The decay of the 1.14 min isomer of Pa234 (UX2)

Nuclear Physics 42 (1963) 642---659, (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E D E C A Y OF T H E 1.14 mln I S O M E R OF Pa :~4 (UX~,) S. B J O R N H O L M and O. B. N I E L S E N

Institute for Theoretical Physics, University of Copenhagen, Denmark Received 19 N o v e m b e r 1962 Abstract: Sources o f 24 d T h m ( U X 1 ) in equilibrium with its daughters, 1.14 rain Pa~U~(UXt) and 6.75 h P a ~ ' ( U Z ) were studied with a six-gap fl-spectrometer, scintillation spectrometers and with e-7 and fl-), coincidence techniques. Quantitative chemical separation o f the three nucleides was performed and the individual y-spectra and disintegration rates were measured. The short lived Pa s" isomer was found to populate three intrinsically excited states in U m , a state with ( K , / , ~t) ---- (0, 1-) at ( 7 9 0 ± 5 ) keV, a (0, 0 +) state at 811 keV and another (0,0 +) state at 1045 keV. Detailed results on fl-, 7'- and electric m o n o p o l e transitions are presented and discussed. By applying three independent methods the relative disintegration rates o f U X , and U Z were found to be 100 to 0.13-t-0.03. A 70-73 keV isomeric transition o f corresponding intensity was found in the conversion line spectrum. The possible assignments o f the Pa ~ levels are discussed.

1. Introduction

In a previous paper 1) we have investigated the fl-decay of the 6.75 h Pa 234 isomer (UZ). The present paper deals mainly with the decay of the other known Pa 234 isomer (UX2). Since its half life is only one minute, it has been studied in equilibrium with its parent Th234 which has a half life of 24 days. The sources contain also some UZ activity, although the equilibrium amount is very small (0.13%). From previous investigations, briefly reviewed in ref. 1), it is clear that the two isomers populate quite different spin states in U 234. In the UX z decay, only low spins are encountered. About 98% of the fl-decays feed the ground state of U 234 directly. The remaining weak fl-branches, together with the weak y-rays and conversion lines, have been studied by numerous authors since 1923, but, the evidence has not led to unambiguous interpretations 2). In view of the fact that the coincidence measurements seem particularly unsatisfactory, it was felt that the six-gap fl-ray spectrometer, having a transmission of 10%, might be used advantageously in establishing the coincidence relations between y-rays and fl-particles or conversion lines and thereby help to clarify the decay scheme. With sources prepared by extraction of Th 234 from about 0.5 mCur of natural uranium the fl-spectra and conversion lines were measured and analysed. Scintillation spectra of the y-rays were recorded for each of the three constituent activities of the UX-complex, and the relative intensities in the equilibrium mixture were measured. Extensive measurements of fl-~ and e-~ coincidences were performed with a Nal642

T H E U X t ISOMER OF P a u4t

643

crystal mounted in a 10~o geometry relative to the source position in the//-spectrometer. The resulting decay scheme accounts for all but about 0.1 ~o of the fl-decays. A number of observations related to the very conspicuous 811 keV electric monopole transition resisted unambiguous interpretation. Turning to the genetic relationship between UX2 and UZ we first established their relative disintegration rates. An unassigned conversion line which seemed to be a candidate for the isomeric transition UX2 ~, UZ in fact was shown to be noncoincident with fl-particles. On the basis of the results obtained, the level structures of Pa T M and U T M are discussed. 2. Experimental

2.1. SOURCE PREPARATION Thorium 234, the daughter of U 23s, was prepared by dissolving three kilograms of uranyl nitrate (UO2(NO3)2,6H20) in nine litres of ether and extracting the solution twice with about 100 ml of water 3). Thorium goes quantitatively into the aqueous phase. This was backextracted several times with ether to remove uranyl nitrate, boiled to reduce the volume, again backextracted with ether, boiled etc., until the volume was reduced to about I0 ml. The sources were prepared by various ion-exchange methods. The one best suited for obtaining thin sources consists in making the above solution 7N in HNO3 and passing it through a 0.5 ml D o w e x 1 anion-exchange column absorbing thorium and letting uranium(VI) pass through 4). After a wash with 7N HNO3 the thorium is eluted with 0.1N HNO3. The active solution is boiled till the volume is about I0/zl, again made 7N in HNO3, absorbed on a 5/A anion-exchange column and finally eluted with 0.1N HNO3. The activity in about 5/A is picked up on a sulphonated polystyreneVYNS foil, 30/zg/cm 2 thick and dried under a heat lamp. Three kilograms of uranyl nitrate represent 475/~Cur. The sources contained 200 gCur, corresponding to a chemical yield of about 50 %. The first preparation leads to fairly thick sources (1 p.p.m o f thorium in the uranium will give one mg thorium on the source), but after the uranium has been extracted a few times, only the thorium 234 activity will grow in, and quite thin sources are obtainable (10-30 gg/cm2). Samples of 6.75 h Pa T M (UZ) were prepared by extracting the protactinium with di-isopropyl ketone from a 6N HCI solution containing the Th234-pa234 activities in equilibrium. After two washings with 6N HCI and allowing time for the decay of the 1.14 rain isomer, the ketone phase could be used directly for y-ray measurements. Samples for fl-counting were prepared by drying an aliquot on a counting disc. The chemical yield, about 90 ~ , was measured by repeating the procedure before any new UZ could have time to grow in. From the ratio of the UZ-activity obtained in the first and in the second extraction the chemical yield can be determined. The 1.14 rnin Pa T M isomer UX 2 was prepared in a similar way; only the two washes of the ketone phase were omitted in order to save time. With this modification,

644

$. BJI~RN'HOLM AND O. B. NIELSEN

and using a fast hand-centrifuge, the time between the separation of the phases and the start of the ~-ray measurements could be reduced to 0.5 min. The B-counting could begin less than one minute after separation. In order to obtain UX 2 free of UZ, the first few preparations were discarded and measurements made in a time short compared to the grow-in time of UZ. The 1.14 rain decay of the protactinium samples was followed over three decades, showing that the decontamination factor with respect to thorium is > 1000 in the first extraction. 2.2. T H E f l - R A Y S P E C T R O M E T E R

Two fl-spectrometers have been used in our experiments, the six-gap "orange" spectrometer described earlier 5) and an enlarged version with doubled linear dimensions. In principle, as well as in practice, both spectrometers operated almost equally well. The instrumental resolution varied from 1.2 ~o to 0.28 ~ with corresponding changes in transmission from 1 0 ~ to about 0 . 5 ~ (cf. table 1). 2.3. T H E E L E C T R O N I C

EQUIPMENT

Spectra of the y-rays were measured with 3.8 c m x 3.8 cm and 7.6 cm x 7.6 cm NaI scintillation spectrometers including a 100-channel pulse-height analyser. The electronic equipment used with the #-spectrometers for/~-y and e-y coincidence experiments, together with the apparatus for measuring ~-~, coincidences, is described in ref. 6). (See also fig. 2 and caption to table 3.) The resolving time 2z of the coincidence circuit is about 2 . 1 0 -7 see. 3. Measurements and Results 3.1. T H E C O N T I N U O U S

fl-SPECTRA

The continuous fl-spectrum was measured from 50 keV to 2350 keV, the high energy end-point region with particular care as described in ref. ~). The low-energy spectrum of Th2a4resolves into two groups with end points 194 keV (67 ~ ) and 100 keV (33 ~), in accord with previously measured values 7). The Fermi plot of the highenergy spectrum of UX 2 was found to be straight from 1500 keV to the end point (2290 +__20) keV (see subsect. 3.5). 3.2 C O N V E R S I O N

LINES

The conversion lines are listed in table 1 (see also figs. 1(a), 1(b) and 7). As indicated in the first column, certain energy regions have not been scanned; for example the interval 250-620 keV where no lines appear in the y-spectrum. On the basis of the observed intensities the high-energy lines ( > 214 keV) can all be assigned to UX2. The assignment of the lines at lower energy is made by measuring the y-rays coincident with each line. The coincident y-spectra of the two UZ-lines are characteristic and well known from ref. 1). The remaining lines coincident with high

645

THE UXs ISOMER OF Paua TABLE 1 Internal conversion lines of the UX-complex Spectrometer conditions Energy interval (keV)

Resolution (~)

Statistics (~)

4 10-80

0.5

1

60-250 0.9

620-735

0.7

0.3

0.1

660-710

0.28

0.4

730-815

0.7

0.3

880-970

0.7

0.1

1375-1590

0.7

0.1

Conversion lines

Assignment Remarks

Energy (keV)

Intensity (~0)

Decay of:

~11 22.6 24.6 26.2 28.4 38.4 42.0 42.8 46.4 52.6

~2 0.25 0.47 0.27 0.15 0.34 } 1.05 0.40 0.10

UXI UXs UXz UX, UXx UXs UXtqUXs UXI UXs

58.0 61.7 65.5 70.6 75.3 78.8

0.24 0.06 0.06 11.00 0.06 0.05

UXI UXI ? UXt UXx UZ

86 90 112 120 (140) 206 214

2.30 0.70 0.023 0.070 <0.005 0.006 0.013

UXI UXI UZ UXI UX, UZ UXs

(631) 650 (675)

<0.0010 0.0045 <0.0010

UXs UXs UXs

(691) 696

~0.0200 0.4000

790 806 810 896 (930)

Shell

LII

43.5

Lnl 43.5 M

43.5

N

43.5

L

70-73

K K

236 255

L

236

K K K

746 765 790

no line observed

UXs UXI

K K

806 811

line questionable

0.0780 0.0270 0.0050

UX, UXs UXs

L M N

811 811 811

0.0057 <0.0008

UXs UXs

K 1001 (K 1045)

<0.0008

isomeric transition UX~ ~ UZ

no line observed

no line observed

no line observed no lines observed

The first three columns specify spectrometer conditions with respect to momentum resolution and counting statistics for the various energy intervals in which a search for conversion lines has been made. The intensities (column five) are measured relative to the 2290 keV//-group and given in percent of all Pan', UXs (or Th ua, UXI)//-decays. The lines are assigned to one of the three possible nucleides in column six. The UX,-lines are further given a shell assignment in column seven. The absolute as well as the relative accuracy of the energy determination is about 0.3 ~o. The accuracy of the intensity measurements varies a great deal with the strength of the lines. The absolute intensity of the stronger lines is considered accurate to 10 ~o.

646

S. IMI~RNHOLM

AND O. B. NIELSEN

energy 7-rays (765 keV and/or 1000 keV) can be assigned to UX2. The lines belonging to the UX~-decay are either coincident with the 64 keV ),-ray or not coincident at all. The 52.6 keV line is discussed in subsect. 3.7. The assignments o f the UX2-1ines to particular transition energies result in the usual way from energy fits and relative intensities o f the (K, L, M) lines. In addition, the measurements o f the ),-spectra coincident with each line help to make assignments. Thus, the L- and M-lines belonging to the 43.5 keV transition and also the K- and L-lines o f the 236 keV transition are found to have identical coincidence ),-spectra C~tSml, x t O -8

C~$SmlmXt 0 "s

ZSO

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s,oo

s~oo

sioo

sioo

sioo

sioo

sioo (reAl

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tSmln~lO"4 u~ ,4" 0 q,..,4

0 0 -1"

t.47

t.46 t.4S

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66'oo

6foo

6;oo

.'oo

Fig. l(b). See caption to fig. l(a).

7O~O.'A

TI~

UXI

ISOMER

Pam

OF

647

3.3. T H E 7-SPECI'RA Scintillation spectra of the three constituents of the UX-complex fig. 2. T h e u p p e r c u r v e w a s m e a s u r e d '

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ENERGIES IN

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are shown in

with the equilibrium mixture in a standard

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ENERGY keY

Fig. 2. Double logarithmic plot of the y-spectra of the UX-complex recorded with a 3.8 cm x 3.8 cm Nal crystal. The dotted line is the spectrum of the UX-complex in equilibrium. The dashed line shows the UZ-spectrum, obtained after chemical separation of protactinium and decay of the 1.14 min UXtisomer. The full line shows the spectrum of UX: obtained by accumulating counts from several chemically separated fractions. The relative positions of the spectra display the relative intensities of the },-lines in the equilibrium mixture. Intensities are given in percent of Th :u #-decays. The photo-peak efficiency curve, applying to the source-crystal geometry used in },-e coincidence measurements with the six-gap #-spectrometer, is shown in the lower part.

648

S. BJORNHOLM AND O. B. NIELSEN

geometry. Subsequently, the UZ was isolated (subsect. 2.1.) and measured in the same geometry. After correction for chemical yield and decay the spectrum was drawn on the same intensity scale as the first spectrum. Finally, UX 2 was extracted and measured. In order to obtain reasonable counting statistics, about ten extractions were made with intervals of a few minutes. With the shape of the UXz-spectrum known, it can be drawn to scale by observing that, at high energies, the UZ- and UX2spectra must sum to form the spectrum of the UX-complex. The above measurements were made with y-rays of 100-1100 keV. The spectra at lower and higher energies were recorded in different runs and joined to the central part of the curve. (The double logarithmic plot is particularly well suited for this procedure.) The intensities shown under the main peaks were determined by comparing a Th234-source with a CslaT-source in identical geometries in the fl-spectrometer as well as in the y-scintillation spectrometer and using the known conversion coefficient of the 662 keV transition in Ba 13~. TABLE 2 Internal transitions following the/~-decay of UXs )'-intensity (~o) E~ (keV)

43.5 K-Xray 236 255-1-5 746=t=5 765 790+5 (806) 811 1001 0045) I160 1440 1750

Multi- K/L polarity exp.

from calc. from total conversion from coinc, adopted )'-spectrum data )'-spectra value

E2 E0 El E1 E2 El EO EO E2 E0

5

0.50 <0.02 0.06 ~ ] 0.36

~0 0.50 0 <0.10 <0.20 0.33 <0.20

<0.008 ~ 0 . 0 4 b) 0.36 <0.04

0 0.05 0.04¢) 0.30 0.02 e)

0.59

~0.03 ~ 0.03

0.66 0

0.59

Remarks

(~o) 2 I)

0 0

5.1

Total intensity

0.09 0.05 0.04 0.30 0.02 <0.03 0.51 0.60 <0.001 0.03 ~0.03

LI_u/Llu = !

questionable

not observed broad lines, possibly complex (fig. 2)

Summary of the results of tables 1 and 3 and ref. to). Column one gives the )'-ray or electric monopole transition energy and column two the multipole assignment. In the subsequent columns the evidence pertaining to multipole assignments and )'-ray intensities is presented. Finally, the adopted (best) value for the ),-ray intensity is given in column seven, and the total transition intensity, including the conversion line intensity, is given in column eight. s) This intensity results from the coincidence experiments (table 3, group no. 1). The 43.5 keV level is fed only to 50 ~o by )'-transitions, whose intensities add up to l ~o- The total intensity must therefore be 2~o. b) See table 3, group no. 4b. c) The sum of the intensities of the 746 keV and 790 keV transitions follows from the limits established by the coincidence experiments (table 3 and fig. 4). The multipole assignments and the ratio of the two intensities are based on the angular correlation measurements made by Wood to).

UX|

THE

I S O M E R O P P a 9~'

649

It is worth noting that UZ contributes substantially to the total y-spectrum, although it represents only a very small proportion of the//-decay rate. Table 2 summarizes the results of conversion line, y-ray and coincidence measurements. 3.4. COINCIDENCE MEASUREMENTS

The UX2 //-spectrum is dominated by the intense ground-state transition; it is therefore difficult to resolve the remaining weak//-groups by direct Fermi analysis. COUMT$/CHANNEL (RELATIVEUNffS)

765 keV 1000 keY

No of ~-ray= coi.c.. p-Roy vith ~'-rOy$ of:(keV) t,.,o, 2551765 TIO00 k.v % % %

Q

450 keV/~" o

o

450

--

0.62 1.12

600

--

0 5 2 ; 1.12

650i--!052

t.05

730

- - =0.48 0.68

870

0.05 0.30 0.45

0301200 - -

0.25 0.18 0.09 0.00

t400 0.00 0.00 ,o,/10.06 0.36 059

~

4

2

O

O

-----

•

,

~'o 4"o 5'0 6'0 7"o do ~o

4400

-----

CHANNEL NO.

Fig. 3(a). Linear plot of the y-spectra obtained with a 3.8 cm × 3.8 cm NaI crystal in coincidence with various/~-ray energies selected with the six-gap spectrometer. The spectra have been corrected for random coincidences. The quantitative results are given in the inserted table.

The//-groups associated with the 765 keV and 1000 keV y-rays are of particular interest, and fig. 3(a) shows 'how the two y-rays appear in coincidence with//-particles of various energies. It is apparent that the 765 keV y-ray is coincident with a//-group of higher maximum energy than the group associated with the 1000 keV y-transition. In a subsequent measurement the paztial B-spectra coincident with y-rays above 400 keV were recorded. Fig. 3(b) shows the Fermi analysis. As a result, end-point energies of the two//-groups are obtained. It is seen that the y-ray energies fit the difference between the Qm,, of the groundstate group and the Qm,, of the partial groups very closely, viz., to within 50 keV.

S. BJORNHOLM AND O. B. NIELSEN

'650

15

% ~,

I0

> .

o

>

[nd point of Iotol $pec!rurn: 22g0*-20 keV

[ i SO0

20'00

'1500

1ooo

kt.v

Fig. 3(b). Fermi plot of the fl-spectrum recorded in coincidence with the 765 keV plus 1000 keV ~,-rays. The quantity Ne is the number of fl-particles (corrected for randoms) coincident with ),-rays above ~ 400 keV.

p~3,.

(UX2)-~

2290~:20 key

.74. %

C,\

Intensities in %

,,0.?Z./o (0.06%

~ ,

>8.0

236

[I OOS

EO 0.09 !

-

1065

I

I 255±5 00" 01

keV

i

l

8~1

io.io. ''°-

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t001 E2 ,0

030

?4G~'S Et =0.06

t

3 ,4

~

t%

S

02

t

43.S

00-+

U 234

Fig. 4. Decay scheme of Pam(UX,) --~ U '~. The thickness of the lines indicates roughly the intensities.

651

THE UXII ISOMER OF Pa |s4~

A m e a s u r e m e n t o f t h e y-rays in c o i n c i d e n c e w i t h t h e 43.5 k e V line (table 3, g r o u p no. 1) s h o w s a s p e c t r u m p r a c t i c a l l y i d e n t i c a l to t h e singles y - s p e c t r u m . Since t h e 43.5 k e V E2 t r a n s i t i o n c a n b e p l a c e d as t h e 2 + to 0 ÷ g r o u n d - s t a t e b a n d t r a n s i t i o n 1), the p o s i t i o n s o f the 765 k e V a n d 1000 k e V y-rays follow, e s t a b l i s h i n g levels in U T M at 811 k e V a n d 1045 keV. T h e 811 k e V electric m o n o p o l e t r a n s i t i o n (cf. t a b l e 2) c a n be p l a c e d b e t w e e n the l o w e r level a n d g r o u n d , a n d t h e 236 k e V t r a n s i t i o n fits t h e e n e r g y difference b e t w e e n t h e t w o u p p e r levels. T h i s i n t e r p r e t a t i o n is c o n f i r m e d by the o b s e r v a t i o n o f t h e K 2 3 6 line b e i n g 45 % c o i n c i d e n t w i t h the 765 k e V y-ray (table 3, g r o u p no. 2). A n a t t e m p t to o b t a i n a f u r t h e r c h e c k by m e a s u r i n g t h e ),-rays in c o i n c i d e n c e w i t h the 811 k e V E0 (table 3, g r o u p no. 3) s h o w s a c o m p l e t e a b s e n c e o f 236 k e V y-rays a n d TABLE 3

The F-rays observed in coincidence with conversion electrons and F-rays

Group No.

Selected transition (keV)

F-rays found in coincidence with selected transition peak energy (keV)

L 43.5

765 1000

K 236

765 (1000)

I

2

3

K 811 ( + K 806?)

4a i)

255y

4b a)

746~,+ 765~,+ 790r

interpreted as (keV) 746 -765 1001 765

(250) 765

236 or 255 ?

765

746+ 790

100 250 (765)

K 236 255 ?

Level indicated no. of coat incidences (keV) (~o)

18 32

811 1045

45 o

1045

<1 3.5

? 790

~ 12 ~ 10 <: 1

Remarks

figs. 6(a) and (b) no quantitative estimate possible absolute coincidence yields correct only within a factor of two

Column two indicates the conversion line to which the spectrometer has been adjusted in the e-y coincidence experiment, (groups I-3) or similarly the selected photo-peak energy of the Nal-scintillation spectrum (group 4). The next column indicates y-peaks observed (or absent) in coincidence with the selected transition. Subsequently, an interpretation in terms of the transitions of table 2 is given and next the observed coincidence frequency in percent of the selected events. This intensity is corrected for solid-angle and photo-peak efficiency of the Nal crystal, using the calibration curve shown in fig. 2. A correction for the contribution from the fl-rays under the conversion lines is also included. *) We are grateful to Dr. G. T. Wood who has carried out these measurements.

652

S. R J ~ R N H O L M

AND

O. B. NIELSEN

leads instead to the E0 assignment of the 236 keV transition. Thus, both levels must have spin zero and positive parity. The 255 keV E1 transition remains. From the table on fig. 3(a) it appears that this transition also occurs between levels below 1045 keV. The y-y coincidence measurements (table 3, group no. 4) show it to be in cascade with a 765 keV y-ray. Since the 811 keV transition is not coincident with 255 keV T-rays, the 765 keV E2 transition cannot be either. One is therefore led to assume that the 765 keV y-peak is complex and a decay pattern as shown on fig. 4 results. This interpretation has been confirmed conclusively by the angular correlation measurements of Wood s). Intensities of individual y-transitions derived from the coincidence measurements are given in column 6 of table 2. 3.5. THE DECAY SCHEME

The scheme on fig. 4 accounts for all the observed transitions with the exception of the weak T-rays of 1160, 1440 and 1750 keV. The intensities and l o g f t values of the /~-groups to excited states are derived from the intensities of the internal transitions.

'

• e

$,

*o

4,

3

2

f

20keV

°o

5oo

~oo

~soo

~0oo

~50o , N

Fig. 5. Fermi plot of the ~-sl~-t£um after subtraction of the 1250 keV and 1530 k©V

~-groups.

u x I ISOMER OF Pa ~

653

The total fl-spectrum, corrected for contributions from the groups to the 811 keV and 1045 keV levels, has been subject to Fermi analysis. Since there are no transitions which could explain the occurrence of further partial groups, the corrected spectrum represents the pure ground-state fl-transition. The Fermi plot given in fig. 5 shows a deviation from linearity below 1300 keV. With the present experimental accuracy it is not possible to draw any conclusion from this. It n~ght be due to instrumental effects, but, even if this possibility is left out o f consideration, the deviations are too small relative to the statistical uncertainties to permit, for example, a distinction ) between an allowed and a first forbidden shape 9). In terms of the usual classification of low-energy excitations in non-spherical even nuclei, the 43.5 keV 2 + state is the first rotational state built on the ground state. The (K, L n) = (0, 1-) level at (790+5) keV is presumably identical with the 788 keV 1 - state reported in the E.C. decay of Np T M (ref. 1o)) and may be classified as an octupole vibrational state. The 811 keV (0, 0 ÷) level, also observed in the or-decay o f Pu 23a (ref. 11)), is identified as a fl-vibrational state from its electric monopole decay mode. The 1045 keV (0, 0 ÷) state differs in this respect by a complete absence of any E0 transition to the ground state (cf. figs. 1(a) and (b)) and may therefore be of a different nature (see also subsect. 4.2). 3.6. COINCIDENCES WITH THE 811 keV ELECTRIC MONOPOLE TRANSITION The decay scheme in fig. 4 accounts in a consistent manner for almost all the experimental results. In this section we discuss some additional information on the 811 keV electric monopole transition related to the question of two-phonon flvibrations. In fig. 6(a), the y-spectrum recorded in coincidence with the 811 keV E0 is seen. A ~-ray of about 765 keV is clearly coincident with the conversion line. The quantitative evaluation of the spectrum shows 3.5 ~ of the conversion electrons to be accompanied by emission of the ~-ray (cf. table 3, no. 3). The simplest explanation would seem to be that a level at 1576 keV feeds the 811 keV level with a weak 765 keV ~-transition. This would also require the strong 765 keV E2 to be coincident with the weak 765 keV ~-ray to the same extent (3.5 ~ ) ; however, ~-~ coincidence measurements fail to confirm this (table 3, group no. 4b). One is then led to assume that the K 811 keV peak contains two lines and the experiments (figs. 6(b) and (c)) lend some support to this assumption, indicating a component 5 keV below the main line with an intensity of about 0.02 ~0, as is also required by the coincidence measurement of fig. 6(a). A conversion line of 0.02~o is relatively strong in the present decay and the most reasonable interpretation is that it represents a 806 keV E0 transition. Thus, the evidence seems to point to the existence of two electric monopole transitions of almost equal energy being in cascade, i.e. to a two-phonon fl-vibration. With the aim to test whether the two E0 transitions actually are coincident, the experiment of fig. 6(d) was performed. If the assumption )

Ref. )) is based on the present measurement.

S. BJORNHOLMAND O. B. NIELSEN

654

were correct, one would expect to observe a sum peak in the output spectrum of the electron scintillation detector of the E-spectrometer when focussing on the double line. If the pulses are further required to be coincident with K-X rays one expects to find a sum peak of intensity 0.9 ~ relative to the single-electron peak.

800 700 ~

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~ _ Coin¢.v. ll-back|e ~ U i • ¢lalverliel ~llllot 1

SO0

~~

°."

400,

".~.

$00

CoiaC,with

0

~7G5 keV

fo

2b

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CMmmol

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Fig. 6(a). The ?'-spectrumcoincident with the strong 696 keV conversion line (K811), full curve. The background spectrum due to/~-particles, dotted line.

~ngles Count~,20min x10-3

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Fig. 6(b). Simultaneous measurement of the strong 696 keV conversion line (K811). Top: singles run. Below: electrons coincident with the 765 keV ?'-peak.The two peaks do not seem to coincide in energy. The fl-spectrometer was found to focus electrons of the same energy simultaneously, as required. However, the intensity of the sum line is an order of magnitude too low and results from coincidences between the K 811 keV electrons and ]/-particles belonging to the partial group which feeds the 811 keV level (of. figs. 3(a) and (b)). A two-phonon beta vibrational state might also decay directly to ground by a weak E0 transition. We have scanned the 1375-1590 keV region of the conversion line spectrum in the search for such a possible line with negative result (see table 1).

C/ts .,.~6 s

go

t00

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Fig. 6(c). High resolution run of the 696 keV conversion line. A weak line 5 keV below the main line seems to be indicated. t05

Single pulses from K-electr ons (K 811) 104

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Channel No. Fig. 6(d). Sum coincidence measurement with the anthracene detector of the ~-spectrometer.['l'be curve shows the pulse spectrum of (696+ 12) keV electrons focussed by the spectrometer (transmission I0 ~ ) . The pulses are further required to be coincident with K-X rays. In addition to the main peak o f single pulses, one sees a peak at twice the energy, corresponding to the simultaneous arrival of two electrons in the counter.

656

S. BJORNHOLM AND O. B. NIELSEN

The measurements thus do not lend support to the assumption of a two-phonon //-vibrational state being populated in the UXz-decay 12), and the findings figs. 6(a), (b) and (c) remain unexplained. 3.7. T H E I S O M E R I C T R A N S I T I O N

UXI-'* UZ

In view of the existing ambiguity of the values for the relative disintegration rates of UX2 and UZ (ref. 7)) we have attempted a re-determination, using three different methods. First, a ratio was obtained from the intensity of the observed weak UZ-conversion lines (table 1) by comparing with their intensity in a pure UZ-source, ref. 1). Second, the quantitative measurements of the ~-spectra (subsect. 3.3) have been used in a similar manner. Finally, we have determined relative disintegration rates by ]/-counting in a windowless, 2~, methane-flow proportional counter. Corrections for backscattering and, in the case of UZ, for the effect of the high proportion of conversion electrons have been made. Since one third of the UZ fl-decays leads to a delayed state in U 234 (ref. 1)), the necessity for further corrections will arise. Recently the lifetime of this state has been determined to be 3.3 x 10 -5 sec (ref. 13)). This is more than the resolving time of the proportional counter and, as the isomeric state decays by emission of several conversion electrons, a 30 ~ correction is required to obtain the true fl-decay rate of UZ. Table 4 shows the results. The three methods give good mutual agreement. Earlier results by Forrest et al. 14) (0.18~) and by Feather and Bretscher 15) (0.15~o) essentially agree with the present determination. The ]/-decay energies of UX2 and UZ indicate UZ to be the lowest state of Pa 234 with UX2 lying 60-t-30 keV higher 1). Since the spin of UZ is unquestionably higher than 2, it is not likely for the even Th 23'* parent to populate UZ directly by ]/-decay. TABLE 4 Ratio o f U Z to U X I //-decay intensities in equilibrium Observed intensity ( ~ ) Type of radiation

Energy (keV)

in decay o f UX-complex

in U Z decay

Branching UX2 --* U Z (~)

conversion lines

78.8 112

0.050 0.023

31 19

0.164-0.05 0.12 :]: 0.04

7"rays

K-Xrays 900

0.060 0.088

50 70

0.12 :[: 0.04 0.13 ± 0 . 0 4

200

0.13t0.O4

0.13:k0.O4")

total E-activity

Weighted m e a n U Z / U X I = 0.13:k0.03 % a) cL text.

TH~ UXI ISOMER oF Pa ~

657

An isomeric transition UX2 --* UZ is the most probable way for the UZ to be populated. Since the 52.6 keV conversion line (table 1) has the correct energy and intensity, it could be an L-line of the isomeric transition. To check this, a coincidence measurement was made between the conversion lines and electrons striking a counter placed behind the source. An isomeric transition should drop out of the coincident conversion line spectrum. Fig. 7 shows the result. The isomeric nature of the 52.6 keV line is strongly indicated. If the line is assumed be to an LI_ . line, then the ~ . line can be masked under the 58.0 keV peak, and if it is an L m line, the L~-n line may lie under the 46.4

5

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o ~

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I N

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1200

1300

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Fig. 7. Comparison of two simultaneously recorded conversion line spectra. (a) single spectrum. (b) the spectrum of electrons coincident with/~-rays (recorded with an anthracene crystal), measured relative to the singles counts. The 52.6 keV line, missing in the coincident spectrum, is ascribed to the isomeric transition U X I - + U Z .

keV peak. Thus, the isomeric transition may be either 70 keV (L m case) or 73 keY ( L H I case). The partial lifetime of the transition is given from the half life t of UX 2 (1.14+ 0.01) min and the UZ/UX2 branching (0.13+0.03)x 10 -2 t] xp = (5.3-t- 1.3) x 104 sec. From the point of view of the lifetime the most likely multipole orders are M4 and E4. The LH~/L m ratio of an M4 transition is 0.18 (ref. x6)), resulting in an L m 70 keV assignment of the 52.6 keV conversion line. The corresponding ratio for an E4 is 1.7, leading to an L~_. 73 keV assignment. The single-particle half life of a 70 keV M4 transition is 4.4 x 105 sec, using the Weisskopf estimate 17) and ( l + c c r ) = ( I + 0 C L × 1 . 4 ) = 2.2x l0 s (ref. 16)). For a 1' T h i s v a l u e h a s b e e n o b t a i n e d in t h e p r e s e n t i n v e s t i g a t i o n .

658

S.B.I~I~NHOLM AND O. B. NIELSEN

73 k e y E4 transition one obtains, similarly, t½ = 1.5 x 10+ see with (1 +aT) = 3.5 x 104. Consequently, the isomeric transition will be about 8 times enhanced if M4 or 3.5 times hindered if E4. 4. Discussion 4.1. THE ODD NUCLEUS Pa~ Protactinium 234 has 91 protons and 143 neutrons. The appropriate Nilsson orbitals [N,n z, A]t2n, are [530]½- for the 91st proton and [743]½- for the 143rd neutron with [631]½÷ lying close by is). Coupling spins parallel 19) we get as the lowest configuration p[530]½-, n[743]½- with K, n = 4 + and as a close lying alternative p[530]½-, n[631]½ + with K, n = 0 - . These assignments explain the presence of the two isomers and the differences in their//-decay modes. Also the occurrence of the isomeric transition is qualitatively consistent with this interpretation, resulting in an M4 assignment. Quantitavely one should, however, expect a somewhat retarded M4, since the transition violates the selection rules in the asymptotic quantum numbers of the Nilsson states involved. A more unambiguous determination of the multipolarity is therefore particularly desirable. (High resolution measurements of the L-conversion lines may provide the answer.) The l o g f t values for the fl-decays Th 234 (0 +) ~ Pa 234 ( 0 - ) and Pa 234 ( 0 - ) -~ U 23~ (0 ÷) are 6.5 and 5.5, respectively. The latter value is somewhat low for a first forbidden unhindered transition. The l o g f t values for the fl-decay to excited states of U 234 are consistent with a 0 - assignment to U X 2. The fl-decay of Th 2a4 has a 33 ~ branch to excited states ~ 94 keV above the 0 - , UX2 level (of. subsect. 3.1). The presence of a 92 keV MI transition indicates a 1state at this energy. In addition, there is a 64 keV E1 transition in cascade with a 29 keV, M 1 and/orE2 transition 2o), indicating a positive parity level very close to the 1- level at 92 keV. Such levels are expected from the available Nilsson orbitals. The 0 - state may further exhibit rotational excitations with even- and odd-spin states displaced. A more detailed interpretation of the two-particle level spectrum of Pa 234 will, however, require further investigation. 4.2. LEVEL STRUCTURE OF U~ The population of three 0 + levels in U 2a" allows for a comparison of the nature of tbese states. The fl-decays to the two excited 0 + states are 30 and 16 times hindered relative to the fl-decay to ground, as seen from the l o g f t values (fig. 4). The excited 0 + states, on the other hand, differ very markedly by their E0 decay mode to ground (figs. 1(a) and (b)). The fl-vibrational level at 811 keV has a/~o value (cf. tables 1 and 2) poSl, _ WK(EO 811) = 0.4 = 1.3+0.3 W~(E2 765) 0.3

THE UXs ISOMER OF PatM

659

in good agreement with the value found in the a-decay o f Pu 23a (ref. t l)), whereas for the 1045 keY level 0.001 pot°45 < - 0.002. 0.60 This result seems to lend support to a distinction between collective//-vibrations, on the one band, and independent-particle 0 + states expected to appear above the energy gap, on the other. At least there seem to exist different 0 + states easily distinguishable by the E0-decay mode. The distinction is rendered less clear though by the observation o f the 236 keV E0 transition between the two states; and in another case, where two excited 0 + states have been observed, H f x78 (ref. 21)), the two branching ratios/~0 turn out to be almost equal. The classification o f 0 + states into collective vibrations and pair excitations can therefore not in general be recognized experimentally through a determination o f the E0/E2 intensity ratio. The authors are indebted to Professor Niels Bohr for the excellent working conditions at his Institute, and to Professors A. Bohr and B. R. Mottelson for stimulating discussions. Thanks are due to K. N y m a n Madsen who assisted with the calculations and Mrs. Helle Nordby for essential aid with the chemical preparations. We would also like to thank Mrs. S. Hellmann for help in preparing the manuscript. References 1) 2) 3) 4) 5) 6) 7) 8) 9) I0) ! 1) 12) 13) 14) 15) 16)

17) 18) 19) 20) 21)

S. Bjornholm and O. B. Nielsen, Nuclear Physics 30 (1962) 488 H. Schneider, P. W. Lange and J. W. L. de Villiers, Nuovo Cim. 14 (1959) 11 M. G. Bouissi6res, N. Marry and 1. Teillac, Comptcs Rend. 237 (1953) 324 L. R. Bunney, N. E. Ballou, Juan Pascual and Stephen Foti, Anal. Chem. 31 (1959) 324 O. B. Nielsen and O. Kofoed-Hansen, Mat. Fys. Medd. Dan. Vid. S¢lsk. 29, No. 6 (1955) P. Gregers Hansen, O. B. Nielsen and R. K. Sheline, Nuclear Physics 12 (1959) 389 E. F. de Haan, G. J. Sizoo and P. Kramer, Physica 21 (1955) 803 G. T. Wood, Phys. Rev. 119 (1960) 2004 C. V. K. Baba, unpublished C. J. Gallagher and T. D. Thomas, Nuclear Physics 14 (1959) 1 S. Bjornholm, M. Lederer, F. Asaro and I. Perlman, (UCRL-9938), to be published I. Marklund, B. van Nooijen and Z. Grabowski, Nuclear Physics 15 (1960) 533 P. G. Hansen, K. Wilsky and S. Bjornholm, to be published I. H. Forreat, S. J. Lyle, G. R. Martin and J. J. Maulden, J. lnorg. Nucl. Chem. 15 (1960) 210 N. Feather and E. Bretscher, Prec. Roy. So(:. A165 (1938) 530 M. E. Rose, Internal c o n v e r s i o n coefficients (North-Holland Publ. Co., Amsterdam, 1958); L. A. Sliv and I. M. Band, Coefficients o f internal conversion o f g a m m a radiation ( A c a d e m y o f Sciences o f the U S S R , M o s c o w ; Leningrad, 1958) G. J. Nijgh, A. H. Wapstra and R. van Lieshout, Nuclear spectroscopy tables (North-Holland Publ. Co., Amsterdam, 1959) F. S. Stephens, F. Asaro and I. Pcrlman, Phys. Rev. 113 (1959) 212 C. J. Gallagher and S. A. Moszkowski, Phys. Rev. 111 (1958) 1282 G. T. Wood, unpublished results, Copenhagen (1959); R. Foucher, J. Merinis, A. G. de Pinho and M. Valadares, Comptes Rend. 255 (1962) 1916 C. J. Gallagher, H. L. Nielsen and O. B. Nielsen, Phys. Rev. 122 (1961) 1590