This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Ahmad, I.

Bacher, A.

Back, B.B.

Berg, G.P.A.

Chasman, R.R.

Foster, C.C.

Greene, J.P.

Ishii, T.

Lozowski, W.R.

Morss, L.R.

Schmitt, W.

Stephenson, E.J.

English

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PARTICLE TRANSFER REACTIONS 
POSSIBLE OBSERVATION OF THE 1/2+[880] ORBITAL IN 249Cm 
I. Ahmad, B.B. Back, R.R. Chasman, J.P. Greene, T .  Ishii, and L.R. Morss 
Argonne National Laboratory, Argonne, Illinois 60439 
G.P.A. Berg, A. Bacher, C.C. Foster, W.R. Lozowski, W. Schmitt, and E.J. Stephenson 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 7408 
A crucial element in theoretical estimates of the half-lives of nuclides in the super- 
heavy region is the spacing of single particle orbitals near the Fermi level. One way to 
get information about these orbitals is to study the spectra of the heaviest actinides. We 
have been engaged in a long term study1 of the actinides and have identified deformed or- 
bitals, such as the Nilsson orbital 112- [521], that have large components from the spherical 
orbitals above the Z=114 gap. The observation of this level has given us detailed informa- 
tion on the splitting of the spherical f712 and fS12 proton  orbital^.^ We have also identified3 
deformed neutron states having large components from the spherical orbitals above the 
N=184 gap, such as the orbitals 112-[761] and 112-[750]. This gives us information on 
the positions of the spherical orbitals hIlI2 and j1312. In addition to the importance of 
these levels in determining the stability of the super-heavy elements (N-184), they also 
play an important role in understanding the stability of the newly discovered4 very heavy 
elements (N- 160). 
The position of the k1712 orbital plays a crucial role in the stability of elements with 
N-184. Because of its large degeneracy, it has a large influence on the magnitude of 
shell corrections in this region. The lowest deformed component of the kI7l2  orbital is the 
Nilsson orbital 1/2+[880], which is expected to lie below 2 MeV in nuclei with neutron 
numbers greater than 152. As yet this orbital has not been observed, and the possibility 
of observing it was the motivation for the experiment described here. In an earlier high 
resolution (d,p) study3 of 251Cf7 we were able to identify all of the neutron single-particle 
states between the well known gap at N=152 and the gap at N=164, and their energies 
are in good agreement with calculated values. Several orbitals above the N=164 gap 
were identified and found to be at excitation energies in fairly good agreement with our 
theoretical estimates. The same calculation that gives these estimates predicts that the 
1/2+[880] orbital should be found at 1400 keV in 251Cf. However, this orbital does not 
have any low 2 components and it would not be populated in ( d , ~ )  reactions. An angular 
momentum decomposition of this orbit a1 indicates that it is -80% k1 7/2. Such orbitals are 
strongly populated in ( 4 ~ e , 3 ~ e )  reac t ions .~ecause  of the intense radioactivity associated 
with 250Cf, the longer-lived isotone, "'Cm, has been used in the present experiment. The 
nucleus 249Cm is an isotone of 251Cf and hence the "'Cm level ordering should be similar 
to that of 251Cf. 
The experiment 248Cm(4He,3He) was performed at the Indiana University Cyclotron 
Facility. The Cm target was prepared by molecular plating the material on a 75 pg/cm2 
carbon foil. A beam of 100 MeV a-particles was incident on the target and the emerging 
3He ions were momentum analyzed with the K600 magnetic spectrometer. The beam 
current was 25 pnA and the spectra were measured at angles of 4") 6") lo0 ,  12' and 16" 
degrees. Spectra with a 208Pb target were also measured at these angles. The effectiveness 
of the (4He,3He) reaction for the identification of high-l states was checked by measuring 
a spectrum (Fig. 1) with a metallic 232Th (0.7 mg/cm2) target. In the spectrum below 
500 keV, the dominant peak is the I=15/2 member of the 712-[743] rotational band at 
252 keV which was previously known. The next strongly populated level is at 988 keV, 
which has been interpreted as the 1512 member of the 912-[734] particle state (u2- 1) 
and has been identified in the isotone 235U at 990 keV. 
Figure 1. 232Th(4He,3He) 
spectrum measured with 
the K600 magnetic spec- 
trograph. The x axis rep- 
resents momentum of out- 
going 3 ~ e  ions. 
4800 5200 5600 6000 
CHANNEL NUMBER ( 3 ~ e  momentum) 
In the 248Cm(4He,3He) spectrum, a peak at 598 keV has been observed at all angles. 
This peak is identified as the 1512 member of the 1112-[725] band. It has been identified 
at 570 keV in the isotone 251 Cf [3]. According to DWBA calculations, the strongest peaks 
in the (4He,3He) spectrum should be the jls12 and kI7l2 orbitals. Calculations indicate 
that the lowest two components of the kI7l2 orbital, the 1/2+[880] and the 3/2+[871] 
orbitals, should occur at -1400 and -1500 keV, respectively. The 1712 member of the 
3/2+ band should be at ~ 2 0 0 0  keV, whereas the 1712 member of the 1/2+ band should 
be at -1500 keV because of the large decoupling parameter. The only component of the 
j1512 orbital which is expected to occur in this energy region is the 1312-[716] Nilsson 
state whose I=15/2 member should occur at - 1600 keV. 
In the spectrum (Fig. 2), the two strongest peaks above 1 MeV occur at 1570 and 
1900 keV. The lower peak has a cross section about twice that of the 1900 keV level. We 
tentatively assign this peak to the 1/2+[880] band because the 1/2+[880] band is expected 
to have a larger cross section than the 1112-[725] band. 
Figure 2. 248 Cm(4He,3 He) 
spectrum measured with 
the K600 magnetic spectro- 
graph. The x axis repre- 
sents the momentum of the 
outgoing 3He ions. 
4500 5000 5500 6000 
CHANNEL NUMBER ( 3 ~ e  momentum) 
This work was supported by the U. S. Department of Energy, Nuclear Physics Division, 
contract No. W-31-109-ENG-38 and by the National Science Foundation Grant PHY 48- 
308-44. 
1. R.R. Chasman, I. Ahmad, A.M. Friedman and J.R. Erskine, Rev. Mod. Phys. 49, 833 
(1977). 
2. I. Ahmad, A.M. Friedman, R.R. Chasman and S.W. Yates, Phys. Rev. Lett. 39, 12 
(1 977). 
3. I. Ahmad, R.R. Chasman, A.M. Friedman and S.W. Yates, Phys. Lett. 251, 338 
(1990). 
4. S. Hofmann e t  al., 2. Phys. A354, 229 (1996). 
5. C.P. Massolo, F. Azaiez, S. Gales, S. Fortier, J .  Guillot, E. Hourani, and J.M. Maison, 
Phys. Rev. C 34, 1256 (1986). 


Possible Observation of the 1/2+ [880] Orbital in 249-Cm

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Possible Observation of the 1/2+ [880] Orbital in 249-Cm

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