stream, or to recoil into the gas, which thermalizes and carries them. Nonvolatile products are thermalized in a carrier gas and transported. Helium is generally used as a carrier gas, and polyethylene or Teflon capillary tubes are used for carrying the gas. Tubes as long as 200 m have been used successfully to transport reaction products. MacFarlane and McHarris [Mac74a] have reviewed in detail the use of gas-jet systems in the study of short-lived nuclides.

Perhaps the earliest application of a carrier gas to transport radioactivity was done in 1900 by Rutherford [Rut00] in his experiments on emanation from thorium compounds. A thick layer of thorium oxide enclosed in paper was placed in a long metal tube. The paper was thick enough, according to Rutherford, “to cut off the regular radiation almost entirely, but allowed emanation to pass through.” The metal tube was attached to a large, insulated cylindrical vessel with an electrode connected to an electrometer. By passing a stream of air, he showed that emanation collected in the cylindrical vessel and ionized the gas. By stopping the air flow, he was able to follow the decay of emanation collected in the vessel.

Transport of a nonvolatile product by gas was used by Ghiorso and coworkers [Ghi58] in their attempt to identify nobelium. A curium target containing 244Cm and 246Cm was bombarded with monoenergetic 12C ions. The recoiling product atoms were thermalized in helium gas; the resulting positively charged atoms were transported to a moving, negatively charged metallic belt. The products deposited on the belt were used for the identification of nobelium. The use of gas to transport nuclear reaction products produced in accelerators increased in the following decades.

In the 1960s and early 1970s, a number of research groups experimented with helium gas-jet systems for thermalizing and carrying reaction products from targets in accelerators. The transport efficiency obtained was low and not reproducible; hence, operating a gas-jet system and obtaining good transport efficiency seemed to be considered more “black magic” than science.

Junglas and coworkers [Jun71] studied the thermalization and transport of 8Li produced by deuteron bombardment of 7Li by 99.999% pure helium. They found that 8Li was attached to large molecular clusters in the helium with mass in the range of 106 to 108. The clusters were found to consist of 59% neutral, 25% positively charged, and 16% negatively charged components. They concluded that the intense ionization of the deuteron beam probably produced charged species; impurity molecules present in helium (e.g., H2O) could build clusters and thermalized reaction products could attach to clusters.

5.1.1 Role of Aerosols

Transport efficiency was uniformly low when reaction products were produced in a surrounding where intense ionization was not produced. For example, Aystö and Valli [Ays73] showed, using recoiling decay products from a 227Ac source, that pure commercial-grade helium transports less than 1% of the products over distances longer than 1 m. At distances shorter than 20 cm, transport efficiencies from 13 to 36% were obtained, depending on other conditions (e.g., target chamber pressure, diameter of the capillary). Cooling helium to liquid-air temperature increased transport efficiency to about 18%, even for a distance of 20 m [Ays74a], while addition of oil vapor to helium increased the efficiency to 75% [Ays74b]. Wien and coworkers [Wie72] found that commercial helium gas carried less than 1% of the thermalized fission fragments produced by spontaneous fission of 252Cf. They found that seeding helium with water vapor and irradiating it with ultraviolet radiation before its introduction into the 252Cf chamber increased the transport efficiency to nearly 55%.

Dautet and coworkers [Dau73] also found that when pure helium or nitrogen was used as a carrier in a gas-jet system to transport fission fragments produced in 14-MeV neutron irradiation of natural uranium, the yield was quite low. Addition of ethylene was found to increase efficiency to nearly 70%. Wilhelm and coworkers [Wil74] studied the effect of different clusters on the transport efficiency of 252Cf fission fragments and 232Th decay products. They mixed the vapor of



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