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The disadvantage factor for thermal neutrons in light-water moderated PuO2-UO2 and UO2 square lattices were obtained from measurements of thermal neutron density distributions in a unit lattice cell, measured with Dy-Al wire detectors. The lattices consisted of 3.4w/o PuO2-UO2 and 2.6w/o UO2 fuel rods, and the water-to-fuel volume ratio within the unit cell was parametrically changed. The PuO2-UO2 and UO2 fuel rods were designed to realize equal fissile atomic number density. The disadvantage factors thus measured were 1.36±0.07, 1. 37±0.08, 1.40±0.06 and 1.38±0.06 in the PuO2-UO2 fuel lattices, and 1.30±0.06, 1.31±0.08, 1.30±0.08 and 1.33±0.06 in the UO2, for water-to-fuel volume ratios, of 1.76, 2.00, 2.38 and 2.95, respectively. This difference in disadvantage factor between PuO2-UO2 and UO2 fuel lattices corresponds to about 8%. Calculated results obtained by multigroup transport code LASER agreed well with the measured ones.

For the study of the low-energy neutron spectrum by the integral method, it is important to develop sensitive neutron temperature detectors. The non-l/ nu property of the effective cross sections of Pu²³⁹ or Lu¹⁷⁶ has been used for this purpose. If the non-l/ nu property of the neutron detector is varied artificially, a very useful technique for the study would be obtained. Gadolinium has a large absorption cross section in the low energy region; the cross section decreases rapidly with increase of neutron energy. Thus, the neutrons filtered through a Gd foil will be devoid of the low energy part in the spectrum. It would be sharply cut off at the energy of 0.1 to 0.5 ev. With this sharp cut-off filter, the shift of Maxwellian distribution of thermal neutrons would be detected very precisely. Two BF₃ counters were located at the same geometries. One of them was bare and the other was covered with the Gd filter. The ratio of their count rates will be defined as the Gd ratio and is very sensitive to the neutron temperature. The neutron temperatures were varied by re-thermalization in a water tank at different temperatures. The pure Maxwellian thermal neutrons were introducedmore » into the tank from the thermal column of the JRR-l reactor. The temperature of the tank was varied very slowly between the range of 15 to 85 deg C. The resolving power of the neutron temperature detector was found to be 2 deg C. (auth)« less