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E0 transitions in 114,116,118Sn
Volume 62B, number 4
PHYSICS LETTERS
E0 TRANSITIONS
21 June 1976
I N 114, I 16,118 S n
A. BA.CKLIN, W. DIETRICH, R. JULIN*, J. KANTELE*, M. LUONTAMA* and L. WESTERBERG**
Institute of Physics, Universityof Uppsala, Uppsala,Sweden Received 10 May 1976 Lowqymg 0+ states have been excited m the (p, p') reaction via the s1/2 isobaric analog resonance in 114,1i6Sn and in radloactwe decay in 118Sn" From conversion electron measurements, values of X = B(E0; 0÷ ~ 0])/B(E2; 0÷ 2"~) are obtained from the 1953 keV state m il4sn, 1757 and 2027 keV states in 116Sn and 1758 and 2056 keV states in i 18Sn"
Even-even nuclei exhibit in general at least one 0 + state at low excitation energy. The 0 + states are of special importance m even Sn nuclei, where two or even three such states are among the very lowest excited states [ 1 - 5 ] . The main body of information on these states has been obtained from two-particle transfer reactions [ 3 - 5 ] . The cross sections for these reactions show a fair over-all agreement with those predicted in various two-quaslparticle models [ 6 - 8 ] and the palnng-vibratlonal model [9, 10]. Although the pair excitation thus must be considered to be an important feature in the Sn spectrum, the exact nature of the 0 + states is not known. For instance, also the 2-phonon quadrupole vibration is expected in the energy interval of interest here. The need for additional experimental data, especially on the E0 and E2 decay of the 0 + states, has therefore been recognized since long [7]. The presently available information on these transitions is scarce [11, 12]. In this letter we present the first data from an attempt to a systematic study of the electromagnetic decay properties of the 0 + states in even Sn nuclei. Ratios of reduced E0/E2 transition probabilities (X-values) are presented for the lowest 0 + states m 116,118Sn and for the lowest state in ll4Sn. The 0 + states in l l 4 S n and l l 6 S n were excited In the (p, p') reaction using the Uppsala EN tandem. The proton energies were 6.40 and 6.92 MeV, respec* Permanent address. Department of Physics, University of Jyvfiskyla, Finland. ** Present address Department of Chemistry, Washington University, St. Lores, Missouri, USA. 402
tlvely, which correspond to the lp = 0 isobaric analog resonances [13]. The 0 + states in 118Sn were excited m the 3-decay of the 3.5 mm l l 8 s b , which was induced in the (p, n) reaction. The targets were metalhc, 0 5 mg/cm 2, self-supporting foils enriched to 64.1% (114Sn), 95.6%(116Sn ) and 97.1% (118Sn) Internal conversion electron spectra were recorded m swept-current and/or fixed-current modes of operation of a combination magnetic lens plus Sl(Li) spectrometer [14]. The spectrometer system was placed at 125 ° to the beam, thus making corrections for angular distributions unnecessary. Fig. 1 gives the electron spectrum (recorded in the swept-current mode of the magnetic lens) for 116Sn and shows that the lines associated with the two lowest 0 + states in l l 6 S n are easily identified. For confirmation of the multlpole character of the transitions of interest, also 7-ray spectra were recorded. The experimental results are summarized in table 1. For some of the states, the main part o f the errors given is due to the fact that the E2 transition turned out to nearly coincide with lines in the contaminating spectra. The branching ratio of the 0~ state in l l 6 S n is In good agreement with earher data [11 ]. The preliminary branching ratios o f the 0~ and 0~ states for l l 8 S n of Kawakarru et al. [12] are about 20 to 30% lower than ours. The X-values for l l 6 S n and l l 8 S n are similar and the ratios X(O~)/X(O~) quite large (about 8 to 10). The situation seems to be different an l l 4 S n , in which the lowest (and only) state we could observe has an Xvalue which is considerably larger than those observed
Volume 62B, number 4
PHYSICS LETTERS
21 June 1976
~000 I
"sSn +0 16,~zaMerle-
I
l
t ~.
t
b~
±
t
~
t~~
"~z)~ - -
t b~
b m
5200
~s
ew..J
~ 2'~00 z
o- 1600
800
L bOO
I 8(30
1200
2000
1800
(;H.~NNEL
2q-O0
?800
3200
3600
Nt/~BER
[~tg. 1, Electron spectrum from the reaction 116Sn(p ' p,) with Ep adjusted to the lp = 0 IAR taken with a magnetic lens plus SffL0 spectrometer Lines which are not m ll6Sn are due to the (p, 7) and (p, n) reactions. for t h e l o w e s t state m the heavier ISOtopes. A possible e x p l a n a t t o n for this c o u l d be t h a t the o b s e r v e d 0 + level in l l 4 s n in fact Is t h e 0~ state; b o t h its e n e r g y a n d its X-value seem c o m p a t i b l e w i t h this. On t h e o t h e r h a n d , n o simple s y s t e m a h c s s h o u l d be r e h e d
u p o n here, since the ( t , p) cross sections for t r a n s i t i o n s to the e x c i t e d 0 + states are m u c h larger in 114Sn than m t h e o t h e r Sn Isotopes. This is i n t e r p r e t e d [4} as an effect o f t h e g7/2 + d5/2 subshell closure, w h i c h dtst u r b s the pairing c o r r e l a t i o n s in the s u p e r - c o n d u c t i n g
Table 1 Summary of present results on I 14,116,118Sn State a) Nucleus
ll4Sn 116Sn tlSSn
0~
0F Energy keV f)
le(02 --* 01) b) [e(O~z ~ 21")
1953.2 1756.8 1757.8
1.00 -+ 0.20 0.38 -+ 0.08 0.32-+0.06
+
+
Energy
le(03 ---"0 t ) b)
Xexp c)
keV
le(O~ ~ 2~)
Xex p
Xvlb d)
0.042 e) 0.0086 0.009'/
2027.3 20565
1.18 -+ 0.20 1.3 ±0.5
0.066 0.I0
0.014 0.013 0013
a)
+ + 02 refers to the first exerted 0 + state, 0s refers to the second excited 0 + state. b) Intensity ratio for the K conversion hnes. -5 c) Xex p = e2R402/B(E2) = 2.56 × 1 0 9 A 4/3- [Ie(O + -. 01.)/le(O÷ --, 21.)] .[aK(E2)/S2K] "£.),,where o K Is the theoretical K-conversion coefficient [ 17], ~ZK the "electronic factor" for the E0 transition [ 18] and E 3, the energy of the E2 transition in MeV. The e~perlmenta! errors are estimated to be about 25% d) Harmonic vibrator value, see text. e) Lowest observed state m this experiment, see text. 0 Best values from the present work and from refs. [19] and [20]. 403
Volume 62B. number 4
PHYSICS LETTERS
ground state and makes p a m n g vibrational excitations possible. No evadence for a lower 0 + state has been found in any other reaction [ 2 - 5 ] , e x c e p t for a very weak m t o n group interpreted as due to an l = 0 transfer to a state at 1.58 MeV in the (d, t) reaction [1 ]. In the (p, p') reaction used in the present w o r k we could see no trace of an EO t r a n s m o n at 1.58 MeV (intensity less than 3% of that of the 1953 keV transition). One should also note that the 0 + state at 2155 keV, which has been well estabhshed m the two-neutron transfer reacnon [4], was not observed in the present measurements (EO intensity less than 5% of that o f the 1953 keV transmon). In table 1 are given for comparison the X-values obtamed for the 2-phonon 0 + state of the simple harmon2 s [e.g. 15]. Experimental lC Oscillator, 1.e., X =/3rm values o f 3 2 s were taken f r o m ref. [161. One may note the agreement w i t h the experimental values for the O~ states in l l 6 S n and l l 8 S n , although this may be accidental. Possible phonon structure o f these states would also c o n f o r m to our preliminary half-hfe measurements on 116Sn which seem to m & c a t e that the E2(O~ --, 2~) t r a n s m o n Is enhenced.
References [1 ] E.J. Schneld, A. Prakash and B.L. Cohen, Phys. Rev. 156 (1967) 1316.
404
2l June 1976
[2] E.J. Schneld, E.W. Hamburgcr and B.L. Cohen, Phys. Rev. 161 (1967) 1208. [ 3] J.H. Bjcrregaard et al., Nucl. Phys. A110 (1968) 1. 141 J.H. Blerregaard et al., Nucl. Phys. AI31 (1969) 481. 15] D.G. Fleming, M. Blaun, H.W. Fulbright and J.A. Robbms, Nucl. Phys. A157 (1970) 1. [6] R. Arvleu and E. Salustl, Nucl. Phys. 66 (1965) 305. [7t J.O. Rasnmssen, Proc. Nucl. Structure Symp., Dubna 1968, IAEA STI/PUB 189, p. 169. [8] D.M. Clement and E.U. Baranger, Nucl. Phys. AI20 (1968) 25. 191 D.R. B& and R.A. Brogha, Nucl. Phys. 80 (1966) 289. [101 R.A. Brogha, C. Rledel, B. Sorcnsen and T. Udagawa, Nucl. Phys. A l l 5 (1968) 273. [11] b. Pleiter, Nucl Phys A184 (1972) 443 [121 H Kawakanu et al., Annual Report of INS 1974, Tokyo unpubhshed. [13] P. Richard, C.F. Moore, J.A. Becker and J.D. Fox, Phys. Rev. 145 (1966) 971. [14] L. Westerberg, L.O. Edvardson, G.Ch. Maducme and J.E. Thun, Nucl. Instr. 128 (1975) 61. [15] K. Kumar, in The electromagnetic interaction in nuclear spectroscopy~ ed. W.D. Hamilton (North-Holland, Amsterdam, 1975) Ch. 3. [ 16] P.H. Stelson and L. Grodzms, Nucl. Data A1 (1965) 21. [17] R.S. Hager and E.C. Seltzer, Nucl. Data A4 (1968) 1. [18] R S. ltagcr and E.C. Seltzer, Nucl. Data A6 (1969) 1. 119] G.H. Carlson, W.L. Talbert and S. Raman, Nuclear Data Sheets 14 (1975) 247. [20] G.H. Carlson, W.L. Talbert and S. Raman, Nuclear Data Sheets 17 (1976) 1.
PHYSICS LETTERS
E0 TRANSITIONS
21 June 1976
I N 114, I 16,118 S n
A. BA.CKLIN, W. DIETRICH, R. JULIN*, J. KANTELE*, M. LUONTAMA* and L. WESTERBERG**
Institute of Physics, Universityof Uppsala, Uppsala,Sweden Received 10 May 1976 Lowqymg 0+ states have been excited m the (p, p') reaction via the s1/2 isobaric analog resonance in 114,1i6Sn and in radloactwe decay in 118Sn" From conversion electron measurements, values of X = B(E0; 0÷ ~ 0])/B(E2; 0÷ 2"~) are obtained from the 1953 keV state m il4sn, 1757 and 2027 keV states in 116Sn and 1758 and 2056 keV states in i 18Sn"
Even-even nuclei exhibit in general at least one 0 + state at low excitation energy. The 0 + states are of special importance m even Sn nuclei, where two or even three such states are among the very lowest excited states [ 1 - 5 ] . The main body of information on these states has been obtained from two-particle transfer reactions [ 3 - 5 ] . The cross sections for these reactions show a fair over-all agreement with those predicted in various two-quaslparticle models [ 6 - 8 ] and the palnng-vibratlonal model [9, 10]. Although the pair excitation thus must be considered to be an important feature in the Sn spectrum, the exact nature of the 0 + states is not known. For instance, also the 2-phonon quadrupole vibration is expected in the energy interval of interest here. The need for additional experimental data, especially on the E0 and E2 decay of the 0 + states, has therefore been recognized since long [7]. The presently available information on these transitions is scarce [11, 12]. In this letter we present the first data from an attempt to a systematic study of the electromagnetic decay properties of the 0 + states in even Sn nuclei. Ratios of reduced E0/E2 transition probabilities (X-values) are presented for the lowest 0 + states m 116,118Sn and for the lowest state in ll4Sn. The 0 + states in l l 4 S n and l l 6 S n were excited In the (p, p') reaction using the Uppsala EN tandem. The proton energies were 6.40 and 6.92 MeV, respec* Permanent address. Department of Physics, University of Jyvfiskyla, Finland. ** Present address Department of Chemistry, Washington University, St. Lores, Missouri, USA. 402
tlvely, which correspond to the lp = 0 isobaric analog resonances [13]. The 0 + states in 118Sn were excited m the 3-decay of the 3.5 mm l l 8 s b , which was induced in the (p, n) reaction. The targets were metalhc, 0 5 mg/cm 2, self-supporting foils enriched to 64.1% (114Sn), 95.6%(116Sn ) and 97.1% (118Sn) Internal conversion electron spectra were recorded m swept-current and/or fixed-current modes of operation of a combination magnetic lens plus Sl(Li) spectrometer [14]. The spectrometer system was placed at 125 ° to the beam, thus making corrections for angular distributions unnecessary. Fig. 1 gives the electron spectrum (recorded in the swept-current mode of the magnetic lens) for 116Sn and shows that the lines associated with the two lowest 0 + states in l l 6 S n are easily identified. For confirmation of the multlpole character of the transitions of interest, also 7-ray spectra were recorded. The experimental results are summarized in table 1. For some of the states, the main part o f the errors given is due to the fact that the E2 transition turned out to nearly coincide with lines in the contaminating spectra. The branching ratio of the 0~ state in l l 6 S n is In good agreement with earher data [11 ]. The preliminary branching ratios o f the 0~ and 0~ states for l l 8 S n of Kawakarru et al. [12] are about 20 to 30% lower than ours. The X-values for l l 6 S n and l l 8 S n are similar and the ratios X(O~)/X(O~) quite large (about 8 to 10). The situation seems to be different an l l 4 S n , in which the lowest (and only) state we could observe has an Xvalue which is considerably larger than those observed
Volume 62B, number 4
PHYSICS LETTERS
21 June 1976
~000 I
"sSn +0 16,~zaMerle-
I
l
t ~.
t
b~
±
t
~
t~~
"~z)~ - -
t b~
b m
5200
~s
ew..J
~ 2'~00 z
o- 1600
800
L bOO
I 8(30
1200
2000
1800
(;H.~NNEL
2q-O0
?800
3200
3600
Nt/~BER
[~tg. 1, Electron spectrum from the reaction 116Sn(p ' p,) with Ep adjusted to the lp = 0 IAR taken with a magnetic lens plus SffL0 spectrometer Lines which are not m ll6Sn are due to the (p, 7) and (p, n) reactions. for t h e l o w e s t state m the heavier ISOtopes. A possible e x p l a n a t t o n for this c o u l d be t h a t the o b s e r v e d 0 + level in l l 4 s n in fact Is t h e 0~ state; b o t h its e n e r g y a n d its X-value seem c o m p a t i b l e w i t h this. On t h e o t h e r h a n d , n o simple s y s t e m a h c s s h o u l d be r e h e d
u p o n here, since the ( t , p) cross sections for t r a n s i t i o n s to the e x c i t e d 0 + states are m u c h larger in 114Sn than m t h e o t h e r Sn Isotopes. This is i n t e r p r e t e d [4} as an effect o f t h e g7/2 + d5/2 subshell closure, w h i c h dtst u r b s the pairing c o r r e l a t i o n s in the s u p e r - c o n d u c t i n g
Table 1 Summary of present results on I 14,116,118Sn State a) Nucleus
ll4Sn 116Sn tlSSn
0~
0F Energy keV f)
le(02 --* 01) b) [e(O~z ~ 21")
1953.2 1756.8 1757.8
1.00 -+ 0.20 0.38 -+ 0.08 0.32-+0.06
+
+
Energy
le(03 ---"0 t ) b)
Xexp c)
keV
le(O~ ~ 2~)
Xex p
Xvlb d)
0.042 e) 0.0086 0.009'/
2027.3 20565
1.18 -+ 0.20 1.3 ±0.5
0.066 0.I0
0.014 0.013 0013
a)
+ + 02 refers to the first exerted 0 + state, 0s refers to the second excited 0 + state. b) Intensity ratio for the K conversion hnes. -5 c) Xex p = e2R402/B(E2) = 2.56 × 1 0 9 A 4/3- [Ie(O + -. 01.)/le(O÷ --, 21.)] .[aK(E2)/S2K] "£.),,where o K Is the theoretical K-conversion coefficient [ 17], ~ZK the "electronic factor" for the E0 transition [ 18] and E 3, the energy of the E2 transition in MeV. The e~perlmenta! errors are estimated to be about 25% d) Harmonic vibrator value, see text. e) Lowest observed state m this experiment, see text. 0 Best values from the present work and from refs. [19] and [20]. 403
Volume 62B. number 4
PHYSICS LETTERS
ground state and makes p a m n g vibrational excitations possible. No evadence for a lower 0 + state has been found in any other reaction [ 2 - 5 ] , e x c e p t for a very weak m t o n group interpreted as due to an l = 0 transfer to a state at 1.58 MeV in the (d, t) reaction [1 ]. In the (p, p') reaction used in the present w o r k we could see no trace of an EO t r a n s m o n at 1.58 MeV (intensity less than 3% of that of the 1953 keV transition). One should also note that the 0 + state at 2155 keV, which has been well estabhshed m the two-neutron transfer reacnon [4], was not observed in the present measurements (EO intensity less than 5% of that o f the 1953 keV transmon). In table 1 are given for comparison the X-values obtamed for the 2-phonon 0 + state of the simple harmon2 s [e.g. 15]. Experimental lC Oscillator, 1.e., X =/3rm values o f 3 2 s were taken f r o m ref. [161. One may note the agreement w i t h the experimental values for the O~ states in l l 6 S n and l l 8 S n , although this may be accidental. Possible phonon structure o f these states would also c o n f o r m to our preliminary half-hfe measurements on 116Sn which seem to m & c a t e that the E2(O~ --, 2~) t r a n s m o n Is enhenced.
References [1 ] E.J. Schneld, A. Prakash and B.L. Cohen, Phys. Rev. 156 (1967) 1316.
404
2l June 1976
[2] E.J. Schneld, E.W. Hamburgcr and B.L. Cohen, Phys. Rev. 161 (1967) 1208. [ 3] J.H. Bjcrregaard et al., Nucl. Phys. A110 (1968) 1. 141 J.H. Blerregaard et al., Nucl. Phys. AI31 (1969) 481. 15] D.G. Fleming, M. Blaun, H.W. Fulbright and J.A. Robbms, Nucl. Phys. A157 (1970) 1. [6] R. Arvleu and E. Salustl, Nucl. Phys. 66 (1965) 305. [7t J.O. Rasnmssen, Proc. Nucl. Structure Symp., Dubna 1968, IAEA STI/PUB 189, p. 169. [8] D.M. Clement and E.U. Baranger, Nucl. Phys. AI20 (1968) 25. 191 D.R. B& and R.A. Brogha, Nucl. Phys. 80 (1966) 289. [101 R.A. Brogha, C. Rledel, B. Sorcnsen and T. Udagawa, Nucl. Phys. A l l 5 (1968) 273. [11] b. Pleiter, Nucl Phys A184 (1972) 443 [121 H Kawakanu et al., Annual Report of INS 1974, Tokyo unpubhshed. [13] P. Richard, C.F. Moore, J.A. Becker and J.D. Fox, Phys. Rev. 145 (1966) 971. [14] L. Westerberg, L.O. Edvardson, G.Ch. Maducme and J.E. Thun, Nucl. Instr. 128 (1975) 61. [15] K. Kumar, in The electromagnetic interaction in nuclear spectroscopy~ ed. W.D. Hamilton (North-Holland, Amsterdam, 1975) Ch. 3. [ 16] P.H. Stelson and L. Grodzms, Nucl. Data A1 (1965) 21. [17] R.S. Hager and E.C. Seltzer, Nucl. Data A4 (1968) 1. [18] R S. ltagcr and E.C. Seltzer, Nucl. Data A6 (1969) 1. 119] G.H. Carlson, W.L. Talbert and S. Raman, Nuclear Data Sheets 14 (1975) 247. [20] G.H. Carlson, W.L. Talbert and S. Raman, Nuclear Data Sheets 17 (1976) 1.