The stopped-flow technique [Ren85b] is of general applicability for the measurement of transport time. In this technique, the time is determined by controlling the gas flow and measuring the fission-product activity collected at a quartz-wool trap. A schematic of the experimental setup is shown in Fig. 25. The gas mixture (nitrogen and ethylene) passed through an electrically operated, three-way valve to the target chamber. The valve was connected in such a way that when power was applied, gas would flow to the target chamber. The outlet from the target chamber was connected to a two-way, electrically operated valve. The fission products carried by ethylene clusters passed through the valve and were retained by a quartz-wool trap in front of a detector. The gas from the filter passed through another two-way valve to pump and exhaust. The activity of the trap was recorded by a multichannel analyzer in the multiscaling mode. The power for all three valves was applied by the operation of a single switch; the switch also initiated the multiscaling. The valves were opened while the reactor was operating steadily; the gas flow adjusted to the desired value, and the system was allowed to reach equilibrium. The valves were closed, the quartz-wool trap was replaced, and flow was initiated by opening the three valves simultaneously, initiating multiscaling. The valves were closed when the multiscaling ended. The cycle was repeated to improve counting statistics. The transport time obtained using this technique was compared with that obtained using the pulsing technique. Figure 26 shows the results obtained using both techniques. The values obtained are in good agreement with each other. A slight modification of this technique made use of a dual multiscaler based on a microcomputer for timing measurements [Gri85].

Figure 25. Schematic of the experimental setup for transport time measurements. [Ren85b; reprinted with permission from Nucl. Instrum. Methods A]

5.3 Examples of Gas-Jet Systems

A large number of gas-jet systems are used in conjunction with on-line isotope separators. There are only a few systems coupled to fast chemical separation systems. In the following paragraphs, selected examples of systems are described.

A helium-jet transport system coupled to an on-line isotope separator has been set up at the Research Reactor Institute of Kyoto University [Oka81]. Figure 27 shows a schematic of the system. The target was 93% enriched 235U electrodeposited as uranium oxide on aluminum; the weight and thickness of the target were 9.0 mg and 0.5 mg cm−2, respectively. The target was covered with approximately 2 mg cm −2 aluminum foil. The system was set up in the through tube of the reactor; the target chamber can be moved along the tube and thus can be in a neutron flux ranging from 1010 to 3 × 1012 n cm−2 s−1. The aerosol generator used dioctyl phosphate or pump oil. The output of the target chamber was fed to the ion source.



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