Study of the 92,96Zr(d, α)90,94Y reactions

Nuclear Physics A233 (1974) 81-90;

@

North-Holland

Publishing

Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

STUDY OF THE 92s96Zr(d, a)90p9’Y REACTIONS S. GILAD, S. COCHAVI, Department

M. A. MOINESTER

of Physics and Astronomy,

Tel-Aviv

and J. ALSTER

University,

Ramat Aviv, Israel

and M. BUENERD and P. MARTIN lnstitur

des Sciences

Nu&aires,

Cedex 257, 38 Grenoble,

France

Received 5 February 1974 (Revised 28 June 1974) Abstract: The hole-hole structure of p*Y was studied via the reaction 96Zr(d, z)~*Y and compared to the particle-hole structure of “‘Y, which was populated by the reaction 92Zr(d, a)90Y. The deuteron beam energy was 28 MeV. Angular distributions of both reactions were obtained for the prominent lines. New states of 94Y were observed at 0.44, 1.17, 1.39, 1.53, 1.82, 1.90, 2.17, 2.33, 2.46 and 2.77 MeV. Our data are consistent with the previously reported 2- assignment of the ground state, and we suggest Jn = 3- for the 0.44 MeV state, these being members of the (n2p+, v2d+)-‘) doublet. The 1.17 state is suggested to be a member of the @pt.-*, rd+-‘) multiplet. The Q-value of the 96Zr(d, a)9’Y reaction was measured to be 7.609&0.020 MeV. The reaction 9*Zr(d, a) was performed at two angles. Several new states of “Y were observed at 0.31, 0.78, 1.03, 1.31, 1.49, 1.69 and 1.89 MeV. E

NUCLEAR REACfIONS 9z*pd. 96Zr(d, a), E = 28 MeV; measured o(E,, f_?). 96Zr(d, a), E = 28 MeV; measured Q. p’s ‘r* 9rY deduced levels, /, n. Enriched targets.

1. Introduction The g.s. of 92Zr, 94Zr and 96Zr have neutrons in the 2d, subshell outside the closed core of 50 neutrons. In the (d, 3He) reaction on the even Zr isotopes the proton may be picked up from the lf+,2p+, 2p,, and lg* orbits; these transitions were observed by Preedom et al. ‘). In the 92*94. 96Zr(d, a) reactions, the low lying transition should be due to the pickup of proton from these orbits, and a neutron mainly from the 2d, orbit, leading to states of the multiplets (~1% ‘, v2d;), (7r2p;‘, v2d;), (1z2p+, v2d;) and (IrIg,, v2d;). In this experiment we investigate the level structure of the hole-hole states in 94Y via the reaction 96Zr(d, a)94Y. For camp arison purposes, we also studied the reaction which populates corresponding particle-hole states in 9oY. The latter 92Zr (d 9 a)“Y, reaction has previously been performed ‘), but no angular distributions were determined. In addition to the known 2- ground state of 94Y, levels are also known at 0.622, 0.724 and 1.428 MeV with J” of (l-, 2-), (2-, l-) and l+ respectively, determined from y-activity following the b-decay of 94Sr [ref. “)I. The ground state doublet p+-d, is well known in 9oY and is expected to be popu81

82

S. GILAD

er al.

lated strongly via the 92Zr(d, X) reaction. The ground state of 94Y is a member of the corresponding hole-hole doublet. This state, as well as the 3- member, should be observed strongly in the 96Zr(d, a) reaction. The g*-d, p ositive parity sextet has been studied in 9oY [ref. “)I. In the 92Zr(d, a) reaction, the 7+ and 5+ members are populated relatively strongly, while the other members are observed weakly or not at all. The corresponding even parity sextet is expected to be populated much weaker in 94Y since the gt proton orbit is nearly empty in 96Zr, as in the 96Zr(d, 3He) study ‘). The pi’-d, multiplet has recently been studied in 9oY by Cochavi et al. ‘) using the reaction 91Zr(d 3He). Members of this multiplet have previously been observed “) by 92Zr(d a),’ but no angular distributions were reported. The corresponding multiplet in “Y is also expected to be seen strongly in the 96Zr(d, z)~~Y reaction. The fi’-d;’ sextet is also expected to be populated in the (d, a) reactions. In the “Zr(d, 3He)90Y study ‘), a level at 2.03 MeV was tentatively identified as the 5- or 4- member of this multiplet. A level at 2.03 MeV was strongly populated in the 92Zr(d, a) previously performed ‘). Some members of the corresponding fi l-d; ’ sextet might be seen in the 96Zr(d, a)94Y reaction. 2. Experiment The experiment was performed with the 28 MeV deuteron beam of the variableenergy cyclotron of the Institut des Sciences Nucleaires at Grenoble. The beam intensity varied from 50 to 300 nA. The target consisted of enriched 92Zr (94.6 o/:) and 96Zr (85.2 %) metallic foils, of thickness 560 and 510 pg/cm’ respectively. The main impurity of the 92Zr target was 90Zr (2.9 %). The main impurities of the 96Zr target were 90Zr (7.2 %),94Zr (3.8 7;) and 92Zr (2.2 0/0).The 94Zr target consisted of a 720 pg/crn’ thick metallic foil, enriched to about 97 “/,. The outgoing a-particles were detected by three surface barrier silicon detectors, each 1000 pm thick. Anticoincidence E-detectors were placed behind the E-detector in order to reject particles passing through the detectors (mainly elastic deuterons). One detector was at forward angles with a very small solid angle while two others, about 14” apart, were used at larger angles. A detector placed at 45” served as a monitor of a-reaction particles. Alpha energy spectra were recorded up to an excitation energy of 4 MeV. The overall energy resolution was 65-80 keV. Figs. I and 2 show typical a-spectra which were analyzed with the aid of the peak unfolding computer code Autofit “). The solid curves in figs. 1 and 2 represent the results of the peak unfolding. The excitation energies of 9oY were calculated from the reported ground state Q-value of the 92Zr(d, a) reaction, and known “) excitation energies of four additional prominent peaks at 0.202, 0.682, I .047 and 1.570 MeV. The Q-value for the 96Zr(d, z) reaction and the excitation energies of 94Y wzre calculated using the energy calibration of the corresponding 9oY spectra. We determine the Q-value to be 7.609+0.020 MeV, compared to the previously reported value of 7.550+0.200 MeV. The uncertainties in the excitation energies of 94Y vary from 10 to 25 keV. Spectra were measured from 10” to 45” in

92.96Zr(d,

83

490.94Y

-

g2Zr(d ,a)% 400

$

Y

Ed=28 Mev 0ru =27.5 deg

300

B

P

2oc

%

”

IOC ‘. ‘..

'

-.-

** . . . .,

*..

..

800

750

700 CHANNEL

NUMBER

Fig. 1. Alpha spectrum of the 92Zr(d, a)90Y reaction. The solid curves represent the results of peak unfolding.

-

Ig6Zr(d,a)

g4 y

Ed = 26 Mev 0~=27.5deg

900.

5

550

650

600 CHANNEL

700

750

NUMBER

Fig. 2. Alpha spectrum of the 96Zr(d, ajg4Y reaction. The solid curves represent the results of peak unfolding.

S. GlLAD et al.

0

I3

30

4.:

0 cb

(:.>l. ;\\w_E Fig. 3. Angular distributions of (~p~-~, vd+) anf4pf,

vd&levels in 90Y compared to the levels in

91.

9hZr(d,

85

490.94y

t

4

0.001: 0

I5

3) (2tl.

45

! 64)

ASGLK

Fig. 4. Angular distributions of prominent lines in the a-spectrum of “‘Y.

steps of 3.5 to 4 degrees, and the resulting distributions are shown in figs. 3, 4 and 5. The 92Zr and g6Zr targets were used alternatively at each angle. The statistical and fitting errors in the differential cross sections are indicated in the figures. The errors of the absolute cross sections are about 10 %. reaction are shown in fig. 6. The excitation Two z-energy spectra of the 94Zr(d, CY) energies were calculated using the method described above for the 96Zr(d, a) reaction. The measured Q-value of 8.278 f0.025 MeV is in good agreement with that of ref. ‘).

S. GILAD et al.

i

Fig. 5. Angular distributions

of prominent

lines in the a-spectrum of 94Y.

TABLE1 Excitation levels of 92Y as obtained from the 94Zr(d, a)9tY reaction (Ed = 28.0 MeV) Excitation (MeV)

g.s.

$

(27.0”)

Wlsr) 15.7*1.0

..$ (34.5”) Olb/sr) 17.510.9

0.3 1 LO.01

10.2fl.l

3.9 &to.4

0.78&0.01

19.6f1.4

7.5kO.8

1.03&0.01

13.3fl.l

8.430.8 4.710.5

1.31 fO.O1

9.5fl.O

1.49~0.01

12.lfl.O

7.1*0.7

1.69f0.01

41.552.1

13.0*1.0

1.89f0.01

19.9fl.2

14.Ohl.l

2.44&0.01

10.2*1.0

8.6kO.7

92.

120

Ed = 28

P6Zr(d,

a)PO.

94y

87

Mev

eLm = 27.0

deg

CHANNEL

NUMBER

Fig. 6. Two z-spectra of the p4Zr(d, x)~‘Y reaction. The solid curves represent the results of peak unfolding.

The resulting uncertainties in the excitation energies of levels in “Y are about 10 keV. Table 1 shows the observed levels of “Y and the differential cross sections of the g4Zr(d a) reaction to these levels at 27.0 and 34.5 degrees. The uncertainty in the absolut; cross sections is about 10 %. 3. Results and discussion Eight prominent lines are observed in the spectrum of 9oY as shown in fig. 1. These lines are the ground state, O-20,0.68, 1.05, 1.57, 1.64, 1.81 and 2.03 MeV states. Two weaker lines of 9oY are observed at 0.95 and 1.42 MeV. The ground state of 92Y, resulting from the 94Zr impurity is also observed. Six prominent lines are observed in the spectrum of 94Y as shown in fig. 2. These are the ground state, 0.44, 1.17, 1.39, 2.46 and 2.77 MeV states. Five weak lines of g4Y are observed at 1.53, 1.82, 1.90, 2.17 and 2.33 MeV. The positions of four known lines of **Y [ref. “)I are shown in fig. 2. These lines are the 0.23, 0.39, 0.71 and 1.32 MeV states, that could result from the 7.2 % “Zr impurity. Also shown are the positions of four additional lines that could result from the 3.8 % g4Zr contaminant. These lines are the 0.78, 1.31, 1.49 and 1.69 MeV states of “Y.

88

S. GILAD er al.

The ground states of 9oY and g4Y are known to have J” = 2- belonging to the p+-di doublets. Their angular distributions in the (d, a) reactions are similar as shown in fig. 3. The 0.20 MeV line of 9oY has J” = 3-, and it is the second member of this doublet. its angular distribution has similar features to that of the 0.44 MeV line in 94Y, as shown in fig. 3. In addition, being the only strong line in the region 0 c E CXE< 1.17 MeV in 94Y, suggests that the 0.44 MeV line should be the 3member of the p+-d, doublet. Fig. 3 shows that the absolute cross section for the g6Zr(d, r) reaction to these levels is about seven times larger than that of the 92Zr(d z) reaction to the corresponding levels. The two-nucleon transfer structure factors’9) for 92Zr and g6Zr, assuming the simplest shell model for these nuclei, yield absolute cross-section values for the p+-d; doublet, about six times larger in 96Zr(d, a) than in 92Zr(d, a), which is consistent with the experimental results. The average two-body interaction energy calculated in the manner of ref. ’ “) for the 2-, 3- doublet of 9oY, referred to a *%r core, is E(2)partic,e_partic,e = -0.38 MeV, and E (2)particle-hole = +0.46 MeV with respect to a 90Zr core. For the 2-, 3doublet of g4Y, referred to a 96Zr core, E(2)ho,c_hole = -0.60 MeV. In a simple shell model, the absolute values should be identical while in fact, there is a discrepancy of up to 220 keV. A discrepancy of 540 keV between the average two-body interaction energies deduced from 42Sc and 4*Sc has previously been noted ’ “) and discussed ’ ’ ) in terms of three-body interactions. However, since E(2) deduced from 92Nb and 96Nb are in rather good agreement, the disagreements between 9oY and 94Y appear surprising. The increased splitting between the 2- and 3- states in 94Y compared to 9oY is likewise of interest. The lines at 0.68 and 1.05 MeV in the 9oY spectrum are known to be J” = 7’ and 5+ respectively, from y-work “). They belong to the gi-d, sextet. Their angular distributions are shown in fig. 4. Another member of this sextet, the 3+ at 0.95 MeV excitation is seen in fig. 1. However, because of its low yield, its angular distribution was not extracted. According to theoretical calculations I’), the even parity g,-d; ’ sextet of 94Y should have its centroid at an excitation energy of about 0.80 MeV. From the spectrum of fig. 2, we conclude that this multiplet is populated very weakly in the g6Zr(d, a) reaction. This is consistent with the low values of C’S for lg+ proton pick-up obtained in the 96Zr(d. 3He) study ‘). A spin and parity of 5- or 4have been suggested “) for the 2.03 MeV line of 9oY. It is a member of the f<‘-d* multiplet. Its angular distribution is shown in fig. 4. From the ‘lZr(d, 3He)90Y reaction the spins of the states at 1.42, 1.57 and 1.81 MeV were limited to J” = (l-4)-, (24)and (l-4)-, respectively. They were identified as members of the pi I-d, multiplet. From the structure amplitudes of the (d, a) reaction, assuming the simple pi1 -d, configuration, the J” = 3- state should be excited about 10 times weaker than the J” = 4- state. Therefore, the most prominent state in “Y at 1.57 MeV is not likely to be 3-. The most prominent line in the 94Y spectrum is at 1.17 MeV. Its angular distribution shows similar structure to that of the 1.57 MeV line in 9oY as shown in fig. 3. Based also on its strength, and that expected

92.

96Zr(d,

a)9%

89

9*y

3.0 2.77z.03

2.462.02

2.5 -

2.33 -‘.02

2.17:.01

1.48

2’

u 153:01 1.43

(1

1.39f.01

1.17’.01

0.2c

( 2= 4-1

32-

2-

PARTICLE-HOLE PREVIOUS CAI_CULATIONS”21 EXPERIMENT”’

2PRESENT

Fig. 7. Level scheme of 9*Y, including previously observed levels, theoretically levels observed by the present work.

WORK

calculated levels and

from (d, a) structure factors 9), we suggest J” = (2--4-) to the 1.17 MeV state of 94Y. The second strongest line in the “Y spectrum at 1.64 MeV has been tentatively identified ‘* “) as the .I” = l- state. The 1.38 MeV line in 94Y shows features similar to both the 1.64 and 1.81 MeV states of 9oY, as shown in figs. 3 and 4. The other members of this multiplet in 9oY are at 1.42 and 1.81 MeV respectively ‘). Their angular distributions are shown in fig. 4. In 9oY, the pi’ -d, centroid energy is about 1.47 MeV higher than the p+-d+ centroid. Since the single particle energy difference between the p+ and p, orbits decreases from 1.51 MeV in *9Y to about 1.26 MeV in 95Y, the pi’-d;’ centroid energy in 94Y should be on 1y 1.22 MeV above the p+-d;’ centroid, at 1.47 MeV excitation. We have already suggested that the 1.17 and 1.39 MeV lines in 94Y belong to the pi r-d; ’ quartet. The other two members of this multiplet should also

90

S. GlLAD et al.

be populated and might be found among the lines having excitation energies of 1.53 MeV and higher. Some of these lines shown in fig. 2 are members of the fi r-d; ’ sextet. Angular distributions of the 1.53,2.46 and 2.77 MeV levels have been obtained and are shown in fig. 5. The information on the 94Y spectrum extracted from this work, the levels predicted by theoretical calculations “), and the states previous to this work “) are summarized in fig. 6. Two a-spectra of the 94Zr(d a)92Y reaction were obtained and are shown in fig. 7. In addition to the ground’state, four excited states are known from y-activity following /?-decay of 92Sr [ref. r3)]. These levels are at 0.241, 0.893, 0.953 and 1.384 MeV with J” = (3-), (2-, 1 -), (l-, 2-) and 1 + respectively. Seven new excited states are observed in fig. 6. These are at 0.3 1,0.78, I .03, 1.3 1, 1.49,1.69 and 1.89 MeV. The state observed at 0.31 MeV is most likely the 3- member of the p+-d, doublet. Previously 13), a 0.241 MeV state in “Y with J” = (3-) was suggested to be the 3member of this doublet, but its weak excitation in the present work makes this unlikely. The excitation energy of the 3- state in the doubly odd Y isotopes thus appears to increase gradually with neutron number. In summary, the Q-value of the 96Zr(d, a)94Y reaction was determined to be 7.609_+0.020 MeV. Ten previously unreported levels of 94Y were observed. Spin and parity of 3- were suggested for the 0.44 MeV level of 94Y as a member of the p,-d;’ doublet. The assignment J” = (2-d-) was suggested to the 1.17 line as a member of the p;‘d;’ multiplet. Seven excited states of “Y were observed. It should be interesting to do a high resolution experiment, to identify the other members of the pi’ -d; 1 quartet, and the members of the fi i-d;’ multiplet. We would like to thank J. Ferme and the cyclotron crew in Grenoble for their cooperation. The Tel-Aviv University group appreciates the very generous hospitality extended to them by the Institut des Sciences Nucliaires. One of us (S.C.) wishes to thank the Bat-Sheva de Rothschild Foundation for its assistance. References I) B. N. Preedom, E. Newman and J. C. Hiebert, Phys. Rev. 166 (1968) I156 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

J. L. Bliick, W. Darcey and M. M. Islam, Nucl. Phys. 79 (1966) 65 W. Grimm and W. Herzog, Z. Phys. 259 (1973) 67 W. Lins, J. Ernst, N. Takahashi, E. Grosse and D. Proetel, Nucl. Phys. Al79 (1972) 161 S. Cochavi, S. Gilad, M. A. Moinester, J. Alster, M. Buenerd and P. Martin, Nucl. Phys. A233 (1974) 73 P. Spink and J. R. Erskine, ANL report 1965, PHY-1965B, unpublished N. B. Gove and A. H. Wapstra, Nucl. Data Tables 11 (1972) Y. S. Park, H. D. Jones and D. E. Bainum, Phys. Rev. C4 (1971) 778 N. K. Glendenning, UCRL report, no. 18270 (1968) M. Moinester, J. P. Schiffer and W. P. Alford, Phys. Rev. 179 (1969) 984 B. J. West and D. S. Koltun, Phys. Rev. 187 (1969) 1315 N. Auerbach and I. Talmi, Nucl. Phys. 64 (1965) B. Parsa, A. Ashari, L. Goolvard and Y. N. Nombar, Nucl. Phys. Al75 (1971) 629