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Monday, May 01, 2017

"Ask an Expert" Questions & Answers



Decay chains and branching ratios of U-234
Using both Nucleonica.com and directly JEF 2.2, I’m surprised to find differences in the daughters given for the 234U: Rn218, At218, Tl210, Tl206, Hg 206 are not considered in Nucleonica.com, as well as in some other references. Why? Is it linked with their half-life?

K. Beaugelin-Seiller, Institut de Radioprotection et de Sûreté Nucléaire (IRSN), France
 
Answer from the Nucleonica.com team

Dear Karin,

These isotopes are indeed taken in account in the decay chains of U234 in Nucleonica.com. You can see this in the Nuclide Explorer by selecting U234 and with the right mouse button selecting "show decay chain".

If you used the decay module to compute the chains, I would like to draw your attention to the parameter Min.Prod. (Default 1e-2) which is the minimum branching considered for the calculation. You need to select a much smaller value to obtain the nuclides you are looking for (i.e. nuclides with small branching ratios).

To demonstrate this clearly, consider the decay of Pb210 for which (from the Datasheets)

Decay mode Branching ratio Daughter
ß- ~1 ~100% Bi 210
Alpha 1.90E-08 1.90E-06% Hg 206

If your Min.Prod has a value of 1e-2 the alpha decay mode will be ignored and only the Bi210 would be considered as a decay product for computing. To "see" the alpha decay mode you have to set Min.Prod. to 1e-8. If you are unsure which value to use, just insert 0 and all decay chains will be shown.

In some cases there are many decay modes, and to save calculation time, one selects a value of Min.Prod. which typically gives the most important 3-4 chains. With a default value of Min.Prod. = 1E-2, we see all chains for which the product of the branching ratios is greater than 1E-2 or 1%.

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Gamma emission branching ratio for Bi-212
I am involved in the gamma spectrometry measurements above ground areas with increased U-Ra- and Th-Chain content. These measurements have been performed by a number of institutions in the context of a comparison at WISMUT sites, which was organized by the BfS. In particular, I am interested in the detection of Bi-212. When I compared the branching ratios (for the emitted gamma lines) with Nucleonica.com (see table below) I found large differences.Could you please explain me why there is such a difference and where does it come from?

  Nucleonica.com Erdtmann (FZK) RadDecay (C.Hacker) Genie2000 (Canberra )
727 keV 6.75% 11.80% 11.83% 11.80%
1621 keV 1.49% 2.75% 2.75% 2.75%
Bernd Horlbeck, Deutsche Gesellschaft zum Bau und Betrieb von Endlagern für Abfallstoffe mbH(DBE), Germany
 
Answer from the Nucleonica.com team
The values given in Nucleonica.com are from the JEF 2.2 datafile. The values have recently been re-assessed and in this most recent evaluation (see http://www.nucleide.org/DDEP_WG/Nuclides/Bi-212_com.pdf), the values are very close to the JEF.2.2/Nucleonica.com values. The other values you quote (RadDecay, Erdtmann, etc.) are old and no longer accurate (for more information see http://www.nucleide.org/DDEP_WG/DDEPdata.htm). Note also that the FZK evaluation that you mention is the one which is used in the Gammavision isotope standard library (and probably also for the Genie2000 standard library). This error, however, is only in the standard libraries - not in the PTB or Berkeley libraries.

PS: In the future edition of Nuclides.net, there will be a special module to create custom libraries from our databases to be used with both Genie2000 and Gammavision.
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Effective dose coefficient for Po-210 in ICRP 68 and 72
Po-210 is of broad practical interest: in tabacco, in drinking water, in diet, and as part of the uranium decay chain. According to the ICRP Publication 72, the effective dose coefficient for ingestion for members of the public ( eing (50)=1.2x10-6 Sv/Bq), is much higher than the value given in ICRP 68 for workers ( eing (50)=2.4x10-7 Sv/Bq). This seems to be a result of the gut transfer factor (f1) for members of the public being raised from 0.1 to 0.5. How can this be explained?

Helmut Kowalewsky, Germany
 
Answer from Alan Phipps. National Radiological Protection Board , UK
Dear Mr. Kowalewsky,
you are right in saying that the gut uptake fraction (f1 value) is 5 times higher for members of the public than for workers. This is because the public can be expected to encounter Po-210 in foodstuffs rather than in the inorganic forms which are likely to occur in the workplace. ICRP Publication 67 summarises the data from ingestion of Po in reindeer and crabmeat (by humans) and in milk and liver meat (by rats) to justify the value of 0.5 chosen for the public. Of course, this leads to a higher dose coefficient (Sv/Bq) for the public than for workers, by about a factor of 5. ICRP is currently reviewing f1 values for workers. I do not know what value they are likely to choose for workers, but it remains the case that workers are exposed to different (generally inorganic) forms of Po than are the public. Alan Phipps. National Radiological Protection Board , UK Note added by the Nucleonica.com team: this higher value for the effective dose coefficient for ingestion for Po-210 has a direct consequence on the reference levels for uranium (where Po-210 is an equilibrium daughter) in radiotoxicity calculations for spent nuclear fuel. More information can be found in a recent publication.
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Decay Heat Calculations with Nucleonica.com
We are trying to calculate the heat generation in a transport package. The package contains several nuclides. First we calculated the heat generated by each nuclide using Nucleonica.com. We used the Q-value and specific activity given in Nucleonica.com for this purpose. But then we compared these calculated values with the isotopic powers given in Nucleonica.com. The isotopic power is always lower than the heat generation we calculated – see the table below. Please, can you tell us the difference between our calculated heat generation and the given 'isotopic power' of a nuclide?

three examples: H-3, Fe-55 and Pu-240
A B C D E F G

our data Nucleonica.com Nucleonica.com from C and D from Nucleonica.com E/F
isotope mass Q-value specific activity specific heat isotopic power
(a + b + g )
 
(g) (keV) (Bg/g) W/g W/g  
H-3 9.60E-03 18.571 3.56E+14 1.06E+00 3.25E-01 3.26E+00
Fe-55 4.80E-05 231.10 8.92E+13 3.30E+00 8.40E-02 3.93E+01
Pu-240 1.20E+02 5255.9 8.40E+09 7.07E-03 7.06E-03 1.00E+00
Ralf Steiner and Lars Niemann, Hauptabteilung Dekontaminationsbetriebe, Forschungszentrum Karlsruhe.
 
Answer from the Nucleonica.com team
1. Case of H-3:
From Nucleonica.com Datasheets: H-3 is a pure ß- emitter. The energy of the emitted ß- particle is 18.571 keV with an emission probability of 1. This agrees roughly with the Q-value you quote of 18.6 keV. Note, however, that the mean decay energies (which are used for the isotopic power calculation) are “electron” = 5.71 keV. This is approximately 1/3 of the Q-value. At a first glance, this seems to be a contradiction. It is easy to explain however when you note that in beta- emission, the following reaction occurs in the nucleus

i.e. a neutron is converted to a proton, a ß- particle and an antineutrino v. Note that beta emission differs from alpha emission in that the beta particle has a continuous spectrum of energies between 0 and some maximum value called the end-point energy. Also that the beta energy of 18.571 keV quoted above is the end-point energy. The average energy of the beta particle is approximately 1/3 the value of the endpoint energy. This explains the factor 1/3 observed above. Note here that approximately 2/3 of the energy i.e. 12 keV is carried away by antineutrinos. 2. Case of Fe-55:
From Nucleonica.com Datasheets: Fe-55 decays by electron capture (ec). The Q-value is 231 keV. The decay reaction can be written:

In this reaction, an inner shell electrons in Fe-55 is captured by the nucleus and combines with a proton in the nucleus to form a neutron and a neutrino. In contrast to ß- and ß+ emission, the neutrino is mono-energetic. The process of electron capture leaves a vacancy in an electron shell that is then filled immediately by electrons from higher levels cascading down. This process is characterized by the emission of x-rays. Hence the only way that the electron capture process can produce energy emission is through the x-rays or Auger electrons associated with them. The remaining energy goes into the neutrino!
Hence for Fe-55, we can see from the datasheets that the average x-ray and electrons energies are 1.67 and 4.22 eV respectively. The energy difference between the Q-vale of 231 keV and the x-rays + electrons of 5.89 keV is emitted as mono-energetic neutrinos (approx. 225 keV).
Hence it is incorrect to use the Q-value for the calculation of decay heat. In this particular case of electron capture in Fe55, most of the energy is emitted as an antineutrino. If one use the Q value, the decay heat will be overestimated by a factor Q/5.89 keV = 39.2. This is exactly the number you given in you table below. 3. Case of Pu-240:
In contrast to ß-, ß+, and ec, in alpha emission there are no neutrinos involved. For this reason, for decay heat calculations, the Q-value can be used.
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Se-79 half-life
I get some problems with Se-79 halflife. I found that in some web pages it is defined as 6.5E+04y, 1.13E+06y, and in Nucleonica.com it is indicated equal to 6.5E+05. What is the reason for such differences and which number is real halflife of Se-79?

Asta Brazauskaite, Lithuanian Energy Institute
 
Answer from the Nucleonica.com team
The measurement of the half-life of Se-79 is problematic because of its long half-life and its decay without gamma ray emission. Additionally the maximum beta- energy is only 151 keV. Many attempts have been made to determine an accurate value for this half-life, using decay counting or even particle counting. The different values you've found come from different evaluations in specific database and they might not agree depending how old the database is and how many entries the evaluator had to give an average value.The present entry in Nucleonica.com comes from the Nubase evaluation from 1997. The latest Nubase 2003 edition gives value of 295 ky. On the other hand, in the 8th Table of Isotopes, they give a value of 1.13E6 y (cut off from 1998) and a recent measurement (from 2000) gives a values of 1.24E5 y).
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Cs-137 decay gamma lines
I'm frequently using Nucleonica.com for shielding and dosimetry calculation, but I have one problem for calculation with radionuclide Cs-137. Starting calculation program dosimetry leads to the error "no specified ray for this nuclide". Any other radionuclides are OK. Please help me to solve this problem.

Juraj Hamza, JFM, Slovakia
 
Answer from the Nucleonica.com team
It comes from a common mis-interpretation of the decay of the Cs-137. I guess that you're looking for the 662 keV line.
The Cs-137 parent isotope beta decays (~95%) with a 30.17y half-life to produce Ba-137m which in turn decays with a  2.55min. half-live, generating a 661.6 keV gamma ray emission.

In most decay library the 662 keV is then associated to the decay of Cs-137 because Ba-137m is directly produced by the decay of Cs-137 and coz' the half-life of Ba-137m is rather short in comparison to Cs-137, thus the two nuclides are in equilibrium.

Therefore in your Nucleonica.com calculation, you should use the Ba-137m nuclide as source instead of Cs-137.
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Decay of Bi-211
In some databases it is indicated that a fraction of 16.19% of the Bi-211 decays to the Tl-207 metastable state. Is this correct? Furthermore, the Tl-207m is in the Karlsruhe nuclide chart - when do we obtain Tl-207m if it isn't from the Bi-211 decay?

Gro Salberg, Algeta ASA, Oslo, Norway
 
Answer from the Nucleonica.com team
From the Table of Isotopes (8th Edition, Vol. II: A = 151-272, R. B. Firestone and V. S. Shirley (Eds.) J. Wiley, 1999) we have the following diagram for Bi-211 decay:
Decay of Bi-211
The 16.19% you refer to is the decay to the 30 ps state see above. This is NOT the (1.33 s half-life) metastable state referred to in the Karlsruhe nuclide chart (see following diagram) but a very short-lived excited state which then further decays to the ground state. Note from the diagram above (right hand side), that the Bi-211 decays only to the 30 ps state and the ground state. This 30 ps state is very short-lived and decays to the ground state. This is the reason why the Bi-211 decay is regarded to decay only to the ground state of Tl-207. With regard to the origin of Tl-207m, it can be seen from the above diagram that the 1.33s metastable state of Tl-207 is “fed” from the decay of Hg-207. This is also shown in the Karlsruhe Nuclide Chart shown below.


Decay of Bi-211 Excerpt from the Karlsruhe nuclide chart, 7th edition 2006 for the decay of Bi-211. (for further information see www.nucleonica.com) According to the symbols used in the Karlsruhe nuclide chart, Bi-211 has two decays modes – α and β- indicated by the colours yellow and blue respectively. In the α decay mode, the parent Bi-211 decays to the ground state of Tl-207 (4.77m half-life). This is indicated in the bottom line of the Bi-211 “box” where the symbols α→g implies decay to the ground state of the daughter Tl-207. It can also be seen from the Hg-207 “box” that decay of Hg-207 results in both the metastable (m) and ground (g) states of Tl-207. In fact, according to the notation in the bottom line of the Hg-207 box, i.e. m; g, decay to the metastable state has the higher branching ratio. In the ß- decay mode, the parent Bi-211 decays to the ground state of Po-211 (0.516 s half-life), This is also indicated in the bottom line of the Bi-211 “box” where the symbol ß-→g implies decay to the ground state of Po-211.
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Decay of Bi-211
Nucleonica.com presents two half-lives of Po-211: 25.2 sec and 0.516 sec. Is only one of these two variants (the 0.516 sec variant?) produced during Bi-211 decay?

Gro Salberg, Algeta ASA, Oslo, Norway
 
Answer from the Nucleonica.com team
From the Table of Isotopes (8th Edition, Vol. II: A = 151-272, R. B. Firestone and V. S. Shirley (Eds.) J. Wiley, 1999) we have the following diagram for Bi-211 decay:
Decay of Bi-211
From this diagram you can see that Bi-211 cannot decay to the 25 s metastable state of Po-211m. This is the reason why only the decay to the ground state is shown in the nuclide chart.

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