High-resolution study of the 230, 232Th(d, 6Li)226, 228Ra reaction at Ed = 45 MeV

Nuclear Physics A422 (1984) 45-60 @ North-Holland Publishing Company

HIGH-RESOLUTION STUDY OF THE 230923?h(d,6Li)226y2mRa REACTION AT Ed = 45 MeV A.M. VAN DEN BERG*, N. BLASI, R.H. SIEMSSEN, W.A. STERRENBURG** and Z. SUJKOWSKI*** Kemfysisch

Versneller Instituut, 9741 AA Groningen, The Netherlands

Received 12 September 1983 (Revised 19 December 1983) Ah&ad: A high-resolution study of the 230,232Th(d,6Li)226*28Rareaction has been performed to search for possible a-clustering effects in the residual nuclei. For **sRa angular distributions were obtained for 12 states, or groups of states, in the laboratory angular range from 7” to 31”. For 226Ra differential cross sections were measured at three different angles for 11 transitions. Strongly populated O+ and 2+ states are found around E. = 0.8 MeV in 228Ra and 1.2 MeV in 22aRa. Low-lying members of the K v = O- band are only weakly populated. The data are compared with hindrance factors from a-decay measurements and with (d, ‘Li) data obtained previously.

E

NUCLEAR REACTIONS 232*23eTh(d,6Li), E = 45 MeV; measured a( 0). 228*226Ra deduced levels, S,, reduced widths, reduced hindrance factors. Finite-range DWBA, enriched targets, magnetic spectrograph.

1. Introduction

Recently several models [refs. ‘-‘)I have been proposed to describe the properties of nuclei in the actinide mass region. An intriguing feature of these nuclei is the observation of strongly excited O+ states at an excitation energy of about 1 MeV in the (p, t) reactions 6). The strength for the population of these states is about 15% of the ground-state strength. Studies of the (t, p) reaction ‘> in the same mass region, in contrast, did not reveal a strong population of these same states. Excited O+ states in the radium isotopes are also populated strongly in a-decay experiments. Measured hindrance factors for the excited O+ states are typically smaller than ten “). A recent study of the 238U(d, 6Li)234Th and the 232Th(d, 6Li)228Ra reactions revealed strongly populated states in the residual nuclei around E, = 0.7 and 1.4 MeV [ref. “)I. An equally interesting feature of nuclei in the A = 230 mass region is the unusually low excitation energy (e.g. 216 keV in 224Ra) of the head of the K” =O- band. Different explanations for the occurrence of this low-lying. negative-parity band have been proposed. Octupole vibrations coupled to a static quadrupole deformed * Present address: Physics Division Argonne National Laboratory, Argonne, Illinois, USA. ** Present address: Vrije Universiteit, Amsterdam, The Netherlands. *** Permanent address: Inst. Nucl. Research, Swierk, Poland. 45

46

A.M. van den Berg et al. / High-resolution study

core were used to explain the excitation energies of members of the K r = 0’ and O- bands in 226Ra [ref. ‘“)I. From the data obtained from P-decay and a-decay experiments x,11,12),on the other hand, it was concluded by Kurcewicz et al. 11) that harmonic octupole vibrations cannot explain the observed spectra. In particular the non-observation of a O+ state at twice the excitation energy of the band head of the K r = O- band was taken as an argument against harmonic octupole vibrations. Therefore these authors suggested that the ground state of the radium isotopes with 224 G A c 228 has a static octupole deformation and that, in addition, there is a Coriolis coupling of the K a = O- band to the K” = l- band. Piepenbring ‘) has alternatively argued that the excitation energies of these band heads can be explained in terms of anharmonic octupole vibrations. Recently Iachello and Jackson “) suggested possible a-clustering effects in these heavy nuclei as an explanation of some of the observed features. They proposed in the framework of the interacting boson approximation (IBA) a phenomenological (algebraic) model to describe these a-clustering effects by introducing modes of excitation consisting of the so-called s* (j = 0) and p* (j = 1) bosons. With the coexistence of quadrupole deformations, which can be described with the conventional s- and d-bosons, the a-clustering represented by the (s”, p*) configurations would manifest itself by the appearance of low-lying natural-parity states in addition to the ground-state rotational band. Due to configuration mixing between these two bands, the positive-parity states would be shifted in energy relative to the states with negative-parity of the “cu-clustering” band. Furthermore, the low-lying naturalparity states will have a low a-hindrance factor and should be populated strongly by a-transfer reactions. In addition, intraband El transitions should be enhanced. The observation of possible candidates for a-clustering states in the actinide region was suggested by Janecke et al. “) who used the (d, ‘Li) reaction at Ed = 55 MeV to study 228Ra and 234Th. In both nuclei these authors observed strongly excited states at excitation energies of about 0.7 to 1.4 MeV, with cross sections comparable to those of the respective ground-state transitions. From their measurements, however, no definite spin assignments could be made because of poor statistics. Several experiments have been performed recently to study the low-lying levels of the radium isotopes. Studies of the P-decay of francium isotopes were done by Kurcewicz et al. *I) and by Ruchowska et al. ‘*). These investigations revealed detailed level schemes of 224,226Raand 228Ra, respectively (see fig. 1 for the level schemes of the two heaviest isotopes). The radium isotope with the longest lifetime (226Ra) has also been studied using multiple Coulomb excitation 13) in which the ground-state band and the K” =O- band up to spin 14h and 13h, respectively, could be identified. In the present experiment the (d, 6Li) reaction was used to study 226Ra and 228Ra. Similar to studies of the (p, t) reaction in this mass region we find strongly excited states at an excitation energy around 1 MeV. These levels most probably are the low-spin members of the K TT= O+ and 2+ bands which were identified before in

A.M. van den Berg et al. / High-resolution study 22SRCl

1.5

226R(3

-13I(“11840

-12’

I

I -II_

1.0 -

-2-

-5

K”;TT

--lo+

I

z;+

_4+

l

-9-

__2+

K*=O+

2

-a+

5 W"

-3+ -2*

K-Q+-

-0’ -

7-

-5-

-o+ K”q,+

0.5 -6+

+

&

-5-

-6+

K*; O+

-3-1K-z O-

-3-4+

0

I

-2+ -0+

zo!-

-4+

i

-2* -o+

Fig. 1. Experimental level schemes of 226Ra [from refs. 11,13)]and **sRa [from ref. I’)].

P-decay experiments ‘1312).In 226Ra most of the strength for the a-pickup reaction is concentrated at states with an excitation energy of about 1.4 MeV, whereas in 228Ra the strength for the (d, 6Li) reaction is distributed almost equally over the members of the ground-state band and those of the K” = O+ band with its head at E, = 0.721 MeV. Strengths and reduced widths for the (d, 6Li) .reaction will be compared with those obtained from other experiments. 2. Results and DWBA analysis For the study of the 232Th(d, 6Li) reaction a self-supporting natural thorium target was used with a thickness of about 200 pg/cm*. The target used in the 230Th(d, 6Li) reaction was made by sputtering 90% enriched 230Th ( Tl,* = 7.5 X lo4 y) material onto a carbon backing. The thickness of the target material was also about 200 pg/cm*. The reaction products from the 45 MeV deuteron-induced reaction were momentum-analyzed with the QMG/2 spectrograph of the KVI [ref. ‘“)I. The standard focal plane detection system “) with an additional energy-loss detector 16) allowed for an almost clean particle identification of the various reaction products. The energy resolution of about 40 keV was mainly determined by the thickness of the targets used in the experiments. Absolute cross sections were obtained by normalizing the angular distributions of the elastically scattered deuterons at the same incident energy to optical-model predictions calculated with the global opticalmodel parameters of Hinterberger ef al. I’). Absolute cross sections are believed to be accurate to about 15%. Because of a very large number of o-particles from

A.M. van den Berg et al. / High-resolution study

48

I 232 8Ok

,

I

I

Th (d, 6Li) ***Ra

c gs.

0.85 0.88‘;

3.06 \

142

400

Channel Fig. 2. Spectra

of z26Ra and 228Ra obtained

at 81,,,=130.

49

A.M. van den Berg et al. / High-resolution study

the 12C(d, a)“B reaction from the target backing, the 226Ra spectra have a higher constant background originating from a-particles than the 228Ra spectra, which were obtained by bombarding a self-supporting target. The angular distribution patterns of L = 0 and L = 2 transitions are more distinct from each other at Ed = 45 MeV than at 55 MeV at which energy the measurements of Janecke et al. ‘) were done. For this reason we have chosen the lower incident energy in the present investigation. Because of the high level density in both residual nuclei the spectra (see fig. 2) had to be decomposed with the code AUTOFIT [ref. ‘“)I. Peak areas were obtained by fixing the positions of the peaks in the spectrum, whereby excitation energies were taken from the known level schemes of 226Ra [refs. 11*13)]and of 228Ra [ref. 12)], respectively. However, those levels which are expected to be populated only weakly in the (d, 6Li) reaction (i.e. unnaturalparity states and states with high-spin values) were excluded from the fitting procedure (see tables 1 and 2). TABLE

1

Reduced widths (at channel radius s = 1.7 A “3 fm) and relative strengths of levels in rz8Ra populated in the (d, “Li) reaction E,(MeV)“)

J”

K”

L

0.000

Of

0+

0

0.064 0.205 0.412 0.474 0.538 0.656 0.721 0.771 0.846 0.880 0.899 ‘)

2+ 4+ 6+

0+ 0+ 0+

2 4 6

1-

0-

1

3s0+ 2’ 2+ 4+ 3+

000+ 0+ 2+ Of 2’

3 5 0 2 2 4

0.967 I 1.05 s) I 1.14y 1.20 k) 1.42 s)

(2) 8) (4) (2) (4) (4) (2) (2) (4)

Sb) -1.0

1.0

&(eV) 75 69

1.1 0.8 1.2

40 <7 <9 <6 <8 38 65 50 63

(0.4) (0.4) (0.3) (0.3) (0.8) (0.3) (0.5) (0.7)

(24) (20) (17) (14) (37) (19) (38) (32)

0.7 co.2 co.2 CO.1

co.2 0.6

‘)

RHF d,

HF ‘)

1.0 1.1 1.9

1 2 15 >834 >lO >44 >215 2 2 3 9

>ll >8 Cl3 >lO 2 1.2 1.5 1.2 (3) (4) (4) (5) (2) (4) (2) (2)

(6) (30) (8) (42) (16) (7) (4) (13)

“) E,. J” and K” were taken from Ruchowska et al. I’) except where noted otherwise (see text). ‘) All strengths S are normalized to the ground-state strength: S = S,/S,(g.s.), with S,(g.s.) = 0.024; for the definition of S, see relation (1) and text. ‘) See relation (2). d, See relation (5). ‘) HF=RHF exp (L(L+~)/(KR~)), with ~Re=0.74 JZAR,/(A+4) [see ref. 3’)]. ‘) Not included in the spectrum decomposition. g, This experiment (see text).

50

A.M. van den Berg et al. / High-resolution study TABLE

2

Reduced widths (at channel radius s = 1.7A I” fm) and relative strengths of levels in Z26Ra populated in the (d, 6Li) reaction E,(MeV) “) 0.000

0.067 0.211 0.254 0.322 0.417 0.447 0.627 r) 0.650 0.670 ‘) 0.824 0.858 ‘) 0.873 0.960 ‘) 1.049 h) 1.070 f,h) 1.14 s) 1.228) 1.33 s) 1.42 g, 1.54s)

J”

K”

L

Sb)

-1.0 0.8 0.4 CO.1 CO.3 <0.2 CO.3

58 43 16 <3 Cl2 ~6 <9


~6

0

0.4

22

2.6

3

2

0.4

20

2.9

5

1

0.4

17

3.4

4

(2) (0) (0) (2) (1)

(1.1) (1.3) (0.7) (1.4) (0.9)

(49) (61) (34) (57) (33)

(1.2) (1.0) (1.7) (1.0) (1.8)

(2) (1) (2) (2) (2)

0+

0+

0

2+ 4+ 136+ 570+ 8+ 0+ 92+ 10+ 12-

0’ 0+ 000’ 000+ 0’ 0’ 00+ 0+ 11-

2 4 1 3 6 5

r%ev)

7

RHF d,

HF ‘)

1.0 1.4 3.6 >19 >5 >lO >6

1 3 29 >24 >17 >741 >143

>lO

>lO

“) E,, J” and K” were taken from Zimmermann r3) except where noted otherwise (see text). b, All strengths S are normalized to the ground-state strength: S = S,/S,(g.s.), with S,(g.s.) = 0.016; for the definition of S, see relation (1) and text. c-g) See footnotes ‘-s) of table 1. ‘) From Kurewicz et al. ‘I).

Angular distributions for the 232Th(d, ‘Li) reaction, measured in the range from elab = 7” to 31”, are shown in fig. 3. Solid curves indicate full finite-range DWBA calculations

performed

with the code DWUCKS

[ref. “)I. The optical-model

para-

meters for this analysis are listed in table 3. The wave function which describes the relative motion of the a-particle with respect to the deuteron in 6Li was generated in a Woods-Saxon potential of which the geometrical parameters were taken from the work of Janecke et al. ‘I). They successfully used this potential for the analysis of data obtained from the investigation of the (d, 6Li) reaction on nuclei in the rare-earth and actinide mass regions. In their analysis they employed a hard core in the potential used to generate the 6Li wave function. For the case of simplicity we did not use such a hard core in the present study as this will only affect absolute spectroscopic factors 21). The transferred nucleons were treated as a spin-zero cluster with total angular momentum L. The number of harmonic oscillator quanta Q = 2N+ L for the nuclear form factor which describes the relative motion of the a-particle with respect to the radium core was chosen to be 22 for even-L and 21

Fig. 3. Angular distributions for the 232Th(d, %i) reaction (see text). Solid curves indicate finite-range DWBA calcn1ation.s. The dotted lines denote shapes corresponding to the empirical angular distribution measured for the transitjon to the first excited 4+ state.

A.M. van den Berg et al. / High-resolution study

52

for odd-L transfer values, where N is the number of nodes in the nuclear radial form factor. The unbound cY-cluster form factors were calculated according to the method of Vincent and Fortune ‘*). The transferred angular momenta were taken from the J” assignments made by Ruchowska et al. ‘*). The a-spectroscopic strength (see table 1) for the population of states in the (d, 6Li) reaction is defined as

(1) where the calculated cross sections are obtained from the analysis mentioned before. The spectroscopic factor S,(6Li = LY+d) was assumed to be unity in this analysis. Because absolute a-spectroscopic strengths S, for the (d, 6Li) reaction depend strongly on the choice of the optical-model parameters we will only use strengths S which are normalized to the ground-state strength for each reaction investigated. The “absolute” ground-state strength S, = 0.024 for the 232Th(d, 6Li) reaction (see table 1) will be used only to facilitate the intercomparison with strengths measured for the lighter radium isotope studied in the present experiment (see table 2). It is seen from fig. 3 that the angular distribution measured for the state at E, = 0.721 MeV, which was identified by Ruchowska et al. 12) to be the head of the K” = O+ band, shows indeed an L = 0 character. Transitions to some other states, however, show angular distributions which are only poorly reproduced by the DWBA calculations. Because of this discrepancy which might be due to coupled channel effects, and because there are a number of states in 228Ra with excitation energies between 1 and 2 MeV for which there is no unambiguous spin assigment, we only analyzed the strongly excited states in this energy range. The DWBA calculations (solid lines) shown for these states indicate a possible value for the transferred angular momentum, whereas the dotted line corresponds to the empirical angular distribution measured for the transition to the 4: state. The level at an TABLE Optical-model

parameters

Particle

V0

6Li db) “a+Ra"

-110 -85.2 '1

“a+d”

-18.7

“) The potentials

“) used for the FR-DWBA

d,

aa

W

1.33 1.25 1.35

0.81 0.66 0.65

-22.5

1.59

0.7

are of the Woods-Saxon

+(2hlm,c)‘(L.

b, An additional

‘Li)Ra

reaction

at I&=45

MeV

4WLl

r1

aI

‘c

Ref.

52

1.53 1.25

0.88 0.98

1.33 1.3 1.35

*O 17i 21)

1.9

21)

form with all lengths in fm and all potential

U(r)=VO{l+exp((r-rOA”3)}-‘+iW{1+exp +4iWoa,(d/dr){l+exp

3

analysis of theTh(d,

((r-r,A”3)/al)}-’

((~-r~A~‘~)/a,)}-‘+ S)( K.,.lrS.,,)(dldr){l

spin-orbit term was used with Vs.,.=-6.0, “) Adjusted to fit the u-particle separation energy. “) No hard core and no radial node beyond the origin.

V,-(r, A’13rc)

+exp ((r-

rS.,A1’3)/a,,,)~F’

r,,, = 1.25 and as.O. = 0.7.

depths in MeV:

A.M. van den Berg et al. / High-resolution study

53

excitation energy of 1.05 MeV might coincide with the .I” = O+ level found from the P-decay of 228Fr [ref. 12)]. Our data, however, do not support this spin assignment. Spectra of the 230Th(d, 6Li) reaction were measured at three different angles only: 8rab= 13”, 17” and 21”, of which the 13” spectrum is shown in fig. 2. It is seen from this figure that states around 1.2 MeV excitation energy are strongly excited with a cross section comparable to that of the ground state. Fig. 4 shows the measured cross sections together with finite-range DWBA calculations using the potentials listed in table 3. The L-values for states up to E, = 1.05 MeV were taken from J’

16%

L

B,,(deg.) Fig. 4. Angular distributions for the ‘?b(d,

6Li) reaction (see text).

A.M. van den Berg et al. / High-resolution

54

study

assignments made by Zimmermann 13). For the states at higher excitation energies L-values were subjectively selected (see table 2). 3. Reduced widths The extracted cY-spectroscopic strength S, of an a-transfer to the reduced width -yt of the a-particle by

reaction can be related

(2) where c$:\,,,( s) is th e value of the normalized a-cluster wave function at the channel radius s (the dimension of the wave function is [(fm)-““1) and p is the reduced mass of the a-particle plus B system. A detailed discussion on this relation between the reduced width amplitude ya, which measures the probability for the dissociation of a nucleus into an a-particle and a residual core, and the spectroscopic strength for an a-particle pickup reaction can be found in e.g. refs. 21*23-25).The channel radius s in relation (2) was chosen as s = 1.7A1’3 with A the mass of the core [cf. refs. “‘“)]. The same nuclear form factor as in the analysis of the (d, 6Li) reaction was used to calculate the reduced widths for the a-transfer reaction listed in tables 1 and 2. It is interesting to compare these results with those from a-decay studies. The branching ratios 1, and the corresponding reduced widths r’ar, for the a-decay of 232Th [ref. ‘“)I and 230Th [ref. “)I are listed in table 4. From the half-life Ti12 of the parent nuclei the reduced widths for a-decay were calculated from RI, In 2 ‘& = 2&(&s) T1,2 ’

Branching Final nucleus 226Ra “)

‘*‘Ra b,

“) Branching b, Branching

ratios

and reduced

TABLE 4 widths (at ~=1.7A”~fm)

(3)

for a-decay

E,(MeV)

L

0.000 0.067 0.211

0 2 4

-0.12 -0.64 -0.82

63 90 20

0.254 0.322 0.417 0.447 0.824 0.873 0.000 0.064 0.205

1 3 6 5 0 2 0 2 4

(-1.7) (-3.0) (-5.1) (-5.1) (-5.5) (-5.9) -0.11 -0.64 -0.70

(1) (0.5) (0.2) (0.2) (10) (21) 73 133 73

ratios from Kurcewicz et al. s). ratios from Horen 26).

w%d

r?dev)

A.M. van den Berg et al. f High-?es~~utia~study TABLE

55

5

Comparisona) of spectroscopic factors and reduced widths for (d.6Lif reactions and n-decay studies leading to the first three members of the ground-state band in the final nuclei Finalnucleus

226Ra ‘)

“*Ra ‘)

“*Ra *)

TIl

a)

L

5-Y

0 2 4 0 2 4 0 2 4 0 2 4

0.016 0.013 0.006 0.024 0.024 0.017 0.019 0.04 0.012 0.017 0.027 0.013

Y& (eV) ‘) (2) (*) (*) (1) (4) (*) (9) (2) (4)

58 43 16 75 69 40 59 120 29 44 68 26

v,‘o (eV) d,

72, tkev) “1

63 90 20 73 133 73 73 133 73 65 82 23

127 230 66 151 277

203

“) Spectroscopic factors S, and reduced widths +y: depend on the choice of the optical-model parameters used in the analysis (see text and relation (1)). Therefore absolute values from this experiment at Ed-45 MeV and from Jlnecke et al. ‘) obtained at J&=55 MeV cannot be compared directly. b, See relation (1) for the definition of S,, numbers between brackets denote the (statistical) error in the last digit; an asterisk denotes pcor agreement between calculated (DWBA) and measured angular distributions. ‘) See relation (2). d, See relation (3). ‘) From a-decay experiments by Rasmussen =). ‘) This experiment from FR-DWBA. a) From Jiinecke et al. 9).

where the penetrability

PL of the emitted a-particle

is given by Milder et al. 27): (4)

The code UMFORM4 [ref. “)I was used to compute the penetrability at the channei radius s = 1.7Arf3 fm. Table 5 lists calculated reduced widths for the a-decay of 230*232Th and 238Uto the ground state and to the first-excited states of their respective daughter nuclei. Values for Q, were obtained from the mass table of Wapstra and Bos 29). Reduced widths calculated by Rasmussen 30) using the WKB approximation are also listed in table 5 (see column 6). The increase of the ground-state reduced width with mass number A as found by Rasmussen 30) is not observed for the reduced a-decay widths obtained from relation (3). These values, listed in column 5 of table 5, in contrast agree well with the present values for &(g.s.) obtained from the DWBA analysis of the (d, 6Li) reaction at Ed = 45 MeV and those obtained at Ed = 55 MeV [ref. 9); see column 4 of table 51. A comparison of the reduced widths for the excited states is done by normalizing to the ground-state reduced width. To this end the “reduced hindrance factor”

56

A.M. van den Berg et al. / High-resolution study

(RHF), defined as

is introduced. The reduced hindrance factors for the a-decay of 230Th and 232Th were obtained using this relation and the values of the reduced widths listed in table 4. Comparison of these a-decay data with those obtained with the present (d, 6Li) reaction (see column 5 of table 6) shows that there is a discrepancy of a factor of two for the population of the 2: level of *“Ra, This is in contrast with the data obtained by JPnecke et al, 9), who found a good agreement between the reduced hindrance factors obtained from the (d, 6Li) reaction at Ed= 55 MeV and the a-decay data for the 2: state. This discrepancy for the present data might be attributed to strong channel coupling effects which are known to be important for deformed nuclei 32). The RHF of the 2: level of 226Ra could not be determined accurately because of the few data points measured for the angular distribution of this state. Quite noteworthy, as can be seen from table 6, the data of the 230Th(d, 6Li) reaction and the a-decay of 230Th both yield small reduced hindrance factors of equal magnitude for the excited levels at E, = 824 and 873 keV in 226Ra. This observation lends further credibility to the intercomparison of (relative) reduced a-widths from reactions and decay. 4. Discussion From the present investigation of the (d, 6Li) reaction it appears that the groundstate strength of 226Ra is reduced by almost a factor of two compared to that of TABLE Reduced

Nucleus 226Ra

**sRa

hindrance

E, (kev)

factors

6

for a-decay

and (d, 6Li) reactions

L

RHF, “)

0

0

Sl

61 211 254 322 417 441 824

2 4 1 3 6 5 0

0.7 3.1 (44) (135) (274) (360) (6)

813 0 64 205

2 0 2 4

(3) =l 0.5 1.0

RHF, b, Cl 1.4 (*) 3.6 (*) r19 >5 210 >6 3 =3 -1 1.1 1.9 (*)

“) Obtained from relation (3) and table 4. ‘) This experiment obtained from relation (2); an asterisk denotes poor agreement between calculated (DWBA) and measured angular distributions (see also table 5).

A.M. van den Berg et al. / High-resolution study

57

the 232Th(d, 6Li)228Ra reaction. However, the lighter isotope has more strength located at excitation energies above 1 MeV. Several features, on the other hand, are common for both isotopes. One such conspicuous feature is the weak population of the members of the Km = O- band (see fig. 1). The first members of the K" = O+ band, in contrast, are strongly excited. The weak population of the K" = O- band in the (d, 6Li) reaction resembles the results obtained in the 24Mg(d, 6Li)20Ne reaction 33), where it was concluded that these states have a predominantly particle-like character. If this is true also in the actinide region, the members of this band should be strongly populated in stripping reactions. Results of (t, p) reactions 34), however, show no evidence for such strong excitations of negative-parity states in the actinide region. Fig. 5 shows the hindrance factor of the first excited l- state in the even-A radium isotopes as a function of the mass number A. There is a strong increase of the HF towards heavier nuclei. The lower limit of 10 for this level in the heaviest isotope studied (228Ra; from the present experiment) and the measured value of 53 for the same level in 226Ra [ref. ‘)I are in sharp contrast with the values quoted for **ORaand ***Ra (HF < 5)

N ,oo,

172 L

' 5

,

I;6

,

140

,,o

1

z5 %lO-

-1g W”

Fig. 5. Hindrance factors (HF) and excitation energies for the first excited l- state in the even-A radium isotopes. Data were obtained from refs. 8~12~35-37). The open circles indicate lower limits for the hindrance factors obtained in the present study.

58

A.M. van den Berg et al. / High-resolution study

[ref. 35) and ref. 36), respectively]. It is seen from fig. 5 that there is no obvious correlation between the hindrance factor of the l- level and its excitation energy, which has a minimum at A = 224. Members of the low-lying K w = O+bands in 226Ra and 228Ra with heads at E,= 824 and 721 keV, respectively, are strongly excited in the (d, 6Li) reaction. In the present experiment we also find evidence for strongly excited O+ states at E, = 1.22 and 1.33 MeV in 226Ra. Unfortunately the high excitation energy of these states makes their observation in the a-decay of 23”Th virtually impossible. Using the reduced hindrance factors obtained from the (d, ‘Li) reaction we can predict the a-decay branching ratios to be smaller than 10e9 for all levels populated with the (d, 6Li) reaction at excitation energies larger than 1 MeV. The strength for the population of the Of states in both residual isotopes shows a remarkable behaviour. The first excited 0’ state in 228Ra at E,= 0.721 MeV is populated strongly (see table 1) in the a-pickup reaction, whereas the 0: state at E, = 0.650 MeV in 226Ra is not, or only weakly populated. This latter state was identified by Zimmermann 13) on the basis of a multiple-Coulomb excitation experiment on 226Ra in which a weak y-ray transition from this level to the J” = l- level at E, = 254 keV was observed. The reduced hindrance factor for the 0: state in 226Ra shows a different behaviour than for the 0: state in the neighbouring radium nuclei. The reduced hindrance factors for these 0’ states are given by RHF = 7 for 224Ra [ref. ‘1, RHF > 10 for 226Ra and RHF = 2 for 228Ra from the present study (see tables 2 and 1, respectively). The reduced hindrance factor for the 0; state in 226Ra, however, roughly has the same value (RHF = 2.6; see table 2) as the first excited O+ state in 228Ra. It was pointed out by Iachello and Jackson “) that a-clustering states could be identified on the basis of low a-hindrance factors. We might therefore conclude that members of the K TI = O- band in these two nuclei are not likely to be pure a-clustering states. The lowest members of the K" = 0: and 2: bands, on the other hand, may be good candidates for such states. In a recent study Daley and Jachello 38) used a phenomenological a-clustering model to calculate the excitation energies and reduced hindrance factors for the low-lying levels of the even-A radium isotopes. They found for the RHF of the 0’ state at E,= 0.916 MeV in 224Ra a value of about 6 which might be compared with the value of 2.6 (2) for the O+ level at E,= 0.824 (0.721) MeV in 226Ra (228Ra). The strong population of exited Of states in the (d, 6Li) reaction, however, cannot be viewed as conclusive evidence for a-clustering without additional information from di-nucleon transfer reactions. Strongly correlated proton or neutron pairs may equally lead to the strong population of excited 0’ states with an a-pickup reaction [cf. ref. “)I. A better criterion for the assignment of a-clustering might therefore be found from an intercomparison of a-transfer, di-proton and di-neutron transfer reactions leading to the same final nucleus.

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59

5. Summary and conclusions High-resolution spectra were obtained for the (d, 6Li) reaction at &=45 MeV leading to low-lying states in 226Ra and 228Ra. Strongly populated groups of states were found around E, = 1.4 MeV in 226Ra and at 0.8 MeV excitation energy in *‘*Ra. In both groups of states we find evidence for an L=O strength which is comparable to the strength for the population of the respective ground states of the final nuclei. These strongly populated excited states in 228Ra were identified before with the (d, 6Li) reaction by Janecke et al. ‘) who also studied 234Th with the same reaction. Similar to the results obtained by these authors the present reduced hindrance factors of these states are relatively small (RHF < 5). The present reduced hindrance factors for 226Ra compare rather well with those obtained from a recent a-decay study performed by Kurcewicz etal. “). In addition we find evidence for strongly excited Oc states at E, = 1.22 and 1.33 MeV in this nucleus which were not known before. In both nuclei studied the members of the K n = O- band are not, or only weakly, populated with the n-pickup reaction. The nature of the excited states in 226Ra and 228Ra which are strongly populated in the (d, 6Li) reaction remains unclear. Theoretical models which predict reduced a-decay widths and/or reduced hindrance factors of nuclei in the actinide region are needed. The investigation of a-stripping reactions and di-proton transfer reactions in this mass region would be highly desirable. Specifically a study of the population of the O+state at E, = 0.721 MeV in 228Ra with a di-proton pickup reaction on 23?h is suggested. We gratefully acknowledge Prof. David for supplying us with an enriched 230Th target and Prof. Janecke for a version of the code UMFORM4 which was used to calculate radial form factors. Stimulating discussions with Prof. Iachello and Dr. Daley during various stages of this study were much appreciated. This work is performed as part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie” (FOM) with financial support from the “Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek” (ZWO). References 1) I. Ragnarsson and R.A. Broglia, Nucl. Phys. A263 (1976) 315 2) R.R. Chasman, Phys. Rev. Lett. 42 (1979) 630 3) GA. Leander, R.K. Sheline, P. Mtiller, P. Olanders, I. Ragnarsson and A.J. Sierk, Nucl. Phys. A388 (1982) 452 4) F. Iachello and A.D. Jackson, Phys. Lett. 108B (1982)151 5) R. Piepenbring, Phys. Rev. C27 (1983) 2968 6) J.V. Maher, J.R. Erskine, A.M. Friedman, J.P. Schiffer and R.H. Siemssen, Phys. Rev. Lett. 25 (1970) 302; J.V. Maher, J.R. Erskine, A.M. Friedman, R.H. Siemssen and J.P. Schiffer, Phys. Rev. CS (1972) 1380

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