Collective states fed by weak α-transltions in the 232U chain

Nue lear P6yalea AZ89 (1977) 1-14: © NortlrEollamd Pr6liakGtp Co., Msterrbmt Not to be repeodaosd b7 plfotoprlot oe micsoelm wIthoat wrhten pamietion ltem tLe pebliehee

COLLECTYVE STATES FED BY ~VEAg ~TBANSYTIONS ïN THE sas U CHAYN InaNtute

w. xulscawicz

of Fxperimxntal

PlYyaica, U aity of Warm~r, 00~81 Warmw, Polandt and GSI, D-61 Darmstadt, Federal Republic of Germemry N. KAFFRRi.i . and N. T1tAUTMANN Inatltute ojNuclaar Ckemdatry, U~aity ojMalms, D~6S MaGrz, Fedeml Republic of Gerneany

A. PLOCHOCRI and J. ZYLICZ Inatttute of Experlnrartal Pbyaica, U nity of Warmw, 0081 Warmw, Poland and M. MATUL and K. STRY~INiBwICZ Institua ojNuclear Reae+wr6, Dept. ofPlryalca, OS-100 Swierk, Poland Received 1 July 1977

Abst5raet: The ~sU decay chain has been investigated down to slalb using C3e(Li) specttnmetas in shlßles and coinddenoe measarementa . In the decay of s~U and ~'Th some 10''-30'' aabtaachings to collective states at ~ 400 keV is s~Th and ssaRa, lespedively, have been ideatiflad. Hindrance factors (HF) calculated for all artraaaitioms are dieataeed in teams of nuclear models. Ia particular, for the B' = 0+ beads with the heads at 831.4 and 916.4 keV in ss°Th and ss4Ra, respectively. the low HF value are oonaistent with the quadru_ polo-pairing vibrational nature of three states indicated by previous (P. t) reaction stodies. No evidence her been found (branching ratio < 2x 10 - ' ~, HF > 10') for an a~laasidon to a hypothetical two-phonon 0* elate is ssaRa which should be expected at Ar 4301oeV, if the 21598 keV l' state wero due to harmonic octupole vibrations . The alternative intarpratetioa of this l' state as a shape isomer is not sported by the analysis of the half-lives of the S and S' rotational states. For the decay of ss4Ra sad its desoeadeats, one new transition and improved energies for the levee are reported . B

I

RADIOACPiVTTY asiÜ. sse~ ssa~+sso~,+sispo~ ~~ I, deduced HF. s °aI'h, sas~ dedneed levels, J, a, T~ . <3e(ü) de~tectois.

, .

1. ïsttrodnccHoa The investigation of the a-decay of even nuclei in the transitional U-Ra region is one of the ftw approaches to study the nature ~ of the K'e = 0+ bands and of the t Permanent addlese.

Oatob~r 1977

w. gURCBw1Cl et al.

2

interesting low-energy K` = 0- excitations. Some positive results obtained already for the decay of z3°U and its daughters 1) stimulated the present study of the z3zU chain. The main results reported here aro deduced from the analysis of singles and coincidence y-ray spectra from the z3zU and zzgTh a~ecays. Very weak y-transitions have been observed for the first timo due to the applicaxion of high-strength sources, low background conditions, a chemical purification from decay products repeated in short time intervals and a long overall duration of the measurements. Based on these data, evidence has been obtained for some 10- s-10- s j a-branchiags to energy levels of zzs.Th and zs~lta between 400 and 1000 keV. Only one of these weak branchings, to the 831 keV 0* level in zzgTh was previously observed by Lederer ~). The hindrance factors calculated for all a-transitions and some of the y-ray branching ratios are discussed in terms of models for collective nuclear excitations. In the case of the zza~ ~ zzo~ ~ zisPo ~ zizPb decays, one new transition and improved energies for the levels are presented. 2. M

fs of y-ny ~r~

The measurements of y-ray spectra were carried out using (io(Li) detectors . Since our aim was, first of all, to search for very weak y-transitions, the background had to bo kept as low as possible. Therefore, the detectors were placed inside thick lead (plus iron and radium) shieldings which roduood the eaternal background by a factor of20 to 40. Tho internal background, resuiting from the y-radiation of the daughter activities growing in, was even more obstructive. The chemical purification of the investigated activities fram their decay products was a very crucial step for this study. Hence, the brie£ description of the measurements, given below, is accompanied by some comments on chemical separation procedures applied . 2.1 . ~II~iQLFS SPECrRUM OF == =LJ

The z3zU activity was purified from the 1 .9 y zzgTh activity and its descendants by partition chromatography in the HN03-THP system . The strength of the sources was up to 100pCi. The measurements were carried out usinga 20 cms Qe(Li) detector of 3.0 keV resolution FVPHM at 1332 keV. Counting was started immediately after the chemical separation. In order to determine the contributions to the y-ray spectrum from the decay products growing in, three 1S h measurements were carried out successively with each source. Several runs were performed, and the spectra from the corresponding 15 h .time intervals were summed, Fig. 1 shows the high-energy part of the spectrum. The data on y-ray energies and intensities are given in table l. s.~. snvar.BS axn corxcm$xc$ srECrxn of == "Th To measure the y-ray spectra from the decay of z=gTh without a too high contribution- from the 3.66 d zzalta daughter activity and its descendants, we had to purify the sources in short time intervals, ranging from 30 to 75 min for different

sasU CHAIN

3

TAmB 1 Tramitions in sssTh followin~ the a-da ;ay of sasU

Transition energy (keV) 37.78fO.OS 129.08f0.05 141 .0 f0 .3 191.0 f0 .2 209.3 f0.3 270.2 f0.2 327.9 f0 .2 332.3 f0.3

Relative yrraY intensity 7920 2630 0.12 1 .2 0.41 113 100 1.9

Transition energy (key

f200 f 80 f 0.03 f 0.1 f 0.10 f 4 f 3 f 0.1

338.1 478 303 .E 547 . 773.4 817') 831')

Relative y-ray intensity

f0.2 f1 f0.3 f1 f0.5 f1

1.43 f 0.053 f 0.36 f 0.039f 0.18 f s~ 0.03 < 0.03

O.OS 0.021 0.03 0.024 0.03

') For wmmcnts see subsect. 3.1 .

ioo

mU ~3 .6

W ~ Z Z

tr w Z

510.7 ~n "511 nNo

0 200

tn U 478

~il 8605

1

1000

1

S "" ~~:?~~iriA71~l;1d~~~/S~rIIy~i{f~x 1

1

1500 2000 CHANNEL NUMBER

1

1

1

1

~

~ ~ the chemical pu Fig. 1. High-energy part of the sasU ~Y s rification. Insert : section of the spectram aaeasared 1S hlater showingthe iaierfei+enoe from growingin descendants .

W. RURCSWICZ et aL

4

series of measurements. The purification was carried out by solvent extraction of thorium into di-(2-e~thylhezyl}orthophosphoric acid (I~SHP) in cyclohoaana (1 :1) from 6 N HCI, ref. 3). At%r washing the organic phase with 6 N HQ, the thorium activity in the HDEHP phase was measured diroctly in order to save time. The strength of the individual ~'a1'h sources amounted to some hnndrod pCi. Both y-singles and two-dimensional coincidence measurements were performed under conditions similar to those described in detail in ref. t). Two Qe(Li) detectors having active volumes of 32 and 104 core aad FWHM resolution at 13321aeV of 1.75 and 2.3 keV respectively wore used. The singles spectrum below 350 1ceV (fig. 2)

1a

Z a

ws

N H

CHANNEL NUMBER 2 The ~~I'h eit~leey-raytpedrum below 330 keV. Seventeenram of 30 min ~i time each Wets wormed. Some Dealer from ~eTh deeoendaata andtom a "'" saw oont:>~ation are present The symbol E elands for summing e8'ects.

asaU CHAIN

S

W Z

v 103 N F Z U

103

10'

mo

goo

soo

CHANNEL NUMBER

eoo

loco

FiQ. 3 . Hi ay part of the y-ray spectrum of aasTh. Forty-0ae runs of 30 min oonadng time each were summed. The symbol Blc~ denotes the peat attributed to baclr~ound. For thrtber comments ct: caption to aB" 2.

was measured using the smaller detector. The biggor one has been applied to study the high-energy part of this spectrum (fig " 3). For the coincidonoo measnremonts S4 runs of 7S min each were performed; rs 3.2 x 106 coincidence events were stored and analyzed. Some of the resultant gated spectra are shown in figs. 4 and S. The stronger q-lines from the ~~aTh chain were used as internal energy calibration standards ~). In addition to the usual eflicilency calibration of the detectors, the attenuation of the low-energy q-raya in the big-volume sources was determined in order to get the correct relative intensities. For this calibration a 16gYb source Prepared in the same way as in the case of ~~gTh was applied. The very low-energy part of the ~~~Th q-ray spectrum was, measured with a high resolution X-ray Ge(Li) detector using specially prcpared thinner sources. In this case, after the chemical procedure already described, the thorium activity was bacYextracted from the HDEHP with 6 N HClJ2 N HF and coprecipitated on ferric hydroxide (1 mg Fe) after destroying the fluorides with boric acid. The results on energies, intensities and coincidence relationships of the ~'gTh q-rays are given in table 2. 2.3 . SINßLES AND COINCIDENCE SPECTRA OF THE aa 4Ra CHAIN

Despite of the chemical purification of the sources and the short counting time a contribution of the ss~lta chain in the 3~'Th y-ray spectra was always observed.

w. RURCSWICZ er oL

6

ßATE :166 .5 keV

80

v

A (216 .01

60

0

100

100

CHANNEL NUMBER

300

Pig. 4. Cismma-ray spectra in comddenoe with selected tteasitions in'asRa The spectra coincident with the Compton dietrlbnticn close to the gated peaks have bay snbtraded. A contribution of random coinddenoes (marked with R) is also present.

Hence, as a by-product of these studies, we have obtained some information on the y-rays of the sse~ activity and its short-lived descendants in equilibrium, table 3. 3. Decay gc6emes All transitions observed in this work were unambiguously placed in the individual decay schemos. Their totalintensities were calculated from the intensities of the y-rays using the theoretical internal-conversion coefilccients 6. ') . The multipolarity of some transitions was known from previous studies a . ~. For othors .it was assumed to be El or E2 depending whether a parity change occurs or not. The total transition intensities were expressedin per cent of a-decays and used to calculate the a-branching ratios (comments on normalization are given in footnotes to tables 2, 3 and 4). The atransition hindrance factors which neglected the angular momentum (HF) and those reduced to account for the centrifugal bamer (1tHF) were calculated according to ref. 1 °). Since the energies of the ac-transitions used in these calcalations were known to a very high accuracy 11) (see values in figs. 6-9), the experimental uncertainty of the HF and RHF valves is determined by the accuracy of the a-branching ratios and thus can easily be estimated. The half-lives (figs. 6-9) were taken from refs. 12, 13) .

sasU CHAIN

7

10 B 6

W Z Z

2 0

W d N F Z

O V

10 8 6

2 0

CHANNEL NUMBER

Fib. S. Singlesy-ray spectrum of sssTh sad the spectnim measured is ooiacidonce with the 216.0 keV line. The spectrum coincident with the Comptoa distribution close to this peal has been subhacted. The peaks marked with Raro caused by random coincides. 3.1 . THE s'sU -i ss'Th

DECAY

The 2'~U decay scheme is given in fig. 6. The energies, spins and parities of the sas l h levels presented here were previously established in radioactive decay and s) the ~ 3°Th(p, t) reaction i a) studies. The transition intensities are based on the data from table l. The multipolarity Fro+F"2 of the 817 keV transition has been established in a study of the ~~aAc decay ls). The intensity of the 8311oeV EO transition is taken from ref. ~). The a-transitions to the 519 keV S- and 874 keV 2* levels have been identified for the first time. For these and the remaining levels the a-branching ratios have been determined and compared in table 4 with the results of. a-spectroscopy experiments s " 16-191 .

w. xvxc$wlcz er ot.

8

T~ 2 (ismma-ray data for the ss°T'h -" sss~ decay i3aeray (teV)

Intemsity (photons/10° alphas)

l3nerpes Cmtensit3en) of ooiaddent y-lines

74.4 f0.1 ') 84.371 f0.003 °) 131.610 f0.004 °) 142.0 f0 .3 °) 166.407 f0.004 °)

4.0 f 1.4 12100 °) f 60 1240 0.013 f 0.004 960 f 50

l31.6(1220ß30), 216.0(= 2390) 131.6(1220 f210),166.4(= 960), 203.9(170 f30) 74(3.7 f1.6), 84(= 12100)

182.2 f0.2°) 203.93 f0 .05 215979 f0.005 °) 228.5 f0 .2 700.5 f0 .3 °) 742.2 f0.3 832.0 f0 .2 992.9 f1 .0

0.052f 0.018 184 f 9 2390 f 130 0.18 f 0.03 sa 0.03 0.014 f 0.004 0.14 f 0.02 sa 0.013

84(= 12100),182(0 .032f0.018), 228(O.15f0.03),742(srt 0.03) 166 84(- 12100~ 142(0.013 f0 .004) 74(4.2 f1 .4), 700(as 0.03) 166 216

') Data >3rom our ooinddence measnremants . °) These valses are tats from ref. ~) . °) This vah~e is taten from ref. °) and used for aormaliTadon . Its uacastainty is f600. Twsra 3 ßamma-ray data for the ss 4lta chain tramitlon in the nucleon sso~ u°Po uo~ aispb

Y-mYs (photonn/10° alphas) 240.981 f0 .003 ") 292.70 f0.10 404.2 f0.2 422.04 f0 .10 549.73 f0.05 643.50 f0.10 8049 f0.2

39300f1300 °) 60 f 7 21f S 29 f 3 930f 80 52 f 9 18f 3

E~iea (intemities) of coincident Y_linas 293(= 60), 404(23f5), 422 (31 f9) 241 241 241

") This value is toton from reP. 4). °) This velue is fates from ref. °) and med for normalization. 3.2 . THS ss°Th ~ sss~ DBCAY

The res~ilts of singles and coincidence y-ray studies, summarized in table 2, led to the decay scheme presented in fig. 7. Only the 84, 132, 166, 206 and 216 keV transitions wore previously lrnown to follow the ~~nTh a-decay g). Those five transitions deeacite the 84, 216, 251 and 290 keV z~~ita. levels having spins and parities 2+, 1' and presumably 4+ sad 3-, respectively. This part of the decay scheme is well snp-

x~2

~U1~

E,~=a3zo .3oio.u)krv 71 .7y

°~

W ôê ~ IÛ

4c~ ¢~RQu~

]r K

E(keV)

2' 0

0' 0

WW

T o~ _ô Ô WWW ~e ui ~é ~ .~ o. °1 QQ X Ia °q,;, Q ~ u+ ~ 4 ro o~=w N ~ p! N 01 ~

5- 0 3' 0 6` 0 1 0

0

4' 0 2' 0 0' 0

N W O N

HF

(RHF)

831 .3

3.2 "10 ° 2.1 " 106

32

l19)

519 .1

5 .4~ 10~

681

(48)

3959 3779 327.9

4.8 .106 5090 5.1 " 10 6280 5.6~10a 120

(1750) (156) (101)

1869

0.32

57 .78 0

,~m,~

ae

Y.

874

31 .7 88.0

10

(1W

1B.8

12.8)

0.98

(0 .58) (i)

Fia. 6. The decay scheme of'a'U. ~Th~

1~

K

EIkrVI

~~ ~ â

7b

HF

IRHFI

~3x 10~° 10 (58) (17t0.3)10~ ~5 (6~

cz` o) 99z.s -,~,-~-~ 0` 0 918.i r

8' 0 f;79.3 5' 0 i32.8

m

(98t2.3) td ° 21000(1L50)

~¢

S~.S =~~~G*4~rs>"rri~>"

~~;t~ t ~t~ji t vt,

2` o

ea.

r~otu

w?'1K~[

o" o

0

a,~

Tutu

Fia. 7. The decay scheme of a'pi'h .

os~ au~ ~

cn

~o

w. xvncswtcz ~ ~r

a

0`

~~ ~ 3.685d E~" oseesss=a~s~ ~.v

533,7

2* 0

?.L0,9

0* 0

0

~- (â9=0~8)10~

'

18

~

5.030 .16

L07

'

9498=0 .16

1

~°Rn,~

FiE. 8. The dray scheme of sum

* ecP~x~x "ornas=amis

.

t

~°Rn,~

r

ot EflWl

~

~k,

.~eas~=a

HF

(Z`3 80~.9~--~^-~(1.8t0.~10a 35

0*

O ~-~-- 100

1

O*

0

-t- 9a9

~Po,~

FiB. 9. The decay Schemas of ss°Rn

and sisPo.

1

w rs

asaU CI~ T~ 4 Hraachings of a~trandtions for the asaU ~ u"Th decay awording to different authors Level energy 0 37 .78 186.9 327.9 377.9 393.9 319.1 831.3 874

r

ref.' 6)

rat: ")

0* 68 2* 32 f1 4* 0.32f0.03 16* 3S' 0* 2*

ref. i")

ltamition intensity ('/,) ref. a)

ref. i~

this work

67.8 fo.7 6e .o ") 68 68 .6 f0.6 31 .4 31 .2 f0.4 32 .2 f0.3 31 .7f0.8 0.37 0.28 f0 .02 0.30f0.09 0.32f0.0~2 (5 .6 f0.2) x 10'' 0.017(2.5 f0.2)x]0 -' (6f1)x10 -' (S .1 f0.3) x 10 -' (1 .7 f0.3) x 10 -4 (2.1 f0.3) x 10 -4 (4.8 f0.6) x 10-' (5.4f0.4)x10 -' (2 .4 f1.0) x 10 -' (2.1 f0.2) x 10'' (3.2 f0 .8) x 10'6

") Thin valae is taken from ref. ri) sad used for normalization.

ported by our study. The new stateax 4331teV is established by coincidence relations. Since q-transitions connect this state with the 4* and 3 - levels, its spin and parity are suggested to be 5 - . Tho levels shown in fig. 7 at 479 and 9161teV are believed to be identical with the 477t21teV (unassignod spin and parity) and 91813 keV (0*) levels populated in the 336` a°(Y t) reaction ~°). The application of the ro~t a~tionahnergy formula (see next section) strongly suggests that the 479 keV level is the 6 + member of the ground-state band . In addition to the twoyr-transitions found to deexcite the 9161ceV level, there should be an F.0 transition to the ground state, a transition which could not be observed in our y-ray studies. The resulting deficit in the a-branching ratio calculated for the 916 keV level is probably insignificant (see the analogous situation of the deexcitation of the 831 keV 0 + state in ~ 3aT'h, fig. 6). The level proposed at 993 1teV may be the first rotational 2 + state built on the 916 YeY 0 + band head. 3.3. TIiB aa4Ra --~ aso~ ,.., sispo ~ uaph DHCAYS The decay scheme of wa its shown in fig. 8, and

the decay schemes of zs°~ and ztspo wen in fig. 9, were established already in previous studies g. ~r). Only the 422 keV transition between the 663 and 241 keV levels in as°~ is new. The abranching ratios deduced from the y-ray intensity data of table 3 are close to those resulting from the analysis of a-ray spectra. However, the present wont allows assignment of more accurate level energies. 4. Dfec~ioo at tbe " t Th aud sss Ra levds 4.1 . ROTATIONAL BANDS AND BRANCIUIVß RATIOS

The excited states of ~ =BTh and ~ =4Ra can be classified in rotational bands with the parameters given in table S. For the ground-state band, as well as for the negative-

w. ~vRCEwICZ ~ ~

lz

parity band, the doviations of the level energies from the simple rotational formula are vary small for a'aTh and sgtnewhat bigger for as4lZa. The ratio of the reduced transition probabilities of the El transitions connecting the 1 - state with the ßrst excited 2+ state and the 0+ ground state in assTh is equal to 2.02 t 0.09, in agreement with the theoretical value of 2 for R = 0 for the initial and final state. The analogous ratio calculated for ~a~Ra is 2.29 f0.17. The RHF values for the at-transitions to the levels of the ground-state band, taloen from the microscopic calculations by Poggenburg et al. ss), are not far from the oaperimental results, the agroement being slightly better for ~aaT'h than for'1~lta (table 6). One may thus conclude that aaaTh is a good rotator while neaps shows some features of a transitional nucleus. This seems to bo consistent with the known difference in the ground-state deformation of ~~eTh and ss 4lta. Tho quadrupole moments of those nuclei, deduced from halflives of the first rotational 2+ states, are 8.46 and 6.25 b, respectively z3) . The corresponding ground~state deformation parameters ß are 0.221 and 0.171 . Tnsia S Parameters ofthe rotational formula Bead head

-

Nucleus

~~r

âl'

A (key

s~Th

0 328 831

00+ Ol00+

216 916

Ol' 00+

9.75 6.78 7.12 14.72 7.18 12 .7

sss~

(keV)

0

00+

Parameters')

B (ev7 -20 1.6

°)

-109 17.6 °)

°) Assamed to be 0. 4.2. NATURE OF THE â" = 0- AND ~ ~ 0+ EXCITATIONS

In the oven nentron~eficient isotopes of thorium and raäium the %I` = Ol as4ita state oocars at as anomaly low~xcitation energy. The position of this state in sa) suggested that these at 216 koV is the lowest of all known until now. Mialler et al. in the potential energy with secondary minima states are associated low~gy difference . The remarkable an oblate octupole deformation surface corresponding to and the R" = 0+ the â~ = 0band of the moment-of-inertia parameter .! for ss
~saU CHAIIQ

13

the R" m 0' band (figs. S and 7). For these transitions we assume the multipolsrity B2 and the same quadrnpole moment Q° as for the ground slats (if tho Q° value were somewhat higher, according to the lower A-valves for the R" ~ 0' band, the final conclusion would not be changed) . After taking into account the appropriate y-ray branching ratios one obtains half-lives of 9.2 x 10'11 s and 1.6 x 10' 11 s for the 290 keV 3' and 433 keV 5' states, respectively . The partial half-lives of the presumably El transitions of 206 and 182 keV are then 1.8 x 10' t ° s and 3 x 10'11 s, respeolively. Compared to the Weisakopf estimate, the 206 keY transition is retarded by a factor FW ~ 8400 while for the 182 keV transition the retardation is F~, rs 1000. The syatematice of half-lives for El y-transitions ss) show that typical retardation factors are of the same order or even higher. There is no reason to claim any big extra retardation in this case to support the idea of shape isomerism. T~ 6 Reduced hindrancefactors for a^traasitions to the ~oimd~tate bands aasU -~ ss~ ua~ aas~ -~ Final state l' theory') exp theory `) . exp 0* 2* 4* 6*

1 1.03 4.6 138

1 O.SSf0.02 2.8 f0 .2 156 f13

1 1.03 4.2 237

1 0.53f0.03 2.1 f0.1 105 f17

") Ref. ").

The alternative interpretation of the l' state as the harmonic octupole vibration cannot bo appliedto ss°Ra as it also cannot be applied to ss 6Th and ~~~Ra, rofa. l' ~ s), This follows from the non-observation of the two-phonon 0+ state at twice of the energy of the one-phonon octupole vibration, i.e. about 4301aeV. Indeed, from the analysis of the coincidence y-ray spectra we have deduced an upper intensity limit of 2 x 10' 6 ~ for the corresponding a-branching, which leads to I3F > 105. It is hard to believe that such an extremely high retardation can be explained by any nuclear structure reasons. The natural conclusion is that there is no 0+ state in the neighbourhood of 430 keV. The first excited 0+ state in z~4Ra has been established at 916 IooV. The ratio of the energies , of this and the 1 ' state exceeds four . Thus, the interpretation of the 916 keV state in terms of the unharmoaic octupole vibrations does not seem justified. On the other hand, the high cross section for the (p, t) reaction ~ °) and the low a-transition IIF value established in this work favour the interpretation of this state as quadrupole-pairing vibrations . For analogous reasons, the quadrupole-pairing nature was proposed in ref. ~~) for the 831 keV 0+ state in ~~gTh. ïn this case, however, the ratio of the energies of

14

W. KURCi3WICZ et d.

the 0+ and 1 - states is 2.5 only and the octupole vibrations] (unhsrmonic) intorpretatioa of both states cannot be excluded. Actually, the microscopic calculations carried outby Ivanova et al. ~~) give an 83 ~ contribution of thotwo-phonon octupole wave function to this 0* state. We are indebted to Mr. K H. Gläsel and Mr. R Heimahn for technical assistance in the experiments. One of ns (W.K.) is grateful to Professor G. Herrmann and his coworlcors from the Institute of Nuclear Chemistry, University of Mainz, for hospitality, and to GSI Darmstadt for financial support. Refereuces 1) W. Karoewicx, N. Kaffrell, N. Trnn A~n A. P]ochocki, J. ~ylicz, K. 3tiycmiewicz and I. Yuthmdov, Nucl . Puys. A270 (1976) 175 2) C. M. Lederer, Thesis, report UCRL-11028, 1%3 3) R. Denig, N. Trautmann gad ß. Herrmaan, J. Radioanal. Chew . S (1970) 223 4) W. Karoewicz, 13 . Ruchoweka, N. Kaûroll and N. Trautmean, Nncl. Insu., to be published S) A. Peghaire, Nacl . Insu. 75 (1969) 66 6) R. S. Halter and & C. Seltzer, Nacl. Data Aa (1968)1 7) O. Dragoon, Z. Pl~jner and F. Schmatzler, Nacl. Data Tabla A9 (1971) 119 8) D. J. Horen, Nacl . Data Sheets 17 U976) 361 9) Y. A. Huis, Nucl . Data Sheets 17 (1976) 351 ;17 (1976) 341 ;17 (1976) 329 10) J. O. Rasmassen, Phys. Rev. 313 (1939) 1393 ;115 (1959)1675 11) A. Rytz, Atomic Data. and Nucl. Data Tables iz (1973) 479 12) K. C. Jordan, ß. W. Otto gad R. P. Ratay, J. laorg. Nncl. Chew. 33 (1971) 1213 13) W. Seelmsan-Bggebert, O. Pfennig sad H. Mùnzel, Chart of the nuclides, 4th ed . (E. Klett Drnckered, Stuttgart, 1974) 14) J. V. Maher, J. R. Brshine, A. M. Friedmann and R. H. 3iernssen, Phys. Rev. CS (1972) 1380 1S) A. Plochocki, Thesis, Institute of Nuclear Research, Swleri, Poland, 1974 16) F. Asaro and I. Penman, Phys. Rev. 99 (1955) 37 17) C~. 3charff-0oldhaber, B. der Mateosian, ß. Harbottle sad M. MacReown, Puys. Rav. 99 (1933) 180 18) S. A. Baranov, I. ß. Aliev, V. M. Kuhilwv and V. M. 3hatiaslcü, Yad. Flz. 4 (1966) 673 [Sov. J. Nncl. Phys . 4 (1%7) 477] 19) J. C. Soares, J. P. Ribeiro, A. (3oncalva, F. Braganoe Oil and J. C~. Ferreira, Comet. Read . BZ73 (1971) 983 20) A. M. Friedmsm, K. Katori, D. Albnght and J. P. 3chilïer, Phys . Rev. C9 (1974) 760 21) Nucl. Data Sheets, Nuclear level schemes A e 45 through A ~ 237, ed. Nuclear Data ßmap (Academic Press, NY, 1973) 22) J. K. Poggenbarg, H. J. Mang and J. O. Raamussen, Phys . Rev. 181 (1969) 1697 23) K. B. LBbaer, M. Vetter and V. HBnig, Nacl. Data Tables A7 (1970) 493 24) P. Melier, 3. C} . Nilssoa and R. K. Sheline, Phys. Lett . 40B (1972) 329 25) C. F. Perdrisat, Rev. Mod. Phya . 3g (1966) 41 26) 3. Bjernhohn, Thesis, in.titate for T7~eoretical Physic, University of Copenhagen, 1965 27) I. Itagaereeon and R. A. Brogue, Nucl . Phys. AZ63 (1976) 315 28) 3. P. Ivanova, A. L. Komov, L. A. Malov and V. ß. Soloviev, Particles and Nacleas 7 (1976) 430