  We propose a new method of alpha($\alpha$)-ray measurement that detects
helium atoms with a Quadrupole Mass Spectrometer(QMS). A demonstration is
undertaken with a plastic-covered $^{241}$Am $\alpha$-emitting source to detect
$\alpha$-rays stopped in the capsule. We successfully detect helium atoms that
diffuse out of the capsule by accumulating them for one to 20 hours in a closed
chamber. The detected amount is found to be proportional to the accumulation
time. Our method is applicable to probe $\alpha$-emitting radioactivity in bulk
material.
