- Email: [email protected]
Thermal-neutron capture gamma-rays from some rare-earth elements
J Nucl. Enwgy II, 1959, Vol. 9, pp. 69 to 74. Pergamon Press Ltd., London. Printed in Northern Ireland
THERMAL-NEUTRON CAPTURE GAMMA-RAYS SOME RARE-EARTH ELEMENTS*
FROM
V. V. SKLYAREVSKII, E. P. STEPANOVand B. A. OBINYAKOV (Received 8 August 1957)
Abstract-Capture y-ray spectra have been measured using a scintillation spectrometer. Strong lines were observedbelow 300keV with Eu, Gd, Dy; Ho, Er, Tm, Hf and Ta. In the spectra of lS8Er and lr*Hf, lines of intensity 0.5-0.8 quanta per capture have been attributed 4+--F 2’ and 2’ + 0+ between rotational levels.
neutron capture holmium, erbium, thulium scintillation spectrometer, 1.2 cm. A diagram of the formed at the RFT reactor,
THERMAL
to transitions of the type
y-ray spectra from europium, gadolinium, dysprosium, and tantalum have been measured below 3DOkeV with a using a NaI(T1) crystal of diameter 3.0 cm and thickness apparatus employed in the experiments, which were peris given in Figure 1. The pulse amplitude spectra from the
5cm.
6
FIG. I.-Diagram of the experiment. l-neutron beam; 2-the target; 3-a boron absorber; 4-the scintillating crystal; 5-photo-multiplier type S; &lead shield; 7-a laminated absorber, consisting of 0.3 mm tantalum + 0.5 mm tin + 0.2 mm copper, to attenuate the K-radiation of lead; 8-a lead collimator; 9-a boron collimator.
photo-multiplier were recorded in a 30-channel analyser. The energy resolution was 15 per cent for 51Cr y-rays at 320 keV and 20 per cent for lj3Gd at 102 keV. Absolute intensities were derived by a method first used by ESTULINet al. ;(I) the areas under the observed spectral peaks were compared with the value obtained for the 480 keV line from 1°B(n, a)‘Li, using a boron target. The intensity of the 480 keV line was taken to be 0.93. It will be seen from Figures 2-4 that the y-ray lines were accompanied by a continuum. This was caused principally by pulses from harder y-rays, also created in the target; but a fraction of it, varying between 5 and 30 per cent with different targets, * Translated from
Atomnaya Energiya 4, 22 (1958). 69
IO
V. V.
SKLYAREVSW,
E. P. STEPANOVand B. A. OBINYAKOV
was a background count. The presence of this continuum makes it difficult to determine the areas under the spectral peaks. Additional measurements were made therefore with tin or lead filters inserted between the target and the crystal. The degree of attenuation in the filter material is known; and as the filter left the background level virtually unaffected, the required areas could be obtained from curves showing the difference between the two spectra, taken with and without the absorber. In some cases this technique causes lines to become evident that would have passed unnoticed in an examination of either spectrum alone. Figure 5 contains an example of such an occurrence.
10
20
30
40
Analyser channel no. FIG. 2.-Capture y-/-‘a),spectrum of erbium. A proposed level scheme for lssEr is shown, transition intensities being marked alongside the arrows.
The energy dependence of the detection efficiency was determined in the usua1 way f2)from the ratio of the number of pulses in a given peak to the total number of y-ray pulses. Mono-chromatic radiation from several different sources was employed; at 102 keV (l%Gd), 145 keV (141Ce),320 keV (Wr), 480 keV (lOB(n, a)‘Li) and 662 keV (WJS). Specimens were prepared in the form of the oxide, a powdered sample of which was pressed into an aluminium capsule 2.0 cm in diameter and 0.3 mm in thickness. An empty capsule was used to measure the spectrum of the background. A rough check on the purity of the specimens was made by measuring the total thermal neutron cross-section in transmission. For the last-mentioned readings a boron target was used as detector, the y-rays from it being recorded in the scintillation spectrometer. No appreciable impurity content was found. RESULTS
Erbium The relative contributions made by the various isotopes of erbium to the absorption cross-section of the natural element are unknown. It will be noticed, how; ever, that in our spectrum, shown in Figure 2, the two lines appearing at 82 and 185 keV have energies in a ratio agreeing with that to be expected from transitions between rotational levels in an even-even nucleus. An appropriate level scheme is
Thermal-neutron
capture gamma-rays from some rare-earth elements
71
shown in an inset to the figure. The great intensity of these lines, in terms of the number of quanta per capture in natural erbium, implies that the thermal neutron capture cross-section of the natural element is mainly due to 16’Er; for 16’Er is the only isotope occurring in nature that is capable of giving rise to an even-even nucleus on neutron capture. The first rotational level has previously been measured in a study of the y-rays from Coulomb excitation ;c3) the energy of 79 keV thus obtained agrees well with our own value of 82. Our result for the second */-ray line agrees with that of FENSTERMACHER et ~1.‘~)who give 188 keV and find an intensity 4.5. We have made measurements of the two y-rays in coincidence, which showed that they are indeed emitted in cascade. Hafnium The thermal neutron capture cross-section of hafnium is due principally to l”Hf. The line at 92 keV seen in Figure 3 corresponds to a transition to the ground state from the first rotational level of 17*Hf, which is known from experiments on Coulomb excitation.t3) The line at 213 keV is essentially due to a transition from the second
0
40
20 Analyser
60
channelno.
FIG. 3.-Capture
y-ray spectrum of hafninrn. A proposed level scheme for l’*Hf is shown, transition intensities being marked alongside the arrows.
rotational level to the first; however, a 215 keV y-ray emitted by 17gHfin an isomeric transition of half-life 20 set overlaps it. By measuring the y-ray spectrum from a hafnium target directly after switching off the neutron beam, the contribution of the isomeric transition was measured, and shown to be 20 per cent of the intensity of the line at 213 keV. Our data confirm the work of BOCKELMAN et a1.,t5)who found lines at 90, 220 and 330 keV in the capture y-ray spectra induced by resonance neutrons. We again made coincidence measurements on our two lines and showed that they are emitted in cascade.
It is knowrP$‘) from the spectra of internal conversion electrons created by thermal neutron capture y-rays that lines are emitted at 79 and 180 keV due to
V. V. SKLYAREVSKII, E. P. STEPANOV and B. A. OBINYAKOV
12
15’Gd(n, y)158Gd, and at 89and 196 keV due to 155Gd(n, y)l%Gd. We have investigated the intensities of these lines using separated isotopes. A 155Gd target was enriched to 973
per cent, and one of 15’Gd to 91.4 per cent.
The measured
spectra are shown
in Figure 4. The high intensities even-even
of the lines 4f -+ 2+ and 2f -+ O+ in the spectra of all four r6sEr, 17sHf, ‘15*Gd and 1sGd show that a substantial proportion
isotopes,
of the transitions
from higher levels descend to the level 4+.
FIG. 4.-Capture
y-ray spectra for gadolinium. using a laaGd target --- using a lJ7Gd target Proposed level schemes are shown for the two isotopes raeGd and r6*Gd formed on neutron capture. Transition intensities are marked alongside the arrows.
Other elements Lines were found in the capture y-ray spectrum of dysprosium which corresponded between the lower levels of la5Dy.(s’ A number of rather intense
to known transitions
lines appeared with europium, tions between
low-lying
holmium,
thulium and tantalum
levels of the odd-odd
The data are as yet insufficient
to establish
CONCLUDING Data energy
for all our observed determinations
level schemes. REMARKS
lines are set out
have an accuracy
and were due to transi-
nuclei, ls2Eu, 16sHo, 170Tu and l@Ta.
of 2-3
in the
accompanying
Table.
The
per cent, those of the y-ray yields
20-30 per cent However, below 100 keV the total intensities of transitions are accurate only to ~40 per cent, as the relevant conversion coefficients are large and not very reliably
known.
compilations
Conversion coefficients for the K and L shells were taken from the of SLIV and BAND(~) and of DRANITSYNA.~~)
Acknowledgements-The authors are much indebted to Academician I. V. KURCHATOV for his interest in this investigation. Thanks are due also to Prof. L. V. GROSHEVand to Drs. V. M. STRUTINSKII and D. P. GRECHUKHIN for valuable advice, and to Prof. I. A. ZAOZERSKII for kindly supplying the rare-earth specime”s. We are very grateful to G. P. MELNIKOVfor maintaining the electronics.
Thermal-neutron
I
0
capture gamma-rays
I
I
I
v
i
_ __.._
from some rare-earth elements
,
rence between the esoanda, and its lines
4raulutioninto !wo
60
40
20 Analyser
73
channel
no.
FIG. 5.-Capture y-ray spectrum of europium. The line at 93 keV obscures a weaker line at 71 keV, shown resolved in the inset. TABLE 1
II
I Emitting isotope
-
-_
y-Ray line ! energy (kevj
r’aHf 168Gd
‘6.5D
/
Y
r=Eu laBHo
r8zTa
,
82 185 92 213 79 183 87 196 78 104 180 12 90 121 142 107 133 170 272 150
0.18 0.64 0.19 0.55 0.104 0.22 0.137 0.271 0.028 0.018 0.16 0.044 0.20 0.20 0.31 0.152 0.30 0.22 0.7 0.073
E2 E2 E2 E2 E2 E2 E2 E2 Ml E3 E2
j
I /
I
Intensity of the
ag ;Y./(l -i- “R-i- ah)
-.--
/ I
rasEr
r=Gd
Line intensity, I Multipolarity yY (numbers of the I of quanta per , transition capture)*
-
-.
I ’
1.6 0.20
j
1;; 0 13
i 1
_. 2.1 0,084 1.7 0.07 2.05 0.09 1.25 0.054 0.68 22.6 0.1
0.22 1.37 0.17 4.1 3.4 0.22
I /
I
--
-. ,
i , 1
_
_
0.85 0.82 0.71 0.66 0.53 0.29 0.50 0.34 0.16 0.49.t 0.21
I
* yy was calculated in accordance with the isotopic absorption cross-sections of Hughes,(rrr except for rsaEr which was assumed to have the U. = 166 barn of the natural element. t This figure for the isomeric transition of half-life 1.3 min essentially determine the fraction, 0.7 according to WEBER,‘~~)of the total neutron capture cross-section of leaDy that may be attributed to the formation of lasmDy.
74
V. V. SKLYAREVSW, E. P. STEPANOVand B. A. OBINYAKOV REFERENCES
1. E~~ULIN I. V., KALINKIN L. F. and MELIORANSKIIA. S., Zh. eksp. teor.fiz. 31, 886 (1956). 2. LINDEN K. and STARFELTN., Ark. Fys. 7,428 (1954). 3. HEYDENBURGN. P. and TEMMERG. M., Phys. Rev. 100,150 (1955). 4. FENSTERMACHER C. A., HICKOK R. L. and SCHULTZ H. L., BUZZ. Amer. Phys. Sot. II 2,41(1957). 5. BOCKELMANC. K., FENSTERMACHER C. A. and DRAPER J. E., BUN.Amer. Phys. Sot. II 2,41(1957). 6. CHURCH E. L. and GOLDHABERM., Phys. Rev. 95,626 (1954). 7. GROSHEVL. v., DEMID~V A. M. and NAIDENOYV. A., Paper read at the Seoenth All-Union Conference on Nuclear Spectroscopy, Leningrad (1957). 8. DZHELEPOV B. S. and PEKER L. K., Decay Schemes of the Radioactive Isotopes. Academy of Sciences Publishing House (1957). 9. SLY L. A. and BAND I. M., Tables of Internal ConversionCoeficients Part 1. Academy of Sciences Publishing House (1956). 10. DRANITSYNAG. E., Internal Conversion Coefficientsfor the LI, Ln and Lm Subshells. Academy of Sciences Publishing House (1957). 11. HUGHESD. J., Pile Neutron Research. Addison Wesley (1953). 12. WEBER G., Zeits. Naturforsch. 9a, 115 (1954).
THERMAL-NEUTRON CAPTURE GAMMA-RAYS SOME RARE-EARTH ELEMENTS*
FROM
V. V. SKLYAREVSKII, E. P. STEPANOVand B. A. OBINYAKOV (Received 8 August 1957)
Abstract-Capture y-ray spectra have been measured using a scintillation spectrometer. Strong lines were observedbelow 300keV with Eu, Gd, Dy; Ho, Er, Tm, Hf and Ta. In the spectra of lS8Er and lr*Hf, lines of intensity 0.5-0.8 quanta per capture have been attributed 4+--F 2’ and 2’ + 0+ between rotational levels.
neutron capture holmium, erbium, thulium scintillation spectrometer, 1.2 cm. A diagram of the formed at the RFT reactor,
THERMAL
to transitions of the type
y-ray spectra from europium, gadolinium, dysprosium, and tantalum have been measured below 3DOkeV with a using a NaI(T1) crystal of diameter 3.0 cm and thickness apparatus employed in the experiments, which were peris given in Figure 1. The pulse amplitude spectra from the
5cm.
6
FIG. I.-Diagram of the experiment. l-neutron beam; 2-the target; 3-a boron absorber; 4-the scintillating crystal; 5-photo-multiplier type S; &lead shield; 7-a laminated absorber, consisting of 0.3 mm tantalum + 0.5 mm tin + 0.2 mm copper, to attenuate the K-radiation of lead; 8-a lead collimator; 9-a boron collimator.
photo-multiplier were recorded in a 30-channel analyser. The energy resolution was 15 per cent for 51Cr y-rays at 320 keV and 20 per cent for lj3Gd at 102 keV. Absolute intensities were derived by a method first used by ESTULINet al. ;(I) the areas under the observed spectral peaks were compared with the value obtained for the 480 keV line from 1°B(n, a)‘Li, using a boron target. The intensity of the 480 keV line was taken to be 0.93. It will be seen from Figures 2-4 that the y-ray lines were accompanied by a continuum. This was caused principally by pulses from harder y-rays, also created in the target; but a fraction of it, varying between 5 and 30 per cent with different targets, * Translated from
Atomnaya Energiya 4, 22 (1958). 69
IO
V. V.
SKLYAREVSW,
E. P. STEPANOVand B. A. OBINYAKOV
was a background count. The presence of this continuum makes it difficult to determine the areas under the spectral peaks. Additional measurements were made therefore with tin or lead filters inserted between the target and the crystal. The degree of attenuation in the filter material is known; and as the filter left the background level virtually unaffected, the required areas could be obtained from curves showing the difference between the two spectra, taken with and without the absorber. In some cases this technique causes lines to become evident that would have passed unnoticed in an examination of either spectrum alone. Figure 5 contains an example of such an occurrence.
10
20
30
40
Analyser channel no. FIG. 2.-Capture y-/-‘a),spectrum of erbium. A proposed level scheme for lssEr is shown, transition intensities being marked alongside the arrows.
The energy dependence of the detection efficiency was determined in the usua1 way f2)from the ratio of the number of pulses in a given peak to the total number of y-ray pulses. Mono-chromatic radiation from several different sources was employed; at 102 keV (l%Gd), 145 keV (141Ce),320 keV (Wr), 480 keV (lOB(n, a)‘Li) and 662 keV (WJS). Specimens were prepared in the form of the oxide, a powdered sample of which was pressed into an aluminium capsule 2.0 cm in diameter and 0.3 mm in thickness. An empty capsule was used to measure the spectrum of the background. A rough check on the purity of the specimens was made by measuring the total thermal neutron cross-section in transmission. For the last-mentioned readings a boron target was used as detector, the y-rays from it being recorded in the scintillation spectrometer. No appreciable impurity content was found. RESULTS
Erbium The relative contributions made by the various isotopes of erbium to the absorption cross-section of the natural element are unknown. It will be noticed, how; ever, that in our spectrum, shown in Figure 2, the two lines appearing at 82 and 185 keV have energies in a ratio agreeing with that to be expected from transitions between rotational levels in an even-even nucleus. An appropriate level scheme is
Thermal-neutron
capture gamma-rays from some rare-earth elements
71
shown in an inset to the figure. The great intensity of these lines, in terms of the number of quanta per capture in natural erbium, implies that the thermal neutron capture cross-section of the natural element is mainly due to 16’Er; for 16’Er is the only isotope occurring in nature that is capable of giving rise to an even-even nucleus on neutron capture. The first rotational level has previously been measured in a study of the y-rays from Coulomb excitation ;c3) the energy of 79 keV thus obtained agrees well with our own value of 82. Our result for the second */-ray line agrees with that of FENSTERMACHER et ~1.‘~)who give 188 keV and find an intensity 4.5. We have made measurements of the two y-rays in coincidence, which showed that they are indeed emitted in cascade. Hafnium The thermal neutron capture cross-section of hafnium is due principally to l”Hf. The line at 92 keV seen in Figure 3 corresponds to a transition to the ground state from the first rotational level of 17*Hf, which is known from experiments on Coulomb excitation.t3) The line at 213 keV is essentially due to a transition from the second
0
40
20 Analyser
60
channelno.
FIG. 3.-Capture
y-ray spectrum of hafninrn. A proposed level scheme for l’*Hf is shown, transition intensities being marked alongside the arrows.
rotational level to the first; however, a 215 keV y-ray emitted by 17gHfin an isomeric transition of half-life 20 set overlaps it. By measuring the y-ray spectrum from a hafnium target directly after switching off the neutron beam, the contribution of the isomeric transition was measured, and shown to be 20 per cent of the intensity of the line at 213 keV. Our data confirm the work of BOCKELMAN et a1.,t5)who found lines at 90, 220 and 330 keV in the capture y-ray spectra induced by resonance neutrons. We again made coincidence measurements on our two lines and showed that they are emitted in cascade.
It is knowrP$‘) from the spectra of internal conversion electrons created by thermal neutron capture y-rays that lines are emitted at 79 and 180 keV due to
V. V. SKLYAREVSKII, E. P. STEPANOV and B. A. OBINYAKOV
12
15’Gd(n, y)158Gd, and at 89and 196 keV due to 155Gd(n, y)l%Gd. We have investigated the intensities of these lines using separated isotopes. A 155Gd target was enriched to 973
per cent, and one of 15’Gd to 91.4 per cent.
The measured
spectra are shown
in Figure 4. The high intensities even-even
of the lines 4f -+ 2+ and 2f -+ O+ in the spectra of all four r6sEr, 17sHf, ‘15*Gd and 1sGd show that a substantial proportion
isotopes,
of the transitions
from higher levels descend to the level 4+.
FIG. 4.-Capture
y-ray spectra for gadolinium. using a laaGd target --- using a lJ7Gd target Proposed level schemes are shown for the two isotopes raeGd and r6*Gd formed on neutron capture. Transition intensities are marked alongside the arrows.
Other elements Lines were found in the capture y-ray spectrum of dysprosium which corresponded between the lower levels of la5Dy.(s’ A number of rather intense
to known transitions
lines appeared with europium, tions between
low-lying
holmium,
thulium and tantalum
levels of the odd-odd
The data are as yet insufficient
to establish
CONCLUDING Data energy
for all our observed determinations
level schemes. REMARKS
lines are set out
have an accuracy
and were due to transi-
nuclei, ls2Eu, 16sHo, 170Tu and l@Ta.
of 2-3
in the
accompanying
Table.
The
per cent, those of the y-ray yields
20-30 per cent However, below 100 keV the total intensities of transitions are accurate only to ~40 per cent, as the relevant conversion coefficients are large and not very reliably
known.
compilations
Conversion coefficients for the K and L shells were taken from the of SLIV and BAND(~) and of DRANITSYNA.~~)
Acknowledgements-The authors are much indebted to Academician I. V. KURCHATOV for his interest in this investigation. Thanks are due also to Prof. L. V. GROSHEVand to Drs. V. M. STRUTINSKII and D. P. GRECHUKHIN for valuable advice, and to Prof. I. A. ZAOZERSKII for kindly supplying the rare-earth specime”s. We are very grateful to G. P. MELNIKOVfor maintaining the electronics.
Thermal-neutron
I
0
capture gamma-rays
I
I
I
v
i
_ __.._
from some rare-earth elements
,
rence between the esoanda, and its lines
4raulutioninto !wo
60
40
20 Analyser
73
channel
no.
FIG. 5.-Capture y-ray spectrum of europium. The line at 93 keV obscures a weaker line at 71 keV, shown resolved in the inset. TABLE 1
II
I Emitting isotope
-
-_
y-Ray line ! energy (kevj
r’aHf 168Gd
‘6.5D
/
Y
r=Eu laBHo
r8zTa
,
82 185 92 213 79 183 87 196 78 104 180 12 90 121 142 107 133 170 272 150
0.18 0.64 0.19 0.55 0.104 0.22 0.137 0.271 0.028 0.018 0.16 0.044 0.20 0.20 0.31 0.152 0.30 0.22 0.7 0.073
E2 E2 E2 E2 E2 E2 E2 E2 Ml E3 E2
j
I /
I
Intensity of the
ag ;Y./(l -i- “R-i- ah)
-.--
/ I
rasEr
r=Gd
Line intensity, I Multipolarity yY (numbers of the I of quanta per , transition capture)*
-
-.
I ’
1.6 0.20
j
1;; 0 13
i 1
_. 2.1 0,084 1.7 0.07 2.05 0.09 1.25 0.054 0.68 22.6 0.1
0.22 1.37 0.17 4.1 3.4 0.22
I /
I
--
-. ,
i , 1
_
_
0.85 0.82 0.71 0.66 0.53 0.29 0.50 0.34 0.16 0.49.t 0.21
I
* yy was calculated in accordance with the isotopic absorption cross-sections of Hughes,(rrr except for rsaEr which was assumed to have the U. = 166 barn of the natural element. t This figure for the isomeric transition of half-life 1.3 min essentially determine the fraction, 0.7 according to WEBER,‘~~)of the total neutron capture cross-section of leaDy that may be attributed to the formation of lasmDy.
74
V. V. SKLYAREVSW, E. P. STEPANOVand B. A. OBINYAKOV REFERENCES
1. E~~ULIN I. V., KALINKIN L. F. and MELIORANSKIIA. S., Zh. eksp. teor.fiz. 31, 886 (1956). 2. LINDEN K. and STARFELTN., Ark. Fys. 7,428 (1954). 3. HEYDENBURGN. P. and TEMMERG. M., Phys. Rev. 100,150 (1955). 4. FENSTERMACHER C. A., HICKOK R. L. and SCHULTZ H. L., BUZZ. Amer. Phys. Sot. II 2,41(1957). 5. BOCKELMANC. K., FENSTERMACHER C. A. and DRAPER J. E., BUN.Amer. Phys. Sot. II 2,41(1957). 6. CHURCH E. L. and GOLDHABERM., Phys. Rev. 95,626 (1954). 7. GROSHEVL. v., DEMID~V A. M. and NAIDENOYV. A., Paper read at the Seoenth All-Union Conference on Nuclear Spectroscopy, Leningrad (1957). 8. DZHELEPOV B. S. and PEKER L. K., Decay Schemes of the Radioactive Isotopes. Academy of Sciences Publishing House (1957). 9. SLY L. A. and BAND I. M., Tables of Internal ConversionCoeficients Part 1. Academy of Sciences Publishing House (1956). 10. DRANITSYNAG. E., Internal Conversion Coefficientsfor the LI, Ln and Lm Subshells. Academy of Sciences Publishing House (1957). 11. HUGHESD. J., Pile Neutron Research. Addison Wesley (1953). 12. WEBER G., Zeits. Naturforsch. 9a, 115 (1954).