GALLEX-GNO. GNO experiments: status and reports. Second International Summer Student School on Neutrino Physics in Memory of Bruno Pontecorvo

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1 GALLEX-GNO GNO experiments: status and reports R. Bernabei Univ. Roma - Tor Vergata and INFN-Roma2 Second International Summer Student School on Neutrino Physics in Memory of Bruno Pontecorvo September 2003

2 Gallex Collaboration Max-Planck-Institut für Kernphysik (MPIK)-Heidelberg, Germany Istitut für Technische Chemie, Forschungzentrum Karlsruhe (FZK) Germany Laboratori Nazionali del Gran Sasso (LNGS) - L'Aquila, Italy Dipartimento di Fisica, Universitá di Milano e INFN - Italy Physik Department E15, Technische Universität München (TUM) - München, Germany Observatoire de la Côte d'azur, Département Cassini - France Department of Environmental and Energy Research, the Weizmann Institute of Science (WI) Israel Dipartimento di Fisica, II Universitá di Roma `Tor Vergata' e INFN, Sezione di Roma 2 - Italy DAPNIA/Service de Physique des Particules, CEA/Saclay France Brookhaven National Laboratory (BNL) - USA

3 GNO Collaboration Dip. Di Fisica dell Università di Milano La Bicocca e INFN sez. Milano INFN Laboratori Nazionali del Gran Sasso Dip. Di Fisica dell Università di Roma Tor Vergata e INFN sez. Roma II Dip. Di Ingegneria Chimica e dei Materiali Università dell Aquila Max Planck Institut fur Kernphysik Heidelberg Physik Dep. E15 Technische Universitaet Muenchen

4 GALLEX/GNO GNO Purpose: measurement of the low energy solar neutrino interaction rate which is related to the sun luminosity (i.e. model-independent), with an accuracy of 5 SNU (GNO) and investigation of its time dependence on a solar cycle with a sensitivity ~15% (GNO). Basic interaction: ν e + 71 Ga e Ge E thr = MeV ~ 1.2 capture/day expected by SSM 71 Ga EC, τ=16.49 days Exp. site: Gran Sasso underground laboratory (3300 m.w.e.) Target: 103 tons of GaCl 3 acidic solution 30 tons of nat Ga (12 tons of 71 Ga) in GaCl 3 + HCl Technique: radiochemical, chemical extraction of 71 Ge every 3-4 weeks; detection of 71Ge decay with gas proportional counters Expected signal (SSM): ~ 9 71 Ge counts detected per extraction LNGS Tank

5 ν s from the SUN Solar neutrino energy spectrum ν signal composition pp+pep 55% 72 SNU 7 Be 27% 35 SNU 8 B 10% 13 SNU CNO 8% 10 SNU Tot: SNU

6 Extraction and counting procedures weeks of exposure to the solar neutrinos (SR) or 1 day for blank run (BR). 71 Ge (GeCl 4 ) extracted in water fluxing ~ 3000 m 3 of nitrogen in the solution 71 Ge (~ 95%-98%) converted in GeH 4 (gas) and used together with Xe gas to fill a miniaturized proportional counter Counting of the 71 Ge nuclei through its decays PLB490(2000)16 PLB314(1993)445 Expected signal (SSM): 1.2 n inter./day, but due to decay during exposure + ineff. ~9 71 Ge counts detected per extraction Miniaturized Proportional Counter

7 Extraction and counting procedures - 2 Decay processes for 71 Ge detection 71 Ge (EC) 71 Ga* 71 Ga (t 1/2 = 11.4 d) % Auger (kev) X-ray (kev) { K fast pulses with the respect to those due to L natural radioactivity M L-window K-window Expected energy distribution RED: background pulse BLU: fast event pulse of 71 Ge decay

8 GALLEX: data taking and result Construction of the detector May 1991 May 1992 GALLEX I data taking: 15 SR + 5 blanks 83.4 ± 19 SNU 14 May Jan 1997 GALLEX 65 SR + Jun Oct 1994 Oct Feb 1996 Feb Apr 1997 Apr 1998 now 36 blanks 51 Cr ν source experiment 11 runs 51 Cr ν source experiment 7 runs Tests with 71 As GNO data taking PLB285(1992)376, PLB285(1992)390 PLB314(1993)445, PLB327(1994)377, PLB357(1995)237, PLB388(1996)384, PLB447(1999)127 PLB342(1995)440 PLB420(1998)114 PLB436(1998)158 FINAL RESULT (1594 days 65 runs): 77.5 ± 7.7 (stat+syst) SNU Observation of: pp fusion in the solar core definitive deficit of 7 Be (or pp) ν not explainable by solar physics + reliability of the radiochemical (solar-ν) experiments (ν-sources, As-test)

9 GALLEX Energy and rise time distributions GALLEX I + II + III

10 Significance of Deficit in Time

11 Why a ν source experiment? To place trustworthiness of the experimental techniques (excluding unforseen effects) How? Exposing the experiment to a ν source of known activity and appropriate energy in the same condition than in the solar exposure Needs >50 PBq build a ν source with activity allowing a precision on 9% in the measurements 51 Cr ν energy close to solar ν detected in the experiments ( ± 0.007) T 1/2 sufficient to transport the source and perform the experiment

12 Chromium enriched in 50 Cr activated at Grenoble nuclear reactor: n + 50 Cr 51 Cr + γ Source construction

13 Response of Gallex experiment to source exposure First observation of low enery neutrino from artificial terrestrial source Confirmation of solar ν deficit as measured by experiments Radiochemical techniques are reliable: it is possible to extract 10 atoms from 30 tons and to count their decay General check of the experiment [1 PBq = Bq = 27.0 kci]

14 Arsenic tests in Gallex Introduction of about 10 5 atoms of 71 As inside the solution Repeated tests under variable respectively purposely unfavorable conditions with respect to: - method and magnitude of carrier addition - mixing-and extraction conditions - standing time To exclude withholdings (classical or hot-atom -effects)

15 Method: triple-batch comparison: As atoms in: - tank sample, - external sample - calibration sample (γspectrometry) Arsenic tests in Gallex Recovery factor = (99.9 ± 0.8) %

16 Newly structured collaboration New analog electronics GNO: the modernized and improved continuation of GALLEX New digital electronics: one Transient Digitizer for each line: led to much improved pulse shape recording New DAQ In-shield counter calibration with X-ray tube Re-measurement of carrier concentration with high accuracy Demanding selection of counters used in solar runs Internal calibration of individual counter efficiencies Reduction of Rn contamination inside counters (from 5-6 to 0-1 atoms/run) Improved internal Rn correction Neural network pulse shape analysis Improved backgrounds counts (from 0.1 to 0.06 counts/day ) GNO I (May98 - Jan00) 19 solar runs GNO II (from Jan00) 44 SR + 12 blanks PLB490(2000)16

17 Improvements from Gallex to GNO Many improvements resulting in a reduction of a factor of 2 in the systematic error Target size Chemical yield Counting efficiency Pulse shape cuts Event sel. (others) Side reactions Rn-cut inefficiency 68 Ge contamination GALLEX GNO GNO+GALLEX Rate (SNU) Stat. err. (SNU) Syst. err. (SNU) Tot. err. (SNU) Tot. err. (%) # of SR Item (active vol determination) Subtotal Subtotal Gallex 0.8% 2.0% 4.0% 2.0% 0.3% 5.0% 1.2 SNU 1.2 SNU +0.7 SNU -2.0 SNU +1.8 SNU -2.6 SNU GNO 0.8% 2.0% 2.2% 1.3% * 0.6% 3.4% 1.2 SNU 0.5 SNU SNU * Neural network analysis Further minor improvements expected in a short time (analysis of counter calibration data,...)

18 Example of the improvement in the systematics 69 Ge and 71 Ge calibration of the PC: Redetermination of carrier solution concentration Pulse shape analysis Rn cut inefficiency: Gallex (9±5)%, GNO new meas. (1.7±4.7)% (ongoing)

19 Neural network analysis better bckg rejection power than rise time analysis Comparison between NN and RT analyses:

20 ... some pictures from GNO! The columns Proportional counter The synthesis line Shielding

21 Example of recorded pulses in GNO A typical 0.5 kev event: 800 mv 1100 mv TDF 1 TDF ns 400 ns

22 GNO Results (31/08/2003) Completed 58 solar runs 1713 days Still counting 5 solar runs 30 days Blanks 12 GNO GALLEX GALLEX+GNO 62.9 ± 5.4 (stat) ± 2.5 (syst) SNU (L 68 ± 9; K 60 ± 7) 77.5 ± 6.2 (stat) ± 4.5 (syst) SNU 69.3 ± 4.1 (stat) ± 3.6 (syst) SNU

23 Energy distribution of fast events t < 50 days t > 50 days (50 d ~ 3τ) L only 69.6 ± 10.3 SNU K only 62.2 ± 8.2 SNU L+K 65.2 ± 7.1 SNU (N.N.) L+K 69.4 ± 7.1 SNU (R.T.)

24 Time dist. of selected events 71 Ge signal ~276 decays GNO 58 runs τ ( 71 Ge) = 16.6 ± 2.1 d τ true ( 71 Ge) = d counts/day/run Time dist. of selected events. GALLEX 65 runs Reduction of the bckg GNO vs Gallex 30% from 0.1 c/day/run to 0.07 c/day/run Time [d]

25 Total exposure time: 3307 d Gallex + GNO results: Davis plot Analysis Energy-RT GALLEX Analyses Energy-PS/NN GNO GNO: Ge in 1713 d exposure time GALLEX 65 SR 77.5 ± 6.2 (stat) ± 4.5 (sys) SNU GNO 58 SR 62.9 ± 5.4 (stat) ± 2.5 (sys) SNU 70.8 ± 4.5 ± 3.8 SNU GALLEX GALLEX+GNO + GNO 123 SR 69.3 ± 4.1 (stat) ± 3.6 (sys) SNU

26 Single run distribution compared with the expectations from a constant ν interaction rate

27 GNO II : example about Blank Runs Blank Runs: to check each 3 month the proper functionality of the whole setup (no tailing effect, background from isotopes other than 71 Ge, etc.) Run Start exp. End exp. Exp Time [d] Chem Yield [%] Counting [d] Excs Cnts Bckg Cnts 160 d A / A / A / A / A / A / Average per run 0.85 ± ± 3.2

28 Time dist. of selected events counts/day/run 71 Ge signal ~276 decays GNO 58 runs Time [d] counts/day/run 6 GNO blanks Time [d]

29 GALLEX +GNO Seasonal variations Data binned with the distance from the Sun Geometrical modulation January July Expected geometrical modulation +2.5 SNU Rate (SNU) perihelion aphelion Flat: χ 2 = 2.7 (5 d.o.f.), P=75% Elliptical: χ 2 = 3.0 (5 d.o.f.), P=70%

30 Interpretation of solar ν expts in terms of neutrino oscillations Typical examples: SNO day-night all expts. McDonald, hep-ex/ Fogli et al., hep-ph/

31 Direct Direct and andabsolute absolute determination determination of of volume volume efficiencies efficiencies of of all all counters counters by by Ge Ge measurements measurements reduce reducesystematic < < 3% 3% (within (within two two years) years) Continued Continued exploration exploration of of joining joining Gallium Gallium from from SAGE SAGE and and GNO GNO in in one one experiment: experiment: potential potential for for tons tons gallium gallium experiment, experiment, for for a a statistical statistical error error of of ±4 ±4 SNU. SNU. Production of a new MCi 51 Production of a new 3 MCi 51 Cr Cr source sourceto to perform performa a 3rd 3rd irradiation irradiation Cross Cross section section determination determination (in (in particular, particular, at at level level kev) kev) to to <5% <5% (important (important for for pp pp and and 7 7 Be Be ν s ν s separation) separation) rem: rem: Gallex Gallex result result (89±7)% (89±7)%(this (this is is a a true true cross cross section section due due to to the the results results from from the the Gallex Gallex As As exp., exp., chemical chemical recovery recovery (100±1)%) (100±1)%) Cr Cr source source production production ar ar RIAR RIAR (Dimitrovgrad) (Dimitrovgrad) from from 11kg 11kg enriched enriched Cr Cr (38% (38% Cr); Cr); expected expected accuracy accuracy < < 8% 8% Allows Allows also also a a model model independent independent proof proof of of non-standard non-standard neutrinos neutrinos from from Gallium Gallium alone alone (exclusion (exclusion of of minimal minimal model, model, SNU SNU [2σ]) [2σ]) Intensity (PBq) exp/theo I (may 1995) ± 0.11 II (sept 1996) ± 0.11 Average 0.93 ± 0.08 (PLB420(1998)114) Later corrections 0.89 ± 0.07 Now take the ε chem from 71 As exp. (1.00 ±0.01)% the source exp. gives results on cross section 71 Ga(ν e,e) 71 Ge

32 Why sub-mev Neutrinos? Solar Physics: 98 % of all solar neutrinos are sub-mev ( Φ 7 ~ 7%, Φ pp ~91% ); the pp-neutrino flux is coupled to the solar luminosity. It is a fundamental astrophysical parameter that should definitely be measured, as precisely as possible. Stringent limitations (or observation) of departures from the standard solar model are obtained if the flux of pp neutrinos could be deduced. Ga SNU contours in the LMA region Neutrino Physics: (a) Below 1 MeV, the vacuum oscillation domain takes over from the matter oscillation domain at >1 MeV. Also there could be hidden effects only at < 1 MeV (e.g., sterile admixtures?). (b) Narrow down on tang2θ 12. To obtain 15%, the pp-flux must be determined to ± 3%. Holanda + Smirnov PRD66(2002)113005

33 Conclusions Gallex experiment has allowed to observe for the 1 st time in 1992 the pp neutrinos Gallex experiment stated the definitive deficit of 7 Be (or pp) ν not explainable by solar physics GNO experiment is running smoothly and with a very high duty cycle since may 1998 (5 years) and GALLEX + GNO since 1991 GNO: reduction of systematic error to 4.0% GNO Blank run zero-compatible The updated GNO results (58 solar runs) is 62.9 ± 5.4 (stat) ±2.5 (syst) and when combined with GALLEX (123 solar runs) is 69.3 ± 4.1 (stat) ± 3.6 (sys) No seasonal variation is observed in the GALLEX+ GNO data Winter-Summer = -7.6 ± 9 SNU

34 Conclusions - 2 GNO exposure time is now equivalent to GALLEX The same statistics (65 solar runs) will be reached at the end of 2003 GNO will continue the solar data taking At present, the gallium experiments are the only ones running with a low energy threshold (pp ν s) Refinement of the rate value to take into account new information on counters 2.5 MCi 51 Cr source Collaboration with SAGE ( better signal/noise; more significant single run result., discussions in progress) Aims: - increasing statistics and decreasing systematics - better knowledge of neutrino cross sections Cr source - Search for time variation (e.g. Sturrock et al.) - Improve the statistical meaning of single run result