In order to test the end-to-end operations of gallium solar neutrino experiments, intense electron-capture sources were fabricated to measure the responses of the radiochemical SAGE and GALLEX/GNO detectors to known fluxes of low-energy neutrinos. Such tests were viewed at the time as a cross-check, given the many tests of ⁷¹Ge recovery and counting that had been routinely performed, with excellent results. However, the four ⁵¹Cr and ³⁷Ar source experiments yielded rates below expectations, a result commonly known as the Ga anomaly. As the intensity of the electron-capture sources can be measured to high precision, the neutrino lines they produce are fixed by known atomic and nuclear rates, and the neutrino absorption cross section on ⁷¹Ga is tightly constrained by the lifetime of ⁷¹Ge, no simple explanation for the anomaly has been found. To check these calibration experiments, a dedicated experiment BEST was performed, utilizing a neutrino source of unprecedented intensity and a detector optimized to increase statistics while providing some information on counting rate as a function of distance from the source. The results BEST obtained are consistent with the earlier solar neutrino calibration experiments, and when combined with those measurements, yield a Ga anomaly with a significance of approximately 4$\sigma$, under conservative assumptions. But BEST found no evidence of distance dependence and thus no explicit indication of new physics. In this review we describe the extensive campaigns carried out by SAGE, GALLEX/GNO, and BEST to demonstrate the reliability and precision of their experimental procedures, including ⁷¹Ge recovery, counting, and analysis. We also describe efforts to define uncertainties in the neutrino capture cross section, which now include estimates of effects at the $\lesssim 0.5$% level such as radiative corrections and weak magnetism. With the results from BEST, an anomaly remains even if one retains only the transition to the ⁷¹Ge ground state, whose strength is fixed by the known lifetime of ⁷¹Ge. We then consider the new-physics solution most commonly suggested to resolve the Ga anomaly, oscillations into a sterile fourth neutrino, ${\nu }_e\to {\nu }_s$. We find such a solution generates substantial tension with several null experiments, owing to the large mixing angle required. While this does not exclude such solutions – the sterile sector might include multiple neutrinos as well as new interactions – it shows the need for more experimental constraints, if we are to make progress in resolving the Ga and other low-energy neutrino anomalies. We conclude by consider the role future low-energy electron-capture sources could play in this effort.

为了测试镓太阳中微子实验的端到端操作，制造了强电子捕获源来测量放射化学 SAGE 和 GALLEX/GNO 探测器对已知低能中微子通量的响应。鉴于常规执行的⁷¹ Ge 回收和计数的许多测试都取得了优异的结果，此类测试在当时被视为一种交叉检查。然而，四次⁵¹ Cr 和³⁷ Ar 源实验产生的速率低于预期，这一结果通常称为 Ga 异常。由于电子俘获源的强度可以高精度测量，它们产生的中微子线由已知的原子速率和核速率固定，并且^{71 Ga 上的中微子吸收截面受到71} Ge寿命的严格限制，没有已经找到了对该异常的简单解释。为了检查这些校准实验，进行了专门的实验 BEST，利用前所未有的强度的中微子源和经过优化以增加统计数据的探测器，同时提供一些有关计数率与源距离的函数的信息。获得的结果与早期的太阳中微子校准实验一致，当与这些测量相结合时，产生显着性约为 4 的 Ga 异常$\sigma$，在保守假设下。但 BEST 没有发现距离依赖性的证据，因此没有新物理学的明确迹象。在这篇综述中，我们描述了 SAGE、GALLEX/GNO 和 BEST 开展的广泛活动，以证明他们的实验程序（包括⁷¹ Ge 回收、计数和分析）的可靠性和精确度。我们还描述了定义中微子俘获截面不确定性的努力，其中现在包括对中微子捕获截面影响的估计$\lesssim 0。5$％水平，例如辐射校正和弱磁性。^{根据 BEST 的结果，即使仅保留到71} Ge 基态的转变，异常仍然存在，其强度由已知的⁷¹ Ge寿命决定。然后我们考虑最常建议解决 Ga 异常的新物理解决方案，即振荡成惰性第四中微子，${\nu }_e\to {\nu }_s$。我们发现，由于需要大的混合角，这样的解决方案通过几次无效实验会产生很大的张力。虽然这并不排除此类解决方案——惰性区可能包括多个中微子以及新的相互作用——但它表明，如果我们要在解决Ga和其他低能中微子异常方面取得进展，就需要更多的实验限制。最后，我们考虑了未来低能电子捕获源在这项工作中可以发挥的作用。