The MUNU experiment

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Nuclear Physics A663&664 (2000) 815c-818c

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www.elsevier.nlllocate/npe

The MUNU Experiment A. Tadsen a for the MUNU Collaboration alnstituto Nazionale di Fisica Nucleare-Sezione di Padova, via Marzolo 8, 1-35131 Padova, Italy The MUNU experiment has been built to measure the neutrino magnetic moment at a reactor. Its central part, at the same time target and detector, is a 1 m 3 TPC filled with 3 bar of CF4. This TPC is surrounded by active and passive vetos. Data taking has started and the measured performance of this detector will be shown. 1. The Neutrino Magnetic Moment

Although the standard model predicts the neutrino magnetic moment to be zero, there exist other models leading to relatively large values [1-6]. With a non vanishing magnetic moment neutrinos will have electromagnetic interactions scattering them into a sterile state. This may be an explanation for the observed low neutrino flux from the sun in the Homestake [7], GALLEX [8], SAGE [9] and KAMIOKANDE experiments [10]. If neutrinos have a magnetic moment in the order of 10- 10 - 1O-12 ttB the magnetic field of the sun would flip them from the left-handed state into a right-handed state, making them invisible for the experiments. To measure the neutrino magnetic moment directly in an experiment we are using u.e: scattering. The cross section for this interaction increases for a non vanishing magnetic moment. It is given by:

e G~me do = -dT - [2 (gA - ()2)m sv - X - 2T- + (gv 21r E

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where E; is the neutrino energy and T the electron recoil energy. The first line gives the contribution from the standard model due to the weak interaction with

2M 2 x = T ( r 2 ) sirr' Ow

(2)

where (r 2 ) is the square charge radius of the neutrino. The second line of eq.l is the additional contribution due to the magnetic moment /-lv' The standard model differential cross section increases linearly with the neutrino energy but the effect of a magnetic moment only logarithmically. Therefore it is advantageous to use low energy neutrinos, as from a nuclear reactor, to measure the magnetic moment of the neutrino. 0375-9474/00/$ - see front matter © 2000 Elsevier Science B.Y. All rights reserved. PH S0375-9474(99)OO776-9

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A. Tadsen/Nuclear Physics A663&664 (2000) 815c-818c

2. Present experimental situation The first experiment measuring [lee- scattering has been performed by Reines at the Savannah River Plant in the mid seventies [12]. Their measures count rates were reanalyzed by Vogel and Engel later [13] using a better determination of the reactor spectrum. They found a higher count rate than expected from the standard model which may be interpreted as a magnetic moment of /-lv = (2 - 4) x 1O-1°/-l B . In the meantime two other reactor experiments have given limits of Pv < 2.4 X 1O-1°/-l B at Kurchakov [14] and /-lv < 1.8 X 10- 10PB at Rovno [15] An experiment has been done at the LAMPF beam-dump giving a limit of Pv < 1.08 X 10-9 PB [11]. Astrophysical observations lead to much lower limits but with larger theoretical errors. The neutrino burst from SN1987A limits the neutrino magnetic moment to a value of /-lv < 1 - 20 X 1O-13 PB assuming neutrinos to be Dirac particles [16-18J. Limits from stellar cooling, /-lv < 10- 12 -10- 11 tie, apply to Dirac and Majorana neutrinos but are less stringent [19,20J.

3. The MUNU Experiment To measure the neutrino magnetic moment with high precision we have built a new detector [21]. Its main part is a TPC filled with CF4 which serves as neutrino target and detector. It has a low energy threshold because it is a gaseous detector. Due to its tracking capabilities background events can be identified and rejected. To complete the detector the TPC is surrounded by a liquid scintillator veto and a passive shielding. Figure 1 shows the detector setup.

f,.;actorf : steel vessel 18.6m I ~.

1'~ IT

anode ( 20 ~lm)

t. cathode (-60 kV) 1m

Figure 1. The MUNU detector, at the center the TPC which is surrounded by the antiCompton and the passive shielding

A. TadsenlNuclear Physics A663&664 (2000) 815c-818c

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Th e experiment is sit ua ted at a nuclear power plant in Bugey, France, inside the reactor building at 18 m from the core. At the experimental site th e neutrino flux is 5 x 102°ve l sec. The shielding due to the concrete of the building reduces the muon flux to 32/m 2 s and the soft cosmic ray component to zero. The detector is housed in a shielding of 15 ern of lead and 8 ern of polyethyl ene to protect it against gammas and neutrons from th e laboratory. It consists of a 10 m 3 stainless steel tank with the TPC mounted inside at th e cente r. The tank is filled with a mineral oil based liquid scintillator. It is viewed by 48 photomultiplier tubes of 20 em diameter and used to veto cosmic muons and as an anti-Compton detector. Th e anti-Compton efficiency is 98% for 1S with an energy larger than 100 keY. The 1 m3 TPC is filled with 3 bar of CF4. This gas has a very high density of 3.68 gil at 1 bar. It has a relatively low Z which reduces multiple scattering and no free hydrogen to avoid the ve + P -7 e" + n background reaction. CF4 is also a suitable gas for a TPC having a high drift speed, a low diffusion and a large attenuation length. The TPC itself is made from a cylindrical acrylic vessel with an inner diameter of 90 em and a length of 162 cm. Inside th e TPC vessel a copper plate is screwed at one end to form the cathode. At the other end a wire grid, anode wires and a xy plane from perpendicular strips are fixed. Field shaping rings outside the TPC define the drift field. The anode wires have a spacing of 4.95 em and are read out all together. The strips have a 3.5 mm pit ch. Each of the 512 strips is read out with a 8 bit FADCs with a 80 ns sampling and a depth of 1024 words. The anode information delivers the dep osited energy and the strip information together with the time the xyz coordinates of the events. In this way tracks are seen in the TPC, allowing direction and fiducial volume det ermination and particle identification.

4. Detector Performance Due to the tracking capability of the TPC, single electrons have a clear signature. They can be easily separated from alph as, muons or coincidences from radioactive chains . This redu ces the background. A fiducial volume smaller th an th e TPC has been defined with out problems to identify background electrons coming from the walls or entering th e TPC. The energy resolution has been measured with the 60 keY gamma line from an 241 Am source. It is 48% FWHM at 3 bar. Electrons with energies down to 20 keY are visible in th e chamber. The actual data taking threshold is at 300keV because we want to measure th e direction of the events and due to the small size of low energy electrons this is not possible below. The angular resolution is a[degree] = 8.3IE[MelfJ + 3.7, corresponding to 20 degree at 500 keY. It has been determined using a full Monte Carlo simulation of the TPC. For th e first time in such an experiment electron energy and direction are measur ed and this is a large advantage. Electrons going backward with respect to the neutrino direction are only background, while forward going electrons are signal plus background together. So the background can be measured on-line with high statistics at the same time as the signal instead of measuring it in the usuall y very short reactor off periods. Electrons with energies above 700-800 keY have a smaller open ing angle and a better angular resolution, so an even more stringent angular cut can be applied , which will

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A. Tadsen/Nu clear Physics A663&664 (2000) 815c-818c

furth er limit the backgroun d. From the electron energy and direction togeth er we are ab le to reconstruct t he incoming neutrin o energy which will reduce syste mat ic errors. To limit the background from natural act ivities th e whole detector has been made from rad iochemical pure ma terials . Th e liquid scintill at or and the acrylic have been tested with neutron acti vation. Th eir conta minat ion is lower th an 10- 12 g/g for 232Th and 238U and 3 x 10- 9 g/g (scintillator ) and 2 x 10- 7 g/g (acrylic) for 40K. All other ma terial s have been carefully selected using I act ivity measurements with Ge detector s. T he expected signal has been calculated using eq. 1. It is 5.4 events per day for a zero magnetic moment and 8.1 events per day for J1.v = lO- IO J1.B, for an energy threshold of 300 keV, taking into account the det ector efficiency. Considering the syst ematic erro r of 5% mainl y from t he reactor spectrum, power and burn-up as well as from th e detecti on effi ciency we expect a sensitivity to the magneti c moment of the neutrino of J1.v rv 3 X 10- 11J1.B after one year of dat a taking .

5. Status Th e detector has been completely installed at the Bugey sit e and works as expected. A first data taking period beginning of this year revealed a high Radon background. After changing th e contaminated filter it disappeared. The count rate is nevertheless still larger th an expecte d. This will be impr oved by an exchange of th e cathode on which we found a 210Pb contamina tion, scheduled for Sept ember. This summer we will take data wit h a t hreshold of 1 MeV where we measured a rate of 6 events/day after all cut s.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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