New limit in the search for spontaneous pion emission from252Cf source

A new experiment was carried out at Modane (Laboratoire Souterrain de Modane, France) to determine the ratio \({{\Gamma _{\pi ^0 } } \mathord{\left/ {\vphantom {{\Gamma _{\pi ^0 } } {\Gamma _{SF} }}} \right. \kern-0em} {\Gamma _{SF} }}\) with a²⁵²Cf source. The very small cosmic ray rate measured on these premises allowed to assign an upper limit of 10^?12 at 95% confidence level to \({{\Gamma _{\pi ^0 } } \mathord{\left/ {\vphantom {{\Gamma _{\pi ^0 } } {\Gamma _{SF} }}} \right. \kern-0em} {\Gamma _{SF} }}\) .