Fig. 7-7 This figure shows distribution data taken by measuring a sample containing about 1.53 g of plutonium with a passive neutron apparatus for 1 hour. The histograms in the figure are the experimental data. The two straight lines represent two components obtained by separating the measured data into two exponential functions. The straight line with the lower slope is called the "random component" and that with the higher slope is called the "time-correlated component." Software developed by a group in JAERI calculates the theoretical distribution by giving the spontaneous fission rate and the (alpha, n) neutron emission rate. A spontaneous fission rate of 205 s-1 as 240Pu and an (alpha, n) neutron emission rate of 1,000 s-1 result in an almost identical distribution of the experimental distribution shown in the figure. Accordingly, it is known that the sample measured has a spontaneous fission rate and an (alpha, n) neutron emission rate nearly equal to the values listed above.

The reprocessing of spent fuels produces high alpha-activity waste, such as americium, curium, some isotopes of plutonium, etc. Although the process is designed to minimize the volume of both high-level and low-level waste, it is important to precisely measure the alpha-activity of both the solid and liquid waste. Indirect measuring techniques that use neutrons having high transmissive power are ordinarily adopted as auxiliary tools to evaluate the alpha-activity of solid waste because most alpha-rays originating in this waste do notpenetrate to the surface of waste. One of these techniques is the "passive neutron method." Alpha-active isotopes emit neutrons through nuclear reactions such as the 18O(alpha, n) reaction when the isotope with the alpha-activity is in a chemical form like an oxide. However, some alpha-active nuclides, such as 244Cm and 240Pu, emit neutrons through spontaneous fission. In analyzing the detected neutron data, it is important to know how many counts come from (alpha, n) reactions and how many come from spontaneous fissions. The characteristics of neutrons emitted independently in (alpha, n) reactions and the neutrons emitted in spontaneous fissions are used to determine these quantities. A group in JAERI has constructed a new apparatus that measures all time intervals between all successive neutron pulses detected. The group has been conducting research using the apparatus. Software is being developed to quickly derive the alpha-activity and the amount of spontaneously fissioning materials in the waste from measured data.

Reference H. Gotoh et al., An Analytical Expression for the Distribution of Time Intervals Between Succesive Detection Pulses of Neutrons from Spontaneous Fissions, JAERI-M 91-138, P.130.

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