Section II. ISOTOPES SELECTION, TAGGING TECHNIQUES AND INSTRUMENTS 



1. Studies of Isotopes and Tagging 



Three criteria - health physics, gamma energy sufficient to be de- 

 tected, and the engineering behavior of tagged sand - were emphasized in 

 the search for isotopes and tagging techniques suitable for sand tracing. 

 Consideration of health physics made low energy radiation, and isotopes 

 that are chemically inert and biologically inactive, desirable. The 

 radioisotope must have a sufficiently high energy gamma ray to permit 

 easy measurement through an overburden of water and untagged sand. 

 Consideration of the engineering behavior of tagged sand called for a 

 new labeling process , one that definitely did not alter the "hydraulic 

 characteristics" of the sand and yet ensured no loss of the radionuclide 

 to the environment. To gain true scientific results from a sediment 

 tracing experiment in a fluid environment, it is imperative that the 

 tagged particle of sand have the same "hydraulic behavior" after tagging 

 as before. It is also necessary to be sure that detected radiation be- 

 longs to the tagged particle and does not represent a radioactive veneer 

 broken off from the particle. 



A solution to shortcomings of labeling procedures presently in use 

 and described by other studies would be to develop a technique that uses 

 sand indigenous to a particular area, uses no external coating, and does 

 not alter grain hydraulic characteristics. Diffusion of gas into a solid 

 offers this solution. Krypton, in a process described by Chleck (19&3) 

 can be diffused at high temperature and pressure into solid materials. 

 However, krypton's long ( 10-year) half life made it generally unsuitable, 

 except perhaps for special applications in tracing studies. Seeking other 

 possible gases, personnel at Oak Ridge National Laboratory (ORNL) developed 

 a process (xenonation) similar to kryptonation whereby a related gas, 

 xenon-133 , is diffused into a solid, in this case, a sand particle. 



The study indicated xenon had numerous suitable characteristics. 

 In terms' of radiation exposure to personnel, xenon is easy to handle. 

 Also, it is biologically inactive. From an engineering standpoint, the 

 suitable half life (5.27 days) and the labeling process, which required 

 no external coating to natural sand, overrode the initial detection and 

 instrumentation problems posed by the low energy (.08 million electron 

 volts) gamma radiation. Consequently, xenon was selected for exclusive 

 use in the initial phases of the program. 



Tests conducted in the CERC Petrology Laboratory indicated xenonation 

 did not alter the specific gravity of sand, nor does it alter grain hy- 

 draulic characteristics. By measuring the time required for sand grains 

 to settle through a column of water, it is possible to test their "hydrau- 

 lic characteristics", i'.e. ,. determine the sediment size distributional 

 characteristics in the fluid medium. Replicate analyses of xenonated 

 and non-xenonated sand from several different locations show only minor 

 differences in the settling curves and consequent statistical parameters. 



