De Broglie wavelength λ associated with neutrons is related with absolute temperature T as:

De broglie wavelength λ=h/mv where v is the velocity of the particle. Let E be the kinetic energy of the neutrons. Thus E=1/2mv² ⇒v=√(2E/m) Therefore λ=h/√(2mE) Since energy of neutrons E∝T, λ∝1/√T