Archive for February, 2021

Possible issue(s) with “inclusion of short-lived daughter radionuclides”

February 23rd, 2021

Qu.) TvD, RIVM, Netherlands
Dear Nucleonica Team,
We are using Nucleonica and we have a question about the following App: Photon Dose Rate Constants++. This app offers the option to include short-lived daughter nuclides.
For Th-232, we get the following results:
Th-232 daughtersThis result occurs for the following two Data sets: 8th ToRI and the JEFF-3.1 (other data sets do not contain short-lived daughters). What is extremely puzzling is the BR product of 2.780E-12 for daughter products Tl-208, Hg-206 and Tl-206. If we look in the help section of this specific app, the following is mentioned under the inclusion of short-lived daughters:
+++
For the calculation of the dose rate constants, some nuclides are considered to be in equilibrium with daughter products. This is the case when a single radioactive decay chain in which radionuclides are present in their naturally occurring proportions, and in which no daughter nuclide has a half-life either longer than 10 days or longer than that of the parent nuclide, shall be considered as a single radionuclide. Such nuclides are denoted with an asterisk in the Nuclide Summary tab e.g. Cs-137*.
In the case of radioactive decay chains in which any daughter nuclide has a half-life either longer than 10 days or greater than that of the parent nuclide, the parent and such daughter nuclides shall be considered as mixtures of different nuclides.
+++

Our issues/questions are as follows:
Issue-1:
If the chain is assumed to be in secular equilibrium, the BR product for Tl-208 (we assume this stands for the product of branching ratios, i.e., the compound branching ratio) should actually amount to 0.36, or better: 0.3593.
Issue-2:
Why are other gamma-emitting nuclides missing, such as: Ra-228 (it has a half life of 5.75 y, but it should be taken into account as well due to the “either-or” formulation above), Ac-228, Th-228, Ra-224, Rn-220, Po-216, Pb-212, and Bi-212?
These are all preceding daughters (with photon emissions) in equilibrium as well.
Issue-3:
Under the same description for included daughters (see above, highlighted text in italics), why does U-238 not have any short-lived daughters? All nuclides could be included as no other daughters have a half life larger than that of the head-of-chain parent radionuclide U-238. Anyhow, I would at least expect the ‘actual’ short-lived nuclides to be included: Th-234 and Pa-234 and Pa-234m.
Issue-4:
What are Hg-206 and Tl-206 doing in this chain?

(Ans. Nucleonica Team)
We have now looked into your question in more detail.
Let’ start with …
Issue 4: Where does Hg-206 and Tl206 come from?
Hg206 and Tl206 come from cluster emission (CE) of the parent Th232. This is a very rare decay mode. They are included in the calculation for completeness although they have no effect on the results. If you look into the Datasheets (JEFF-3.1) for Th-232 you will see the cluster emission involves the emission of the nuclei Hg-206 and Hg-208. The nuclide Tl-208 arises through the decay of the cluster emitted Hg-208. (The Tl-208 is somewhat confusing since it arises both through cluster emission CE and from the Po-212 in the Th-232 decay chain. So the three nuclides shown in your table 1, Tl-208, Hg-206 and Tl206, all arise from CE and as such are not important.
Th-232 CE

Regarding the rules in italics…
– Th-232 (alpha) Ra-228: T/2(Ra-228) = 5.75 y > 10 d: not a short lived nuclide and thus no further short nuclides

– Th-232 (2ß) U-232: T/2(U-232) = 69.8 y > 10 d: not a short lived nuclide and thus no further short nuclides

– Th-232 (Ne-26) Hg-206: T/2(Hg-206) = 8.15 min < 10 d < T/2(Th-232) = 1.405e10 y: Hg-206 is a short lived nuclide,

Hg-206 (ß-) Tl-206: T/2(Tl-206) = 4.202 min < 10 d < T/2(Hg-206): Tl-206 is a short lived nuclide

Tl-206 (ß-) Pb-206 stable

– Th-232 (Ne-24) Hg-208: T/2(Hg-208) = 42 min < 10 d < T/2(Th-232): Hg-208 is a short lived nuclide, but has no radiations in JEFF-3.1;

Hg-208 (ß-) Tl-208: T/2(Tl-208) = 3.053 min < 10 d < T/2(Th-232): Tl-208 is a short lived nuclide

Tl-208 (ß-) Pb-208 stable

Issue-1: Here it is sufficient to say that the Th-232 is NOT in secular equilibrium with the (normal) daughters, only the CE daughters.

Issue-2: I think there is a misunderstanding of the rule in italics here. Ra-228 has a half-life of 5.75y and as such >10 days. So it should not be included in the calculation.

Additionally the italics text is not “either or”. The half-life must be less than 10 days AND less than the half-life of the parent nuclide. (this is the IAEA rule). In other words…In fact one follows the decay chains only as long as daughter nuclides having a half-life shorter than 10 days and shorter than that of the parent nuclide are found.

Issue-3: U-238 : the first daughter is Th-234 with a half-life 24.09 days.(i.e >10days). For this reason th228 and all daughters are not included.

I hope this manages to answer the questions you raise.

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Updated activity limits A1, A2 from IAEA SSR-6 ( Rev.1) 2018

February 18th, 2021

The latest values of the activity limits (A1, A2,…) have been updated in Nucleonica based on the Regulations for the Safe Transport of Radioactive Material 2018 Edition, IAEA, SSR-6 (Rev. 1). New and updated values are available, for example, for the nuclides Tb-149, Sr-83, Ni-57, Ir-193m, Kr-79, Ge-69, Ba-135m, Tb-161, Te-121m. These new and updated values are available within the e-Ship++ application.
SSR-6 2018
Reference
Regulations for the Safe Transport of Radioactive Material 2018 Edition, IAEA, SSR-6 (Rev. 1), page 25 (page 45 in pdf)

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