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Isomeric cross-section ratios for 134mCs, 134Cs and 133mBa, 133Ba formed in (d, p) and (d, 2n) reactions
3. Inorg. Nucl. Chem., 1966, VoL 28, pp. 5 to 9. Pergamon P r e a Ltd. Printed in Northern Ireland
ISOMERIC CROSS-SECTION RATIOS FOR ls4mCs, 184Cs AND 18SmBa, XSSBa FORMED IN (d,p) AND (d, 2n) REACTIONS* A. MOCOROA, H. VIGNAU,M. C. CARACOCHE and S. J. NASStn~ Institute of Physics, La Plata National University and National Atomic Energy Commission, Argentina (Received 24 January 1965)
Abstract--Ratios between the cross-sections ~,~/~g as a function of deuteron energy have been experimentally determined for the isomers 18"~Cs, ls'Cs produced by the reaction mCs (d, p)xs~C and for the isomers lU"Ba and ~aaBaproduced by the reaction mCs (d, 2n)mBa. IT is thought, in general, that nuclear reactions are produced by compound nucleus formation at energies below 30 MeV and by the knock-on mechanism at energies over 100 MeV. cl) Both mechanisms m a y lead to an intermediate nucleus with approximately the same excitation energy but with different angular m o m e n t u m distributions. This would result in different yields of end products when they differ greatly in spin, as in the case of nuclear isomers. ~s-e) A study of the ratio of the cross-sections for an isomeric pair in a given reaction could help in the interpretation of changes in the reaction mechanism with energy. At low energies, the spins of the initial nucleus as well as those of the isomers seem to be the main factor determining the yield ratio, the tendency observed being that the more frequently formed isomer is the one whose spin is nearest to the target nucleus. At higher energies where the compound nucleus is produced in a wider range of spin levels this effect may disappear. EXPERIMENTAL (a) Irradiations Cesium chloride p.a. samples were irradiated with a deuteron beam of the 72 in. diameter C.N.E.A. Synchrocyclotron. For this experience the stacked-foils technique was employed. The cesium chloride salt was wrapped in silver foils of about 25 mg/cm~. This technique allowed simultaneous bombardment over a wide range of energies. The irradiations were performed in the orbit corresponding to incident energies of 28.0 MeV and 29'2 MeV for the (d, p) and the (d, 2n) reactions respectively, with a maximum beam dispersion estimated at -I-1-2MeV. * The present work was partially supported by the Comisi6n Especial de Fisica At6mica y Radiois6topos (C.E.F.A.R.) ~1~j. M. BLATTand V. F. WmSSKOPF,Theoretical Nuclear Physics. 3. Wiley, New York (1952). cs) R. V^NDEI'mOSCHand J. R. HtntZ~GA, Phys. Reo. 120, 1305 (1960); 120, 1313 (1960). ~s~j. K. ~ w s , R. M. DL~,tONDand R. A. SHARP,Phys. Rev. 102, 190 (1956). ~' B. LINDER and R. A. JAMES,Phys. Rev. 114, 322 (1959). ~5~S. M. BAndY, U.C.R.L. Report A.E.C.U. 8710 (1959). ~s~E. S~sk and A. C. I-IEL~IOTZ,Rev. Mod. Phys. 21, 271 (1949). 5
A. MOCOROAet ai.
6
The deutron energy in each sample was calculated from the range-energy of AARONet alJ~) As the excitation functions of each isomer were not determined separately, the deuteron beam intensity variation through the target was not taken into consideration. (b) Chemical separations The cesium chloride irradiated was dissolved in a few milliliters of water; Bas+ was added as carrier. Ba 9+was precipitated as BaSe4 which, after filtration and washing was collected on a lucite plate. In this sample the isomeric pair xaS=Ba, XaSBawas measured. The solution was dried and afterwards some drops of 6 M NaOH and a few milliliters of 70 ~. HCIO, were added. The solution was then evaporated by constant shaking until white fumes started. Once cold the solution was treated with 15 ml of ethanol. The xuCs, lu'~2s pair was measured in the CsCIO, precipitate. r- 134 8(-) 55,-sr9
(5+)4(+)
(21~h)
<~1% O~13~0,010
r 134 55~,S79
(2,2y)
0~
4+
~,+; i
1,97 1,77
,
I ii1
I I; I
I
168
I
-~"I
I I
~
f I I
'
56Bo~4
BO133
(39,h!,, o,28T
(3/2÷)
i
0,436 ( I / 2 + } ~ L ' ~
~,3
(3/2)+
l
5/2+
,
o
la4uCs.
(~w2-)
7/2+
0,60
11
f I
FIG. l . - - D o c a y scheme 184r~CSand
I/2{+)
,.o 1,57
1 ,
O,OI 2
(7,2y)
0
82 t
'
i',
o:o8,
55Cs~3 FIG. 2.--Decay scheme of 18a=Ba and xSa0Ba.
(c) Counting techniques The gamma spectra were measured by means of 2 in. x 2 in. NaI(TI) crystal and 100 channel analyser. Areas under the photo-peaks were determined to obtain intensities. To prevent pile-up effects, foils of 0.331 g Sn/cra=and 0.181 g Me/era2 were used as absorbers. Corrections were applied for absorption and efficiencydependence on energy. Decay schemes, branching ratios and conversion coefficients were used as given in the Nuclear Data Sheets.(s) The decay scheme of luCs and xu'~.s is shown in Fig. 1. The population of the 2.9 hr isomer was determined by measuring the 127 keV line and the population of the 2.2 years level by measuring the 600 keV and the 1168 keV lines, both at suitable times after the irradiation. (The contribution of the 1570 keV line to the decay of lu~Cs is negligible.) Examples of the spectra used are shown in Figs. 3. (~) W. A. AARON,B. L. HOFrML~Nand F. C. Wzu.~,Ms, U.C.R.L. Report A.E.C.U. 663 (1951). Nuclear Data Sheets, National Academy of Sciences, N.R.C. 61-2-97.
(s)
Isomeric cross-section ratios for ls4mCs, ~aaCs and ~sa'~Ba, ~SSBa
IC
7
2
I
KeV Fro. 3.--Gamma spectrum of one of the 18'Cs samples measured 11.0 hr after the end of irradiation. The 127 keV ray of la4'~Cs can be seen and 670 keV gamma belonging to 18~Cs, which masks the 601) keV ray of za4gCs.
u~ r,
,~
~oo'
~
,oo'.....
KeV
s~o
"o
Fzo. 4.--Spectrum taken of 184Cs 103 days after irradiation. The 800 keV and 600 keV gamma rays of z~gCs are observed, the latter not resolved from the 563 and 569 ~,-rays, which also belong to the z~4gCs. and 4. The intensity of the 600 keV line was corrected for the presence of the 563 keV line (assumed to occur with an intensity equal to 14 per cent of that of the 600 keV line) and of the 569 kvV line (assumed to occur with an intensity equal to 11.5 per cent of that of the 600 la~V line). The 1168 keV line was assumeA to have an intensity equal to 2 per cent of that of the 600 keV line.
8
A. MOCOROAet al.
I
I
I00
200
300 KeY
400
500
FIo. 5.--F-ray spectrum of ~UmBa 45.8 hr after irradiation.
~0
g
2 I
I00
200
300 KeV
400
.....
~D
-==
FIG. 6.--7-ray spectrmn of ~3aaBa 3060 hr after irradiation. The decay scheme of lS3Ba and 13a=Ba is shown in Fig. 2. lU=Ba was measured by means of the 275 keV gamma, la3~Ba by moans of the 355 keV line. The scintillograms of these isomers are shown in Fig. 5 and 6. The observed intensity of the 355 keV line was corrected for the presence of 382 keV gammas which wc assumed to occur with intensity equal to 1% of that of the first mentioned line.
Isomeric cross-section ratios for 1s4mCs, la4Cs and ~mBa, Xa*Ba
9
RESULTS AND DISCUSSION T a b l e s 1 a n d 2 show the e x p e r i m e n t a l r a t i o ' s a ( 8 - ) / a ( 5 + a n d 4 +) for ~ C s l ~ C s a n d a(11/2-)[~(I/2 + a n d 3/2 +) for 13SCs (d, 2 n ) ~ B a .
(d, p)
TABLE 1 .---CRoSS-SECTION RATIOS FOR THE REACTION laaCs (d, p ) C s TM
Ea(MeV)
~r,~la"
9"4 11"3 13"3 16'8 18"4 19"8 23"7 24"9 26"2 27"4
0.78 4- 0"01 0.82 4- 0"03 0-72 -{- 0.04 0'72 4- 0"12 0.77 -4- 0"03 0.85 _t_ 0-07 0.87 4- 0"02 0-92 -{- 0.02 1"08 -4- 0.08 1"18 ± 0.13
TABLE 2.----CROSS-SECTIONRATIOS FOR THE REACTION laaCs (d, 2n)XSaBa
Ea(MeV) 13"5 15"6 16"0 17"9 19"4 21-0 21-3 22"8 24"0 24"3 25"6 27"0 28-5
,r./a, 0"52 4- 0"03 0-71 4- 0-03 0.73 4- 0"07 0"76 -4- 0-03 0.87 4- 0.05 0.99 4- 0-02 1.09 ± 0.10 1-22 q- 0.12 1"48 -t- 0"02 1.59 -4- 0-03 1"53 -t- 0"08 1.30 -4- 0"02 1-13 ± 0"02
T h e am/og-values a r e averages o f three m e a s u r e m e n t s p e r f o r m e d a t different times after the e n d o f the i r r a d i a t i o n . T h e errors i n d i c a t e d are the greatest deviations f r o m t h e average. I t is d e m o n s t r a t e d once m o r e c9-14~ t h a t nuclides with low spin are p r o d u c e d p r e f e r a b l y a t low excitation energies, a n d t h a t the p r o b a b i l i t y o f the f o r m a t i o n o f high spin i s o m e r s goes u p as the energy increases.
Acknowledgements--We would like to express our appreciation to the members of the synchrocyclotron crew for performing the irradiations. Gratitude is also expressed to Mr. A. Gtncnou for his assistance throughout the course of this work. ~9~A. C. PAPPASand R. A. SHARP,J. Inorg. NucL Chem. 10, 173 (1959). ~10~H. B. LEvY, Ph.D. Thesis, U.C.R.L. Report 2305 (1953). ~11~j. W ~ o , A.N.L. Report 6598 (1962). t12~S. AH~CASISand S. J. NASSrFF,J. Inorg, Nucl. Chem. 17, 206 (1961). ~a~ H. VIC3NAUand S. J. NASSn~F,Nuclear Physics 26, No 1, 108 (1961). ~4~ S. J. NASS~, S. ABECASISand A. MocoRoA, J. Inorff. Nucl. Chem. 24, 1131116 (1962).
ISOMERIC CROSS-SECTION RATIOS FOR ls4mCs, 184Cs AND 18SmBa, XSSBa FORMED IN (d,p) AND (d, 2n) REACTIONS* A. MOCOROA, H. VIGNAU,M. C. CARACOCHE and S. J. NASStn~ Institute of Physics, La Plata National University and National Atomic Energy Commission, Argentina (Received 24 January 1965)
Abstract--Ratios between the cross-sections ~,~/~g as a function of deuteron energy have been experimentally determined for the isomers 18"~Cs, ls'Cs produced by the reaction mCs (d, p)xs~C and for the isomers lU"Ba and ~aaBaproduced by the reaction mCs (d, 2n)mBa. IT is thought, in general, that nuclear reactions are produced by compound nucleus formation at energies below 30 MeV and by the knock-on mechanism at energies over 100 MeV. cl) Both mechanisms m a y lead to an intermediate nucleus with approximately the same excitation energy but with different angular m o m e n t u m distributions. This would result in different yields of end products when they differ greatly in spin, as in the case of nuclear isomers. ~s-e) A study of the ratio of the cross-sections for an isomeric pair in a given reaction could help in the interpretation of changes in the reaction mechanism with energy. At low energies, the spins of the initial nucleus as well as those of the isomers seem to be the main factor determining the yield ratio, the tendency observed being that the more frequently formed isomer is the one whose spin is nearest to the target nucleus. At higher energies where the compound nucleus is produced in a wider range of spin levels this effect may disappear. EXPERIMENTAL (a) Irradiations Cesium chloride p.a. samples were irradiated with a deuteron beam of the 72 in. diameter C.N.E.A. Synchrocyclotron. For this experience the stacked-foils technique was employed. The cesium chloride salt was wrapped in silver foils of about 25 mg/cm~. This technique allowed simultaneous bombardment over a wide range of energies. The irradiations were performed in the orbit corresponding to incident energies of 28.0 MeV and 29'2 MeV for the (d, p) and the (d, 2n) reactions respectively, with a maximum beam dispersion estimated at -I-1-2MeV. * The present work was partially supported by the Comisi6n Especial de Fisica At6mica y Radiois6topos (C.E.F.A.R.) ~1~j. M. BLATTand V. F. WmSSKOPF,Theoretical Nuclear Physics. 3. Wiley, New York (1952). cs) R. V^NDEI'mOSCHand J. R. HtntZ~GA, Phys. Reo. 120, 1305 (1960); 120, 1313 (1960). ~s~j. K. ~ w s , R. M. DL~,tONDand R. A. SHARP,Phys. Rev. 102, 190 (1956). ~' B. LINDER and R. A. JAMES,Phys. Rev. 114, 322 (1959). ~5~S. M. BAndY, U.C.R.L. Report A.E.C.U. 8710 (1959). ~s~E. S~sk and A. C. I-IEL~IOTZ,Rev. Mod. Phys. 21, 271 (1949). 5
A. MOCOROAet ai.
6
The deutron energy in each sample was calculated from the range-energy of AARONet alJ~) As the excitation functions of each isomer were not determined separately, the deuteron beam intensity variation through the target was not taken into consideration. (b) Chemical separations The cesium chloride irradiated was dissolved in a few milliliters of water; Bas+ was added as carrier. Ba 9+was precipitated as BaSe4 which, after filtration and washing was collected on a lucite plate. In this sample the isomeric pair xaS=Ba, XaSBawas measured. The solution was dried and afterwards some drops of 6 M NaOH and a few milliliters of 70 ~. HCIO, were added. The solution was then evaporated by constant shaking until white fumes started. Once cold the solution was treated with 15 ml of ethanol. The xuCs, lu'~2s pair was measured in the CsCIO, precipitate. r- 134 8(-) 55,-sr9
(5+)4(+)
(21~h)
<~1% O~13~0,010
r 134 55~,S79
(2,2y)
0~
4+
~,+; i
1,97 1,77
,
I ii1
I I; I
I
168
I
-~"I
I I
~
f I I
'
56Bo~4
BO133
(39,h!,, o,28T
(3/2÷)
i
0,436 ( I / 2 + } ~ L ' ~
~,3
(3/2)+
l
5/2+
,
o
la4uCs.
(~w2-)
7/2+
0,60
11
f I
FIG. l . - - D o c a y scheme 184r~CSand
I/2{+)
,.o 1,57
1 ,
O,OI 2
(7,2y)
0
82 t
'
i',
o:o8,
55Cs~3 FIG. 2.--Decay scheme of 18a=Ba and xSa0Ba.
(c) Counting techniques The gamma spectra were measured by means of 2 in. x 2 in. NaI(TI) crystal and 100 channel analyser. Areas under the photo-peaks were determined to obtain intensities. To prevent pile-up effects, foils of 0.331 g Sn/cra=and 0.181 g Me/era2 were used as absorbers. Corrections were applied for absorption and efficiencydependence on energy. Decay schemes, branching ratios and conversion coefficients were used as given in the Nuclear Data Sheets.(s) The decay scheme of luCs and xu'~.s is shown in Fig. 1. The population of the 2.9 hr isomer was determined by measuring the 127 keV line and the population of the 2.2 years level by measuring the 600 keV and the 1168 keV lines, both at suitable times after the irradiation. (The contribution of the 1570 keV line to the decay of lu~Cs is negligible.) Examples of the spectra used are shown in Figs. 3. (~) W. A. AARON,B. L. HOFrML~Nand F. C. Wzu.~,Ms, U.C.R.L. Report A.E.C.U. 663 (1951). Nuclear Data Sheets, National Academy of Sciences, N.R.C. 61-2-97.
(s)
Isomeric cross-section ratios for ls4mCs, ~aaCs and ~sa'~Ba, ~SSBa
IC
7
2
I
KeV Fro. 3.--Gamma spectrum of one of the 18'Cs samples measured 11.0 hr after the end of irradiation. The 127 keV ray of la4'~Cs can be seen and 670 keV gamma belonging to 18~Cs, which masks the 601) keV ray of za4gCs.
u~ r,
,~
~oo'
~
,oo'.....
KeV
s~o
"o
Fzo. 4.--Spectrum taken of 184Cs 103 days after irradiation. The 800 keV and 600 keV gamma rays of z~gCs are observed, the latter not resolved from the 563 and 569 ~,-rays, which also belong to the z~4gCs. and 4. The intensity of the 600 keV line was corrected for the presence of the 563 keV line (assumed to occur with an intensity equal to 14 per cent of that of the 600 keV line) and of the 569 kvV line (assumed to occur with an intensity equal to 11.5 per cent of that of the 600 la~V line). The 1168 keV line was assumeA to have an intensity equal to 2 per cent of that of the 600 keV line.
8
A. MOCOROAet al.
I
I
I00
200
300 KeY
400
500
FIo. 5.--F-ray spectrum of ~UmBa 45.8 hr after irradiation.
~0
g
2 I
I00
200
300 KeV
400
.....
~D
-==
FIG. 6.--7-ray spectrmn of ~3aaBa 3060 hr after irradiation. The decay scheme of lS3Ba and 13a=Ba is shown in Fig. 2. lU=Ba was measured by means of the 275 keV gamma, la3~Ba by moans of the 355 keV line. The scintillograms of these isomers are shown in Fig. 5 and 6. The observed intensity of the 355 keV line was corrected for the presence of 382 keV gammas which wc assumed to occur with intensity equal to 1% of that of the first mentioned line.
Isomeric cross-section ratios for 1s4mCs, la4Cs and ~mBa, Xa*Ba
9
RESULTS AND DISCUSSION T a b l e s 1 a n d 2 show the e x p e r i m e n t a l r a t i o ' s a ( 8 - ) / a ( 5 + a n d 4 +) for ~ C s l ~ C s a n d a(11/2-)[~(I/2 + a n d 3/2 +) for 13SCs (d, 2 n ) ~ B a .
(d, p)
TABLE 1 .---CRoSS-SECTION RATIOS FOR THE REACTION laaCs (d, p ) C s TM
Ea(MeV)
~r,~la"
9"4 11"3 13"3 16'8 18"4 19"8 23"7 24"9 26"2 27"4
0.78 4- 0"01 0.82 4- 0"03 0-72 -{- 0.04 0'72 4- 0"12 0.77 -4- 0"03 0.85 _t_ 0-07 0.87 4- 0"02 0-92 -{- 0.02 1"08 -4- 0.08 1"18 ± 0.13
TABLE 2.----CROSS-SECTIONRATIOS FOR THE REACTION laaCs (d, 2n)XSaBa
Ea(MeV) 13"5 15"6 16"0 17"9 19"4 21-0 21-3 22"8 24"0 24"3 25"6 27"0 28-5
,r./a, 0"52 4- 0"03 0-71 4- 0-03 0.73 4- 0"07 0"76 -4- 0-03 0.87 4- 0.05 0.99 4- 0-02 1.09 ± 0.10 1-22 q- 0.12 1"48 -t- 0"02 1.59 -4- 0-03 1"53 -t- 0"08 1.30 -4- 0"02 1-13 ± 0"02
T h e am/og-values a r e averages o f three m e a s u r e m e n t s p e r f o r m e d a t different times after the e n d o f the i r r a d i a t i o n . T h e errors i n d i c a t e d are the greatest deviations f r o m t h e average. I t is d e m o n s t r a t e d once m o r e c9-14~ t h a t nuclides with low spin are p r o d u c e d p r e f e r a b l y a t low excitation energies, a n d t h a t the p r o b a b i l i t y o f the f o r m a t i o n o f high spin i s o m e r s goes u p as the energy increases.
Acknowledgements--We would like to express our appreciation to the members of the synchrocyclotron crew for performing the irradiations. Gratitude is also expressed to Mr. A. Gtncnou for his assistance throughout the course of this work. ~9~A. C. PAPPASand R. A. SHARP,J. Inorg. NucL Chem. 10, 173 (1959). ~10~H. B. LEvY, Ph.D. Thesis, U.C.R.L. Report 2305 (1953). ~11~j. W ~ o , A.N.L. Report 6598 (1962). t12~S. AH~CASISand S. J. NASSrFF,J. Inorg, Nucl. Chem. 17, 206 (1961). ~a~ H. VIC3NAUand S. J. NASSn~F,Nuclear Physics 26, No 1, 108 (1961). ~4~ S. J. NASS~, S. ABECASISand A. MocoRoA, J. Inorff. Nucl. Chem. 24, 1131116 (1962).