Work in Greece on delayed neutrons

At present, two programmes on delayed neutron research are in progress in Greece. DELAYED NEUTRON ENERGY SPECTRUM MEASUREMENTS. In an experiment at the 1 MW Democritus reactor, the energy of the delayed neutrons from 235U fission is measured by the time-of-flight technique and β-n-coincidence using a special fast system for irradiation of the sample. The beta particle preceding the emission of the delayed neutron is used as a start pulse while the delayed neutron provides a stop signal. A maximum flight time of approximately 100 nsec is allowed over a flight distance of approximately 30 cm (longer flight paths could be used for better energy resolution, but this would decrease the efficiency of detecting the delayed neutron). To avoid false measurements due to other beta particles which do not correspond to delayed neutrons, an anti-coincidence system is used to cancel the measurement whenever two beta particles come in a time interval shorter than 100 nsec. Coincidences due to delayed neutrons are relatively rare in comparison with chance coincidences or β-γ-coincidences. The spectrum obtained in a first run of measurements is discussed. An attempt will be made to measure the spectrum of each individual group by varying the irradiation time and the measuring time. Various irradiation, transfer and measuring times can be obtained with the existing experimental set-up. A minimum transfer time of 300 msec can be achieved. Very thin foils (a few mg/cm2) are used to minimize the beta-particle absorption in the sample. DELAYED NEUTRON GROUP RELATIVE ABUNDANCES AND PERIODS. In a second experiment, measurements to determine the relative abundances and periods of the delayed neutrons for various fissile materials are now under way. The arrangement for delayed neutron group measurement allows the sample to be transferred in a minimum time of approximately 800 msec. Accordingly, delayed neutrons of quite short half-lives can be detected. The experimental facility permits of very accurate repetition of each cycle of measurement which results in good statistical accuracy. In earlier work, measurements of delayed neutron precursor relative abundance for 235U fission by thermal neutrons have been performed using the radiochemically known half-lives of iodine and bromine. (author)