Looking for spectral changes occurring during storage of ultra-cold neutrons

It seems that the spectrum of ultra-cold neutrons does change. The measured data indicate with 5σ reliability, that a small heating by about 2·10-10 eV (∼ 2 mm of rise height against the earth's gravity) occurred during the initial ∼ 103 wall reflections, and no change thereafter. The reason of this effect is searched for. (author)