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Spin, lifetime and magnetic moment of a new isomeric state in 120Sb
Volume 64B, number 2
PHYSICS LETTERS
13 September 1976
SPIN, LIFETIME AND MAGNETIC MOMENT OF A NEW ISOMERIC STATE IN 120Sb A. IORDACHESCU, E.A. IVANOV, D. PLOSTINARU, 0. ECHT*, G. PASCOVICI, S. VAJDA, B. CONSTANTINESCU and P. RACOLTEA Institute of Atomic Physics, P.O. Box 5206, Bucharest, Romania
Received 29 July 1976 A 3+ isomeric state has been identified in 120Sb by pulsed beam time-differential ‘y-ray measurements following the 12’Sn(p, n) reaction and the interactions of this state with magnetic and electric fields have been investigated. The results of the measurements are: Tr12 = 247i: 1 ns.ET=69.8* 0.1 keV,aK=0.43+ 0.04,~=+2.571 iO.006 n.m. and (e2qQ/h) = 38.4 f 0.1 MHz in tin at 470 K.
In the low-mass even antimony isotopes the 3+ state appears as the ground state (l14y116Sb) or first excited state (118,122Sb). The presence of the 3+ short-lived isomeric states in l18Sb [l] and 122Sb [2] suggests the possibility of the existence of a 3+ isomerit state in 120Sb too. Here we report on the identification of a new 3+ short-lived isomeric state in 120Sb and on measurements of its electric quadrupole interaction and magnetic moment. The experiments were performed at the cyclotron of IAP, Bucharest. To excite the states in 120Sb the reaction 120Sn(p, n) on isotopically enriched polycrystalline metallic targets has been used. The 11 MeV proton beam was pulsed with a pulse width of 4 ns at a repetition time of 20 /..Is.The y- and KX-rays were detected with Si(Li) (250 eV resolution for the 6.403 keV line of 57Co) and thin NaI(T1) detectors. The gfactor has been measured on a molten target. The delayed y-ray spectra show three lines of 26.42 + 0.04,29.83 f 0.04 and 69.8 + 0.1 keV with the same half-life of 247 f 1 ns. A delayed y-ray spectrum registered with a thin NaI(Tl) detector together with the time spectrum for the 69.8 keV y-ray is presented in fig. 1, The 69.8 keV line was attributed to the isomeric transition because no such line could be observed in the prompt y-ray spectra. The other two lines were assigned to the K-conversion X-rays of antimony. Their intensities corrected for absorption and for the long-lived X-ray components correspond to a * Present address: Fachbereich Physik, Universitlt Konstanz, Germany.
60
60
100
120
CHANNEL
140
160
160
NUMBER
Fig. 1. Delayed r-ray spectrum registered with a thin NaI(TI) crystal and time spectrum for the 69.8 keV line.
conversion coefficient oK = 0.43 -t 0.04 which is consistent with pure El character for the 69.8 keV transition. The differential perturbed angular distribution (DPAD) pattern of the quadrupole interaction of an isomeric nucleus with an axially symmetric electric field gradient is, specially at low spins, strongly dependent on the spin. Based on this fact we determine the spin of the isomeric level in 120Sb. The quadrupole interaction of the new isomeric state in 120Sb has been measured in tin at a temperature of 470 K by the DPAD method. The 69.8 keV y-rays were detected with two thin NaI(T1) crystals positioned at f3 = 0” 151
Volume 64B, number 2
PHYSICS LETTERS
1
13 September 1976
‘20Sn,,,,,(p,nl
“‘Sb
r.' --004 0.10
I
I
J=3
0.05 EO
0
K -0.02
-0.05
-0.04
-0.o.q
0
-0.10
I
I
0.2
I 0.4
TIME
I 0.6
OJsl
I 0.6
1D
1.2
Fig. 2. DPAD perturbation amplitudes of lzoSb (3+ state) in Sn metal obtained at a temperature of 470 K. The solid lines are the least-squares fits to the theoretical function A 2 G2 (t) for spins 2 and 3.
and 90”, respectively. The experimental data are presented in fig. 2 together with the fits for spins 2 and 3. The best fit was obtained for a spin 3 and we assigned this vaIue to the spin of the new isomeric state in 120Sb. From systematics of the levels in even isotopes of antimony we assumed a positive parity for this level. Hjorth [3] has observed a state with 1, = 0 at 88 keV excitation energy in 120Sb by the 121 Sb(d, t) reaction, which most likely can be identified with our isomeric level. The magnetic interaction of the new isomeric state with an external magnetic field has been measured by the DPAD method and the g-factor has been determined. A typical time-differential pattern is presented in fig. 3. For the g-factor of the 3+ isomeric level in 120Sb a value of g = to.856 + 0.002 has been obtained by averaging over several runs at different magnetic fields. As the Knight shift of antimony in tin is not known the g-factor was corrected only for diamagnetic shielding [4] . The corrected value is g = i-O.857 ?r 0.002 CJ = t2.571 + 0.006 n.m.). For the similar 3+ states in the neighbouring nuclei the following values have been reported: g(l18Sb) = to.870 [ I] and g(122Sb) = to.988 [2]. The configurations of the ground and isomeric states of odd-odd antimony nuclides are obtained by coupling one of the proton states, 2d,/2 or 1g7/2, with one of the neutron states, 3s1/2,2d3/2 or lh,I,z, of neighbouring odd-neutron nuclei. Using these states, the theoretical and empirical g-factor for the 3+ iso I52
0
1
I
I
0.2
0.4
06
TIME
I 0.8
I 1.0
(j.1s.1
Fig. 3. Intensity modulation of the 69.8 keV T-ray time spectra showing the Larmor precession of “‘Sb and the proposed level scheme for the new isomeric state in “‘Sb.
merit states in l18Sb and 122Sb have been calculated for various possible configurations [ 1,2] . The compari son of our experimental result for the g-factor of the 3+ state in 120Sb with the calculated values shows an agreement for the configuration
k cdc
= to.838
f 0.055 from ref. [2])
in agreement with the (d, t) measurement [3]. The ground state of 120Sb hasJn = I+ [5]. A pure El transition from the 3+ isomeric level implies the presence of an intermediate level which most probably has J” = 2-. The angular distribution coefficient of the 69.8 keV y-ray has a value A2 = -0.054 + 0.004 which eliminates the J = 3 possibility. A state with Jr = 4- at low excitation energy in lzoSb is very unprobable. A 2- state also appears in 122Sb as the ground state. If our 3+ level is identical with the 3+ level at 88 keV excitation energy found by Hjorth, it results that the 2- state has to have an excitation energy of about 18 keV. In connection with the 2- + l+ transition a tentative experiment has been done to search for a low energy y-ray corresponding to this transition by using a Si(Li) detector. We obtained negative results and established that such a line has to have a maximum energy of at most 2.5 keV for any isomeric activity in 120Sb with a half-life in the range T1/2 =2min-1Oh. We also looked for a y-line corresponding to a pos-
Volume 64B, number 2
PHYSICS LETTERS
sible 3+ + l+ transition, similar to that known in ll*Sb [l] and have not observed such a line. We concluded that if such a transition exists the branching ratio for this decay is less than 1%. The proposed level scheme is given as an insert in fig. 3. The strength of the isomeric transition, calculated with a value crt = 0.47 for the total conversion coefficient, is DexP = T$j2/TTy$(l + czt) = 2.2 X lo@ W.U. similar to that for the same transition in 122Sb [6] . The authors would like to express their appreciation to Mrs. Stefania Niculescu for her kind cooperation in the data processing. In particular, we would like to thank the Cyclotron Operations staff for their conscientious efforts far beyond minimum requirements.
13 September 1976
References [l] D. Plostinaru, E.A. Ivanov, G. Pascovici and A. IordXchescu, Phys. Lett. 57B (1975) 235. [2] P. Heubes et al., Phys. Rev. C7 (1973) 2128. [3] S.A. Hjorth, Arkiv Fysik 33 (1966) 183. [4] V.S. Shirley, in Hyperfine structure and nuclear radiations, eds. E. Matthias and D.A. Shirley (North-Holland, A Amsterdam, 1968) p. 985. [S] A.D. Jackson, E.H. Rogers and G.J. Garrett, Phys. Rev. 175 (1968) 65. [6] E. Der Mateosian and M.L. Sehgal, Phys. Rev. 129 (1963) 2195.
153
PHYSICS LETTERS
13 September 1976
SPIN, LIFETIME AND MAGNETIC MOMENT OF A NEW ISOMERIC STATE IN 120Sb A. IORDACHESCU, E.A. IVANOV, D. PLOSTINARU, 0. ECHT*, G. PASCOVICI, S. VAJDA, B. CONSTANTINESCU and P. RACOLTEA Institute of Atomic Physics, P.O. Box 5206, Bucharest, Romania
Received 29 July 1976 A 3+ isomeric state has been identified in 120Sb by pulsed beam time-differential ‘y-ray measurements following the 12’Sn(p, n) reaction and the interactions of this state with magnetic and electric fields have been investigated. The results of the measurements are: Tr12 = 247i: 1 ns.ET=69.8* 0.1 keV,aK=0.43+ 0.04,~=+2.571 iO.006 n.m. and (e2qQ/h) = 38.4 f 0.1 MHz in tin at 470 K.
In the low-mass even antimony isotopes the 3+ state appears as the ground state (l14y116Sb) or first excited state (118,122Sb). The presence of the 3+ short-lived isomeric states in l18Sb [l] and 122Sb [2] suggests the possibility of the existence of a 3+ isomerit state in 120Sb too. Here we report on the identification of a new 3+ short-lived isomeric state in 120Sb and on measurements of its electric quadrupole interaction and magnetic moment. The experiments were performed at the cyclotron of IAP, Bucharest. To excite the states in 120Sb the reaction 120Sn(p, n) on isotopically enriched polycrystalline metallic targets has been used. The 11 MeV proton beam was pulsed with a pulse width of 4 ns at a repetition time of 20 /..Is.The y- and KX-rays were detected with Si(Li) (250 eV resolution for the 6.403 keV line of 57Co) and thin NaI(T1) detectors. The gfactor has been measured on a molten target. The delayed y-ray spectra show three lines of 26.42 + 0.04,29.83 f 0.04 and 69.8 + 0.1 keV with the same half-life of 247 f 1 ns. A delayed y-ray spectrum registered with a thin NaI(Tl) detector together with the time spectrum for the 69.8 keV y-ray is presented in fig. 1, The 69.8 keV line was attributed to the isomeric transition because no such line could be observed in the prompt y-ray spectra. The other two lines were assigned to the K-conversion X-rays of antimony. Their intensities corrected for absorption and for the long-lived X-ray components correspond to a * Present address: Fachbereich Physik, Universitlt Konstanz, Germany.
60
60
100
120
CHANNEL
140
160
160
NUMBER
Fig. 1. Delayed r-ray spectrum registered with a thin NaI(TI) crystal and time spectrum for the 69.8 keV line.
conversion coefficient oK = 0.43 -t 0.04 which is consistent with pure El character for the 69.8 keV transition. The differential perturbed angular distribution (DPAD) pattern of the quadrupole interaction of an isomeric nucleus with an axially symmetric electric field gradient is, specially at low spins, strongly dependent on the spin. Based on this fact we determine the spin of the isomeric level in 120Sb. The quadrupole interaction of the new isomeric state in 120Sb has been measured in tin at a temperature of 470 K by the DPAD method. The 69.8 keV y-rays were detected with two thin NaI(T1) crystals positioned at f3 = 0” 151
Volume 64B, number 2
PHYSICS LETTERS
1
13 September 1976
‘20Sn,,,,,(p,nl
“‘Sb
r.' --004 0.10
I
I
J=3
0.05 EO
0
K -0.02
-0.05
-0.04
-0.o.q
0
-0.10
I
I
0.2
I 0.4
TIME
I 0.6
OJsl
I 0.6
1D
1.2
Fig. 2. DPAD perturbation amplitudes of lzoSb (3+ state) in Sn metal obtained at a temperature of 470 K. The solid lines are the least-squares fits to the theoretical function A 2 G2 (t) for spins 2 and 3.
and 90”, respectively. The experimental data are presented in fig. 2 together with the fits for spins 2 and 3. The best fit was obtained for a spin 3 and we assigned this vaIue to the spin of the new isomeric state in 120Sb. From systematics of the levels in even isotopes of antimony we assumed a positive parity for this level. Hjorth [3] has observed a state with 1, = 0 at 88 keV excitation energy in 120Sb by the 121 Sb(d, t) reaction, which most likely can be identified with our isomeric level. The magnetic interaction of the new isomeric state with an external magnetic field has been measured by the DPAD method and the g-factor has been determined. A typical time-differential pattern is presented in fig. 3. For the g-factor of the 3+ isomeric level in 120Sb a value of g = to.856 + 0.002 has been obtained by averaging over several runs at different magnetic fields. As the Knight shift of antimony in tin is not known the g-factor was corrected only for diamagnetic shielding [4] . The corrected value is g = i-O.857 ?r 0.002 CJ = t2.571 + 0.006 n.m.). For the similar 3+ states in the neighbouring nuclei the following values have been reported: g(l18Sb) = to.870 [ I] and g(122Sb) = to.988 [2]. The configurations of the ground and isomeric states of odd-odd antimony nuclides are obtained by coupling one of the proton states, 2d,/2 or 1g7/2, with one of the neutron states, 3s1/2,2d3/2 or lh,I,z, of neighbouring odd-neutron nuclei. Using these states, the theoretical and empirical g-factor for the 3+ iso I52
0
1
I
I
0.2
0.4
06
TIME
I 0.8
I 1.0
(j.1s.1
Fig. 3. Intensity modulation of the 69.8 keV T-ray time spectra showing the Larmor precession of “‘Sb and the proposed level scheme for the new isomeric state in “‘Sb.
merit states in l18Sb and 122Sb have been calculated for various possible configurations [ 1,2] . The compari son of our experimental result for the g-factor of the 3+ state in 120Sb with the calculated values shows an agreement for the configuration
k cdc
= to.838
f 0.055 from ref. [2])
in agreement with the (d, t) measurement [3]. The ground state of 120Sb hasJn = I+ [5]. A pure El transition from the 3+ isomeric level implies the presence of an intermediate level which most probably has J” = 2-. The angular distribution coefficient of the 69.8 keV y-ray has a value A2 = -0.054 + 0.004 which eliminates the J = 3 possibility. A state with Jr = 4- at low excitation energy in lzoSb is very unprobable. A 2- state also appears in 122Sb as the ground state. If our 3+ level is identical with the 3+ level at 88 keV excitation energy found by Hjorth, it results that the 2- state has to have an excitation energy of about 18 keV. In connection with the 2- + l+ transition a tentative experiment has been done to search for a low energy y-ray corresponding to this transition by using a Si(Li) detector. We obtained negative results and established that such a line has to have a maximum energy of at most 2.5 keV for any isomeric activity in 120Sb with a half-life in the range T1/2 =2min-1Oh. We also looked for a y-line corresponding to a pos-
Volume 64B, number 2
PHYSICS LETTERS
sible 3+ + l+ transition, similar to that known in ll*Sb [l] and have not observed such a line. We concluded that if such a transition exists the branching ratio for this decay is less than 1%. The proposed level scheme is given as an insert in fig. 3. The strength of the isomeric transition, calculated with a value crt = 0.47 for the total conversion coefficient, is DexP = T$j2/TTy$(l + czt) = 2.2 X lo@ W.U. similar to that for the same transition in 122Sb [6] . The authors would like to express their appreciation to Mrs. Stefania Niculescu for her kind cooperation in the data processing. In particular, we would like to thank the Cyclotron Operations staff for their conscientious efforts far beyond minimum requirements.
13 September 1976
References [l] D. Plostinaru, E.A. Ivanov, G. Pascovici and A. IordXchescu, Phys. Lett. 57B (1975) 235. [2] P. Heubes et al., Phys. Rev. C7 (1973) 2128. [3] S.A. Hjorth, Arkiv Fysik 33 (1966) 183. [4] V.S. Shirley, in Hyperfine structure and nuclear radiations, eds. E. Matthias and D.A. Shirley (North-Holland, A Amsterdam, 1968) p. 985. [S] A.D. Jackson, E.H. Rogers and G.J. Garrett, Phys. Rev. 175 (1968) 65. [6] E. Der Mateosian and M.L. Sehgal, Phys. Rev. 129 (1963) 2195.
153