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Protactinium-237
I.D.2 : 1.E.I : 4.E
Nuclear Physzcs 15 (1960) 664--677, (~) North-Holland Pubhsh~ng Co, Amsterdam Not to be reproduced by photoprmt or microfilm ~athout written permission trom the pubhsher
PROTACTINIUM-237 K
T A K A H A S H I t and H
MORINAGAt
Department o/ Physics, T~kohu University, Sendal, Japan Recexved 1 December 1959 Protactmmrn-237 was produced from t h e reaction U2SS(y, p ) P a ~87 b y t h e 25-MeV b r e m s s t r a h l u n g T h e actlvxty produced was chemically separated with t h e a~d of a recoil m e t h o d from flssxon p r o d u c t s T h e half-hie of this new isotope was found to be T j = 394-3 m m T h e radiations were measured w i t h a scintillation spectrometer Three b e t a components were identified wxth the end-points of 2 304-0 05 MeV, 1 35 MeV, a n d a b o u t 0 8 MeV M a n y g a m m a rays were also found A decay scheme was constructed w i t h t h e aid of N d s s o n ' s umfled model
Abstract:
1. Introduction
The asotope protactinium-237 was first reported by Crane and Iddings 1) in 1954, from the reaction U ~s (d, 2pn)Pa 2s7 by the 190-MeV deuterons from a synchro-cyclotron. The half-hfe of the beta decay of this new isotope was determined to be about l l rain. No detailed investigation about the characteristics of this nucleide has been made since then. Usually, in the case of such a high energy reaction of fissile elements, it is likely that the reactions become very much complicated. Moreover, Pa 2s7 as not the most abundant isotope among protactinium isotopes produced in high energy spallation reactions. On the other hand, in the case of photoreaction with the 25-MeV bremsstrah]ung, there are only four competing reactions, viz., the (y, n), the (y, 2n), the (y, p) and the (y, f) reactions on fissile elements. Pa ~7, therefore, is the ma]or protactinium isotope produced by low energy photoreactions on natural uranium. Difficulty, however, is that the cross sections of the (y, p) reactions m the region of uranium are about the order of some hundredth of those of the (7, n) or the (y, f) reactions. The products of the latter reactions, having m a n y specaes of activities with various half-lives and various chemical behaviours, will complicate any separation procedure for protactinium In order to remove this difficulty, a physical separation technique of spallation products from flssaon products was employed, in addition to the specific chemical procedure to separate protactinium Recently, Nakai and Yajima *) showed that UO 2 powder of about 8 microns in grain saze embedded in oxalic acid powder with the mixing ratio 1 : 70, loses fissaon products into bedding material almost completely when it is activated by pale neutrons. They also showed that almost complete separation is achieved in the case of Now at D e p a r t m e n t of Physics, U m v e r s l t y of T6kyo, B u n k y 6 k u , T6kyo, J a p a n 664
PROTACTINIUM-23?
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UO~ (NO3) 2 " 6H~ O powder of about the same grain size embedded in amorphous graphite powder with the same mixing ratio, as in the case of UO2--C2H204 • 2H, O mixture. The spallation products such as (y, n), (y, p), (y. 2n) products however, have extremely low recoil energies, of the order of 10 keV, or extremely short ranges when produced in the photoreactions Therefore, they can not get out from the phase of fissile material in such a mixture and will remain in the same phase making it possible to separate themselves from fission products We have tried to employ this principle using the above two recipes for eliminating the disturbing fission product activities before finally separating the protactinium activity, b y usual chemical separation methods Of late years, methods for isolation of protactinium have been studied b y m a n y investigators and several standard procedures have well been established 3). In the present work, two types of these methods were employed after the physical separation of fission products from UO 2 powder: one is the codeposition technique with manganese dioxide precipitates from the nitric acidic solution of UOa and the other is the solvent extraction with methylisobutylketone (Hexon) from the nitric acidic aqueous solutlon of UO 2. 2. E x p e r i m e n t a l
Procedure
In most of this work, a mixture of UO 2 powder of small grains (diameter 8#) mixed with oxalic acid powder of about the same grain size with the ratio of 1 : 40 in weight was used. The total weight of each sample was about 41 g Some of irradiations were carried out with uranyl nitrate-graphite mixture in which the former is 1 : 40 in weight ratio to the latter, the amorphous graphite They were bombarded with the 25-MeV bremsstrahlung from the T6hoku University betatron at the position of approximately 500 r/min for about 40 minutes. After each irradiation, in the case of UO2-oxalic acid mixture, the sample was dissolved into hot water and the residue, the UO 2 powder, was centrifuged This residue was then washed once or twice with a hot diluted oxalic acidic solution in order to avoid the Cn activity to contaminate the final Pa activity, and also to minimize the fission product activities which could adsorb onto the surface of the fine UO2 powder. The UO2 was then dissolved into 8N H N 0 3 solution. In the case of uranyl nitrate-graphite mixture, the irradiated sample was dissolved into hot diluted nitric acidic solutlon and the graphite was filtered out. The filtrate was boiled and dried in order to remove the N 13 activity. The assay was then dissolved into the chluted solution of hthium carbonate. This step was added so that the Cn produced from graphite and mixed into the uranyl nitrate phase could be driven out at the next step when the solution was again made acidic. Then the solution was again made 8N nitric acidic
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Two independent methods were employed in order to separate protactinium from such nitric acidic solution. One was the codeposition technique with manganese dioxide precipitate from nitric acidic solution of UO 24). To the nitric acidic solution, a saturated solution of Mn(NO3), was added and then 0.1N KMnO 4 solution was dropped so that it was just enough for making the precipitation of Mn02. This MnO, was centrifuged and then filtered and dried onto a filter paper. It took about 15 minutes to complete this procedure. Another method used was the solvent extraction of protactinium with methyl-isobutylketone (Hexon) from the nitric acidic aqueous solution of UOS, 6). After dissolving the UO 2 into 8N HNO3 solution, it was shaken with equal volume of hexon. Before starting to count the activity of this solution it did not take more than a few minutes. For long counting the hexon wad dried. This extraction method was used only for the samples of UO, mixed with oxalic acid. The measurement of the half-life was made with a G-M counter and from the change of beta and gamma spectra with time measured b y a scintillation spectrometer. The beta ray end-point energy was measured with a beta scintillation spectrometer; the detector was a plastic scintillator of 1 inch thickness and 2 inches in diameter with a 7 # aluminium window. For the observation of gamma rays, a conventional multi-channel NaI (T1) scintillation spectrometer was used. The sizes of the sodium iodide cyrstal used were 1½ inches in thickness and 121 inches in diameter for most of the measurements, and 4 " × 4" for the detection of high energy gamma rays. Gamma-gamma coincidence measurements were also carried out for some intense gamma rays, such as 145, 205, 275, 330 and 460 keV, using a slow coincidence circuits (2~ = 5/~sec). Conventional single channel pulse height analyser was used for energy discrimination of gamma ray pulses from one of the detectors. Output pulses from the analyser controlled the coincidence gate of the multi-channel pulse height analyser, which received gamma pulses from another detector. The performance of this coincidence circuit system was checked for the 330-keV gamma r a y from Aul~e(T½ = 5.6 d) which has the 356-keV coincident gamma ray.
3. Experimental Results Fig. 1 shows the decay curve of the activities obtained according to the above recipe from a sample of uranium nitrate mixed with graphite powder. Quite similar results were obtained from samples of UO~--Cg.H204 mixture. In fig. 1, the long-lived activity is due to the beta decay of T h ~ ( T t = 24.1 d) which mixed into the Mn02 precipitates b y a very small amount (about 0.3 percent). The 6.66-h activity comes from Pa ~4 which is in the radioactive
667
PROTACTINIUM-~7
equilibrium in natural uranium. Referring to the amount of this Pa =* activity, the chemical yield of protactinium throughout this chemical procedure was estimated to be approximately 30 percent. After the subtraction of these well known activities, an appreciable amount of activity with a half-life of 394-3 rain was observed. This activity must be either due to Pa =T, or to some fission products which followed this chemical procedure, or m a y be due to
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12
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Fig 1 Decay of the p r o t a c t m m m actlvltles chemically separated from a sample of the u r a n m m mtrate-graphlte mixture b o m b a r d e d with the 25-MeV bremsstrahlung.
other impurities. There is, however, no fission product activity which has only such a half-life. It is also difficult to attribute this 39-rain activity to any conceivable impurity. Therefore, it is suspected that this activity is assigned to Pa =~. No 1 l-rain activity 1) was observed with an intensity expected from the (7, P) yield systematics. The beta ray end-point energy of this new activity was measured with a plastic phosphor. Following the decay of the beta spectrum, similar value of half-life was obtained after the subtraction of long-lived activities. The Kurie
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MORINAGA
plot for the 39-min component is shown in fig. 2 (a). The end-point energy of the highest energy beta ray was determined to be 2 3 0 i 0 05 MeV. This value agrees well with the value estimated by Seaborg et al. e) for Pa ~s~ from the energy systematics of the alpha and beta decays of heavy elements. Here, no correction for the finite resolution of the phosphor was applied. Since the Kurie plot is linear in rather large portion of the spectra, the extrapolation of the
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Fig 2 Kurle plots of P a 2s7 b e t a spectrum (a) total, (b) after the subtraction of the 2 30-MeV component, (c) after the subtraction of the 2 30-MeV and 1,35-MeV components
linear part was used for determining the end-points. The Kurie plot of the beta spectrum begins to deviate upwards from the straight line at the vicinity of 1.3 MeV. In order to check the linearity of the Kurie plots of beta spectra, an examination of the performance of our spectrometer was made with the beta rays from In 114 which has almost simple allowed spectrum. The result is shown in fig. 3. Comparing this to the spectra of the 39-min activity, it is concluded that other lower energy components should be included in the decay of Pa 28~. After the Kurie plot analysis, two more branches of lower energies
PROTACTINIUM-~7
669
were detected. The Kurie plots of these components are also shown in fig. 2 (b) and (c), respectively: the higher energy one having E• max at 1.35 MeV and, the lowest one at approximately 0.8 MeV. The branching ratios of these three components are roughly 60 : 30 : 10. For the lowest component, the value of
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Fzg 3 Kurle plot of t h e b e t a s p e c t r u m of I n 11:
its end-point energy and its branching should have rather large uncertainties because of the poor resolution and difficulty of the subtraction of the higher energy components. Results of the beta ray measurements with considerations of gamma ray spectra and the results of other experiments, are listed in table 1 TABLE 1 S u m m a r y of the b e t a decay of P a =8~ Eo
Intensity
(MeV)
(%)
2 30 1 35
~08
60
{2,
7 ~10
Log
/t
72 73 74 >~65
Clasmflcatlon
lu lu ah ah
The gamma ray spectrum was measured with a conventional scintillation spectrometer The decay of the spectrum was followed for several hours and almost all the peaks were found to decay with a half-life of about 39 rain. Small fractions of the long component activity comlng from Pa ~4 were also
670
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TAKAHASHI AND H
MORINAGA
observed. Therefore, those gamma rays are assigned to the new isotope Pa =v. After the subtraction of the long component ganima ray spectrum and the background spectrum, the gamma ray spectrum for Pa ~ was obtained. This is shown in fig. 4. Because of the low counting efficiencies for high energy gamma rays, some of high energy gamma rays might have been missed. The intense 90-keV line is considered to be mainly due to the K X-ray from U =v. I
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Fig. 4 S c m t d l a t l o n s p e c t r u m of t h e g a m m a r a y s f r o m P a "7 o b s e r v e d w i t h a 1½"× 1½" NaI(T1) c r y s t a l G a m m a r a y e n e r g m s a r e g i v e n m keV.
For the observation of high energy gamma rays, a 4" × 4" NaI (T1) was used. In order to cut off the low energy gamma rays, a ~"-thick lead shield was placed at the face of the crystal. Fig. 5 shows the observed spectrum. The analysis of the gamma ray spectra was carried out using, as a guide, the spectral shapes obtained in the same geometry from Inn% Hg ~°8, Na 2., Cs 13v and Zn ss sources. After the subtraction of the back-scattering and the Compton peaks of higher energy components, relative heights of the photopeaks were obtained. Therefore, relative intensities of lower energy components m a y
PROTACTINIUM-~7
671
have considerable uncertainties. Because of the rather large number of gamma rays quite a few peaks m a y actually be double or triple lines. Especially at lower energies the separation of peaks could have bad ambiguities. The intensity ratios of the observed gamma rays were determined using photopeak efficien-
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NUMBER OF C H A N N E L Fig. 5 Scmtfllatlon s p e c t r u m of t h e g a m m a r a y s f r o m P a zs7 o b s e r v d w i t h a 4 " × 4 " crystal G a m m a r a y energms are given m keV
NaI(T1)
cies of the present geometry of the spectrometers. They are listed in table 2. Since the activity produced was weak and its half-life is short, gammagamma coincidence measurements m a y give only qualitative results. The 145-keV gamma ray was observed coincident with many other gamma rays as listed in table 2. The 205- and 275-keV gamma rays were also found to be in
672
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MORINAGA
coincidence with the 90-, 145- and 460-keV gamma rays. Similar results were observed for the 330-keV gamma ray but were not conclusive. A measurement with the 460-keV gamma ray gave a consistent result with the above data. It is interesting to know the yield ratio of the reaction U23S(7, p) Pa 23~ relative to the reaction U23s(7, n)U 237 at the 25-MeV bremsstrahlung energy. The chemical yield of Pa ~37 was estimated with reference to that of Pa 234 TABLE 2 I n t e n m t l e s of t h e g a m m a r a y s from P a ~a~
Energy
(keY)
90 145 205 275 330 405 460 550 590 750 805 860 880 915 1045 1320 1420
Relative mtens,ty
50 45 55 20 40 30 100 30 25 50 45
Relative mtenslty coinc,dent w i t h t h e 145-keV ~-ray 5 3 3 25 15
(5) t 35 (5) t 25
100 100 35 10 15
t Figures m parentheses m a y not be entlrely due to P a s87, slnce such coincidences m a y from the decay of Pa ~34
arise
which was present in natural uranium in a condition of radioactive equilibrium. Then, we can calculate the yield of U~S(y, p)Pa ~a7 using the Cn activity which was produced in the sample of the mixture as the monitor of the beam intensity. The measured value of the yield ratio was one five-hundredth relative to the (7, n) reaction of U 23s. This value of yield ratio is in good agreement in the order of magnitude with the value expected from the (7, P) yield systematics. This further supports t h a t this 39-min activity is due to Pa 237. 4. D i s c u s s i o n
As it is well known, A = 237 is in the strongly deformed region Through recent studies 7, 8), it has been established t h a t the description of nuclear states
PROTACTINIUM-237
673
by the unified model is quantitatively successful. Here, we t r y to analyze our experimental results in the light of such theories The Kurie plot analysis of beta ray spectrum of Pa ~v shows that the beta branching to the ground state (and its vicinity) of U ~ is about 60 percent of the total disintegration and that the end-point energy of this ground state transition is 2 30 MeV Since the measured value of the half-hfe IS about 39 rain, the log ]t value of this ground state transition turns out to be about 7.2. The ground state spin and parity of Pa ~3~ is {--- from the systematics of the ground state properties of several known Pa isotopes. According to the analysis using Nilsson diagram, this is the first rotational state belonging to the ½-- [530J Intrinsic state s). It seems to be unreasonable that i comes lower at A = 237 since the decoupling factor does not change so much with the neutron number. On the other hand, the ground state of U ~3~ has been found to be a !2+ state and has been assigned to the ½+ E631J intrinsic state from the investigations of the alpha decay of Pu ~1 9) Therefore, the beta transition from Pa 2a~ to the ground state and its vicinity of U z3~is classified as first forbidden and unhindered (abbreviated as 1.u.) transition. This is quite consistent with the log/t value of 7 2 obtained above, in the view of the recent studies on the systematics of the selection rules for beta transitions ~) The second beta group has an end-point energy of 1.35 MeV and its branching was about 30 percent (cf fig. 2 and table 1). Since the log [t value of this group was estimated to be about 7, the transition must be classified as either 1.u. or allowed but hindered (abbreviated as a h ). From the theoretical examinations of the Nilsson diagram for neutron levels near N -=-- 145 and the analysis of the level schemes of U23Z(N = 141), we can expect an intrinsic hole-state of ~-+ ~631] in U t33 among low energy states This state appears at about 313 keV in U ~8. In Um, the ½+L631J state is also observed, but as an excited particle-state at about 400 keV This means that for U 238 the separation of ½ + [631~ and { + [631] state is about 700 keV. Since the energy dependence of the states on deformations IS very similar for these two states, the separation of the corresponding states in U 23~ should be close to 700 keV. Since the {+E631J state can be reached by the 1 u. transition from Pa ~ ground state, one m a y suspect that the 1 35-MeV beta branching includes such a 1.u transitxon. As shown in table 2, four gamma rays that have energies of 750, 805, 860 and 915 keV, were observed with large intensities. If the ~-+E631J level exists at about 910 keV, the direct transltlons to the ground state and its vicinity would occur from this level Such transitions are the unhindered M1 transitions From these facts we can tentatively assign the 910-keV level as the _3+ 2 E631~ Nilsson level. Recent investigations of the alpha decay of Pu 241 show that in U z~ the { + ~622J state appears at 145 keV. The first member of the rotational band of
K TAKAHASHI AND H
674
MORINAGA
this {-+[622] level has also been observed at 200 keV having a spin of 7 In analogy to Pu m~, the ground-state rotational band should be observed between the ground state and the { + [622] level. In the observed gamma ray spectra, however, the photopeak of the 915-keV line is wider than the energy resolution of our spectrometer (fig. 4): this suggests that more higher energy at)rain
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components may be included in this 915-keV line. This component may probably be the M1 transition to the 3 + rotational level of the ground state from the level a little above the 910-keV level of { + [631]. Therefore, we can expect a possible state of { belonging to the rotational family of {+[631] at an energy of about 950 keV. Thus, it might be inferred that the transitions to these rotational members of ½+ [631] and { + [622], occur from the ground 3 and the rotational { states of the { + [631] level in the unhindered M1 transitions. The above-mentioned gamma rays of 750, 805, 860, and 915 keV (actually two lines) are considered to correspond to these transitions.
PROTACTINIUM-.~7
675
Taking account of the beta and gamma counting efficiencies and the photopeak efficiencies of our spectrometer, we can calculate the intensity of the 1 35-MeV beta branching on the assumption that the above-described M1 transitions are the only transitions to take place. Since such M 1 transitions are expected to be the fastest ones, this assumption is plausible. The total intensity of the 1 u. beta transitions to the { + [631] level and its ~- rotational member was thus estimated to be about 23 percent. The log fl value is about 7.0. Approximately a half of the 23 percent branching seems to decay to the { + state and the rest decays to the { + state. In this case, the log/t value turns out to be 7.3 for each transition. This value is consistent with the systematics of the beta decay selection rules for the 1 u. transition. As shown m table 2, another intense gamma ray was observed at 460 keV. In gamma-gamma coincidence experiments, this gamma ray was observed in coincidence with the gamma rays of the energies of 145, 205, 275 and 330 keV. Very little information is available, because the weakness of the source intensity of Pa 237 made it difficult to get enough statistics However, we have observed that both the 275- and 330-keV gamma rays were coincident with the 145-keV gamma ray. We m a y therefore postulate two new intrinsic states, the {--[743J Nilsson level at about 475 keV and the {--[752] state at about 935 keV, from which an intense M1 gamma transition of 460 keV decays to the former state Analyses of the Nilsson diagram for neutron levels near N ----- 145 and of the level schemes of Pu ~ag, Pu .3~, U *a6 and U 2s3 would seem to support these assignments of energies and the intrinsic states. The ground state of Pu ~a~(N = 143) has been almost surely assigned ~ - - [743] and a state at 145 keV is considered to be { + [ 6 3 1 ] . On the other hand, Pu 2a9 (N ----- 145) has the ground state of {+[631J, and has the {--[743] state at 392 keV The energy difference between these two intrinsic states changes fairJy rapidly with the change of neutron number. Such a tendency is also observed in the level structures of UZaS(N = 143) and U2a3(N -----141) The ground state of ~--[743] occurs m U ~6, as in the case of Pu ~ , but the state of { + [ 6 3 1 ] appears very close to the ground state. In U~L however, these two states appear as the excited states: the { state appears at 400 keV and the !2 state is observed at about 295 keV. The energy difference between these states is about 100 keV. In the case of the {--[743] state in U ~ ( N = 145), it will be expected from the above discussions that this level would be observed between 300 keV and 550 keV. Thus the ~--[743] orbital can reasonably be assigned to a state at about 475 keV, as expected before. Therefore, the intense 460-keV transition to the ~ - - [743] state can reasonably be explained to occur from the ~ - - [752] state at about 935 keV m the M1 transition as expected before. Since the level postulated at 953 keV is { - - [7521, the a h. transition of the beta ray from the
676
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3__ ground state of Pa ~7 is expected to occur. Actually such a beta branching 2 must be assumed to exist being mixed in the spectrum of the 1.35-MeV mentioned above. According to the estimation from the intensity ratios of the 460-keV gamma ray and of the 1.35-MeV beta rays, and the beta a n d g a m m a counting efficiencies, this transition is expected to correspond to the remaining 7 percent branching of 1.35 MeV. The experimental l o g / t value was calculated to be about 7.4 for this branching. The value is indeed what is expected for the a.h. transition of the beta ray. Thus, it m a y be concluded that the {--[752] state decays to the 7--[743] state at about 475 keV with the emission of the 460-keV gamma ray in M1. From this level, many gamma rays of such energies as 205, 275, 330 and 405 keV lead to the lower states; they are the ] + state at about 270 keV, the ~ + state at 200 keV, the ~ + state at 145 keV and the { + state at about 70 keV. The first three states are probably the members of the rotational band of the { + [622] Nilsson level at 145 keV and the last one is the rotational member of the ½+[631] orbital Results of high energy gamm~ ray measurements show that the 1420-, 1320-, 1045- and 1010-keV gamma,rays lead to the lower states, and suggest that another intrinsic state and its rotational members appear in the vacinity of 1500 keV. Therefore, the beta branching whose energy is about 0.8 MeV can be attributed to the decay to these highly excited states of U mr. The experimental l o g / t value of the 0.8-MeV beta branching was estimated to be about 6.5 and this value implies that the transition should be classified as either a.h or 1 u. from the systematics Referring again to the Nilsson diagram, we can tentatively assign the states as the ~--[761] state and the first member of its rotational band at around 1500 keV. The intensity ratios of such gamma rays emitted from these levels as those of 1420, 1320, 1045, 1010, 590 and 550 keV, can be explained quite consistently, if one assumes that the {--[761] ground and its first rotational states appear at 1480 and 1530 keV. According to the above discussion, we can construct a very tentative decay scheme of Pa roT. The proposed decay scheme is shown in fig. 6, where the log It values are given in italics and the gamma ray transition energies are in keV. The energy assignments of the excited states which have ambiguities are shown in parentheses. It is interesting to see such a large number of intrinsic states having been observed in the decay of a radioisotope. This is due to its high disintegration energy Because of the intrinsically poor resolution of the scintillation spectrometer it was difficult, in general, to resolve gamma rays to or from levels in the same intrinsic structure. They are separated usually b y several tens of keV. Therefore, our assignments of rotational members m a y not be so certain. As for the intrinsic states, the assignments seem to be more certain. It will be very interesting to make more elaborate analysis of this decay scheme using stronger source of Pa 237
PROTACTINIUM-237
677
The authors wish to express their appreciations to Professor S. Kachi and Dr. S. Yajima for their helpful suggestions and advices concerning the chemical procedures References 1) W W T Crane and G M Iddlngs, Phys Rev 95 (1954)1702 2) T. Nakal and S Yapma, J Chem Soc J a p a n (Pure Chemmtry) 79 (1958) 1267 3) J J Katz and G T Seaborg, The Chemistry of the Actmxde Elements (John Wiley and Sons, New York, 1957) Chap IV 4) J Golden and A G Maddock, J Inorg and Nuclear Chem 2 (1956)47 5) G H Mornson and H Frelser, Solvent Extraction in Analytical Chemistry (John Wiley and Sons, New York, 1957) p 140 6) B M Foreman Jr., and G T Seaborg, J. Inorg and Nuclear Chem 7 (1958) 305 7) B R Mottelson and S G Nllsson, Mat Fys Skr Dan Vld Selsk 1 No 8 (1959) 8) F S Stephens, F Asaro and I Perlman, Phys Rev 113 (1959) 212 9) M S Freedman, F Wagner, J r , a n d G W Engelkemelr, Phys Rev 88 (1952) 1155
Nuclear Physzcs 15 (1960) 664--677, (~) North-Holland Pubhsh~ng Co, Amsterdam Not to be reproduced by photoprmt or microfilm ~athout written permission trom the pubhsher
PROTACTINIUM-237 K
T A K A H A S H I t and H
MORINAGAt
Department o/ Physics, T~kohu University, Sendal, Japan Recexved 1 December 1959 Protactmmrn-237 was produced from t h e reaction U2SS(y, p ) P a ~87 b y t h e 25-MeV b r e m s s t r a h l u n g T h e actlvxty produced was chemically separated with t h e a~d of a recoil m e t h o d from flssxon p r o d u c t s T h e half-hie of this new isotope was found to be T j = 394-3 m m T h e radiations were measured w i t h a scintillation spectrometer Three b e t a components were identified wxth the end-points of 2 304-0 05 MeV, 1 35 MeV, a n d a b o u t 0 8 MeV M a n y g a m m a rays were also found A decay scheme was constructed w i t h t h e aid of N d s s o n ' s umfled model
Abstract:
1. Introduction
The asotope protactinium-237 was first reported by Crane and Iddings 1) in 1954, from the reaction U ~s (d, 2pn)Pa 2s7 by the 190-MeV deuterons from a synchro-cyclotron. The half-hfe of the beta decay of this new isotope was determined to be about l l rain. No detailed investigation about the characteristics of this nucleide has been made since then. Usually, in the case of such a high energy reaction of fissile elements, it is likely that the reactions become very much complicated. Moreover, Pa 2s7 as not the most abundant isotope among protactinium isotopes produced in high energy spallation reactions. On the other hand, in the case of photoreaction with the 25-MeV bremsstrah]ung, there are only four competing reactions, viz., the (y, n), the (y, 2n), the (y, p) and the (y, f) reactions on fissile elements. Pa ~7, therefore, is the ma]or protactinium isotope produced by low energy photoreactions on natural uranium. Difficulty, however, is that the cross sections of the (y, p) reactions m the region of uranium are about the order of some hundredth of those of the (7, n) or the (y, f) reactions. The products of the latter reactions, having m a n y specaes of activities with various half-lives and various chemical behaviours, will complicate any separation procedure for protactinium In order to remove this difficulty, a physical separation technique of spallation products from flssaon products was employed, in addition to the specific chemical procedure to separate protactinium Recently, Nakai and Yajima *) showed that UO 2 powder of about 8 microns in grain saze embedded in oxalic acid powder with the mixing ratio 1 : 70, loses fissaon products into bedding material almost completely when it is activated by pale neutrons. They also showed that almost complete separation is achieved in the case of Now at D e p a r t m e n t of Physics, U m v e r s l t y of T6kyo, B u n k y 6 k u , T6kyo, J a p a n 664
PROTACTINIUM-23?
665
UO~ (NO3) 2 " 6H~ O powder of about the same grain size embedded in amorphous graphite powder with the same mixing ratio, as in the case of UO2--C2H204 • 2H, O mixture. The spallation products such as (y, n), (y, p), (y. 2n) products however, have extremely low recoil energies, of the order of 10 keV, or extremely short ranges when produced in the photoreactions Therefore, they can not get out from the phase of fissile material in such a mixture and will remain in the same phase making it possible to separate themselves from fission products We have tried to employ this principle using the above two recipes for eliminating the disturbing fission product activities before finally separating the protactinium activity, b y usual chemical separation methods Of late years, methods for isolation of protactinium have been studied b y m a n y investigators and several standard procedures have well been established 3). In the present work, two types of these methods were employed after the physical separation of fission products from UO 2 powder: one is the codeposition technique with manganese dioxide precipitates from the nitric acidic solution of UOa and the other is the solvent extraction with methylisobutylketone (Hexon) from the nitric acidic aqueous solutlon of UO 2. 2. E x p e r i m e n t a l
Procedure
In most of this work, a mixture of UO 2 powder of small grains (diameter 8#) mixed with oxalic acid powder of about the same grain size with the ratio of 1 : 40 in weight was used. The total weight of each sample was about 41 g Some of irradiations were carried out with uranyl nitrate-graphite mixture in which the former is 1 : 40 in weight ratio to the latter, the amorphous graphite They were bombarded with the 25-MeV bremsstrahlung from the T6hoku University betatron at the position of approximately 500 r/min for about 40 minutes. After each irradiation, in the case of UO2-oxalic acid mixture, the sample was dissolved into hot water and the residue, the UO 2 powder, was centrifuged This residue was then washed once or twice with a hot diluted oxalic acidic solution in order to avoid the Cn activity to contaminate the final Pa activity, and also to minimize the fission product activities which could adsorb onto the surface of the fine UO2 powder. The UO2 was then dissolved into 8N H N 0 3 solution. In the case of uranyl nitrate-graphite mixture, the irradiated sample was dissolved into hot diluted nitric acidic solutlon and the graphite was filtered out. The filtrate was boiled and dried in order to remove the N 13 activity. The assay was then dissolved into the chluted solution of hthium carbonate. This step was added so that the Cn produced from graphite and mixed into the uranyl nitrate phase could be driven out at the next step when the solution was again made acidic. Then the solution was again made 8N nitric acidic
666
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Two independent methods were employed in order to separate protactinium from such nitric acidic solution. One was the codeposition technique with manganese dioxide precipitate from nitric acidic solution of UO 24). To the nitric acidic solution, a saturated solution of Mn(NO3), was added and then 0.1N KMnO 4 solution was dropped so that it was just enough for making the precipitation of Mn02. This MnO, was centrifuged and then filtered and dried onto a filter paper. It took about 15 minutes to complete this procedure. Another method used was the solvent extraction of protactinium with methyl-isobutylketone (Hexon) from the nitric acidic aqueous solution of UOS, 6). After dissolving the UO 2 into 8N HNO3 solution, it was shaken with equal volume of hexon. Before starting to count the activity of this solution it did not take more than a few minutes. For long counting the hexon wad dried. This extraction method was used only for the samples of UO, mixed with oxalic acid. The measurement of the half-life was made with a G-M counter and from the change of beta and gamma spectra with time measured b y a scintillation spectrometer. The beta ray end-point energy was measured with a beta scintillation spectrometer; the detector was a plastic scintillator of 1 inch thickness and 2 inches in diameter with a 7 # aluminium window. For the observation of gamma rays, a conventional multi-channel NaI (T1) scintillation spectrometer was used. The sizes of the sodium iodide cyrstal used were 1½ inches in thickness and 121 inches in diameter for most of the measurements, and 4 " × 4" for the detection of high energy gamma rays. Gamma-gamma coincidence measurements were also carried out for some intense gamma rays, such as 145, 205, 275, 330 and 460 keV, using a slow coincidence circuits (2~ = 5/~sec). Conventional single channel pulse height analyser was used for energy discrimination of gamma ray pulses from one of the detectors. Output pulses from the analyser controlled the coincidence gate of the multi-channel pulse height analyser, which received gamma pulses from another detector. The performance of this coincidence circuit system was checked for the 330-keV gamma r a y from Aul~e(T½ = 5.6 d) which has the 356-keV coincident gamma ray.
3. Experimental Results Fig. 1 shows the decay curve of the activities obtained according to the above recipe from a sample of uranium nitrate mixed with graphite powder. Quite similar results were obtained from samples of UO~--Cg.H204 mixture. In fig. 1, the long-lived activity is due to the beta decay of T h ~ ( T t = 24.1 d) which mixed into the Mn02 precipitates b y a very small amount (about 0.3 percent). The 6.66-h activity comes from Pa ~4 which is in the radioactive
667
PROTACTINIUM-~7
equilibrium in natural uranium. Referring to the amount of this Pa =* activity, the chemical yield of protactinium throughout this chemical procedure was estimated to be approximately 30 percent. After the subtraction of these well known activities, an appreciable amount of activity with a half-life of 394-3 rain was observed. This activity must be either due to Pa =T, or to some fission products which followed this chemical procedure, or m a y be due to
TV2=24. I d QO0 0~
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12
14
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Fig 1 Decay of the p r o t a c t m m m actlvltles chemically separated from a sample of the u r a n m m mtrate-graphlte mixture b o m b a r d e d with the 25-MeV bremsstrahlung.
other impurities. There is, however, no fission product activity which has only such a half-life. It is also difficult to attribute this 39-rain activity to any conceivable impurity. Therefore, it is suspected that this activity is assigned to Pa =~. No 1 l-rain activity 1) was observed with an intensity expected from the (7, P) yield systematics. The beta ray end-point energy of this new activity was measured with a plastic phosphor. Following the decay of the beta spectrum, similar value of half-life was obtained after the subtraction of long-lived activities. The Kurie
668
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TAKAHASHI
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plot for the 39-min component is shown in fig. 2 (a). The end-point energy of the highest energy beta ray was determined to be 2 3 0 i 0 05 MeV. This value agrees well with the value estimated by Seaborg et al. e) for Pa ~s~ from the energy systematics of the alpha and beta decays of heavy elements. Here, no correction for the finite resolution of the phosphor was applied. Since the Kurie plot is linear in rather large portion of the spectra, the extrapolation of the
(c)
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Fig 2 Kurle plots of P a 2s7 b e t a spectrum (a) total, (b) after the subtraction of the 2 30-MeV component, (c) after the subtraction of the 2 30-MeV and 1,35-MeV components
linear part was used for determining the end-points. The Kurie plot of the beta spectrum begins to deviate upwards from the straight line at the vicinity of 1.3 MeV. In order to check the linearity of the Kurie plots of beta spectra, an examination of the performance of our spectrometer was made with the beta rays from In 114 which has almost simple allowed spectrum. The result is shown in fig. 3. Comparing this to the spectra of the 39-min activity, it is concluded that other lower energy components should be included in the decay of Pa 28~. After the Kurie plot analysis, two more branches of lower energies
PROTACTINIUM-~7
669
were detected. The Kurie plots of these components are also shown in fig. 2 (b) and (c), respectively: the higher energy one having E• max at 1.35 MeV and, the lowest one at approximately 0.8 MeV. The branching ratios of these three components are roughly 60 : 30 : 10. For the lowest component, the value of
20
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Fzg 3 Kurle plot of t h e b e t a s p e c t r u m of I n 11:
its end-point energy and its branching should have rather large uncertainties because of the poor resolution and difficulty of the subtraction of the higher energy components. Results of the beta ray measurements with considerations of gamma ray spectra and the results of other experiments, are listed in table 1 TABLE 1 S u m m a r y of the b e t a decay of P a =8~ Eo
Intensity
(MeV)
(%)
2 30 1 35
~08
60
{2,
7 ~10
Log
/t
72 73 74 >~65
Clasmflcatlon
lu lu ah ah
The gamma ray spectrum was measured with a conventional scintillation spectrometer The decay of the spectrum was followed for several hours and almost all the peaks were found to decay with a half-life of about 39 rain. Small fractions of the long component activity comlng from Pa ~4 were also
670
K
TAKAHASHI AND H
MORINAGA
observed. Therefore, those gamma rays are assigned to the new isotope Pa =v. After the subtraction of the long component ganima ray spectrum and the background spectrum, the gamma ray spectrum for Pa ~ was obtained. This is shown in fig. 4. Because of the low counting efficiencies for high energy gamma rays, some of high energy gamma rays might have been missed. The intense 90-keV line is considered to be mainly due to the K X-ray from U =v. I
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For the observation of high energy gamma rays, a 4" × 4" NaI (T1) was used. In order to cut off the low energy gamma rays, a ~"-thick lead shield was placed at the face of the crystal. Fig. 5 shows the observed spectrum. The analysis of the gamma ray spectra was carried out using, as a guide, the spectral shapes obtained in the same geometry from Inn% Hg ~°8, Na 2., Cs 13v and Zn ss sources. After the subtraction of the back-scattering and the Compton peaks of higher energy components, relative heights of the photopeaks were obtained. Therefore, relative intensities of lower energy components m a y
PROTACTINIUM-~7
671
have considerable uncertainties. Because of the rather large number of gamma rays quite a few peaks m a y actually be double or triple lines. Especially at lower energies the separation of peaks could have bad ambiguities. The intensity ratios of the observed gamma rays were determined using photopeak efficien-
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NUMBER OF C H A N N E L Fig. 5 Scmtfllatlon s p e c t r u m of t h e g a m m a r a y s f r o m P a zs7 o b s e r v d w i t h a 4 " × 4 " crystal G a m m a r a y energms are given m keV
NaI(T1)
cies of the present geometry of the spectrometers. They are listed in table 2. Since the activity produced was weak and its half-life is short, gammagamma coincidence measurements m a y give only qualitative results. The 145-keV gamma ray was observed coincident with many other gamma rays as listed in table 2. The 205- and 275-keV gamma rays were also found to be in
672
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TAKAHASHI
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coincidence with the 90-, 145- and 460-keV gamma rays. Similar results were observed for the 330-keV gamma ray but were not conclusive. A measurement with the 460-keV gamma ray gave a consistent result with the above data. It is interesting to know the yield ratio of the reaction U23S(7, p) Pa 23~ relative to the reaction U23s(7, n)U 237 at the 25-MeV bremsstrahlung energy. The chemical yield of Pa ~37 was estimated with reference to that of Pa 234 TABLE 2 I n t e n m t l e s of t h e g a m m a r a y s from P a ~a~
Energy
(keY)
90 145 205 275 330 405 460 550 590 750 805 860 880 915 1045 1320 1420
Relative mtens,ty
50 45 55 20 40 30 100 30 25 50 45
Relative mtenslty coinc,dent w i t h t h e 145-keV ~-ray 5 3 3 25 15
(5) t 35 (5) t 25
100 100 35 10 15
t Figures m parentheses m a y not be entlrely due to P a s87, slnce such coincidences m a y from the decay of Pa ~34
arise
which was present in natural uranium in a condition of radioactive equilibrium. Then, we can calculate the yield of U~S(y, p)Pa ~a7 using the Cn activity which was produced in the sample of the mixture as the monitor of the beam intensity. The measured value of the yield ratio was one five-hundredth relative to the (7, n) reaction of U 23s. This value of yield ratio is in good agreement in the order of magnitude with the value expected from the (7, P) yield systematics. This further supports t h a t this 39-min activity is due to Pa 237. 4. D i s c u s s i o n
As it is well known, A = 237 is in the strongly deformed region Through recent studies 7, 8), it has been established t h a t the description of nuclear states
PROTACTINIUM-237
673
by the unified model is quantitatively successful. Here, we t r y to analyze our experimental results in the light of such theories The Kurie plot analysis of beta ray spectrum of Pa ~v shows that the beta branching to the ground state (and its vicinity) of U ~ is about 60 percent of the total disintegration and that the end-point energy of this ground state transition is 2 30 MeV Since the measured value of the half-hfe IS about 39 rain, the log ]t value of this ground state transition turns out to be about 7.2. The ground state spin and parity of Pa ~3~ is {--- from the systematics of the ground state properties of several known Pa isotopes. According to the analysis using Nilsson diagram, this is the first rotational state belonging to the ½-- [530J Intrinsic state s). It seems to be unreasonable that i comes lower at A = 237 since the decoupling factor does not change so much with the neutron number. On the other hand, the ground state of U ~3~ has been found to be a !2+ state and has been assigned to the ½+ E631J intrinsic state from the investigations of the alpha decay of Pu ~1 9) Therefore, the beta transition from Pa 2a~ to the ground state and its vicinity of U z3~is classified as first forbidden and unhindered (abbreviated as 1.u.) transition. This is quite consistent with the log/t value of 7 2 obtained above, in the view of the recent studies on the systematics of the selection rules for beta transitions ~) The second beta group has an end-point energy of 1.35 MeV and its branching was about 30 percent (cf fig. 2 and table 1). Since the log [t value of this group was estimated to be about 7, the transition must be classified as either 1.u. or allowed but hindered (abbreviated as a h ). From the theoretical examinations of the Nilsson diagram for neutron levels near N -=-- 145 and the analysis of the level schemes of U23Z(N = 141), we can expect an intrinsic hole-state of ~-+ ~631] in U t33 among low energy states This state appears at about 313 keV in U ~8. In Um, the ½+L631J state is also observed, but as an excited particle-state at about 400 keV This means that for U 238 the separation of ½ + [631~ and { + [631] state is about 700 keV. Since the energy dependence of the states on deformations IS very similar for these two states, the separation of the corresponding states in U 23~ should be close to 700 keV. Since the {+E631J state can be reached by the 1 u. transition from Pa ~ ground state, one m a y suspect that the 1 35-MeV beta branching includes such a 1.u transitxon. As shown in table 2, four gamma rays that have energies of 750, 805, 860 and 915 keV, were observed with large intensities. If the ~-+E631J level exists at about 910 keV, the direct transltlons to the ground state and its vicinity would occur from this level Such transitions are the unhindered M1 transitions From these facts we can tentatively assign the 910-keV level as the _3+ 2 E631~ Nilsson level. Recent investigations of the alpha decay of Pu 241 show that in U z~ the { + ~622J state appears at 145 keV. The first member of the rotational band of
K TAKAHASHI AND H
674
MORINAGA
this {-+[622] level has also been observed at 200 keV having a spin of 7 In analogy to Pu m~, the ground-state rotational band should be observed between the ground state and the { + [622] level. In the observed gamma ray spectra, however, the photopeak of the 915-keV line is wider than the energy resolution of our spectrometer (fig. 4): this suggests that more higher energy at)rain
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components may be included in this 915-keV line. This component may probably be the M1 transition to the 3 + rotational level of the ground state from the level a little above the 910-keV level of { + [631]. Therefore, we can expect a possible state of { belonging to the rotational family of {+[631] at an energy of about 950 keV. Thus, it might be inferred that the transitions to these rotational members of ½+ [631] and { + [622], occur from the ground 3 and the rotational { states of the { + [631] level in the unhindered M1 transitions. The above-mentioned gamma rays of 750, 805, 860, and 915 keV (actually two lines) are considered to correspond to these transitions.
PROTACTINIUM-.~7
675
Taking account of the beta and gamma counting efficiencies and the photopeak efficiencies of our spectrometer, we can calculate the intensity of the 1 35-MeV beta branching on the assumption that the above-described M1 transitions are the only transitions to take place. Since such M 1 transitions are expected to be the fastest ones, this assumption is plausible. The total intensity of the 1 u. beta transitions to the { + [631] level and its ~- rotational member was thus estimated to be about 23 percent. The log fl value is about 7.0. Approximately a half of the 23 percent branching seems to decay to the { + state and the rest decays to the { + state. In this case, the log/t value turns out to be 7.3 for each transition. This value is consistent with the systematics of the beta decay selection rules for the 1 u. transition. As shown m table 2, another intense gamma ray was observed at 460 keV. In gamma-gamma coincidence experiments, this gamma ray was observed in coincidence with the gamma rays of the energies of 145, 205, 275 and 330 keV. Very little information is available, because the weakness of the source intensity of Pa 237 made it difficult to get enough statistics However, we have observed that both the 275- and 330-keV gamma rays were coincident with the 145-keV gamma ray. We m a y therefore postulate two new intrinsic states, the {--[743J Nilsson level at about 475 keV and the {--[752] state at about 935 keV, from which an intense M1 gamma transition of 460 keV decays to the former state Analyses of the Nilsson diagram for neutron levels near N ----- 145 and of the level schemes of Pu ~ag, Pu .3~, U *a6 and U 2s3 would seem to support these assignments of energies and the intrinsic states. The ground state of Pu ~a~(N = 143) has been almost surely assigned ~ - - [743] and a state at 145 keV is considered to be { + [ 6 3 1 ] . On the other hand, Pu 2a9 (N ----- 145) has the ground state of {+[631J, and has the {--[743] state at 392 keV The energy difference between these two intrinsic states changes fairJy rapidly with the change of neutron number. Such a tendency is also observed in the level structures of UZaS(N = 143) and U2a3(N -----141) The ground state of ~--[743] occurs m U ~6, as in the case of Pu ~ , but the state of { + [ 6 3 1 ] appears very close to the ground state. In U~L however, these two states appear as the excited states: the { state appears at 400 keV and the !2 state is observed at about 295 keV. The energy difference between these states is about 100 keV. In the case of the {--[743] state in U ~ ( N = 145), it will be expected from the above discussions that this level would be observed between 300 keV and 550 keV. Thus the ~--[743] orbital can reasonably be assigned to a state at about 475 keV, as expected before. Therefore, the intense 460-keV transition to the ~ - - [743] state can reasonably be explained to occur from the ~ - - [752] state at about 935 keV m the M1 transition as expected before. Since the level postulated at 953 keV is { - - [7521, the a h. transition of the beta ray from the
676
K
TAKAHASHI AND
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MORINAGA
3__ ground state of Pa ~7 is expected to occur. Actually such a beta branching 2 must be assumed to exist being mixed in the spectrum of the 1.35-MeV mentioned above. According to the estimation from the intensity ratios of the 460-keV gamma ray and of the 1.35-MeV beta rays, and the beta a n d g a m m a counting efficiencies, this transition is expected to correspond to the remaining 7 percent branching of 1.35 MeV. The experimental l o g / t value was calculated to be about 7.4 for this branching. The value is indeed what is expected for the a.h. transition of the beta ray. Thus, it m a y be concluded that the {--[752] state decays to the 7--[743] state at about 475 keV with the emission of the 460-keV gamma ray in M1. From this level, many gamma rays of such energies as 205, 275, 330 and 405 keV lead to the lower states; they are the ] + state at about 270 keV, the ~ + state at 200 keV, the ~ + state at 145 keV and the { + state at about 70 keV. The first three states are probably the members of the rotational band of the { + [622] Nilsson level at 145 keV and the last one is the rotational member of the ½+[631] orbital Results of high energy gamm~ ray measurements show that the 1420-, 1320-, 1045- and 1010-keV gamma,rays lead to the lower states, and suggest that another intrinsic state and its rotational members appear in the vacinity of 1500 keV. Therefore, the beta branching whose energy is about 0.8 MeV can be attributed to the decay to these highly excited states of U mr. The experimental l o g / t value of the 0.8-MeV beta branching was estimated to be about 6.5 and this value implies that the transition should be classified as either a.h or 1 u. from the systematics Referring again to the Nilsson diagram, we can tentatively assign the states as the ~--[761] state and the first member of its rotational band at around 1500 keV. The intensity ratios of such gamma rays emitted from these levels as those of 1420, 1320, 1045, 1010, 590 and 550 keV, can be explained quite consistently, if one assumes that the {--[761] ground and its first rotational states appear at 1480 and 1530 keV. According to the above discussion, we can construct a very tentative decay scheme of Pa roT. The proposed decay scheme is shown in fig. 6, where the log It values are given in italics and the gamma ray transition energies are in keV. The energy assignments of the excited states which have ambiguities are shown in parentheses. It is interesting to see such a large number of intrinsic states having been observed in the decay of a radioisotope. This is due to its high disintegration energy Because of the intrinsically poor resolution of the scintillation spectrometer it was difficult, in general, to resolve gamma rays to or from levels in the same intrinsic structure. They are separated usually b y several tens of keV. Therefore, our assignments of rotational members m a y not be so certain. As for the intrinsic states, the assignments seem to be more certain. It will be very interesting to make more elaborate analysis of this decay scheme using stronger source of Pa 237
PROTACTINIUM-237
677
The authors wish to express their appreciations to Professor S. Kachi and Dr. S. Yajima for their helpful suggestions and advices concerning the chemical procedures References 1) W W T Crane and G M Iddlngs, Phys Rev 95 (1954)1702 2) T. Nakal and S Yapma, J Chem Soc J a p a n (Pure Chemmtry) 79 (1958) 1267 3) J J Katz and G T Seaborg, The Chemistry of the Actmxde Elements (John Wiley and Sons, New York, 1957) Chap IV 4) J Golden and A G Maddock, J Inorg and Nuclear Chem 2 (1956)47 5) G H Mornson and H Frelser, Solvent Extraction in Analytical Chemistry (John Wiley and Sons, New York, 1957) p 140 6) B M Foreman Jr., and G T Seaborg, J. Inorg and Nuclear Chem 7 (1958) 305 7) B R Mottelson and S G Nllsson, Mat Fys Skr Dan Vld Selsk 1 No 8 (1959) 8) F S Stephens, F Asaro and I Perlman, Phys Rev 113 (1959) 212 9) M S Freedman, F Wagner, J r , a n d G W Engelkemelr, Phys Rev 88 (1952) 1155