Solar neutrino unit

The solar neutrino unit (SNU) is a unit of Solar neutrino flux widely used in neutrino astronomy and radiochemical neutrino experiments. It is equal to the neutrino flux producing 10⁻³⁶ captures per target atom per second.^[1] It is convenient given the very low event rates in radiochemical experiments. Typical rate is expected to be from tens SNU to hundred SNU.^[2]

There are two ways of detecting solar neutrinos: radiochemical and real time experiments. The principle of radiochemical experiments is the reaction of the form

$_{N}^{A}Z+\nu _{e}\longrightarrow _{N-1}^{A}(Z+1)+e^{-}$ .

The daughter nucleus's decay is used in the detection. Production rate of the daughter nucleus is given by $R=N\int \Phi (E)\sigma (E)dE$ , where

$\Phi$ is the solar neutrino flux
$\sigma$ is the cross section for the radiochemical reaction
$N$ is the number of target atoms.

With typical neutrino flux of 10¹⁰ cm⁻² s⁻¹ and a typical interaction cross section of about 10⁻⁴⁵ cm², about 10³⁰ target atoms are required to produce one event per day. Taking into account that 1 mole is equal to 6.022×10²³ atoms, this number corresponds to ktons of the target substances, whereas present neutrino detectors operate at much lower quantities of those.