Epithermal yields

Work on mercury alkyls has been done by Heitz and Adloff (31-33), who studied Hg(CH3)2, Hg(C2Hj)2 and HgPh2. They found no isotope effect between " Hg, Hg, and ° Hg, and no correlation with the respective conversion coefficients. They also noted that the retentions could not be satisfactorily explained by exchange of the respective ligands, and thus concluded that the molecules are reformed by an epithermal not by a thermal process. Parent yields were typically 74, 15, and 8% for the diphenyl-, dimethyl- and diethylmercury, respectively. 

The several polymeric metal carbonyls studied have led to some surprisingly high yields [e.g., Fe3(CO),2 and Ruj(CO)j2 in Table IV] but to no substantiated mechanisms. The 17% yield of Fe3(CO),2 in neutron-irradiated Fe(CO)j was interpreted as a reaction of Fe(CO)4 with the Fe(CO)5, but no further evidence is available. The study of Mn2(CO),o has been fruitful (44, 46). The insensitivity of the parent yield MnMn(CO),o to heat indicates that the molecule is formed by a reaction quite early in the sequence, perhaps epithermal. The discovery (46) of a species which reacts rapidly with I2 and exchanges with IMn(CO)5 led to the conclusion that the Mn(CO)5 radical is produced prominently (4.5%) by nuclear reactions in the solid decacarbonyl. The availability of this labeled Mn(CO)5 has made possible several interesting observations about the exchange properties of this radical in the solid (45) and in solution (42). 

In a later report, Schmidt and Allen (1970) extended their measurement to 38 pure liquids and mixtures at room temperature and to 5 liquids as a function of temperature. The free-ion yields are arranged by the alkanes and their isomeric and cyclic counterparts, which show considerable differences in the results. Thus, the free-ion yield in neopentane (NP) is about seven times that in n-pentane. Some of the results are shown in Table 9.1. In mixtures of NP with CC1, or CS, the observed decrease of Gf with the additive concentration has been interpreted by Mozumder and Tachiya (1975) as due to epithermal electron scavenging (vide infra). 

The Williams Brook property has yielded high gold assays. Preliminary work presents evidence suggesting that Au mineralization occurs in two forms 1) as refractory Au in mm scale massive sulfide (dominantly pyrite) and disseminated veins that cut potassically altered rocks, and 2) as vuggy quartz veins as finegrained (< mm) free gold. Trace element correlations indicate that Au may be related to epithermal mineralization as there are good correlations of Au with Ag,... 

Here Mu is assumed to be formed as a result of combination of and an excess electron. This view is the same as for the spur model of positronium (Ps) formation. While the spur model has received strong support for positronium yield in condensed phases, the validity of the same model for Mu formation is not clear. Figure 11 presents the original form of the spur model of Mu formation, since it helps to contrast the difference between the epithermal model (Fig. 2) and the spur model of Mu formation. Alternatively, the part of Mu formation, i.e., p and excess electron combination, in Fig. 11 may be replaced with the picture of Mu... 

In contrast, In/Al alloy wire can be manufactured relatively simply employing enriched In and the wire can be activated to contain the desired level of radioactivity in a moderate-size nuclear reactor within a rather short irradiation time. The thermal neutron cross section and epithermal resonance integral for In(n,y) In are 8.1 b and 220 b, respectively. The expected yield of "In for 1 week of irradiation at a neutron flux of 1 x 10 n cm s is 6.7 GBq mg ( 180 mCi mg ) of In. A typical seed wire, 2 cm in length, 0.03 cm in diameter, containing --- 0.12 mg of In (2% In), would contain 0.74 GBq ( 20 mCi) of In. Experimental results are summarized in O Table 38.5. As seen, theoretical calculations are within a factor of 2 of experimental results. 

Calculations performed assuming the same contents in all tanks indicate that the reactivity of the tank next to the concrete reflector exceeds that of the other tanks. This is due to enhanced reflection of epithermal neutrons by the concrete. Solutions of U, which are more reactive in slab geometry than solutions of Pu or U, will only be placed into the new tanks located away from the concrete wall. (This prevents contamination of other tanks by radioactive decay daughters.) Calculations assuming that the two tanks closest to the wall are filled with Pu(N03)4 and that the two outer tanks are filled with U02(N03)2, yield keff = 0.8S8 0.006. This is about the same as when all tanks are Filled with Pu solution, and indicates that the reactivity of a polyethylene-reflected tank of solution docs not exceed the reactivity of a conciete-refleeted tank of Pu solution. Additional calculations show that the subcriticality of the four-tank grouping is not jeopardized by off-normal construction well beyond that to which control is readily achievable. 

The calculated low energy reaction yields can be reduced—though not eliminated—through removal of the restoring correction term from (t). However, a more effective suppression technique involves reduction of the ambient temperature to 100°K or below. From Table III and Fig. 12 the epithermal contamination approaches 20 of the total hot yield at 300°K and 99 mole Ar concentration. 

A recalculation of California Research Corporation test results on diphenyl based upon revised flixx figures yields an average specific damage rate of 27% per 10 nvt epithermal. In this case epithermal flvix includes all neutron energies above the cadmium cut-offo ... 

Dysprosium-aluminum wire has been chosen to measure the thermal-neutron spatial distribution. Dysprosium has a high cross section for thermal-neutron activation and a convenient half-life of 140 min. Thermal-neutron flux distributions will be made through the glory hole on both sides of core center and along the fine control rod when it is fully inserted. An absolute thermal-flux measurement will be made at the core center with a standard gold foil. This foil will be counted on an end-window GM counter whose efficiency for the standard gold foil has been determined from a previous standard pile irradiation. The results from the two activation detectors will yield the average thermal flux in the reactor core as described in Section II. A cadmium-ratio measurement of AGN-201 fuel can also be made in order to determine the fraction of the total fission rate due to epithermal neutrons. 

The threshold reaction contributions to the total fission rate can be assumed small for the AGN-201 reactor, since its moderator-to-uranium volume ratio is appreciable and its fuel is enriched with the isotope. Very fast fission is normally accounted for in the four-factor formula by the factor e, the number of neutrons produced by all fissions divided by the number produced by thermal fission. In the AGN-201, nonthermal fission is predominately resonance fission, since has finite fission cross sections at all energies. The amount of epithermal fission can be determined by a simple cadmium-ratio measurement of AGN-201-type fuel. The fission product activity of a bare and cadmium-covered fuel sample can be counted on a proportional counter after two similar irradiations in the reactor core. Their ratio will yield the amount of nonthermal fission to total fission after proper corrections for differences of sample weight, irradiation times, and, power level have been made. The final expression for power level then becomes, . . f... 

---