Energy levels in even actinide isotopes from (t, p) reactions

I

~

Nuclear Physics A217 (1973) 116--124; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

E N E R G Y L E V E L S I N EVEN A C T I N I D E I S O T O P E S F R O M (t, p) R E A C T I O N S t B. B. B A C K t t , t t t , E. R. F L Y N N

a n d O L E H A N S E N tt

Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico, USA 87544 and R. F. C A S T E N a n d J. D. G A R R E T T

Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico, USA 87544 and Brookhaven National Laboratory, Upton, New York, USA 11973 Received 20 A u g u s t 1973 Abstract: Energy levels in 232,234Th ' 236.23s. 24o U a n d in 2 5 ° C m have been m e a s u r e d u s i n g the (t, p) reaction. A n g u l a r distributions were o b t a i n e d for the 234"23sU targets a n d evidence for second order effects in t h e direct reaction m e c h a n i s m was found.

E

N U C L E A R R E A C T I O N S 234"236U(t, p), E = 15 MeV, 23sU(t, p), E ~ 16 MeV, 2ao. 232Th(t ' p), E = 20 MeV; m e a s u r e d cr(Ep, 0). 24aCre(t, p), .E = 15 MeV; m e a s u r e d tr(Ev, 0), Q. 232.234Th, 236.23s. 240U, 250Cm deduced levels, J. ~.

1. Introduction The nature of low lying 0 + states in the actinide region has been studied intensely over the past few years ~-5). A search for such 0 + -0 0 + (t, p) transitions was published in ref. 4) and the present note describes the remainder of these (t, p) data.

2. Experimental procedures and results The 24SCm target s was prepared by isotope separation at the Argonne National Laboratory while the remaining targets were made by vacuum evaporation. Carbon foils were used as backings in all cases. The targets ranged from ~ 100 #g/cm 2 to ~ 300/~g/cm 2 in thickness and were of > 95 % enrichment with the exception of 23°Th, which was 90 % enriched. The tritons were accelerated in the Los Alamos Van de Graaff accelerators to energies of 15 MeV (24SCm, 234, 236U) ' 16 MeV (23SU) or 20 MeV (230, 232Th). The reaction protons were m o m e n t u m analyzed in a broad range spectrograph and detected in photographic emulsions placed along the focal plane. The energy resolution was f r o m 15 to 20 keV F W H M . An example of a spectrum is shown in fig. 1 from the 234U(t, p)Za6U reaction. It t Work performed under the auspices of the US Atomic Energy Commission. tt Now at the Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark. **t Supported by Statens naturvidenskabelige Forskningsr~d, Denmark. ** We are indebted to J. Lerner and A. Friedman for their courtesy in supplying this target. 116

E V E N A C T I N I D E ISOTOPES

~

'X] ]

JU

2:

~1074 ,.bi

~{~1 ~7,, ~ B

0

~80gs

P~

1592 ]

2-

z

T254U(I],236U *

96

117

I t

Itt

i

li :

.::'..

ss~

98 r00 DISTANCE ALONG PLATE (crn)

,~o i/f!,

102

104

Fig. l. P r o t o n energy s p e c t r u m f r o m the 234U(t, p)236U reaction m e a s u r e d at a lab angle o f 35 ° a n d at a n incident energy o f 15 MeV. T h i s s p e c t r u m was chosen at a n angle c o r r e s p o n d i n g to a m i n i m u m in the g r o u n d - s t a t e a n g u l a r distribution (see fig. 2). G r o u p s c o r r e s p o n d i n g to reaction p r o d u c t s f r o m light target impurities are labeled by the c o r r e s p o n d i n g final nucleus or by an I. 234U ( t , p ) 256U

• ,!

t~

~%7s i

~" I:'k

:-

:2

•

'

•

46

,

,o' i

!

r

i

752 : ;

i

i

t

io%- : IO°~- !

sss! ;

:

i074 ~ 101~ ioO~-

i ~

;~loO~

0

30

60

0

30

60

i

:.~ ;592~

I0°.~- ;

i

0

30

4 60

8c.m.(deg) Fig. 2. A n g u l a r distributions o f reaction products c o r r e s p o n d i n g to 2s~U(t, p) transitions to various final states in 236U. T h e incident triton energy was 15 M e V . 238U{t,p) 240 U

....

-' y:l

I' L 0.1 ~=

' 45

• •

•

0.Ol I

1545 1 i q O.Ol

!,

t

0.1"~-

•

a

I......

, [

'

iI

'

1596

~-

'

'792 1

1

°°J r

1895

0.01F 0.01F" 0"1 t

O.I~-E

~ , ; ; ' • • 1040 ; "

0.01~ 0.001~ ~ •

!

i

f

f

0.@ -

o.4'160 ; ~; ~ t

1670

0.01~- , ' ' F

I

=

1929

,708 1 o,o,_~- ,"

b

-! , ;

0.01_.k ~

!

I 0.01iF '

60 [i o.ool~8:.m (deg}

Fig. 3. A n g u l a r distributions o f reaction p r o d u c t s c o r r e s p o n d i n g to 238U (t, p) transitions to various final states in 24°U. T h e incident triton energy was 16 MeV.

1240

5.0 2.5 2.8

4.5 5.0

3.1 ,< 2

110 26 10 7.5

am,~ (arb. units)

7 5 5

6 5

3

8 6 4 4

0+ (2 +)

0+ 2÷ 4+ 6+ 8+ 2-

no. dTr b) of angles

11694-5 1237~8

10544-5

0 454-2 1544-5

Ex a) (keV)

1270 1416.8

0 44.92 148.41 307.21 518.3 676 732 776.6 993 1035 1058 1077.8 1127

Ex b) (keV)

236U(t, p)238U,

18 3

2.6

< 1.5

81 20 10

am,x (arb. units)

Ela b ~

3 3

2

3 3 2

no. of angles

15.0 MeV

6+ 14 +

0÷ 2÷ 4+ 6÷ 8+ 1310 + 0+ 2+ 2+ 12 + 4+

j~r b)

0 454-1 1514-2 10404-5 11604-5 15454-5 1596z~5 16704-5 17084-5 17564-5 17924-5 18934-5 19294-5 20104-5

E~ ") (keV)

0

E~ c) (keV)

0.30 0.10 0.023 0.014 0.06 0.024 0.05 0.03 0.03 0.025 0.045 0.02 0.03 0.03

O'max (mb/sr)

7 7 7 5 7 5 6 6 7 6 4 5 5 5

0+ 2+ 4+

no. j~r c) of angles

238U(t, p)24°U, Eta b = 16.0 MeV

b) M. Schmorak, C. Bemis, M. Zender, N. Gove a n d P. Dittner, Nucl. Phys. A178 (1972) 410; C. Lederer, J. Hollander and I. Perlman, Table o f Isotopes, 6th ed. (Wiley, N e w York, 1967); a n d ref. 2). ~) G r o u n d state J from its doubly-even character; 2 + and 4 + assignments from the systematics o f the energy levels.

a) Present data.

12674-5 13554-10 13924-5

10744-5 12084-5

920 959

0 45.24 149.48 309.79 522.18 687.6

0 464-1 1504-2 3134-5

7524-5

Ex b) (keV)

Ex a) (keV)

234U(t, p)236U, EIa b = 15.0 MeV

TABLE 1

U(t, p) results

7;

>

oo

a.,ax

3

2

< 2 13

12

3 2 2

of angles

no.

73 25 7

(arb. units)

- ' ) See footnotes to table 1. d) C. Lederer et al. o f b).

10774-5

1095 1138.5

0 49.37 162,12 333.6 556.9 730 774 778 827.8 873 1023 1045

0 484-2 1644-5

776jz5

E~ ~ )

(keV)

Ex a)

(keV)

12 +

0÷ 2+ 4+ 6+ 8+ 0+ 2+ 2+ 10 + 4+ 6+ 6÷

j,~ b)

2a°Th(t, p) 232Tb, Ela b = 20.1 MeV

TABLE 2

E~ a)

0 484-2 1604-5

(keV)

0 48 160

Ex d) (keV)

83 30 3

units)

O-max (arb.

2 2 2

no. of angles

0÷ 2+ 4+

j1r

2a2Th(t, P) 234Th, Et~b = 20.1 MeV

Th and 24SCm(t, p) results

0 494-2 Qo = 2064 k20 keV

(keY)

E, a)

3 3

of angles

no.

0+ 2+

1 . c)

24SCm(t, p) 2S°Cm, Elnb ~ 15.0 MeV

O -t 0 r~

>

Z

< m

120

B . B . BACK. et al.

demonstrates the basic experimental difficulty: the (t, p) yields from the actinide targets are low and the spectrum is obscured by strong contaminant groups from light target impurities. Each spectrum represents about 20 h of accelerator time, i.e. from 15 to 20 mC of integrated beam current. Angular distributions were observed only for the 234U and 23sU targets, and they are shown in figs. 2 and 3, respectively. We have used a somewhat more lenient criterion than usual for assigning an energy level: all groups observed at three angles or more are cited in tables 1 and 2. Groups corresponding to known levels are cited if observed at two or more angles, and upper limits on cross sections to the excited 0 + states observed 1. z) in (p, t) also are listed. Absolute cross sections were measured for the case of z3su by observing elastic scattering and (t, p) reactions simultaneously with a counter telescope and normalizing the elastic scattering to Rutherford scattering at the most forward angles. An external error of +_20 % is assigned to the differential cross sections. A Q-value of 2064+20 keV was measured for the zgScm(t, p) transition to the 250Cm ground state. This value corresponds to a binding energy of 73014+ 20 keV for the 25OCm ground state compared to 73070 keV predicted from mass systematics 6).

3. Angular distribution shapes and DWBA analysis A study of the angular distributions presented in figs. 2 and 3 reveals some simple trends. The 0 ÷ ~ 0g+ and 0 ÷ -~ 2g+ distributions are characteristic and do not change from 2agu to 23sU. [A subscript g indicates a member of the ground state rotational band.] The 0 ÷ -~ 4+ distributions are different in the two cases, with the 16 MeV data (2asU)showing less structure than the 15 MeV results (234U). Distributions for higher lying states are generally featureless, rising steadily toward larger angles. It is not possible to assign L-values on an empirical basis except for the characteristic L -- 0 case. These results are similar to those obtained at a lower bombarding energy in ref. 7) and are consistent with the (p, t) measurements of ref. 2). DWBA predictions are shown in figs. 4 and 5 for L = 0, 2, and 4 transfers; pure configurations were used and the optical model parameters obtained from elastic triton scattering s) on Pb were employed combined with the Perey proton parameters 9) (see table 3). Similar optical model potentials have been successful in reproducing two-neutron transfer angular distributions in the Pb [ref. 1o)] and actinide [ref. 4)] regions. This proceedure should be reasonable judging from the results of analysis of transfer reactions on deformed nuclei in this and other regions of the periodic table 1~- 13). The agreement between the predicted L = 0 angular distributions and that of the observed ground-state (t, p) transitions is satisfactory; however, similar agreement was n o t obtained for the L = 2 transitions (figs. 4 and 5). This result is similar to other (t, p) data on deformed nuclei 11, 14, is), where the observed 0 + -~ 2 + shapes vary markedly from state to state and from nucleus to nucleus. The difference in the

EVEN ACTIN1DE ISOTOPES 10 3

i

l

121

I --F- 7--

234 U (l, P) 236U

E t : 15.0 MeV

/

10 2

FO j

c

.,Q o

102¸

lo3

46keV

2+

2 150 keV 4 *

4

P

I0 ° L

0

1.

':

.~ . ~

_~,

30

60

8c.m. (deg)

Fig. 4. Comparison of measured angular distributions (points) and DWBA predictions (lines) for 2a4U(t, p) transitions to the jTr = 0 +, 2 + and 4 + members of the 2~6U ground-state rotational band. t w o 0 + ~ 4 7 distributions is not reflected in the D W B A predictions and in fact is a n e w feature o f (t, p) reactions on deformed nuclei. In the l s z w ( t , p) case 11) the L = 4 transitions were well accounted for by D W B A calculations. The failure o f the simple D W B A recipe (which works so well on spherical nuclei) to account for the angular distribution shapes in a systematic fashion for deformed

122

B . B . B A C K e t al. 1,0 F

i

i -7--F258U(t,p) 240 u

T-

I

~

Et: 16.0 MeV

o.l~.

z~ ;'

d 3

45keV 2 + b

O.OI

J

1

~

'/

o.ool I

tSI keV 4 +

I

r

[

0

J

[

30

~

!

I

60

ec.m.(deg) Fig. 5, C o m p a r i s o n o f m e a s u r e d a n g u l a r distributions (points) a n d D W B A predictions (lines) for 23sU(t, p) transitions to the j~r = 0 +, 2 ÷ and 4 + m e m b e r s of the 24°U g r o u n d - s t a t e rotational b a n d .

TABLE 3 Optical m o d e l p a r a m e t e r s

tritons protons neutrons

V

r0

(l

W

W"

r0'

(1'

156.7 53.1 a)

1,16 1.25 1.25

0.752 0.65 0.65

10.0 0.0

0.0 70.8

1.498 1.25

0.817 0.47

roc

1.30 1.25

Well depths are in MeV, geometry p a r a m e t e r s in fro. 3) Real well adjusted to give each n e u t r o n a b i n d i n g energy o f ~ ><(two-neutron separation energy). A T h o m a s spin-orbit t e r m was used with a strength o f 32 T h o m a s units.

EVEN ACTINIDE ISOTOPES

123

nuclei, suggests that higher order effects are important in the reaction mechanism. As sho~vn by several authors ~6-18) the inelastic channels (leading to excitation of the 2~- and 4~- states) are strongly coupled to the elastic channels and hence lead to modifications in the differential cross section for the two-nucleon transfer reactions. 4. Discussion

The selective excitation of the ground-state rotational band demonstrates the superfluid nature of the ground states. In the intrinsic system, to lowest order, the groundstate band is degenerate. Each member receives the superfluid two-nucleon transfer enhancement, well known from spherical superfluid cases [see e.g. ref. 19)]. The ratio a(2g+)/a(0~) is similar to what was observed for other deiormed nuclei such as the heavy Sm and Nd isotopes 14, ~5), namely a ratio from 0.25 to 0.30. In this respect the anomalously small a(2+)/a(0~ -) ratio observed in the 182W(t, p)lS~W reaction al) differs from all the other cases. The 0 ÷ ---, 0g+ transitions all have characteristic shapes, and experience in (t, p) reaction from the rare earth region [r~fs. 11, 14, ~s, 2o)] and from (p, t) studies in the actinides 2) implies that excited 0 + states have angular distributions similar to those of the ground state transitions. The lack of excited 0 ÷ strength in the present cases as discussed in detail in ref. 4) therefore is an important result in spite of the inadequacy of first order DWBA theory. The difference between the two 0 ÷ --* 4~ shapes observed here is interesting. If coupling to the inelastic degrees of freedom is responsible for this feature, one would qualitatively expect that the inelastic excitation of the 4 ÷ target or final state combined with the enhanced L = 0 transition either between 0g+ --* 0g+ or 4g+ ~ 4 + is the most important second order route; hence the present result would point to a different role of the f14 hexadecapole deformation in the 234U ~ 2 3 6 U and z38U ~ 24°U cases. This speculation, so far, has no support from other sources. It may be concluded that coupled channel effects are important and that detailed (t, p) spectroscopy in the actinide region requires an elaborate theoretical analysis as well as considerably more extensive systematic data than have so far become available. The help of T. J. Mulligan and S. Orbesen in the data taking process is warmly acknowledged. We would like to express our appreciation to J. Gursky for providing the evaporated targets and to J. Lerner and A. Friedman for supplying the isotope separated Z48Cm target. H. C. Britt is thanked for many helpful discussions. References

1) J. Maher, J. Erskine, A. Friedman, J. Schiffer and R. Siemssen, Phys. Rev. Len. 25 (1970) 302 2) J. Maher, J. Erskine, A. Friedman, R. Siemssen and J. Schiffer, Phys. Rev. C5 (1972) 1380 3) A. Friedman and K. Katori, Phys. Rev. Lett. 30 (1973) 102

124

B.B. BACK et al.

4) R. Casten, E. Flynn, J. Garrett, O. Hansen, T. Mulligan, D. B6s, R. Broglia and B. Nilsson, Phys. Lett. 40B (1972) 333 5) R. Griffin, E. Jackson and E. Volkov, Phys. Lett. 36B (1971) 281; W. I. van Rij and S. H. Kahana, Phys. Rev. Lett. 28 (1972) 50; S. Abdolvagabova, S. Ivanova and N. Pyatov, Phys. Lett. 38B (1972) 215; R. Chasman, Phys. Rev. Lett. 28 (1972) 1275; D. B6s, R. Broglia and B. Nilsson, Phys. Lett. 40B (1972) 338 6) A. Wapstra and N. Gove, Nucl. Data A9 (1971) 267 7) R. Middleton and H. Marchant, Proc. Second Int. Conf. nuclear masses, Vienna, 1963, ed. W. M. Johnson (Springer Verlag, Wien, 1964) p. 329 81) E. Flynn, D. Armstrong, J. Beery and A. Blair, Phys. Rev. 182 (1969) 1113 9) F. Perey, Phys. Rev. 131 (1965) 745 10) G. Igo, P. Barnes and E. Flynn, Ann. of Phys. 66 (1971) 60 11) R. Casten and O. Hansen, Nucl. Phys. A210 (1973) 489 12) E. Flynn, G. Igo, P. Barnes, R. Casten and J. Erskine, Nucl. Phys. A159 (1970) 598; R. Siemssen and J. Erskine, Phys. Rev. Lett. 19 (1967) 90 13) T. Braid, R. Chasman, J. Erskine and A. Friedman, Phys. Rev. C1 (1970) 275; C4 (1971) 247 14) J. Bjerregard, O. Hansen, O. Nathan and S. Hinds, Nucl. Phys. 86 (1966) 145 15) R. Chapman, W. McLatchie and J. Kitching, Nucl. Phys. A186 (1972) 603 16) R. Ascuitto, N. Glendenning and B. Sorensen, Phys. Lett. 34B (1972) 17; C. King, R. Ascuitto, N. Stein and B. Sorensen, Phys. Rev. Lett. 29 (1972) 71 17) T. Tamura, D. R. B6s, R. Broglia and S. Landowne, Phys. Rev. Lett. 25 (1970) 1507 [erratum: 26 (1971) 156] 18) D. Bronschweig, T. Tamura and T. Udagawa, Phys. Lett. 35B (1971) 273 19) E. Flynn, J. Beery and A. Blair, Nucl. Phys. A159 (1970) 225 20) J. Garrett, B. Back and E. R. Flynn, private communication