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Excitation of giant resonance modes in 90Zr and 120Sn by 200 MeV protons
Volume 103B, number 4,5
PHYSICS LETTERS
30 July 1981
EXCITATION OF GIANT RESONANCE MODES IN 9°Zr AND"12°Sn BY 200 MeV PROTONS F.E. BERTRAND, E.E. GROSS, D.J. HOREN and J.R. WU 1 Oak Ridge National Laboratory 2, Oak Ridge, TN 37830, USA J. TINSLEY and D.K. McDANIELS University of Oregon 3, Eugene, OR 97403, USA L.W. SWENSON Oregon State University 3, Corvallis, OR 97331, USA and R. LILJESTRAND University of Alberta, TRIUMF 4, Vancouver, British Columbia, Canada V6T 2A3 Received 30 April 1981
Giant resonances in 90Zr and 120Sn have been studied using inelastic scattering of 200 MeV protons. The isovector giant dipole and isoscalar giant quadrupole and giant octupole (L = 3,3hto) resonances are clearly observed. An upper limit of .~ 5% is placed on the 2h~o hexadecapole strength in the quadrupole resonance peak. For 9°Zr a peak is observed which is consistent with recently reported MI strength.
During the past ten years isoscalar (T = 0), giant resonances have been discovered and intensively studied [1]. There is firm evidence for giant quadrupole (GQR) and giant monopole (GMR) resonances and strong indications [2] of the excitation of the 3hco giant octopole resonance (GOR). While predictions have been made for other, higher-L, T = 0, giant resonances, none have yet been observed. Of particular interest are predictions [3] of a 2hco, giant hexadecapole (E4) resonance (GHR) located at the same excitation energy as the GQR and which might exhaust 3 0 - 5 0 % of the L = 4, T = 0 energy weighted sum rule (EWSR). The new isoscalar giant resonances have been stud-
1 Present address: Bell Laboratories, NaperviUe, IL 60540, USA. 2 Operated by Union Carbide Corporation under contract W-7405-eng-26with the US Department of Energy. 3 Work supported in part by a grant from the NSF. 4 Under the auspices of a contract with the Canadian NRC. 326
ied using inelastic scattering of electrons and a wide variety of hadrons. While hadronic probes provide clear excitation of giant resonance peaks, positive identification of the resonance multipolarity is often lacking. This comes about since angular distributions from inelastic hadron scattering are often not strongly characteristic of m o m e n t u m transfer. The use of 2 0 0 - 4 0 0 MeV protons for giant resonance studies offers several advantages over probes previously used. The most notable advantage lies in the clearly characteristic angular distribution shapes for neighboring m o m e n t u m transfers. For example, according to DWBA calculations, the difference between a "pure" L = 2 giant resonance and an L = 2 + 4, even for as little as 1 0 - 2 0 % L = 4 EWSR depletion, is very large. Another interesting aspect of 200 MeV proton inelastic scattering is the very large small-angle cross section predicted for Coulomb excitation of the giant dipole resonance (GDR). Although it is assumed that the GDR is excited in inelastic proton scattering, di-
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rect evidence for GDR excitation has here-to-fore not been provided. Furthermore, using 2 0 0 - 4 0 0 MeV protons, spin-flip excitations, such as M1 states, may be enhanced at forward angles. In this letter results from 200 MeV proton excitation of giant resonances in 90Zr and 120Sn are reported Strong excitation of the GDR, GQR, and GOR resonances is observed and an upper limit of 5% is established for L = 4 EWSR strength depleted within the GQR. Further, for 90Zr a peak is observed at forward angles which is suggested to arise from excitation of a M1 state. These results demonstrate that medium energy inelastic proton scattering is an important tool for studies of a variety of giant multipole resonances. Protons of 200 MeV from the TRIUMF accelerator were inelastically scattered from ~60 mg/cm 2 targets and detected in the focal plane of the MRS, a broad range magnetic spectrograph facility [4]. Typical spectra covered an excitation energy range of ~40 MeV for a single field setting of the spectrograph. The MRS detector system consists of a thin plastic scintillator and a 12.5 cm by 12.5 cm wire chamber located in front of the magnet, two larger wire chambers after the magnet which intersect the focal plane, followed by a thick plastic scintillator. The front wire chamber determines the acceptance solid angle, allowing absolute determination of cross sections. The scintillators provide time-of-flight particle identification, and along with the fast wire chamber pulses form the event trigger. We have measured the spectrograph response and beam quality at zero degrees using a reduced intensity beam and find no spurious background that would affect our results. At the smallest angles studied, observations were made using blank target frames; no deleterious background was observed. Typical beam intensities ranged from 0 . 1 - 1 . 5 hA. The absolute beam current was determined using a calibrated monitor of p r o t o n - p r o t o n scattering from a thin CH 2 target located up-stream of the spectrograph. The energy resolution was typically 9 0 0 - 1 1 0 0 keV (FWHM) although some data on low-lying states were taken with about 450 keV (FWHM) resolution. Measurements were made every two degrees between 4 and 20 degrees. Our measurements of proton scattering cross sections from hydrogen using a CH 2 target agree to within 5% with phase-shift values [5]. Fig. 1 shows spectra from 90Zr and 120Sn at angles which should provide maximum cross sections for
30 July 1981
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Fig. 1. Spectra from inelastic scattering of 200 MeV protons from 9°Zr and 12°Sn. The multipolarities shown for the resonances are discussed in the text. The dashed line shows the shape and magnitude assumed for the nuclear continuum underlying the resonance peaks.
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L = 1,2 and 3 excitations. The broad peaks which are obvious in the high excitation energy continua arise from excitation of various multipolarity giant resonances the assignments of which we discuss below. The inelastic nuclear continuum is slowly varying in magnitude for the angles we have studied. This fact provides increased confidence in the extrapolation, indicated by the dashed lines in fig. 1, of the shape and magnitude of the continuum beneath the resonance peaks. At 4 °, a large peak is observed which is located at the excitation energy of the GDR. The solid curve shown on the 4 ° data is the GDR shape and energy from (% n) measurements [6], with magnitude fitted to our data. The GDR accounts for virtually the entire observed peak at 4 ° . The excellent agreement between the (% n) and (p, p ' ) spectra provides a clear demonstration of inelastic proton excitation of the GDR. The GDR cross sections at 4 ° are 27 + 5 mb/sr and 25 +- 4 mb/sr, respectively, for 90Zr and 120Sn. The GMR is located [1] at nearly the same energy as the GDR but DWBA calculations indicate that the GMR cross section is at least an order of magnitude smaller than that of the GDR at 4 °. Since the E1 cross section drops rapidly with increasing angle, the E0 cross section becomes increasingly more important at larger angles. There has been a report [7] of GDR excitation in 2°8pb by 218 MeV protons which was based on location of the peak and a rapidly rising cross section at forward angles. An interesting aspect of the 4 °, 90Zr spectrum is the presence of a narrow peak at 9.1 +- 0.4 MeV. This peak is not observed in larger angle spectra. We suggest that this peak may arise from excitation of M1 states in 90Zr. Recent (e, e')measurements [8] report M1 states located at 9.371,9.000 and 8.233 MeV in 90Zr. In addition, a resonance has been observed in the 90Zr(p, n) reaction (corresponding to an excitation of 8.3 -+ 0.4 MeV in 90Zr) which is suggested to represent the analogue of parent M1 states [9]. The cross section obtained in the (p, n) measurement [10] at 200 MeV is ~ 4 mb/sr which would imply a (p, p') cross section of ~ 1 0 mb/sr, in good agreement with our measurement of 7.2 +- 2.0 mb/sr. It is clear that we have not shown conclusively that the 9.1 MeV peak arises from excitation of an M1 state. However, considering the poor resolution of the (p, p ' ) and (p, n) measurements, the agreement among the three measurements certainly 328
30 July 1981
provides strong suggestion of an M1 assignment for the peak. No similar state is seen in our 120Sn, 4 °, spectrum, nor has an M1 state been reported from other measurements on 12°Sn. The 8 ° and 10 ° spectra shown infig. 1 were obtained at a maximum for the L -- 2 angular distribution for 90Zr and 1208n, respectively. The solid curve, taken from the shape and location of the GQR as determined from (c~, a')measurements [11], provides excellent agreement with the present data. The cross section for GDR at the larger angles is greatly reduced from that observed at 4 ° . We assume that the cross section in the excitation energy region where GDR is located is comprised of GDR and GMR components in unestablished proportions. In the 12 ° spectra broad peaks seen somewhat less clearly at 8 ° and 10°,are observed at 25 -+ 1 and 27 +- 1 MeV in 120Sn and 90Zr, respectively. The width (FWHM) of the peaks is, respectively, 8 +- 1 and 9 -+ 1 MeV for 120Sn and 90Zr. As discussed below we interpret these peaks as arising from excitation of the 3hco, GOR (E3). Our values for the width and energy of the GOR are in agreement with previous [2] measurements and are close to the trends calculated for the GOR by Nix and Sierk [12]. Fig. 2 shows angular distributions for the GQR and GOR peaks. The GQR cross sections were obtained by fitting the (c~, a ' ) GQR shape to the 90Zr and 120Sn data at each angle after an appropriate underlying continuum was subtracted. For the GOR the energy and shape of the peak was determined at angles where the L = 3 cross section is a maximum and this shape was used at each angle. For the larger angle data, only an upper limit could be set for the GOR cross section. The uncertainties shown on the measured cross sections arise mostly from the uncertainty in the assumptions made for the shape and magnitude of the nuclear continuum underlying the resonance peaks. Parameters used in the DWBA calculations shown in fig. 2 were derived from energy-and mass-dependent optical potentials deduced from recent proton elastic scattering measurements [13] in the 1 0 0 - 2 0 0 MeV energy range. The comparisons in fig. 2 show that the GQR cross sections are well described by the L = 2 DWBA calculation. The sensitivity of our results to higher multipole contributions in the GQR peak is shown by the dashed curve which results fl'om a mixture of only 5% of the L = 4, EWSR, with the L = 2
Volume 103B, number 4,5 ~0
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30 July 1981
cates that there is no major L = 4 strength within the GQR peak shown in fig. 1. A 2boo, E4, resonance may be many MeV wide and thus, escape detection in the present experiment. There have been several earlier suggestions of L = 4 and L = 6 strength in the GQR peak based on rather arbitrary combinations of calculated angular distributions [14,15]. A difficulty with these previous efforts is that the calculated L = 2, 4 and 6 angular distributions have quite similar shapes in low-energy (p, p') and (a, a ' ) measurements. Thus, many arbitrary combinations of multipole strengths might improve the fits to the " G Q R " data. However, angular distributions for 200 MeV protons are dramatically different for L = 2 and L = 4, which provides, for the first time, the sensitivity needed to set an upper limit on the L = 4 strength. From the DWBA calculations we derive an EWSR depletion for the GQR of 30 -+ 8% in 90Zr and 35 -+6%in 120Sn. These values are low by about a factor of two when compared with values [1 ] from other inelastic reactions. Similarly low EWSR depletions for the GQR were noted in measurements [16] performed several years ago using inelastic scattering of 155 MeV protons. Due to the poor energy resolution of most of our measurements only limited data is available on lowlying states. We have compared DWBA calculations using the parameters of ref. [13] with our measurement of the angular distributions of the 1.33 MeV, 2 +, and 4.04 MeV, 3 - , levels of 60Ni and the 2.61 MeV, 3 - , level of 2°8pb. The calculations reproduce the shape of the measured 2 + and 3 - angular distributions, although a somewhat deeper first minimum is predicted than is observed. For the 60Ni levels the deformation parameters (/32,3) we obtain are in good agreement with previous measurements [17]. However, for the 2.61 MeV 3 - state in 208pb, use of the well established B(E3) [ 18] provides calculated cross sections a factor of two larger than the data. It has been suggested [19] that for proton energies of 150 to 200 MeV, use of the standard prescription relating transition rate and deformation parameter will lead to poor agreement with the data. It is quite possible that the "simple" collective model interaction does not provide an adequate description of 200 MeV proton scattering and use of a more microscopic calculation such as the folding model may be in order. 329
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The measured angular distribution for the peak labeled E3 in fig. 1 agrees with the L = 3 DWBA calculation, and does not agree at all with the L = 4 or L = 2 calculation. These results provide definitive evidence for the existence of the 3hoe giant octupole resonance. Within the limitation discussed above, our results for the GOR when compared to the DWBA calculations, yield 14 +- 3% and 18 +- 4%, T = 0, L = 3, EWSR depletion for 120Sn and 90Zr, respectively. These values are in agreement with the 800 MeV proton measurements of ref. [3], b u t are lower than the values from the (3He, 3He') observations. For 90Zr, the location of the GOR is nearly identical to the energy of a 2xT= 1 quadrupole resonance observed in (e, e') measurements [20]. It is well known [1] that (p, p ' ) e x c i t a t i o n of a A T = 1 quadrupole resonance should be much weaker than for a A T = 0 quadrupole. Fig. 2 shows that the GQR cross section is only approximately a factor of two or three larger than that for the 27 MeV state, certainly a smaller ratio than would be expected if the 27 MeV state was a L = 2, A T = 1 state. Further, the angular distribution for the 27 MeV state agrees very well with the L = 3 calculation, not with that for L = 2. Our results provide definite evidence for the existence of the 3/~co giant octupole resonance. In the 12 ° spectra of fig. 1, structure is visible between 5 and l 0 MeV of excitation energy. The peaks near 5.5 MeV and 8.0 MeV for 12°Sn and 90Zr, respectively, arise from excitation of the so-called low-energy octupole resonance [21]. At larger angles our data for 90Zr show a shift of the centroid of this peak to higher excitation energy and for 120Sn new peaks appear at ~ 9 MeV of excitation. This result indicates considerable higher multipolarity (L~>4) contributions to the spectra in the regions below the GQR. To quantify these observations higher resolution data are needed. These results demonstrate that inelastic scattering of 200 MeV protons is a powerful tool with which to study giant multipole resonances. We find distinct excitation of the giant dipole, giant quadrupole and giant octupole resonances. Further the strongly L-characteristic angular distributions enable us to set a very
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low upper limit on any admixture o f L = 4 strength in the GQR peak. A serious problem is that the EWSR depletions derived from these data do not agree with previous results. This difficulty may arise from a failure [19] of the collective model to describe adequately the interaction at 200 MeV. We would like to thank G.R. Satchler for many helpful discussions and J.G. Rogers, D.A. Hutcheon and G.H. Mackenzie for their many hours of help and advice concerning the MRS system and beam development.
References [1] F.E. Bertrand, Ann. Rev. Nucl. Sci. 26 (1976) 457; Giant multipole resonances - perspectives after ten years, Proc. Intern. Conf. oh Nuclear physics (Berkeley, 1980), Nucl. Phys. A345 (1981) 153c. [2] T.A. Carey et al., Phys. Rev. Lett. 45 (1980) 239; H.P. Morsch et al., Phys. Rev. Lett. 45 (1980) 337; T. Yamagata et al., submitted for publication. [3] P. Ring and J. Speth, Phys. Lett. 44B (1973) 477; P. Ring and J. Speth, Nucl. Phys. A235 (1974) 315; I. Hamamoto, Nucl Phys. A196 (1972) 101. [4] TRIUMF annual report (1977) p. 82; further details can be found on pp. 4-46 of the TRIUMF users manual. [5] R.A. Arndt et al., Phys. Rev. C9 (1978) 555. [6] B.L. Berman and S.C. Fultz, Rev. Mod. Phys. 47 (1975) 713. I71 C. Djaloli et al., Z. Phys. A298 (1980) 79. [8] D. Meuer et al., Nucl. Phys. A349 (1980) 309. [9] D.E. Bainum et al., Phys. Rev. Lett. 44 (1980) 1751. [10] J. Rapaport et al., private communication. [11] F.E. Bertrand et al., Phys. Rev. C22 (1980) 1832. [12] J. Rayford Nix and A.J. Sierk, Phys. Rev. C21 (1980) 396. [13] A. Nadasen et al., Phys. Rev. C23 (1981) 1023. [14] E.C. Halbert et al., Nucl. Phys. A245 (1975) 189. [15] H.P. Morsch et al., Phys. Rev. C22 (1980) 489. [161 N. Marty et al., Nucl. Phys. A238 (1975) 93. [17] R.L. Auble, Nucl. Data Sheets 28 (1979) 103. [18] M.B. Lewis, Nucl. Data Sheets B5 (1971) 243. [19] V.A. Madsen New aspects of the DWBAfor giant resonances, Proc. Conf. on Giant multipole resonances (Oak Ridge, TN, October 1979), ed. F.E. Bertrand (Harwood A.P., 1980) p. 93. [20] S. Fukuda and Y. Torizuka, Phys. Lett. 62B (1976) 146. [21] J.M. Moss et al., Phys. Rev. Lett. 37 (1976) 816.
PHYSICS LETTERS
30 July 1981
EXCITATION OF GIANT RESONANCE MODES IN 9°Zr AND"12°Sn BY 200 MeV PROTONS F.E. BERTRAND, E.E. GROSS, D.J. HOREN and J.R. WU 1 Oak Ridge National Laboratory 2, Oak Ridge, TN 37830, USA J. TINSLEY and D.K. McDANIELS University of Oregon 3, Eugene, OR 97403, USA L.W. SWENSON Oregon State University 3, Corvallis, OR 97331, USA and R. LILJESTRAND University of Alberta, TRIUMF 4, Vancouver, British Columbia, Canada V6T 2A3 Received 30 April 1981
Giant resonances in 90Zr and 120Sn have been studied using inelastic scattering of 200 MeV protons. The isovector giant dipole and isoscalar giant quadrupole and giant octupole (L = 3,3hto) resonances are clearly observed. An upper limit of .~ 5% is placed on the 2h~o hexadecapole strength in the quadrupole resonance peak. For 9°Zr a peak is observed which is consistent with recently reported MI strength.
During the past ten years isoscalar (T = 0), giant resonances have been discovered and intensively studied [1]. There is firm evidence for giant quadrupole (GQR) and giant monopole (GMR) resonances and strong indications [2] of the excitation of the 3hco giant octopole resonance (GOR). While predictions have been made for other, higher-L, T = 0, giant resonances, none have yet been observed. Of particular interest are predictions [3] of a 2hco, giant hexadecapole (E4) resonance (GHR) located at the same excitation energy as the GQR and which might exhaust 3 0 - 5 0 % of the L = 4, T = 0 energy weighted sum rule (EWSR). The new isoscalar giant resonances have been stud-
1 Present address: Bell Laboratories, NaperviUe, IL 60540, USA. 2 Operated by Union Carbide Corporation under contract W-7405-eng-26with the US Department of Energy. 3 Work supported in part by a grant from the NSF. 4 Under the auspices of a contract with the Canadian NRC. 326
ied using inelastic scattering of electrons and a wide variety of hadrons. While hadronic probes provide clear excitation of giant resonance peaks, positive identification of the resonance multipolarity is often lacking. This comes about since angular distributions from inelastic hadron scattering are often not strongly characteristic of m o m e n t u m transfer. The use of 2 0 0 - 4 0 0 MeV protons for giant resonance studies offers several advantages over probes previously used. The most notable advantage lies in the clearly characteristic angular distribution shapes for neighboring m o m e n t u m transfers. For example, according to DWBA calculations, the difference between a "pure" L = 2 giant resonance and an L = 2 + 4, even for as little as 1 0 - 2 0 % L = 4 EWSR depletion, is very large. Another interesting aspect of 200 MeV proton inelastic scattering is the very large small-angle cross section predicted for Coulomb excitation of the giant dipole resonance (GDR). Although it is assumed that the GDR is excited in inelastic proton scattering, di-
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rect evidence for GDR excitation has here-to-fore not been provided. Furthermore, using 2 0 0 - 4 0 0 MeV protons, spin-flip excitations, such as M1 states, may be enhanced at forward angles. In this letter results from 200 MeV proton excitation of giant resonances in 90Zr and 120Sn are reported Strong excitation of the GDR, GQR, and GOR resonances is observed and an upper limit of 5% is established for L = 4 EWSR strength depleted within the GQR. Further, for 90Zr a peak is observed at forward angles which is suggested to arise from excitation of a M1 state. These results demonstrate that medium energy inelastic proton scattering is an important tool for studies of a variety of giant multipole resonances. Protons of 200 MeV from the TRIUMF accelerator were inelastically scattered from ~60 mg/cm 2 targets and detected in the focal plane of the MRS, a broad range magnetic spectrograph facility [4]. Typical spectra covered an excitation energy range of ~40 MeV for a single field setting of the spectrograph. The MRS detector system consists of a thin plastic scintillator and a 12.5 cm by 12.5 cm wire chamber located in front of the magnet, two larger wire chambers after the magnet which intersect the focal plane, followed by a thick plastic scintillator. The front wire chamber determines the acceptance solid angle, allowing absolute determination of cross sections. The scintillators provide time-of-flight particle identification, and along with the fast wire chamber pulses form the event trigger. We have measured the spectrograph response and beam quality at zero degrees using a reduced intensity beam and find no spurious background that would affect our results. At the smallest angles studied, observations were made using blank target frames; no deleterious background was observed. Typical beam intensities ranged from 0 . 1 - 1 . 5 hA. The absolute beam current was determined using a calibrated monitor of p r o t o n - p r o t o n scattering from a thin CH 2 target located up-stream of the spectrograph. The energy resolution was typically 9 0 0 - 1 1 0 0 keV (FWHM) although some data on low-lying states were taken with about 450 keV (FWHM) resolution. Measurements were made every two degrees between 4 and 20 degrees. Our measurements of proton scattering cross sections from hydrogen using a CH 2 target agree to within 5% with phase-shift values [5]. Fig. 1 shows spectra from 90Zr and 120Sn at angles which should provide maximum cross sections for
30 July 1981
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Fig. 1. Spectra from inelastic scattering of 200 MeV protons from 9°Zr and 12°Sn. The multipolarities shown for the resonances are discussed in the text. The dashed line shows the shape and magnitude assumed for the nuclear continuum underlying the resonance peaks.
327
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L = 1,2 and 3 excitations. The broad peaks which are obvious in the high excitation energy continua arise from excitation of various multipolarity giant resonances the assignments of which we discuss below. The inelastic nuclear continuum is slowly varying in magnitude for the angles we have studied. This fact provides increased confidence in the extrapolation, indicated by the dashed lines in fig. 1, of the shape and magnitude of the continuum beneath the resonance peaks. At 4 °, a large peak is observed which is located at the excitation energy of the GDR. The solid curve shown on the 4 ° data is the GDR shape and energy from (% n) measurements [6], with magnitude fitted to our data. The GDR accounts for virtually the entire observed peak at 4 ° . The excellent agreement between the (% n) and (p, p ' ) spectra provides a clear demonstration of inelastic proton excitation of the GDR. The GDR cross sections at 4 ° are 27 + 5 mb/sr and 25 +- 4 mb/sr, respectively, for 90Zr and 120Sn. The GMR is located [1] at nearly the same energy as the GDR but DWBA calculations indicate that the GMR cross section is at least an order of magnitude smaller than that of the GDR at 4 °. Since the E1 cross section drops rapidly with increasing angle, the E0 cross section becomes increasingly more important at larger angles. There has been a report [7] of GDR excitation in 2°8pb by 218 MeV protons which was based on location of the peak and a rapidly rising cross section at forward angles. An interesting aspect of the 4 °, 90Zr spectrum is the presence of a narrow peak at 9.1 +- 0.4 MeV. This peak is not observed in larger angle spectra. We suggest that this peak may arise from excitation of M1 states in 90Zr. Recent (e, e')measurements [8] report M1 states located at 9.371,9.000 and 8.233 MeV in 90Zr. In addition, a resonance has been observed in the 90Zr(p, n) reaction (corresponding to an excitation of 8.3 -+ 0.4 MeV in 90Zr) which is suggested to represent the analogue of parent M1 states [9]. The cross section obtained in the (p, n) measurement [10] at 200 MeV is ~ 4 mb/sr which would imply a (p, p') cross section of ~ 1 0 mb/sr, in good agreement with our measurement of 7.2 +- 2.0 mb/sr. It is clear that we have not shown conclusively that the 9.1 MeV peak arises from excitation of an M1 state. However, considering the poor resolution of the (p, p ' ) and (p, n) measurements, the agreement among the three measurements certainly 328
30 July 1981
provides strong suggestion of an M1 assignment for the peak. No similar state is seen in our 120Sn, 4 °, spectrum, nor has an M1 state been reported from other measurements on 12°Sn. The 8 ° and 10 ° spectra shown infig. 1 were obtained at a maximum for the L -- 2 angular distribution for 90Zr and 1208n, respectively. The solid curve, taken from the shape and location of the GQR as determined from (c~, a')measurements [11], provides excellent agreement with the present data. The cross section for GDR at the larger angles is greatly reduced from that observed at 4 ° . We assume that the cross section in the excitation energy region where GDR is located is comprised of GDR and GMR components in unestablished proportions. In the 12 ° spectra broad peaks seen somewhat less clearly at 8 ° and 10°,are observed at 25 -+ 1 and 27 +- 1 MeV in 120Sn and 90Zr, respectively. The width (FWHM) of the peaks is, respectively, 8 +- 1 and 9 -+ 1 MeV for 120Sn and 90Zr. As discussed below we interpret these peaks as arising from excitation of the 3hco, GOR (E3). Our values for the width and energy of the GOR are in agreement with previous [2] measurements and are close to the trends calculated for the GOR by Nix and Sierk [12]. Fig. 2 shows angular distributions for the GQR and GOR peaks. The GQR cross sections were obtained by fitting the (c~, a ' ) GQR shape to the 90Zr and 120Sn data at each angle after an appropriate underlying continuum was subtracted. For the GOR the energy and shape of the peak was determined at angles where the L = 3 cross section is a maximum and this shape was used at each angle. For the larger angle data, only an upper limit could be set for the GOR cross section. The uncertainties shown on the measured cross sections arise mostly from the uncertainty in the assumptions made for the shape and magnitude of the nuclear continuum underlying the resonance peaks. Parameters used in the DWBA calculations shown in fig. 2 were derived from energy-and mass-dependent optical potentials deduced from recent proton elastic scattering measurements [13] in the 1 0 0 - 2 0 0 MeV energy range. The comparisons in fig. 2 show that the GQR cross sections are well described by the L = 2 DWBA calculation. The sensitivity of our results to higher multipole contributions in the GQR peak is shown by the dashed curve which results fl'om a mixture of only 5% of the L = 4, EWSR, with the L = 2
Volume 103B, number 4,5 ~0
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25
30
35
Oc.m.
Fig. 2. Angular distributions for the GQR and GOR compared with DWBAcalculations (solid and dashed curves). angular distribution. Clearly, the GQR peak contains very little, if any, contribution from excitation o f a 2hco, E4, giant resonance. This statement only indi-
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cates that there is no major L = 4 strength within the GQR peak shown in fig. 1. A 2boo, E4, resonance may be many MeV wide and thus, escape detection in the present experiment. There have been several earlier suggestions of L = 4 and L = 6 strength in the GQR peak based on rather arbitrary combinations of calculated angular distributions [14,15]. A difficulty with these previous efforts is that the calculated L = 2, 4 and 6 angular distributions have quite similar shapes in low-energy (p, p') and (a, a ' ) measurements. Thus, many arbitrary combinations of multipole strengths might improve the fits to the " G Q R " data. However, angular distributions for 200 MeV protons are dramatically different for L = 2 and L = 4, which provides, for the first time, the sensitivity needed to set an upper limit on the L = 4 strength. From the DWBA calculations we derive an EWSR depletion for the GQR of 30 -+ 8% in 90Zr and 35 -+6%in 120Sn. These values are low by about a factor of two when compared with values [1 ] from other inelastic reactions. Similarly low EWSR depletions for the GQR were noted in measurements [16] performed several years ago using inelastic scattering of 155 MeV protons. Due to the poor energy resolution of most of our measurements only limited data is available on lowlying states. We have compared DWBA calculations using the parameters of ref. [13] with our measurement of the angular distributions of the 1.33 MeV, 2 +, and 4.04 MeV, 3 - , levels of 60Ni and the 2.61 MeV, 3 - , level of 2°8pb. The calculations reproduce the shape of the measured 2 + and 3 - angular distributions, although a somewhat deeper first minimum is predicted than is observed. For the 60Ni levels the deformation parameters (/32,3) we obtain are in good agreement with previous measurements [17]. However, for the 2.61 MeV 3 - state in 208pb, use of the well established B(E3) [ 18] provides calculated cross sections a factor of two larger than the data. It has been suggested [19] that for proton energies of 150 to 200 MeV, use of the standard prescription relating transition rate and deformation parameter will lead to poor agreement with the data. It is quite possible that the "simple" collective model interaction does not provide an adequate description of 200 MeV proton scattering and use of a more microscopic calculation such as the folding model may be in order. 329
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PHYSICS LETTERS
The measured angular distribution for the peak labeled E3 in fig. 1 agrees with the L = 3 DWBA calculation, and does not agree at all with the L = 4 or L = 2 calculation. These results provide definitive evidence for the existence of the 3hoe giant octupole resonance. Within the limitation discussed above, our results for the GOR when compared to the DWBA calculations, yield 14 +- 3% and 18 +- 4%, T = 0, L = 3, EWSR depletion for 120Sn and 90Zr, respectively. These values are in agreement with the 800 MeV proton measurements of ref. [3], b u t are lower than the values from the (3He, 3He') observations. For 90Zr, the location of the GOR is nearly identical to the energy of a 2xT= 1 quadrupole resonance observed in (e, e') measurements [20]. It is well known [1] that (p, p ' ) e x c i t a t i o n of a A T = 1 quadrupole resonance should be much weaker than for a A T = 0 quadrupole. Fig. 2 shows that the GQR cross section is only approximately a factor of two or three larger than that for the 27 MeV state, certainly a smaller ratio than would be expected if the 27 MeV state was a L = 2, A T = 1 state. Further, the angular distribution for the 27 MeV state agrees very well with the L = 3 calculation, not with that for L = 2. Our results provide definite evidence for the existence of the 3/~co giant octupole resonance. In the 12 ° spectra of fig. 1, structure is visible between 5 and l 0 MeV of excitation energy. The peaks near 5.5 MeV and 8.0 MeV for 12°Sn and 90Zr, respectively, arise from excitation of the so-called low-energy octupole resonance [21]. At larger angles our data for 90Zr show a shift of the centroid of this peak to higher excitation energy and for 120Sn new peaks appear at ~ 9 MeV of excitation. This result indicates considerable higher multipolarity (L~>4) contributions to the spectra in the regions below the GQR. To quantify these observations higher resolution data are needed. These results demonstrate that inelastic scattering of 200 MeV protons is a powerful tool with which to study giant multipole resonances. We find distinct excitation of the giant dipole, giant quadrupole and giant octupole resonances. Further the strongly L-characteristic angular distributions enable us to set a very
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low upper limit on any admixture o f L = 4 strength in the GQR peak. A serious problem is that the EWSR depletions derived from these data do not agree with previous results. This difficulty may arise from a failure [19] of the collective model to describe adequately the interaction at 200 MeV. We would like to thank G.R. Satchler for many helpful discussions and J.G. Rogers, D.A. Hutcheon and G.H. Mackenzie for their many hours of help and advice concerning the MRS system and beam development.
References [1] F.E. Bertrand, Ann. Rev. Nucl. Sci. 26 (1976) 457; Giant multipole resonances - perspectives after ten years, Proc. Intern. Conf. oh Nuclear physics (Berkeley, 1980), Nucl. Phys. A345 (1981) 153c. [2] T.A. Carey et al., Phys. Rev. Lett. 45 (1980) 239; H.P. Morsch et al., Phys. Rev. Lett. 45 (1980) 337; T. Yamagata et al., submitted for publication. [3] P. Ring and J. Speth, Phys. Lett. 44B (1973) 477; P. Ring and J. Speth, Nucl. Phys. A235 (1974) 315; I. Hamamoto, Nucl Phys. A196 (1972) 101. [4] TRIUMF annual report (1977) p. 82; further details can be found on pp. 4-46 of the TRIUMF users manual. [5] R.A. Arndt et al., Phys. Rev. C9 (1978) 555. [6] B.L. Berman and S.C. Fultz, Rev. Mod. Phys. 47 (1975) 713. I71 C. Djaloli et al., Z. Phys. A298 (1980) 79. [8] D. Meuer et al., Nucl. Phys. A349 (1980) 309. [9] D.E. Bainum et al., Phys. Rev. Lett. 44 (1980) 1751. [10] J. Rapaport et al., private communication. [11] F.E. Bertrand et al., Phys. Rev. C22 (1980) 1832. [12] J. Rayford Nix and A.J. Sierk, Phys. Rev. C21 (1980) 396. [13] A. Nadasen et al., Phys. Rev. C23 (1981) 1023. [14] E.C. Halbert et al., Nucl. Phys. A245 (1975) 189. [15] H.P. Morsch et al., Phys. Rev. C22 (1980) 489. [161 N. Marty et al., Nucl. Phys. A238 (1975) 93. [17] R.L. Auble, Nucl. Data Sheets 28 (1979) 103. [18] M.B. Lewis, Nucl. Data Sheets B5 (1971) 243. [19] V.A. Madsen New aspects of the DWBAfor giant resonances, Proc. Conf. on Giant multipole resonances (Oak Ridge, TN, October 1979), ed. F.E. Bertrand (Harwood A.P., 1980) p. 93. [20] S. Fukuda and Y. Torizuka, Phys. Lett. 62B (1976) 146. [21] J.M. Moss et al., Phys. Rev. Lett. 37 (1976) 816.