Answer to Question #12241 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

The following question was answered by an expert in the appropriate field:

Q

How can I find out which nuclear reactions occur and what main products and byproducts are produced when we irradiate natural rhenium in a reactor? I know that when producing rhenium-186 (186Re) and rhenium-188 (188Re) by irradiating natural rhenium in a reactor, other products will be produced in this process. How can I know the complete nuclear reactions that will happen? Is there a website or software for this?

A

If you intend to irradiate rhenium in a typical thermal reactor, the major expected radionuclides from activation of the rhenium-185 (185Re) and rhenium-187 (187Re) isotopes will be, as you have noted, 186Re and 188Re. Other possible products may result from other nuclear reactions on the stable rhenium isotopes if the energetics are favorable and the available neutron fluences are sufficient. In addition, there may be impurities present in the rhenium that may themselves be capable of being activated. We shall first consider other nuclear reactions on rhenium. We can hypothesize a number of possible reactions, although many may not be likely, and I will give only a couple of examples.

For instance, we might postulate an (n,p) reaction on 185Re to yield tungsten-185 (185W). We can first check the energy requirements for this reaction by noting that the energy released by or required to initiate the reaction is given by the difference in binding energies between the products and the reactants. Using values taken from the Radiological Health Handbook (Department of Health, Education, and Welfare 1970), the binding energy of the 185W is 1478.611 megaelectronvolts (MeV) and that of 185Re is 1478.257 MeV, and the difference is 0.354 MeV—slightly positive, indicating the reaction is energetically possible without the addition of any further energy beyond that of a thermal neutron. If we were to look at a possible (n, 2n) reaction of the 185Re, we would find the difference in binding energies between the rhenium-184 (184Re) product and the initial 185Re to be -7.8 MeV. Here, the negative sign indicates that the reaction would not occur with thermal neutrons; in fact, more than 7.8 MeV of energy would have to be provided. This could be available from the kinetic energy of fast neutrons (>7.8 MeV) present in the reactor, but the reaction yield would likely be quite low.

You can determine the energy requirements of nuclear reactions by looking up binding energies (or atomic masses) in available tables or using Q-value calculations. Here is a link to one such calculator; I suggest you try it out to see how it works. Additionally, the feasibility of reactions producing any significant product depends on the magnitude of the interaction cross section. Thus, while the 185Re(n,p)185W reaction appears energetically possible at thermal neutron energies, the actual cross section for this reaction is close to zero. Here is a link to a Brookhaven National Laboratory site for cross sections. Using this website, fill out the boxes for the reaction of interest (for example, target of 185Re and reaction of n,p), click the submit button, and you will see several sets of results from different references. You can pick a reference with an option to click "s," and that option will show numerical listings of cross sections. For example, for the second listed reference you will see the estimated cross section at thermal energy (0.0253 electronvolts [eV]) is about 3.3 × 10-11 barns (b); the third listed reference gives 1.0 × 10-20 b. If you look at results for the (n,2n) reaction and select "plot" for the first listed reference, you will see that the cross section is zero close to 7 MeV, and rises to a maximum of about 2.1 b at about 15 MeV. The fast-neutron fluence rate above 7 MeV in a thermal reactor would not likely be sufficient to produce much of the 184Re product.

As noted earlier, other neutron-capture events in impurities present may also be possible. A good deal of available rhenium, as I understand, comes from processing of molybdenite concentrates that have come from copper-processing operations. As such, even after chemical isolation of rhenium and/or after electrodeposition to prepare the metallic form, small amounts of molybdenum may remain, generally less than 0.1%. It is possible that one could observe activation to molybdenum-99 (99Mo) (and its decay product technetium-99m [99mTc]) a short time after reactor irradiation. Reports of other trace elements include copper, aluminum, calcium, magnesium, iron, and platinum. Depending on how much of these are present, one might possibly see some thermal-neutron activation products associated with them under some circumstances. I shall not list all possible radionuclides; you can easily determine what the product radionuclides might be to compare any unidentified gamma radiations that you might observe in your irradiated samples with possible impurity activation products.

George Chabot, PhD

Reference

Department of Health, Education, and Welfare. Radiological health handbook. Washington, DC: U.S. Public Health Service; 1970.

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