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Nuclear spectroscopy of 173Tm
Nuclear Physics 4 8 (1963) 675----685; (~) North-Holland Publishing Co., Amsterdam Hot to be reproduced by photoprint or microfilm witl~out written permission from the publisher
N U C L E A R S P E C T R O S C O P Y OF l~3Tm T. K U R O Y A N A G I
a n d T. T A M U R A
Linear Accelerator Laboratory, Department of Physics, Japan Atomic Energy Research Institute, Japan Received 1 July 1963 Abstract: T h e 17rI'm nucleide was p r o d u c e d by t h e (y, p) reaction by u s i n g a n intense b r e m s s t r a h l u n g f r o m a linear electron accelerator. A l u Y b target enriched to 99 % was used. T h e radiations f r o m XTaTm were m e a s u r e d by m e a n s o f a beta a n d g a m m a scintillation spectrometer. T w o beta-ray g r o u p s with e n d - p o i n t energies o f 1.26 a n d 0.86 M e V were found, a n d K X-ray, 395 a n d 460 keV g a m m a rays were observed. T h e excited states at 395 a n d 460 keV in ~v3Yb were identified as ½+ [651 ] a n d ½- [521 ] intrinsic states, respectively. It was f o u n d that the 395 keV state is an isomeric level with a h a l f life o f 2.9 psec. T h e a s s i g n m e n t s were m a d e with the aid o f electron - K X - r a y a n d electron - beta coincidence m e t h o d s .
1. Introduction Thulium-173 was first produced from the photonuclear reaction of natural ytterbium by using an internal bombarding techrrique with a betatron 1). Since a much stronger beam became available at our Linear Electron Accelerator Laboratory, a detailed investigation of the decay of this nucleide became possible.
2. Experimental Procedures Thulium-173 was produced by the ( ? , p ) reaction on a 9 mg of ytterbium oxide sample enriched to 98.7 % in mass number 174. The bremsstrahlung was converted from the 21 MeV and 40 uA electron beam of J A E R I linear electron accelerator. The sample was bombarded for about 3 h in an water cooled converter-target assembly which is shown in fig. 1. The intensity ot bremsstrahlung was estimated to be about 5 x 106 r/min from an activation o f 63Cu by using the well known energy-yield curve of the (?, n) reaction 2). This value was checked by a rSntgen rate meter. Because of a high enrichment of the target and because of the absence of strong background radiations due to the (~,, n) proctuct, no chemical separation was done. The amount of the activity was 0.05 pCur for the 3 h bombardment. In the measurement of the beta ray, the sample was folded with aluminium foil of 2 mg/cm 2 thickness. The gamma-ray spectrometer consisted of a 7.7cm ~ × 7.7 cm NaI(TI) crystal and a 6363 photomultiplier tube with a conventional 256-channel pulse-height analyser. In order to eliminate a contribution of beta rays, a 2 cm lucite absorber was used. The energy calibration was performed by employing the photopeaks of g a m m a rays of 196Au, lt4In, 137Cs and 22Na sources. 675
T. KUROYANAGIAND T. TAMURA
676
The beta ray was viewed by a 3.8 cm ~ x 1.3 cm anthracene crystal which has the resolution of 10 ~o for the 624 keV line of conversion electron of 137mBa. The energy calibration curve and the reference Kurie plot were made from the beta-ray spectra of 19SAu ' 114in ' 169Er ' and 137mBa conversion line. Figs. 2 (a) and 2 (b) show the examples. O.Imm AI WINDOW
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Fig. 2(a). K u r i e plot o f beta-ray s p e c t r u m o f l°SAu.
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NUMBER
Fig. 2(b). Beta-ray s p e c t r u m o f ls~Cs.
In the coincidence measurements, a conventional fast slow coincidence circuit with a resolving time of 80 nsec was used• In order to determine the efficiencies of the beta and gamma scintillation crystals, the coincidence circuit mentioned above was used. The accuracy of the intensity measurements was within 5 ~.
NUCLEAR
3.
677
SPECTROSCOPY
Results
3.1. GAMMA- AND BETA-RAY SPECTRA The gamma-ray spectrum of the sample irradiated with the 21 MeV bremsstrahlung was measured from several minutes after the end of bombardment to about one week. Several hours after the bombardment, outstanding peaks of 460, 395 and 52 keV energy were found, and a very weak peak at 510 keV was seen (fig. 3(a)). The spectrum taken at about a hundred hours after the irradiation was found to be that from the g a m m a rays of 17SYb (fig. 3(b)). This was confirmed by following the decay of each peak. 10 5 -
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Fig. 3. Gamma-ray spectra of the irradiated sample of separated isotope of ytterbium obtained at (a) 8 h after and (b) about 100 h after the termination of the bombardment.
............... 0
50
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I00
__
TIME (h)
Fig. 4. Decay curve of the peak observed at the energy of 395 keV.
Following the decay, the g a m m a rays of 460, 395 and 52 keV were found to have a half life of 8.2 h. The decay curve is shown in fig. 4. The weak peak of 510 keV could not be assigned to the g a m m a ray o f ~73Tm because o f the weak intensity and slight contamination of 64Cu. The intensity ratio of those g a m m a rays were estimated by using a photopeak efficiency and successive peeling off technique. In order to see if the 8.2 h component really belongs to 173Tm, an irradiation by the beam of 13 MeV bremsstrahiung was made. Then, none of the 8.2 h component was found. The beta-ray spectrum was analysed in the same way as the gamma-ray measurement. After a sufficiently long time (200 h), the spectrum showed beta rays due to 17Syb and 172Tm, which was known from the fact that the end point energies were
678
T. KUROYANAGIAND T. TAMURA
0.47 and 1.8 MeV with half lives of 4.1 d and 69 h, respectively. The decay curve of the beta ray taken above the energy of 0.5 MeV shows that it consisted of 8.2 h and some short lived components. After an analysis of the decay curve, the half lives of the short components were found to be 10 rain and 32 min, which were attributed to the contamination o f Cu and CI from these end point energies. After the subtraction of the beta-rays of 172Tm and 175Yb, beta-ray groups of the 8.2 h half life with end-point energies o f 1.26 and 0.86 MeV were found. The Kurie plots of the beta-ray spectrum measured at eight hours after the bombardment are shown in fig. 5. In fig. 5 the contributions o f 172Tm and 175yb have been subtracted.
40
I O0
30
50
2(3
0 0.5
0.5
ID 1.5 ENERGY (I~iV)
1.0
2D
Fig. 5. Kurie plots of beta-ray spectrum of 17aTm. The left and right side show a Kurie plot of the total spectrum and a Kurie plot after the subtraction of 1.26 MeV component, respectively. 3.2. BETA-GAMMA A N D G A M M A - G A M M A COINCIDENCE
The beta-gamma and gamma-gamma coincidence measurements were performed in order to construct a decay scheme. The beta-gamma coincidence spectra were taken by setting the single-channel window at 52, 395 and 460 keV. In the cases of both the 52 and 460 keV window, the same end point energy of the coincidence spectra oftbe beta ray was found to be 0.80 MeV from the Kurie plots, and the coincidence ratio was computed to be about 100 %. When the coincidence gate was opened by the photopeak o f 395 keV gamma ray, no coincident beta-ray was observed. Gamma-gamma coincidence gates were set at the photopeak of the K X-ray, 395and 460 keV gamma rays. No coincident gamma ray was measured.
NUCLEAR
679
SPECTROSCOPY
The results of the beta-gamma and g a m m a - g a m m a coincidence measurements, which have no cascade relation except 0.80 MeV beta ray to the K X-ray and the 460 keV g a m m a ray, suggested that the 395 keV g a m m a ray is delayed. Therefore, the beta-gamma and g a m m a - g a m m a delayed coincidence measurements were performed using a 256-channel time analyser. When the time analyser was triggered by the beta-ray pulses, the g a m m a - r a y pulses selected by the single-channel window were accepted in the time analyser. F r o m this measurement, it was found that the 395-keV g a m m a ray is retarded with a half life of 2.9 #see. Fig. 6 shows the decay curve. In this
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/JSec Fig. 6. Time spectrumobtainedfrom the delayedcoincidencemeasurementbetweenbeta-rays and the 395 keV gamma ray. figure the excess count of the first channel is considered to be the prompt coincidence between the Compton pulses of the 460 keV g a m m a ray and the 800 keV beta ray. And the fiat part in the higher channels is the accidental coincidence count. The g a m m a - g a m m a delayed coincidence relation was found in the coincidence measurement between the K X-ray and the 395 keV g a m m a ray with the same half life of 2.9 #see. Finally, slow coincidence measurements of the resolving time of 2T = 2#sec were tried. The beta-ray spectrum in coincidence with 395 keV g a m m a ray was found to have an end point energy of 860 keV. A g a m m a - g a m m a coincidence relation between the K X-ray and 395 keV g a m m a ray was also obtained.
680
T. K U R O Y A N A G I A N D T . TAMURA
F r o m a straight forward consideration o f the experimental data, a cascade relation o f beta-gamma and g a m m a - g a m m a was proposed as shown in fig. 13. Relative intensities o f the beta and g a m m a rays were estimated and are listed in table 1. TABLE 1 Relative intensities of the gamma and beta ray of 17aTm Gamma ray energy (keY)
Percent of total decay
K X-ray 395 460
21 ±1 91 ±5 7.5!0.2
Beta ray energy (MeV)
Percent of total decay
0.80 0.86 1.26
28 71 0.5
4. Multipole Order Assigmnents of the Gamma Transitions Although the cascade relation in the decay of the 173Tm could be constructed from the fast-slow, slow and delayed coincidence experiments, the spin and parity assignments of the levels were not uniquely determined. Further studies of multipole order •o f the g a m m a radiations were made, since properties of the radiations are not specified from the half life because of large fluctuations o f the hindrance factor involved in the g a m m a decay. 4.1. THE 395 keV TRANSITION In order to determine the multipole order of the 395 keV g a m m a ray, the internal conversion coefficient of the K shell was measured by means o f conversion electron-K X-ray coincidence. An internal conversion coefficient is defined by the following equation: -
/ek
-
/ek [k .
(1)
In this equation, Ik/I7 is the intensity ratio computed from the single spectrum of the g a m m a ray, and Iek/I k is a coincidence ratio in the electron-K X-ray coincidence measurement. The same coincidence arrangement and fast-slow coincidence circuit as mentioned above were used. The spectrum of the internal conversion line of 137mBa taken with this arrangement is shown in fig. 7. Better checks were performed by using a 175yb source (fig. 8). The crystal efficiency for peak area of the energy of 335 k e y was determined to be 22__+2 ~o with the Yb source. In this measurement, the energy
NUCLEAR i fj 5 ----
681
SPECTROSCOPY
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Fig• 8• Internal conversion line spectra• The left side is the internal conversion lines of 411 keV transition in the decay of l°aAu in coincidence with the beta ray, and the right side is the internal conversion line of 396 keV tr~n~itlt~n
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Fig. 9• Electron spectrum of 17aTm in coincidence with the K X-ray•
682
T.
KUROYANAGI
AND
T.
TAMURA
calibration curve o f the conversion electron line was made by using sources of ~37Cs, sTmSr and H4=In with an accuracy of at least 10 keV. A pulse height spectrum o f the beta-ray obtained by the electron - K X-ray coincidence of tTaTm is shown in fig. 9. It was clearly found that an electron line spectrum superposed on a beta-ray spectrum has an energy of 345 keV. The peak could be identified as the K conversion o f the 395 keV transition, taking into account the fact that there is no fast coincident g a m m a ray with the K X-ray and that the energy just fit to the energy of the K conversion electron for the 395 keV transition. The portion of the continuous beta-ray spectrum is the same one as the beta-gamma coincidence spectrum. After the subtraction of the 0.80 MeV beta ray, a K conversion coefficient of the 395 keV transition was computed to be 0.0085+0.0018 from eq. (1). The K conversion coefficient of the 395 keV transition agrees best with an E1 assignment. Io' 400 W
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Fig. 10. Electron spectrum of 17STm in coincidence with the beta ray. 4.2. THE 460 keV TRANSITION In order to determine the multipolarity of the 460 keV transition, a beta-electron coincidence was tried. In this measurement, the gamma-ray counter was replaced by a beta-ray counter which consisted of a 2.5 cm O x 1.3 cm anthracene crystal and a D u m o n t 6292 photomultiplier tube. In this case, eq. (1) for conversion coefficient is modified as follows: ct -
I~
I~
-
I= Ip
lp I:,
,
(2)
where lp/l 7 is the ratio between the total beta transition and the g a m m a transition of 460 keV, and l,/Ip is a coincidence ratio of the electron-beta coincidence measurement.
NUCLEAR SPECTROSCOPY
683
Fig. 8 shows the electron line spectrum in the decay of 19SAu gated by the beta-ray pulses above the energy of 500 keV. The K-LM ratio was in good agreement with the previously observed value, and then the detection efficiency for the electron line was determined. When the coincidence gate was opened by beta ray pulses of l?aTm, an electron line of the energy of 400 keV besides the beta-ray spectrum was observed (fig. 10). The beta-ray spectrum is due to the coincident pulses for the Compton electrons of the 460 keV gamma ray. The peak area of the electron line was evaluated after the subtraction of a contribution of the beta ray. From the coincidence ratio, the K-conversion coefficient of the 460 keV transition was calculated to be 0.015-1-0.003 by using eq. (2). A comparison between the experimental and theoretical values suggested that the multipole order o f the transition is E2. In this measurement, the theoretical value for the K-LM ratio was used assuming an E2 transition, since the K and LM line could not be discriminated. The experimental and theoretical values a) o f the internal conversion coefficient are listed in table 2. TABLi~ 2 Experimental Energy
(keV)
and theoretical internal conversion coefficients
ctk ( e x p )
0tk ( t h e o r ) E1
E2
MI
395
( 8 . 5 4 - 1 . 8 ) x 10 - s
8.5 x 10 -a
2 . 4 x 10 -2
6 . 2 x 10 -2
460
(1.5 ={=0.3) X I 0 -~
6.0 x 10 -a
1.7 X 10 -2
4 . 2 x 10 -2
5. Spin and Parity Assignment and Decay Scheme The ground state of 1 7 3 y b has been well established to be the ~ - [512] Nilsson state. On the other hand, from the Nilsson diagram 4) the ground state o f 173Tm is considered to be ½+ [411], which is consistent with the systematical knowledge of ground state properties of thulium isotopes of odd mass number. The weak beta ray of the end point energy of 1.26 MeV is interpreted to be ground to ground beta transition, which is also consistent with beta-decay energy systematics. The l o g f t value of 9.5 fit to the systemtatical values for the first forbidden unhindered ~ shape transition of beta decay in deformed odd mass nuclei. The 395 keV level is uniquely assigned to be ½÷ state, because the 0.86-MeV beta ray has a l o g f t value of 6.3, which is classified either allowed hindered or first forbidden unhindered, and the 395 keV gamma transition has the E1 characteristic. In the Nilsson diagram, an orbital of ~+ cannot be seen in the region we are considering, except for those which come from the higher N shell. As shown in fig. 11, the lowest shooting down orbital from g| single particle state is ½+ [65t] orbital, which may appear around the orbitals of ~ - [512] or ½- [521]. It was considered that an assignment of the ½+ [651] orbital for the 395 keV state may be correct if an inversion
684
T. KUROYANAGI
AND
T. TAMURA
of the rotational band is taking place. A computation of decoupling parameters s) for the ½+ [651] orbital was tried by using the Nilsson wave function 4) in the following relation:
a = ( - ) l E (a2o + 2x/l(l + 1)a,1 a,o). 1
E Q 6.75
5 4 6.50
3 2 I
0
°°I Fig.
o
od
o:~
o.,3
o
2
4
6
o
-I
x
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11. Portion of Nilsson diagram for
Fig. 12. Plot of deeoupling parameter a as a function of deformation parameter. Circles show the calculated values, and cross point shows expected value for the case of ~?aTm.
neutrons.
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~(azoS+2¢~a;lalo).
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I=1
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Fig. 13. Decay scheme of l?aTm.
We obtained the values of +5.5, +1.3 and -0.65 for deformation parameters of r / = 2, 4 and 6, respectively, which are plotted in fig. 12. Since the deformation parameter of r/ = 7 is expected, the value of the decoupling parameter less than - 1.0
NUCLEAR
SPECTROSCOPY
685
is plausible. Thus the 395 keV level could be considered to be a ½+ member of an inverted rotational band built on the ½+ [651] intrinsic state. A hindrance factor of the E1 transition from the 396 keV state was evaluated to be 8 x 108 as compared to Moszkowski's estimate. Since the 395 keV level is a ~+ rotational member of the ½+ [651] orbital, a ½+ intrinsic state is expected to lie closely together with the ½+ state, and some beta-ray branching must be fed to the ½+ state. Therefore the l o g f l value of the beta transitions to the ½+ and ~+ states m a y be smaller than the measured value of 6.3. The 460 keV level is assigned to an ½- [521 ] intrinsic state, since the E2 character of the 460 keV g a m m a transition and the first forbidden unhindered character of the 0.80-MeV beta transition then are interpreted without "any difficulty. The spin parity assignments thus made are included in the decay scheme (fig. 13). The authors are deeply indebted to Professor H. Morinaga of the University of Tokyo for m a n y discussions. Appreciation is also expressed to Mr. K. Kaneko and Mr. H. Saito for their assistance during the course of measurement. They are grateful to the operating crew of J A E R I linear electron accelerator for performing the irradiation. References 1) T. Kuroyanagi, H. Yuta, K. Takahashi and H. Morinaga, J. Phys. Soc. Japan 16 (1961) 2393
2) 3) 4) 5)
R. Montalbetti, L. Katz and J. Goldemberg, Phys. Rev. 91 (1953) 659 M. E. Rose, Internal conversion coefficients (North-Holland Publ. Co., Amsterdam, 1958) B. R. Mottelson and S. G. Nilsson, Mat. Fys. Skr. Dan. Vid. Selsk. 1, No. 8 (1959) S. G. Nilsson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 16 (1955)
N U C L E A R S P E C T R O S C O P Y OF l~3Tm T. K U R O Y A N A G I
a n d T. T A M U R A
Linear Accelerator Laboratory, Department of Physics, Japan Atomic Energy Research Institute, Japan Received 1 July 1963 Abstract: T h e 17rI'm nucleide was p r o d u c e d by t h e (y, p) reaction by u s i n g a n intense b r e m s s t r a h l u n g f r o m a linear electron accelerator. A l u Y b target enriched to 99 % was used. T h e radiations f r o m XTaTm were m e a s u r e d by m e a n s o f a beta a n d g a m m a scintillation spectrometer. T w o beta-ray g r o u p s with e n d - p o i n t energies o f 1.26 a n d 0.86 M e V were found, a n d K X-ray, 395 a n d 460 keV g a m m a rays were observed. T h e excited states at 395 a n d 460 keV in ~v3Yb were identified as ½+ [651 ] a n d ½- [521 ] intrinsic states, respectively. It was f o u n d that the 395 keV state is an isomeric level with a h a l f life o f 2.9 psec. T h e a s s i g n m e n t s were m a d e with the aid o f electron - K X - r a y a n d electron - beta coincidence m e t h o d s .
1. Introduction Thulium-173 was first produced from the photonuclear reaction of natural ytterbium by using an internal bombarding techrrique with a betatron 1). Since a much stronger beam became available at our Linear Electron Accelerator Laboratory, a detailed investigation of the decay of this nucleide became possible.
2. Experimental Procedures Thulium-173 was produced by the ( ? , p ) reaction on a 9 mg of ytterbium oxide sample enriched to 98.7 % in mass number 174. The bremsstrahlung was converted from the 21 MeV and 40 uA electron beam of J A E R I linear electron accelerator. The sample was bombarded for about 3 h in an water cooled converter-target assembly which is shown in fig. 1. The intensity ot bremsstrahlung was estimated to be about 5 x 106 r/min from an activation o f 63Cu by using the well known energy-yield curve of the (?, n) reaction 2). This value was checked by a rSntgen rate meter. Because of a high enrichment of the target and because of the absence of strong background radiations due to the (~,, n) proctuct, no chemical separation was done. The amount of the activity was 0.05 pCur for the 3 h bombardment. In the measurement of the beta ray, the sample was folded with aluminium foil of 2 mg/cm 2 thickness. The gamma-ray spectrometer consisted of a 7.7cm ~ × 7.7 cm NaI(TI) crystal and a 6363 photomultiplier tube with a conventional 256-channel pulse-height analyser. In order to eliminate a contribution of beta rays, a 2 cm lucite absorber was used. The energy calibration was performed by employing the photopeaks of g a m m a rays of 196Au, lt4In, 137Cs and 22Na sources. 675
T. KUROYANAGIAND T. TAMURA
676
The beta ray was viewed by a 3.8 cm ~ x 1.3 cm anthracene crystal which has the resolution of 10 ~o for the 624 keV line of conversion electron of 137mBa. The energy calibration curve and the reference Kurie plot were made from the beta-ray spectra of 19SAu ' 114in ' 169Er ' and 137mBa conversion line. Figs. 2 (a) and 2 (b) show the examples. O.Imm AI WINDOW
,---
...........
.........
I
T1
I SAMPLE ~'~ / COOLINGWATER , 5ore
'~'
Fig. 1. Converter target assembly.
0.625 MeV ~j
15 [
k
104
~.
y
~
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-
;
•
• .
v
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*.
I03
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iiii
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--
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NUMBER
Fig. 2(a). K u r i e plot o f beta-ray s p e c t r u m o f l°SAu.
i 0
50 CHANNEL
i ioo
J
NUMBER
Fig. 2(b). Beta-ray s p e c t r u m o f ls~Cs.
In the coincidence measurements, a conventional fast slow coincidence circuit with a resolving time of 80 nsec was used• In order to determine the efficiencies of the beta and gamma scintillation crystals, the coincidence circuit mentioned above was used. The accuracy of the intensity measurements was within 5 ~.
NUCLEAR
3.
677
SPECTROSCOPY
Results
3.1. GAMMA- AND BETA-RAY SPECTRA The gamma-ray spectrum of the sample irradiated with the 21 MeV bremsstrahlung was measured from several minutes after the end of bombardment to about one week. Several hours after the bombardment, outstanding peaks of 460, 395 and 52 keV energy were found, and a very weak peak at 510 keV was seen (fig. 3(a)). The spectrum taken at about a hundred hours after the irradiation was found to be that from the g a m m a rays of 17SYb (fig. 3(b)). This was confirmed by following the decay of each peak. 10 5 -
~--395
•KXray
"
r-
1 0 4 ~ / ~ " ~
.j w z <[ 1c,)
o.
•
: Kiray s
cu
"'l
.460 ° i "%J
~, 114
.
Io) 396 ;
282
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Z
•
)
.,
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Jo'
Tr/2:82 a" OIh
.
".4
:
" • °°
:
: • (hi
i
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\
$% •
0 I00 CHANNEL NUMBER
\ .\
10 2
150
Fig. 3. Gamma-ray spectra of the irradiated sample of separated isotope of ytterbium obtained at (a) 8 h after and (b) about 100 h after the termination of the bombardment.
............... 0
50
\
I00
__
TIME (h)
Fig. 4. Decay curve of the peak observed at the energy of 395 keV.
Following the decay, the g a m m a rays of 460, 395 and 52 keV were found to have a half life of 8.2 h. The decay curve is shown in fig. 4. The weak peak of 510 keV could not be assigned to the g a m m a ray o f ~73Tm because o f the weak intensity and slight contamination of 64Cu. The intensity ratio of those g a m m a rays were estimated by using a photopeak efficiency and successive peeling off technique. In order to see if the 8.2 h component really belongs to 173Tm, an irradiation by the beam of 13 MeV bremsstrahiung was made. Then, none of the 8.2 h component was found. The beta-ray spectrum was analysed in the same way as the gamma-ray measurement. After a sufficiently long time (200 h), the spectrum showed beta rays due to 17Syb and 172Tm, which was known from the fact that the end point energies were
678
T. KUROYANAGIAND T. TAMURA
0.47 and 1.8 MeV with half lives of 4.1 d and 69 h, respectively. The decay curve of the beta ray taken above the energy of 0.5 MeV shows that it consisted of 8.2 h and some short lived components. After an analysis of the decay curve, the half lives of the short components were found to be 10 rain and 32 min, which were attributed to the contamination o f Cu and CI from these end point energies. After the subtraction of the beta-rays of 172Tm and 175Yb, beta-ray groups of the 8.2 h half life with end-point energies o f 1.26 and 0.86 MeV were found. The Kurie plots of the beta-ray spectrum measured at eight hours after the bombardment are shown in fig. 5. In fig. 5 the contributions o f 172Tm and 175yb have been subtracted.
40
I O0
30
50
2(3
0 0.5
0.5
ID 1.5 ENERGY (I~iV)
1.0
2D
Fig. 5. Kurie plots of beta-ray spectrum of 17aTm. The left and right side show a Kurie plot of the total spectrum and a Kurie plot after the subtraction of 1.26 MeV component, respectively. 3.2. BETA-GAMMA A N D G A M M A - G A M M A COINCIDENCE
The beta-gamma and gamma-gamma coincidence measurements were performed in order to construct a decay scheme. The beta-gamma coincidence spectra were taken by setting the single-channel window at 52, 395 and 460 keV. In the cases of both the 52 and 460 keV window, the same end point energy of the coincidence spectra oftbe beta ray was found to be 0.80 MeV from the Kurie plots, and the coincidence ratio was computed to be about 100 %. When the coincidence gate was opened by the photopeak o f 395 keV gamma ray, no coincident beta-ray was observed. Gamma-gamma coincidence gates were set at the photopeak of the K X-ray, 395and 460 keV gamma rays. No coincident gamma ray was measured.
NUCLEAR
679
SPECTROSCOPY
The results of the beta-gamma and g a m m a - g a m m a coincidence measurements, which have no cascade relation except 0.80 MeV beta ray to the K X-ray and the 460 keV g a m m a ray, suggested that the 395 keV g a m m a ray is delayed. Therefore, the beta-gamma and g a m m a - g a m m a delayed coincidence measurements were performed using a 256-channel time analyser. When the time analyser was triggered by the beta-ray pulses, the g a m m a - r a y pulses selected by the single-channel window were accepted in the time analyser. F r o m this measurement, it was found that the 395-keV g a m m a ray is retarded with a half life of 2.9 #see. Fig. 6 shows the decay curve. In this
v
~n'~lOI0~ __Z ~. ~l- ~ . i i
2=2 85'usec
\
Z ~0 0
JCf~.
.
.
~°°
• °°°~
°.°
0
•
°..°°.
• ~
.
,. \ .\ % ° '\
~\
0
I
I0
I
20
~0
30
I
40
/JSec Fig. 6. Time spectrumobtainedfrom the delayedcoincidencemeasurementbetweenbeta-rays and the 395 keV gamma ray. figure the excess count of the first channel is considered to be the prompt coincidence between the Compton pulses of the 460 keV g a m m a ray and the 800 keV beta ray. And the fiat part in the higher channels is the accidental coincidence count. The g a m m a - g a m m a delayed coincidence relation was found in the coincidence measurement between the K X-ray and the 395 keV g a m m a ray with the same half life of 2.9 #see. Finally, slow coincidence measurements of the resolving time of 2T = 2#sec were tried. The beta-ray spectrum in coincidence with 395 keV g a m m a ray was found to have an end point energy of 860 keV. A g a m m a - g a m m a coincidence relation between the K X-ray and 395 keV g a m m a ray was also obtained.
680
T. K U R O Y A N A G I A N D T . TAMURA
F r o m a straight forward consideration o f the experimental data, a cascade relation o f beta-gamma and g a m m a - g a m m a was proposed as shown in fig. 13. Relative intensities o f the beta and g a m m a rays were estimated and are listed in table 1. TABLE 1 Relative intensities of the gamma and beta ray of 17aTm Gamma ray energy (keY)
Percent of total decay
K X-ray 395 460
21 ±1 91 ±5 7.5!0.2
Beta ray energy (MeV)
Percent of total decay
0.80 0.86 1.26
28 71 0.5
4. Multipole Order Assigmnents of the Gamma Transitions Although the cascade relation in the decay of the 173Tm could be constructed from the fast-slow, slow and delayed coincidence experiments, the spin and parity assignments of the levels were not uniquely determined. Further studies of multipole order •o f the g a m m a radiations were made, since properties of the radiations are not specified from the half life because of large fluctuations o f the hindrance factor involved in the g a m m a decay. 4.1. THE 395 keV TRANSITION In order to determine the multipole order of the 395 keV g a m m a ray, the internal conversion coefficient of the K shell was measured by means o f conversion electron-K X-ray coincidence. An internal conversion coefficient is defined by the following equation: -
/ek
-
/ek [k .
(1)
In this equation, Ik/I7 is the intensity ratio computed from the single spectrum of the g a m m a ray, and Iek/I k is a coincidence ratio in the electron-K X-ray coincidence measurement. The same coincidence arrangement and fast-slow coincidence circuit as mentioned above were used. The spectrum of the internal conversion line of 137mBa taken with this arrangement is shown in fig. 7. Better checks were performed by using a 175yb source (fig. 8). The crystal efficiency for peak area of the energy of 335 k e y was determined to be 22__+2 ~o with the Yb source. In this measurement, the energy
NUCLEAR i fj 5 ----
681
SPECTROSCOPY
. . . . . . . . . . . . . . . . 625 !
:.
Jz
I ©4 ,
Z I L)
I
• (/3 I--z
I
i0 ~ r
r " ' : ' " " • ' ' % : "'" "
I
O
i i
4i
°"
i
°"
o
50 CHANNEL
i
Ioo NUMBER
Fig. 7. Spectrum of the internal conversion line of as~mBa obtained from K X-ray and electron coincidence. Ad" I O~
e-p
ioa
330(K)
335
:;39rL
d
335 (KJ
z
Z -iIo
~,o' 2-
Ufo'
.
z
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•
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)
g 0( J
.
id
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fd!
p o
;o
2b
Go
o
20
NUMBER
CHANNEL
4'0
J 6O
Fig• 8• Internal conversion line spectra• The left side is the internal conversion lines of 411 keV transition in the decay of l°aAu in coincidence with the beta ray, and the right side is the internal conversion line of 396 keV tr~n~itlt~n
in
thP
d~,r'~v
of
175Yh
in
¢~oint'id~nee
°.
1
0
2
40 CHANNEL
0
80
100
NUMBER
Fig. 9• Electron spectrum of 17aTm in coincidence with the K X-ray•
682
T.
KUROYANAGI
AND
T.
TAMURA
calibration curve o f the conversion electron line was made by using sources of ~37Cs, sTmSr and H4=In with an accuracy of at least 10 keV. A pulse height spectrum o f the beta-ray obtained by the electron - K X-ray coincidence of tTaTm is shown in fig. 9. It was clearly found that an electron line spectrum superposed on a beta-ray spectrum has an energy of 345 keV. The peak could be identified as the K conversion o f the 395 keV transition, taking into account the fact that there is no fast coincident g a m m a ray with the K X-ray and that the energy just fit to the energy of the K conversion electron for the 395 keV transition. The portion of the continuous beta-ray spectrum is the same one as the beta-gamma coincidence spectrum. After the subtraction of the 0.80 MeV beta ray, a K conversion coefficient of the 395 keV transition was computed to be 0.0085+0.0018 from eq. (1). The K conversion coefficient of the 395 keV transition agrees best with an E1 assignment. Io' 400 W
.--':..".
~
~I0
% "~,
PZ -.
\:.~
0 u
o •
Io'
:Y'~.. • ".7"" o. • o.
0
20 '
40' .' 0 8 b I00 ' CHANNEL NUMBER
120 '
Fig. 10. Electron spectrum of 17STm in coincidence with the beta ray. 4.2. THE 460 keV TRANSITION In order to determine the multipolarity of the 460 keV transition, a beta-electron coincidence was tried. In this measurement, the gamma-ray counter was replaced by a beta-ray counter which consisted of a 2.5 cm O x 1.3 cm anthracene crystal and a D u m o n t 6292 photomultiplier tube. In this case, eq. (1) for conversion coefficient is modified as follows: ct -
I~
I~
-
I= Ip
lp I:,
,
(2)
where lp/l 7 is the ratio between the total beta transition and the g a m m a transition of 460 keV, and l,/Ip is a coincidence ratio of the electron-beta coincidence measurement.
NUCLEAR SPECTROSCOPY
683
Fig. 8 shows the electron line spectrum in the decay of 19SAu gated by the beta-ray pulses above the energy of 500 keV. The K-LM ratio was in good agreement with the previously observed value, and then the detection efficiency for the electron line was determined. When the coincidence gate was opened by beta ray pulses of l?aTm, an electron line of the energy of 400 keV besides the beta-ray spectrum was observed (fig. 10). The beta-ray spectrum is due to the coincident pulses for the Compton electrons of the 460 keV gamma ray. The peak area of the electron line was evaluated after the subtraction of a contribution of the beta ray. From the coincidence ratio, the K-conversion coefficient of the 460 keV transition was calculated to be 0.015-1-0.003 by using eq. (2). A comparison between the experimental and theoretical values suggested that the multipole order o f the transition is E2. In this measurement, the theoretical value for the K-LM ratio was used assuming an E2 transition, since the K and LM line could not be discriminated. The experimental and theoretical values a) o f the internal conversion coefficient are listed in table 2. TABLi~ 2 Experimental Energy
(keV)
and theoretical internal conversion coefficients
ctk ( e x p )
0tk ( t h e o r ) E1
E2
MI
395
( 8 . 5 4 - 1 . 8 ) x 10 - s
8.5 x 10 -a
2 . 4 x 10 -2
6 . 2 x 10 -2
460
(1.5 ={=0.3) X I 0 -~
6.0 x 10 -a
1.7 X 10 -2
4 . 2 x 10 -2
5. Spin and Parity Assignment and Decay Scheme The ground state of 1 7 3 y b has been well established to be the ~ - [512] Nilsson state. On the other hand, from the Nilsson diagram 4) the ground state o f 173Tm is considered to be ½+ [411], which is consistent with the systematical knowledge of ground state properties of thulium isotopes of odd mass number. The weak beta ray of the end point energy of 1.26 MeV is interpreted to be ground to ground beta transition, which is also consistent with beta-decay energy systematics. The l o g f t value of 9.5 fit to the systemtatical values for the first forbidden unhindered ~ shape transition of beta decay in deformed odd mass nuclei. The 395 keV level is uniquely assigned to be ½÷ state, because the 0.86-MeV beta ray has a l o g f t value of 6.3, which is classified either allowed hindered or first forbidden unhindered, and the 395 keV gamma transition has the E1 characteristic. In the Nilsson diagram, an orbital of ~+ cannot be seen in the region we are considering, except for those which come from the higher N shell. As shown in fig. 11, the lowest shooting down orbital from g| single particle state is ½+ [65t] orbital, which may appear around the orbitals of ~ - [512] or ½- [521]. It was considered that an assignment of the ½+ [651] orbital for the 395 keV state may be correct if an inversion
684
T. KUROYANAGI
AND
T. TAMURA
of the rotational band is taking place. A computation of decoupling parameters s) for the ½+ [651] orbital was tried by using the Nilsson wave function 4) in the following relation:
a = ( - ) l E (a2o + 2x/l(l + 1)a,1 a,o). 1
E Q 6.75
5 4 6.50
3 2 I
0
°°I Fig.
o
od
o:~
o.,3
o
2
4
6
o
-I
x
"l
11. Portion of Nilsson diagram for
Fig. 12. Plot of deeoupling parameter a as a function of deformation parameter. Circles show the calculated values, and cross point shows expected value for the case of ~?aTm.
neutrons.
a=(--) i
~(azoS+2¢~a;lalo).
I/z+ [4Li] f~Tm',~. 8.2 h " ~
\\ 080 2 a ~ . 6 . 5 / ~
~/2- [szq
2.9 psec /~
I=1
'\ o 70
ybl~'s 103
~'z- [~-'z]
stoble
Fig. 13. Decay scheme of l?aTm.
We obtained the values of +5.5, +1.3 and -0.65 for deformation parameters of r / = 2, 4 and 6, respectively, which are plotted in fig. 12. Since the deformation parameter of r/ = 7 is expected, the value of the decoupling parameter less than - 1.0
NUCLEAR
SPECTROSCOPY
685
is plausible. Thus the 395 keV level could be considered to be a ½+ member of an inverted rotational band built on the ½+ [651] intrinsic state. A hindrance factor of the E1 transition from the 396 keV state was evaluated to be 8 x 108 as compared to Moszkowski's estimate. Since the 395 keV level is a ~+ rotational member of the ½+ [651] orbital, a ½+ intrinsic state is expected to lie closely together with the ½+ state, and some beta-ray branching must be fed to the ½+ state. Therefore the l o g f l value of the beta transitions to the ½+ and ~+ states m a y be smaller than the measured value of 6.3. The 460 keV level is assigned to an ½- [521 ] intrinsic state, since the E2 character of the 460 keV g a m m a transition and the first forbidden unhindered character of the 0.80-MeV beta transition then are interpreted without "any difficulty. The spin parity assignments thus made are included in the decay scheme (fig. 13). The authors are deeply indebted to Professor H. Morinaga of the University of Tokyo for m a n y discussions. Appreciation is also expressed to Mr. K. Kaneko and Mr. H. Saito for their assistance during the course of measurement. They are grateful to the operating crew of J A E R I linear electron accelerator for performing the irradiation. References 1) T. Kuroyanagi, H. Yuta, K. Takahashi and H. Morinaga, J. Phys. Soc. Japan 16 (1961) 2393
2) 3) 4) 5)
R. Montalbetti, L. Katz and J. Goldemberg, Phys. Rev. 91 (1953) 659 M. E. Rose, Internal conversion coefficients (North-Holland Publ. Co., Amsterdam, 1958) B. R. Mottelson and S. G. Nilsson, Mat. Fys. Skr. Dan. Vid. Selsk. 1, No. 8 (1959) S. G. Nilsson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 16 (1955)