We can use the de Broglie wavelength formula:
$$\lambda = \frac{h}{p} = \frac{h}{mv}$$
where $\lambda$ is the wavelength, $h$ is Planck's constant ($6.626 \times 10^{-34} \text{ J s}$), $m$ is the mass of the neutron, and $v$ is its velocity.
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