Measurement of the Solar Neutrino Capture Rate by the Russian-American Gallium Solar Neutrino Experiment During One Half of the 22-Year Cycle of Solar Activity

TL;DR

This paper reports the measurement of the solar neutrino capture rate in gallium by the SAGE experiment over roughly half of a 22-year solar activity cycle, focusing on the low energy pp neutrinos accessible via the $^{71}$Ga to $^{71}$Ge radiochemical channel with a $233$ keV threshold.Using a ~50 t liquid gallium target in seven reactors at the Baksan Neutrino Observatory, SAGE performs ~4-week exposures, chemically extracts $^{71}$Ge, and counts decays in low-background proportional counters, with careful controls on extraction and counting efficiencies and backgrounds.The combined analysis of data from 1990–2001 yields a capture rate of $70.8^{+5.3}_{-5.2}$ SNU (stat) with a total systematic uncertainty of $^{+3.7}_{-3.2}$ SNU, which is about 55% of the standard solar model prediction and provides strong evidence for depletion of the low energy solar neutrino flux.By integrating results with other solar neutrino experiments, the study derives the solar pp flux at Earth as $(4.6 \,\pm\ 1.2)\times 10^{10}$ cm$^{-2}$ s$^{-1}$, compatible with the standard solar model when oscillations to active flavors are considered, and supports the MSW LMA oscillation scenario while highlighting the need for improved low-energy measurements.

Abstract

We present the results of measurements of the solar neutrino capture rate in gallium metal by the Russian-American Gallium Experiment SAGE during slightly more than half of a 22-year cycle of solar activity. Combined analysis of the data of 92 runs during the 12-year period January 1990 through December 2001 gives a capture rate of solar neutrinos with energy more than 233 keV of 70.8 +5.3/-5.2 (stat.) +3.7/-3.2 (syst.) SNU. This represents only slightly more than half of the predicted standard solar model rate of 128 SNU. We give the results of new runs beginning in April 1998 and the results of combined analysis of all runs since 1990 during yearly, monthly, and bimonthly periods. Using a simple analysis of the SAGE results combined with those from all other solar neutrino experiments, we estimate the electron neutrino pp flux that reaches the Earth to be (4.6 +/- 1.1) E10/(cm^2-s). Assuming that neutrinos oscillate to active flavors the pp neutrino flux emitted in the solar fusion reaction is approximately (7.7 +/- 1.8) E10/(cm^2-s), in agreement with the standard solar model calculation of (5.95 +/- 0.06) E10/(cm^2-s).

Figures (5)

• Figure 1: Capture rate for all SAGE extractions as a function of time. Error bars are statistical with 68% confidence. The combined result of all runs in the $L$ peak, the $K$ peak, and both $L$ and $K$ peaks is shown on the right side. The last 3 runs are still counting and their results are preliminary.
• Figure 2: Upper panel shows a histogram of energy vs rise time for all events observed during the first 22.86 days after extraction for all runs that could be counted in both $L$ and $K$ peaks (except May 1996). The live time is 1169.9 days. The approximate expected location of the ${}^{71}$Ge $L$ and $K$ peaks as predicted by calibrations is shown darkened. Lower panel shows the same histogram for all events that occurred during an equal live time interval beginning at day 100 after the time of extraction.
• Figure 3: Count rate for all runs from January 1990 in $L$ and $K$ peaks. The solid line is a fit to the data points with the 11.4-day half-life of ${}^{71}$Ge plus a constant background. The vertical error bar on each point is proportional to the square root of the number of counts and is shown only to give the scale of the error. The horizontal error bar is $\pm$5 d, equal to the 10-day bin size.
• Figure 4: Cumulative distribution function of the measured neutrino capture rate for all 158 SAGE data sets (jagged curve) and the expected distribution derived by 1000 Monte Carlo simulations of each set (smooth curve). The capture rate in the simulations was assumed to be 70.8 SNU.
• Figure 5: Combined SAGE results for each year. Shaded band is the combined best fit and its uncertainty for all years. Error bars are statistical with 68% confidence.