Experimental limit for nucleon stability against charge-nonconserving decay

The result of a search for a nucleon decay bound in a nucleus has been described in this work for the charge-nonconserving decay <SUP>87</SUP>Rb --> <SUP>87m</SUP>Sr+neutrals. <SUP>87m</SUP>Sr activities separated from 800 g of Rb<SUB>2</SUB>CO<SUB>3</SUB> chemically were counted in a twin Si(Li) detector system using the K-conversion electron and K x-ray coincidence technique by looking at the 2.8-h isomeric decay of 388.4-keV M4 transition in <SUP>87m</SUP>Sr. With modest shielding the background level has been reduced to 3&#215;10<SUP>-4</SUP> keV<SUP>-1</SUP>h<SUP>-1</SUP>, yielding a lower limit of greater or equal 7.5&#215;10<SUP>19</SUP> yr on the mean life against the charge-nonconserving decay of a nucleon. This result sets an upper limit on the ratio of the charge-nonconserving to the normal weak-interaction decay width of a nucleon, &#915;(n-->p+v<SUB>e</SUB>+v<SUP>-</SUP> <SUB>e</SUB>)/&#915;(n-->p+e<SUP>-</SUP>+v<SUB>-</SUB> <SUB>e</SUB>)&#60;=7.9&#215;10<SUP>-21