1+ and 0+ states in 90Y

1+ and 0+ states in 90Y

Volume 72B, number 2 PHYSICS LETTERS 19 December 1977 1+ A N D 0 + S T A T E S I N 9Oy* H.T. FORTUNE, S.C. HEADLEY and L.R. MEDSKER :1: Physics Dep...

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Volume 72B, number 2

PHYSICS LETTERS

19 December 1977

1+ A N D 0 + S T A T E S I N 9Oy* H.T. FORTUNE, S.C. HEADLEY and L.R. MEDSKER :1: Physics Department, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA Received 7 October 1977 The 88Sr(3He, p)90y reaction allows the identification of 1+ states at excitation energies of 1816 -+5 and 2313 +- 10 keV, very close to the predictions of a recent core-coupling shell-model calculation. Three additional 0+ or 1+ states are observed above 3 MeV.

Two recent shell-model calculations for 90y have appeared: a hybrid model by Dalton and Robson [1] and a two-particle core-coupling model by HoffmannPinther and Adams [2]. The former treats negativeparity 90y states as particle-hole excitations on 90Zr and the positive-parity states as two-particle states outside 88Sr. Because the only positive-parity proton orbital considered is the 1g9/2, even with four neutron orbitals 2d5/2, 3Sl/2, 2d3/2, and lg7/2 only one 1÷ and no 0 + states are predicted. Ref. [2] describes 9 0 y as two particles coupled to the 0 + ground state and 1.84 MeV 2+ state of 88Sr. The positive-parity two-particle basis is identical to that of ref. [1], but the inclusion of coupling to the 2+ core produces many additional levels. In particular they predict three low-lying 1+ states - at 1.7, 2.2, and 2.3 MeV, and a 0 + state at 2.2 MeV. Ref. [2] published excitation energies and wave functions for the negative-parity states but only energies for the positive-parity levels. We have reproduced the positive-parity calculations. Because of the near de-

generacy, the l + wave functions depend strongly on the precise parameters of the model. With the parameters of ref. [2] the two 1÷ states near 2.2 MeV contain most of the lg9/2 lg7/2 intensity, 0.35 and 0.44; the intensity of the remaining state is 0.12. Slight variations in the interactions significantly change the mixing. In this model the l+(3He, p) strength is proportional to the square of the lg9/2 lg7/2 amplitude - all other amplitudes have zero overlap with the 88Sr ground state. Appearance of more than one 1+ state near 2 MeV with significant strength would be a triumph for the core-coupling model. The 0 ÷ model state has zero predicted strength since it is the 2 + core state coupled to a 2 + two-particle state. Neither model is expected to be accurate above about 3 MeV because of the restricted proton basis space. No previous 1+ states are known in 90y. We have used the 88Sr(3He, p)90y reaction and a multiangle spectrograph to search for 1+ states. Distorted-wave (DW) calculations using the code DWUCK [3] with standard optical-model parameters [4, 5] (listed in table 1) show that at our bombarding energy (18 MeV) the angular distribution shapes are excellent signatures of the transferred L-value.

* Work supported by the National Science Foundation. * Present address: Florida State University, Tallahassee, Florida 32306, USA

Table 1 Optical-model parameters used in the analysis of the 88Sr(aHe, p)9Oy reaction. Lengths are in fm and strengths in MeV. Channel

Vo

r0

a

W

W' = 4 WD

rb

a'

Vso

rso

aso

roc

3He a) p b) Bound state

172 Vp(E) c) -

1.14 1.17 1.26

0.75 0.75 0.60

20 d) .

0 e)

1.60 1.32

0.80 0.51

0 6.2 h = 25

1.01 1.26

0.75 0.60

1.26

.

.

.

a) Ref. [41 . b) Ref. [5]. c) Vp = 54 - 0.32 Ep + 0.4Z/A 1/3 + 24(N - Z)/A. d) W= 0.22E - 2.7/> 0. e) W' = 4(11.8 - El4 + 1 2 ( N - Z)/A). 173

Volume 72B, number 2

PHYSICS LETTERS

19 December 1977

720

8eSr ( 3 H e , p ) 9 O y ~)Lob = 1 1 2 5 °

Or) Y ¢0 ,,~

S

480

0

I--" LL 0 Og hi m 240

z

~r

z

z

--

m~ ~N

¢o

O~ m~

\

oa

0 '---"

o

~n ~O

}

Z3 Z

=

"a .¢.

"'{ ..'v.i.. o",,%.¢. ~,k." ";

4D

-

'l b

!

3.0

. . . .

2.0 ':

EXCITATION

1.0 ENERGY

O.O

(MeV)

Fig. 1. Spectrum of the 88Sr(3He, p)90y reaction at a bombarding energy of 18 MeV and a laboratory angle of 11¼°.

A spectrum is displayed in fig. 1. A number of L = 0 angular distributions are observed for states at excitation energies o f 1816 -+ 5, 2313 +- 10, 3432 + 5, 3490 -+ 5, and 3625 -+ 6 keV. They are exhibited in fig. 2. The first two and the fifth clearly possess an additional L = 2 component, so that if they represent single states, they must have Jn = 1÷. The other two are consistent with pure L = 0 and, hence, they have Jn

in 89y(d, p ) 9 0 y , implying j~r = (1, 2, 3 ) - • Neither L = 1 n o r L = 3 gives even a marginal fit to the (3He, p) data and if the previous result is correct, we must be observing a different state. Our 2313 keV state may be the state listed at 2325 -+ 10 in a recent compilation [6] with no J~ information. Our energies are compared with those from the compilation in table 2. The existence of two strong L = 0 states near 2 MeV gives credence to the core-coupling model. The other three states above 3 MeV undoubtedly consist mainly

= 0 + or 1 +.

The 1816 keV state is very close to a level at 1813 + 2 keV [6] reported [7] to be populated with l n = 2 lO00k

Ex-1816

~.~_._t-~.'

",,

100p

%

t~9/z

\~

100~

Ig r/Z

~

,:o-2

Ex: 3.432

~0

aL

#

10

Ex : 2 313

z

~, I

............. .......

0

"

filled

............... 0

20 °

40 ° OC.m

'

\.

sum

I I

IO

! 60"



20 °

40" ec.m

60 °

I 0°

20 °

Fig. 2. Angular distributions of 88Sr(SHe, p)90y that exhibit L = 0 character.

174

L=o+zz (zn 5/z)-

40 ° Oc m

60 °

Volume 72B, number 2

PHYSICS LETTERS

19 December 1977

Table 2 . . . . Excitation energies of levels observed with L = 0 angular distributions in Nucl. Data Sheets a)

Present work

88

3

St( He, p)9Oy.

89y(d, p)90y b)

E x (keY)

L

j1r

E x (keV)

jlr

Ex

In

1 8 1 6 -+ 5 2313 -+ 10 3432 ± 5 3490 ± 5 3625± 6

0+2 0+ 2 0(+2) 0(+2) 0+2

1÷ 1÷ 0 ÷ or 1 ÷ 0 ÷ or 1÷ 1÷

1 8 1 3 -+ 2 2325 -+ 10 3420 3495 -

(1-3)-

1814 3634

2 (3 or 4)

a) Ref. [6]. b) Ref. [7].

o f c o m p o n e n t s t h a t lie o u t s i d e t h e m o d e l space o f refs. [1] a n d [ 2 ] . It is c o n c e i v a b l e , o f c o u r s e , t h a t a t w o -

References

p a r t i c l e c a l c u l a t i o n in a larger basis c o u l d p r o d u c e t w o l o w - l y i n g 1+ s t a t e s , e v e n in t h e a b s e n c e o f c o r e e x c i t a -

[1] B.J. Dalton and D. Robson, Nucl. Phys. A210 (1973) 1. [2] P. Hoffmann-Pinther and J.D. Adams, Nucl. Phys. A229 (1974) 365. [3] P.D. Kunz, University of Colorado, private communication. [4] J.R. Comfort and J.P. Schiffer, Phys. Rev. C4 (1971) 803. [5] F.D. Becchetti and G.W. Greenlees, Phys. Rev. 182 (1969) 1190. [6] D.C. Kocher, Nucl. Data Sheets 16 (1975) 55. [7] R.E. Goans and C.R. Bingham, Phys. Rev. C5 (1972) 914.

t i o n . H o w e v e r , w e feel t h a t is u n l i k e l y . N e v e r t h e l e s s , s u c h a n e n l a r g e d c a l c u l a t i o n w o u l d b e u s e f u l in f u r t h e r understanding the properties of 90y.

175