1.E.3: I Nuclear Physics AI04 (1967) 588--608; (~) North-Holland Publishing Co., Amsterdam

3.A [ Not to be reproduced by photopriut or microfihn without written permission from the pubhsher

T H E N U C L E A R g - F A C T O R S OF T H E ~-[523],[, STATES IN 2aTNp AND 2a9Np

AND T H E LEVEL S T R U C T U R E OF 237Np

CHRISTIAN GONTHER-t and DANIEL R. PARSIGNAULT ll" CaliJbrma Institute of Technology, Pasadena, Califorma Tt?

Received 17 July 1967

Abstract: The nuclear g-factors of the 59 keV level in 2aTNp and the 75 keV level m eagNp were measured by the a-y perturbed angular correlation technique in a d~fferential and inte~al measurement, respectively. Liqmd sources were used to minimize perturbation by internal fields. The ratio of the g-factors is g.~- (~3VNp)/g~.- (-~rNp) - 1.04±0.09. The Np ions are expected to be m the 6 valency state (5f~). With a paramagnetic correction factor fl estimated to be 1.68t:0.11, g.i.(~a~Np )= 0.762-t 0.060 and g l--('z39Np) = 0.793--0.097. The experimental values of the g-factors are in good agreement with the theoretical predictions of the Nilsson model.

The y-spectrum following the decay of ~41Am was studied with a Ge(L0 detector. The new results are discussed.

E RADIOACTIVITY ~lAm, "-43Am; measured :~7(0, H, t), Ey, I~.. ~39Np level deduced g. 2~;Np deduced levels, g, J, ~, B().). Ge(Li) detector.

I. Introduction

Up to now, only one at tempt to measure a g-factor using the ~-y angular cor-

relation technique 1-3) is known, namely the measurement of the g-factor of the

59 keV level in 237Np by Krohn et al. 4). I t is the most favourable state to study many

of the part icular difficulties of the ~-y angular correlat ion method because of its long

half-life (63 nsec) and a strong c~-7 cascade which is known to have a large anisotropy.

Since the early measurements of Krohn et al. have a low accuracy and are doubtful

because of u n k n o w n internal per turbat ions 5), we have re-investigated this case.

Furthermore, we have measured the g-factor of the 75 keV level of 239Np (T.} = 1.40

nsec) under similar conditions. Therefore the determinat ion of the ratio of the g-

factors is independent of the paramagnet ic correction. The results of these measure-

ments are presented in this paper. An evaluat ion of the paramagnet ic correction is

also made. In addit ion, an investigation of the nuclear structure of 237 N p is presented.

2. Experimental procedure

The experimental set-up used in the e-': angular correlat ion measurements de-

* Now at the lnstitut ftir Strahlen- und Kernphyslk der Umversitat, Bonn, Germany. ~t Now at the Van de Graaff Laboratory, The Ohio State Umversity, Columbus, Ohio.

ttt This work was performed under the auspices of the U.S. Atomic Energy Commission. Prepared under Contract (04-3)-63 for the San Francisco Operations Office, U.S. Atomic Commission.

588

Np ISOTOPES 589

scribed below consisted of a conventional fast-slow coincidence system and a small table magnet ( ~ 20 kG, with ~ 6 mm pole pieces and 4 mm gap). The s-particles were detected by a 60 #m thick silicon surface barrier detector which was mounted inside a vacuum chamber which surrounded the pole pieces. For the °/-detector, we used a 56 AVP photomultiplier with a 3.8 cm × 4.4 cm Nal(T1) crystal. A schematic diagram of the arrangement is shown in tig. 1. For the differential measurements a commercial time-to-pulse-height converter (E.G. & G. Model TAC 107) was used in connection with a RIDL 400-channel analyser.

u v ~

MAGNETIC SHIELDING

LEAD SHIELDING

p

~. / ':(" i [q \ - a DETECTOR

WATE?~ COOLING ~ ...............

2 ~, R!DL 31-18 ~[--~A~-- PREAMP I

3 1 _ ~ . . . . . . . . . . . TO SLOW s,g:~-I colNc

R e d RSSR 107

I FAST

12R_~A~_ ~_

1 TO FAST

COINC

0 i ~ C ~ .... J

SCALE

Fig. 1. Schematic diagram of thc ~-7 pcrturbed angular correlation apparatus

In the measurement of the 7-spectrum following the decay of 241Am and 243Am, a 1 cm 2 x 4 mm Ge(Li) detector was used. The detector was connected to a Tennelec TC 130 FET preamplifier, TC 200 amplifier and a Nuclear Data 4096-channel analyser. The energy resolution of this system was 2.4 keV at 120 keV.

Liquid sources had to be used in the ~-'/angular correlation measurements in order to minimize perturbations due to internal fields. Such were prepared in the following way: the radioactive A m oxide, which was obtained from Oak Ridge National Laboratory, was dissolved in 6N HCI. The sources used in the experiment were pre- pared by evaporating the HCI solution to dryness and dissolving the chloride in HCI

590 c. GUN'rlIER AND D. R PARSIGNAULT

solutions of the desired normality. The liquid was enclosed in small polyethylene bags ( ~ 3 m m x 4 m m and ~ 0.01 mm thick), which were prepared by heat welding. The energy resolution was completely lost due to the relatively thick layer of liquid. Finally, it proved necessary to circulate continuously water-vapour saturated helium gas through the vacuum chamber to prevent the source from drying out.

3. The 0-factors of the 59 keV and 76 keV levels in 23~Np and 239Np

3.1. MEASUREMENT OF THE g-FACTORS

The ~t-~ angular correlations used to determine the 0-factors of the {- isomeric states in 237Np and 239Np are those between the or-particles in the decay of 241Am

Am 243

keV I I°I~ 117.8 7 TI/2~ lOOps ~879o/°

Z 74.6 ~ - Tl/2=l.4Ons /

+

o {+

9 159 ~- -

105.O { -

59.6 { -

o -~+

N 239

Am241 5

33."7-//7-- '- 2 + i - ~ / //12"7~

Ti/2 = 80 ps ----.-/RR.(3°/"

TV2 = 63ns /

N~ 37

Fig. 2. Partial disintegration schemes of UIAm and 24aAm.

and 243Am, respectively, populating the 2 s- states and the depopulating 7-transitions to the ground state (see fig. 2). Both angular correlations have a vanishing .44 co- efficient due to the El character of the y-transition 3). The attenuation of the angular correlation in the decay of 241Am has previously been measured for various liquid and solid sources. Krohn et al. 5) found pure exponential attenuations of moderate strength for three inorganic solutions (Z2 ~ 107 sec-~). Very strong integral at- tenuations (G 2 < 0.1) were found by Flamm and Asaro for various solid sources 3).

We have repeated the time differential measurement of this angular correlation with sources of Am chloride dissolved in dilute HCI. Measurements were with 2N and 6N HC1 solutions. Our values for the attenuation parameter 22 are larger than the value measured by Krohn et al. Also we find, in contrast with these authors, a

Np ISOTOPES 591

dependence of the attenuation on the concentration of the HCI solution. For 2N HC1 solutions the attenuation was approximately 1.5-2 times smaller than for 6N HC1, varying somewhat for different sources. The variation can be attributed to slight changes in the normality of the source solutions during the experiment. In fig. 3, one of our measurements with a 6N HC1 solution is shown (2 z = (3.40+0.21) • 107

sec- 1); this measurement was with a time resolution of ~ 7 nsec. The variations in the attenuation might be caused by a change of the valency

states of the Np ions for different normalities; this would suggest that the attenuation results from an interaction of the nuclear magnetic moment with the magnetic field from the 5f shell. An estimate with reasonable values for the magnetic field and the

!' t i -05 ~ A2(T) = A2(O) e -xzT ~,~ = (340-+021)10 r s-'

-03 ~},, ,

_o 'L : . ,, _

-0 03 ' -

-00Zr-i i

-0 01 I I ~ ! 0 I0 20 30 40 50 ~" In ns

F i g . 3. A t t e n u a t i o n p a r a m e t e r f o r t h e ~ -7 a n g u l a r c o r r e l a t i o n in t h e d e c a y o f 24*Am, s o u r c e s o l u t i o n

6 N H C 1 .

relaxation time of the 5f shell shows that this mechanism might at least partly be responsible for the observed attenuation. In order to avoid systematic errors in the ,q-factor measurements due to different paramagnetic corrections resulting from different valency states, we used in both measurements reported below sources made from solutions with the equilibrium concentration of 6N HC1.

One of the differential measurements of 237Np made in a magnetic field of 12.5 kG at 0 = 90 ° and 180 ° is shown in fig. 4. The data were fitted with a least-squares routine for i > 10 nsec to a cosine function including a shift of the time zero point and of the horizontal axis. The attenuation parameter was not varied in the fit but taken from a separate measurement using the same source. For the Larmor fre-

592 C. G [ I N T H E R AND D. R P A R S I G N A U L T

quency, we obtain (DL(237Np) -~ -(7.650-0.35) • 107 sec -1

The sign of co L is taken from ref. 4).

The result for the time zero shift is of special interest because it can give some in-

dication of the speed with which the electronic shell reaches a stable configuration. It is an important question for the integral a-factor measurement of the short-lived 75 keV state in 239Np. As a result to our fit, we obtain a time zero shift of 2.7 nsec,

which can alternatively be interpreted as a rotation of the angular correlation by

12 ° . Approximately 3 ° of this rotation can be attributed to the rotation of the s-particles in the magnetic field. Unfortunately, our experimental arrangement was

o2!

i ! ,[ i , !,.. ~,, i ~ ! , , I

ooo ,, !' I

,)l !i' ! [

o ib z-'o ~b .... 4~ "~ In rIs

. l I

i i I I ~~ ! ] ' ]

j!l i J L

I'

"7"-

I _ _ 1 50 60 70 8O

Fig. 4. Differential measurement of the ~-7 perturbed angular correlation xn °~'tXAm.

very sensitive against a misalignment of the source due to the short distance between

the ~-detector and the source (1.5 cm). We realized this only after completion of the measurement and a check showed, that the remaining rotation is probably due to such a misalignment. We can therefore only givc an upper limit of ~ 2 nsec for the time in which the electronic shell restores itself after the :~-decay.

Our result for COL (237Np ) can be compared with a recent measurement by Heisen-

berg 6). Using our values for the magnetic field, his result of COL = --(7.45__+0.03)" 107 sec-1 is in good agreement with our value.

The a-factor of the 75 keV state of 239Np was determined by measuring the integral ~-~ angular correlation in a field of 18.5 kG. The result is shown in tig. 5. The fit of the data to the theoretical function included a decentering (T ~') as indicated

Np ISOTOPES 593

in the figure. The rotat ion of the ~-particles in the magnetic field was calculated as

2 .5+0 .5 °. The result for OJLZ is

fOL,r(239Np ) = _ 0.238__+ 0.017.

It includes a 7 ~ correction due to internal perturbations, (G 2 = 0.935+0.010), which was calculated from the 2 z of 237Np and T~ = 1.40+0.06 nsec for the 75 keV

J a j ~ a decentenng b spin rotation

12 c rotation of a - particle

I0

01

1 9b m l 120 150 L~ 180

Fig. 5. Integral measurement of the "~-'F perturbed angular correlation m 2a'~Am.

state 7). Corrections due to ~ 12 ~/o coincidences proceeding over the 118 keV level and a ,~ 2 % contaminat ion of 24tAm are negligible. With the above values, we obtain for the ratio of the g-factors

g~-- (239Np) -- 1 .04+0 .09 . g ~_ (237Np )

In deriving this value, we have assumed that the field at the nucleus reaches a

594 C. G(JNTHER AND D. R. PARSIGNAULT

stable value in a time short in comparison with l nsec. We cannot confirm this from our experiment. However, there are strong arguments that this assumption is valid. From experiments with rare-earth elements, it is known that this condition is fulfilled for the 3-- rare-earth ions in dilute solutions of the HC1 after "s-decay and electron capture decay 8,9). Especially the latter decay leads to strong ionizations of the ions similar to the ionizations following e-decay 1o.~); this shows that the electronic shell reaches its stable configuration, even after high ionization, in a time short in comparison with nsec if the ion is in thermal equilibrium with its surrounding. The stopping time for the recoiling nuclei after :~-decay in water can be calculated from their range 12) ( ~ 0.1/~m) and assuming a constant deceleration to be psec. Thus the thermal equilibrium should be reached in a time of the order of the correlation time in water re ~ 10 psec, which is sufficient to ensure the validity of the above assumption.

3.2. T H E P A R A M A G N E T I C C O R R E C T I O N

In order to derive the g-factors of the two ~ states separately, the effective field Hcfr at the neptunium nucleus has to be known. The external field Hcx t is modified by an induced field Hi.e, which results from an orientation of the unfilled 5f shell of the Np ion in the external field. For liquid sources, Herr can be written in the form

Herr = H~xt + H,.d = f lH, x,. From our measured value of COL, we obtain: g~-(237Np) = (1.28+0.06)//3. One can get an estimate of the paramagnetic correction factor/3 from the experi-

mental values of the ground state g-factor 9~÷ = 1.28+__0.24 and the ratio 9.~-/9~÷ =

0.537 [ref. x3)] as /3 = 1.85+0.37. In an attempt to calculate/3 for solutions of Np in HCI, a number of problems arise.

Basic for the calculation of [3 is the knowledge of the electronic structure of the Np ions. In acidic aqueous solutions, Np exists 14) in valency states from 3 + to 6 ÷, the most stable being the 6 + state. The 3 ÷ and 4 + ions are thought to be simple cations, whereas the 5 + and 6-' states can also exist in the form of NpO~- and NpO2++, respectively. However, the latter molecules are presumably formed in times much longer than those relevant for this experiment (0.1 psec) [ref. 14)]. In all Np ions, the electrons beyond the radon core are in the 5f shell. After the ~-decay, one expects high ionizations of the atom 1 o, 11). Ionizations of shells other than the 5f shell should be reduced 8,9) in a time < 0.1 nsec, therefore we expect that the Np ions are in the 6 + or eventually in the 7 + state.

Next, one has to consider possible crystalline effects. Paramagnetic ions in liquids are believed to form stable paramagnetic complexes together with the surrounding solvate shell. The Brownian motion of the surrounding molecules causes reorien- tations and vibrations of the paramagnctic complexes 15). For example, in ref. is), the paramagnetic ions of the rare earths in liquid solutions are considered as being surrounded by six water molecules in the form of a distorted octahedron. For the rare-earth ions, the paramagnetic correction factors without taking into account

N:) ISOTOPES 595

these crystalline effects are in good agreement with experiments. However, the crystal field splittings for the transuranic elements are much larger than for the rare earths [see for example ref. 16)], and the crystalline effects, if present, would completely de- termine the paramagnetic correction factor. We do not know of any information concerning these crystalline structures of transuranic elements in liquids and will therefore neglect them. An argument in favour of this might be that it seems likely that these complexes are formed in times > 0.I psec.

Under the above assumptions and taking into account the large spin-orbit coupling of the 5f shell, /3 takes the form

/3 = 1 + . ( J z ) Hsr _ 1 gSpB(J+ 1) l i s t , J Hex t 3 k T

where <J~>/J describes the average orientation of the 5f shell in the external field and Hsr the expectation value of the field produced by the 5f shell at the nucleus for G = J .

The calculations of /3 for the various valency states is still forbiddingly com- plicated. No reliable calculation of Hsf can be performed because of its strong de- pendence on the departures of the 5f wave functions from Russel-Saunders coupling (ref.17)). Fortunately, for the two valencies considered most likely in our case, the situation simplifies considerably. The 7 + state has no paramagnetic correction, e.g. // = 1; this case seems thus excluded. For the 6 + state, the difficulty resulting from intermediate coupling does not exist, since there is only one 5f electron. The experi- mental informations of refs. 18,, 3) can be used with our value for e) L to derive 93- and fl for 237Np. For the hyperfine constant of the excited ~- state of 237Np in the

free 6 + valency state, one obtains A / h e = 0.0237 c m - ~ (x = 0); this gives fl = 1 +0.52/ 9,~- and together with our experimental value for [I • 9~-

9! - = 0.762+0.060, fl = 1.68+0.11.

Our results for /3 is in agreement with the above estimated value. As a further check, one can calculate with these values the quantity ( r -3>s r for the Np +6 ion as <r-3>s~ = 7.3+0.6 a.u. There are several estimates of this quantity available in the literature. The most reliable ones seem to come from a use of the constancy of the ratio ~/(r -3> for all the actinides, where ~ is the spin orbit coupling c(;nstant ~9,2o). Eisenstein and Price 21) calculate ( r - 3 > = 6.0 a.u. from their experimental value of and the ratio ~/<r -3> = 360cm-~/a .u Another value can be obtained from the experimental ,6) value ( r - 3 > 5.33 a.u. for +4 = Pa , which has also one electron in the 5f shell. Using again the spin-orbit coupling constants for Pa +4 and Np +6, one obtains <r - 3 ) = 8.0 a.u. for Np ~6. Thus, a value ( r - 3> = 7.0+ 1.0 a.u. should be fairly reliable and would be in good agreement with our value.

The above arguments seem to support the assumption that the Np ions exist as free N p +6 ions for times longer than ~ 10 -7 sec after the ~-decay of 24~Am. In a recent experiment, Heisenberg 6) determined/3 experimentally from the temperature

596 C, G i J N T H E R A N D D, R, P A R S I G N A U L T

dependence of ~o L assuming fl - 1 o c 1/T. He obtains a value # = 1.26_+0.06 which disagrees with the "theoretical" value obtained here. Using his value for fl, we would obtain 9~- = 1.01 +_0.05. As will be discussed below, this value seems to be too large,

F ~ 59 --99~7=i05 - ................................

1 :~ ~ ! 2 5

I0 ~

,o'~k , ~

125~

• 115 ~

[ ~ il 1 4 6 2 0 7

I tlimC 170 f', i ;;~ ',65~ ~:~

" - " " ~ ~. ~ i t 221 265

k l T -~ L~za ~

~bo s ~

i o ~ 3 5 2 7 ' F 32.2 *t°, 569

f : f ! t 3~e -~ ,- , t i ,

g F "-9 L

I

,'~o5- r 6 6 3 4~o . . . . . ~ e:,o 1 iO'~- I i 77_3

i ! 642 ~? It 689, ; i ; i , . ~ I , ! . ,"

~696 ~,o.: ' 4 I 0~1 . - 4

[ h',J~ I ,.. '~ . . . .

• ~:_;. ~ ~ ,. ; ~.- ". - . ' , : " ?~ • -j

: . ?~ . . . ' ~ : . . ~ . . . , ~ . . . . .,,,-~: ~f,, i" ' " " "-~-~-¢ - * • '~" "~ • "

" " " " " ' " " " " ~ - i i " ' " I

IOIL . . . . t. , J

¢,,00 700 800 900 GAMMA RAY ENERGY keg

597, !

]

Fig. 6. T h e ) ,-ray s p e c t r u m of ~aTNp.

Np ISOTOPES 597

since it would require a rotational g-factor gR = 0.78__0.06. We would therefore like to conclude that, although fl around 1.7 seems rather likely, additional in- dependent experiments are necessary to get a reliable value for g~-. Especially, a direct measurement of the ground state magnetic moment would be of great help.

4. The gamma-transitions in 237Np

The gamma-transitions following the ~t-decay of 241Am have been the topic of several investigations. Following the completion of the present investigation, two studies of the ~,-ray spectrum of 237Np using Ge(Li) detectors have been published (refs. 23,24)). Reference to earlier work can be found in these papers.

A typical "/-ray spectrum of 2aVNp is shown in fig. 6. Since most of the ~-decay populates the 59.6 keV and the 103.0keV levels (84 ~ and 1 4 ~ , respectively), an absorber made of 1.6 mm of Sn and 1.7 mm of Cu was used to absorb the 59.6 keV 7-rays.

Table l lists the energies and relative intensities of the "/-ray transitions. The errors in the intensities include estimated errors from the background subtraction, the absorption correction and the sensitivity of the detector 25). The intensities of the 60 keV, 99 keV and 103 keV 7-rays were derived in a separate run without absorber. The intensity of the 99 keV line was corrected for the KX-rays with the help of the intensity of the 114 keV line and the known ratios for the different X-ray K-lines.

Our intensities are compared with the most recent data of Michaelis 22,23) and Lederer et al. 24). Our intensities are in good agreement with the results of Michaelis. The agreement with the data of Lederer et al. is somewhat less; above 600 keV, the intensities reported by these authors are approximately a factor of 2 larger than ours, far outside the experimental uncertainties. Some of the intensities from the K = ) bands can be compared with the data of Yamazaki and Hollander 26) in the decay of 237 U. All intensity ratios for which a comparison is possible, are in excellent agreement and prove the correct interpretation of these lines.

We have also made an attempt at measuring the gamma-ray transitions in 239Np. The nucleide 243Am (T1 = 7650 y) alpha-decays to 239Np, which in turn beta- decays to 239pu with a half-life of 2.35 d. The latter decay leads to a complicated gamma-ray spectrum which completely masks the weak gamma lines in the 243Am decay, if both isotopes are in equilibrium. Therefore, an ion exchange was made to separate 243Am from its daughter product 239Np [ref. 27)]. Although we obtained a

complete separation, attempts to measure the gamma transitions following the alpha-decay of 243Am were unsuccessful: the amount of 239Np grown in within the first 3 h following the ion exchange separation was already ~ 10 ~o. Therefore, this method of direct measurement is insufficient in the case of 2 a9Np"

598 C. GL'NTHER AND 1). R. PARSIGNAULT

T A B L E l

l=nergms and relative intcnsttms o f g a m m a rays in ~SrNp

Energies a) (keY)

59.6 e) 99.0 e)

103.0 ") 113.9 122.8 125.3 139.2 0 .55--

Relative intensities

Mtchaehs ~') Th~s work

ref. '-'~) ref.

(4.8 - 1.5)106 (7 :" 1 )10 ~ (6.34- (2.7 _-- 0.4)10 z (3.5.= 1.0)10 ~ (3.3-- ( 2 . 8 : 3 - 0 . 4 ) 1 0 a (2.7." 0.8)10 ~ (2.74- 68.4 :- 8.2

163 :L33 220 4- 55 180 :" 537 _k16 667 --170 470 t:

0.11 146.5 e) 55 :- 8 149.9 17 8 a- 3.5 164.7 11.1 ± 0.7 169.6 21.9 -'-- 0 7 175.5 2.334- 0.23 191.7 2.45_4- 0.25 207.8 100 ~- 5 221.2 5.44_'--_ 0.33 232.9 0.53 -'.- 0.05 247.1 0.34-': 0.12 264.6 4-1.0 1.42-1 0.36 267.2 3.47 ~: 0.35 275.6:'..:I.5 0.67~: 0.13 293.5 2 .28:4:0 .20 311.9 ~- 1.0 1.42_-~: 0.30 322.1 209 :k 1.0 332.34-1.0 28.8 4- 4.3 335.2:1-1.0 67.4 ! : 4.0 369.0 1 371.0 j 41.3 4- 2.0

376.4 19.5 -- 1.0 383.6 4.22 ,: 0.42 391.6 0 . 5 9 c 0.15 419.3-a- 1.0 3.78::t: 0.20 426 44-1.0 3 .46~ 0.21 431.7!=1.0 0 . 1 0 ± 0.07 435 .3 ' -1 .0 0.30_'-- 0.07 444.04-1.5 0.304- 0.18 451 .8=1 .0 0.61 -I: 0.45 455.1 ~1.0 1.20"_ 0.25 459.24-1.0 0.50-'.: 0.15 514.0-'-.2.0 0.58 i: 0.15 597.9--1 .0 0.66-',: 0.08 619.4 7.234- 0.40 642.4- : 1.0 062~- 0.16 653.5 3.21 ": 0.22 663 0 46.1 ~- 2.5 681.4 0 .26: : 0.08 689.5 1.60=- 0.12

37 4- 13 19 !: 7 24 4- 6

8 :i: 3

100 : . 20

16 - - 4

11 4- 4

19 4- 4 80 4- 16

50 ~ 12

11 ~_= 3

7 !: 2

13 i: 3

58 '-- 12

13 -~ 4

80 :l:

20 ;- 23 :=

3.7-- 100 4-

4 . 7 ~

3.3 -!=

2 .8-- 12 .~ 28 --

78 L

33 4-

18 k 3 . 7 ± 1.5 t_ 4.84- 4.5_::

0 5)-

2.34- 2.3 ~_

1.5-- 6,0 :.

7 3-4- 50 zi:

1.6-- 332=

696.04-1.0 0.65'-- 0.10

Lederer et al. ~ ) __A_ssi_g_nmen.t__

• ,,s) ref. °-'1) Ir"r~Kl Iv-qK~

1.0)10 ~ (6 4 + 0 . 1 ) 1 0 n ~- :~ ~- ,~ 1.1)10 a (4 3.'--.0.5)103 ,~- :~ ~ 0.8)10 ~ 3 .4 . 103 }- ~. ~

125 ~: 70 KX-rays 60 "<360 4>t- ,~ ~- ]. . : g

95 890 ._360 ~ • g .}+ ~-

29 4- 11 .~ f~ .?~,~ 20 4.3 ~ - ~ - .~.-' ~-

6 7.3-- 3.7 ~ ½ .}-~- 7 18 4- 7 42--:s, • . ~ - ~

1.8 20 I00 ~= 18 ~---~ ~-

1.6 6 , 2 ~ 1.4 -~-½ .~- • ~- ½- ,, ~.-,~ ~-~

1.1 4.8 ~- 1.2 ~_ ~ .~, ,~

~--~ ~-~ 9. i -~ 0.9 3.7z+- 1.2 ~"½ 2 '

4 4 .8- - 1.4 contaminaUon '~) 6 29 4- 5 ~-½ J t- .- i

32 4- 7 ½-½. ~~ 16 102 h 18 ,2s+ t2 ~ ~.

~+½ ~,-~ 7 50 4- 11 ~., ~. .~+

5 29 4- 5 ~.~½ 9 + 1.5 5 . 2 5 1.4 .}r ½ ,~+f] 0.7 1.5 6.42: 1.8 ~." ½ {+ ~- 1.4 5.9-= 1.6 {" ~- ~+ ,~

803 keV :~ ~ 803 keV ~ ½

0.3

0.7 < 3 . 0 ~ ~ }- ½ 0.7 ,~'-½ :~'- ~

~-~ ~--~

0 9 1 .8= 0.9 } - 3 ~ - ~ 1.8 15.0-L 0 9 ~ - / / .{- '-]

1.8_'.+_ 0.5 771 keV J~+ .) 3.6 9.8_"- 0 5 ~ - f l ~ - ~

,~.. [~ ~-~ 15 95 4- 5 .~ o.8 1.o.> 0.3 .~- ~ ~+ 1.3 7 . 7 2 : 0 . 9 .~-/3 ~," ~

1 .8: . 0.4 / 771 keV ~-~

Np ISOTOPES

TABLE 1 (continued)

599

Relative intensities

Energies a) This work ....... Mlchaehs ~) Lederer et aI. t,) Assigmnent (kcV) ref. 22) ref. z3) ref. z~) It.~lK1 I ~ K f

710.3 0.58-: 0.12 1.6,., 0.4 771 keV :~- 7227 242 1:1.2 30 !.6 28 -- 9 50 _--_ 4 ~-[/ ~ ?, 7307:'.-1.5 0.11 _iz 0.03 738.1 !=1.5 0.83 - 0.12 2.1.! 0.4 771 keV z- 757.1:i_1.5 1.12" 0.05 1.3-t- 06 2.3-- 0.4 ~-/q ~ ~ 768.6:1_-2.0 0.87 i: 0.09 1.9__ m 0.9 1.4_~ 0.5 803 keV 7~, 771.3±2.0 073=_ 0.07 1.8--__ 05 771keV ~:~ 803 0~2.0 0.13-= 0.03 803 keV ~" 863.61:2.0 0.12-:: 0.03 872 7:t.:2.0 0 10)= 0.02

a) The energies are given with a precision of -+_0 5 keV unless otherwise noted. b) Normahzed to rot. (207.8 keV) = 100. c) The intensmes for these hnes were measured with no absorber. a) Compare fig. 1 of ref. ze,) wxth fig. 6 in thas paper.

5. Discussion

5.1. THE LEVEL SCHEME OF ~7Np

Fig. 7 shows the level scheme o f 237Np. The nuclear levels in this decay scheme

are largely based on the invest igat ion o f the ~-decay o f 241Am by Baranov et al. 28).

More recently, the decay scheme has been confirmed and extended by Michael is 23)

and Lederer et al. 24). In the fol lowing we discuss the different bands with special

emphas is on the new results of our investigation.

5.1.1. The ~ + [642]T band. Aside f rom the well s tudied -} + and ~ + levels in this band,

this s tudy confirms the existence o f the )2 ~-+ level at 129.5_+0.5 keV. The state was first tentat ively p roposed by Baranov et al. 28) and also pos tu la ted in refs. 23,2,).

Our ass ignment is based on two g a m m a t ransi t ions popu la t ing this level; the )3_-

(K = ~r) --' -~2J-+(K = ~}) t ransi t ion (175.5 keY) and the 9+(K = ½) ~ t@+(K = z s)

t ransi t ion (322.1 keV). The reduced t ransi t ion rates, which involve these t ransi t ions ,

are included in tables 3 and 4. They s t rongly confi rm the given assignment.

Fur the rmore , we pos tu la te tentat ively the _½_5+ state o f the g round state band at

256.4-t-0.9 keV. I t is based on the in te rpre ta t ion o f the 139.2+0.5 keV line as the

~s-- (K = -}) ~ )~s-+(K = s) t ransi t ion, which is suggested by the energy and intensity

o f this line (see subsect. 5.1.2.).

The energies of the [642]T ro ta t iona l band can be compared with the ro ta t ional fo rmula

E = E o + A I ( I + 1 ) + B I 2 ( I + 1) 2 . (1)

Wi th A = 4.74 keV and B = 0 derived from the energies o f the ~ + and 9z + states,

the calcula ted energies of the ~2!-+ and )2 -5+ states are 128.1 keV and 260.7 keV

c o m p a r e d with the exper imenta l values o f 129.3+0.5 keV and 256 .4+0 .9 keV. The

600 C. GtiNTHER AND D. R. PARSIGNAULT

!n 0 0

l~.'p I S O T O P E S 6 0 1

7 7 , 03 r - ~

~ , , iI o, le4

~ ~ ~"7"------~

z ~

OU O 0 0 +1 +~ +~ +1 0 ~c .D (~DO~D OU b.- ff'~ P,I QD b.- I~ b.-

+ +

o ~ 0 r ~ - ~

I - - l e J

i 1

LO +

+ + + +

- - ~'~ I ~" u~

o o + l & l 0

602 C . G U N T H E R A N D D . R . P A R S I G N A U L T

d i sag reemen t is n o t surpr is ing. As has been po in t ed ou t by R a s m u s s e n *, one expects

large Coriol is in t e rac t ion for Ni l sson states derived f rom shel l -model states with

h igh j. The [642]]" g r o u n d state in 237Np results f rom thei-½ -3.- shel l -model state, the

p r o t o n state with the highest observed j . In fact, there are two close- lying states

([633] T a n d [651]T, see e.g. table I I l c in ref. 30)), wi th very large Cor io l is coup l ing

ma t r ix e lements ( ( K > I J+ i K < ) ~ 6). A s t rong mix ing of these states has been no ted *t

TABLE 2 Relative intensity hmits of gamma rays in 2aTNp

Intensity Energy Intensities hmits Assignment

(keV) Mtchaelis 2~) Lederer e t al . 24) this work l i . ' hK t lr.-trKt

157.3--1.5 27 _+_8 < 0.2 ~- ~- .~-+ ~ 260.4--1.5 1.0_4-0.4 < 0.1 ~ ~ ~,~+ 299 8 _-k 1.5 3.0_-k. 0.9 < 0.3 ~- ~ ~- 330.2--.0.7 < 3.0 ~+ ½. ~"- ,} 348.5~2.0 1.3!~0.6 < 0.3 .~' ~ g- ~ 358 . . . . -~-2 2.3 ~_1.2 < 0 3 ~5-x.2 ~+ ~2s 531.1±0.5 ~1.2 <0.15 ~-~ ~ - ~ 549 --2 < 0.12 ,)-~ ~+ 564.2-~0.5 0.9 < 0.10 ~-:~ 9- 5 - - . '2 "2

570.2 ~2.0 1.8 ±0.9 < 0.16 not assigned 676 ±2 ~0.4 < 0.05 not assigned 783.0± 1.5 0.8 ::0.4 < 0.03 not assigned

TABLE 3 El transition probabihties for the [523] -~ [642] ~-transitions

B(EI, I l - 7 I t ) / B ( E 1 , ~ --~ ~)

Admixture of Transition r~,(nsec) Exper. No mixing K t = ~ K~ ~ ~

.~--7 ~- 250 1.48 2.30 0.255 ~ - -7 ~-+ 3640 1.17 0.919 0.102 45 ~- - 7 .}+ 40.1 £:6.1 1.0-1-0.25 1.0 1.0 1.0

.h t_ - ~ ~ 13.8i3.9 1.7--0.5 1.17 1.59 4.68 ~ - --~ ~ - 11.2--2.0 1.3:1_-0.3 1.27 2.26 11.4 ½~-- -7 ~+ 8.0'--1.6 3.7-t20.9 0.315 5.05 153

in the s imi lar case o f 249Bk. W e have pe r fo rmed some trial d iagona l i za t ions , which

used r ea sonab le values for the energies a n d m o m e n t s of iner t ia 31) and inc luded a

pa i r ing r educ t ion of the Cor io l is ma t r ix e l emen t 32). As a result we conc lude tha t the

dev ia t ions of the energies f rom f o r m u l a (1) can be accoun ted for by the Cor io l is

in te rac t ion .

t See ref..09), p. 935. tt See ref. ~9), p. 947.

Np ISOTOPES 603

5.1.2. The 5-[523],~ band. I t is popu la t ed up to spin ~2 -s- by the a lpha decay o f

2~lAIn and is well es tabl ished by prev ious inves t iga t ions 28). We derive accura te

values for the energies o f the two highest m e m b e r s (305.0_+0.5 and 395.6_+0.5 keV,

respect ively) f rom the i n t r a b a n d _13_ ~ ~ (146.5 keV) a n d J~---+ :~-~- (179.6 keV)

t rans i t ions . The energy spac ing can be descr ibed very accura te ly by eq. (1) with

A = 6.251 keV and B = - 1.88 eV.

TABLE 4 Branching ratios m ~TNp

Assumed B(L, 11 -> Ir)/B(L, Ll -> It,) lmtml It Kt ~t It I t, multipolarlty

intrinsic L exper, theor.

½- [530] ~- 2 ~-' g_ 2 El 6.6 • 1.7 10.7 ~)

~ [400] .~ ~ 4- ., .~ E2 0.11 :t_0.02 b) 0.75

-.~-

.~ ..~

~--[5211 ~. ½ - ~ ½

~-- [3 vibr. ~ ~ + ~ .~

M1 2.72 r1~0.27 b) 2.50 a) E2 3.32 -I-0.33 i,) 2.37 E2 0.067 -t:_ 0.007 0.97

Mt 8.7 ±2.6 10.7 a) E2 10.1 5:_3.0 6.0 M 1 1.66 .'-0.20 1.53 a) E2 2 05 ±0.24 1.59 E2 < 1.49 0.47

E2 9+'~ 17.5 MI 7.1 !_0.5 6.36 a) E2 8.9 --0.6 3.83 MI 1.70 !.:0.12 1.20 E2 2.31 -I-0.16 1.31 E2 <0.015 0.42

M1 0.76 j~0.25 0.80

El 0.076±0.007 0.40 M1 0.1924-0.015 0.40 E2 0.22 --0.02 1.33

7 2 ~- ~ 2 El 0.32 ±0.11 1.82 .~ - + 7. 2 M 1 > 5 . 0 1.86

E2 > 5.7 0 027 ~ ~ MI 0.28 -4-0.04 0.98

E2 0.34 zlz0.05 40

-~) Calculated with the generalized intensity rule, formula (4.4.12) of rel\ a0). l,) Ref. °-6).

O n e can ca lcula te the reduced E1 t r ans i t i on p robab i l i t i e s f rom the [523]$ b a n d

to the g r o u n d - s t a t e b a n d f rom the in tens i ty ra t ios o f these t r ans i t ions to the i n t r a b a n d

E2 t r ans i t ions wi th in the [523]~, b an d . U s i n g the wel l -es tabl ished f o r m u l a o f the

ro t a t i ona l mode l for the E2 t r ans i t ions a n d an in t r ins ic q u a d r u p o l e m o m e n t

Qo = 10.9__+l.lb [ref. 33)], one ob ta ins the va lues collected in table 3. The g iven

604 c. O'[JNTHER AND D R PARSIGNAULT

values for the transitions from the ~- state a~re derived from the known half-life of this state and are from ref. a4). It is apparent from table 3 that the transitions cannot be described by the direct [523] ~ [642] transition, which follows also from the ab- solute values of B(E1) from the 2 ~- state 35). Also, the most likely admixtures to the ground state band included in table 3, cannot account for the B(E1 ) values. The latter values were calculated with an amplitude of admixture of ~i = e-o x/( I - K<)(I+ K>).

5.1.3. The ½- [530]~" band. The two lowest members of this band have recently been identified in the fl-decay of 2 3 7 U by Yamazaki and Hollander 26). The a-decay of 241Am populates only the lower 3- state of these two levels. Excited states of this band up to spin -~- have been postulated by Lederer et al.24). The interpretation of the high-spin members of this band by these authors is mainly based on the alpha feeding of the states involved as found by Baranov et al. 28) and on thc energy spacing.

Lederer et al. point out that the high-spin states of the [530]T band should decay partly by interband transitions. We observe transitions from the -~- and the-l~-- states to the [523]~ band. We interpret the 221.2 keV and 264.6 keV lines as the -}-(~-) -~ ~-(}) and ~-(;2)-~ ~-(~2) transitions, respectively. An alternative inter- pretation 26) of the 221.2 keV line as the transition from the 281 keV state to the 60 keV state is excluded by the observed intensity of this line. The observed intensity ratio is in reasonable agreement with the theoretical expected value (see table 4). The expected transition from the ~-(~2) state to the ~-(~2-) state (165.7 keV) is masked by stronger lines in the 7-ray spectrum. The two above transitions fix the encrgy of the ~' - level of the [530]T band to 324.2+0.4 keV.

1 1 - ( ~ ) _ _ From the - ~ - state one would expect transitions to the -~-, ~2 ~- and ~3-- states of thc [523]~ band with approximate energies of 280, 212 and 132 keV. The only available line is the 275.6___0.5 keV line, which we therefore interpret as the ~ - ( ½ ) ~ o -(~) transition. The other two possible transitions are masked by stronger lines. This tLxes the energy of the-~ ~-- member of the [530]T band at 434.1 _ 1.0 keV.

The energies of thc [530]T rotational band can be dcscribed accuratcly by the simple

rotational formula E, = E o + A [I(I+ 1 ) + ( - ) ' + ~a(I+ ½)]. (2)

From the experimental energies of the ½-, k- and ~- states, we derive A = 6.59 + 0.04 keV and a = -1 .697+0.004. For the other members of the band, one gets then the energies 356.6 keV (~-), 433.9 keV ( ~ - - ) and 484.7 keV (~-) compared with the experimental energies of 357 + 2, 434.1 _ 1.0 and 485 + 2 keV.

5.1.4. The }+[400]T band. The three lowest members of this band, 332.36 keV (½+), 368.59 keV (}+) and 370.94keV (~+), are well established both from the alpha decay 28) of 24~Am and the beta decay 26) of 237U. The two upper states, 459.5+_0.4 keV (~-+) and 452.5+_0.4 keV (}+), have been found in the a-decay o f 241Am by Baranov et al. 28) and have recently been confirmed by Michaelis 13}

and Lederer et al. 24).

Np ISOTOPES 605

The reduced transition ratios from the rotational levels of this band to the ground- state band are listed in table 4. All transition ratios are in agreement with predominant- ly M1 multipolarity. Especially, the values for the ratios, which involve pure E2 transitions with AI = 2, prove that the AI = 1 transitions involved are > 90 ~ M1; this is in agreement with the results of Yamazaki and Hollander 26), who found for several transitions > 85 ~ MI from K conversion coefficients, it is interesting to note that the general intensity rule of Bohr and Mottelson gives good results for all M! transition ratios.

The energy spacing of the [400]]" band cannot be described by the simple rotational formula

E, = Eo+A[I(I+ 1)+(-)'+½a(l+-~-)l+B[I(l+ 1 )+ ( - ) ' +~a ( l+~ - ) ] z. (3)

Corrections for possible Coriolis coupling must be included. Two types of corrections have been discussed in the literature;

(i) first-order Coriolis coupling with K = ] states leads 36) to a correction term

( - ) ' + ~c(I- ½)(z+ D(z+ 2). (ii) Second-order Coriolis coupling of the K = ±½ components has a correction

term with the form 30) (_)I+~C(I+½)I(I+ 1). One cannot distinguish between the two correction terms within the experimental accuracy. From the five experimental energies, we derive the parameters

A = 6.250±0.016keV, C = 2 4 ± 9 e V ,

B = - 2 . 4 ± 1 . 8 e V , a = 1.054+0.002.

5.1.5. The az-[521]~" band. Baranov et al. as) assigned the 434 keV state as the ground state of the [521]T rotational band. However, this state is most likely the 1 1 - member of the [530]T band. From our,,,-ray data, we can define a state at 514.5 ± 0.5 keV, which would have the expected properties of the 2-[521]T state.

(i) It depopulates to the ~-- and 3.- members of the [530]T band and the ~+ [642]]" ground state, and possible to the 2 s-- [523]~ state (455 keV transition). The latter two transitions would have El and MI multipolarity and would be strongly hindered relative to the M1 transitions to the [530]]" band. With the Weiskopf estimate, one obtains for the relative hindrance

Hre~(E1, 2-[521] ~ ~+[642]) = 6.4.103,

Hre~(M1, ~-[521] ~ 2-[523]) > 4.

Both transitions are asymptotically forbidden (An~ = 2 and AA = 2), and comparable hindrance factors have been found for the transitions between the corresponding neutron states (see for example ref. 37), table 2). The experimental intensity ratio for the transitions to the [530]T band is B(M1, ~.-(~.) ~ 3--(½))/B(M1, ~-(~) ~ ½-(½)) = 0.76+0.25 compared to the theoretical ratio of 0.80.

6 0 6 C. GIJNTHER AND D. R. PARSIGNAULT

(ii) The ~-population of the 514 keV state can be estimated from the intensity of the depopulating gamma rays. The result is I~(514 keV)/l~(438 keV) ~ _1. kederer et al. 24) note that the alpha intensity to the 438 keV state is approximately two orders of magnitude larger than the expected population of the ~-[521]T state. Thus the c~- population of the 514 keV state would have the right magnitude for an interpretation

of the 514 keV level as the ~- [521]]" state. (iii) Baranov et al. find a level at 549+2 keV, which they interpret as the ~-

member of the [521]T band. However, as pointed out by Lederer et al., this inter- pretation leads to a rotational constant of A -- 9.4 keV, which is improbably high. Assuming the 549 keV state to be the ~- member of the [521 ]T band, we derive A = (6.9_+0.4) keV, in perfect agreement with the expected value.

It must be born in mind however, that there is only comparatively weak evidence for the existence of these states. There are also some arguments against the given

interpretation: (i) The c~-population of the 549 keV state (10 -4) found by Baranov et al. seems

somewhat high. However, this can possibly be understood by an alternating intensity pattern. Such a pattern is also observed for the transitions to the [400] 1 and [530]T bands 24-).

(ii) No transitions from the 549 keV state to the ground state and [530]T bands are observed. The reason could be that the 549 keV state decays mostly by an intra-

band transition to the 514 keV state.

5.1.6. The vibrational levels. Aside from the well-understood rotational levels discussed above, four levels with excitation energies of 723, 757, 770 and 803 keV have been found 22-23,24,28). They are assigned by previous investigators as vi-

brational levels. An extensive discussion of their nature is given in ref. 24). Our data do not provide an additional information concerning these levels. For completeness, we include our reduced transition ratios in table 4. The difficulty in using them as an aid in the interpretation of the levels is apparent.

5.1. THE MAGNETIC PROPERTIES OF THE [642]~ AND THE [523] v BAND 1N '/37Np

In an attempt to interpret the magnetic moments of the ~- excited states in 237Np and 239Np, two comparisons are suggested. First, the ground state of 241Am is the same Nilsson state as the ~- states, and one would then expect very similar magnetic moments for these states. The .q-factor of the 24.*Am ground state has recently been measured as) directly as 0.642+0.001, which is somewhat smaller than the value 0.76 + 0.06 derived for 237Np with the above calculated paramagnetic correction (see

discussion in subsect. 3.2). Second, the magnetic moments of the ground state and first excited bands in 237Np

together with the M1 transition probabilities within these bands can be interpreted in terms of the Nilsson model. The predictions of the Nilsson model for the g-factors

Np ISO I'OPI.:S 607

and the M1/E2 mixing ratio 62 are 30,39)

K z

g = gR + (gK -- gR) 1(1+ 1 ) ' (4)

1 11.5"10 5 (gK--gR] 2, - ( I - 1)(I+ l) \ ~ o / (5) 52 E 2

where E~ is in keY. Values for (~2 c a n be obtained from the measured L-subshell ratios of refs. 26,40).

The 62 values derived from the data of these authors and the conversion coefficients

TABLE 5

g K - - g R for the [64211" and the [523]~ bands

Transi tmn Subshell 5 '~ gK -g_R] g K - - g R a) ratio [ Qo [

~.+ ~ ~.. LI/LII h) 0.0132--0.0024 (33.2 keV) LI/L m b) 0.0151 --0.0018

LII/Llll h) 0.0193 ?-0.0044 assumed value 0 0144=-0.0015 (7.5 :t-0.4 ) - 10 -~ 0.82_4_0.09

7.-_> ~.- L1/Lll e) 0.166 .+_0.022 (43.4 keV) LI/LII I e) 0.158 ±0.018

Ln/LII I e) ~0.1 assumed value 0.161 +0.014

- .~. ~ - LI/LI! e) 0.20 -I-0.05 (55.56 keV)

(3.01-t-0 13) • 10 -2 0 . 3 3 - 0 . 0 4

(2.64"=0.33)- 10 -z 0.29 k0.05

a) Calculated with Qo = 10.9.-=1.1. h) Rcf. zs). e) Ref. 40).

of ref. 4t) are included in table 5 with the derived values for gK--gR. The sign of the latter values follows fi'om the measured g-factors. Using the average values

(gK--gR)[642]T = 0.82--+0.09 (gK--gR)[523]$ = 0.32-+0.04

and the rotational g-factor gR ----- 0.38 fi'om ref. 38), one obtains

g~- = 0.61+0.03, g~+ = 0.97-+0.07, g~.-/g~ . . . . 0.63-t-0.06.

The ratio of the two g-factors is in agreement with the experimental value. The value for g~ - is again somewhat lower than our experimental value and in agreement with the ground state g-factor of 24tAm.

Finally, one can calculate values for gs, erf for both bands. Using Nilsson wave functions for the deformation fi = 0.25, one obtains gs, err -- 0.49 -+ 0.19 and 0.67 + 0.07 for the [642]T and [523].~ bands, respectively. The value for the [523]J. band has the right order of magnitude t). The value for the ground-state band is somewhat low, which can probably be attributed to the large Coriolis mixing of this band 32).

608 C. GUNTHER AND D. R. PARSIGNAULT

I t is a g r ea t p l e a s u r e to a c k n o w l e d g e t he k i n d h o s p i t a l i t y o f P r o f e s s o r F. B o e h m

a n d o f his g r o u p . W e w o u l d l ike to t h a n k D r . E. Se l tze r f o r m a n y f ru i t fu l d i s c u s s i o n s

a n d m o s t p a r t i c u l a r l y D r . D. B o w m a n w h o bu i l t t h e t a b l e m a g n e t f o r t h e s t i m u l a t i n g

d i s c u s s i o n s a n d e n c o u r a g e m e n t s d u r i n g th i s e x p e r i m e n t . T h e a u t h o r s w o u l d a l so l ike

to a c k n o w l e d g e p a r t i a l s u p p o r t b y t h e B u n d e s m i n i s t e r i u m fiir W i s s e n s c h a f t , F e d e r a l

R e p u b l i c o f G e r m a n y a n d t he N A T O F e l l o w s h i p C o m m i t t e e , Par i s , F r a n c e .

References

1) E. Bodenstedt and J. D. Rogers, in Perturbed angular correlations, ed. by E. Karlsson (North- Holland Publ. Co., Amsterdam, 1964) chapt. IIl

2) H. Frauenfelder and R. M. Steffen, in Alpha-, beta- and gamma-ray spectroscopy, ed. by K. Steg- hahn (North-Holland Publ. Co., Amsterdam, 1965) chapt. XIX

3) E. Flamm and F. Asaro, Phys. Rev. 129 (1963) 290 4) V. E. Krohn, T. B. Novey and S. Raboy, Phys. Rev. 98 (1955) 1187 5) V. E. Krohn, T. B. Novey and S. Raboy, Phys. Rev. 105 (1956) 234 6) J. Heisenberg, thesis (1966) University of Hamburg, unpublished 7) P. R. Christensen, Nuclear Physics 41 (1963) 17 8) C. Gianther et al., Z. Phys. 183 (1965) 472 9) R. W. Bauer and M. Deutsch, Phys. Rev. 128 (1962) 751

10) K. Gunter, F. Asaro and C. Helmholz, Phys. Rev. Lett. 16 (1966) 362 11) N. Perrin and W. deWiecpawick, Compt. Rend. B262 (1966) 511 12) Experimental nuclear physics, Vol. II1 ed. by E. Segr6 (John Wiley and Sons, New York, 1953)

p. 178 13) J. A. Stone and W. L. Pdlinger, Bull. Am. Phys. Soc. 11 (1966) EG8 14) G. T. Seaborg, The transuramc elements (Addison-Wesley Publ. Co., Reading, Mass., 1958) 15) S. A. Al'tshuler and K. A. Vahev, JETP (Soy. Phys.) 8 (1959) 661 16) J. D. Axe, H. J. Stapleton and C. D. Jeffrics, Phys. Rev. 121 (1961) 1630 17) B. G. Wybourne, Spectroscopic properties of rare earths (lnterscience Publ. Co., New York,

1965) 18) J. C. Eisenstem and M. H. L. Pryce, Proc. Roy Soc. A255 (1960) 181 19) M. E. Foglio and M. H. L. Pryce, Mol. Phys. 4 (1961) 287 20) C. J. Lenander, Phys. Rev. 130 (1963) 1033 21) J. C. Eisenstein and M. H. L. Pryce, J. Res. Natl. Bur. Std. 69A (1965) 217 22) W. Michaelis, Z. Phys. 186 (1965) 42 23) W. Mlchaehs, Z. Phys. 194 (1966) 395 24) C. M. Lederer et al., Nuclear Physics 84 (1966) 481 25) C. Gianther and D. R. Parslgnault, Phys. Rev. 153 (1967) 1297 26) T. Yamazaki and J. M. Hollander, Nuclear Physics 84 (1966) 505 27) G. R. Choppin, B. G. Harvey and S. G. Thompson, J. Inorg. Nucl. Chem. 2 (1956) 66 28) S. A. Baranov, V. M. Kulakov and V. M. Shatmsky, Nuclear Physics 56 (1964) 252 29) E. K. Hyde, I Perlman and G. T. Seaborg, The nuclear properties of the heavy elements, Vol. II

(Prentice-Hall, Englewood ChiTs, New Jersey, 1964) 30) O. Nathan and S. G. Nllsson, m Alpha-, beta- and gamma-ray spectroscopy, ed. by K. Siegbahn

(North-Holland Publ. Co., Amsterdam, 1965) chapt. X 31) B. R. Mottelson and S G. Nilsson, Mat. Fys. Skr. Dan. Vld. Selsk 1, No. 8 (1959) 32) R. T. Brockmeler, S. Wahlborn, E. J. Seppi and F. Boehm, Nuclear Physics 63 (1965) 102 33) J. O. Newton, Nuclear Physics 5 (1958) 218 34) C. F. Perdrlsat, Revs. Mod. Phys. 38 (1966) 41 35) K. E. G. Lobner and S. G. Malmskog, Nuclear Physics 80 (1966) 505 36) R. M. Dmmond, B. Elbek and F. S. Stephens, Nuclear Physics 43 (1963) 560 37) M. N. Vergnes and J. O. Rasmussen, Nuclear Physics 62 (1965) 233 38) L. Armstrong, Jr., and R. Marrus, Phys. Rev. 144 (1966) 994 39) P. Alexander, F. Boehm and E. Kankelext, Phys. Rev. 133 (1964) B284 40) J. L. Wolfson and J. J. H. Park, Can. J. Phys. 42 (1964) 1387 41) I.. A. Shv and I. M. Band, in Alpha-, beta- and gamma-ray spectroscopy, ed. by K. Slegbahn

(North-Holland Publ. Co. Amsterdam, 1965) appendix 5