particles. The reason for this is that alpha particles might never escape 

 the labelled simulated sediment due to self -absorption or the particle 

 would be degraded by interaction with surrounding matter very rapidly in rela- 

 tively minute distances, consequently, an alpha particle would not be detected. 

 While a beta particle does have a longer useful path than alpha particles, 

 beta particles are also rapidly attenuated. Further beta particles display a 

 spectrum of energies ranging from the high of any given radio element downward. 

 Gamma photons, on the other hand, have constant energy upon emission and are 

 not attenuated nearly as rapidly as either alpha or beta particles. As a 

 consequence gamma photons are detectable even when the labelled particle is 

 covered with sand and/or water. That is to say between the labelled particle 

 and the detector there are some sand grains and water. With respect to the 

 energy of the photon, it is advantageous that the gamma photons have energies 

 of 1 Mev or more, in order to facilitate transmission and reduce attenuation 

 to a minimum. Another advantage for seeking a radioisotope which emits a 

 gamma photon of high energy is that during detection (when using a single channel 

 pulse-eight analyzer in the system) the natural background in the 1 Mev or 

 greater range is much lower than is found with lower energy photons. 



The normal 50-hour test described earlier requires approximately 3 weeks of 

 operational time. This fact has, of course, a very strong influence on the 

 choice of radioisotope. For safety reasons it is desirable that no more than 

 approximately 50 millicuries of radioisotope be used for any test. These two 

 parameters (50 millicuries of activity and 3 weeks of test time) strongly suggest 

 an isotope with a half life of approximately 60 hours. If such an isotope 

 is chosen and the labelled simulated sediment placed upon the beach and bottom 

 sediments at 8 A. M. on Monday and at this time the activity is 50 millicuries, 

 then 3 working weeks or some 456 calendar hours later, the remaining activity 

 is approximately 0.25 Millicurie and in one more calendar week the remaining 

 activity is approximately 35 microcuries. Such an arrangement is desirable 

 in that at the end of 3 weeks the remaining activity will be on the order of 

 the minimum detectable amount and in one more week another test could be started 

 without serious interference from the activity of the prior test. If the test 

 were 2 weeks and 50 millicuries of activity were used then the suggested half 

 life becomes approximately 40 hours. Thus for a 2-week period or 288 calendar 

 hours the activity remaining the last day would be approximately 0.35 millicurie 

 and one week later the remaining activity would be approximately 20 microcuries. 

 It can be readily seen, therefore, that half life is of critical importance in 

 the choice of the radioisotope. 



The chemistry of the element chosen must permit the element to be homogen- 

 eously dispersed throughout the glass without any reaction with the glass. Fur- 

 ther it is necessary that once included in the glass the material will not be 

 leached out of the glass by the surrounding water. 



A final criterion in any test of this sort is the economics of the test 

 for it is common knowledge that desirable tests are sometimes delayed by un- 

 favorable economics. The costs of those parts of the test which are discussed 

 in this paper are most reasonable. The special density glass may be prepared, 

 ground, rounded and analyzed by spectrographic means for approximately $250. 

 The irradiation of the glass and activation of the tracer element is a service 

 irradiation which costs on the order of $100. Miscellaneous expenses such as 



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