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Topic: Discovery of Element 118 by Oganessian, Don't Call it Ununoctium.  (Read 13166 times)

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Offline Mitch

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Element 118 has had a sorted past. It literally is a wonderful case study in scientific fraud and how the culture of science always eventually corrects and catches the dishonest individual. Disregarding the juicy aspects of that salacious back-story and moving forward to the recent discovery claim, by Oganessian for element 118. Surprisingly the article wasn't published in Nature or Physical Review Letters, but in Phys Rev C (PRC). PRC is a journal for nuclear scientists and it just seems odd to me that they didn't pick a journal with a higher impact factor. Then again Oganessian is already the man in heavyelement's research so I guess he doesn't need to be in the lime-light, although he does seem to have a disproportionate number of first author papers. The 118 decay chain is shown below:


Usually, nuclear scientists in the field will make a new element and check to see if it was really made by seeing if the daughters have the same decay characteristics as those already published, we call this a genetic correlation. In this paper we would check to see if the decay characteristics of element 116, 114, and 112 matches with what we have seen previously. The problem is, no one else but the Russian's have ever made those isotopes of 116, 114, and 112. So as of yet, they are unconfirmed and still need to be further investigated themselves! That's the problem of taking a blind leap of faith and running a 118 experiment, the daughters haven't been investigated, so you end up trying to prove you made what you made by using various theoretical models that point to similar decay characteristics as you expected. To Organessian's great credit they did not publish their results on 118, which they technically discovered in 2002, but waited until they did further investigations of 116 and 114 and even several chemistry runs for 112. So, in this paper it all matches up, the decay of 118 matches what they saw for 116 and so on.

Unfortunately, the paper has no smoking gun for 118. I define a smoking gun as being an EVR-a-a-a-sf, the bold indicating that the decay occurred when the beam was turned off. Even without that, the background is low enough, that the probability of random correlations seems low and the data still good. One wearisome thing about the 118 claim are the two 118 alphas with 11.65MeV in energy. See their table below highlighted in red.

212Pom has the exact same alpha energy of 11.65MeV. One would also expect to see 212Pom as a common transfer product contaminant in these reactions. Also, from what I understood from their data analysis code, the first alpha in strip 3 should of turned off the beam, and I didn't see where they explained why it didn't. Also, having a mean life-time, for 116, ranging from 98ns to 42ms seems too large, even for just 3 events.   

In summary, the problem with the work is that 118 doesn't decay into the known and investigated regions of the periodic table, like element 110(Ds) and 108(Hs). So we can't say with 100% certainty they made what they said they did. Until the rest of the nuclear community "catches-up" to the Russians with thourough systematic studies all the way from Roentgenium to element 118, we will not be able to evaluate the claim thoughtfully. And since the Americans and the German's haven't been able to prove Russia's 112 element claim thus far, I doubt anyone will be running a 118 confirmation experiment in the near future.

Note 1: You can download the paper here: Oganessian's element 118 claim
Note 2: No one in the field will ever call element 118 "ununoctium", so please don't embarrass yourself.
Note 3: Edited some of my nonsense because of Grejak's comment below.
Note 4: I think the Swiss did do a 112 chemistry experiment from the short-lived 114 alpha decay, but that is still unpublished to my knowledge.

Mitch
« Last Edit: November 18, 2006, 04:48:05 PM by Mitch »
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Offline constant thinker

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Re: Oganessian's recent 118-294 claim. New element 118 discovery.
« Reply #1 on: October 16, 2006, 09:12:54 PM »
I was waiting for you to post something about 118, and the validity of the claim.


In summary, the problem with the work is that 118 doesn't decay into the known and investigated regions of the periodic table, like element 110(Ds) and 108(Hs). So we can't say with 100% certainty they made what they said they did. Who knows, maybe pxn exit channels become probable in that region and what they saw was the discovery of element 117. Until the rest of the nuclear community "catches-up" to the Russians with thourough systematic studies all the way from Roentgenium to element 118, we will not be able to evaluate the claim thoughtfully. And since the Americans and the German's haven't been able to prove Russia's 112 element claim thus far, I doubt anyone will be running a 118 confirmation experiment in the next couple of years.

Mitch

From your post it seams as if though that the results won't be duplicated by a 3rd party for awhile.
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Offline Grejak

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Hi Mitch,
Thanks for the heads up about this post.  I have not had the time to do an in-depth analysis of the paper yet.  Nonetheless, there are a few things that are quite interesting.


Quote
Surprisingly the article wasn't published in Nature or Physical Review Letters, but in Phys Rev C (PRC).
This surprised me as well initially.  Looking over the papers I have, 114 was published in PRL and Nature, but 116 only warranted a PRC publication.  Since the Dubna group has previously published part of the 118 results in both a JINR Preprint and also in NPA, the paper may not have been “new” enough for nature/PRL.

Quote
212Pom has the exact same alpha energy of 11.65MeV. One would also expect to see 212Pom as a common transfer product contaminant in these reactions.
Agreed.  If you look at figure 1, it clearly shows a peak corresponding to 212Po, so you would expect that 212Pom is also populated in the transfer reactions and would provide some background at the higher energies.  This would not explain the EVR-alpha correlations though, as 212Pom has a half-life of 45.1 seconds and the two alphas assigned to 118 were correlated to EVR’s within 3 ms.

I also find the 11.8(5) MeV alpha event quite interesting.  Mostly because this is listed as an “escaped alpha particle registered by focal-plane detector without position signal because of low deposited energy.”  Correct me if I am wrong, but 11.8 MeV seems like more then enough energy to register a position.  The absence of a position leads me to believe that the alpha energy listed was not the energy observed in the focal plane.  I suspect that the listed energy is actually an estimated based off of energy deposited and possible angles of escape, which I do not like. 

Quote
Also, from what I understood from their data analysis code, the first alpha in strip 3 should of turned off the beam, and I didn't see where they explained why it didn't.
This work contains results from 2 experiments separated in time by a few years.  In the first set of experiments with 48Ca projectiles, it was assumed that all decay chains would end in a spontaneous fission.  The early experiments did not employ fast shut-offs, instead they looked for spontaneous fissions and then searched back in time for alpha chains.  It was only in the later experiments, after they knew alpha energies for the first few isotopes, that fast shut-offs were employed.  The first chain listed comes from experiments performed prior to the use of fast shut-offs for superheavy elements, the other chains come from a later experiment. 


Quote
Also, having a mean life-time, for 118, ranging from 98ns to 42ms seems too large, even for just 3 events.
116 actually, but you are correct.  I went through and ran a Monte Carlo simulation that randomly generated  1 000 000 sample sets with the same half-life (9.98 ms) and number of events (3).  Of these 1 000 000 sets, only 1.4% had a lifetime distribution broader than the experimental one.  Including the 3 decays from the 118 chains, there are a total of 10 decays of 290116 where a lifetime was recorded.  I did the same simulation with these values (10 events, 7.08 ms half-life).  Again, only 1.9% of randomly generated sets were had a wider lifetime distribution.  If I ignore the 290116 decays from the 118 chain then the results are a little better with 5.1% of trials having a wider distribution.  The other possibility is that there is an isomeric state in 290116, although that would be difficult to distinguish based off of the alpha energies given.


Quote
Who knows, maybe pxn exit channels become probable in that region and what they saw was the discovery of element 117.
No, that does not work very well.  The efficiency of the DGFRS for a pxn reaction is likely much less than 1% when it is tuned to look for the xn reaction products.  By evaporating the additional proton, your average charge state will change a lot more than it would if you were evaporating just neutrons.  Additionally, that would not explain the chemistry experiments on 112. Specifically the most recent ones where they made 114, which decayed in the recoil chamber, and then looked for the decays of 112 on gold plated detectors.  There they saw mercury like behavior, which would indicate 112, not 111.


At the moment, I am finding the 116 chains to be much more interesting than the 118 chains.  The first thing of interest is the decay times. I already mentioned that the distribution of 290116 half-lives is abnormally wide.  However, 2 more decays down, the distribution of 282112 half-lives is abnormally narrow.  Based off of the 5 events, only 1.6% of randomly distributed sets were narrower.  Eventually I will get around to adding in all the 282112 decays to see if that changes.

The alpha events listed for 290116 are also quite interesting.  Of the 12 events listed (including the ones from 118), three were not observed, three were only detected in the upstream detectors and one was an escape.  Again, the escape event has no position listed as it is said to have had too low of an energy for position resolution, although 11.15(31) MeV seems like plenty of energy for position resolution.  Unfortunately, not much more information about these are given in the paper.

Two of the spontaneous fissions listed in the table are out more than 3 mm from the EVR.  For one of these events (out 3.4 mm), the FWHM position resolution is 1.1 - 2.2 mm (I assumed 2.2), giving a standard deviation of 0.93 mm, making the SF event out more 3.6 standard deviations.  For the second event, the FWHM position resolution for EVR-SF events is 0.4 - 0.8 mm (I assumed 0.8 ), which gives a standard deviation of 0.35.  The SF listed in the correlation is then 14.6 standard deviations outside of the mean.  In the paper they state “…one of them was detected during a beam-off period and the probability of observing it as a random event is extremely low.  For the second SF, this probability if about 1%”.  However, I seem to recall that the DGFRS has a small SF background.  One of these SF events follows an escape alphas, so I suspect that it is actually a background event.  For the other event, I am weary of anything being 14.6 standard deviations out.


Anyway, that is all I have had time to think about, I still need to go over the paper in detail.

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Offline Mitch

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Offline Grejak

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Mitch,
You are correct that the Swiss/Dubna results on 112 have not yet been published.  They have submitted a paper to nature, so hopefully that will come out soon.  Until then, you will just have to make due with a presentation from Gäggler on the results.  Unfortunately, you can not clearly see the slide with the background rates :-(.  When they are talking about background free environment, I would not completely believe that.  The spectra that they showed was from one detector and for only 4 hours.  They did not show an alpha spectra from the entire experiment for all detectors.  That said, short alpha-SF is a pretty good indicator that their results are valid.

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I am just wondering what you guys think about the next magic number. Will someone ever reach that and if there's a possibility that the element would be stable?
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Offline constant thinker

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Even if the next magic number isn't like U-238 in it's 4.5 some odd billion year half-life, it would be cool if it had a long enough half-life to observe it's chemical and physical properties.

I guess 118 is a step closer to it. I like felixe's question.
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Offline constant thinker

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I mean it to be a magic number. I meant it to say that it would be nice if the magic number had a half-life like U-238, but it doesn't matter if it's half-life length is as long as U-238. I only want it to have a half life long enough to study its chemistry. I didn't mean to say that U-238 was a magic number, since that's how you interpreted it.

What is the next magic number supposed to be by the way? I think it's an isotope of element 126.
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Offline Dan1195

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Re: Discovery of Element 118 by Oganessian, Don't Call it Ununoctium.
« Reply #9 on: November 11, 2006, 05:20:20 PM »
Verification of decay chains where that do not end with known nuclides, especially those detached from known decay series by a few neutrons, is problematic.  Chemical studies alone , even if successful, are of no use in determining mass number.  You also need to measure the actual mass, rather difficult when dealing with such low cross sections and the need to verify decay properties at the same time.

Perhaps the best, but very time consuming, method is to sythesize members of all chains in between the known nuclides and the ones observed by the Russians.  More knowledge is known of SF half-life tendancies and likely fission barriers now that is it easier to determine if the nuclide has an even or odd number of neutrons. Then the excitation function can be studied to place each chain in place.  e.g. the (x,5n & 6n) chain is already known, the decay properties end in a 5 ms SF half-life and (x,2n) is not energetically feasible so (x, 4n) chain is almost certainly the observed chain. 

Of course its not foolproof.  The presence of isomeric states can be problematic (See Rf-261 and various Db & Lr isotopes for examples). Finding suitable targets and beams at a reasonable cross section can also be a problem. But it may be the best option we have at the present time.

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