Evidence for quasifission in the sub-barrier reaction of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Si</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>30</mml:mn></mml:mrow></mml:mmultiscripts></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">U</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>238</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>

Fragment mass distributions for fission after full momentum transfer were measured for the $^{30}\mathrm{Si}$$+$$^{238}\mathrm{U}$ reaction at bombarding energies around the Coulomb barrier. At energies above the Bass barrier, the mass distributions were Gaussian with mass symmetry. An asymmetric fission channel with mass ${A}_{\mathrm{L}}/{A}_{\mathrm{H}}$ $\ensuremath{\approx}$ $90/178$ emerged at the sub-barrier energies, where competition between fusion and quasifission was suggested from the evaporation residue (ER) cross section produced in the fusion $^{30}\mathrm{Si}$$+$$^{238}\mathrm{U}$. We thus conclude the asymmetric channel is attributed to quasifission. It was supported by a model calculation using the Langevin equation to give the fragment mass distribution, where fusion-fission and quasifission were separated. The observed mass asymmetry for quasifission as well as the calculation is significantly smaller than those observed in actinide targets bombarded with heavier projectiles, which suggests that the system $^{30}\mathrm{Si}$$+$$^{238}\mathrm{U}$ approaches the shape of the compound nucleus before disintegrating as quasifission.

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