= Solution
The <mass dependence of fixed-barrier tunnelling> follows by holding <energy> and the potential-energy barrier fixed: <quantum tunnelling> has the leading <WKB approximation>
$$
T\propto\exp\!\left[-\frac2\hbar\int_{x_1}^{x_2}\sqrt{2m(V-E)}\,dx\right].
$$
The alpha-particle <mass> is about $7.3\times10^3$ <electron> <masses>, so its decay exponent is about \b[85 times larger] for the same $V-E$ profile. Exponential suppression therefore makes transmission much smaller. This comparison holds the <potential energy> and incident <energy> fixed: the same electrostatic potential is not the same potential-energy barrier for charges of different magnitude and sign.
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