The MUNU experiment

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Nuclear Physics B (Proc. Suppl.) 66 (1998) 218-221

PROCEEDINGS SUPPLEMENTS

The MUNU Experiment C. Broggini a * a INFN Padova, via Marzolo 8, 1-35131 Padova, Italy We built a low background detector based on a 1 m 3 time projection chamber surrounded by an active antiCompton shielding. The detector has been installed near a nuclear reactor in Bngey for the experimental study of the ~ e - scattering. A low threshold, around 500 keV, can be set on the electron recoil energy, giving the experiment a sensitivity to the Pe magnetic moment down to 3.10 -11 Bohr magnetons.

1.

Introduction

MUNU was designed to study the ~ e - --+ Feescattering with the antineutrinos from a nuclear reactor. The aim of MUNU is to get a precise measurement (~ 5%) of the Y~e- differential cross section down to 0.5 MeV kinetic energy of the recoiling electron. This way the experiment can be sensitive to a neutrino magnetic moment extending down to 3.10 -11 Bohr magnetons. In fact, a non vanishing neutrino magnetic moment produces P~e- scattering events in addition to the ones due to the weak interaction. In the standard model neutrinos are massless Dirac particles and have vanishing magnetic moment. But models exist in which the magnetic moment can become relatively large, while the neutrino mass remains small (see for instance [I]

[2] [3] [4]). The magnetic moment gives electromagnetic interaction to the neutrino and its most spectacular effect is the precession of neutrinos from left handed VtL states to right handed vl'R states in the presence of a transverse magnetic field /~j.. In fact, solar neutrino astronomy triggered the recent interest in the magnetic moment of the neutrino, which might be responsible for the observed low flux of u~ from the Sun [5] [6] [7]. The total eros section for the weak and electromagnetic interactions in the Fee- --+ Yee- scattering are proportional to Ev and log(Ev) respectively, whereas the weak differential cross section *On behalf of the MUNU Collaboration 0920-5632/98/$19.00 © 1998 ElsevierScienceB.V. All fightsreserved. PII S0920-5632(98)00040-1

is a polynomial in T and the electromagnetic one goes as 1/T (E~, is the neutrino energy and T is the kinetic energy of the recoiling electron). The different dependence of the weak and electromagnetic cross sections on the energy put the following costraints to any neutrino-electron scattering experiment willing to have a high sensitivity to the neutrino magnetic moment: a low energy beam and a low threshold electron detector. Two experiments have measured the Fee- cross section in the few MeV energy range [8,9]. Both the experiments were running close to the core of a nuclear reactor and they measured only the kinetic energy of the recoiling electron with a plastic or liquid scintillator. The signal was marginal in comparison with the background and the threshold on the recoiling electron energy was rather high (1.5 MeV and 3.2 MeV respectively). The sample of Fee- interactions, obtained by comparing the reactor-on and reactor-off rate, consists of about 460 [8] and 170 [9] events. The limits on the magnetic moment is ,,~ 3.10 -1° Bohr magnetons. 2.

The experiment

MUNU, a second generation experiment, has two substantial improvements with respect to previous experiments: • the electron energy and direction will be both determined * about 3000 neutrino events can be collected during 1 year of data taking.

C Broggini/NuclearPhysics B (Proc. Suppl.) 66 (1998) 218-221 The central detector is a 1 m 3 time projection chamber (TPC). The electrons of the filling gas are the target for the Fee- interaction. Their scattering angle can thus be measured and this allows for a "Kamiokande-like" experiment: i.e. a simultaneous measurement of signal plus background events in the forward direction and background events only in the backward direction. Background can thus be measured on-line, while the reactor is on, and subtracted. Moreover the T P C gives the possibility of defining a fiducial volume into which a single recoiling electron can be easily identified, thus helping in keeping the background low. Since the expected event rate is low (,,d0/day) it is necessary to minimize all possible background sources.

In particular each part of the detector has been constructed from selected low radioactivity material (for instance the T P C vessel is made from acrylic) and the T P C itself is immersed into 10 m a of liquid scintillator working as anti-Compton. The detector has been placed at 18 m from the core of a nuclear reactor in Bugey. The reactor is a 2800 M W t h one which emits about 5.1020 Y~ s -1, it is on 11 months out of 12 and it has for a long time been used to search for neutrino oscillations. The spectrum is known with uncertainties of ,~ 3% in the energy region between 2 M e V and 8 MeV, where there is only the contribution from the beta decay of the fission fragments. At lower antineutrino energy, in particular below 1.2 M e V [10], there is also the contribution from the beta decay of the nuclei which are produced due to neutron radiative capture both in the fission fragments inside the core and in the surrounding materials. The error on the spectrum is large in this region, however only very few events of our sample are produced by very low energy antineutrinos (because of the 0.5 M e V threshold on the electron kinetic energy). In addition, from the electron recoil energy and scattering angle we reconstruct the incoming antineutrino energy and we can select different energy ranges. A laboratory has been installed underneath the reactor core. It has an overburden of steel, con-

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crete and water corresponding to 45 m of water, which suppresses the soft component of the cosmic rays and reduces the muon flux to 32 rn -2 • s -1. The g a m m a activity has been measured to be comparable to the one of a standard physics laboratory and no neutron flux associated with the reactor has been detected in the oscillation experiment, placed closer to the reactor core. 3.

The central detector

The central detector is an acrylic vessel TPC, a cylinder of 90 cm inner diameter and 162 cm long. The vessel is made with as little material as possible to minimize the inactive volume. The cylinder wall thickness is 0.5 cm, whereas the two lids are 1.2 cm thick plates. The total weight of the acrylic vessel, which has been tested to stand a pressure difference between inside and outside up to +100 mbar, is ,~100 kg. All the parts of the vessel are made from low radioactivity acrylic selected by neutron activation measurements with sensitivity of--~ lO-12gr/gr to 2a2Th and 23Su contaminations and --~ lO-13gr/gr to 4°K contaminations. The filling gas of the T P C is CF4 at 5 bar. CF4 was chosen because of its high density (3.7 g . 1-1 at 1 bar), of the relatively low atomic number of C and F, which reduces the multiple scattering, and because it does not contain free protons, which suppresses the ~eP -4 e+n background reaction. In addition, the cosmogenic activation of C and F is rather low. The T P C drift volume is limited by a cathode at one end of the vessel, and a grid at the other one. Behind the grid there are the anode plane, which gives the energy signal, and a plane containing two sets of isolated perpendicular strips, of 3.5 m m pitch, to pick up the induced signals and to give the x and y coordinates. The coordinate along the drift field is determined from the time evolution of the strip signals, each of which is entering a channel of a 25 M H z ADC. The T P C has been tested before being moved to Bugey, with the CF4 continuosly circulating through an Oxysorb filter and a cold trap to remove oxygen, water and possible freon contami-

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C Broggini/Nuclear Physics B (Proc. Suppl.) 66 (1998) 218-221

nations. An electron attenuation length longer than 16 m, a 20% FWHM energy resolution at 500 k e V and a space resolution of 1.6 m m were measured [11]. 4.

T h e active veto

The acrylic vessel is mounted inside a stainless steel tank (3.8 m long and 2 m in diameter) filled with a mineral oil based liquid scintillator. The liquid scintillator, which is 50 cm thick, serves to veto the cosmic muons and as antiCompton detector. Its radioactive contamination was checked by neutron activation and it is as low as the acrylic vessel one whereas the light attenuation length was measured to be 8 m at 430 rim. The scintillator is viewed by 48 hemispherical photomultipliers, 24 on each lid, of 20 cm diameter and made with low activity glass. They are immersed in the scintillator, along with their bases, and held in place by a polyethylene structure. The anti-Compton efficiency of the scintillator is about 98% for 3' energies above 100 keV. The liquid scintillator and the steel vessel also serve as low activity shielding. In addition, outside the steel vessel, there are a 8 cm thick boron loaded polyethylene shielding and a 15 cm thick lead shielding to absorb neutrons and gammas entering the detector from outside. The steel vessel is pressurized to 5 bar so as to have always a small pressure difference between the inside and the outside of the acrylic vessel TPC. A pressure equalizing system keeps this difference below -4-100 mbar not only while running, but also during pumping and filling the detector with CF4.

5. Signal and background The expected rates at 18 m from the 2800 M W t h Bugey reactor core was calculated, for Pv = O, assuming W and Z exchange only, with sin20~ = 0.2325 , and for pv = 10-1° PB, and are given in table 1 (the electromagnetic signal goes as pv2). These rates have to be compared to the background. The background rate was calculated assuming a

Table 1 Event rates T(MeV)

0.5-1 >I

Acceptance (Contained) 0.85 0.65

Fee- Events/day p =0 pv = 10-1° 5.3 8.1 4.2 5.3

100 k e V threshold for the anti-Compton rejection and a forward angle selection. The threshold in the TPC was set to 500 keV. Two main sources of background were identified: cosmic rays and natural radioactivity. Only cosmic ray muons go through the 45 m water equivalent shielding existing above the Bugey reactor. While muons crossing the detector are easily identified and rejected, muon interactions in the surrounding material or in the detector itself can create radioactive nuclei, giving 7's when decaying, and neutrons, giving 7's when captured. The 7's in turn can produce Compton electrons inside the TPC volume. Muon interactions within the TPC volume itself can induce j3 activity through stopped muon capture and spallation nuclei production. In all, we estimate a cosmic ray background of about 2 events per day. The other important background component is that from natural activities. The radioactivity of the materials entering in large quantity near the TPC fiducial volume was measured by neutron activation. Low background Ge detectors were used to measure the radioactivity of all the others materials with a sensitivity of ~ l O - l ° g r / g r to 232Th and 23SU contaminations. From the measured activities of the various materials or different components used for the MUNU detector we estimate a background rate due to natural radioactivity of about 4 events per day, essentially due to Compton electrons inside the TPC volume. The background generated by -~p ~ e+n interactions in the acrylic and in the liquid scintillator was calculated to be negligible. The total background is thus estimated to be about 6 events per day.

C Broggini/Nuclear PhysicsB (Proc. Suppl.) 66 (1998) 218-221 6. Capability and status o f the experiment Considering the signal rates at 18 m from the reactor core, as given in table 1, a statistical error less than 3 % should be achievable in the bin 0.5 < T ( M e V ) < 1 in one year measuring time. Combined with a systematic error of 5 %, essentially from the reactor spectrum (3 %), reactor power and burn-up (2 %) and detection efficiency (3 %), this leads to a sensitivity to the magnetic moment of the neutrino of#~ ~ 3.10 -11 #B (90% C.L.), an order of magnitude better than in previous experiments. This sensitivity is mainly limited by the systematic uncertainties and changes only slowly as a function of the signal versus background ratio. The limit value on/4, would be around 4 • 10-11 #B if the background were a factor 4 higher. The contribution from the magnetic moment term is different in the two energy bins given in table 1. Using the ratio of the rates in these two energy bins we will be able to cross check with reduced systematics the contribution of the magnetic moment. Also the study of the angular distribution of the recoil electrons will provide a usefull cross check. Depending on the actual background situation it may be possible, after the first data taking period at 5 bar, to lower pressure and threshold without loosing too much in event rate since the electron recoil spectrum peaks at low energies. A sensitivity around 2 - 10-11 PB seems then achievable. We observe that a change of 5 % in sin 20W changes the event rate by 4.3 % in the energy bin 0.5-1 MeV, and by 5.8 % above 1 MeV. A 5 % determination of sin 2 6w appears thus possible in our experiment, assuming a negligible magnetic moment. This accuracy is rather good, considering that we are dealing with a purely leptonic process. It is comparable to that achieved by the CHARM II Collaboration in the study of the v~,e- scattering at CERN [12]. More generally, we will be sensitive to any new physics effect producing a change bigger than 5% in the Fee- cross section at low energy. The status of the experiment is the following: the different components of the detector have been

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constructed and succesfully tested in the various laboratories and the detector has been completely mounted in the Bugey laboratory. We are now going to start the debugging phase of the whole apparatus and we plan to begin the data taking early next year.

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