Why the mixture should not be strongly heated during sublimation

because strong heated makes temperatures and pressures reach the substance's triple point in its phase diagram, So the substance can be in a liquid case , and that is against the sublimation process meaning

which define as the transition of a matter or a substance  from the solid phase to the gas phase directly, without passing through the intermediate liquid phase

sublimation process happens by absorbing of heat which makes molecules overcome attractive force between each other and changing to gas phase

but in the previous case it will pass through the liquid phase so it will not be sublimation process any more

the answer for sodium is 0.186

the answer for rubidium is 0.251

particle   p is an electron and particle q is a neutron