▁ 1 5 5 G d , ▁ 1 5 6 G d , ▁ 1 5 7 G d , ▁ 1 5 8 G d ▁and ▁ 1 6 0 G d , ▁and ▁ 1 ▁radio is ot ope , ▁ 1 5 2 G d , ▁with ▁ 1 5 8 G d ▁being ▁the ▁most ▁abund ant ▁( 2 4 . 8 4 % ▁natural ▁abund ance ). ▁The ▁predicted ▁double ▁beta ▁decay ▁of ▁ 1 6 0 G d ▁has ▁never ▁been ▁observed ; ▁only ▁a ▁lower ▁limit ▁on ▁its ▁half - life ▁of ▁more ▁than ▁ 1 . 3 × 1 0 2 1 ▁years ▁has ▁been ▁set ▁experiment ally . ▁ ▁Th irty ▁radio is ot op es ▁have ▁been ▁character ized , ▁with ▁the ▁most ▁stable ▁being ▁alpha - dec ay ing ▁ 1 5 2 G d ▁( n atur ally ▁occurr ing ) ▁with ▁a ▁half - life ▁of ▁ 1 . 0 8 × 1 0 1 4 ▁years , ▁and ▁ 1 5 0 G d ▁with ▁a ▁half - life ▁of ▁ 1 . 7 9 × 1 0 6 ▁years . ▁All ▁of ▁the ▁remaining ▁radio active ▁is ot op es ▁have ▁half - l ives ▁less ▁than ▁ 7 4 . 7 ▁years . ▁The ▁majority ▁of ▁these ▁have ▁half - l ives ▁less ▁than ▁ 2 4 . 6 ▁seconds . ▁G ad olin ium ▁is ot op es ▁have ▁ 1 0 ▁met ast able