The 6− analog-antianalog states of 28Si

Volume 45B, number 2

PHYSICS LETTERS

9 July 1973

THE 6 - ANALOG-ANTIANALOG STATES OF 28 Si* G.F. NEAL and S.T. LAM

Department of Physics, University of Alberta, Edmonton, Alberta, Canada Received 16 April 1973 A resonance is observed in the 27Al(p, 7)2SSi reaction at Ep = 2876 • 2 keV, which corresponds to an excitation energy of 14356 -+2 keV. The 14.36 MeV level decays to a new level at 11577 e 2 keV, which is turn decays to the known level at 9701.8 • 0.5 keV. With previous information on the 9.70 MeV level and the present 7-ray angular distributions, obtained from singles spectra as recorded by a 40 cm3 Ge(Li) detector, the spins of the three levels can be limited to J = 5, 6;J = 6; andJ = 5, respectively. Transition strength arguments based on measurements of the strength of the 2876 keV resonance and the lifetime of the 11.58 MeV level indicate that the 14.36 MeV level has j n = 6 , T = 1 and that the 11.58 MeV level hasf tr = 6-, T = 0.

A search for a 6 - , T = 1 isobaric analog state as a resonance in the 27Al(p, 3,)28Si reaction and the subsequent 3,-ray decay of this level to a 6 - , T = 0 state was p e d o r m e d with a CN Vah de Graaff. Large-angle, high m o m e n t u m transfer inelastic electron scattering on 28Si [ 1] indicates the possible presence of a 6 - , T = 1 level at Ex = 13.9 MeV. Further, particle-hole shell model calculations predict both a 6 - , T = 1 and a 6 - , T = 0 state in 28Si at 13.63 and 10.98 MeV, re- 1 , f7/2) configuration spectively, based on the tt d 5/2 [2]. Given the isobaric analog nature of such a state and the lp = 3 transfer required, such a resonance should be observable in the (p, 3,) reaction. In the present investigation, a resonance is found at Ep = 2876 -+ 2 keV in the 27Al(p, 3,)28Si reaction (E x = 14356 :l: 2 keV), which has the characteristics of a 6 - , T = 1 state. This resonance was also observed by Miehe et al. [3]. A 40 cm 3 Ge(Li) detector was used to measure the 3,-ray transitions from the Ep = 2876 keV resonance. The 27A1 target was 15 #g/cm 2 thick evaporated onto a thick tantalum backing. Direct water cooling made it possible that 10/aA of beam current could be used without damaging the target. Fig. 1 shovcg the decay scheme deduced for the resonance. It was constructed by matching the cascading 3,-ray intensities and energies. Gamma rays resuiting from the decay of the 9.70 MeV level were quickly identified. In addition, two "),-rays of energy *

Research supported in part by the Research Corporation and the University of Virginia Center for Advanced Studies.

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127

Volume 45B, number 2

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Fig. 2. Gamma-ray angular distributions of the R ~ 11.58 and 11.58 --* 9.70 MeV transitions. The curves are from calculations using the formation parameters, spins and multipoles as shown in the diagram. The calculated angular distributions, W(O) are also shown in the diagram. The dotted arrow indicates that the "r-ray transition is unobserved. 2778.4 +- 1.1 keV and 1874.2-+ 1.1 keV were observed, the energy sum of which plus the energy o f the 9.70 MeV level matches the resonance energy derived from the accelerator calibration to within 2 keV. At 0 °, the former 3,-ray was found to show a full Doppler shift and is therefore identified as corresponding to the primary transition from the resonance to a new level at 11577.0 -+ 1.5 keV. It then follows that the 1874 keV 3,-ray, which showed an attenuated Doppler shift, corresponds to the 11.58 --* 9.70 MeV transition. Further, a comparison of the intensities of these two transitions indicates that the 11.58 MeV level decays entirely to the 9.70 MeV level (to within 5%). The excitation 128

9 July 1973

energy of the 9.70 MeV level is found to be 9701.8 -+ 0.5 keV. Fig. 2 shows the 3'-ray angular distributions o f the R --* 11.58 and 11.58 --* 9.70 MeV transitions. The curves are from angular correlation calculations using the formation, the spin sequence, and the multipole radiation as shown in the figure. The Rose and Brink convention for the phase o f the mixing ratio is used [4]. The interpretation of these data depends on the spin of the 9.70 MeV level. Combining the 28Si(a, ot')28Si and 27Al(z, d)28Si work of refs. [5] and [6], the j n o f this level can be restricted to 1 - , 3 - , 5 - . The work of Lam et al. at the 1724 keV resonance in the 27Al(p, 3')" 28Si reaction further limited this t o J 7r = 5 - , 3 - , with 5 - being much preferred [7]. Consequently, fits to the two "),-ray angular distributions were undertaken for assumptions of both J = 5 a n d J = 3 for the 9.70 MeV level. Of the over 40 different spin and formation combinations tried, only the following sequences fit the data below the 0.1% confidence limit of X2: 6 --* 6 ~ 5 and 5.-'* 6 -* 5. No combinations of spins based on J = 3 could be made to fit the data for any value o f variable parameters. The first sequence required lp = 3 or 4, 61 (quadrupole/dipole) = 0, and /52 (quadrupole/dipole) = 0% where the/5's are multipole mixing ratios. The strength of the resonance was found to be or), = ( 2 / + l)l-'pl".t/F = 4.1 -+ 0.9 eV as derived with the thick target technique relative to co'), = 9.5 +- !.5 eV for the 1317 keV resonance [10]. From the assumption Fp ~ 1-', F.r can be deduced. The 5 ~ 6 --* 5 sequence requires 151 = 0 . 5 0 -+ 0.05 for the primary transition which results in an E2 strength o f 108 +-26 W.u. or an M2 strength o f ( 3 . 2 +- 0.7) X 103 W.u. Clearly, these are too large (see e.g. ref. [9]) and the 6 -'~- 6 ~ 5 sequence is the only acceptable combination. The implied strengths of the primary transition in this case are then tM(M1)I 2 = 0.70 -+ 0.15 W.u. or IM(E1) 12 = 0.023 -+ 0.004 W.u. This M1 strength is in conformity with the characteristic j~r ~ j r , A T = 1 spin-flip transition in a self-conjugate nucleus [8], whereas the E1 strength is some 10 to 20 times the average E1 strength in this region of nuclei [9], or, compared to the findings o f ref. [7] specifically for 28Si some 100 to 1000 times too large. The attenuated Doppler shift of the 3,-ray corresponding to the 11.58 --* 9.70 MeV transition was de-. termined in a separate measurement. This gives a life-

Volume 45B, number 2

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9 July 1973

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Fig. 3. Gamma-ray angular distributions of the transitions from the 9.70 MeV level. The curves are from c~alculationsusing the same parameters as shown in fig. 2 as well as the spins and multipolarities involved in the decay of the 9.70 MeV level. The calculated angular distribution, 1t,'(0)is also shown in the diagram. The dotted arrow indicates that the "y-raytransition is unobserved. time o f 0.34 -+ 0.10 ps. Since the present observation indicates that this state decays 100% to the 9.70 MeV level by pure quadrupole radiation, the above lifetime implies the following enhancements: IM(E2)I 2 = 20 -+ 6 W.u. or IM(M2)I 2 = (6.1:1: 1.8) X 102 W.u., o f which the M2 value is clearly too strong. Hence, combining the previous information with the present investigation, the 14.36 MeV level can be assigned J ~ = 6 - , T = 1 ; the 11.58 MeV level J~ = 6 - , T = 0, and the 9.70 MeV level j r = 5 - . In further support of the above assignments, fig. 3 shows the angular distributions of four of the five 3'rays de-exciting the 9.70 MeV level. They are in good agreement with the angular correlation calculations. As stated previously, 6 - levels of T = 1 and T = 0

are predicted in this region of excitation energy by the particle-hole calculations of ref. [2]. The above analysis of the 7-ray angular distributions allows formation o f the resonance by capturing f7/2 protons only. Hence, the 14.36 MeV level is likely to have a configuration of (d~2[ , f7/2)- From the analysis o f ref. [5], the 5 - , T = 1 level at E x -- 13.25 MeV is mostly (d~-/1, f5/2)" Hence, in spite o f the difficulties and complexities in explaining the structure o f 28Si, the high-spin, negative parity levels seem to be well predicted by the simple one particle - one hole calculation. The authors would like to thank Professors A.E. Litherland, R.E. Azuma and R.C. Ritter for their 129

Volume 45B, number 2

PHYSICS LETTERS

helpful discussions and their interests in the work. They would also like to thank Miss P.B. Dworken for providing the "}-ray angular correlation program, and Dr. G. Walter for communication of results prior to publication. The computing facilities provided by the University of Virginia Computer Science Center is gratefully acknowledged.

References [I ] T.W. DonneUy,J.D. Walecka, G.E. Walker and i. Sick, Phys. Lett. 32B (1970) 545. [21 S.A. Faxrisand J.M. Eisenberg, Nucl. Phys. 88 (1966) 241.

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9 July 1973

[3] C. Miehe, J.P. Gonidec, A. Huck and G. Walter, Conf. on Nuclear structure and nuclear reactions, Birmingham, April, 1972, and private communication. [41 HA. Rose and D.M. Brink, Revs. Mod. Phys. 39 (1967) 306. [5] M. Kieran, A.M. Baxter, J.A. Kuehner and R.W. Oilerhead, Bull. Amer. Phys. Soc. 14 (1969) 550. [6] S. Hinds and R. Middieton, Proc. Phys. Soc. 76 (1960) 545. [71 S.T. Lam, A.E. Litherland and R.E. Azuma, Can. J. Phys. 49 (1971) 685. [8] P.M. Endt, Third Symposium on the Structure of lowmedium mass nuclei in the 2s-ld shell, ed.J.P. Davidson (University Press of Kansas, 1968) p. 73. [9] S.J. Skorka, J. Hertel and T.W. Retz-Schmidt, Nucl. Data A2 (1966) 347. [10] R. Nordhagen and A. Tveter, Nucl. Phys. 56 (1964) 337.