Figure 7. Full-scale 80-μm thick balloon inserted in a water pool of ATOX Co., Ltd. It simulates the straight insertion of the mini-balloon into the existing KamLAND detector.

6. Summary KamLAND is providing precise and visualized reactor ν¯e oscillation data. Recently released preliminary geoneutrino data will contribute to the global analysis of geoneutrino physics. The construction of KamLAND-Zen has been started to search for 0νββ decay from 136 Xe. R&D and construction are ongoing successfully, including miniballoon development. I am grateful to the organizers and their families, for the fruitful workshop and their warm hospitality also to my family. REFERENCES 1. K. Eguchi et al. (KamLAND), Phys. Rev. Lett. 90, 021802 (2003). 2. S. Abe et al. (KamLAND), Phys. Rev. Lett. 100, 221803 (2008). 3. For recent review, J. W. F. Valle, J. Phys.: Conf. Ser. 203, 012009 (2010). (doi:10.1088/1742-6596/203/1/012009) 4. T. Araki et al. (KamLAND), Phys. Rev. Lett. 94, 081801 (2005). 5. G. L. Fogli, E. Lisi, A. Marrone, A. Palazzo, and A. M. Rotunno, Phys. Rev. Lett. 101, 141801 (2008).

Figure 8. Full-scale 25-μm thick test balloon fabricated by Daizo-Skypia Co., Ltd.

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