Temperature Dependence of the Probability of "small Heating" and Total Losses of UCNs on the Surface of Fomblin Oils of Different Molecular Mass

We measured the temperature dependence of the probability of small heating and total losses of UCNs on the PFPE Fomblin Y surface with various molecular mass $$M_{{\bar{W}}}$$ (2800, 3300, 6500 amu) in the temperature range of 100–300 K. The probability of small heating sharply decreases with increasing $$M_{{\bar{W}}}$$ and decreasing temperature. The probability of total loss weakly decreases with decreasing temperature and takes the minimum value at $$M_{{\bar{W}}} =3300 \, \hbox {amu}$$ . As this oil provides a homogeneous surface with minimal probabilities of small heating and total losses of UCNs, it is the preferred candidate for experiments on measuring the neutron lifetime.