A sample containing $_{88}^{224} \mathrm{Ra},$ which decays by $\alpha$ -particle emission, disintegrates at the following rate, expressed as disintegrations per minute or counts per minute $(\mathrm{cpm}): t=0,1000 \mathrm{cpm} ; t=1 \mathrm{h}$
$992 \mathrm{cpm} ; t=10 \mathrm{h}, 924 \mathrm{cpm} ; t=100 \mathrm{h}, 452 \mathrm{cpm}$ $t=250 \mathrm{h}, 138 \mathrm{cpm} .$ What is the half-life of this nuclide?