[gippgig]

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.

[Mitch]

Yeah, I was actually 2nd author on that thing. My work focused on the 3n reaction channel though.

[Dan1195]

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.

[Grejak]

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, t_1/2 = 2.91 s), obscured any evidence of alpha decays of ²⁵⁹Lr (Ealpha = 8.45 MeV, Ialpha = 0.77, t_1/2 = 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...

[Grejak]

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.

[Dan1195]

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.

[Grejak]

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).

[Dan1195]

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.