Small angle neutron scattering from D2O in the critical region

Small angle neutron scattering experiments were carried out on D2O on the c ritical isochore. Apparatus was used which enabled the control of temperatu re to +/-0.01 K and pressure to 10 mbar. The corrected scattering data were used to extract values for the correlation length of the critical density fluctuations. xi, and the thermodynamic limit of the structure factor, S(0) . Both quantities are found to show divergent power-law behaviour with redu ced temperature, and have exponents nu = 0.62 +/- 0.03 and gamma = 1.14 +/- 0.05, respectively. These results confirm that D2O belongs to the three-di mensional Ising universality class. A value of xi(0) = 1.30 +/- 0.23 Angstrom was calculated for the characteri stic correlation length of D2O, a result that compares favourably with that inferred from thermodynamic data and calculated theoretically.