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Topic: New transactinide results  (Read 11829 times)

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Offline gippgig

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New transactinide results
« on: April 16, 2008, 02:07:17 PM »
A recent paper (Physical Review C 77 034603) reports on the production of 258-261Rf in the 238U(26Mg,xn)264-xRf reaction. 258Rf was found to have a surprisingly high 31% alpha (9.05 MeV) decay branch (why wasn't this detected before?). 259Rf was found to have a 15% EC branch but little or no SF. SF previously ascribed to this isotope (which now forms the basis for Dubna's claim to have discovered element 104) probably is from a known 25% SF branch of the EC daughter 259Lr.

The April 12 issue of Science News reports that preliminary results from an experiment by the Paul Scherrer Institute at Dubna have found unexpected noble gas-like behavior of element 114.

Offline Mitch

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Re: New transactinide results
« Reply #1 on: April 16, 2008, 02:49:48 PM »
Yeah, I was actually 2nd author on that thing. My work focused on the 3n reaction channel though.
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Offline Dan1195

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Re: New transactinide results
« Reply #2 on: April 18, 2008, 11:41:51 AM »
Recent Rf studies have discovered a couple of things:

First is Fission hinderance for odd-N Rf isotopes is greater than initially believed. New results on 257Rf give a 1.3% SF branch to this nuclide w/7 min partial SF half-life with SF hinderance for the odd-N nucleus of >104. This may help with sorting out the properties of 261-263Rf.

Second is the presence of significant EC branches. These often went undetected previously because EC can be difficult to distinguish from straight SF of a nuclide or even straight alpha decay without a sufficent data sample, knowledge of daughter decay properties and accurate systematics in the region. Even with this it can still be very hard in some cases.


Offline Grejak

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Re: New transactinide results
« Reply #3 on: April 18, 2008, 02:46:19 PM »
A recent paper (Physical Review C 77 034603) reports on the production of 258-261Rf in the 238U(26Mg,xn)264-xRf reaction. 258Rf was found to have a surprisingly high 31% alpha (9.05 MeV) decay branch (why wasn't this detected before?). 259Rf was found to have a 15% EC branch but little or no SF. SF previously ascribed to this isotope (which now forms the basis for Dubna's claim to have discovered element 104) probably is from a known 25% SF branch of the EC daughter 259Lr.

Mitch may have been second author on the paper, but I was the first.  (Arrogant enough? ;)).

Anyway, the typical way for detecting electron capture is to see decays from the daughter, in this case 259Lr.  Previous experiments that have directly produced 259Rf have not used some sort of preseparator (e.g. the BGS).  So contamination from transfer reaction products, specifically 256No which has very similar decay properties (Ealpha = 8.43 MeV, Ialpha = 0.995, t1/2 = 2.91 s), obscured any evidence of alpha decays of 259Lr (Ealpha = 8.45 MeV, Ialpha = 0.77, t1/2 = 6.3 s).

As to why it was not seen in 259Rf produced as the decay product of 263Sg…Isomers dude.  The same thing has been seen before where the isomers populated in direct vs. indirect production are different (263Sg comes to mind, ironically).

My apologies if this is too brief/basic, I do not know how much you know about the subject.  Although I am always happy to go into more detail...

Offline Grejak

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Re: New transactinide results
« Reply #4 on: April 18, 2008, 04:31:45 PM »
A recent paper (Physical Review C 77 034603) reports on the production of 258-261Rf in the 238U(26Mg,xn)264-xRf reaction. 258Rf was found to have a surprisingly high 31% alpha (9.05 MeV) decay branch (why wasn't this detected before?). 259Rf was found to have a 15% EC branch but little or no SF. SF previously ascribed to this isotope (which now forms the basis for Dubna's claim to have discovered element 104) probably is from a known 25% SF branch of the EC daughter 259Lr.

The April 12 issue of Science News reports that preliminary results from an experiment by the Paul Scherrer Institute at Dubna have found unexpected noble gas-like behavior of element 114.
So, I realized that I did not actually answer your main question re the alpha branch in 258Rf. 

There are two main things at play here:
1) 262Sg has only been observed to decay via SF, so 258Rf cannot be produced through alpha-decay.

2) Only three other experiments have directly produced 258Rf.  Wild et al. looked only at the SF properties.  It is likely that they had too much alpha activity to see anything interesting as no separation was performed and no alpha results are discussed.  Somerville et al. detected the activity using mica track detectors, so would also only be sensitive to SF and not alpha.  Ghiorso et al. were sensitive to alpha decay, but contamination of the spectra by 254No and other isotopes of Rf with more significant alpha-decay branches prevented detection.

The third way to make 258Rf is by EC of 258Db and then detection of 254No.  Unfortunately, the alpha-daughter of 258Db also has an EC branch, so the 254No observed in decay chains of 258Db can be due to either alpha-decay of 258Rf or EC of 254Lr.  Heßberger and friends did propose an ~13% alpha decay branch in 258Rf by looking at the alphas preceding decays of 254Lr and 254No.  They saw that 254No was normally preceded by a 9.0 MeV alpha while 254Lr was preceded with alphas of multiple energies.  However, this could have been due to either isomers or alpha-decay of 258Rf.

Offline Dan1195

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Re: New transactinide results
« Reply #5 on: April 20, 2008, 08:24:41 PM »
W/EC branches reported in both 257,259Rf and SF reported in 257,263Rf (SF likely not observed in 259Rf due to insufficent number of decays observed) one has to wonder about possible SF and/or EC branches in 261Rf. Some latest spectroscopy suggests the 68 s state may the ground state, which has the lower alpha energy.

The EC daughter would be the 39 minute 261Lr. which decays by either EC or SF (seems unclear which). Obviously the longer lifetime would make detecting the SF end product tougher to separate from other decay products, unless an experiment was specifically designed for this, as with the 263Db->263Rf study. As for SF, I am not sure how sensitive direct formation studies of the 68 second state have been to SF. The alpha parents of this nucleus seem to mostly decay to the ~4 s isomer, so they wont help much here. I have no seen any max SF % branch estimates for the 68 second state.

Offline Grejak

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Re: New transactinide results
« Reply #6 on: April 21, 2008, 03:44:50 PM »
Henderson et al. looked into the 261Rf EC branch thing.  They separated out a Lr fraction from bombardments of 248Cm and 18O and observed a 44 min SF activity.  If contamination due to the p,4n channels is negligible and the 261Lr activity was due to 261Rf, then the EC branch in 261Rf would be 14±1.4% (Chapter III.C of his thesis).

Offline Dan1195

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Re: New transactinide results
« Reply #7 on: April 21, 2008, 09:58:51 PM »
With that info in mind one has to look now at the 263Rf decay, which has a reported large SF and small alpha branch. Unfortunately the potential EC daughter is unknown here, so any positive identification on the source of the SF is problematic.

Hopefully in the future RIB's (radioactive ion beams) will be able to shed some light on that region of the nuclear chart. From what I have read they will be most useful initally for Z=104-108 near the N-162 shell enclosure, where there is a paucity of data.

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