Measurement of the solar neutrino capture rate with gallium metal. III: Results for the 2002--2007 data-taking period

TL;DR

This study reports a long-running radiochemical measurement of solar neutrinos using gallium, employing improved Ge extraction and GeH4 counting to determine the 71Ge production rate from solar νe interactions with 71Ga. The 2002–2007 data yield a solar-neutrino capture rate of 65.4 SNU (stat) and ≈65–66 SNU when systematics are included, broadly agreeing with standard solar models and oscillation physics. By combining with Gallex and GNO results, the Gaia-like global rate is 66.1 ± 3.1 SNU, supporting a pp-dominated solar neutrino flux and enabling a contemporary estimate of the solar pp flux near 6.0 × 10^{10} cm^{-2} s^{-1} at the Sun. The work also highlights a low source-experiment ratio (~0.87) that may point to cross-section uncertainties for the lowest excited states in 71Ge, motivating further dedicated measurements and neutrino-source tests. Overall, the results reinforce the consistency between direct solar neutrino measurements, solar models, and neutrino oscillation phenomenology, with time-variation analyses showing no significant departure from a constant solar νe production rate over the observation period.

Abstract

The Russian-American experiment SAGE began to measure the solar neutrino capture rate with a target of gallium metal in Dec. 1989. Measurements have continued with only a few brief interruptions since that time. We give here the experimental improvements in SAGE since its last published data summary in Dec. 2001. Assuming the solar neutrino production rate was constant during the period of data collection, combined analysis of 168 extractions through Dec. 2007 gives a capture rate of solar neutrinos with energy more than 233 keV of 65.4 (+3.1)(-3.0) (stat) (+2.6)(-2.8) (syst) SNU. The weighted average of the results of all three Ga solar neutrino experiments, SAGE, Gallex, and GNO, is now 66.1 +/- 3.1 SNU, where statistical and systematic uncertainties have been combined in quadrature. During the recent period of data collection a new test of SAGE was made with a reactor-produced 37Ar neutrino source. The ratio of observed to calculated rates in this experiment, combined with the measured rates in the three prior 51Cr neutrino-source experiments with Ga, is 0.87 +/- 0.05. A probable explanation for this low result is that the cross section for neutrino capture by the two lowest-lying excited states in 71Ge has been overestimated. If we assume these cross sections are zero, then the standard solar model including neutrino oscillations predicts a total capture rate in Ga in the range of 63-66 SNU with an uncertainty of about 4%, in good agreement with experiment. We derive the current value of the neutrino flux produced in the Sun by the proton-proton fusion reaction to be (6.0 +/- 0.8) x 10^(10)/(cm^2 s), which agrees well with the pp flux predicted by the standard solar model. Finally, we show that the data are consistent with the assumption that the solar neutrino production rate is constant in time.

Figures (7)

• Figure 1: Upper panel: Count rate vs energy and rise time for events during the first 30 days of counting. Regions where the $L$ and $K$ peaks are predicted to occur based on ${}^{55}$Fe calibrations are shown darkened. There are 427 counts in the $L$-peak region and 287 counts in the $K$-peak region. The counts in both regions are a combination of events from ${}^{71}$Ge decay and background. Lower panel: Equivalent graph for all events that occurred during an equal live time interval beginning at day 100 after extraction. There are 226 counts in the $L$-peak region and 94 counts in the $K$-peak region.
• Figure 2: Combined SAGE results for each year. Shaded band is the combined best fit and its uncertainty for all years. Vertical error bars are statistical with 68% confidence.
• Figure 3: Results of all neutrino source experiments with Ga. Gallex results are from the recent pulse shape analysis of Kaether Kaether_thesis; SAGE results are from Refs. CrPRC and ArPRC. Hashed region is the weighted average of the four experiments.
• Figure 4: Measured capture rate for all 310 SAGE data sets (jagged curve) and the expected distribution derived by 100 Monte Carlo simulations of each set (smooth curve).
• Figure 5: Lomb power spectrum from all 168 SAGE data runs. The mean time of exposure was used as the time of measurement.