Activation cross section

Rare earth elements, with relatively high thermal neutron activation cross-sections, have been tested or considered as tagging species for this purpose. At GA (Ref 8), preliminary expts were conducted with 0.38 cal ammo using dysprosium (Dy) and europium (Eu) deposited on the wall of the cartridge case and in the gunpowder, and Dy, hoKnium (Ho) and indium (In) in the primer. 

Sieve trays have holes 0.25-0.50 in. dia, hole area being 10% of the active cross section. 

Valve trays have holes 1.5 in. dia each provided with a liftable cap, 12-14 caps/sqft of active cross section. Valve trays usually are cheaper than sieve trays. 

The natural abundance of 64Z is 48.88% and its activation cross-section to thermal neutrons is 0.50 b. The half-life of 65Zn is 245 d. 

Here d = 2r, where r is the radius of the colliding particles. For two different particles in a gas mixture d = rx + r2. d is sometimes called the collision radius (RM) to denote an active sphere of radius R and active cross-section nR2. ... 

If the plastic foam failure in fact occurs in this manner, it may be assumed that each value of strain (e,) corresponds to some actual cross-sectional area S,(e,) which is different from the original area So because some rods have been destroyed by the force P. Since the active cross-section which experiences the load is proportional to the number of unfailed joints, then ... 

A liquid level is maintained with an overflow weir while the vapor comes up through the perforated floor at sufficient velocity to keep most of the liquid from weeping through. Hole sizes may range from 1/8 to 1 in., but are mostly l/4-l/2in. Hole area as a percentage of the active cross section is 5-15%, commonly 10%. The precise choice of these measurements is based on considerations of pressure drop, entrainment, weeping, and mass transfer efficiency. The range of conditions over which tray operation is... 

The rate of formation is given by fvN, where / is the activating flux in particles per square centimeter per second, square centimeters for the reaction, and N the number of nuclei of the isotope involved. The rate of decay is XN, k being the decay constant of the radioactive species of which N atoms are present. The over-all rate of growth is then given by ... 

Equation 5 can be simplified by replacing the number of atoms N by W /M, where W is the weight of the element present, is the abundance of the isotope activated, and M the chemical atomic weight. If o-, the activation cross section, is expressed in barns (1 bam is equal to 10 square centimeters) it reduces to... 

With elements of short half-life, the sensitivity of the method, besides being dependent on the flux and activation cross section, will also depend on the time which must elapse between the termination of activation and the measurement of the activity. With short-lived nuclides this period frequently determines the over-all sensitivity of the method. 

The N, which constitutes 99.6 percent of natural nitrogen, is present as residual nitrogen impurity in oxide fuel of water reactors and fast-breeder reactors, as air dissolved in the coolant of water-cooled reactors, and as residual nitrogen in the graphite of HTGRs. The activation cross section for 2200 m/s neutrons is 1.85 b. 

Natural chlorine contains 75.77 percent C1, for which the 2200 m/s activation cross section is 43 b. Assuming 3 ppm chlorine in the fabricated HTGR fuel [HI], the estimated yearly production of C1 from a 1000-MWe reactor is 1.02 Ci. 

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