- Email: [email protected]
A 1.310 MeV isomeric state in 240Pu
Wapstra, Goudsmit,
Physica
A. H.
37
273-276
P. F. A.
1967
A 1.310 MeV 1SOMERIC by A. H. WAPSTRA
STATE
IN 24OPu
and P. F. A. GOUDSMIT
Instituut voor Kernphysisch Onderzoek, Amsterdam, Nederland
Synopsis The 67 min 240Np isomer in 24’3Pu. Nilsson-model
is shown
assignments
to decay
to a 165 f
10 ns isomer
at 1.310 MeV
are discussed.
Two isomers with half-lives of 7.5 min and 67 min (see e.g. ref. 1) exist in 240Np. On the basis of the Nilsson model, the assignments2) {p 5/2-[523] YZ5/2+[622]) 0- and (# S/2+(642] ti 5/2+(622]} 5+ are to be expected, the 7 min isomer being the low spin state2) 3). The decay of the 67 min isomer is reported4) to occur by a 900 keV p- transition followed by a cascade of 565 and 595 keV y rays with a 1160 keV cross-over, the final state of the cascade being undecided. Accepting these data would lead to adopting of a high spin state at about 700 keV; this being somewhat unexpected, we decided to reinvestigate this decay. Samples were prepared by bombarding natural uranium with 35 MeV alpha particles followed by a chemical separation; they were measured with a Ge(Li) semiconductor gamma spectrometer. In addition to the lines in table I assigned to 67 min 240Np, only known lines of 23sNp and 239Np were found. The two energy sum rules 567.2 + 601.1 = 1167.6 and 601.1 = = 448.2 + 152.0 not only confirm Lessler and Michel’s resultsa) but indicate in addition that the ,9 decay leads to a 1309.5 keV level decaying to a spin 5 level at 742.6 keV, which in its turn decays to the spin 4 and 6 levels of the ground-state rotational band, knowns) from the c( decay of 244Cm. In scintillation y - y coincidence measurements, y rays of 150, 450 and 600 keV were indeed found coincident with a channel at 500 keV. Sum rules allowed tentative positions of several more y rays as shown in fig. 1. Delayed coincidence measurements showed the half-life of the 742.6 keV level to be less than 2 ns; its transitions to the ground-state band are, therefore, not strongly K-forbidden. This suggests its interpretation as 5level of the octupole vibrarion band, the ground state of which is known at 597 keV (ti2/21 = 5.2 keV; y rays of 507.2 and 607 keV suggest that its 3- member is also excited in the decay of this isomer). Feeding of this 5-
273
-
5’ 5’
G+c”i cn $ljJmV) *S+c!
C!$ F
QF3z; @Q!=LW
if 100%.
logft=
5.8
7426
t2ns
7;if
lOO%.logft:6.2
51.Oh
for conversion.
Arrows
attached
to intensity
values identify
the parent
nuclides.
Fig. 1. Decay scheme of 67 min 24ONp and of some other nuclides leading to 24OPu. Levels and y rays not found in the decay of the 67 min isomer are dotted. The intensities of the 147, 152, 193 and 271 keV y rays have been corrected
0.89;
A
1.310 MeV
ISOMERIC
TABLE
STATE
IN 24aPu
275
I
;amma rays in the decay of 67 min s4sNp. ‘he intensity
values in column 2 are given
pproximately
as percents per disintegration,
the energies in keV *)
EY
EY
I,
0.4
847.0
5.0
147.2
1.5
867.2
9.0
152.0
9.0 1.0
804.9 896.5
271.5
7.3 9
4.0 1.2 14
915.2
1.5
295.0
0.7
958.7
2.5
307.0 448.2
1.5 18
462.2 467.4 507.2
182.6 193.1
567.2 601.1 607
j3 particles
I,
134.6
890.6
973.9
23 5.0
1.5
988 1074.4
2.2
1131.8
2.0 29
1163 1167.6
0.7 0.7
22
1179
1.0
5.0 0.7
1.7
and the 567 keV gamma ray yielded
indeed a half-life
f
of 165 f
10ns. Comparison with Weiskopf estimates for the 567 and 1168 keV transitions gives hindrance factors Fw = 4.1 x 106 (M 1) and 3.0 x 1010 (E 1) if the parity of the 1309 keV level were negative, and Fw = 5.2 x 10s (E 1) and 2.3 x 10s (M 1) if it were positive; comparison with systematicss) shows that the first choice is to be preferred. This allows identification of the 1309 keV level with the low-lying {$ 5/2+(642] + 5/2-[523]) 5- two-quasiparticle level predicted by Veresh et aZii6). The intensity balances in the provisional decay scheme of fig. 1 suggest that the remaining part of the decay of the 1309 keV level occurs by M 1 transitions to at least two negative parity bands which should, then, have K values around 2; this notwithstanding the fact that the energies of the proposed 1002 and 1162 keV levels agree reasonably with those expected for the spin 4 and 6 members of the iSvibrational band of which the spin 0 and 2 members are found at 859 and 901 keV. A K = 3 level at 1032 keV is found in the decay of 24aAm, according to a re-interpretation of the results of Glass et aZii7). Though this level appears to be excited in the decay of 67 min 24’JNp, the direct feeding of the band built upon this level from the 1309 keV level is weak. Veresh et a&6) suggest a {n$+[631] n5/2+[622]} 3+ level at about the right energy; its being a two-neutron level whereas the 1309 keV one is a two-proton one may explain the above fact. The authors thank drs P. Polak for performing the chemical separations *)
Accidentally,
a 175.0 keV y ray, I,, =
6.5, has been left out.
276
1.3 10 MeV ISOMERIC STATE
A
and professor
dr. R. van
Lieshout
IN
24OPu
for his interest
in this work, which is
part of the research program of this Institute and financially supported by the Foundation of Fundamental Research of Matter (FOM) and the Organization for Pure Scientific Received
Research
(ZWO).
13-3-67
REFERENCES 1) Qaim, 2)
man, 3)
S. M., Nuclear
Asaro,
Phys. 84 (1966) 411.
I:., Stephens
Jr.,
F. S., Hollander,
J. M., Hulet,
E. K., Hoff,
R. W. and Perl-
I.; see ref. 8.
Bunker,
M. E., Dropesky,
Rev. 110 (1959)
B. J.,
4)
Lesslet,
R. M. and Michel,
5)
Liibner,
K. E. G. and Malmskog,
6)
Veresh,
7)
1053. Glass,
R. A., Carr,
8)
Hyde,
E. K., Perlman,
Prentice
Knight,
J. D., Starrier,
J. W. and Warren,
B., Phys.
143.
T., Soloviev,
M. C., Phys.
V. G. and Siklos, R. J. and Gibson,
Hall, Englewood
Rev. 118 (1960) 263.
S. G., Nuclear
I. and Seaborg,
Phys. 80 (1966) 505.
T., Bull. Acad.
Science
USSR
Phys. Sec. 2.5 (1962)
W. M., J. inorg. nuclear Chem. 13 (1960) G. T., “Nuclear Properties of the Heavy
Cliffs, N. J. (1963).
181. Elements”,
Physica
A. H.
37
273-276
P. F. A.
1967
A 1.310 MeV 1SOMERIC by A. H. WAPSTRA
STATE
IN 24OPu
and P. F. A. GOUDSMIT
Instituut voor Kernphysisch Onderzoek, Amsterdam, Nederland
Synopsis The 67 min 240Np isomer in 24’3Pu. Nilsson-model
is shown
assignments
to decay
to a 165 f
10 ns isomer
at 1.310 MeV
are discussed.
Two isomers with half-lives of 7.5 min and 67 min (see e.g. ref. 1) exist in 240Np. On the basis of the Nilsson model, the assignments2) {p 5/2-[523] YZ5/2+[622]) 0- and (# S/2+(642] ti 5/2+(622]} 5+ are to be expected, the 7 min isomer being the low spin state2) 3). The decay of the 67 min isomer is reported4) to occur by a 900 keV p- transition followed by a cascade of 565 and 595 keV y rays with a 1160 keV cross-over, the final state of the cascade being undecided. Accepting these data would lead to adopting of a high spin state at about 700 keV; this being somewhat unexpected, we decided to reinvestigate this decay. Samples were prepared by bombarding natural uranium with 35 MeV alpha particles followed by a chemical separation; they were measured with a Ge(Li) semiconductor gamma spectrometer. In addition to the lines in table I assigned to 67 min 240Np, only known lines of 23sNp and 239Np were found. The two energy sum rules 567.2 + 601.1 = 1167.6 and 601.1 = = 448.2 + 152.0 not only confirm Lessler and Michel’s resultsa) but indicate in addition that the ,9 decay leads to a 1309.5 keV level decaying to a spin 5 level at 742.6 keV, which in its turn decays to the spin 4 and 6 levels of the ground-state rotational band, knowns) from the c( decay of 244Cm. In scintillation y - y coincidence measurements, y rays of 150, 450 and 600 keV were indeed found coincident with a channel at 500 keV. Sum rules allowed tentative positions of several more y rays as shown in fig. 1. Delayed coincidence measurements showed the half-life of the 742.6 keV level to be less than 2 ns; its transitions to the ground-state band are, therefore, not strongly K-forbidden. This suggests its interpretation as 5level of the octupole vibrarion band, the ground state of which is known at 597 keV (ti2/21 = 5.2 keV; y rays of 507.2 and 607 keV suggest that its 3- member is also excited in the decay of this isomer). Feeding of this 5-
273
-
5’ 5’
G+c”i cn $ljJmV) *S+c!
C!$ F
QF3z; @Q!=LW
if 100%.
logft=
5.8
7426
t2ns
7;if
lOO%.logft:6.2
51.Oh
for conversion.
Arrows
attached
to intensity
values identify
the parent
nuclides.
Fig. 1. Decay scheme of 67 min 24ONp and of some other nuclides leading to 24OPu. Levels and y rays not found in the decay of the 67 min isomer are dotted. The intensities of the 147, 152, 193 and 271 keV y rays have been corrected
0.89;
A
1.310 MeV
ISOMERIC
TABLE
STATE
IN 24aPu
275
I
;amma rays in the decay of 67 min s4sNp. ‘he intensity
values in column 2 are given
pproximately
as percents per disintegration,
the energies in keV *)
EY
EY
I,
0.4
847.0
5.0
147.2
1.5
867.2
9.0
152.0
9.0 1.0
804.9 896.5
271.5
7.3 9
4.0 1.2 14
915.2
1.5
295.0
0.7
958.7
2.5
307.0 448.2
1.5 18
462.2 467.4 507.2
182.6 193.1
567.2 601.1 607
j3 particles
I,
134.6
890.6
973.9
23 5.0
1.5
988 1074.4
2.2
1131.8
2.0 29
1163 1167.6
0.7 0.7
22
1179
1.0
5.0 0.7
1.7
and the 567 keV gamma ray yielded
indeed a half-life
f
of 165 f
10ns. Comparison with Weiskopf estimates for the 567 and 1168 keV transitions gives hindrance factors Fw = 4.1 x 106 (M 1) and 3.0 x 1010 (E 1) if the parity of the 1309 keV level were negative, and Fw = 5.2 x 10s (E 1) and 2.3 x 10s (M 1) if it were positive; comparison with systematicss) shows that the first choice is to be preferred. This allows identification of the 1309 keV level with the low-lying {$ 5/2+(642] + 5/2-[523]) 5- two-quasiparticle level predicted by Veresh et aZii6). The intensity balances in the provisional decay scheme of fig. 1 suggest that the remaining part of the decay of the 1309 keV level occurs by M 1 transitions to at least two negative parity bands which should, then, have K values around 2; this notwithstanding the fact that the energies of the proposed 1002 and 1162 keV levels agree reasonably with those expected for the spin 4 and 6 members of the iSvibrational band of which the spin 0 and 2 members are found at 859 and 901 keV. A K = 3 level at 1032 keV is found in the decay of 24aAm, according to a re-interpretation of the results of Glass et aZii7). Though this level appears to be excited in the decay of 67 min 24’JNp, the direct feeding of the band built upon this level from the 1309 keV level is weak. Veresh et a&6) suggest a {n$+[631] n5/2+[622]} 3+ level at about the right energy; its being a two-neutron level whereas the 1309 keV one is a two-proton one may explain the above fact. The authors thank drs P. Polak for performing the chemical separations *)
Accidentally,
a 175.0 keV y ray, I,, =
6.5, has been left out.
276
1.3 10 MeV ISOMERIC STATE
A
and professor
dr. R. van
Lieshout
IN
24OPu
for his interest
in this work, which is
part of the research program of this Institute and financially supported by the Foundation of Fundamental Research of Matter (FOM) and the Organization for Pure Scientific Received
Research
(ZWO).
13-3-67
REFERENCES 1) Qaim, 2)
man, 3)
S. M., Nuclear
Asaro,
Phys. 84 (1966) 411.
I:., Stephens
Jr.,
F. S., Hollander,
J. M., Hulet,
E. K., Hoff,
R. W. and Perl-
I.; see ref. 8.
Bunker,
M. E., Dropesky,
Rev. 110 (1959)
B. J.,
4)
Lesslet,
R. M. and Michel,
5)
Liibner,
K. E. G. and Malmskog,
6)
Veresh,
7)
1053. Glass,
R. A., Carr,
8)
Hyde,
E. K., Perlman,
Prentice
Knight,
J. D., Starrier,
J. W. and Warren,
B., Phys.
143.
T., Soloviev,
M. C., Phys.
V. G. and Siklos, R. J. and Gibson,
Hall, Englewood
Rev. 118 (1960) 263.
S. G., Nuclear
I. and Seaborg,
Phys. 80 (1966) 505.
T., Bull. Acad.
Science
USSR
Phys. Sec. 2.5 (1962)
W. M., J. inorg. nuclear Chem. 13 (1960) G. T., “Nuclear Properties of the Heavy
Cliffs, N. J. (1963).
181. Elements”,