(*data1= Import["Documents/Barium137.csv","csv"];*)

data1= Import["http://physicsx.pr.erau.edu/Courses/CoursesS2019/PS315/NonLinearFit/radio.csv","csv"];

data2=Table[{data1[[i,1]],data1[[i,3]]},{i,24,632}];

wts=Table[1./data2[[ii,2]],{ii,1,Length[data2]}];

nlm=NonlinearModelFit[data2,a+((Log[2])/t12) N0 (E^(-((Log[2])/t12) t)),{a,{N0,800000.},t12},t,Weights->wts]

Normal[nlm]

Show[ListPlot[data2],Plot[nlm[t],{t,0,Length[data2]}],Frame->True]

nlm["ParameterErrors"]

nlm["BestFitParameters"]

nlm["CovarianceMatrix"]//TableForm