Strong L=5 transition in Te(p, p′) reactions

Volume 45B, number 5

PHYSICS LETTERS

20 August 1973

STRONG L =5 TRANSITION IN Te(p, p') REACTIONS M. MATOBA, M. HYAKUTAKE and J. NIIDOME Department of Nuclear Engineering, Kyushu University, Fukuoka, Japan and K. YAGI, Y. AOKI and K. SATO* Department of Physics, Osaka University, Osaka, Japan Received 25 June 1973 Strong L=5 transitions have been found in the (p, p') reaction on 122,124,126,128,13OTe"The possible 2S-pole collective motion in Te nuclei is discussed.

Recently, Kerek et al. [ 1 ] have investigated the systematics of 5 - states in 122'124'126'128'13°Te at E x ~ 2 MeV, from the 12°,122,124Sn(t~, 2n~) reactions and from decay studies of 128, 130Sb produced in 235U fission. They have suggested that the lowest 5 - states are mainly due to the (hll/2, s1/2) configuration. Also extensive studies o f 122Te'and I24Te by the (p, t) and (r, d) reactions [2] and of 124Te from the decay of 124Sb [3] have been reported, in which, however, there was no evidence of the existence of these 5 states. Five even-A Te isotopes which have generally been considered to be spherical provide a good subject to investigate the collective properties as a function of neutron number because the B(EL) values for the onephonon quadrupole and octupole vibration in 122Te are three or four times larger than those for 130Te [4]. The angular distributions for inelastic scattering of 50 MeV protons have been proved to be a good probe to study the collective 2 +, 3 - , 4 + and 7 - states in medium-weight nuclei [5, 6]. The 51.9 MeV proton beam from the INS synchrocyclotron was used to investigate the 5 - states in 122,124,126,128,130Te. The experimental procedure was similar to that described in refs. [5, 6]. The targets used were.isotopically enriched metallic foils. The emitted protons were magnetically analyzed and a typical momentum spectrum for 126Te is shown in fig. la. Overall energy resolution was 80 keV. Three states were strongly excited in the E x ~ 2 MeV region * Present Address, Institute for Nuclear Study, the University of Tokyo, Tokyo, Japan.

of which the angular distributions were reproduced by DWBA calculations for L = 3, 4 and 5, respectively [7]. We were interested in the 2.22 MeV 5 - state for which the angular distribution and DWBA fit are shown in a)

1.5

~zo t .~2.5

2.o2..',(,,+) -

2, 3 8 M e V ( 3 " )

~_

2 . 2 2 M e V (~)'1

l~3"Ot 126T e ( P' P' ) 3,5 i {)L = 34.4' , i

~ _ i

iO00

i

i

i

COUNTS

b) e e/ ~e+ ~

12r"l"e (p,p')

+i -

*',

x.£

.o

,,,,,.v

DWBA

E

v

0"010°

I

I

3I0 •

i

i

I 60 e

I

I

gO*

Ocrn

Fig. 1. a) Momentum spectrum of protons from the 126Te(p,p') reaction at O = 34.4° . b) Angular distribution of the 5 - state in the 126Te(p, p') reaction. The curve shows the L = 5 DWBA prediction. 463

Volume 45B number 5

PHYSICS LETTERS

Table 1 Summary of data on 5- states in Te isotopes (p, p') Ex(MeV)

.in

GLa) (spu)

122Te 126Te 128Te 13°Te

2.41 2.22 2.13 2.11

5555-

3.9 3.2 2.5 2.1

+ 1.3 ± 0.3 -+0.2 ± 0.2

a) For a uniform charge distribution, one lms GL = [(3 +L)2/47r(2L + 1)]Z 213~.

fig. lb. The optical potential used in this calculation was of the standard type with parameters taken from ref. [8]. For the macroscopic form factor for the collective excitation, a complex coupling type was used. For 122Te, 128Te and 13°Te, the spectra and the angular distributions were similar to those for 126Te. No clear indication o f L = 5 excitation in 124Te has been found. However, the angular distribution for the 2.29 MeV 3 - state was such that the fit could be improved by admixing an L = 5 contribution. Consequently, this state might be a doublet. The excitation energies and the transition strengths obtained in singleparticle units (spu) are summarized in table 1. The present l'esults support the existence of 5 states at E x ~ 2 MeV in even-A Te isotopes and especially that of the 2.41 MeV 5 - state in 122Te, of which the assignment had been uncertain in ref. [1]. As is known from table 1, these states are more or less enhanced by the inelastic scattering of 50 MeV protons and the transition strength decreases as the neutron number approaches the closed neutron shell, N = 82. It should be noticed that under an assumption of a uniform charge distribution the transition strength or the B(EL) value obtained from inelastic scattering

464

is generally underestimated for the higher multipolarity [9] and in this mass region a correction factor o f 1.5 for L = 5 excitation has been given by Bernstein

[4].

Nucleus

± 0.01 ± 0.01 ± 0.01 ± 0.01

20 August 1973

There should be two or more two-quasi-particle 5 states in the E x ~, 2 ~ 4 MeV region, as suggested theoretically for the Sn isotopes which have a similar number o f neutrons as the Te isotopes [10]. However, only the lowest 5 - states were excited strongly in the (p, p') reaction on all the Te isotopes studied. In conclusion, both the enhancement of the lowest 5 - states and its dependence on the neutron number suggest the 25-pole motion in even-A Te isotopes. The authors would like to express their thanks to Mr. H. Murakami for his help with the experiment.

References [1] A. Kerek, Nucl. Phys. A176 (1971) 466, A185 (i972) 497; A. Kerek, P. Carlg and J. Mc Donald, Nucl. Phys. A198 (1972) 466. [2] R.L. Auble and J.B. Ball, Nucl. Phys. A179 (1972) 353. [3] K.R. Baker, J.H. Hamilton, A.V. Ramayya and G. Highland, Nucl. Phys. A186 (1972) 493. [4] A.M. Bernstein, Advances in nuclear physics eds. M. Baranger and E. Vogt, vol. 3, (1969) p. 325. [5] K. Yagi et al., Nucl. Phys. A l l l (1968) 129; M. Koike et al., Nucl. Phys. A125 (1969) 161. [6] M. Matoba et al., Phys. Lett. 44 (1973) 159. [7] M. Kawai, K. Kubo and H. Yamaura, INS report (University of Tokyo) PT-9 (1965). [8] C.B. Fulmer, J.B. Ball, A. Scatt and M.L. Whiten, Phys. Rev. 181 (1969) 1565. [9] L.W. Owen and G. R. Satchler, Nucl. Phys. 51 (1964) 155. [10] M. Gmitro, A. Rimini, J. Sawicki and T. Weber, Phys. Rev. 173 (1968) 964.