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A new isomer of bromine: 82mBr
J. Inorg.NucLChem.. 1965. ~ol. 27. pp. 903 Io 906. PerBiimonPrcs~Lid. Printed,n Northern Irtland
A NEW ISOMER OF BROMINE: s2"Br* J. F. EMERY Analytical Chemistry Division, Oak Ridge National Laboratory, Oal~ Ridge, Tennessee
(Received 13 August 1964) ~--The decays of "~"Br have been studied by ,,-ray scintillation spectroscopy and a beta proportional counter. This new isomer was observed to decay with a half-life of 6.20 q- 0-05 rain, exciting a highly converted ~'-ray transition in S2Br of 46 :t 2 keV. aK is 268. A total combined beta branching of 2.4 per cent was also observed. The thermal neutron activation cross section and resonance integral for 8S'Br are 3.0 and 34 barns, respectively, and the corresponding values for the total production of StBr arc 3.23 =k 0-2 and 41.3 ~- i barns. A NEW isomer o f SSBr has been discovered, 6.2 rain S2mBr,which decays primarily from a highly c o n v e r t e d 46 keV level to the g r o u n d state o f 8~Br. Beta b r a n c h i n g a c c o u n t s for the r e m a i n d e r o f the decay. A f t e r a short n e u t r o n irradiation, the observed y - r a y s p e c t r u m was distinctly different f r o m the expected SSBr spectrum. A d d i t i o n a l g a m m a - r a y s p e c t r a indicated n B r was growing. A literature survey revealed that in 1951 BERTHELOT et al. a) a t t e m p t e d to find a n i s o m e r o f 35 hr eSBr. Their experiment, the S z i l a r d - C h a l m e r ' s m e t h o d for s e p a r a t i o n o f isomers, indicated that not m o r e than 1.5 per cent o f the t o t a l d e c a y o f the 35 hr UBr could t a k e place by means o f isomeric transition followed b y a b e t a p e r i o d o f 18 rain or longer. T h e y m a d e no a t t e m p t to find a short-lived i s o m e r p r e c e d i n g the decay o f S~Br. Since the first report, ~zl by this a u t h o r , for 8Z'~Br a n d its half-life, a d d i t i o n a l experiments have d e t e r m i n e d the decay m o d e s a n d energies a n d a decay scheme p r o p o s e d . EXPERIMENTAL
METHODS
The 8~"Br activity was prepared by irradiation of NaB*Br (99.62 % *lBr) in the pneumatic tube of the Oak Ridge Research Reactor. Irradiation times were confined to 20 secs or less to minimize the direct production of ~ZBrand the growth of 8SBr by the decay of BS"Br during irradiation. The y-ray spectroscopy measurements were started 35 sees after removal from the re.actor. Since the 'SBr activity grows-in very rapidly, and :J-ray spectral measurements require count intervals of I-2 mins and read-out times are also of I rain, this precludes the determination of the half-life with any degree of accuracy by this technique. Therefore, half-life ~ t a were made on the X-ray and beta radiations. Bromine X-ray activity was followed for over 3 hr, and a half-life value of 6.20 =L-0.03 rain was resolved from a CLSQ decay curve computer progranm~, tn~ The half-life determined by the computer programme from the beta decay data was 6.15 4- 0-09 rain. For X-ray and gamma spectral studies, a scintillation spectrometer system compomxI o f a 3 in x 3 in * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. "~ A. BI~RTHELOT,L. P^rINF.AUand C. HEREzra3, C.R. Acad. Sci., Paris. 232, 498 (1951). ~s~j. F. EMERY, Analyt. Chem. Div. Quart. Progr. ReptJune 15, 1963, ORNL-CF 63-7-18, p 53. qs~G. D. O'KELLEY, (Editor), Applications of Computers to Nuclear and Rad/gchem/,ffry, p. 25. NAS-NS-3107. From OTS Department of Commerce, Washington 25, D.C. 1 9O3
904
J. I-. EMERY
diameter thallium activated sodium iodide crystal attached to a DuMont 6363 multiplier phototube and a 2(D-channel pulse-height analyser was used. The efficiency of this detector assembly is 21 per cent, and its resolution is 38 per cent for bromine X-rays. The observed K X-rays/0.046 MeV ;/-ray ratio is 155. Correction for fluorescent yield"' gives a value of 268 for ajr. To determine the beta branches and cross-sections, the detector assembly was calibrated for S=Br by a previously 4~-/3-y coincidence calibrated '=Br source. Figure I shows the X-ray region and the unconverted 46-keV gamma ray. Figure 2 is a series of spectra showing the growth of R=Br. tO5
T
I
I
I
I
I
1
I
I
I t I 40 50 60 70 CHANNEL NUMBER
1 80
1 90
Br X-r0y
~0
4
I-Z
o
o ..J ,¢[ 1.-o
4 6 Key
I--
t0 3
~0 z 0
[ fO
I 20
I 30
tOO
F=G. I The ratio of 'JBr produced from the isomeric state to the "=Br produced directly was calculated using the usual parent-daughter equations and measurement at two different decay times. A value of 9.0 =i= 0.3 was found, which indicates that 90 per cent of the a=Br produced is via the isomer. The CLSQ decay curve analysis programme also determines the activity at reactor discharge time of the various components of the decay curve. By use of the activity at t = 0 values computed from the //-decay data, efficiency of the beta proportional counter and the previously determined value of the direct production ofS~Br, a value of 2.4 per cent was calculated for the total beta branching. From the decay scheme cs~ of a=Br (Fig. 3), it is seen that all de-excitations have to go through the 0-777 or the 1.48 MeV level. Integration of these photopeaks and correction for the decay from reactor discharge time to the midpoint of the count period (,--.0. ! t,/=) gave values of 0.23 and 0.06 per q" H. L. HAGEOOORNand A. H. WAPSTRA, Nucl. Phys. 15, 146 (1960). '~' N. BE~CZEa-KOLLER, Dissertation, Columbia University, CU-177 0958).
905
A new isomer of bromine: RZ"Br
t04 • 35 sec Decoy • 5 0 0 sec Decay
• 2 h Decay Geometry • 2 t 5 o
cm
10 3
o t~ Z 0 ,0 )-
i0 z
o
10
02
04
06 08 t0 GAMMA ENERGY (MeV)
12
14
t6
FIG. 2 4
-
0.444
-< ./~oo%,
!~,5__- 0.0i46 ;
36"'~5Br:72
5.~
~A~-"~-~408~7
I
2.648
2I
\\\
&0922
\ \
¢-
\\v ~o.
O-
1.3t7
I I
t.044
I
;
;
I ;'o.,,,
I
0.777
i
Stoble
,
147s
"/
i
82 36Kr46
Fro. 3 cent for the respective 0.777 and !.48 MeV levels. If one assumes that all other {J-decays have to go to the ground state of "=Br, then this ground state beta branch will be 2"1 per cent. The proposed decay scheme for 8S"Br is shown in Fig. 3. CROSS S E C T I O N S T h e r m a l n e u t r o n cross sections a n d resonance integrals were d e t e r m i n e d for the isomer and the g r o u n d state of*aBr. Dilute alloys of gold a n d cobalt were used to determine the n e u t r o n flux. The thermal flux. (~th, determined for these m e a s u r e m e n t s
J. F. E~.RY
906
TABLE I.----CRO~S-SECTIONDATAFOR NEUTRONMONITORS Nuclide produced
#thCbsrn.~
l"Au S°Co
I,~sra,~
Cd ratio
1558 75
2-61 .-t=0"05
99.5 38
TABLE 2.--EXPERIMENTALLY DETERMINED CRO~ SECTIONS FOR SSBr PRODUCTION
Nuclide produced
a,h~rns~
/.Cbarns)
Cd ratio
~*~'Br S'~Br atBr
3"3 0.26 3.23 :i: 0.2
36"5 7"0 41.3 -t
3-40 2.0 3.09 + 0"1
Total
I
was 5.16 × 10~8 neutron cm -s sec -1, and the resonance flux, ~,, was 1.95 × 10II neutron cm -2 sec-1. The cross-section for the total production of StBr was calculated directly from the disintegration rate, irradiation time, weight of SlBr present, decay time, cadmium ratio, and neutron flux, while the values for the isomer and the direct production of UBr were calculated from the absolute number of atoms produced, N~2,~B, and N~2B,, obtained from the above experiment. Table i lists the cross sections for the neutron flux monitors, and Table 2 summarizes the experimentally determined cross-sections for 8~Br. DISCUSSION SIEGBAHN'S(6) tables for Z = 35 and K = 0"1 list a value of 180 for M-3 transition. Since ~ is 268, the transition is probably M-3. Other evidence that indicates a 2 (--) level for s*'~Br are the log Oct) values for the beta branches. The log OCt) value of the beta group to the upper 2 ( + ) level is 7"85 and to the lower 2 ( + ) level is 7.9. These correspond to a fl- transition of first forbidden non-unique with zero spin change and a change in parity. The log OCt) value of the beta group to ground state is 7.5. This value is somewhat, but not unreasonably, low for a first forbidden unique AJ = 2, A~r yes, beta transition. For this type of transition the average log (ft) value is 8-5 :t= 0.7. Using the above data and arguments, a spin assignment of 2- is proposed for ~mBr. Acknowledgement--The author wishes to thank N. R. JOHNSONand W. S. LYON,Jn. for many helpful discussions and comments. ~s) "Beta and Gamma-ray Spectroscop)','" (Edited by K. SIEGBAHN). Interscien~. New York (1955).
A NEW ISOMER OF BROMINE: s2"Br* J. F. EMERY Analytical Chemistry Division, Oak Ridge National Laboratory, Oal~ Ridge, Tennessee
(Received 13 August 1964) ~--The decays of "~"Br have been studied by ,,-ray scintillation spectroscopy and a beta proportional counter. This new isomer was observed to decay with a half-life of 6.20 q- 0-05 rain, exciting a highly converted ~'-ray transition in S2Br of 46 :t 2 keV. aK is 268. A total combined beta branching of 2.4 per cent was also observed. The thermal neutron activation cross section and resonance integral for 8S'Br are 3.0 and 34 barns, respectively, and the corresponding values for the total production of StBr arc 3.23 =k 0-2 and 41.3 ~- i barns. A NEW isomer o f SSBr has been discovered, 6.2 rain S2mBr,which decays primarily from a highly c o n v e r t e d 46 keV level to the g r o u n d state o f 8~Br. Beta b r a n c h i n g a c c o u n t s for the r e m a i n d e r o f the decay. A f t e r a short n e u t r o n irradiation, the observed y - r a y s p e c t r u m was distinctly different f r o m the expected SSBr spectrum. A d d i t i o n a l g a m m a - r a y s p e c t r a indicated n B r was growing. A literature survey revealed that in 1951 BERTHELOT et al. a) a t t e m p t e d to find a n i s o m e r o f 35 hr eSBr. Their experiment, the S z i l a r d - C h a l m e r ' s m e t h o d for s e p a r a t i o n o f isomers, indicated that not m o r e than 1.5 per cent o f the t o t a l d e c a y o f the 35 hr UBr could t a k e place by means o f isomeric transition followed b y a b e t a p e r i o d o f 18 rain or longer. T h e y m a d e no a t t e m p t to find a short-lived i s o m e r p r e c e d i n g the decay o f S~Br. Since the first report, ~zl by this a u t h o r , for 8Z'~Br a n d its half-life, a d d i t i o n a l experiments have d e t e r m i n e d the decay m o d e s a n d energies a n d a decay scheme p r o p o s e d . EXPERIMENTAL
METHODS
The 8~"Br activity was prepared by irradiation of NaB*Br (99.62 % *lBr) in the pneumatic tube of the Oak Ridge Research Reactor. Irradiation times were confined to 20 secs or less to minimize the direct production of ~ZBrand the growth of 8SBr by the decay of BS"Br during irradiation. The y-ray spectroscopy measurements were started 35 sees after removal from the re.actor. Since the 'SBr activity grows-in very rapidly, and :J-ray spectral measurements require count intervals of I-2 mins and read-out times are also of I rain, this precludes the determination of the half-life with any degree of accuracy by this technique. Therefore, half-life ~ t a were made on the X-ray and beta radiations. Bromine X-ray activity was followed for over 3 hr, and a half-life value of 6.20 =L-0.03 rain was resolved from a CLSQ decay curve computer progranm~, tn~ The half-life determined by the computer programme from the beta decay data was 6.15 4- 0-09 rain. For X-ray and gamma spectral studies, a scintillation spectrometer system compomxI o f a 3 in x 3 in * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. "~ A. BI~RTHELOT,L. P^rINF.AUand C. HEREzra3, C.R. Acad. Sci., Paris. 232, 498 (1951). ~s~j. F. EMERY, Analyt. Chem. Div. Quart. Progr. ReptJune 15, 1963, ORNL-CF 63-7-18, p 53. qs~G. D. O'KELLEY, (Editor), Applications of Computers to Nuclear and Rad/gchem/,ffry, p. 25. NAS-NS-3107. From OTS Department of Commerce, Washington 25, D.C. 1 9O3
904
J. I-. EMERY
diameter thallium activated sodium iodide crystal attached to a DuMont 6363 multiplier phototube and a 2(D-channel pulse-height analyser was used. The efficiency of this detector assembly is 21 per cent, and its resolution is 38 per cent for bromine X-rays. The observed K X-rays/0.046 MeV ;/-ray ratio is 155. Correction for fluorescent yield"' gives a value of 268 for ajr. To determine the beta branches and cross-sections, the detector assembly was calibrated for S=Br by a previously 4~-/3-y coincidence calibrated '=Br source. Figure I shows the X-ray region and the unconverted 46-keV gamma ray. Figure 2 is a series of spectra showing the growth of R=Br. tO5
T
I
I
I
I
I
1
I
I
I t I 40 50 60 70 CHANNEL NUMBER
1 80
1 90
Br X-r0y
~0
4
I-Z
o
o ..J ,¢[ 1.-o
4 6 Key
I--
t0 3
~0 z 0
[ fO
I 20
I 30
tOO
F=G. I The ratio of 'JBr produced from the isomeric state to the "=Br produced directly was calculated using the usual parent-daughter equations and measurement at two different decay times. A value of 9.0 =i= 0.3 was found, which indicates that 90 per cent of the a=Br produced is via the isomer. The CLSQ decay curve analysis programme also determines the activity at reactor discharge time of the various components of the decay curve. By use of the activity at t = 0 values computed from the //-decay data, efficiency of the beta proportional counter and the previously determined value of the direct production ofS~Br, a value of 2.4 per cent was calculated for the total beta branching. From the decay scheme cs~ of a=Br (Fig. 3), it is seen that all de-excitations have to go through the 0-777 or the 1.48 MeV level. Integration of these photopeaks and correction for the decay from reactor discharge time to the midpoint of the count period (,--.0. ! t,/=) gave values of 0.23 and 0.06 per q" H. L. HAGEOOORNand A. H. WAPSTRA, Nucl. Phys. 15, 146 (1960). '~' N. BE~CZEa-KOLLER, Dissertation, Columbia University, CU-177 0958).
905
A new isomer of bromine: RZ"Br
t04 • 35 sec Decoy • 5 0 0 sec Decay
• 2 h Decay Geometry • 2 t 5 o
cm
10 3
o t~ Z 0 ,0 )-
i0 z
o
10
02
04
06 08 t0 GAMMA ENERGY (MeV)
12
14
t6
FIG. 2 4
-
0.444
-< ./~oo%,
!~,5__- 0.0i46 ;
36"'~5Br:72
5.~
~A~-"~-~408~7
I
2.648
2I
\\\
&0922
\ \
¢-
\\v ~o.
O-
1.3t7
I I
t.044
I
;
;
I ;'o.,,,
I
0.777
i
Stoble
,
147s
"/
i
82 36Kr46
Fro. 3 cent for the respective 0.777 and !.48 MeV levels. If one assumes that all other {J-decays have to go to the ground state of "=Br, then this ground state beta branch will be 2"1 per cent. The proposed decay scheme for 8S"Br is shown in Fig. 3. CROSS S E C T I O N S T h e r m a l n e u t r o n cross sections a n d resonance integrals were d e t e r m i n e d for the isomer and the g r o u n d state of*aBr. Dilute alloys of gold a n d cobalt were used to determine the n e u t r o n flux. The thermal flux. (~th, determined for these m e a s u r e m e n t s
J. F. E~.RY
906
TABLE I.----CRO~S-SECTIONDATAFOR NEUTRONMONITORS Nuclide produced
#thCbsrn.~
l"Au S°Co
I,~sra,~
Cd ratio
1558 75
2-61 .-t=0"05
99.5 38
TABLE 2.--EXPERIMENTALLY DETERMINED CRO~ SECTIONS FOR SSBr PRODUCTION
Nuclide produced
a,h~rns~
/.Cbarns)
Cd ratio
~*~'Br S'~Br atBr
3"3 0.26 3.23 :i: 0.2
36"5 7"0 41.3 -t
3-40 2.0 3.09 + 0"1
Total
I
was 5.16 × 10~8 neutron cm -s sec -1, and the resonance flux, ~,, was 1.95 × 10II neutron cm -2 sec-1. The cross-section for the total production of StBr was calculated directly from the disintegration rate, irradiation time, weight of SlBr present, decay time, cadmium ratio, and neutron flux, while the values for the isomer and the direct production of UBr were calculated from the absolute number of atoms produced, N~2,~B, and N~2B,, obtained from the above experiment. Table i lists the cross sections for the neutron flux monitors, and Table 2 summarizes the experimentally determined cross-sections for 8~Br. DISCUSSION SIEGBAHN'S(6) tables for Z = 35 and K = 0"1 list a value of 180 for M-3 transition. Since ~ is 268, the transition is probably M-3. Other evidence that indicates a 2 (--) level for s*'~Br are the log Oct) values for the beta branches. The log OCt) value of the beta group to the upper 2 ( + ) level is 7"85 and to the lower 2 ( + ) level is 7.9. These correspond to a fl- transition of first forbidden non-unique with zero spin change and a change in parity. The log OCt) value of the beta group to ground state is 7.5. This value is somewhat, but not unreasonably, low for a first forbidden unique AJ = 2, A~r yes, beta transition. For this type of transition the average log (ft) value is 8-5 :t= 0.7. Using the above data and arguments, a spin assignment of 2- is proposed for ~mBr. Acknowledgement--The author wishes to thank N. R. JOHNSONand W. S. LYON,Jn. for many helpful discussions and comments. ~s) "Beta and Gamma-ray Spectroscop)','" (Edited by K. SIEGBAHN). Interscien~. New York (1955).