A particle having electric charge $3 \times 10^{-19} \mathrm{C}$ and mass $6 \times 10^{-27} \mathrm{~kg}$ is accelerated by applying an electric potential of 1.21 V . Wavelength of the matter wave associated with the particle is $\alpha \times 10^{-12} \mathrm{~m}$. The value of $\alpha$ is $\_\_\_\_$ . (Take Planck's constant $=6.6 \times 10^{-34} \mathrm{~J} . \mathrm{s}$ )