There are currently four solar neutrino experiments [16].
The Kamiokande water
Cerenkov experiment [17] can observe only the highest energy 
neutrinos. The Homestake [18] radiochemical
chlorine experiment also has its largest sensitivity at
the highest energies, but has some sensitivity to the lower energy
parts of the 
spectrum and to the higher 
line.
The two radiochemical gallium experiments, SAGE [19]
and GALLEX [20], are sensitive to the low energy pp
neutrinos, as well as to the higher energy neutrinos. The GALLEX
experiment has recently demonstrated its detection efficiency
using an intense 
Cr source, for which they observed 
times the expected numbers of counts [21].
The results of the experiments are compared with the predictions of two standard solar models [22], that of Bahcall and Pinsonneault (BP) [23] and that of Turck-Chieze and Lopes (TCL) [24], in Table 1. It is seen that all of the observed rates are well below the theoretical predictions.
Table: Predictions of the BP and TCL standard solar models for the
Kamiokande, Homestake, and Gallium experiments compared with the
experimental rates. The Kamiokande flux is in units of 
, while
the Homestake and gallium rates are in SNU (
interactions
per atom per s).
The experimental rates
relative to the theoretical predictions are shown in the last two
columns, where the first uncertainty is experimental and the second is
theoretical. All uncertainties are 1 
.
The solar neutrino problem has two aspects. The older and less significant
is that all of the experiments are below the SSM predictions. This was
never a serious concern for the Kamiokande and Homestake
experiments individually, which
are mainly sensitive to the high energy 
neutrinos, which are the least
reliably predicted. However, the predictions for the gallium experiments
are harder to modify due to the constraint of the solar luminosity,
and the statistics on the gallium experiments are now good
enough that the deficit observed there is hard to account for.
A second and more serious problem is that the Kamiokande rate indicates
less suppression than the Homestake rate. The Homestake experiment has a
lower energy threshold, and the lower observed rate suggests that there is
more suppression in the middle of the spectrum (the 
line and the
lower energy part of the 
spectrum) than at higher
energies [25]-[31].
This is
very hard to account for by astrophysical or nuclear physics mechanisms: the
is made from 
so any suppression of 
\
should be accompanied by at least as much suppression of 
. Furthermore,
all known
mechanisms for distorting the 
decay spectrum are
negligible [32].