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ULTRAFAST CHEMICAL SEPARATIONS
passed through the target chamber containing 235U located in a beam hole in the reactor. The halides investigated (KF, KCl, KBr, NaCl, and NaBr) showed a plateau for yield as a function of furnace temperature; the plateau was about 80°C wide and occurred 100 to 150°C below the respective melting points. Their results showed that the alkali halides produced aerosols of optimum size over a narrow temperature range.
Mazumdar and coworkers [Maz81 ] determined, efficiencies for ZnBr 2, PbCl2, MgCl2, CdBr2, and PbF2 for transporting fission products. All the materials showed narrow peaks for efficiency as a function of oven temperature. All materials except PbF2 showed an efficiency of nearly 60%; PbF2 showed lower efficiency. Other materials used for aerosol were AgCl, silver, and tellurium (all of which showed 60% yield) and cadmium, with a 40% yield [And81 ]. Thus, it is evident that a variety of inorganic materials can be used as efficient carriers for reaction products.
The selection of aerosols for a gas jet depends on the nature of the work to be performed with the collected reaction products. Ethylene left a sizeable deposit. The deposit left by n-decane was nearly one-tenth of that observed from ethylene [Sch77b]. If alpha or fission-fragment activity is to be determined, then of course an aerosol leaving minimum deposit should be used. If chemical separations are to be performed on the products transported by gas jet, the nature of chemical separation dictates the proper aerosol. For aqueous chemistry or gas-phase synthesis involving high temperature, the inorganic aerosols (especially alkali halides) offer several advantages. They provide complete dissolution in aqueous solutions and also will allow high-temperature (~900° C), gas-phase chemistry. Silva and coworkers showed that organic clusters resulted in incomplete dissolution [Sil77].
5.2 Transport Time
Gas-jet transport systems are used in particle accelerators and nuclear reactors for transport and study of short-lived radionuclides. It is essential to know the transport-time characteristics of the system and the influence of different parameters on transport time. Terms like transport time, transit time, and sweepout time have been used in the literature with different meanings. In this paper, we will use the following definitions:
“Transport time” will be used to refer to the total time taken by the product generated to reach the detector position.
“Sweepout time” is the time taken by the recoiling product to appear outside the target chamber.
“Transit time” is the time taken by the product to travel through the capillary tube from the outlet of the target chamber to the detector position.
Several authors have calculated the transit time using gas dynamics. Details of calculations will not be discussed here; the reader is referred to the articles by Zirnheld [Zir74a] and Weiffenbach and coworkers [Wei75]. A number of different techniques have been used for measuring the transport time. Dautet and coworkers [Dau73] measured the transport time as a function of flow rate of ethylene by injecting a pulse of helium into the carrier gas and using a helium-leak detector to signal the arrival of helium at the collection chamber. They observed a smooth decrease in transport time with increasing flow rate of the carrier gas.
The most common technique for the measurement of transport time utilizes the pulsing capability of accelerators and reactors (if available). For example, Weiffenbach and coworkers [Wei75] measured the transport time by recording the total activity observed at the collector in multiscaling mode; the multiscaling was initiated by the pulse which turned on the cyclotron beam. The counting was repeated several times, each with a new collector surface, to obtain good statistics. The results of a typical measurement are shown in Fig. 22. They found a decrease in transport time with increase in target-chamber pressure. Mazumdar and coworkers [Maz80] used the reactor in the pulsed mode to determine the transport time in the gas-jet system coupled to an