Nuclear Physics A417 (1984) 24-36 @ North-Holland ~bl~shi~g Company

A STUDY OF THE 230Th (a, a’f) REACTION AT 50 h&V

P. DAVID, J. DEBRUS, J. HARTFIEL, H. JANSZEN and R. VON MUTIUS

Institut ftir Strahlen- und Kernphysik, UniversitBt Bonn, Nussallee 14-16, D-5300 Bonn, Germany

.&&act: The total kinetic energy release (TKE) of the ~ssion~g nucleus 230Th is measured as a function of the excitation energy E, for various mass splittings. A small rise in the TKE(E,) of 0.2tO.l MeV per I MeV rise in E, is observed. The mass-yield distributions show tine structure for E, around 6.7 MeV. The mass yield is presented as function of the TKE.

E NUCLEAR REACTIONS, Fission 13*Th(a,dff E = 50 MeV; measured fission fragment

energies, mass djstr~butions, excitation functions.

1. Introduction

In this paper we report on the totai kinetic energy (TKE) release and mass distributions of the fragments of z3”Th for fission induced by ineiastic scattering of 50 MeV a-particles as a function of the excitation energy.

It is particularly interesting to investigate the thorium isotopes in the fission process, since phenomena have been observed here which were not seen in other nuclei : resonances were identified, barrier structures extracted and a trlpIe-humped barrier was proposed for the isotopes 23*‘233Th [ref. ‘)I. These interpretations have thus given insight mainly into the static properties of the nuclear potential energy surface.

The dynamic properties of the fission process can be studied by the TKE and mass distributions. If this is done as a function of excitation energy of the fissioning system, the influence of shell effects might be reveafed, resonances or narrow states identified and dissipative phenomena observed. The latter have been shown in a systematical survey of the TKE(E,) to change with mass number A. A general phenomenon is observed for TKE(E,) in the excitation energy range for E, from the barrier Bf up to the barrier for second-chance fission B,, in going from the

24

P. David et al. / 230Th(a, a’f) 25

actinium isotopes 2, over the isotopes 231*232,233Th to heavier elements 3); the TKE(E,) decreases for actinium from the barrier on, while for the thorium isotopes they rise from the barrier up to about 8 MeV and they decrease again for the heavier elements from the barrier on. The investigation of the fission process as a function of the excitation energy by the (u, cr’f) reaction gives the possibility to simultaneously measure excitation functions of the observables under identical conditions. The reaction is selective and only by comparison with other measurements, in particular the (e,e’f) and (y, f) reactions, will the picture of the fissioning nucleus be complete.

This work is a continuation of the study of the TKE on 232Th at 120 MeV [ref. “)I. Further aims of the present experiment are: to find out whether the TKE of the 230Th fission fragments show similar effects to those observed for 232Th (i.e. structures superimposed onto a continuous rise with E,) and to see whether the structure of the mass-yield distributions changes with E,.

In this context one must also be aware of observations which show the TKE of special mass splittings or narrow mass groups to be clearly higher than the kinetic energies of their neighbours 4), of differences in the barrier heights for the different cleavage processes 5, and of tunneling effects 6). In sect. 2 the experimental set-up is outlined briefly; sect. 3 contains the results, and their discussion is given in sect. 4 together with the conclusions.

2. Experimental procedure

Triple-coincidence events between the inelastically scattered a-particles and the two fission fragments were measured in the reaction 230Th(a, a’f). The 50 MeV analyzed beam of the ISKP isochronous cyclotron was used to bombard the 230Th02 target of 47 pg/cm2 supported by a 40 pg/cm’ carbon foil. Inelastically scattered a-particles were detected in a AE -E telescope for excitation energies of the fissioning nucleus 230Th between 4 and 13 MeV. The telescope was set at e a’lab = 30” to avoid contamination lines in the excitation energy region of the fission barrier. The solid angle of the telescope was 7.2 msr. The thicknesses of the surface-barrier detectors of the telescope were 200 pm (AE) and 1000 pm (E). An aluminum foil of 14 pm was mounted in front of the AE detector to protect it against radiation damage by fission fragments. The total energy resolution of the a-

spectrum is 200 keV. The fission fragments were detected in two 60 pm cooled surface-barrier

detectors situated opposite to each other and at the recoil angle of 65” of the 230Th nucleus excited to 6 MeV. The detectors subtended a solid angle of 124 msr each. In fig. 1 the experimental set-up is sketched.

The triple coincidences between the pulses from the fission-fragment detectors and the telescope were written in list-mode on tape, allowing the subtraction of

26 P. Da&d et al. / 230Th(a, a’f,J

Fig. 1. Experimental set-up.

random events which constitute part of the prompt peak. The count rate in the fission detectors was kept around 3 kHz at an average beam current of 200 nA. The pulse-height signals from the two fission detectors were further used to obtain masses and energies of the fission fragments.

3. Mass and energy determination

The procedure for generating masses and energies from the coincident fission- fragment signals in the fission detectors is the same as was outlined, in ref. 3). For the fissioning nucleus 230Th the prompt-neutron distribution v,(A) has not been measured as a function of the fragment mass A. Only the ratio of the average values of V, of ‘*~Th(n~~,f) and 235U(nn(h,f), resulting in a value of + (230Th)/~ (236U) = 0.864~0.08 [ref. ‘)I, was measured. Therefore, to get mass- dipendent talues v,(A) for 230Th, the data of Terre11 “) measured in the reaction 235U (qh,f) were taken and multiplied with the factor obtained by Lebedev and Kalashnikova :

v,(230Th) = 0.864~~(‘~~U).

The excitation energy dependence of yP was not taken into account. The error introduced by this can be estimated from the increase of TKE(E,) by about 0.6 MeV if v,(E,) increases by one neutron.

In correcting the energy loss of fission fragments in the target the tables of Ziegler et al. ‘) were used.

27

TKE IMeVi 170

170

165

160

I t I t

(a> Z30Th(a,a'f)

Ea40MeV

W 232Thin,f)

Bf . TRMHON et dl w??91 m

“--I- :

6 7 6 9 IO 11 12 13 [I

Fig. 2. Excitation functions of TKE averaged over the masses for the thorium isotopes: (a} 23dTh [this work], (b) 23Yi’b [reE ““)I, (cc) “‘Th [ref. “)I and (d) ‘ssTh [refs. “**“)].

28 P. David et al. / 230Th(a, a’f)

4. Results

4.1. THE TOTAL KINETIC ENERGY RELEASE OF “‘Th

Fig. 2a shows TKE(E,) averaged over the masses for the range of excitation energies from the fission threshold B, up to B,, the threshold for second-chance fission. The data are summed in 200 keV intervals near the barrier and in 1 MeV intervals above. A region contaminated by carbon and oxygen lines is left out.

For comparison in figs. 2b-d results from the literature are included for the nuclei 231Th [ref. ‘“j], 232Th [ref. “)I and 233Th [refs. 11S12)]. The slope for 230Th dTKE(E,)/dE, is 0.2+0.1. Within the experimental resolution LIE of about 200 keV no stricture can be identified in this excitation function.

In fig. 3a the TKE averaged over the excitation energies is displayed versus the heavy fragment mass m,; fig. 3b shows the corresponding widths 6.

4.2. THE Z30Th MASS DISTRIBUTIONS

Figs. 6 display the mass distributions for various bins of excitation energy; the E, interval in fig. 6a corresponds to the interval of fig. 3a. Clearly the mass

m [Met

180

l-

‘1

170

160

230Th(a,a'f) E,=SO,MeV E,=S,!i-7,s

andg1ie:3

(4 120 140 160 m&l 120 140 160 m,iul

:b)

230Th(a,a'fI E,=SOMeV

E,=S,S-7,s and 9,513 MeV

Fig. 3. (a) TKE averaged over the excitation energy intervals given in the figure, as a function of the heavy fragment mass of Z30Th (ma). (b) Same as (a), but for the widths (r of the TKE.

P. David et al. / 230Th(a, a’f) 29

30 P. David et al. / 230Thfa, Klf,

TABLE 1

Neutron-binding energies and fission thresholds (B, = second barrier) for some Th isotopes “*r6)

Target

‘=‘Th 230Th 23’Th 232Th 233-,~,

(GA) (ML) (h%) (I&)

5.249 6.3 12.368 6.791 6.5 13.09 12.040 5.1205 6.22 11.62 11.9116 6.4364 6.15 12.66 il.557 4.786 6.28 10.94 11.2228

distribution shown in fig. 6c reveals prominent structure and it is therefore interesting to compare this with the mass distribution obtained in the reaction 229Th(n,,,,f) for a similar excitation energy of E, = 6.79 MeV (cf. table 1). The results from ref. 13) are therefore also included in the figure.

Of particular interest is the change of the mass distribution with the TKE. Therefore fig. 7 displays the mass yields for E, = 5.5 to 7.5 and 9.5 to 13 MeV for energy bins given in the figure. For the highest TKE observed the asymmetry decreases, the light and the heavy mass yields get narrower and the maximum yield is at around mH = 134 I)r 3 u. Table 2 summarises the centers of gravity of the light- and heavy-mass-yield distributions and their corresponding widths for various excitation energies of the compound nucleus 230Th and for several TKE bins.

TABLE 2

Centres of gravity and widths of the light- and heavy-mass-yield distributions displayed in figs. 6ad, for four bins of E, and in fig. 7 for various bins of the TKE

%W %@I *,w %(Uf

E, WW 5.5 7.5 9.5-13.0 3 9.5-13 6.5- 7.5

5.5 6.5

TKE (E,) (MeV) 180-190 170-180 160-170 150-160 12&150

89.5 + 0.24

90.6 f0.28 89.4 + 0.25 89.2 + 0.25

5.3kO.17 140.5 f 0.24 5.3kO.17

5.7&0.21 139.4kO.28 5.7iO.21 5.2kO.18 140.6&0.25 5.2kO.18 5.2kO.18 140.8 kO.25 5.2kO.18

95.0k0.78 4.4kO.52 135.0+0.78 4.4kO.53 92.3 kO.72 4.5+0.50 137.7k0.72 4.5 f0.50 89.6f0.71 4.6 + 0.50 140.4+0.71 4.6kO.50 87.2kO.71 4.8kO.47 142.8 kO.71 4.8 + 0.47

85.7 +0.68 5.8 *to.47 144.3 kO.68 5.8 f0.48

P. David et al. / Z30Th(a, a‘f) 31

,rr-- - -2-T

l +c--- -- -e-_ --cr

‘6,

-2 -4

- s -- --,.

--WA E

-- i

-t=r o

_-=- $2

-=e.__ --_&is

32 P. David et al. / 230Th(a, a’f,J

P-

8-

7-

6/-

60 80 100 120 lb0 160 At&l]

1,

a-

I-

6-

S-

b-

3-

2-

t l(

t t

t i t

1t 4 t

I

’ t 1 t

TKL 170-110 IHCV1

# t 1 4 t

._. . , 't,._*+&@t' . t

. .- .

60 80 1W 120 NO t60 AkIt

60 60 100 120 lC0 160 Alul

I: : .; 4’ ; ; ‘( ‘t+l@& ; : ; 4, :.- ; : I 60 80 100 120 140 (60 AhI

60 80 100 120 110 160 Aid

Fig. 7. Mass-yield distributions from the reaction 23aTh(a, a’ff for various TKE bins given in the figure.

P. David et al. / z30Thfa, a’f) 33

5. Discussion and conclusion

The comparison of the TKE(E,) for several Th isotopes in fig. 2 shows that for the nucleus 230Th TKE(E ) X also rises with excitation energy. For excitation energies E, from 6 to 13 MeV the slope is dTKE(E.JdE, = 0.2kO.l. The average value TKE = 163.8+0.12 MeV is in good agreement with the value calculated from the semi-empirical formula of Viola I’). For comparison fig. 4a and b shows results from Asghar et ai. 13) from the reaction 229Th+nth (E, = 6.791 MeV) and fig. 5 contains data of Unik et al. i4) for the same reaction. The comparison of the TKE averaged over E, and plotted versus mH in figs. 3, 4 and 5 gives good agreement of our data with the results of Unik et al. 14) and Asghar et al. 13) as regards the maximum value. For m, larger than 145 u the TKE values of Unik et al. are lower by about 2 MeV. Also the data of Unik et al. show a dip of the TKE at mH approximately 131 u, which is not indicated in the present (a, cr’f) reaction, though the shoulder at mH 145 u is. The more recent Z29Th(n,,,f) measurement of Asghar et al. 13) gives lower TKE(m,) values if compared to the results of Unik et al. 14) and to the present data, the difference being about 3 MeV. The widths crrKE are systematically larger for the present data as compared with the results of Asghar et al. 13) d p an es ecially for the more s~metric masses they increase. This may be due to the subtraction procedure of the random events and is also caused by the thicker target used in the (CY, a’f) reaction, giving rise to a higher-energy loss. The trend of the asymmetric mass bumps to move towards symmetric masses for increasing TKE is the result of fig. 7. This reflects the approaching of the Q-value of the cleavage process. The fragments are less excited, less deformed and by this they are in a closer configuration resulting in a higher TKE at inlinity. The widths of both the mass peaks have decreased at the highest TKE bin and the yield concentrates around mH = 134f 3 u. This phenomenon is observed also in other light and heavy actinide nuclei ; for 232Th see ref. 3, and for a comparison here figs. 8a, b show the mass yields of 248Cms.T. [ref. la)] and 2s2Cfs.f. [ref. “)], where again at the highest TKE the yield around mH = 134 u is dominant, Similar results for Th and light U isotopes were obtained by Asghar et al. ‘O). The most illuminating experiment in this respect, performed with Lohengrin at Grenoble by Quade et al. 21), reveals that the masses mu = 134 and 144, 154 u have the highest Q-values and therefore survive in looking for the highest TKE. The thin target used in the present experiment allowed us to get mass distributions comparable to those obtained in neutron-induced fission reactions. The comparison given in fig. 6c for E, around 6.7 MeV with data from Asghar et al. 13) shows good agreement of the absolute yield of two quite different reactions and also shows the same structure. This result within the error bars again supports the general statement that only the excitation energy is important to generate the mass yield in low-energy fission processes regardless how it is brought into the nucleus. Figs. 7 reveals, furthermore, that the mass yield is quite different for different excitation energies. In table 3 the

34 P. David et al. / zsOTh(a, u’fi

I%1 TKE=2l&22Ot?eV N-194

11,

f I

f%l bc

TKE=186-~~88e~ N=57873 :. :.

f%l TKE=218-22OPW bl067

TKE=lS4-W&V

80 100 120 l&O 160

A tu3 Fig. Sb. Same as fig. 7, but from the spontaneous

fission of *fZCf~,r,

P. David et al. / “‘Th(a, a’f) 35

TABLE 3

Comparison of the centres of gravity of the light- and heavy-mass-yield distributions, of their widths and of the TKE and their widths as obtained in thermal neutron-induced ftssion and in this experiment

2z9Tb(n,h, 0 [ref. 13)]

229Th(n,,, f) [ref. “)I

*“Th(a, a’ f) [this work]

%.(u) e” (u) CL(U) OH (u) target

events TKE (MeV) o (MeV)

89.8 140.2

4.7 4.1

0.914 &cm2 ThOt

133 pg/cm2 Ni backing

lo6 163.6kO.5

8.6

89.6 140.4

4.7 4.7

lo5 163.6+0.5

8.2

89.5 k 0.2 140.5 * 0.2

5.3 + 0.2 5.3 kO.2

40 fig/cm’ ThO,

47 fig/cm2 C backing

lo4 163.8 +0.2 10.2kO.l

numerical values of TKE, the centres of gravity of the light and heavy fragment

mass-yield distributions and their widths are listed.

In conclusion we have found a small but positive slope of TKE(E,) similar to what was found in other Th isotopes. This may indicate barrier penetration effects. The mass-yield distributions for highest TKE values converge to a more symmetric shape with maximum yield around ma = 134) 3 u. Shell effects are most probably the origin of the structure observed in the mass distribution of E, around 6.7 MeV. These change at the higher excitation energies.

This work has been performed as part of the research program of the Institute fiir Strahlen- und Kernphysik of the University of Bonn with financial support of the Bundesministerium fi.ir Forschung und Technologie der Bundesrepublik Deutschland.

References

1) S. B$mholm and J. E. Lynn, Rev. Mod. Phys. 52 (1980) 725 2) E. Konecny, H. J. Specht and J. Weber, Proc. IAEA Symp. on physics andchemistryoffission, Rochester

1973, vol. II (1974) 3 3) P. David, J. Debrus, I-i. Janszen, J. Schulze M. N. Harakeh, J. van der Plicht and A. van der Woude, Nucl.

Phys. A380 (1982) 27 4) D. Belhafaf, J. P. Bocquet, R. Brissot, Ch. Ristori, J. Cranqon, H. Nifenecker, J. Mougey and V. S.

Ramamurthy, Z. Phys, A309 (1983) 253 5) S. K. Lisin, L. N. Morozov, V. A. Pchelin, L. V. Chistiakov, V. A. Shiginand U. M. Schubko, Sov. J. Nucl.

Phys. 24 (1976) 570 6) J. F. Berger, M. Girod and D. Gogny, Lecture Notes in Physics 158 (1982) 82 7) V. T. Lebedev and V. I. Kalashnikova, JETP (Sov. Phys.) 8 (1959) 370

36 P. David et al. / Z30Th(a, a’f)

8) J. Terrell, Phys. Rev. 127 (1962) 880 9) J. F. Ziegler, Handbook of stopping cross sections for energetic ions in all elements, vol. 5 (Pergamon,

Oxford, 1980) 10) A. Sicre, G. Auchampaugh, H. C. Britt, A. Gavron, Y. Patin, J. van der Plicht and J. B. Wilhelmy,

LA-UR preprint (1981) unpublished 11) A. I. Sergachev, G. G. Vorob’eva, B. D. Kuz’minov, V. B. Mikhailov and M. Z. Tarasko, Sov. J. Nucl.

Phys. 7 (1968) 475 12) J. Trochon, H. Abon Yehia, F. Brisard and Y. Pranal, Nucl. Phys. A318 (1979) 63 13) M. Asghar, F. Caitucoli, B. Leroux, M.Maurel, P. Perrin and G. Barreau, Nucl. Phys. A373 (1982) 225 14) J. P. Unik, J. E. Gindler, L. E. Glendenin, K. F. Flynn, A. Gorski and R. K. Sjoblom, IAEA

Conf. on physics and Chemistry of fission, Rochester, vol. 2 (1973) 19 15) A. H. Wapstra and K. Bos, Atom. Nucl. Data Tables I9 (1977) 175 16) B. B. Back, 0. Hansen, H. C. Britt and J. D. Garrett, Phys. Rev. C9 (1974) 1924;

B. B. Back, H. C. Britt, 0. Hansen, B.Leroux and J. D. Garrett Phys. Rev. Cl0 (1974) 1948 17) V. E. Viola, Jr., Nucl. Data Al, no. 5 (1966) 391 18) J. Hartfiel, diplome, University of Bonn (1981). Institut fur Strahlen- und Kernphysik 19) P. David, J. Debrus, F. Ltibke, H. Mommsen and R. Schoenmackers Phys. Lett. 360 (1976) 445, and

unpublished data from ‘s2Cf cabribation runs 20) M. Asghar, P. D’Hondt, C. Guet, P. Perrin and C. Wagemans, Nucl. Phys. A292 (1977) 225;

M. Asghar, C. Guet and P. Perrin Nucl. Phys. A298 (1978) 13; M. Asghar, F. Caitucoli, P. Perrin and C. Wagemans Nucl. Phys. A311 (1978) 205

21) See e.g. P. Armbruster, in Dynamics ofnuclear fission and related collective phenomena, Lectures Notes in Physics 158 (1982) 1