This work was supported by the National Science Foundation Grant NSF PHY 78-22774 A02 & A03 and by Indiana Universit

Jacobs, W.W.

Kailas, S.

Karwowski, H.J.

Singh, P.P.

Soga, F.

Throwe, T.G.

Vigdor, S.E.

English

IUScholarWorks

SYSTEMATICS OF THE K X-RAY MULTIPLICITY FOR (Li,xn) PRODUCTS WITH 180 A A 210 
S.E. Vigdor, H.J. ~arwowski*, T.G. Throwe, W.W. Jacobs, S. Kailas, P.P. Singh, 
and F. Soga 
Indiana University Cyclotron Facility, Bloomington, Indiana 47405 
As part of our program of measurements of the Such a structure peculiarity might, of course, arise in 
decay characteristics of compound nuclei formed at high a few anomalous cases from some accidental 
angular momentum and high excitation in Li-induced near-coincidence among various quasiparticle 
fusion reactions,l) we have developed a new technique excitations, but the effect in our case appears to be 
for determining absolute total cross sections, a,, for no accident. This is illustrated by the compilation of 
(heavy-ion, xn) reactions. The technique involves our multiplicity measurements in Figs. 1 and 2. 
measurements of the mean multiplicity RiK) of K X-rays 
emitted during the de-excitation cascades in the xn 
residual nu~lides.~) The X- rays arise from internal -T- --f- - - x  
3.0 / / I-----,--- 
conversion during the y-cascades, so that RIK> is 
sensitive to the multipolarity and to the energy of the 
nuclear transitions through which the y-decay proceeds. 
/ 
This sensitivity is of little consequence for the 
determination of a,, but provides the opportunity to 
A 
in a mass region just below the N-126 shell closure. 
In all the cases we have studied so far, we have 
found 1.0 <MK> 5 3.0. It is not a trivial matter to 
account in detail for the origin of as many as three 
X-rays per cascade. We have already argued in ref. 2, 
on the basis of various experimental constraints, that 
in the neutron-poor T1 and Pb isotopes studied, 
approximately two K X-rays arise from a narrow region 
I 
learn something about the structure of the populated 2 0.0 
V residues as a side benefit. Our results to date, for 
3.0- (Li,xn) reactions with a variety of target nuclei and 
bombarding energies, suggest an intriguing systematic 
2.0 behavior of RiK), and hence of the nuclear structure, 
Elab (MeV) 
. 
b) 6 ~ i  
/ -----!-- 
0 0 \ 
0 \ 
\ 
- 0 - 
, /+/  ' 
Figure 1. Measured K X-ray multiplicities for (6~i,xn) 
reactions on five targets as a function of bombarding 
energy. The dashed curves are intended to guide the 
eye. 
of spin (12 5 J 20) dominated by low-energy M1 In Fig. l(a) we observe a similar dependence on 
transitions. (The probability of K-shell conversion bombarding energy for 1 9 8 ~ t ( 6 ~ i  ,xn) and 1 9 7 ~ ~ ( 6 ~ i  ,xn) , 
per unit spin change is an order of magnitude greater with RiK) remaining remarkably constant over the range 
for M1 than for E2 transitions in the Z = 80 region.) from 75 to 124 MeV, despite significant changes over 
this range in the mass and spin distributions of the 
populated residues. The observed reduction in <MK> for 
both targets at Elab = 55 MeV is consistent with our 
expectation of a lower spin cutoff (J = 12) on the 
region of highly converting transitions. In 
particular, we have evidence that the low-energy 
falloff is not associated with the change in neutron 
excess of the dominant residues between 55 and 75 MeV: 
a measurement for 7 ~ i  + lg7Au at Elab = 68 MeV (not 
included in Fig. l), populating the same residues 
(Ig7 ,lg8pb) as 55-MeV 6 ~ i  + lg7Au, but at appreciably 
higher spin, yielded <MK> = 2.04 2 0.16, in excellent 
agreement with the higher-energy Au results. 
The results for other targets in Fig. l(b) seem to 
complicate the issue, exhibiting quite different energy 
dependences of <MK> for different target nuclei. 
I I 1 I 1 I I 1 1 
90 100 110 I20 130 
Neutron Number of Dominant (Li, xn) Residue 
Figure 2. Compilation of measured K X-ray 
multiplicities for (Li,xn) reactions Induced on the 
targets shown at bombarding energies from 75 to 124 
MeV, plotted vs. the neutron number Npeak corresponding 
to the peak in the residual-nuclide production 
distribution appropriate to each target and energy. 
The values of Npeak are deduced from systematics 
established by y-singles measurements in the present 
experiment and in the work of ref. 3. The daspd 
triangle represents a 55-MeV measurement for ~ i + ~ ~ ~  pb. 
The dashed curves are intended to guide the reader's 
eye to the author's conclusion (see text). 
However, an apparent simplicity in the variation of 
<MK) is restored in Fig. 2, where we have plotted the 
measurements (including a few for prajactile-target 
combinations not presented in Fig. 1) not against curves" suggested in the figure. 
If we indeed find continued evidence for a simple energy, but rather as a function of the neutron number 
Npeak corresponding to the peak in the mass systematic behavior of <MK>, what will this tell us 
distribution of residual nuclides appropriate to each 
target and energy. (We have omitted 55-MeV results 
about nuclear structure in the region? This is still a 
matter of speculation, but the following scenario seems 
from Fig. 2, with the exception of the dashed triangle 
for 2 0 8 ~ b ,  since they are l o w  for reasons independent 
of Npeak.) While the fall-offs in <MK> for N ,$I10 and 
N 2120, indicated by the dashed lines in the figure, 
most plausible to us. Suppose that at moderately high 
spins (5212) in the transitional-shape nuclei with 110 
5 N A  120, the y-cascades proceed at least partly 
through strongly coupled (deformation-aligned) 
rotational bands built upon mildly deformed (probably, are not very well established by measurements to date, 
the existence of two separate plateaus (for even-Z and 
for odd-Z compound nuclei) of high and remarkably 
though not necessarily, oblate), high-K, few-neutron 
quasiparticle intrinsic states. Adjacent levels within 
constant multiplicity in the intermediate-N region such a band differ by one unit of spin, and can be 
seems clear. We intend to carry out new measurements connected by either M1 or E2 transitions of energy 
shortly to fill in gaps in Fig. 2, in order to 
establish whether the variation of <MK> with N in this 
mass region really follows the simple "universal 
where is the moment of inertia f6r rotations about an 
axis orthogonal to the symmetry axis. Assuming axial 
and R-symmetry for the nucleus, the ratio of leading- 
order collective MI-to-E2 transition probabilities is 
given by4) 
B(M1; KIi+K12) 
0 
B(E2; KIl+K12) 
The B(Ml)/B(E2) ratios for transitions within such 
bands would thus be enhanced by the postulated large 
values of K (easily accessible because of the 
availability of high-j particle orbitals) and small 
quadrupole moments QO (characteristic of the 
transitional nuclei). In addition, the microscopic 
configuration would have to be appropriate to yield a 
significant difference between the intrinsic (gK) and 
collective rotation (gR) g-factors. 
Despite the mild deformation, the moments of 
ineftia for such heavy nuclei are relatively large, and 
the rotational transition energies correspondingly 
small enough to give large K-shell conversion 
probabilities. The unit difference observed in <MK> for 
even-Z vs. odd-Z compound nuclei might be attributed to 
similar bands at lower spin built, for example, upon an 
hg/2-particle state for the unpaired proton (as are 
known to exist in the relevant odd T1 isotopes, see 
refs.5,6). A significant decrease in <MK) would be 
expected for NL110, as we enter the strongly deformed 
region, where yrast cascades are dominated by 
collective E2 transitions, and as we approach spherical 
symmetry at the N-126 shell closure, where collective 
rotational bands should disappear. 
Detailed verification of this speculation 
throughout this transitional region would require a 
very extensive program of y-ray spectroscopy, which we 
are not proposing. We have already performed a y-y 
coincidence experiment aimed primarily at investigating 
the presently unknown level scheme in lg8pb at J > 12, 
where the observed X-rays originate. The results of 
that experiment are still being analyzed. It is our 
hope that by combining such detailed data for a 
selected case with more extensive (and easily acquired) 
~yStematiC8 of <MK> vs. Npeakr we can provide 
sufficient evidence to support or reject the above 
nuclear structure scenario. 
*permanent address : Institute for Nuclear Research, 
Swierk, Poland 
1) S.E. Vigdor, lectures at the XIII-th Masurian Summer 
School on Nuclear Physics, Mikolajki, Poland, Sept. 
1-11, 1980, to be published in Nukleonika. 
2) H.J. Karwowski et al., Phys. Rev. Lett. 42, 1732 
(1979). 
3) J. Kropp et al., 2. Physik A280, 61 (1977). 
4) A. Bohr and B.R. Mottelson, Nuclear Structure, Vol. 
I1 (W.A. Benjamin, Reading, Massachusetts, 1975). 
5 )  F.S. Stephens, in "Elementary Modes of Excitation in 
Nuclei," ed. A. Bohr and R.A. Broglia 
(North-Holland, Amsterdam, 1977), p. 172. 
6) A.J. Kreiner et al., Nucl. Phys. A308, 147 (1978). 


Systematics of the K X-Ray Multiplicity for (Li,xn) Products with 180 < A < 210

https://scholarworks.iu.edu/dspace/bitstream/2022/2831/1/iucf_1980_108-110.pdf

Systematics of the K X-Ray Multiplicity for (Li,xn) Products with 180 < A < 210

Abstract

Similar works

Full text

Available Versions

IUScholarWorks