Dear Dr. Strub,

                          I think the environment of the experiment should matter both from the theoretical physics as well as the engineering physics point of view, especially since you are dealing with materials of rare use value (rare earths). Please refer to my twin published papers on String Theory of Stock Market Nanostructure systems and Haag's theorem resolution of stock exchanges on my RG page, if you will. Its some other properties can similarly be discussed Econophysically. SKM QC

I guess you may have tried to look for the 511 keV annihilation photons in your samples. The lowest-lying 4+ state in Mo-98 is 1510 keV leaving insufficient energy for beta+ decay to proceed. Maybe there is electron capture, and this would be more difficult to detect against the beta- emission. So (not knowing enough about the nuclear physics) I speculate there may be some EC decay branch but this hasn't been measured yet as it will be masked by the beta-?

I suppose that if any "red" decay will happen, it should be EC like with most of low A-isotopes of Tc. I think Andy´s hint to energy level shows the problem and you should find out EC, but it would be really difficult besides the dominating beta-minus branch and the low production rate of TC-88 and the long half life. .

@Andy K Pearce:

In fact, I have no sample. I just checked the nuclide chart and the mass tables. You are right, the energy difference is not big enough to populate the 1510 keV level by beta plus, but electron capture should be possible. However, I do not find any data on this decay mode for Tc-98, so I suspect I am missing a point somewhere.

I think there are other examples along the valley of (beta) stability. The point is that 98Ru is more stable (i.e. lower mass at rest) compared to 98Mo. Hence the strong preference for 98Tc to decay to 98Ru (i.e. beta-). See the video below for a nice flight over the valley in 3D.

http://portail.cea.fr/multimedia/Pages/videos/culture-scientifique/physique-chimie/physique-nucleaire-au-cea.aspx

@Emmeric Dupont: The stability difference is not so big. It seems to me the case is similar to e.g.  Pm-146.

Of course, due to the selection rules of beta decay, a decay into the ground state of 98Mo is not very probable. But the same is true for the decay into 98Ru. But both these nuclei have 4+ levels at energies below that could be populated from 98Tc (from an energetic point of view).

You are right that EC data are available for similar cases. I could trace back the absence of data in Table of Isotopes and other similar compilations to the paper below, which says that "the EC decay was expected from mass and decay scheme data with emission of two y-rays, 0.722 MeV and 0.787 MeV, in cascade. But no clear photopeaks were observed and the upper limit for EC/B- was estimated to be

Thank you very much for the hint! This is the kind of information I was looking for... Do you have a strategy how you look for such "absent data"? I tried several serach terms containing "98Tc" and "Tc-98", but with these, of course, I missed this paper.  

Hello Erik,

I think your question is very interesting, since there is Technetium100, which has a small amount of EC. A short look on the nuclide Chart shows no further case, of a pure beta (-)isotop with a lower mass than an other isotope with an EC-Proportion of the same element. There must be a specific reason, why the rule "isotopes with less Neutrons have equal or more EC (or beta (+))" is here invalid. 

best regards

@ Erik

The clue was in Nuclear Data Sheets, which is the reference for all compilations of radioactive decay data. See below the NDS issue related to the mass 98.

http://dx.doi.org/10.1006/ndsh.2003.0003

Back in 1951, there was a report published by ORNL claiming positron decay of Tc98 to the first excited level of Mo98 followed by gamma radiation to the ground level. This was the only decay scheme listed in the extensive Table of Isotopes in the Reviews of Modern Physics in 1953. Now it’s well-known and accepted that Tc98 decays by emitting electron. If you dig in the literature you might perhaps find why the decay reported by ORNL and accepted in the Reviews of Modern Physics has been abandoned.

In the updated version of the Table of Isotopes published in 1955 in the Reviews of Modern Physics in 1955, the positron decay is replaced by the electron decay to the second excited state (1.39 MeV) of Ru98, the decay scheme published in Phys. Rev. vol. 99, but you probably know it already. 

Dear Ron, could you provide a paper?

I checked the old issues of the table of isotopes in Reviews of Modern Physics), 1948, 1953 and 1958. 1948 is including Tc-98, attributing "beta-, K(?)" but with a half-life that is much too short (2.7d), so it is probably a wrong attribution. The 1953 does not include anything about Tc-98 at all, while the 1958 edition includes beta- with a half life of 1.5e6 years.

However, all this is the pre-semiconductor era - if the ec branching is only around some % (as the 1993 publication suggests) I would not expect a detection, especially as the 722 keV line will be hidden in the shoulder of the 745 keV peak.

Dear Hanno,

It looks like a wrong call. I've read it in an old faded copy of a Russian article. Now that I look under magnifying glass it looks more like Tc93.

Does simple principle of N/Z or Q value of nuclear reaction do not guide the decay? Inherently Q value should be postive for decay to occur. I am also interested to know the exact answer.