  The decay of the primordial isotopes $^{238}\mathrm{U}$, $^{235}\mathrm{U}$,
$^{232}\mathrm{Th}$, and $^{40}\mathrm{K}$ have contributed to the terrestrial
heat budget throughout the Earth's history. Hence the individual abundance of
those isotopes are key parameters in reconstructing contemporary Earth model.
The geoneutrinos produced by the radioactive decays of uranium and thorium have
been observed with the Kamioka Liquid-Scintillator Antineutrino Detector
(KamLAND). Those measurements have been improved with more than 18-year
observation time, and improvements in detector background levels mainly by an
8-year almost rector-free period now permit spectroscopy with geoneutrinos. Our
results yield the first constraint on both uranium and thorium heat
contributions. Herein the KamLAND result is consistent with geochemical
estimations based on elemental abundances of chondritic meteorites and mantle
peridotites. The High-Q model is disfavored at 99.76% C.L. and a fully
radiogenic model is excluded at 5.2$\sigma$ assuming a homogeneous heat
producing element distribution in the mantle.
