N U C L E A R PHYSICS A

ELSEVIER Nuclear Physics A649 (1999) 271c-274c

Analog Giant Resonances Excited by (p, n), (n, p), (d, ~He), (t, SHe) and (SHe, 3H) Reactions

Stanley S. Hanna Physics Department, Stanford University, Stanford, California 94305 U.S.A.

The increased availability of facilities for producing charge-exchange reactions has made it possible to excite and study analog giant resonances which are the isospin multiplets, with Tz = -T...T, of giant resonances with isospin T. See Fig. 1 [1]. The analog giant E1 resonances (GDRs) have now been observed for A = 9, 12, and 13 nuclei [2] by means of the (n,p) [3], ('a'~,"x °) [21, ( r - , 7 ) [41, (Tr+,Tr - ) [5,61 and C t, SHe) [7] reactions. In the (n,p) reaction on laC at E,, = 65 MeV [3] the analog GDR in laB was identified by its proton angular distribution (Fig. 2) and by comparing the proton spectrum with that of the gamma spectrum from pion capture (Fig. 3) [4]. An M2 (spin-dipole) analog resonance was also identified in A = 13 nuclei in the same (n,p) reaction. In this case the ,dentificatlon was made also on the basis of the proton spectrum and angular distribution m (n,p) and by a model dependent comparison with M2 levels in I~B [3]. The work wlttt the single-charge-exchange pion reactions [2] featured the observation of the decay properties of the GDR components of the isovector E1 resonance in these nuclei. The lsospin purity of these resonances was displayed by their decay properties, as well as the clear separation of the E1 strength into T = 3/2 and 1/2 levels.

In the above reaction ~SC(n,p)lSB [3], the 0 ° cross section measured for the ground state gave a value of 179 4-20 MeV fm 3 for V~¢ which is the spin-isospin central part of the volume integral of the effective N-N interaction. A theoretical calculation, 181.3 MeV fm s, is in good agreement. Distorted wave impulse approximation calculations were compared with the experimental angular distributions in Fig. 2. Good agreement was obtained for the dominant Gamow-Teller transition to the ground state of laB (peak at E v = 51 MeV at 31°), and fair agreement for the transition to the unresolved doublet at 6.5 and 7.6 MeV assuming it is spin-dipole in character and for the broad resonance at 10.2 MeV assuming it is the analog of the giant E1 resonance in xsC. It can be seen that the GDR peak at 44 MeV has a very different angular distribution than the unresolved M2 peak at 40 MeV at 31 °.

The giant isovector analog E1 resonances were observed by means of the pion single- charge-exchange reactions (~-+, ~r °) [8] (see Fig. 4). The giant T = 1 analog E1 resonances in 12B and 12N, built on the ground state of 12C, were observed in the l~C(r~-,r°) reactions (see Fig. 4). The giant T = 3/2 resonances in lSB and 9Li, built on the ground states of lsC and 9Be, were studied with the xsC, 9Be(~r-,Tr °) reactions (see Fig. 4). Analogs of these resonances were found in the lsC, 9Be(~r+, lr °) reactions along

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272c S.S. Hanna~Nuclear Physics A649 (1999) 271c-274e

with the lower lying T = I12 resonances. These resonances together with the higher T = 3/2 resonances constitute isospin-splitting in the nuclei, ZaN and 9Be (see Fig. 4). These isospin assignments were made by comparing results from the (Tr ±, 7r °) reactions and studying the measured angular distributions. The assignments wcrc confirmed by determining the decay properties of the resonances with coincident (~r°p) measurements in the (Tr+,~r°p) reactions. The proton angular distributions measured relative to the momentum transfer direction were consistent with a resonant dipole process.

T=6

Z=39 ~ \ /1~ / / / . . . . "~ Tz= 6

" ~ >TfST=~ N •50 Z =40 ~ - . ~ Tz= 5 ~

T=4 N =49 Z =41 Tz= 4

(n,p) (d,2H¢) (p,n) (t,3Hc) (y,x) (3He,t) (~-,~0) (c,¢'x) (~+,~0) (re--, 7) (p,p'x) (~'+,7) (13-) (~,~'x) (13 + )

_laC~(n.p)'~3e ' I A 'r 4o, s~..6suey~. ,.JL -

I '

501 - 9 °

251 -

( u , i I n I , u I

30{ - 1 7 ° ] ~ n -

30(

c -..4. I t i l t . i [

IO0- - 23,=, ~

~00 ~ -

o J - n I n n J I ' ,t l

~00 - 31 ° , = . J ~ t ~ --

50 ! r r " " " ~

o 1 I i I . t I

50 39 ° -

I I t l f . v ]

0 20 40 60 (MeV)

Figure i. Schematic diagram of analog tran- sitions and probes.

Figure 2. Spectra of znO(n,p)1~B. The proton peaks at ]~p = 50, 44, and 40 MeV at 31 ° correspond to levels in 13B at 0, 7, and 10.2 MeV [3].

The isospin character of giant resonance states can be studied by comparing the analog

S.S. Hanna~Nuclear Physics A649 (1999) 271c-274c 273c

components excited by charge-exchange reactions with the parent resonances excited by inelastic scattering [1]. Recently, the use of (SHe, t) reactions under high resolution has been revived in these studies [9, 10]. For N = Z nuclei, Tg~ -- 0 and (SHe, t) can excite only T = 1 analog states, whereas inelastic (e,e'), (p,p')... reactions can excite both T = 0 and T = 1 reactions. Detailed comparisons have been carried out for 2aS[10]. In this example the T = 0 --4 T = 1 transitions excite the well-studied Gamov-Teller (GT) transitions which are merely analogs of giant M1 resonances in the parent nuclei. In the general case Tg~ > 1, one excites GT states with AT = 0, +1 which are analogs of M1 states. A detailed study was made for the T = 1 nucleus SaNi [9]. Another very useful probe is the virtual (d,2He) reaction [11].

R E F E R E N C E S

1. S.S. Hanna, in Isospin in Nuclear Physics, Ed. D.H. Wilkinson (North-Holland, Am- sterdam, 1969) p. 592.

2. S.S. Hanna, in Spin-Isospin Responses and Weak Processes in Hadrons and Nuclei, Eds. H. Ejiri, Y. Mizuno, and T. Suzuki (North-Holland, Amsterdam, 1994) 173c.

3. K. Wang, C.J. Martoff, D. Pocanic, S.S. Hanna, F.P. Brady, J.L. Romero, C.M. Castaneda, J.R. Drummond, B. McEachern, and D.S. Sorenson, Phys. Rev. C 53 (1996) 1718.

4. C.J. Martoff, J.A. Bistirlich, C.W. Clawson, K.M. Crowe, M. Koike, J.P. Miller, S.S. Rosenblum, W.A. Zajc, H.W. Baer, A.H. Wapstra, G. Strassner, and P. Truol, Phys. Rev. C 27 (1983) 1621. G.R. Burleson, G.S. Blanpied, G.H. Daw, A.J. Viescas, C.L. Morris, H.A. Thiessen, S.J. Greene, W.J. Braithwaite, W.B. Cottingame, D.B. Holtkamp, I.B. Moore, and C.F. Moore, Phys. Rev. C 22 (1980) 1180. S. Mordechai, N. Auerbach, S. Greene, C.L. Morris, J.M. O'Donnell, H.T. Fortune, G. Liu, M. Burlein, A. Wousmaa, S.H. Yoo, and C.F. Moore, Phys. Rev. C 40 (1989) 850. S.M. Austin, private communication 1998. S.S. Hanna, Nucl. Phys. A577 (1994) 173. Y. Fujita, H. Akimune, I. Daito, M. Fujiwara, M.N. Harakeh, T. Inomata, J. Janecke, K. Katori, H. Nakada, S. Nakayama, A. Tamii, M. Tanaka, H. Toyokawa, M. Yosoi, Phys. Left. B 365 (1996) 29.

10. Y. Fujita, H. Akimune, I. Daito, M. Fujiwara, M.N. Harakeh, T. Inomata, J. Janecke, K. Katori, C. Luttge, S. Nakayama, P. von Neumann-Cosel, A. Richter, A. Tamii, M. Tanaka, H. Toyokawa, H. Ueno, and M. Yosoi, Phys. Rev. C 55 (1997) 1137.

11. H. Sakai, private communication and H. Ohnuma, Phys. Rev. C 47 (199 3) 648.

5.

6.

7. 8. 9.

274c S.S. Hanna~Nuclear Physics A649 (1999) 271c-274c

f: "It' (138) (MeV)

20 1,5: 10 5 0

i | ~E*("C)(M,V) , ~, I . | ' 11,~ 12B÷ n

800

600

0

2 0 O

0

' ' ' 1 0 2 ' '

~3C(2.P)'=s ~. 11/3i 5 o

100 1 2 0 1 4 0 160 180 2 0 0

C h a n n e l s

mass 13 Ex (MeV)

t2

8

4

0

I 0 0

> 100

; o

% loo

I00

40 20 O

* i i

'~ . . , . , .~ ~ , . ! , , . , .

1 2 0 [40 180

T.® (MeV)

Figure 3. Comparison 0¢ spectra from 13C(n, p)Z3B and 13C(~r-, 7)ZSB [3J.

Figure 4. Spectra from 12'laC(lr~=, 7r °) at T,~ = 165 MeV and O = 0 compared with each other and indicated photon nuclear reactions [8].