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Spin, lifetime and magnetic moment of a new isomeric state in 121Te
Volume 90B, number 1,2
PHYSICS LETTERS
11 February 1980
SPIN, LIFETIME AND MAGNETIC MOMENT OF A NEW ISOMERIC STATE IN 121Te
M. IONESCU-BUJOR, A. IORDACHESCU, E.A. IVANOV and D. PLOSTINARU Institute for Physics and Nuclear Engineering, Bucharest, Romania Received 4 December 1979
A 7/2 + isomeric state has been identified in 121 Te by pulsed-beam time-differential 3,-ray m e a s u r e m e n t s following the 118Sn(a, n) and 121Sb(d, 2n) reactions, and the interaction of this state with an external magnetic field has been investigated. The results of the m e a s u r e m e n t s are: TI/2 = 85.3(5) ns, E~, = 212.3(1) and 231.0(1) keV, j1r = 7/2 + and At = +0.738(10) n.m.
Gamma transition rates and magnetic moments of excited nuclei provide important information about many aspects of nuclear structure. Short-lived isomers offer a good opportunity for experimental investigations on static and dynamic electromagnetic properties of excited nuclear states. In this letter we report on the identification and a detailed study of a new isomeric state in 121Te. The experiments were performed at the U-120 cyclotron of IPNE, Bucharest. The new isomeric state in 121Te has been excited by the ll8Sn(a, n) and 121Sb(d, 2n) reactions with 24 MeV a-particles and 12 MeV pulsed deuteron beams, respectively (pulse width 4 ns at a repetition time of 5/as), on isotopically enriched polycrystalline metallic targets. The 7-rays were detected with Ge, Ge(Li) and NaI(T1) detectors. The delayed 3,-ray spectra showed two lines of energies 212.3(1) and 231.0(1) keV with the same half life of 85.3(5) ns (see fig. 1). A 3/2 + excited state of 121Te at E x = 212.19 keV is known from the decay of 121I [1 ]. We assumed that this level represents the intermediate state it/the observed isomeric decay. Direct evidence for a genetic link between the two transitions has been obtained in a sum-coincidence experiment. The pulsed beam extracted in air through a thin foil hit the target positioned on top of a large NaI(T1) single crystal. The summing effects of the coincidence events, observed in the single delayed "y-ray spectrum, support our assumption about the cascade decay. The 231 keV line was assigned to the isomeric transition because no such line could be observed in the prompt ~,-ray spectrum (see fig. 1).
TIME (Fls) 0/-. 06
0.2 I
,
i
l
i
m~
1~-
I
360
,
i
E':24MeV 1
I
"~
0.8 i
118Sn(~'n)121Te t los
xx.x
o
~
l
-..
1
2310
400 440 CHANNEL
103
I
480
Fig. 1. P r o m p t and delayed "),-ray spectra from the ll8Sn(c~, n)121Te reaction registered with a Ge detector together with t h e time spectrum for b o t h lines detected with a NaI(T1) crystal.
The low-lying levels of 1 2 1 T e [ 1] including our experimental data are presented as an insert in fig. 2. The spin and parity j~r = 7/2 + for the short-lived isomeric level have been established on the basis of good consistency with the following experimental data. 65
Volume 90B, n u m b e r 1,2
7/2*
PHYSICS LETTERS
1112-
[ 231.0 [ E2 [
4/,:~3 853 ns 293.9 15.,.d
3/2*
21Z31
212.3 0.06n$
118SDliquid (o(. nj121Te H=106
kG
HI * 5.3*/*E2] 112 +
R(t} 0.1
m
0
16.8 d
-0.1 I
0
0.2
I
I
0.~
I
TIME (ps)
0.6
Fig. 2. DPAD spectrum o f the 85.3 ns state in 121Te and the level scheme o f 121Te including the new short-lived isomeric state.
The anisotropy of the 231 keV transition measured on a Sn target is positive pointing to a stretched L = 2 transition [2]. The multipolarity of the isomeric transition has been deduced from a careful investigation of the relative intensities of the delayed 7-lines. The intensity ratio [Lr(212)/I,r(231)] cal has been calculated as follows: 0.94(E1), 0.98(M1), 1.01(E2)and 1.23(M2), where the oL characteristics in brackets refer to the assumed pure multipolarity of the 231 keV transition; in this calculation we took into account that the 212 keV 7-ray belongs to an M1 transition with a 5.3% E2 contribution [3]. Measurements of this ratio have been performed with high-resohition detectors (Ge and Ge(Li)) and an Sb target, in the time interval between 100 and 180 ns after the beam pulse. In this experiment the delayed 7-rays were isotropic due to the fast loss of alignment of the isomeric state on account of the strong quadrupole interaction with the crystal electric field gradient. The experimental intensity ratio [I,r(212)/I,r(231)] exp = 1.02(2) is consistent with E2 character of the 231 keV transition. This multipolarity assignment is supported by the half-life measurement. The E2 231 keV transition is retarded by a factor of 3.9 compared to the Weisskopf estimate. For M1 or E1 character it would be retarded by 5 X 104 and 4 × 106, respectively, while an M2 transition would be enhanced by a factor of 20. We carefully searched for a possible cross-over transition to the ground state of 121Te [1 ]. No such line 66
11 February 1980
could be observed and we established an intensity upper limit of 10 - 2 for the branching in the isomeric decay; this result is consistent with the j~r value assigned to the 443 keV level. The magnetic interaction of the isomeric state with an external magnetic field has been measured by the differentially perturbed angular distribution method and the g-factor has been determined. A molten metallic tin target and two NaI(T1) detectors positioned at 0 = -+135 ° have been used in this measurement. The sign o f the g-factor has been deduced from additional measurements with the two detectors placed at 0 ° and 90 ° with respect to the beam direction. A typical timedifferential pattern is presented in fig. 2. A value o f g = +0.211(3) (/1 = +0.738(10) n.m.) has been obtained by averaging over several runs at different magnetic fields. This result was not corrected for the Knight shift ( ~ - 0 . 5 % ) and diamagnetic shielding (~0.5%), as these corrections are small and almost cancel each other. From the systematics of odd spherical nuclei with even Z around 50 and from the shell model one expects E2 transitions in 121Te to occur between the g7/2 and d3/2 neutron states. The pairing correlation [4] accounts for the E2 transition strength observed. Our results are consistent with a g7/2 neutron level at E x = 443 keV; this level is also suggested by (3He, a) measurements [5]. The measured value of the magnetic moment differs by almost a factor 2 from the single-particle value for a g7/2 neutron state,/~sp = +1.488 n.m. A similar deviation has been observed for another 7/2 + isomeric state: #exp(7/2 +, llSSn) = +0.682 n.m. [6]. The main contribution to the deviation between the experimental magnetic moment and the single-particle value in spherical nuclei is considered to be the correction due to core polarization. Using the configuration mixing theory of Noya et al. [7] we have estimated the magnetic moment of the 7/2 + isomeric state in 121Te. From the (d, t), (3He, a) [5] and (d, p) [8] nuclear reaction data on the even isotopes o f tellurium we established the most appropriate pure single-particle configuration for this state: [Tr(gT/2)2u(g7/2)-l(d5/2) 6 (d3/2)2(hl 1/2) 4 ] .,2÷. The value calculated with this configuration, ~c/a~ = +1.045 n.m., with an interaction strength parameter C = 40 MeV, is not in good agreement with the experimental result. Better agreement with experiment is obtained for the experimentally de-
Volume 90B, number 1,2
PHYSICS LETTERS
termined n e u t r o n o c c u p a t i o n n u m b e r s from ref. [5] : /'/cal = + 0 . 7 6 2 n,m. This agreement together with the E2 transition strength justifies the description of the 7/2 + isomeric level as a p r e d o m i n a n t l y single-particle state.
References [1 ] C.M. Lederer and V.S. Shirley, Table of isotopes, 7th Ed. (Wiley, New York, 1978).
11 February 1980
[2] T. Yamazaki, Nucl. Data 3A (1967) 1. [3] H.S. Sahota, Indian J. Phys. 47 (1973) 729. [4] L.S. Kisslinger and R.A. Sorensen, Rev. Mod. Phys. 35 (1963) 853. [5] M.A,G. Fernandes and M.N. Rao, J. Phys. G3 (1977) 1397. [6] E.A. Ivanov, A. Iord~chescu and G. Pascovici, Rev. Roum. Phys. 20 (1975) 141. [7] H. Noya, A. Arima and H. Horie, Prog. Theor. Phys. Suppl. 8 (1958) 33. [8] J.R. Lien et al., Can. J. Phys. 55 (1977) 463.
67
PHYSICS LETTERS
11 February 1980
SPIN, LIFETIME AND MAGNETIC MOMENT OF A NEW ISOMERIC STATE IN 121Te
M. IONESCU-BUJOR, A. IORDACHESCU, E.A. IVANOV and D. PLOSTINARU Institute for Physics and Nuclear Engineering, Bucharest, Romania Received 4 December 1979
A 7/2 + isomeric state has been identified in 121 Te by pulsed-beam time-differential 3,-ray m e a s u r e m e n t s following the 118Sn(a, n) and 121Sb(d, 2n) reactions, and the interaction of this state with an external magnetic field has been investigated. The results of the m e a s u r e m e n t s are: TI/2 = 85.3(5) ns, E~, = 212.3(1) and 231.0(1) keV, j1r = 7/2 + and At = +0.738(10) n.m.
Gamma transition rates and magnetic moments of excited nuclei provide important information about many aspects of nuclear structure. Short-lived isomers offer a good opportunity for experimental investigations on static and dynamic electromagnetic properties of excited nuclear states. In this letter we report on the identification and a detailed study of a new isomeric state in 121Te. The experiments were performed at the U-120 cyclotron of IPNE, Bucharest. The new isomeric state in 121Te has been excited by the ll8Sn(a, n) and 121Sb(d, 2n) reactions with 24 MeV a-particles and 12 MeV pulsed deuteron beams, respectively (pulse width 4 ns at a repetition time of 5/as), on isotopically enriched polycrystalline metallic targets. The 7-rays were detected with Ge, Ge(Li) and NaI(T1) detectors. The delayed 3,-ray spectra showed two lines of energies 212.3(1) and 231.0(1) keV with the same half life of 85.3(5) ns (see fig. 1). A 3/2 + excited state of 121Te at E x = 212.19 keV is known from the decay of 121I [1 ]. We assumed that this level represents the intermediate state it/the observed isomeric decay. Direct evidence for a genetic link between the two transitions has been obtained in a sum-coincidence experiment. The pulsed beam extracted in air through a thin foil hit the target positioned on top of a large NaI(T1) single crystal. The summing effects of the coincidence events, observed in the single delayed "y-ray spectrum, support our assumption about the cascade decay. The 231 keV line was assigned to the isomeric transition because no such line could be observed in the prompt ~,-ray spectrum (see fig. 1).
TIME (Fls) 0/-. 06
0.2 I
,
i
l
i
m~
1~-
I
360
,
i
E':24MeV 1
I
"~
0.8 i
118Sn(~'n)121Te t los
xx.x
o
~
l
-..
1
2310
400 440 CHANNEL
103
I
480
Fig. 1. P r o m p t and delayed "),-ray spectra from the ll8Sn(c~, n)121Te reaction registered with a Ge detector together with t h e time spectrum for b o t h lines detected with a NaI(T1) crystal.
The low-lying levels of 1 2 1 T e [ 1] including our experimental data are presented as an insert in fig. 2. The spin and parity j~r = 7/2 + for the short-lived isomeric level have been established on the basis of good consistency with the following experimental data. 65
Volume 90B, n u m b e r 1,2
7/2*
PHYSICS LETTERS
1112-
[ 231.0 [ E2 [
4/,:~3 853 ns 293.9 15.,.d
3/2*
21Z31
212.3 0.06n$
118SDliquid (o(. nj121Te H=106
kG
HI * 5.3*/*E2] 112 +
R(t} 0.1
m
0
16.8 d
-0.1 I
0
0.2
I
I
0.~
I
TIME (ps)
0.6
Fig. 2. DPAD spectrum o f the 85.3 ns state in 121Te and the level scheme o f 121Te including the new short-lived isomeric state.
The anisotropy of the 231 keV transition measured on a Sn target is positive pointing to a stretched L = 2 transition [2]. The multipolarity of the isomeric transition has been deduced from a careful investigation of the relative intensities of the delayed 7-lines. The intensity ratio [Lr(212)/I,r(231)] cal has been calculated as follows: 0.94(E1), 0.98(M1), 1.01(E2)and 1.23(M2), where the oL characteristics in brackets refer to the assumed pure multipolarity of the 231 keV transition; in this calculation we took into account that the 212 keV 7-ray belongs to an M1 transition with a 5.3% E2 contribution [3]. Measurements of this ratio have been performed with high-resohition detectors (Ge and Ge(Li)) and an Sb target, in the time interval between 100 and 180 ns after the beam pulse. In this experiment the delayed 7-rays were isotropic due to the fast loss of alignment of the isomeric state on account of the strong quadrupole interaction with the crystal electric field gradient. The experimental intensity ratio [I,r(212)/I,r(231)] exp = 1.02(2) is consistent with E2 character of the 231 keV transition. This multipolarity assignment is supported by the half-life measurement. The E2 231 keV transition is retarded by a factor of 3.9 compared to the Weisskopf estimate. For M1 or E1 character it would be retarded by 5 X 104 and 4 × 106, respectively, while an M2 transition would be enhanced by a factor of 20. We carefully searched for a possible cross-over transition to the ground state of 121Te [1 ]. No such line 66
11 February 1980
could be observed and we established an intensity upper limit of 10 - 2 for the branching in the isomeric decay; this result is consistent with the j~r value assigned to the 443 keV level. The magnetic interaction of the isomeric state with an external magnetic field has been measured by the differentially perturbed angular distribution method and the g-factor has been determined. A molten metallic tin target and two NaI(T1) detectors positioned at 0 = -+135 ° have been used in this measurement. The sign o f the g-factor has been deduced from additional measurements with the two detectors placed at 0 ° and 90 ° with respect to the beam direction. A typical timedifferential pattern is presented in fig. 2. A value o f g = +0.211(3) (/1 = +0.738(10) n.m.) has been obtained by averaging over several runs at different magnetic fields. This result was not corrected for the Knight shift ( ~ - 0 . 5 % ) and diamagnetic shielding (~0.5%), as these corrections are small and almost cancel each other. From the systematics of odd spherical nuclei with even Z around 50 and from the shell model one expects E2 transitions in 121Te to occur between the g7/2 and d3/2 neutron states. The pairing correlation [4] accounts for the E2 transition strength observed. Our results are consistent with a g7/2 neutron level at E x = 443 keV; this level is also suggested by (3He, a) measurements [5]. The measured value of the magnetic moment differs by almost a factor 2 from the single-particle value for a g7/2 neutron state,/~sp = +1.488 n.m. A similar deviation has been observed for another 7/2 + isomeric state: #exp(7/2 +, llSSn) = +0.682 n.m. [6]. The main contribution to the deviation between the experimental magnetic moment and the single-particle value in spherical nuclei is considered to be the correction due to core polarization. Using the configuration mixing theory of Noya et al. [7] we have estimated the magnetic moment of the 7/2 + isomeric state in 121Te. From the (d, t), (3He, a) [5] and (d, p) [8] nuclear reaction data on the even isotopes o f tellurium we established the most appropriate pure single-particle configuration for this state: [Tr(gT/2)2u(g7/2)-l(d5/2) 6 (d3/2)2(hl 1/2) 4 ] .,2÷. The value calculated with this configuration, ~c/a~ = +1.045 n.m., with an interaction strength parameter C = 40 MeV, is not in good agreement with the experimental result. Better agreement with experiment is obtained for the experimentally de-
Volume 90B, number 1,2
PHYSICS LETTERS
termined n e u t r o n o c c u p a t i o n n u m b e r s from ref. [5] : /'/cal = + 0 . 7 6 2 n,m. This agreement together with the E2 transition strength justifies the description of the 7/2 + isomeric level as a p r e d o m i n a n t l y single-particle state.
References [1 ] C.M. Lederer and V.S. Shirley, Table of isotopes, 7th Ed. (Wiley, New York, 1978).
11 February 1980
[2] T. Yamazaki, Nucl. Data 3A (1967) 1. [3] H.S. Sahota, Indian J. Phys. 47 (1973) 729. [4] L.S. Kisslinger and R.A. Sorensen, Rev. Mod. Phys. 35 (1963) 853. [5] M.A,G. Fernandes and M.N. Rao, J. Phys. G3 (1977) 1397. [6] E.A. Ivanov, A. Iord~chescu and G. Pascovici, Rev. Roum. Phys. 20 (1975) 141. [7] H. Noya, A. Arima and H. Horie, Prog. Theor. Phys. Suppl. 8 (1958) 33. [8] J.R. Lien et al., Can. J. Phys. 55 (1977) 463.
67