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Excitation of giant resonances in 208Pb using inelastic 14N scattering
Volume 93B, number 1,2
PHYSICS LETTERS
2 June 1980
EXCITATION OF GIANT RESONANCES IN 2°8pb USING INELASTIC 14N SCATTERING U. GARG, P. BOGUCKI, J.D. BRONSON, Y.-W. LUI, K. NAGATANI, E. TAKADA, N. TAKAHASHI, T. TAMAYA and D.H. YOUNGBLOOD Cyclotron Institute, Texas A & M University, 'College Station, TX 77843, USA
Received 29 February 1980
Inelastic 14N scattering at 266 MeV was used to investigate the giant resonance region in 2°spb. The giant quadrupole resonance is excited quite strongly, but there is no evidence for the excitation of the recently reported 3- and 5- resonances.
In recent years, there has been special interest in the excitation of nuclear collective modes using heavy-ion inelastic scattering, primarily because the angular-momentum-matching conditions favor large L-transfer and make heavy ions a promising probe for investigating new collective modes. Several reports of the excitation of giant resonances using various heavy-ion projectiles have appeared in the past two years [ 1 - 4 ] . In particular, Doll et al. [3], in inelastic scattering of ~ 2 0 MeV/nucleon 160 from 208pb, have reported structures at high excitation (E x ~ 20 MeV) which they attributed to 3 - and 5 - giant resonances; no such structures were observed in the earlier light ion (a or 6Li) inelastic scattering measurements [1,5]. Moreover, these 160 results have been cited by Broglia et al. [6] as a crucial test of their "coherent state" model of heavy-ion reactions. In this letter, we report results from inelastic 14N scattering experiments on 208pb at 19 MeV/nucleon. In this measurement, the giant quadrupole resonance (GQR) at E x ~ 11 MeV was excited quite strongly, however the spectra at higher excitation were smooth; there were no discernible peaks which could be associated with the reported 3 - or 5 - strength. This study affords a close comparison with the 160 measurements because the very similar projectile-mass and energy-per-nucleon conditions should lead to similar excitations of giant resonances. 266 MeV 14N beams from the Texas A & M 224
cm cyclotron bombarded a 3.8 mg c m - 2 thick, selfsupporting, metal-foil target of 208pb (enriched to >95%) placed at the center of a 1.2 m scattering chamber. A conventional A E - E solid state detector telescope was used to detect inelastically scattered 14N particles; particle identification was achieved usir~g standard electronic circuitry. Considerable care was taken to minimize background contributions from various secondary scatterings, and an energy resolution of ~ 8 0 0 keV was obtained, which is satisfactory for our purpose. Another solid state detector, placed at ~ 2 0 ° in the scattering chamber, was used to monitor the beam integration. Data were taken at laboratory angles 12 °, 16 °, and 20 ° for 2°8pb and, to obtain an energy calibration, on a 12C target at several angles between 6 ° and 20 ° . The experimental techniques and data analysis procedures are similar to those used in inelastic a-scattering measurements described earlier [5]. The 14N inelastic scattering spectrum for 208pb at 0 (lab) = 16 ° is shown in fig. 1. In this spectrum only one giant resonance peak, located at E x ~ 11 MeV, is apparent in addition to low-lying states. Both the energy and width of this peak are consistent with its identification as well-known GQR [5,7] ; a gaussian with parameters from a-scattering (E x = 11.0 MeV, I" = 2.7 MeV) is shown superimposed on the data in fig. 1. There is no evidence for any significant giant resonance strength around E x ~ 20 MeV or at 39
Volume 93B, number 1,2 5500
PHYSICS LETTERS I
2 June 1980
I
208pb ( 14N, 14N') 208pb ELAB= 266 MeV
5000
8LAB = 16° 2500
I-Z
2000
GQR
1
0 ('-) 1500
I000
500
•
6O
50
40
50
20
I0
0
EXCITATION ENERGY (MeV)
Fig. 1. Inelastic 14N scattering spectrum for 2°aPb. The GQR position is indicated by the arrow; the solid line shows a gaussian calculated with GQR parameters from a-scattering.
any excitation energy above the GQR. This contradicts the interpretation by Doll et al. [3] of their 160 inelastic scattering results, where a broad hump in their spectrum, corresponding to E x ~ 20 MeV, was attributed to collective 3 - and 5 - excitations in 208pb. Straightforward DWBA calculations suggest that with 20 MeV/nucleon 14N and 160 the excitation of giant resonances should be comparable; indeed the cross section that we obtained for the GQR is very similar to that obtained by Doll et al. It seems unlikely that there is anything unique about 160 as a projectile which would highlight the collective states at E x ~ 20 MeV. In fact, the 3 - and 5 - resonances reported in the 160 measurements have not been observed in any other heavy-ion inelastic scattering measurements on 208pb. Previous measurements, done with 26 MeV/nucleon 6Li [1] and 10-12 MeV/ nucleon 12C and 14N [2] have significantly different dynamical conditions which might be less favorable for their excitation. The cross section obtained for the GQR with 17 MeV/nucleon 12C ions [4] was about a factor four less than the present work, suggesting somewhat different dynamical conditions for this reaction also. The present results strongly indicate 40
that the broad hump at E x -~ 20 MeV, observed only in 160 measurements, does not represent a collective state in 208pb but rather arises from other effects. This hump may result from processes involving threebody final states such as pick-up reactions leading to particle-unstable states in 17F and 170. As a result of the subsequent decay of these states, the detected particle is again 160. Similar structures, arising from the decay of 5He and 5 Li, have been identified earlier in inelastic a-scattering spectra [8]. In a recent report, Sandorfi [9] has shown that such a broad structure, resulting from 17F decay, would coincide with this hump. An investigation of the variation in the position of this hump as a function of the bombarding energy would be useful in resolving this question. Such contributions should be weaker and more diffuse for 14N beams due to the high thresholds for decay back into the projectile plus nucleon channel in 15N (10.8 MeV) and 150 (7.3 MeV) relative to 170 (4.1 MeV) and 17F (0.6 MeV'). The results of 160 inelastic scattering from 208pb have been cited by Broglia et al. [6] as a crucial test of their "coherent state" model of heavy-ion reactions. Their calculations predict the excitation of the 3hw component of the L = 5 giant resonance and the
Volume 93B, number 1,2
PHYSICS LETTERS
giant octupole resonance (L = 3) at an energy E x 20 MeV; the total ( 3 - + 5 - ) cross section is predicted to be comparable to the GQR cross section. Our results, however, suggest that the 3 - and 5 states are not populated as strongly as predicted. Indeed, the non-observation of these predicted excitations in 14N inelastic scattering measurements points to the need of further explorations, both theoretical and experimental, to resolve this discrepancy. This work was supported in part by the National Science Foundation and the Robert A. Welch Foundation.
2 June 1980
References [1 ] H.J. Gils, H. Rebel, J. Buschman and H. Klewe-Nebenius, Phys. Lett. 68B (1977) 427. [2] M. Buenard et al., Phys. Rev. Lett. 40 (1978) 1482. [3] P. Doll et al., Phys. Rev. Lett. 42 (1979) 366. [4] R. Kamermans, J. van Driel, H.P. Morsch, J. Wylczinski and A. van der Woude, Phys. Lett. 82B (1979) 221. [5] D.H. Youngblood et al., Phys. Rev. C13 (1976) 994. [6] R.A. Broglia, G. PoUarolo, A. Vitturi and A. Winther, Phys. Lett. 89B (1979) 22. [7] F. Bertrand, Ann. Rev. Nucl. Sci. 26 (1976) 457. [8] D.R. Brown, J.M. Moss, C.M. Rozsa, D.H. Youngblood and J.D. Bronson, Nucl. Phys. A313 (1979) 157, and references therein. [9] A. Sandorfi, in: Proc. on Heavy-ion physics from 10 to 200 MeV/amu (BNL, 1979), eds. J. Barette and P.D. Bond (Brookhaven National Laboratory, Associated Universities Inc., Upton, NY, 1979) p. 405.
41
PHYSICS LETTERS
2 June 1980
EXCITATION OF GIANT RESONANCES IN 2°8pb USING INELASTIC 14N SCATTERING U. GARG, P. BOGUCKI, J.D. BRONSON, Y.-W. LUI, K. NAGATANI, E. TAKADA, N. TAKAHASHI, T. TAMAYA and D.H. YOUNGBLOOD Cyclotron Institute, Texas A & M University, 'College Station, TX 77843, USA
Received 29 February 1980
Inelastic 14N scattering at 266 MeV was used to investigate the giant resonance region in 2°spb. The giant quadrupole resonance is excited quite strongly, but there is no evidence for the excitation of the recently reported 3- and 5- resonances.
In recent years, there has been special interest in the excitation of nuclear collective modes using heavy-ion inelastic scattering, primarily because the angular-momentum-matching conditions favor large L-transfer and make heavy ions a promising probe for investigating new collective modes. Several reports of the excitation of giant resonances using various heavy-ion projectiles have appeared in the past two years [ 1 - 4 ] . In particular, Doll et al. [3], in inelastic scattering of ~ 2 0 MeV/nucleon 160 from 208pb, have reported structures at high excitation (E x ~ 20 MeV) which they attributed to 3 - and 5 - giant resonances; no such structures were observed in the earlier light ion (a or 6Li) inelastic scattering measurements [1,5]. Moreover, these 160 results have been cited by Broglia et al. [6] as a crucial test of their "coherent state" model of heavy-ion reactions. In this letter, we report results from inelastic 14N scattering experiments on 208pb at 19 MeV/nucleon. In this measurement, the giant quadrupole resonance (GQR) at E x ~ 11 MeV was excited quite strongly, however the spectra at higher excitation were smooth; there were no discernible peaks which could be associated with the reported 3 - or 5 - strength. This study affords a close comparison with the 160 measurements because the very similar projectile-mass and energy-per-nucleon conditions should lead to similar excitations of giant resonances. 266 MeV 14N beams from the Texas A & M 224
cm cyclotron bombarded a 3.8 mg c m - 2 thick, selfsupporting, metal-foil target of 208pb (enriched to >95%) placed at the center of a 1.2 m scattering chamber. A conventional A E - E solid state detector telescope was used to detect inelastically scattered 14N particles; particle identification was achieved usir~g standard electronic circuitry. Considerable care was taken to minimize background contributions from various secondary scatterings, and an energy resolution of ~ 8 0 0 keV was obtained, which is satisfactory for our purpose. Another solid state detector, placed at ~ 2 0 ° in the scattering chamber, was used to monitor the beam integration. Data were taken at laboratory angles 12 °, 16 °, and 20 ° for 2°8pb and, to obtain an energy calibration, on a 12C target at several angles between 6 ° and 20 ° . The experimental techniques and data analysis procedures are similar to those used in inelastic a-scattering measurements described earlier [5]. The 14N inelastic scattering spectrum for 208pb at 0 (lab) = 16 ° is shown in fig. 1. In this spectrum only one giant resonance peak, located at E x ~ 11 MeV, is apparent in addition to low-lying states. Both the energy and width of this peak are consistent with its identification as well-known GQR [5,7] ; a gaussian with parameters from a-scattering (E x = 11.0 MeV, I" = 2.7 MeV) is shown superimposed on the data in fig. 1. There is no evidence for any significant giant resonance strength around E x ~ 20 MeV or at 39
Volume 93B, number 1,2 5500
PHYSICS LETTERS I
2 June 1980
I
208pb ( 14N, 14N') 208pb ELAB= 266 MeV
5000
8LAB = 16° 2500
I-Z
2000
GQR
1
0 ('-) 1500
I000
500
•
6O
50
40
50
20
I0
0
EXCITATION ENERGY (MeV)
Fig. 1. Inelastic 14N scattering spectrum for 2°aPb. The GQR position is indicated by the arrow; the solid line shows a gaussian calculated with GQR parameters from a-scattering.
any excitation energy above the GQR. This contradicts the interpretation by Doll et al. [3] of their 160 inelastic scattering results, where a broad hump in their spectrum, corresponding to E x ~ 20 MeV, was attributed to collective 3 - and 5 - excitations in 208pb. Straightforward DWBA calculations suggest that with 20 MeV/nucleon 14N and 160 the excitation of giant resonances should be comparable; indeed the cross section that we obtained for the GQR is very similar to that obtained by Doll et al. It seems unlikely that there is anything unique about 160 as a projectile which would highlight the collective states at E x ~ 20 MeV. In fact, the 3 - and 5 - resonances reported in the 160 measurements have not been observed in any other heavy-ion inelastic scattering measurements on 208pb. Previous measurements, done with 26 MeV/nucleon 6Li [1] and 10-12 MeV/ nucleon 12C and 14N [2] have significantly different dynamical conditions which might be less favorable for their excitation. The cross section obtained for the GQR with 17 MeV/nucleon 12C ions [4] was about a factor four less than the present work, suggesting somewhat different dynamical conditions for this reaction also. The present results strongly indicate 40
that the broad hump at E x -~ 20 MeV, observed only in 160 measurements, does not represent a collective state in 208pb but rather arises from other effects. This hump may result from processes involving threebody final states such as pick-up reactions leading to particle-unstable states in 17F and 170. As a result of the subsequent decay of these states, the detected particle is again 160. Similar structures, arising from the decay of 5He and 5 Li, have been identified earlier in inelastic a-scattering spectra [8]. In a recent report, Sandorfi [9] has shown that such a broad structure, resulting from 17F decay, would coincide with this hump. An investigation of the variation in the position of this hump as a function of the bombarding energy would be useful in resolving this question. Such contributions should be weaker and more diffuse for 14N beams due to the high thresholds for decay back into the projectile plus nucleon channel in 15N (10.8 MeV) and 150 (7.3 MeV) relative to 170 (4.1 MeV) and 17F (0.6 MeV'). The results of 160 inelastic scattering from 208pb have been cited by Broglia et al. [6] as a crucial test of their "coherent state" model of heavy-ion reactions. Their calculations predict the excitation of the 3hw component of the L = 5 giant resonance and the
Volume 93B, number 1,2
PHYSICS LETTERS
giant octupole resonance (L = 3) at an energy E x 20 MeV; the total ( 3 - + 5 - ) cross section is predicted to be comparable to the GQR cross section. Our results, however, suggest that the 3 - and 5 states are not populated as strongly as predicted. Indeed, the non-observation of these predicted excitations in 14N inelastic scattering measurements points to the need of further explorations, both theoretical and experimental, to resolve this discrepancy. This work was supported in part by the National Science Foundation and the Robert A. Welch Foundation.
2 June 1980
References [1 ] H.J. Gils, H. Rebel, J. Buschman and H. Klewe-Nebenius, Phys. Lett. 68B (1977) 427. [2] M. Buenard et al., Phys. Rev. Lett. 40 (1978) 1482. [3] P. Doll et al., Phys. Rev. Lett. 42 (1979) 366. [4] R. Kamermans, J. van Driel, H.P. Morsch, J. Wylczinski and A. van der Woude, Phys. Lett. 82B (1979) 221. [5] D.H. Youngblood et al., Phys. Rev. C13 (1976) 994. [6] R.A. Broglia, G. PoUarolo, A. Vitturi and A. Winther, Phys. Lett. 89B (1979) 22. [7] F. Bertrand, Ann. Rev. Nucl. Sci. 26 (1976) 457. [8] D.R. Brown, J.M. Moss, C.M. Rozsa, D.H. Youngblood and J.D. Bronson, Nucl. Phys. A313 (1979) 157, and references therein. [9] A. Sandorfi, in: Proc. on Heavy-ion physics from 10 to 200 MeV/amu (BNL, 1979), eds. J. Barette and P.D. Bond (Brookhaven National Laboratory, Associated Universities Inc., Upton, NY, 1979) p. 405.
41