The majority of the clocks involved are caesium clocks; the International System of Units (SI) definition of the second is based on caesium.
ðə mədʒˈɔːɹᵻɾi ʌvðə klˈɑːks ɪnvˈɑːlvd ɑːɹ sˈiːsiəm klˈɑːks; ðɪ ˌɪntɚnˈæʃənəl sˈɪstəm ʌv jˈuːnɪts sˈiː dˌɛfɪnˈɪʃən ʌvðə sˈɛkənd ɪz bˈeɪst ˌɔn sˈiːsiəm.