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Energy levels of 56Co studied by the 58Ni(d , α)56Co reaction near 0°
Nuclear Physics @ North-Holland
A425 (1984) 41 l-422 Publishing Company
STUDIED RENCHENG
Tandem Accelerator
ENERGY LEVELS OF %Co BY THE “Ni(& 01)‘~Co REACTION SHANG*,
Laboratory,
A.A. PILT, M.C. VETTERLI, and J.A. KUEHNER MeMaster Received
University, 6 February
Hamilton,
NEAR
0’
A.J. TRUDEL
Ontario, Canadu L&S 4K1
1984
Abstract: Tensor analyzing powers, Tr,,, of outgoing a-particles in the ‘sNi(d, a)%o reaction at detection angles near 0” have been measured for excited states in ‘%Zo for beam energies of 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. Thirty-seven spin-parity combinations for 96Co excited states have been deduced. Previous J” ambiguities for 11 of these states have been eliminated, and results in conflict with existing assignments for the levels at 3.235 and 3.378 MeV have been obtained. A search for O+ states was carried out from angular distribution measurements at forward angles of the unpolarized (d, a) reaction. The combined results from this and previous experiments were found to be in reasonable agreement with calculated level schemes.
E
NUCLEAR REACTIONS 4”); ‘*Ni(d, a)?~,
58Ni(polarized d, IX)‘~CO, E = 6.75-9.5 E = 7.0 MeV; measured o(E,, e). %o
MeV: measured T_,a(0, = levels deduced J, rr.
1. Introduction The %o nucleus, which is close to the doubly magic 56Ni, is important for shell-model considerations. It cannot be reached via single-particle transfer reactions; thus, two-particle transfer plays an important role in the study of 56Co. The ‘*Ni(d, r~)~%o and 56Fe(31-Ie, p)‘%Zo reactions have been extensively studied r4). Essentially, these reaction studies made use of the predominantly direct process of the reactions at high energies. Angular distributions of the outgoing particles were fitted to DWBA predictions and L-values thereby extracted. Several spin and parity assignments or limits were made to ‘%Zo excited states when the results were combined with earlier studies of P-decay and electron capture 5-7). Few studies of the (p, 31-Ie) reaction have been undertaken “*‘*) due to the very negative Q-value. Some other studies of %o have been primarily devoted to the search for high-spin states by utilizing the selectivity of high-energy (q d) and (d, a) reactions 28729*3’) or to the isovector deformation parameter from charge exchange 30). A number of y-ray studies yielded lifetimes and y-branching ratios of 56Co bound states 32-37). Of these, the most pertinent to the present work was the determination of spins of bound states using particle-y correlations 36*37). * Permanent address:
Tsinghua
University,
Beijing, 411
People’s
Republic
of China,
412
R. Shang et al. / 56Co
A number of calculations of the 56Co levels have also been done *-I’). McGrory’s calculation lo) included lpl h and 2p2h configurations and is in good agreement with the experimental results below 3 MeV in excitation. Nevertheless, some discrepancies remain. For instance, a pair of natural parity states at 2.78 MeV (2’) and 2.87 MeV (6+) was predicted, while previous experiments found only a single weakly excited state at 2.79 MeV. Such a pair of natural-parity states was found in the present work. Goode and Zamick 13) suggested the existence of considerable configuration mixing in “%o excited states from the abnormally slow K-capture decay from the 56Ni ground state to the lowest l+ states in %o. The inclusion of 2p2h configurations led to the prediction of eight l+ states in the region between 1.72 and 3.8 MeV in excitation. Prior to the present work, there was little solid evidence for these 1’ states. Three O+ states, at 1.45,3.5 1 and 3.59 MeV, were studied using the 56Fe(3He, p)?Jo reaction 15-‘*). The 3.59 MeV state was identified as the isobaric analogue (IAS) of the 56Fe ground state. It is predominantly T = 2, but there is some mixing from the mainly T = 1 O+ state at 3.5 1 MeV. The (‘He, p) work also led to the suggestion that the 1.45 MeV state was the anti-analogue (AAS). At higher energies, where the reaction is primarly direct, Oi states are not expected to be populated in the (d, a) reaction. In the present work, beam energies near the Coulomb barrier were selected, where the reaction mechanism is expected to be predominantly compound nuclear. In this case, one expects the AT =0 selection rule to operate but, in addition, the anti-analogue state should not be seen *‘). In this experiment, the tensor analyzing powers, T20, of the outgoing o-particles from the ‘“Ni(d, (Y)%o reaction were measured near 0” and natural or unnatural parity for the excited states was deduced following the method of Kuehner et al. 19). Because 0’ states have no yield at 0” in such an experiment, angular distributions of outgoing a-particles from the unpolarized (d, CX)reaction were also measured to identify possible O+ states.
2. Experimental
procedure
A polarized deuteron beam was obtained from the McMaster University Lambshift polarized ion source and accelerated through the FN tandem accelerator. The cY-particles from the (d, (Y)reaction were momentum analyzed with an Enge split-pole magnetic spectrograph and detected with a resistive-wire gas proportional counter which is composed of two electrically independent counters within the same cavity mounted on the focal plane. Measurements were taken at 4” (lab) at beam energies 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. An aluminum foil of suitable thickness was placed between the two counters, sufficiently thick to absorb the a-particles but to pass the multiply scattered deuterons. Operation of the second counter in anti-coincidence was used to suppress the background from scattered deuterons.
R. Shnng et al. / “Co
413
Targets of enriched (-99%) 58Ni were vacuum evaporated to a thickness of approximately 30 pg/cm2 onto a 30 pg/cm2 carbon backing. A resolution of 1520 keV was obtained for the a-spectra observed. For each incident beam energy, alternate runs were carried out for deuterons polarized predominantly in the I??= 0 and rn = 1 substates, respectively, where the quantization axis is along the beam direction. The fractional polarization of the beam was 0.70* 0.10, which was measured both by the quench-ratio method and by using analyzing powers of some well-known natural-parity states in ‘?Zo as calibrants. An unpolarized deuteron beam of 7.0 MeV was used to measure angular distributions at small angles (5O-40”) to search for O+ states and other states which have no yield at 0” as well as to reveal contaminant peaks.
3. Analysis and results For the 58Ni(d, CU)~‘COreaction, more than 40 a-particle groups to different excited states in 56Co were observed. Fig. 1 shows a pair of spectra at a beam energy of 7.0 MeV and for deuteron substates m = 0 and m = 1, respectively. The peaks from the (d, a) reaction on I60 and 13C are also shown. The broad peak from 160(d, cr)14N obscures several 56Co peaks and makes the identification of levels above 3.71 MeV excitation very difficult. The excitation energies of only a few strongly excited states above 4 MeV are tentatively indicated. The a-peak from 13C(d, (Y)“B was located near the position of the 2.059 MeV state in 56Co. This was perceived from the shift of that peak relative to the other peaks with change in angle. Except for these two contaminant peaks, all the others are from the ‘“Ni(& (Y)~~CO reaction. From the spectra shown, it is clear that the peak areas of the natural-parity states in the M = 1 spectrum are obviously larger than the corresponding peaks in the m = 0 spectrum. Spin-parity assignments for 37 states were deduced from the tensor analyzing powers T,, measured at several different beam energies. The spin-parity assignments and excitation energies are tabulated in table 1, and the tensor analyzing powers, T20, of a number of states selected from table 1 are shown for the various beam energies in fig. 2. T20 values close to 0.7 indicate natural parity if this holds true for all beam energies, whilst the 7’20’sof unnatural-parity states could lie anywhere between - 1.4 and +0.7 [refs. “‘-“)I. The previous ambiguous assignments, ‘3’2*‘8720)of 11 states have been removed leading to the following new definite assignments: 1.922 MeV, 3+; 2.222 MeV, 2+; 2.357 MeV, I+; 2.610 MeV, 3’; 2.636 MeV, l+; 2.663 MeV, 3’; 2.725 MeV, l+; 2.969 MeV, 2+; 3.060 MeV, 5+; 3.140 MeV, 3+ and 3.297 MeV, 4+. The present work is in agreement with previous conclusions for all the low-lying states below 1.720 MeV. For the other levels, only natural- or unnatural-parity classification, or possible spin-parity assignments, can be given because of limited previous results available. Tentative assignments were made to states at 2.301 (2’) and 2.469 (4+).
R. Shang et al. ,t sdCo
.... 1’. it I’
I’;
73NNM3
83d SLN002
R. Shang ‘.720
1.922
-r-
2 ‘222
1
YPi
v
I:
11
_____
2.725
$5
+I
et al. j “Co
$I
t
‘.926
3.140
ik
-AL----
1
tt
1 ___--
1
---_
_ ----
1
t
1 1II P
i
.----
____-‘.
i
Fig. 2. Tensor analyzing powers, T,,, of 12 selected excited states in %o., For each state, T,, is presented (left to right) for the beam energies 6.75, 7.0, 7.5, 9.0 and 9.5 MeV, respectively. The limits of natural parity (T,, = 0.707) and i” = O- ( T2,, = - 1.414) are indicated by the dashed lines. The excitation energies and deduced parities (N or U denoting natural or unnatural parity, respectively) are also given.
Furthermore, the present results (natural parity) are in disagreement J” = I‘ assignments to states at 3.235 and 3.378 MeV (see table 1).
with previous
4. Discussion 4.1. COMPARISON
WITH
THEORETICAL
PREDICTIONS
The early lplh calculation by Vervier’) low-lying states, but was unable to predict the many l+ states. McGrory lo) included
was in good agreement with the known the experimentally observed O+, 7’ and 2p2h configurations in his calculation,
and this refinement led to much better agreement up to 3 MeV. The results of the present work are shown together with McGrory’s predictions in fig. 3. The agreement between theory and experiment is much improved, with many predicted states now having experimental counterparts. However, it still appears that there are more observed than predicted 1’ states in the region of interest. As mentioned in the introduction, levels corresponding to the predicted states at 2.78 and 2.87 MeV had not been found; only one level had been seen near this excitation. In the present work, a new, weakly populated level was observed at 2.77 MeV. The tensor analyzing powers of this state are close to 0.7, but with very large uncertainties; thus suggesting (but not conclusively) natural parity. The unpolarized beam angular distribution measurement, shown in fig. 4, indicated a cross section falling to zero at forward angles. This is not inconsistent with a O+ assignment, but a definite spin-parity assignment cannot be made owing to its relative weakness and high background upon which it rests.
R. Shang et al. / 56Co
416
TABLE
Excitation
energies
and spin-parity
J”
E, (MeV) previous “) a )
:+ 2+ 5+ 3+ 0+ 1+
I+, 2+, 3+ 7+
2.225 2.283 2.305 2.357 2.469 2.609 2.636 2.647 2.665 2.730 2.773 2.791 2.926 2.973 3.048 3.061 3.074 3.140 3.178 3.235 3.257 3.295 3.363 3.378 3.431 3.489
other
+
0.576 0.830 0.970 1.009 1.114 1.451 1.720 I .930
(3+, 4+) ‘) 3+(2+) ‘) 3+(2+) ‘)
(3,4,5)+
‘) “) “)
(I,&
3+ “)
‘) 1+
4+(5+) ‘) 2+, 3+ ‘) 3+(2+) ‘)
(1,2,3)+ ‘) ‘)
(I+)‘) (4,5)+ ‘)
‘)
“) 3.54 P)
I”)
7’0)
“) Ref. 12) except where noted. b, Uncertainties in excitation energies ‘) A denotes agreement with previous
1
combinations
are *5 keV. unambiguous
E, WW present work ‘)
for “Co
excited
states
Spin-parity determination in this work
New spin and parity limits
0.577 0.832 0.967 1.008 1.109 1.450 1.720 1.922
U N N U U o+ *) U U
A’)
2.222 2.281 g*h) 2.301 a) 2.357 g, 2.469 2.610 2.636
N
2+ A
L) U
(2+) 1+
(N) ‘) U U
(4+) 3+ 1+
2.663 2.725
U U
3+ 1+
2.770 2.789 2.926 2.969 3.048 3.060 3.077 3.140 3.180 3.234 3.255 3.297 3.366 3.382 3.436 3.493 3.521 3.544 3.610 3.717
O+“)orNm)
assignment:
A A A A A A 3+
N “) N N U U U U U N N N U N U U N U U U
D denotes
(2+) “) 2+ 5’ 1+,3+ 3+ 1+,3+ D 4+ D;2+ A
AY
disagreement.
R. Shang et al. / 56Co 4.2. CONCERNING
417
I+ STATES
Goode and Zamick 13) predicted seven J” = l+ states below the “giant” spinisospin state at 3.8 MeV on the basis of their 2p2h calculation. In the present work, six 1+ states were confirmed at 1.72,2.36,2.63,3.08 and 3.43 MeV. In ref. 14)additional If states were found at 3.38, 3.52 and 4.39 MeV. However, the present work has demonstrated that the 3.38 and 3.52 MeV levels have natural parity. Combining this result with the L = 2 assignment from the two-particle transfer experiments 2*“) we obtain J” (3.38) = 2’. The 4.39 MeV state was not observed in the present work.
4.3. CONCERNING
0+ STATES
at 1.45, 3.51 There are three suspected O+ states in 56Co below 4 MeV excitation, and 3.59 MeV. The two states at 3.51 and 3.59 MeV are predominantly T = 1 and ‘5,‘6), but are isospin-mixed to a considerable amount (0.3 in T = 2, respectively intensity). These two states were obscured by other, stronger, states in the present work. The 1.45 MeV state is believed to be the anti-analogue ‘*2,“) predicted by McGrory lo) to lie near 2 MeV. The unpolarized-beam angular distribution measurements (fig. 4) shows little or no yield at forward angles, consistent with a 0’ level. Different wave functions have been suggested for this anti-analogue state. Laget et al. 2, assumed $(T = 1) = J0.75[(~f;f,)o,(~p,,zvp,,,)o,lo,
-J0.25[(~f;f,)o,(~p,,,~p,,,)o,lo,
,
(Cr(T= 1) = ~0.60[(7l.f;:,)o,(~rrP3/2~p~,~)o~lo~
-~0.40[(~f;f,>o,(~lrP5/2YPS/Z)01101
,
or
“) The decreasing forward-angle cross section in the (d, (x) reaction is consistent with the O+ assignment (see text and fig. 4). ‘) Ref. ‘s). ‘) Refs. 36*37). g, T,, obtained from only one beam energy. This is normally sufficient to exclude natural parity if T,, f 0.7 1, but is not enough to allow a definite N-assignment to be made if T,, - 0.7 I. h, Probably a multiplet. ‘) Ref. ‘). ‘) T,,= 1.16kO.25 at the one energy at which this state was observed. This is consistent with natural parity, but see footnote a). ‘) Data from four energies strongly suggests natural parity although unnatural parity cannot be strictly ruled out. ‘) Ref. 30). “) The T,,of this partially resolved doublet is near the natural parity limit, Tzo= 0.7 1,at all energies studied, strongly suggesting an N-assignment to both states. “) Partially resolved doublet (see text). “) The high level density in 56Co above 3.5 MeV excitation makes a definite correspondence between previous and present work very tenuous. p, Refs. 29,3’). q, Assuming that the same state is observed in the two (d, (Y) reactions, as appears reasonable.
et al. / 56Co
R. Shang
418
3234 3180 314030603~B 3.047 29692.9.%-
tt33 7;
6+-
3.70
4+-
3./6
-$r%~
301 299
2+ 2+
?87
%k%26632725 2 647-----2636-
-
2.6/O
-
?78 ?72 2+-
I+
264
3*-.
2+
222-
2&59-
s:--
123
o+71922P
3 >
/720-
/+
/ /+-
I 68
5+-
/38
3+-
1. 16
?i
/
o+
/450-
1109
-
lOas_ 0967-
3 5’ 2+/
2+-
099 0.92
0.832
-
4/4+
0.64
0./53-
3+-----
cw-
4+-
3+-
023
4+
0.0
PRESENT WORK TliEORYfM&RORY~ Fig. 3. Comparison of the energy lqvels of ?o as obtained from a synthesis of previous and the present work with those predicted by McGrory I”). The ground state and first excited state of 56Co were beyond the range of the detector in this experiment, and some of the states indicated were not observed due to their low cross section.
R. Shang et al. / 56Co
I45 o 2.77 l
E=7.0
MeV
100 Fig. 4. Angular
AN&LAB)
419
MeV MeV
3oo
<;d
4c
distributions of the differential cross sections for the 1.45 and 2.77 MeV levels in %o. the solid curves are drawn to guide the eye. For details, see text.
where subscripts 01, etc. refer to J and T, respectively. They invoked this kind of wave function to explain the experimental ratio between the (3He, p) cross sections to the IAS and AAS. Belote et al. 3, assumed [(f;f2)o,(p:,z)o,]o,, which allowed the state to be strongly excited by the transfer of an L = 0 pair with S = 0, T = 1 into the ZP,,~ orbitals outside the 54Fe core in the (3He, p) reaction. We have recently suggested 25) that anti-analogue states in odd-odd nuclei can, in many cases, not be populated in a (d, a) reaction at any angle, as exemplified by the 42Ca(d, (w)~‘K reaction. Model wave functions for the IAS and AAS in “Co of the form
where p represents one of p3,*, fs12, or p,/*, as suggested by Bruge and Leonard ‘I), would give rise to this selection rule. In the present experiment, however, the 1.45 MeV state is observed, albeit quite weakly, at angles beyond 10”. The most likely explanation of the non-zero cross section is produced by the assumption of
420
more complicated
R. Shang
configurations,
particularly
function. In fact, the 56Ni ground admixtures; thus, 4p2h contributions
4.4. DISCUSSION
OF INDIVIDUAL
et al. / “Co
4p2h components,
state is known 13) to have to %o are not unlikely.
LEVELS
for the AAS wave appreciable
2p2h
IN %2~
@,I 1.9Z.?MeV;3’. This state was measured by the t3He, p) [refs. 2*3)], (d, CY) [ref. ‘)] and (p, 3He) [ref. “)I reactions to have an L=2 shape of the angular distribution. The c(d, a)/a(p, 3He) ratio preferred 3’, but 2+ is not definitely excluded. Angular distribution calculations using McGrory’s wave functions suggested the three states at 1.922,2.059 and 2.222 MeV to be 3+, 2’ and 2*, respectively. In this work, the 1.922 and 2.222 MeV states have been assigned 3+ and 2’, respectively. The 2.059 MeV state cannot be analyzed because of the interference of the strong a-peak from the 13C(d, LY)“B reaction. (b) 2.281 MeV, 7+. This state is very strong in the (d, a) reaction. Previous ‘9”) (p, 3He) and (d, (u) measurements both showed L = 6 angular distributions. Since the (d, o) reaction particularly favours stretched configurations such as (f,,2);2,, the 7’ assignment seems most probable. The unnatural-parity assignment in the present work and the high-spin measurement ‘“) both confirm the 7’ assignment. c’c) 2.301 MeV, (2’). This level was not resolved from the 2.281 MeV state in previous (d, cu), (p, 3He) and t3He, t) experiments. The (3He, p) reaction in ref. ‘) resolved these two levels and observed an L=2 shape. The present measurement still could not resolve these two levels very well. A tentative N-assignment can be given on the basis of a two-component gaussian fit. (d) 2.357 Me V, 1 +. Previously measured ‘) angular distributions in the (d, a) reac-
tion showed an apparent L = 0 shape, which unnatural-parity assignment in this work is The 2.371 MeV level was not resolved from weak near 0” at all beam energies, while the
led to a l+ assignment. The confident in agreement with the ‘l+ conclusion. the 2.357 MeV level But it was very corresponding 2.357 MeV level is very
strong. Hence, the unresolved 2.37 1 MeV state should have no effect on the unnaturalparity assignment for the 2.357 MeV state. (e) 2.469 MeV, (#+). The previous measurements of this level were in conflict. The (3He, p) reaction ‘) favoured L=4, while the (d, CX) angular distribution ‘) suggested either L=4 or L= 3. But the (p, 3He) reaction ‘I) gave L= 0. It was suggested “q’2) that this “state” in fact is a doublet containing an additional J” = l+ state near 2.47 MeV. In the present (d, a) spectra, the peak width does not very obviously show whether a singlet or doublet is to be preferred. But the T&, measurements appeared contradictory. The result from four beam energies showed naturalparity characteristics, while at another energy, the opposite result obtained. It may be that at the first four energies the 2.469 MeV (4+) state is dominant, while at the fifth energy, the nearby unnatural-parity state (1’) is dominant. The 2.469 MeV state can be considered to correspond to the 2.43 MeV 4+ state in McGrory’s calculation.
R. Shang et al. / 56Co (f)
2.610, 2.636 and 2.663 MeV. The three
with the level at 2.641 MeV unobserved
421
states are members
in this experiment.
of a quadruplet,
The 2.610 and 2.663 MeV
states were measured in previous works ‘) to have L = 2 angular distributions in the (d, a) reaction. The unnatural-parity assignment from the present experiment leads to J” = 3+. (g) 2.770 and 2.789 MeK Both components of this barely resolved doublet exhibited T2,, values near 0.7 at all angles studied; thus a N-parity assignment follows for both states. The 2.77 MeV state is, however, quite weakly populated. The angular distribution of the unpolarized (d, (Y) reaction showed the fall-off in intensity at forward angles characteristic of a J” = 0’ state. Such a 0’ state, however, would be surprising since it would not appear to have any theoretical counterpart. Nevertheless the J” O+ assignment cannot be ruled out. We thus conclude J” (2.277 MeV) = O+ or N and J” (2.789) = N. (h) 2.926 and 2.969 MeV. A state at 2.963 MeV was determined to have an L = 2 angular distribution in the (3He, p) reaction *). In fact, that measurement could not resolve that state from the 2.93 MeV level, which was also suggested to have an L = 2 shape ‘). The (3He, t) charge-exchange reaction ‘s) also gives L= 2 for the 2.97 MeV level. Combining these results with the natural-parity assignments from the present work leads to J” = 2+ assignments for at least the 2.969 MeV level and probably the 2.93 MeV level as well. (i) 3.048, 3.060 and 3.077 MeV. Very few previous results are available for the 3.048 MeV state. The 3.060 MeV state was measured to have an L = 4 shape in the (d, a) reaction. Combining with other results, spins of 4+ and 5+ were assigned in ref. ‘). Our unnatural-parity assignment selects 5’ for the state. The unnatural-parity assignment for the 3.077 MeV state is in agreement with the J” = l+ from ref. 12). These two states possibly correspond to predicted 5+ and l+ states at 3.01 and 2.99 MeV, respectively. (j) 3.140 MeV. On combining results from the (3He, t) and (d, (w) reaction, ref. ‘) gave 3+(2+) to this state. Our unnatural-parity assignment implies 3+. (k) 3.234 and 3.255 MeVstates. In many experiments, these two states were not separated. Only in ref. ‘) was a possible (I+) assignment given to the 3.234 MeV state from a plausible L = 0 transfer in the (d, (Y) reaction. The natural-parity assignments for these two states from the present work are in conflict with such a conclusion. (J) 3.384 MeV state. The determination of natural parity in the present work is in conflict with the l+ assignment given in ref. I*). The previous (p, ‘He) and (d, (Y) observation showed L = 2 angular distribution; combining with the T,, measurement, J” = 2+ is clearly indicated. 5. Conclusion The present T2,, measurements have led to model-independent spin-parity assignments to 37 of the excited states in 56Co. Several previous uncertain J” assignments
422
R. Shang et af. / 56Co
have been resolved. The good agreement up to 3 MeV excitation of McGrory’s, and Goode and Zamick’s theoretical prediction with the experimental results is encouraging. We thank J.W. McKay for the operation of the polarized ion source and Dr. Y. Peng for fabricating the targets. Dr. J. McGrory is especially thanked for providing us with the results of his unpublished calculations for 56Co. References I) 2) 3) 4) 5) 6) 7) 8) 9) IO) I I) 12) 13) 14) IS) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37)
M.J. Schneider and W.W. Daehnick, Phys. Rev. C4 (1971) 1649 J.M. Laget and J. Gastebois, Nucl. Phys. Al22 (1968) 431 J.A. Belote, W.E. Dorenbusch and I. Rapaport, Nucl. Phys. A109 (1968) 666 T. Caldwell, 0. Nathan, 0. Hansen and H. Bork, Nucl. Phys. A202 (1973) 225 D.O. Wells, S.L. BIatt and W.E. Meyerhof, Phys. Rev. I30 (1963) 1961 R.C. Jenkins and W.E. Meyerhof, NucI. Phys. 58 (1964) 417 H. Ohnuma, Y. Hashimoto and I. Tomita, Nucl. Phys. 66 (1965) 337 D.O. Wells, Nucl. Phys. 66 (1965) 562 J. Vervier, Nucl. Phys. 78 (1966) 497 J.B. McGrory, private communication G. Bruge and R.F. Leonard, Phys. Rev. C2 (1970) 2200 R.L. Auble, NucI. Data Sheets 20 (1977) 253 P. Goode and L. Zamick, Phys. Rev. Lett. 22 (1969) 958 M. Beckerman, Phys. Rev. Cl4 (1976) 1648 T.G. Dzubay, R. Sherr, F. Becchetti and D. Dehnhard, Nucl. Phys. A142 (1970) 488 D.P. Balamuth and E.G. Adefberger, Phys. Rev. Cl6 (1971) 928 L.D. Rickertsen, M.J. Schneider, J.J. Kraushaar, W.R. Zimmerman and H. Rudolph, Phys. Lett. 6OB (1975) I9 W.R. Zimmerman et al., Phys. Rev. Cl6 (1977) 2432 J.A. Kuehner, P.W. Green, G.D. Jones and D.T. Petty, Phys. Rev. Lett. 35 (1975) 423 CM. Lederer and V.S. Shirley, ed., Table of isotopes, 7th ed. (Wiley, New York, 1978) p. 162 ff D.T. Petty et al., Phys. Rev. Cl4 (1976) 12 D.T. Petty, thesis, McMaster University (1976), unpublished D.T. Petty et al., Phys. Rev. Cl4 (1976) 908 Shang Rencheng, A.A. Pilt, J.A. Kuehner, M.A.M. Shahabuddin and A. Trudel, Nucl. Phys. A366 (1981) 13 A.A. PiIt er al., Phys. Lett. 1OOB (1981) I14 D.G. Sarantites, J. Urbon and L.L. Rutledge, Jr., Phys. Rev. Cl4 (1976) 1412 H.O. Meyer and J.A. Thomson, Phys. Rev. C8 (1973) 1215 K. Okada, J. Kawa and T. Yamagata, Nucl. Phys. A349 (1980) I25 H. Nann er af., Phys. Rev. CZd (1981) 1984 H. Orihara et af., Phys. Lett. B106 (1981) I71 H. Nann et aL, Phys. Lett. 3109 (1982) I75 R. DelVecchio, R.F. Gibson and W.W. Daehnick, Phys. Rev. C5 (1972) 446 L.E. Samuelson er al., Phys. Rev. C7 (1973) 2379 J.H. Barker and D.G. Sarantites, Phys. Rev. Cl0 (1974) 1407 C. Moazed et al., Nucl. Phys. AZ39 (1975) 242 W.D. Kampp and S. Buhi, Z. Phys. A284 (1978) I I7 W.D. Kampp, K.H. Bodemiller, A. Nagel and S. Buhl, Z. Phys. AZ%? (1978) 167
A425 (1984) 41 l-422 Publishing Company
STUDIED RENCHENG
Tandem Accelerator
ENERGY LEVELS OF %Co BY THE “Ni(& 01)‘~Co REACTION SHANG*,
Laboratory,
A.A. PILT, M.C. VETTERLI, and J.A. KUEHNER MeMaster Received
University, 6 February
Hamilton,
NEAR
0’
A.J. TRUDEL
Ontario, Canadu L&S 4K1
1984
Abstract: Tensor analyzing powers, Tr,,, of outgoing a-particles in the ‘sNi(d, a)%o reaction at detection angles near 0” have been measured for excited states in ‘%Zo for beam energies of 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. Thirty-seven spin-parity combinations for 96Co excited states have been deduced. Previous J” ambiguities for 11 of these states have been eliminated, and results in conflict with existing assignments for the levels at 3.235 and 3.378 MeV have been obtained. A search for O+ states was carried out from angular distribution measurements at forward angles of the unpolarized (d, a) reaction. The combined results from this and previous experiments were found to be in reasonable agreement with calculated level schemes.
E
NUCLEAR REACTIONS 4”); ‘*Ni(d, a)?~,
58Ni(polarized d, IX)‘~CO, E = 6.75-9.5 E = 7.0 MeV; measured o(E,, e). %o
MeV: measured T_,a(0, = levels deduced J, rr.
1. Introduction The %o nucleus, which is close to the doubly magic 56Ni, is important for shell-model considerations. It cannot be reached via single-particle transfer reactions; thus, two-particle transfer plays an important role in the study of 56Co. The ‘*Ni(d, r~)~%o and 56Fe(31-Ie, p)‘%Zo reactions have been extensively studied r4). Essentially, these reaction studies made use of the predominantly direct process of the reactions at high energies. Angular distributions of the outgoing particles were fitted to DWBA predictions and L-values thereby extracted. Several spin and parity assignments or limits were made to ‘%Zo excited states when the results were combined with earlier studies of P-decay and electron capture 5-7). Few studies of the (p, 31-Ie) reaction have been undertaken “*‘*) due to the very negative Q-value. Some other studies of %o have been primarily devoted to the search for high-spin states by utilizing the selectivity of high-energy (q d) and (d, a) reactions 28729*3’) or to the isovector deformation parameter from charge exchange 30). A number of y-ray studies yielded lifetimes and y-branching ratios of 56Co bound states 32-37). Of these, the most pertinent to the present work was the determination of spins of bound states using particle-y correlations 36*37). * Permanent address:
Tsinghua
University,
Beijing, 411
People’s
Republic
of China,
412
R. Shang et al. / 56Co
A number of calculations of the 56Co levels have also been done *-I’). McGrory’s calculation lo) included lpl h and 2p2h configurations and is in good agreement with the experimental results below 3 MeV in excitation. Nevertheless, some discrepancies remain. For instance, a pair of natural parity states at 2.78 MeV (2’) and 2.87 MeV (6+) was predicted, while previous experiments found only a single weakly excited state at 2.79 MeV. Such a pair of natural-parity states was found in the present work. Goode and Zamick 13) suggested the existence of considerable configuration mixing in “%o excited states from the abnormally slow K-capture decay from the 56Ni ground state to the lowest l+ states in %o. The inclusion of 2p2h configurations led to the prediction of eight l+ states in the region between 1.72 and 3.8 MeV in excitation. Prior to the present work, there was little solid evidence for these 1’ states. Three O+ states, at 1.45,3.5 1 and 3.59 MeV, were studied using the 56Fe(3He, p)?Jo reaction 15-‘*). The 3.59 MeV state was identified as the isobaric analogue (IAS) of the 56Fe ground state. It is predominantly T = 2, but there is some mixing from the mainly T = 1 O+ state at 3.5 1 MeV. The (‘He, p) work also led to the suggestion that the 1.45 MeV state was the anti-analogue (AAS). At higher energies, where the reaction is primarly direct, Oi states are not expected to be populated in the (d, a) reaction. In the present work, beam energies near the Coulomb barrier were selected, where the reaction mechanism is expected to be predominantly compound nuclear. In this case, one expects the AT =0 selection rule to operate but, in addition, the anti-analogue state should not be seen *‘). In this experiment, the tensor analyzing powers, T20, of the outgoing o-particles from the ‘“Ni(d, (Y)%o reaction were measured near 0” and natural or unnatural parity for the excited states was deduced following the method of Kuehner et al. 19). Because 0’ states have no yield at 0” in such an experiment, angular distributions of outgoing a-particles from the unpolarized (d, CX)reaction were also measured to identify possible O+ states.
2. Experimental
procedure
A polarized deuteron beam was obtained from the McMaster University Lambshift polarized ion source and accelerated through the FN tandem accelerator. The cY-particles from the (d, (Y)reaction were momentum analyzed with an Enge split-pole magnetic spectrograph and detected with a resistive-wire gas proportional counter which is composed of two electrically independent counters within the same cavity mounted on the focal plane. Measurements were taken at 4” (lab) at beam energies 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. An aluminum foil of suitable thickness was placed between the two counters, sufficiently thick to absorb the a-particles but to pass the multiply scattered deuterons. Operation of the second counter in anti-coincidence was used to suppress the background from scattered deuterons.
R. Shnng et al. / “Co
413
Targets of enriched (-99%) 58Ni were vacuum evaporated to a thickness of approximately 30 pg/cm2 onto a 30 pg/cm2 carbon backing. A resolution of 1520 keV was obtained for the a-spectra observed. For each incident beam energy, alternate runs were carried out for deuterons polarized predominantly in the I??= 0 and rn = 1 substates, respectively, where the quantization axis is along the beam direction. The fractional polarization of the beam was 0.70* 0.10, which was measured both by the quench-ratio method and by using analyzing powers of some well-known natural-parity states in ‘?Zo as calibrants. An unpolarized deuteron beam of 7.0 MeV was used to measure angular distributions at small angles (5O-40”) to search for O+ states and other states which have no yield at 0” as well as to reveal contaminant peaks.
3. Analysis and results For the 58Ni(d, CU)~‘COreaction, more than 40 a-particle groups to different excited states in 56Co were observed. Fig. 1 shows a pair of spectra at a beam energy of 7.0 MeV and for deuteron substates m = 0 and m = 1, respectively. The peaks from the (d, a) reaction on I60 and 13C are also shown. The broad peak from 160(d, cr)14N obscures several 56Co peaks and makes the identification of levels above 3.71 MeV excitation very difficult. The excitation energies of only a few strongly excited states above 4 MeV are tentatively indicated. The a-peak from 13C(d, (Y)“B was located near the position of the 2.059 MeV state in 56Co. This was perceived from the shift of that peak relative to the other peaks with change in angle. Except for these two contaminant peaks, all the others are from the ‘“Ni(& (Y)~~CO reaction. From the spectra shown, it is clear that the peak areas of the natural-parity states in the M = 1 spectrum are obviously larger than the corresponding peaks in the m = 0 spectrum. Spin-parity assignments for 37 states were deduced from the tensor analyzing powers T,, measured at several different beam energies. The spin-parity assignments and excitation energies are tabulated in table 1, and the tensor analyzing powers, T20, of a number of states selected from table 1 are shown for the various beam energies in fig. 2. T20 values close to 0.7 indicate natural parity if this holds true for all beam energies, whilst the 7’20’sof unnatural-parity states could lie anywhere between - 1.4 and +0.7 [refs. “‘-“)I. The previous ambiguous assignments, ‘3’2*‘8720)of 11 states have been removed leading to the following new definite assignments: 1.922 MeV, 3+; 2.222 MeV, 2+; 2.357 MeV, I+; 2.610 MeV, 3’; 2.636 MeV, l+; 2.663 MeV, 3’; 2.725 MeV, l+; 2.969 MeV, 2+; 3.060 MeV, 5+; 3.140 MeV, 3+ and 3.297 MeV, 4+. The present work is in agreement with previous conclusions for all the low-lying states below 1.720 MeV. For the other levels, only natural- or unnatural-parity classification, or possible spin-parity assignments, can be given because of limited previous results available. Tentative assignments were made to states at 2.301 (2’) and 2.469 (4+).
R. Shang et al. ,t sdCo
.... 1’. it I’
I’;
73NNM3
83d SLN002
R. Shang ‘.720
1.922
-r-
2 ‘222
1
YPi
v
I:
11
_____
2.725
$5
+I
et al. j “Co
$I
t
‘.926
3.140
ik
-AL----
1
tt
1 ___--
1
---_
_ ----
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i
Fig. 2. Tensor analyzing powers, T,,, of 12 selected excited states in %o., For each state, T,, is presented (left to right) for the beam energies 6.75, 7.0, 7.5, 9.0 and 9.5 MeV, respectively. The limits of natural parity (T,, = 0.707) and i” = O- ( T2,, = - 1.414) are indicated by the dashed lines. The excitation energies and deduced parities (N or U denoting natural or unnatural parity, respectively) are also given.
Furthermore, the present results (natural parity) are in disagreement J” = I‘ assignments to states at 3.235 and 3.378 MeV (see table 1).
with previous
4. Discussion 4.1. COMPARISON
WITH
THEORETICAL
PREDICTIONS
The early lplh calculation by Vervier’) low-lying states, but was unable to predict the many l+ states. McGrory lo) included
was in good agreement with the known the experimentally observed O+, 7’ and 2p2h configurations in his calculation,
and this refinement led to much better agreement up to 3 MeV. The results of the present work are shown together with McGrory’s predictions in fig. 3. The agreement between theory and experiment is much improved, with many predicted states now having experimental counterparts. However, it still appears that there are more observed than predicted 1’ states in the region of interest. As mentioned in the introduction, levels corresponding to the predicted states at 2.78 and 2.87 MeV had not been found; only one level had been seen near this excitation. In the present work, a new, weakly populated level was observed at 2.77 MeV. The tensor analyzing powers of this state are close to 0.7, but with very large uncertainties; thus suggesting (but not conclusively) natural parity. The unpolarized beam angular distribution measurement, shown in fig. 4, indicated a cross section falling to zero at forward angles. This is not inconsistent with a O+ assignment, but a definite spin-parity assignment cannot be made owing to its relative weakness and high background upon which it rests.
R. Shang et al. / 56Co
416
TABLE
Excitation
energies
and spin-parity
J”
E, (MeV) previous “) a )
:+ 2+ 5+ 3+ 0+ 1+
I+, 2+, 3+ 7+
2.225 2.283 2.305 2.357 2.469 2.609 2.636 2.647 2.665 2.730 2.773 2.791 2.926 2.973 3.048 3.061 3.074 3.140 3.178 3.235 3.257 3.295 3.363 3.378 3.431 3.489
other
+
0.576 0.830 0.970 1.009 1.114 1.451 1.720 I .930
(3+, 4+) ‘) 3+(2+) ‘) 3+(2+) ‘)
(3,4,5)+
‘) “) “)
(I,&
3+ “)
‘) 1+
4+(5+) ‘) 2+, 3+ ‘) 3+(2+) ‘)
(1,2,3)+ ‘) ‘)
(I+)‘) (4,5)+ ‘)
‘)
“) 3.54 P)
I”)
7’0)
“) Ref. 12) except where noted. b, Uncertainties in excitation energies ‘) A denotes agreement with previous
1
combinations
are *5 keV. unambiguous
E, WW present work ‘)
for “Co
excited
states
Spin-parity determination in this work
New spin and parity limits
0.577 0.832 0.967 1.008 1.109 1.450 1.720 1.922
U N N U U o+ *) U U
A’)
2.222 2.281 g*h) 2.301 a) 2.357 g, 2.469 2.610 2.636
N
2+ A
L) U
(2+) 1+
(N) ‘) U U
(4+) 3+ 1+
2.663 2.725
U U
3+ 1+
2.770 2.789 2.926 2.969 3.048 3.060 3.077 3.140 3.180 3.234 3.255 3.297 3.366 3.382 3.436 3.493 3.521 3.544 3.610 3.717
O+“)orNm)
assignment:
A A A A A A 3+
N “) N N U U U U U N N N U N U U N U U U
D denotes
(2+) “) 2+ 5’ 1+,3+ 3+ 1+,3+ D 4+ D;2+ A
AY
disagreement.
R. Shang et al. / 56Co 4.2. CONCERNING
417
I+ STATES
Goode and Zamick 13) predicted seven J” = l+ states below the “giant” spinisospin state at 3.8 MeV on the basis of their 2p2h calculation. In the present work, six 1+ states were confirmed at 1.72,2.36,2.63,3.08 and 3.43 MeV. In ref. 14)additional If states were found at 3.38, 3.52 and 4.39 MeV. However, the present work has demonstrated that the 3.38 and 3.52 MeV levels have natural parity. Combining this result with the L = 2 assignment from the two-particle transfer experiments 2*“) we obtain J” (3.38) = 2’. The 4.39 MeV state was not observed in the present work.
4.3. CONCERNING
0+ STATES
at 1.45, 3.51 There are three suspected O+ states in 56Co below 4 MeV excitation, and 3.59 MeV. The two states at 3.51 and 3.59 MeV are predominantly T = 1 and ‘5,‘6), but are isospin-mixed to a considerable amount (0.3 in T = 2, respectively intensity). These two states were obscured by other, stronger, states in the present work. The 1.45 MeV state is believed to be the anti-analogue ‘*2,“) predicted by McGrory lo) to lie near 2 MeV. The unpolarized-beam angular distribution measurements (fig. 4) shows little or no yield at forward angles, consistent with a 0’ level. Different wave functions have been suggested for this anti-analogue state. Laget et al. 2, assumed $(T = 1) = J0.75[(~f;f,)o,(~p,,zvp,,,)o,lo,
-J0.25[(~f;f,)o,(~p,,,~p,,,)o,lo,
,
(Cr(T= 1) = ~0.60[(7l.f;:,)o,(~rrP3/2~p~,~)o~lo~
-~0.40[(~f;f,>o,(~lrP5/2YPS/Z)01101
,
or
“) The decreasing forward-angle cross section in the (d, (x) reaction is consistent with the O+ assignment (see text and fig. 4). ‘) Ref. ‘s). ‘) Refs. 36*37). g, T,, obtained from only one beam energy. This is normally sufficient to exclude natural parity if T,, f 0.7 1, but is not enough to allow a definite N-assignment to be made if T,, - 0.7 I. h, Probably a multiplet. ‘) Ref. ‘). ‘) T,,= 1.16kO.25 at the one energy at which this state was observed. This is consistent with natural parity, but see footnote a). ‘) Data from four energies strongly suggests natural parity although unnatural parity cannot be strictly ruled out. ‘) Ref. 30). “) The T,,of this partially resolved doublet is near the natural parity limit, Tzo= 0.7 1,at all energies studied, strongly suggesting an N-assignment to both states. “) Partially resolved doublet (see text). “) The high level density in 56Co above 3.5 MeV excitation makes a definite correspondence between previous and present work very tenuous. p, Refs. 29,3’). q, Assuming that the same state is observed in the two (d, (Y) reactions, as appears reasonable.
et al. / 56Co
R. Shang
418
3234 3180 314030603~B 3.047 29692.9.%-
tt33 7;
6+-
3.70
4+-
3./6
-$r%~
301 299
2+ 2+
?87
%k%26632725 2 647-----2636-
-
2.6/O
-
?78 ?72 2+-
I+
264
3*-.
2+
222-
2&59-
s:--
123
o+71922P
3 >
/720-
/+
/ /+-
I 68
5+-
/38
3+-
1. 16
?i
/
o+
/450-
1109
-
lOas_ 0967-
3 5’ 2+/
2+-
099 0.92
0.832
-
4/4+
0.64
0./53-
3+-----
cw-
4+-
3+-
023
4+
0.0
PRESENT WORK TliEORYfM&RORY~ Fig. 3. Comparison of the energy lqvels of ?o as obtained from a synthesis of previous and the present work with those predicted by McGrory I”). The ground state and first excited state of 56Co were beyond the range of the detector in this experiment, and some of the states indicated were not observed due to their low cross section.
R. Shang et al. / 56Co
I45 o 2.77 l
E=7.0
MeV
100 Fig. 4. Angular
AN&LAB)
419
MeV MeV
3oo
<;d
4c
distributions of the differential cross sections for the 1.45 and 2.77 MeV levels in %o. the solid curves are drawn to guide the eye. For details, see text.
where subscripts 01, etc. refer to J and T, respectively. They invoked this kind of wave function to explain the experimental ratio between the (3He, p) cross sections to the IAS and AAS. Belote et al. 3, assumed [(f;f2)o,(p:,z)o,]o,, which allowed the state to be strongly excited by the transfer of an L = 0 pair with S = 0, T = 1 into the ZP,,~ orbitals outside the 54Fe core in the (3He, p) reaction. We have recently suggested 25) that anti-analogue states in odd-odd nuclei can, in many cases, not be populated in a (d, a) reaction at any angle, as exemplified by the 42Ca(d, (w)~‘K reaction. Model wave functions for the IAS and AAS in “Co of the form
where p represents one of p3,*, fs12, or p,/*, as suggested by Bruge and Leonard ‘I), would give rise to this selection rule. In the present experiment, however, the 1.45 MeV state is observed, albeit quite weakly, at angles beyond 10”. The most likely explanation of the non-zero cross section is produced by the assumption of
420
more complicated
R. Shang
configurations,
particularly
function. In fact, the 56Ni ground admixtures; thus, 4p2h contributions
4.4. DISCUSSION
OF INDIVIDUAL
et al. / “Co
4p2h components,
state is known 13) to have to %o are not unlikely.
LEVELS
for the AAS wave appreciable
2p2h
IN %2~
@,I 1.9Z.?MeV;3’. This state was measured by the t3He, p) [refs. 2*3)], (d, CY) [ref. ‘)] and (p, 3He) [ref. “)I reactions to have an L=2 shape of the angular distribution. The c(d, a)/a(p, 3He) ratio preferred 3’, but 2+ is not definitely excluded. Angular distribution calculations using McGrory’s wave functions suggested the three states at 1.922,2.059 and 2.222 MeV to be 3+, 2’ and 2*, respectively. In this work, the 1.922 and 2.222 MeV states have been assigned 3+ and 2’, respectively. The 2.059 MeV state cannot be analyzed because of the interference of the strong a-peak from the 13C(d, LY)“B reaction. (b) 2.281 MeV, 7+. This state is very strong in the (d, a) reaction. Previous ‘9”) (p, 3He) and (d, (u) measurements both showed L = 6 angular distributions. Since the (d, o) reaction particularly favours stretched configurations such as (f,,2);2,, the 7’ assignment seems most probable. The unnatural-parity assignment in the present work and the high-spin measurement ‘“) both confirm the 7’ assignment. c’c) 2.301 MeV, (2’). This level was not resolved from the 2.281 MeV state in previous (d, cu), (p, 3He) and t3He, t) experiments. The (3He, p) reaction in ref. ‘) resolved these two levels and observed an L=2 shape. The present measurement still could not resolve these two levels very well. A tentative N-assignment can be given on the basis of a two-component gaussian fit. (d) 2.357 Me V, 1 +. Previously measured ‘) angular distributions in the (d, a) reac-
tion showed an apparent L = 0 shape, which unnatural-parity assignment in this work is The 2.371 MeV level was not resolved from weak near 0” at all beam energies, while the
led to a l+ assignment. The confident in agreement with the ‘l+ conclusion. the 2.357 MeV level But it was very corresponding 2.357 MeV level is very
strong. Hence, the unresolved 2.37 1 MeV state should have no effect on the unnaturalparity assignment for the 2.357 MeV state. (e) 2.469 MeV, (#+). The previous measurements of this level were in conflict. The (3He, p) reaction ‘) favoured L=4, while the (d, CX) angular distribution ‘) suggested either L=4 or L= 3. But the (p, 3He) reaction ‘I) gave L= 0. It was suggested “q’2) that this “state” in fact is a doublet containing an additional J” = l+ state near 2.47 MeV. In the present (d, a) spectra, the peak width does not very obviously show whether a singlet or doublet is to be preferred. But the T&, measurements appeared contradictory. The result from four beam energies showed naturalparity characteristics, while at another energy, the opposite result obtained. It may be that at the first four energies the 2.469 MeV (4+) state is dominant, while at the fifth energy, the nearby unnatural-parity state (1’) is dominant. The 2.469 MeV state can be considered to correspond to the 2.43 MeV 4+ state in McGrory’s calculation.
R. Shang et al. / 56Co (f)
2.610, 2.636 and 2.663 MeV. The three
with the level at 2.641 MeV unobserved
421
states are members
in this experiment.
of a quadruplet,
The 2.610 and 2.663 MeV
states were measured in previous works ‘) to have L = 2 angular distributions in the (d, a) reaction. The unnatural-parity assignment from the present experiment leads to J” = 3+. (g) 2.770 and 2.789 MeK Both components of this barely resolved doublet exhibited T2,, values near 0.7 at all angles studied; thus a N-parity assignment follows for both states. The 2.77 MeV state is, however, quite weakly populated. The angular distribution of the unpolarized (d, (Y) reaction showed the fall-off in intensity at forward angles characteristic of a J” = 0’ state. Such a 0’ state, however, would be surprising since it would not appear to have any theoretical counterpart. Nevertheless the J” O+ assignment cannot be ruled out. We thus conclude J” (2.277 MeV) = O+ or N and J” (2.789) = N. (h) 2.926 and 2.969 MeV. A state at 2.963 MeV was determined to have an L = 2 angular distribution in the (3He, p) reaction *). In fact, that measurement could not resolve that state from the 2.93 MeV level, which was also suggested to have an L = 2 shape ‘). The (3He, t) charge-exchange reaction ‘s) also gives L= 2 for the 2.97 MeV level. Combining these results with the natural-parity assignments from the present work leads to J” = 2+ assignments for at least the 2.969 MeV level and probably the 2.93 MeV level as well. (i) 3.048, 3.060 and 3.077 MeV. Very few previous results are available for the 3.048 MeV state. The 3.060 MeV state was measured to have an L = 4 shape in the (d, a) reaction. Combining with other results, spins of 4+ and 5+ were assigned in ref. ‘). Our unnatural-parity assignment selects 5’ for the state. The unnatural-parity assignment for the 3.077 MeV state is in agreement with the J” = l+ from ref. 12). These two states possibly correspond to predicted 5+ and l+ states at 3.01 and 2.99 MeV, respectively. (j) 3.140 MeV. On combining results from the (3He, t) and (d, (w) reaction, ref. ‘) gave 3+(2+) to this state. Our unnatural-parity assignment implies 3+. (k) 3.234 and 3.255 MeVstates. In many experiments, these two states were not separated. Only in ref. ‘) was a possible (I+) assignment given to the 3.234 MeV state from a plausible L = 0 transfer in the (d, (Y) reaction. The natural-parity assignments for these two states from the present work are in conflict with such a conclusion. (J) 3.384 MeV state. The determination of natural parity in the present work is in conflict with the l+ assignment given in ref. I*). The previous (p, ‘He) and (d, (Y) observation showed L = 2 angular distribution; combining with the T,, measurement, J” = 2+ is clearly indicated. 5. Conclusion The present T2,, measurements have led to model-independent spin-parity assignments to 37 of the excited states in 56Co. Several previous uncertain J” assignments
422
R. Shang et af. / 56Co
have been resolved. The good agreement up to 3 MeV excitation of McGrory’s, and Goode and Zamick’s theoretical prediction with the experimental results is encouraging. We thank J.W. McKay for the operation of the polarized ion source and Dr. Y. Peng for fabricating the targets. Dr. J. McGrory is especially thanked for providing us with the results of his unpublished calculations for 56Co. References I) 2) 3) 4) 5) 6) 7) 8) 9) IO) I I) 12) 13) 14) IS) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37)
M.J. Schneider and W.W. Daehnick, Phys. Rev. C4 (1971) 1649 J.M. Laget and J. Gastebois, Nucl. Phys. Al22 (1968) 431 J.A. Belote, W.E. Dorenbusch and I. Rapaport, Nucl. Phys. A109 (1968) 666 T. Caldwell, 0. Nathan, 0. Hansen and H. Bork, Nucl. Phys. A202 (1973) 225 D.O. Wells, S.L. BIatt and W.E. Meyerhof, Phys. Rev. I30 (1963) 1961 R.C. Jenkins and W.E. Meyerhof, NucI. Phys. 58 (1964) 417 H. Ohnuma, Y. Hashimoto and I. Tomita, Nucl. Phys. 66 (1965) 337 D.O. Wells, Nucl. Phys. 66 (1965) 562 J. Vervier, Nucl. Phys. 78 (1966) 497 J.B. McGrory, private communication G. Bruge and R.F. Leonard, Phys. Rev. C2 (1970) 2200 R.L. Auble, NucI. Data Sheets 20 (1977) 253 P. Goode and L. Zamick, Phys. Rev. Lett. 22 (1969) 958 M. Beckerman, Phys. Rev. Cl4 (1976) 1648 T.G. Dzubay, R. Sherr, F. Becchetti and D. Dehnhard, Nucl. Phys. A142 (1970) 488 D.P. Balamuth and E.G. Adefberger, Phys. Rev. Cl6 (1971) 928 L.D. Rickertsen, M.J. Schneider, J.J. Kraushaar, W.R. Zimmerman and H. Rudolph, Phys. Lett. 6OB (1975) I9 W.R. Zimmerman et al., Phys. Rev. Cl6 (1977) 2432 J.A. Kuehner, P.W. Green, G.D. Jones and D.T. Petty, Phys. Rev. Lett. 35 (1975) 423 CM. Lederer and V.S. Shirley, ed., Table of isotopes, 7th ed. (Wiley, New York, 1978) p. 162 ff D.T. Petty et al., Phys. Rev. Cl4 (1976) 12 D.T. Petty, thesis, McMaster University (1976), unpublished D.T. Petty et al., Phys. Rev. Cl4 (1976) 908 Shang Rencheng, A.A. Pilt, J.A. Kuehner, M.A.M. Shahabuddin and A. Trudel, Nucl. Phys. A366 (1981) 13 A.A. PiIt er al., Phys. Lett. 1OOB (1981) I14 D.G. Sarantites, J. Urbon and L.L. Rutledge, Jr., Phys. Rev. Cl4 (1976) 1412 H.O. Meyer and J.A. Thomson, Phys. Rev. C8 (1973) 1215 K. Okada, J. Kawa and T. Yamagata, Nucl. Phys. A349 (1980) I25 H. Nann er af., Phys. Rev. CZd (1981) 1984 H. Orihara et af., Phys. Lett. B106 (1981) I71 H. Nann et aL, Phys. Lett. 3109 (1982) I75 R. DelVecchio, R.F. Gibson and W.W. Daehnick, Phys. Rev. C5 (1972) 446 L.E. Samuelson er al., Phys. Rev. C7 (1973) 2379 J.H. Barker and D.G. Sarantites, Phys. Rev. Cl0 (1974) 1407 C. Moazed et al., Nucl. Phys. AZ39 (1975) 242 W.D. Kampp and S. Buhi, Z. Phys. A284 (1978) I I7 W.D. Kampp, K.H. Bodemiller, A. Nagel and S. Buhl, Z. Phys. AZ%? (1978) 167