Energy levels in 237Pu and 239Pu

Nuclear Physics A211 (1973) 541--558; (~) North-Holland Publishing Co., Amsterdam

Not to be reproducedby photoprint or microfilmwithout written permissionfrom the publisher

ENERGY LEVELS 1N ZaTPu AND 2agPu T. GROTDAL, L. LOSET, K. NYBO and T. F. THORSTEINSEN Institute of Physics, University of Bergen and Niels Bohr Institute, University of Copenhagen

Received 29 March 1973 (Revised 29 May 1973) Abstract: The energy levels of 237pu have been studied by the (d, t) reaction and the energy levels of 2agPu by the (d, p) and the (d, d') reactions at deuteron bombarding energies of 12 and 13 MeV. The scattered particles were analysed in a broad range magnetic spectrograph at angles of observation of 90°, 120°, 125° and 150°. In 2aTPu a total of five and in 239pu four Nilsson orbitals have been identified which were previously unknown. A K~ = ½- octupole band has been assigned in 239pu by means of the (d, d') reaction. EI

I

NUCLEAR REACTIONS 238pu(d,t), 238pU(d,p), 239pU(d,d'), Ea = 12.1 MeV and 13.1 MeV, measured tr(0). 237.239pu deduced levels, J, zc, 1.

1. Introduction The previous information about energy levels in 237pu comes from studies of the ~-decay t,2) of 241Cm and the electron capture decays a, 4) of 237Am and 2a7pu. Based on the 241Cm or-decay 1) and the 237pu electron capture decay a) the 743T Nilsson orbital was established as the ground state in 2aTpu. Also based on the ~-decay work the 6314 orbital was established and the 6221" orbital was tentatively assigned from Z37Am electron capture decay 4). The energy levels of 239pu have been studied in the fl-decay 5) of 239Np, the electron capture decay 6) of 2a 9Am and the ~-decay 7) of 24aCm. F r o m these experiments the bandheads of the 6314, 622 T, 743 T, 501J, and 6244 (or possibly 6334) orbitals have been observed at 0, 285.5, 391.6, 469.8 and 511.9 keV. In the present work the 2a7pu and 239pu nuclei have been studied by the 238pu(d, t)237pu, 238pu(d, p)239pu and 239pu(d, d')Z39pu reactions. The results from the previous experiments are confirmed except for the K ~ = ½- band at 469.8 keV which earlier was given a definite Nilsson assignment, but here is found to be mainly of collective nature. In addition five orbitals in 2aTpu and four orbitals in 239pu have been assigned. The identification of the levels is based on the absolute cross sections, the characteristic intensity patterns of the rotational bands, the angular distributions, the energy systematics of the orbitals and the cross section ratios from the different angles 541

542

T. GROTDAL

et aL

of measurement, especially the 90°/125 ° cross section ratio. Detailed angular distributions have been measured for the (d, t) and (d, p) reactions at 13 MeV bombarding energy with 238U as target 28). The angular distributions are relatively structureless at this bombarding energy and the cross section ratios from the 90 ° and 125 ° measurements provide an important guide in estimating the/-value of the transferred neutron to approximately one unit.

2. Experimental procedure and results The measurements were performed by bombarding targets of 238pu and 239pu with 12 and 13 MeV deuterons from the tandem accelerator at the Niels Bohr InstiEXCITATION ENERGY(keV)

1300 1200

1500

1000

500

0

t

I

I

I

2 3 8 P U (D,T) 2 3 7 P U E d = 1 2 , 1 0 8 MEV A N G L E = 90.0 DEG. CHARGE = 3 0 0 0 0 pC

1100 1000 E E

900

--~

800

o.

700

z

600

5011 3s

501t

g

i 6311 J o 7 5

(D

500

631 I i

4O0

,

i

I 63 ~ 622

300

II 7&31

503~

2O0

,/~2,

100 0 70

72

74

76

78

DISTANCE

ALONG

PLATE (cm)

80

Fig. l. Triton spectrum for the =aSPu(d, t)2a~Pu reaction.

82

1

23"/. 239pu L E V E L S

543

tute. The targets had an isotopic purity > 99 % and were prepared by evaporation on 40 #g/cm 2 carbon backings. The thickness of the targets ranged from 40 to 80/~g/cm 2. The average beam intensity was 0.5 #A and the total charges were from 20000 to 30000 p,C.

The charged particles from the reactions were analysed in a broad range magnetic spectrograph at laboratory angles of 90 °, 120°, 125 ° and 150° and detected with 25 #m EXCITATION

ENERGV(keV)

1500

1000

500

0

I

I

I

I

238PU (D,P) 239PU Ed = 12.105 MEV ANGLE = 125.0 DEG CHARGE = 2 1 0 0 0 ~ C

1500 1400

f

613

1300

620t 9

7

f 1

1200

1:::

1100 1000

or" iii 13_

900 11!

800 I-, Z Z3 O

631 | ? 31

700 600 500 400 300 200 I'

100

I

0

23

24.

25 26 27 28 DISTANCE ALONG P L A T E ( c m )

Fig. 2. Proton spectrum for the 2aSpu(d, p)2agPu reaction.

29

544

T. GROTDAL et al.

Ilford K2 nuclear emulsions covered by aluminium absorbers in order to separate tritons, deuterons and protons. The plates were scanned in ¼ mm strips. A solid state detector in the target chamber monitored the elastically scattered deuterons at an angle of 90 ° relative to the incoming deuteron beam. The absolute cross sections were obtained by normalisation to the combined yield of the elastic scattering and the scattering to the first excited state. These yields were recorded on short exposures taken before and after the main exposures. The scattering cross sections used for normalisation were found by extrapolation of data a, 9) from neighbouring elements. The adopted values were 235___25, 694-10, and 60 4- 6 mb/sr for 90 °, 120 ° and 125 ° respectively with 12.1 MeV deuterons and 174-5 mb/sr for 150 ° with 13.1 MeV deuterons. Spectra for the reactions 238pu(d, t)237pu, 23SPu(d, p)239pu and 239pu(d, d')Z39pu EXCITATION ENERGY (keV) 1000

500

239PU (D,D') 239PU E = 12.092 MEV ANGLE = 120.0 DEG. CHARGE = 26000 ~C

1400

0

3/2",1/2*

1200

E E

1000

,
r,,LI EL

800

U3 Z

o

r,j

7!

600

400

200

512+

50

52 54. DISTANCE ALONG PLATE (cm)

t

56

Fig. 3. Spectrum of deuterons scattered from 2agPu.

237. 239pu LEVELS

545

TABLE 1 Levels populated in 237pu Energy average (d, t) (keV)

0 5O 106 146 156 183 199 224 257 280 305 371 405 437 454 471 486 513 545 582 591 655 691 716 741 757 775 809 840 852 884 933 964 1000 1014 1053 1104 1189 1216 1250 1264 1348 1383 1397 1463 148 I 1534

Assignment

da/d.Q(d, t) 90 °

½743~ ~743~ ~743~ ½631~

~631~ ~-743~ 631~ ½6314 ~743~ ~622~ 631~ 622~ 631~, {633~ ½ 6334 ½631~ ½624~ ~6334

~631~ ½5o1~ (~ 5o1~) 5o14 (½ 5o1~ )

(~ 503~)

({ 501~)

<1

1 11 36 95 5 3 7 9 22 29 44 159 14 6 5 21 38 352 20 74 34 2 25 4 2 10 8 13 34 21 3 4 14 143 5 3 18 4 5 5 20 35 7 6 5 4

(#b/sO 125 ° 1

1 14 31 89 4 4 8 21 20 50 59 162 28 12 8 42 67 537 53 123 67 9 37 6 3 12 12 17 60 48 5 7 28 239 14 8 32 8 6 35 49 6 14 10 9

546

T. G R O T D A L et aL

TABLE 2 Observed and calculated energy levels and differential cross sections in the 2aSpu(d, t)2aTpu reaction Nilsson orbital

Excitation energy (keV)

Differential cross section, Q = 0, 0 = 90 °, Ea = 12.1 MeV (#b/sr) theor,

]743~ 4} 4} ½ 631~

½

(0)

1

50

0 a~

I

106 183 257

16 a) 0 a) 26 a)

17 9 17

146

67

58

156

72

199 224 305

9 8 12 1

157 5

280

8 o 28 1

40

20 29 47 17

322 b) 30 45

4} j 622~

371 ¥ 6334

405 437 486

4} J631~ 405 454 513 4} ak 5014

expt.

545 591 582

4}

0.3 110 8 112 lO 2 801 159 91 8

<1

11 54

85

322 b) 13 84

807 !75 47 6

<0.1 503~

852

437 11 3
106

501~

1014

917 110 24 <0.1 <0.1

543

4}

4} a) Coriolis coupling included. b) Summed cross sections of ~ 6334 and ] 6314.

237.239pu LEVELS

547

TABLE 3 Mixing amplitudes and cross sections for the 622~, 631~, 633~ and 624~, Nilsson orbitals Excitation energy theor, (keV)

&r/dO(d, t) (/zb/sr) Q = 0 expt. (keV)

374

½

theor, pure mixed 0.3

expt.

622~ 278.8

0.2

278 398 409

280 405

8 20 110

319 436 453 475

(321) 437 454 471

0 29 8 1

373 487 510 538

371 486 513

28 47 112 3

51 12 131 16

1

4

439

Nilsson orbitals Band heads (keV)

5 4 133 0.4 42 0.2 1

631~ 374.1

633~ 401.8

624~ 468.7

0.999 40 322

0.995 0.055 --0.077

0.095 --0.546 0.831

-0.004 0.835 0.549

30 13 10

0.988 0.084 --0.114 0.048

0.145 --0.427 0.877 --0.154

--0.012 0.840 0.337 --0.422

--0.034 0.320 0.318 0.892

0.979 0.109 --0.146 0.072

0.188 --0.358 0.888 --0.205

--0.023 0.833 0.224 --0.503

--0.052 0.404 0.367 0.836

0.969

0.227

--0.034

--0.067

85 45 84

are shown in figs. 1-3. T h e overall r e s o l u t i o n is 10 k e V for the t r i t o n spectrum, 15 k e V for the d e u t e r o n s p e c t r u m a n d 13 keV for the p r o t o n spectrum. The excitation energies a n d the cross sections are listed in tables 1, 5 a n d 6 with assignments, a n d t h e level schemes are shown in figs. 5 a n d 6. T h e m e a s u r e d Q-values are - 0 . 7 4 6 _ _ 0 . 0 1 0 M e V for the 238Pu(d, t)237pu r e a c t i o n a n d 3.423___ 0.010 M e V for the 23Spu(d, p)239pu reaction. This is in g o o d a g r e e m e n t w i t h the values - 0 . 7 4 0 _ + 0 . 0 0 6 M e V a n d 3.431 _+0.003 M e V given in the 1971 m a s s table b y W a p s t r a a n d G o v e 1 o).

3. Calculations I n the early w o r k s l i , 12) o n the N i l s s o n m o d e l the values x = 0.05, # = 0.45, 0.448 a n d 0.434 for N = 5, 6 a n d 7 orbitals respectively were used for the strength p a r a m e t e r s in the oscillator p o t e n t i a l (set A ) . M o r e recently the values x = 0.0641 - 2.6 × 10- 6 A a n d # = 0 . 6 2 4 - 1 . 2 3 4 x 10- 3 A, w h e r e A is the mass n u m b e r o f the actual nucleus, have been f o u n d 13) to r e p r o d u c e the e x p e r i m e n t a l sequence o f g r o u n d states in b o t h the rare e a r t h a n d the actinide region o f d e f o r m e d nuclei (set B). F o r the N = 7 orbitals originating f r o m the j ~ shell m o d e l state a n d for the N - 5 orbitals o b s e r v e d in the actinide region the two sets lead to very similar wave functions; however, for the N = 6 orbitals in the s a m e region the wave functions a n d consequently the p r e d i c t e d fingerprints are r a t h e r different, as illustrated in fig. 4.

548

T. GROTDAL e t al. X2=27

-4 £;,= 0.202

E2= 0.184 _~ E4=-0.0 57

£4=- 0.0/.2 1

I

,

-4 631 f i

~37U

-4 633 i -4 -4

I

~=0.22

I

32=14

~ ~=0.202

X2= 7

i

z

,~/~ ,,/~ 3l~ ,/~ ~/~ ~/~

rr v--

X2=36

_~ E 2 = 0.208 E~ =-O.03&

127 <

,~/~ ,,/~ ~/~ ,/, ~/~ E~= 0.226 E~.= -0.021

239U

,I ,~ =0.251

-/2 "/2 9/, 7, °t~

X2=12

62 z I

~_1

_

X~=9

/

X2=17

£2= 0.209 Ej-0.039

[__ 231Th

z

I I

x2=13

631

l

ill

239pu

I

X2=6 ]

5 =0.22

t E2= 0.226 64 = - 0.02 l

x 2= 6

245Cm

620 f

2~ 9Cm. i

x2: 2

N =0.251

X2 = 0.8 ![

l

~=0.251

,3/, ,,/, ,/, ,/~

Fig. 4. Fingerprints for N = 6 Nilsson orbitals calculated with the new (upper graph) and the old (lower graph) parameter set, ,~ and/*. The experimental cross sections (middle graph) are taken from ref. 22), ref. 16), ref. 2a) and the present work for Cm, Th, U and Pu respectively. The Z2 is calculated without regard to the absolute values, whereas the three graphs for each orbital are drawn to the same scale. TABLE4 Optical-model parameters used in the DWBA calculations Particle

d p t n

V (MeV)

Ws (MeV)

ro (fm)

a (fm)

ro" (fro)

a' (fm)

Re/'.

108 57 150 40-60

68 74 64

1.15 1.25 1.40 1.25

0.81 0.65 0.65 0.65

1.34 1.25 1.40

0.68 0.47 0.65

25) 26)

27)

In the calculations with set B the values o f the deformation parameters (e2, ca) given in ref. *4) were used, a l t h o u g h the inclusion o f the hexadecapole term is n o t essential. The deformation parameter 8 used in c o n n e c t i o n with set A was c h o s e n to give roughly the same quadrupole m o m e n t as that obtained with set B. From fig. 4 it is clear that in the prediction o f fingerprints set A is in better agreement with experiment than set B. Set A was therefore used throughout this work. The deformation parameter 5 was set to 5 = 0.22.

z37.2agpu LEVELS

549

The DWBA calculations were carried out by means of the code JULIE with the parameters given in table 4. Comparison of experimental cross sections with theoretical ones were made at Q = 0 for the (d, t) and Q = 3 MeV for the (d, p) reaction. The experimental cross section were reduced to these Q-values by the use of the calculated Q-dependence for an I = 2 transition. The influence of Coriolis interaction was investigated by means of the code SNOOPY 15) which includes a search routine for fitting the measured excitation energies. In most cases the changes in cross sections introduced by the Coriolis interaction account for only a fraction of the discrepancies between observed and calculated cross sections. (See tables 2, 3 and 7.) 4. Discussion of the energy levels in 237pu

The 743 T orbital. Studies of the 241Cm or-decay have identified the ground state as the ~- member and the level at 47.7 keV as the ~- member of the 743 T band in analogy with the level scheme of 235U. Later measurements 3) of logft values in the electron capture decay of 237pu and the large hindrance factors observed 2) for the ~-transitions to the ground state rotational band in 237pu are consistent with these assignments. In the (d, t) reaction the ~- and ~- states are weakly populated. This is in accordance with the calculated fingerprints of the 743T orbital, and thus further supports the previous assignments. In the present work the -~--, ~-~-- and ~ - members of the 743T band are observed at excitation energies of 106, 183 and 257 keV. The 90°/125 ° cross section ratio for the level at 106 keV indicates an l = 5 transition whereas the ratio for the level at 257 keV is typical for l = 7. The other band members are too weakly populated to give meaningful ratios. The 743T rotational band is somewhat compressed compared to the other bands observed in 237pu and in the (d, t) reaction the 3~_- and ~ - states are populated with twice the predicted intensity. This can be understood in terms of the strong Coriolis interaction, with matrix elements ( j _ ) , ~ 7, between Nilsson orbitals originating from the j~ shell. Coriolis calculations were performed with the eight N = 7 orbitals in question included. The excitation energies were calculated by means of the Nilsson model. The rotational parameter was kept equal for the bands. The best fit to the observed excitation energies was obtained with the rotational parameter A = 7.28 keV and the Coriolis matrix element reduced to 53 ~ of the Nilsson model value. The 631J, orbital. This orbital is previously known from studies of the 24tCm or-decay ~, 2) and the electron capture decay 4) of 237Am, where the members of the rotational band up to ~+ were identified. In this work the characteristic cross section pattern of the rotational band is observed with excitation energies of 146, 156, 199, 224 and 305 keV for the five lowest spins. Also the ratio of the experimental cross sections at 90 ° to that of 125° supports the identification of the band. The absolute

550

T. GROTDAL et aL 237 P U

3/2.-~C 1014 501t 5/2--.~-C852 503~.

3/2 A 591

5/2 "-~-582 1/2 - - 545

501~

UNASSIGNED 9/2...~B486 9/2 A 513 7/2.~-B t.Tl 624

~j2_~_B437 7/2~4~ 512~&05 633 .i.

5 / 2 ~ A t.05

631 f 9/2~A 305 5/2..~.-A20o 15/2"-~-A257 7/2~22t., 622 T 13/2._A._A183 512--199 11/2oA 106 9 / 2 ~ 50 7/2~ 0 7z,3

6311.

Fig. 5. Level scheme for 237pu. cross sections are larger than predicted, the discrepancy increasing with spin. This was the case 16) also in 233Th and has recently been discussed by Erskine 17) who concluded that the disagreement probably results from inadequate treatment of the asymptotic tail of the bound state wave function in the D W B A calculations. The 631 + orbital is almost unaffected by the Coriolis interaction, and the rotational parameter A = 6.21 keV might therefore be considered as typical for the N = 6 orbitals in 237pu" The decoupling parameter is a = - 0 . 4 7 which can be compared with the Nilsson model value a = - 0 . 9 6 . The 622 T orbital. The levels at 280 and 371 keV are ascribed to the ~r+ and ~+ members of the 622i" band based on energy systematics and the 90°/125 ° cross section ratios. Other band members are not observed in the (d, t) reaction. The absolute cross sections are 3-4 times larger than predicted. The discrepancy is considerably reduced when the Coriolis interaction is taken into account (cf. table 3). The parameters used in the calculations are discussed in connection with the 633+ and 6311" orbitals, The assignment of the level at 280 keV is in agreement with previous observations 4).

237.239pu LEVELS

551

The 631T, 633~ and 624~ orbitals. The 6311" orbital is characterized by strongly populated ~+ and ~+ states. From energy systematics and absolute cross sections the levels at 405 and 513 keV are ascribed to the ~÷ and ~+ members of the 631~ band. The small group at 454 keV fits into the rotational band and is ascribed to the ~ + state. The 90°/125 ° cross section ratios support these assignments. The ½+ and 11 + states are very weakly populated and the calculated energies indicate moreover that they are concealed by the groups belonging to the ~ 6221" state at 371 keV and the 501J, state sat 591 keV. With regard to the 633~ orbital the groups at 437 and 486 keV have cross sections which agree with the theoretical cross sections of the ½+ and ~+ states respectively, and the 90°/125 ° cross section ratios indicate 1 = 4 transitions for both groups. These groups are therefore ascribed to the 5 + and ~+ members of the 633~ rotational band. From this assignment the ~+ state is expected at approximately 400 keV, and is consequently not observed due to the strong group belonging to the ~ 6311" state. The assignments of the ~+ and ~+ members of the 631T and 633J, bands are consistent with recent results from the study of 237Am electron capture decay 4). In the isotone 235U the 633~ and 6311" orbitals are observed at 333 and 393 keV respectively. This level ordering is preserved also in the nuclei 233U, 231Th and 233Th contrary to the observations in 237pu" The 90°/125 ° cross section ratio for the group at 471 keV indicated l = 4 and this group is ascribed to the ~ 624~ state, in agreement with recent electron capture decay studies 18). Because of the small energy difference between the 631 T, 633~ and the 624~ orbitals the Coriolis interaction is expected to have a considerable effect upon the band structures. Coriolis calculations were performed which included the orbitals 624~, 6221", 631~, 6311` and 633~. The influence of other N = 6 orbitals was found to be negligible. G o o d energy fits were obtained with the values A = 6.3 keV, for the rotational parameter, and a = - 0 . 4 7 for the decoupling parameter of the 631J, orbital. The Coriolis matrix elements were reduced to 25 ~ of the Nilsson model value. The resulting cross sections and mixing amplitudes are given in table 3. Although the states are strongly mixed the cross sections were not much changed, even if the matrix elements were increased to approximately 50 ~ of the Nilsson model values. The energy fit was in the latter case considerably worsened. The 501~ orbital The ½ 501J, state is the most strongly populated state in all actinide nuclei which have been investigated by the (d, t) reaction, and is therefore unambigously identified. In 237pu the group at 545 keV, which has a cross section of 352 #b/sr at 90 °, is ascribed to the ½- state. The group at 591 keV is ascribed to the ~- member of the rotational band on the basis of the 90°/125 ° cross section ratio and the absolute cross section. The group at 582 keV is tentatively assigned to the ~ 501~ state. Both the absolute cross section and the angular distribution are consistent with such assignment. The deduced band parameters are A = 6.77 keV and a = 1.27. The decoupling parameter predicted by the Nilsson model is a = 0.9.

552

T. GROTDAL et aL

The 503~ and 501T orbimls. According to the Nilsson scheme the sequence of N = 5 orbitals in the actinide region is 5015, 503~ and 501T in order of increasing excitation energy. Both the ½ 5015 and ~ 501T states have predicted cross sections of the order of 1000 pb/sr at 90 ° for Q = 0. In the reaction 232Th(d, t)231Th the ½ 501~. and g25011` states were observed at 551 and 869 keV respectively, both with the expected cross sections. In analogy to 23~Th the 1014 keV level is ascribed to the ~ 501T state. The 90°/125 ° cross section ratio is in agreement with an l = 1 transition, but the absolute cross section is only 50 Y/ooof the theoretical value. Also in analogy to the case of 231Th the level at 852 keV tentatively is ascribed to the ~z 5035 state. As for the {: 501T state the cross section is only half the expected, and the unassigned groups in the triton spectrum cannot account for the lack of cross section, 5. Discussion of the energy levels in

239pu

The 631~ orbital. The ground state spin of 239pu is -12 as determined by atomic beam x 9) and paramagnetic resonance 2 o) measurements. Studies of the E-decay 5) o f 239Np,the electron capture decay 6) of 239Am and the ~-decay 7) of 243Cm have led to a ½ 631~ assignment for the ground state and to the identification of the five lowest members of the rotational band. A level at 193 keV has tentatively been ascribed to the -~J-+ state. The ½+ and {r+ states are separated by 8 keV only and the corresponding groups are experimentally not resolved in the (d, p) spectra. To obtain the cross sections a least squares peak fitting routine was employed with a constraint on the energy separation. The ~+ 2 state is very weakly populated in the (d, p) reaction and was not observed, whereas the remaining band members up to and including the ½___Ll+state were observed with the cross section pattern characteristic for the 6315 rotational band. The three point angular distributions support these assignments. The 622T orbital The previously established rotational band associated with the 622T orbital is confirmed by the present experiment. The 6221" orbital is easily identified in most cases due to the characteristic strong ~+ state, and the {r+ state which has normally ~ 25 ~ of the ~+ 2 cross section at 90 °. In addition to the ~+ and ~+ states which are observed at 284 keV and 386 keV respectively, the ½+ state is observed at 326 keV with a cross section of 1/zb/sr. The group at 464 keV is partly ascribed to the 12--~-+state and partly to the ½- octupole state discussed below. The K ~ = ½- octupole band. The two prominent groups at 505 and 558 keV observed in the 239pu(d, d')z39pu reaction both have a 90°/125 ° cross section ratio which is characteristic for E3 transitions 24). The summed cross section of the two levels is 69/tb/sr at 90 °. In a concurrent study 21) o f the 238pu(d, d')238pu reaction a 3- state at 659 keV belonging to the K ~ = 0 - octupole vibrational band is populated with a cross section of 85/tb/sr for the same angle and bombarding energy. This

237. 2 3 9 p u L E V E L S

553

239 P u

9/2-~.-C1233

613'[" 11/2 +1137

9/2.-~-C1409 912 C 1359 7/2(:1342 712C~1311 5/2,....~_C1289 5/2~C 1261 3/2-'-1261 312"3"-'~233 622~ 1/2--1214 620't

5/2 --1100 7/2 --~ 1038

1/2--'1017 3/2-'~'990 7615

15/2 A---620

t1/2-~-8 63t,.

7/2---.-A558 A 505 1112----A488 5/2 3/2~/-..88 743t ' 1/2"~-464 11/2C'~-464

9/2 '--~'-B565

7i2C507 624,~

UNASSIGNED

(631~,Q3o) 9/2~A 386 7/2---@-8326 5/2 A--~.-28& 622t 1112--,~-A191 9/2--'=-163 7/2 A-~- 75

312A 8 1/2~ 0 6314,

Fig. 6. Levelscheme for 2a9Ptl. suggests that the levels at 505 and 558 keV in 239pu are the { - and ~- members of a K ~ = ½- rotational band which is mainly described as the K ~ = 0- octupole vibration built on the 631~ orbital. The four lowest spin members of this band have previously been observed at excitation energies of 469, 492, 505 and 556 keV in studies 5) of the fl-decay of 239Np, but the band was then proposed as the 501,L Nilsson orbital. The ½-, 3 - and { - members have also been observed in the (d, p) spectra, but the fingerprint pattern does not resemble those of the low-lying ½- Nilsson model states. This is true even if part of the cross section of the 488 keV level is ascribed to the ~ - 7431" state. It is therefore indicated that several orbitals contribute to the (d, p) population of this band. The (d, d') data clearly demonstrate the collective nature of the band in 239pu and probably represents the only case in which clear evidence has been obtained of an octupole excitation in an odd-A actinide nucleus. The 7431" orbital. Only the ~ - - and -~-- members of the 7431" band are populated with observable strength, according to theory. The ~-, $- and ~ - - states have pre-

554

T. G R O T D A L et aL TABLE 5 Levels populated in 2agPu by the 2aaPu(d, p)239pu reaction Energy average

Assignment

E a = 12.1MeV 90 ° 125 °

(d, p) (keV)

0

½ 631~

8

~ d31~

75 163 191 284 326 386 464 488 507 538

565 620 634 658 716 749 780 885 901 990 1017 1038 1100 1137 1174 1214 1233 1261 1289 1311 1342 1359 1390 1409 1437 1465 1488

d~r/d.Q(d, p) (/zb/sr)

½ 631~ ~ 631~ ~ 631~ ~ 6224 ½ 6224 ~ 622~ ~ 6 2 2 t , ½(631~, Qao) ~ 7 4 3 ~ , ~(631~, Qso) ]624~, ~(631~, Q3o)

~ 624~ a~ 7434 ~ 624~

~ ~ ~ ~ ~-

761~ 761~ 761~ 761~ 761~

½ 6204 ~6214, ~ 6134 ]6204, (~ 622~) ( t 622~) ~ 6204 (½ 6224) ~ 620~ (~ 6224)

84 110 19 46 5 27

42 60 16 43 5 16

1 129 11 27 28 <5 52 4 9 3 <3 5 14 43 15 193 33 180 36 34 43 275 207 244 126 54 58 50 15 55 61 26 40

E a = 13.1MeV 150 ° 16 39 8 28 5 14

<5 128 6 15 11 6 46 11 13 3 2 3 10 30 8 98 44 147 37 40 33 153 149 164 87 50 51 60 22 45 50 14 36

84 4 5 6 9 28 11 13 3 4 <3 <32 < 17 59 36 66 30 42 23 78 91 90 52 38 30 47 21 31

viously been tentatively assigned a) from decay data. In the present work the -~-state coincides with a ½- state at 488 keV. The level at 620 keV is ascribed to the ~-~state on basis of the ratio of the cross sections at 90 ° and 125 ° which clearly indicates high spin.

237.239pu LEVELS

555

TABLE6 Levels populated in 2s9ptl by the 239pu(d, d')2S9pu reaction Energy average

Assignment

(d, d') (keV)

0 59 505 558 751 777 798 825 855 903

½,~ 631~ ~ , ] 631~ i(631~, Qao) ½(631~, Q3o)

da/d~2(d, d') ~b/sr) 90 °

120 °

229500 5500 31 38 10 8 16 12 13 13

65250 3750 53 63 11 7 8 12 15 16

The 624J, orbital. A level at 511.8 keY with spin and parity { + or 3 + has previously been established 5) and the proposed assignments are 633~ or 624J,. The levels observed in the present work at 565 and 634 keV constitute together with the 512 keV level a rotational band. The 90°/125 ° cross section ratio clearly indicates that the level at 634 keV corresponds to an 1 = 6 transition and this level is therefore ascribed to the ~ - 624~ state. The level at 565 keV has a typical I = 4 cross section ratio and is ascribed to the ~ 624+ state. The 3 + state is weakly populated in the (d, p) reaction, but might be part of the group at 507 keV. In the isotone zsTU the 624~ orbital is observed at 426 keV and the 633~ orbital at ~ 900 keV. The energy systematics therefore supports the present assignment of the 624~ orbital in 2a9pu. The 761J, orbital. This orbital has a fingerprint which is practically identical with that of the 6201' orbital and the angular momentum transfers differ by only one unit for corresponding members of the two bands. On the basis of the fingerprints alone it is therefore not possible to distinguish between the two orbitals. Both the 6201" and the 761~ orbitals are identified in 235U and in 249Cm. In 235U the 761~ orbital is found at lower excitation energy than 620T whereas in 249Cm the opposite level ordering is observed. In both cases the 620 T orbital is observed at an excitation energy close to that of the 6131" Nilsson orbital in accordance with the Nilsson level scheme. In 239pu the actual fingerprint is observed both at ~ 1000 and ~ 1200 keV of excitation energy. Since the strong ~ 613 T group is observed at 1233 keV it is concluded that in the case of 239pu the 761~ orbital is found at ~ 1000 keV. The strongly populated levels at 990 and 1038 keV are assigned to the ~ - and 3 members of the 761~ rotational band while the group at 1017 keV is assigned to the ½- state. The band parameters calculated from these states are for the rotational parameter A = 6.60 keV and for the decoupling parameter a = - 2 . 3 6 as compared with the theoretical value a = - 3 . 5 for 6 = 0.22. The calculated energies for the ½- and-~-- states are 1101 and 1139 keV respectively

556

T. GROTDAL et aL

TABLE 7 Observed and calculated energy levels artd differential cross sections in the 2aSpu(d, p)Z39pu reaction Nilsson orbital

631~

a~. 622~

Excitation energy (keV)

Differential cross section, Q = 3.0 MeV, 0 = 90 °, Ea -----12.1 MeV (,ub/sr) theor,

expt.

0 8

96 [10

102 132

75 163 191

9 9 18 2

22 52 5

284

½

386

743~ 488

35 o 102 3 0 0 < 1

28 130

26 a)

0

a~

621 624~ (565) (634)

< 1

3

7 17 12

33 8

<1

½ 7614

½ 620~

(1017) (990) (1100) (1038)

25 149 26 137

(1137)

27 123 22 135 7 23

1214 1238 1261 1311 1359

195 23 188 47 33

193

25

168 b) 36 33

2

6224

(1261) (1289) (1342) (1390)

114 72 79 22 2

a) The ~ 743I' and j (6314, 0 - ) states are not experimentally resolved. b) The ~ 620~' and ~- 6224 states are not experimentally resolved.

168 b) 78 38 9

asT. z39pu LEVELS

557

and the groups observed at 1100 and 1137 keV are ascribed to these levels. Finally the ~ - state is predicted to be a weakly populated level at 1238 keV but cannot be experimentally resolved from the strong groups observed in that energy region. The cross section pattern is in good agreement with theory. The 620T orbital. As discussed in connection with the 761~ orbital the fingerprint corresponding to the 620~ orbital is found at ~ 1200 keV of excitation energy. The group at 1214 keV has one of the largest cross sections observed in the 238pu(d, p)239pu reaction, and the ratio of cross sections at 90 ° and 125 ° clearly indicates low spin. This group is therefore ascribed to the ½+ state. The groups at 1261 keV and 1311 keV are ascribed to the {+ and 5 + states respectively, based on the cross section patterns for the 6201' rotational band and on the cross section ratios. When compared to the fingerprints of the well established 620T bands observed in the 248Cm(d, p)249Cm reaction 22), the -~+ member at 1261 keV in 239pU appears to be too strongly populated. This may be explained by the presence of the ~ 622+ state in the 1261 keV group. The assignments of the three groups lead to the band parameters a = 0.17 and A = 6.13 keV. The theoretical value of the decoupling parameter is a = 0.09. The calculated excitation energy of the ~:+ state is 1235 keV. The group corresponding to this state is concealed by the strong group at 1233 keV, which is assumed to contain the ~z 613T state. The ~ 620T state is assigned to the group at 1359 keV which is close to the calculated energy of 1357 keV. The absolute cross section and the 90°/125 ° cross section ratio are both in agreement with this assignment. The 622~ orbital. The proton groups at 1289, 1342 and 1409 keV are tentatively ascribed to the {+, 5 + and ~+ z members o f the rotational band built on the 6225 orbital. The assignments are based on the cross section pattern. The proton group corresponding to the ~:+ state is assumed to be part of the strong group at 1261 keV. The 613T orbital. This orbital is characterized by the strongly populated ~+ state, which is the only member of the rotational band with observable cross section. This state is tentatively assigned to the group at 1233 keV, the only group with large cross section which does not fit into a rotational band. The excitation energy is also in agreement with the predictions from the Nilsson energy level scheme. 6. Conclusion

The sequence of orbitals observed is in agreement with the Nilsson model. The fingerprints calculated with the new set of Nilsson potential parameters, ~ and /~, are rather different from those calculated with the original set, and only the latter gives satisfactory agreement with experiment. Out of the three strongly populated N = 5 orbitals, 501,[, 503J, and 501 T, only the 501 J, orbital is observed with its full strength in the 238Pu(d, t)ZaTpu reaction, whereas the 503,[ orbital is populated with ~ 25 % and the 5011" with ,,~ 50~o of the predicted strength.

558

T. GROTDAL et at.

The 239pu(d, d ' ) 239Pu reaction clearly d e m o n s t r a t e d the collectiveness of the ~ state at 505 keV a n d the ½- level at 556 keV of excitation energy, a n d it is concluded that these states are m e m b e r s o f a n K ~ = 0 - octupole v i b r a t i o n a l b a n d based o n the 631,[ Nilsson orbital. The a u t h o r s w a n t to t h a n k dr. Geir Sletten for the p r e p a r a t i o n of targets. We t h a n k Irene Jensen, R a g n h i l d F a g e r b a k k e a n d I n g a Nilsen for careful plate scanning, a n d L i n d a Stoltz Olsvik for assistance with the peakfitting program. T. G r o t d a l a n d K. N y b o acknowledge s u p p o r t from Det Videnskapelige F o r s k n i n g s f o n d av 1919, a n d T. F. Thorsteinsen acknowledges a g r a n t f r o m Norges Almenvitenskapelige Forskningsr~td. The c o l l a b o r a t i o n between the Niels Bohr Institute a n d the Institute o f Physics, University of Bergen, has been supported by N o r d i s k K u l t u r f o n d .

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28)

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