Studies of the alpha decay of 233U

I 1.E.4:5 [

Nuclear Physics A100 (1967) 609--618; (~) North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

S T U D I E S O F T H E A L P H A D E C A Y O F 233U O. A. TROJAN *, K. G. McNEILL andN. R. STEENBERG Department of'Physics, University of Toronto, Toronto 5, Ontario Received 24 April 1967 Abstract: The decay of 233U through the first and second excited states of 229Th has been studied using a uranium oxide source. The half-life of the first excited state of 229Th was found to be less than 1.9 nsec; it decays by an almost pure M1 transition. The half-life of the second excited state of 229Th was found to be 0.5~0.2 nsec; this level de-excites by E2 cross-over radiation in 96 % of events. Contrary to expectations, an anisotropic alpha-gamma angular correlation was not found, and the probable reason for this is discussed.

E

RADIOACTIVITY z33U; measured Ix, ~, ~7-delay, o:7(0), 2°'gTh deduced levels, Tff, branching, cc, mixing ratios.

I. Introduction

The d e c a y scheme 1) o f 233U is shown in fig. 1. D e c a y o f the ~ g r o u n d state o f 233U to the g r o u n d state o f 229Th or to excited states can t a k e place by the emission o f a l p h a particles with different o r b i t a l a n g u l a r m o m e n t a l with a m i n i m u m value o f l g o v e r n e d b y c o n s e r v a t i o n o f a n g u l a r m o m e n t u m , a n d higher values being p r o g r e s sively u n i m p o r t a n t . T h e relative phases o f the p a r t i a l waves will alter the a n g u l a r d i s t r i b u t i o n o f a l p h a particles f r o m oriented nuclei or, for instance, the a n g u l a r c o r r e l a t i o n o f g a m m a rays emitted f r o m d a u g h t e r nuclei with respect to the direction o f the c o r r e s p o n d i n g a l p h a rays. A s a l p h a - d e c a y p r o b a b i l i t i e s in d o u b l y even nuclei show great regularity a n d the f a v o u r e d a l p h a transitions in o d d - m a s s nuclei show g r e a t similarity to the d o u b l y even nuclei, it is expected t h a t the l = 0, 2 phase r e l a t i o n s h i p s h o u l d be the same for all such decays. H o w e v e r , it is n o t clear t h a t this is so. K r o h n et al. 2) w o r k i n g with Z41Am f o u n d l = 0 a n d l = 2 waves to be nearly in p h a s e with each other. M u r p h y 3) also f o u n d with 243Am an in-phase relationship. These results s u p p o r t a conclusion 4) t h a t the a l p h a wave d i s t r i b u t i o n on the nuclear surface has a m i n i m u m at the equator. But w o r k i n g with 233U, R o b e r t s et al. suggested 5) that l = 0 a n d l = 2 waves were o u t o f phase. I n the p r e s e n t w o r k an a t t e m p t has been m a d e to d e t e r m i n e the a l p h a - g a m m a a n g u l a r c o r r e l a t i o n in 233U with the p u r p o s e o f finding out m o r e a b o u t the relative phases o f the p a r t i a l a l p h a waves involved in the decay o f the p a r e n t nucleus. A l t h o u g h 233U has m a n y a d v a n t a g e s f r o m the p o i n t o f view o f i n t e r p r e t a t i o n , it is a very * Present address: Atomic Energy of Canada Ltd., Sheridan Park, Ontario. 609

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o.A. TROJAN et al.

difficult nucleus to study experimentally. Since the probability of alpha emission is very sensitive to energy, in the daughter nucleus only levels less than 100 keV are significantly excited, and the ensuing g a m m a transitions are strongly converted. To avoid effects due to alpha particle scattering, only thin sources ( < 1 mg/cm 2) can be used, but this coupled with the long half-life means that only weak sources can be used. Moreover, the daughter nuclei recoil with energies of 82.5 keV through the source, resulting in the possibility of perturbation of angular correlations by the interaction of the daughter nuclei and surrounding electromagnetic fields. 54

U233(I.6 x 105y}

Fig. 1. The alpha decay scheme of 23aU.

2. Source preparation For reasons discussed above, the 2 3 3 U s o u r c e had to be thin. A thickness of 1 mg/cm 2 and a circular shape of 2.5 cm diam. was chosen. To prepare the source, electroplating was used, the process being basically the one developed 6) at Chalk River, in which uranium is deposited as oxide from an electrolyte of uranyl nitrate dissolved in 0.2 M solution of a m m o n i u m oxalate. The pH of the electrolyte was adjusted to 8 by the addition of ammonia. The initial concentration was 1 mg of U per ml of electrolyte. Before deposition, the cathodes were degreased by successive washings in carbon tetrachloride and acetone. The cathodes were then pickled for 30 sec in a 1 : 1 aqueous HC1 solution at 60 ° C. After being rinsed in hot water, the cathode was further etched in a 3 : 3 : 2 solution of HNO3, H 2 0 and H F at room temperature for 5 sec. Deposition was started immediately after the above chemical stripping process. Plating was carried out at 7 6 + I°C with a current of 30 m A / c m 2 supplied from a rectifier. Convection currents aided by gas evolution provided stirring.

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c~-DECAY OF 233U

Dry runs with natural uranium showed that 95 ~ or more of U was deposited in 30 rain with the above conditions. When using 233U, 4 h were however required for 94 ~ deposition. N o sure explanation is advanced, but this is possibly due to the ionization caused by the radioactivity throwing to the right the ionic equilibrium U O ~ + C 2 0 2 ~ U O 2 C z O 4. After deposition, the amount of uranium deposited and the uniformity of the source was checked using a gas-flow, proportional counter. It should be noted that the uranium is deposited in oxide form rather than as pure metal. The possible effect of this is discussed later. A very thin source was also prepared by rubbing a 0.12 m m piece of aluminium on the electrodeposited source; some 233 U adhered to the aluminium. For calibration purposes, a 226Ra source was made available by AECL, Chalk River; this source was prepared by vacuum sublimation from a hot filament onto a cold platinum surface.

3. Experimental methods To measure angular correlations and lifetimes, a fast-slow coincidence system was set up. A block diagram is shown in fig. 2.

i

Fig. 2. A block diagram of the electronic apparatus used. The source was placed in a cylinder of plastic scintillator (NE 102) of 12.7 cm diam. and 12.7 cm height which was split along its mid-plane. Essentially it formed a 4re counter (fig. 3). Portholes (one in each of the two halves of the plastic) were however viewed by alpha-ray and gamma-ray counters, respectively, and these windows could be moved relative to one another to enable angular correlation

612

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TROJAN et al.

experiments to be carried out. Two photomultipliers at opposite ends of the cylindrical block fed an anticoincidence circuit. The object of the 4n counter and anticoincidence circuit AC was to veto pulses from 43 keV gamma rays which were in coincidence with cascade gamma rays from the 99 keV level. The alpha particle counter was a 51 #m thick by 2.5 cm diam. NE102 plastic scintillator. The gamma-ray counter was a 5.1 cm × 5.1 cm N a I crystal in the angular correlation work or a N E I 0 2 scintillator in the lifetime measurements.

LUCITE LIGHT PIPE--',. I

I /---I----'AXIS

OF ROTATION

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NE 102 PLAST SCINTILLATOR

-BRASS TUBE

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REGION

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FIXED NE 102 PLASTIC SCINTILLATOR

GAMMA

LUCITE LIGHT TUBE

Fig. 3. A cross-sectional view of the 4~z counter.

The supervisory coincidence circuit SCC had a resolving time of 26 ns. The timeto-amplitude converter TAC was of the design of Green and Bell 7) and fed a 100channel analyser, itself gated by a triple coincidence circuit TCC. Because of the low specific activity source, counting times were lengthy, and care had therefore to be taken over stabilization of components. Drift was in fact reduced to less than 1 part in 50 over periods of 100 h. Measurements were then taken of the gamma-ray and alpha spectra of 2 a3U, of

c~-DECAY OF 233U

613

the lifetimes of excited states using delayed coincidence techniques and of the alphag a m m a angular correlations for 226Ra and 2 3 3 U .

4. Results

4.1. BRANCHING RATIOS F r o m the gamma- and X-ray spectrum of 233U (fig. 4), it is possible to measure the internal conversion coefficient for the 99 keV and 43 keV g a m m a transitions and thus the E2/M1 branching ratio. F r o m the spectrum, assuming the decay scheme of fig. 1 and that as a first approximation the contribution of the 56 keV line is zero, it is found that the internal conversion coefficient for the 43 keV transition is 34 with an expected error of about 10 %. Th X-RAYS

I 3 Irr z z o o

99

I

40 80 ENERGY (t~eV)

120 ---

Fig. 4. The gamma-ray spectrum obtained from the decay o f 23~U using a 100-channel, pulse-height

analyser. Statistical errors on all 100 points are less than the width of the line drawn.

This value is 7 9/oolower than the theoretical L-shell coefficient for a pure M 1 transition according to Rose s) and 4% lower than the corresponding coefficient as determined by Sliv and Band 9). Pure E2 would however, give a coefficient of 502 according to Rose 8). This suggests that an admixture of 1 % of E2 would increase the IC coefficient by 12 % to 41. It would appear therefore that the 43 keV transition is almost if not completely pure M1. This result is in agreement with that of Salusti lo) who used experimental results of the quadrupole moment fitted to a nuclear model and found that E2 contributed 0.5 % to this transition. Also from the same spectrum (fig. 4), the ratio of the number of 56 keV g a m m a rays resulting from a cascade de-excitation of the 99 keV level to the number of 99 keV g a m m a ray is approximately 1 : 5 0 . Assuming as a first approximation that the internal conversion coefficients differ by a factor of two [ref. 8)] and that the

o . A . TROJAN e t aL

614

efficiencies of the detector are the same for the two gamma rays, then the 99 keV level de-excites by cascade in 4 ~ of cases and by cross-over radiation to the ground state in 96 ~ of cases. 4.2. L I F E T I M E S

Measurements were made using standard delayed coincidence techniques on the half-lives of the first excited state of ZEZRn and the first two excited states of 2a9Th. For the work on the first excited state of 2a9Th, the alpha source was moved to within 1 mm of the alpha detector to minimize flight time differences between alphas

I0 LIJ ~-t 3 n~ t.9 _z

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0,10

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Fig. 5. The results o f half-life d e t e r m i n a t i o n s o f the 187 k e V level in e2~Rn and o f the 99 k e V level in

~Z~Th. Statistical errors are s h o w n .

slowed down to different energies in the source; this type of geometry also removed the need for vacuum conditions. The source sizes were reduced to 1 mm 2, and a thin (rubbed on A1) source of U was used. A NaI detector had to be used in order to distinguish between 43 keV gamma rays and others. This severely limited the time resolution of the equipment. As a result, it is only possible to set an upper limit of 1.9 ns on the half-life of this state. With the other two levels investigated, a plastic scintillator could be used. The alpha sources were prepared by rubbing small amounts of the active materials directly onto the alpha detector, which was a 2.5 cm diam. x 0.005 cm thick NE102 plastic scintillator. In the case of the 2 3 3 U decay, a discriminator was set so that interference

~ - D E C A Y OF

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615

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/

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w(e)

/ I/

\\

'

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~

\ 1

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!

/ 90 °

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120 ° ANGLE,

150" O

180 °

•

Fig. 6. A graph showing the alpha-gamma angular correlation from ~26Ra. (a) The theoretical prediction o f the unattenuated correlation. (b) The predicted attenuated correlation averaged over 18 °. Experimental points at 90 °, 120 °, 135% 150 ° and 180 ° with statistical errors. l

1.15

1 w(e)

0.85

90 =

120 ° ANGLE 0

150 °

180 =

Fig. 7. The alpha-gamma, time-integrated, angular correlation obtained from the decay o f sasU through the second excited state (99 keV) o f 229Th.

616

o.A. TROJANet al.

from 43 keV g a m m a rays was eliminated. Fig. 5 shows the coincidence curves obtained. F r o m the slopes upper limits of 0.61 +0.05 ns and 0.66+0.03 ns were obtained from t h e / Z 6 R a and z a 3 u sources for the 187 keV and 99 keV levels, respectively. Using the value of 0.32 ns for the half-life of the 187 keV state given by Bell 11), a p r o m p t coincidence curve was unwrapped from the 226Ra curve. By applying this to the 23aU case, a value of 0.51 ns was obtained for the half-life of the 99 keV level. Because of uncertainties in the unwrapping procedure, a value of 0.5_+0.2 ns is assigned to this level. 4.3. ANGULAR CORRELATIONS As a check on the apparatus, the alpha-gamma angular correlation was measured in 226Ra. The results are shown in fig. 6. This correlation has been measured 12) by Milton and Fraser using a finer geometry. Taking into account attenuation effects and geometric "smearing-out" effects, the present results agree well with those of the authors quoted, as shown in the figure. The fact that the 43 keV transition is almost pure M1 means that the highestorder terms in the angular correlation will be cos20 terms. The relative amounts of l = 2 and l = 4 partial waves in the alpha decay may be calculated using a formula due to Bohr, FrtJman and Mottelson 13), and for the two cases of "in-phase" and "out-of-phase" the angular correlations should be Win (0) = 1 +0.010 P2 (cos 0), Wout (0) = 1 - 0 . 0 2 0

P2 (cos 0).

Neither of these expressions is significantly different from isotropy. The experimental result found in the present work was in fact isotropy within 2 %. The angular correlation for the 99 keV level is not expected to be isotropic, as will be discussed later. The experimental results shown in fig. 7 are however isotropic to within 2 %. As N a I was used as the g a m m a detector in the angular correlation work, and this resulted in a time resolution of the order of 10 times the half-life of the 99 keV level of 2Z9Th, the angular correlation obtained is a "time-integrated" one. Some attempt was made to detect a variation of anisotropy with time by taking delayed coincidence curves at different angles, but the result did not differ significantly from isotropy.

5. Discussion

Using an intensity ratio of l = 4 partial waves to l = 2 waves of 0.175 for the 99 keV level 4, 13), the theoretical angular correlation expressions for the in-phase and out-of-phase situations respectively are Win (0) = 1 +0.263 Pz (cos 0)+0.064 P4 (cos 0), Wout (0) = 1 +0.442 Pz (cos 0 ) - 0 . 2 1 8 P4 (cos 0).

o~-DECAY OF 233U

617

A significant anisotropy was therefore expected. Indeed no ratio for the l = 2, 4 intensities will fit the isotropic result found. Assuming that the above theory relating to the relative transmission amplitudes of the various allowed partial waves in alpha emission is correct, an isotropic result of the time-integrated spectrum therefore indicates that perturbations have taken place which have destroyed the anisotropy. Such perturbations may be due to either static or changing electromagnetic fields. If static fields perturb the intermediate state, the angular correlation should not be attenuated below "hard-core" values 14). Under these conditions the angular correlation expressions for the source under consideration can at worst deteriorate to Wi, (8) = 1 +0.053 P2(cos 8)+0.007 P4(cos 0), Wout (8) = 1 +0.088 P2(cos 8 ) - 0 . 0 2 4 P4(cos 0), which would still result in measurable anisotropies. The fact that isotropy is obtained experimentally indicates therefore that dynamic perturbations due to changing electromagnetic interactions must be involved. These are discussed in the following paragraphs. Following alpha decay the recoiling ionized atom will distort the lattice around its final rest position. The host lattice accommodates quickly, i.e. within 10-12 sec [ref. 15)]. Fluctuating electric field gradients at the nuclear site therefore do not exist for a long time. Normally electric field gradients are of the order of 101 s V/cm 2, thus with an electric quadrupole moment 16) of 4.6 b the precession frequency is 50 MHz. Even if values of the field gradient an order of magnitude higher than the above exist for short times, no significant perturbation of the nuclear orientation can take place. Similar arguments apply during the time of actual recoil of the daughter atom ( ~ 10 -13 sec). The magnetic dipole moment of 229Th is a6) 0.41 n.m., and if this were to interact with steady fields of 107 G such as may occur in alkali atoms at nuclear sites due to unpaired electrons, a precession would occur with a frequency of 103 MHz. This is capable of deorienting the nuclei, but as stated above, the static field should not destroy any angular correlation completely. From other work, it would appear that the removal of angular correlations may be related to the conductivity of the host lattice. Both Flamm 17) and Murphy 3) find that if the recoil nuclei are allowed to enter a conducting medium the a-y correlation is less perturbed than when they enter a non-conducting medium. This agrees with results on 7-Y correlations summarized by Steffen is). The conductivity of the medium will govern, amongst other things, the rate of de-excitation and de-ionization of the daughter atom. Changes in the state of atomic excitation will cause fluctuations in the magnetic fields seen by the nuclei. If such fluctuations have a time constant of the order of the lifetime of the excited state, the random perturbations may destroy measurable angular correlations.

618

o. A, TROJANet al.

U r a n i u m oxide is a s e m i c o n d u c t o r ; the electrical conductivity at r o o m temperature has been measured by a n u m b e r of authors, the quoted values 19,20) r a n g i n g from 10- 2 (f2 • c m ) - 1 to 10-6 (~c~ • c m ) - 1. The recovery time for de-excitation is defined 21) by z = e/a, where e is the dielectric c o n s t a n t a n d a the conductivity of the material. N o value for the dielectric c o n s t a n t of U O 2 could be f o u n d in the literature. The highest relevant values seem to be a b o u t 20, e.g. that for lead oxide [ref. 22)]. T a k i n g this value of 20 for e a n d the range of values for a, z ranges from 0.2 ns to 2 /~s. A lower value of e will give correspondingly lower values of z. The greatest d e o r i e n t a t i o n will take place if the recovery time is that c o r r e s p o n d i n g to the 10 - 2 (f2 • c m ) - 1 conductivity value. I n fact, with this value of cr a n d a L a r m o r precession frequency of 700 M H z , the a n g u l a r correlation predicted 14) will be reduced to below the experimental limits of the present experiment. It is therefore suggested that it is the use of a u r a n i u m oxide source prepared as described above which is responsible for the measured isotropic distribution. I t is thus suggested that further attempts be m a d e to measure this ~-~ correlation using a pure metal source o n a c o n d u c t i n g backing. W e would like to t h a n k Professor R. E. Jervis for radioactive l a b o r a t o r y facilities a n d help in p r e p a r i n g the 2 3 3 U source, Dr. T. A. Eastwood (Atomic Energy of C a n a d a Ltd.) for providing the 226Ra source a n d Professor J. M. Daniels for help in the p r e p a r a t i o n of the manuscript. This work was assisted by grant T-84 of the N a t i o n a l Research C o u n c i l of C a n a d a .

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)

D. Strominger, J. M. Hollander and G. T. Seaborg, Revs. Mod. Phys. 30 (1958) 585 V. E. Krohn, T. B. Novey and S. Raboy, Phys. Rev. 105 (1957) 234 E. S. Murphy, Jr., ANL-6685 (1963) I. Perlman and J. O. Rasmussen, Handbuch der Physik, Vol. 42 ed. by S. Fltigge (SpringerVerlag, Berlin, 1957) p. 109 L. D. Roberts, J. W. T. Dabbs and G. W. Parker, ORNL Report No. 2204 (1956) J. M. McKenzie, Nucleonics 17 (1959) 60 R. E. Green and R. E. Bell, Nucl. Instr. 3 (1958) 127 M. E. Rose, Internal conversion coefficients (Interscience Publ., New York, 1958) L. A. Sliv and I. M. Band, Coefficients of internal conversion of gamma radiation Part 2, L-Shell, NP-TR-217. (UC-34) (Academy of Sciences of the USSR, Moscow-Leningrad, 1958) E. Salusti, Nuovo Cim. 30 (1963) 171 R. E. Bell, S. Bjornholm and J. C. Severiens, Mat. Fys. Medd. Dan. Vid. Selsk. 32, No. 12 (1960) J. C. D. Milton and J. S. Fraser, Phys. Rev. 95 (1954) 628 A. Bohr, P. O. Fr6man and B. R. Mottelson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 10 (1955) A. Abragam and R. V. Pound, Phys. Rev. 92 (1953) 943 C. Kittell, Introduction to solid state physics (Wiley and Sons, New York, 1953) V. N. Egorov, Opt. i. Spectroskopiya 16 (1964) 549 E. Flamm, University of California Radiation Laboratory Report (1960) UCRL-9325 R. M. Steffen, Advan. Phys. 4 (1955) 293 W. Hartmann, Z. Phys. 102 (1936) 709 R. K. Willardson, J. W. Moody and H. L. Goering, J. Inorg. Nucl. Chem. 6 (1958) 19 J. A. Stratton, Electromagnetic theory (McGraw-Hill, New York, 194t) p. 15 Handbook of chemistry and physics, 45th ed. (Chemical Rubber Publishing Co., Cleveland)