Measurement of cold fission for 229Th(nth, f), 232U(nth, f) and 239Pu(nth, f) with the Cosi fan tutte spectrometer

NUCLEAR PHYSICS A

Nuclear Physics A560 (1993) 677-688 North-Holland

Measurement of cold fission for 229Th(nt,, f>, 232U(nt,, f) and 239Pu(nt,, f) with the Cosi fan tutte spectrometer M. Asghar ‘, N. Boucheneb 1,2,G. Medkour ‘, P. Geltenbort ’ Institut de Physique, VSTHB, Algiers, Algeria ’ Centre d’Etudes Nuclkaires, Bordeaux-Gradignan, 3 Institut Laue-Langevin, Grenoble, France

3, B. Leroux *

France

Received 2 December 1992 (Revised 20 April 1993)

Abstract

The light-fragment-group mass-energy correlations for 229Th(n,,,, f), 232U(n,,, f) and 239Pu (Nth, 0 measured with the Cosi fan tutte, have been used to determine the cold fission probability for these fissioning systems. These are the first results on cold fission for 229Th(n,,, f) and 232U(n,,, f). For **‘Th(n,,, 0 cold fission is realised for the whole mass range M, = 80-99 present in the mass spectrum, but the coldest fission shows up for M, = 85. For 232U(n,,, f) cold fission is present for M, = 84-107, with the coldest fission manifesting for M, = 100-105. In the case of UgPu(n,,, f), cold fission exists for M, - 95-112 and here the coldest fission is produced for M, = 105-110. This systematic study shows that the shells in the nascent fragments seem to play a decisive role in the realisation of cold fission. These results are discussed in terms of the existing concepts and ideas on this phenomenon.

Key words: NUCLEAR

FISSION 229Th(n,,, f), 232U(n,hr f), 239Pu(n,,, f), E = thermal; measured fragment mass-energy correlations for the light fragment group; deduced cold fission probability as a function of fragment mass.

1. Introduction In nuclear fission the reaction energy Q for any fragmentation Ml/M2 is shared out between binary fragments as the kinetic energy E,, the intrinsic excitation energy Eiz and the deformation energy E, such that Q=Ek+E;+ED.

(1)

Ultimately, the Ez + E, energy ends up as the excitation energy E * of fragments. These fragments lose their excitation mostly through evaporation of a cascade of prompt neutrons. Since the kinetic energy E, has a rather wide and almost gaussian distribution, the yield surface Y(M, EJ contains a small fraction of 0375-9474/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

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events for which the E, is such that the resulting value of E * = Eiz + E, is less than the neutron binding energy B, of either of the binary fragments. Here, fission occurs without any emission of prompt neutrons and the fragments lose their excitation energy through -y-ray emission. In this case, we deal with the phenomenon of cold fission. The truly cold fission corresponds to the limiting case, where E, = Q, and the fragments are expected to be produced without any final excitation energy. The cold fission probability p0 depends on the fissioning nucleus; it decreases from pO = 3% for 235U(nth,f) to Pa = 0 .2% for 252Cf(s,f). This behaviour results from the fact that both the average reaction energy (Q) and the average kinetic energy ( Ek) increase linearly with the mass of the fissioning nucleus A,; however, the (Q) increases faster with A, than ( Ek), and the corresponding increase in the width of E, distribution does not compensate it completely. Cold fission has been studied for 233U(n,,, f), 235U(n,,, f) 239Pu(n,,, f) 248Cm(s,f) and 252Cf(s,f) using mostly a double ionisation chamber, and the Lohengrin mass separator installed at the Grenoble high-flux reactor [l-9]. An attempt has been made to analyse the cold-fission phenomenon in terms of compact scission configurations, where the ground-state properties such as deformation of nascent fragments are considered [lo]. It is known that the effective potential energy surface explored by a fissioning system on its way towards fission is a beehive of neutron and proton shells [ll]. Most of these shells do not correspond to the ground-state configurations of the nascent fragments. If some of the deformed shells are strong enough to counteract the liquid-drop model deformation energy of the corresponding fragments, the scission configurations for fragmentations with neutron and/or proton numbers corresponding to these strong, deformed shells, may result in cold fission, through these fragment “shape isomeric states”. Here, because of decreased mutual Coulomb repulsion between the nascent fragments, the total energy of the system in the “shape isomeric state” scission configuration may be less than when it is in its ground state. In order to deepen our understanding of the cold fission phenomenon and to see whether or not these highly deformed non-ground state shells play some part in this context, we have looked for cold fission for 229Th(n,,, f), 232U(n,,, f) and *“‘Pn(n,,, f) using the Cosi fan tutte mass spectrometer operating at the Grenoble high-fIux reactor. This instrument is based on one time-of-flight set-up and one axial ionisation chamber that measures the kinetic energy of the same fragment that passes first through the time-of-flight system [12], see sect. 2. Although, as pointed out above, cold fission has already been investigated for 239Pu(n,,, f), these are the first results on cold fission for 229Th(n,,, f> and 232U(n,,, f). 229Th and 232U are the lighest fissioning nuclei that one can study with a reasonable counting statistics in a reasonable time, because the cold fission probability for these light nuclei is expected to be a lot higher than for the heavier fissioning systems.

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2. Experimental set-up and data recording The study of 22gTh(n,,,, f), 232U(n,,, f) and 239Pu(n,,, f) was carried out with the Cosi fan tutte spectrometer installed at the Grenoble high-flux reactor. The details on the spectrometer, and the way the measurements are done, have already been published [12]. However, briefly, the Cosi configuration used in this work consists of a time-of-flight device with a flight path 1 = 107 cm, and an axial ionisation chamber with an entrance window, where the fragments are stopped. The signals of pulse height Ph from the chamber reflect the kinetic energies of the fragments. The chamber is run with isobutan gas at a pressure of 160 mb. This chamber also allows one to carry out Bragg spectroscopy and to measure the Bragg parameter Q,, which is sensitive to the fragment nuclear charge. The details on the fission targets used and the other pertinent data are given in Table 1. Each recorded event consists of three correlated parameters: one time of flight T, one pulse height Ph and one Bragg parameter Q,. The data are recorded event-by-event for an off-line analysis. The Q, parameter is also used to eliminate [9] a good part of fission events that suffer small angle scattering on the support grid of the entrance window of the ionisation chamber and lose energy.

3. Data analysis In the yield Y(t, Ph) matrices of the light group for 229Th(n,,, f), 232U(n,,, f) and 239Pu(n,,, f), one observes reasonably separated mass lines. These mass lines are fitted to the relation: $14V2 = +M( l/T)2

= (II + a’M)Ph + b + b’M,

(2)

where a, a’, b, b’ are the constants to be determined, and M is the fragment mass. The mass resolution values obtained for the three fissioning systems are given in

Table 1 Pertinent data for targets and experiment Fission target

Duration of data recording

Statistic for light group

Mass resolution for light group (amu)

22gTh02, 106.7~g/cm2 evaporated on a 0.1 mm Al backing 232U02, 31 pg/cm2 evaporated on a 0.1 mm Al backing 23gPuF3, 131 wg/cm* evaporated on 57.5 pg/cm2 polyimide coated with 25 pg/cm gold.

3 months

2.4 x lo5

0.64

7 months

4.3 x 105

0.74

2 x 106

0.68

15 days

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Table 1. The individual mass yields are obtained by unfolding the mass spectrum via a non-linear least-squares analysis with a sum of gaussians. The yield Y(M, E) matrix is constructed event-by-event from the fragment kinetic energy E-values that are obtained from the precise time-of-flight data with the relation E = #(1/T12 These E-values are corrected for the energy loss in the fission target and in the START detector foil.

4. Access to cold fission

and discussion

In order to study cold fission, we determined the light-fragment kinetic energy for a certain low value of fission E L,max values for different fragmentations probability pf per MeV of energy. Clearly, this pf depends on the number of counts in the yield Y(M, El matrix. Then, the E,_,,JMJ values were transformed into the corresponding total-fission-fragment kinetic energy Ek,max(ML) via the relation E k,max =E L,max‘AdA,

-ML,

(3)

where A, is the mass of the fissioning nucleus. As Table 1 shows, our counting statistics are rather modest. However, in spite of this limitation, these data lead to significant results on cold fission. We next discuss each of these fissioning nuclei separately: 4.2. 229Th(n,,, f) Fig. la shows the curve for Ek,max(M,) for this sytem for a fission probability window pf = (2-.5)10P5/MeV along with the curve for the maximum reaction energy Q,,,(Mr) calculated from the Liran and Zeldes mass table [13]. For a given fragmentation MJM,, the Q,,, corresponds to the charge division 2,/Z, that maximises this quantity. One notices in Fig. la, that the E,(M,) lines calculated from the fixed E, (= 110 and 112 MeV) values, are parallel to the Q,,,(ML) cum forM, = 75-98. The Q,,,(Mr) curve shows important oscillations with a period of = 2 to 3 mass units, due to neutron and proton pairing energies present in the masses. The Ek,max CM,) curve also shows some oscillations. However, one notices that these oscillations are important only for certain fragmentations like M, = 82, 84, 86, 90 and 95. This behaviour seems to be related to the important difference in the observed mean proton and neutron odd-even effects 6p and an, where Sp(Gn) is defined as the relative excess, in per cent, of the sum of the even-Z (even-N) fragment yields over the odd-Z (odd-N) fragment yields: for example, for E, = 107 MeV, 6p = 60% and 6n = 10% (refs. [9,14]). This difference in 6p and 6n means that the probability of breaking a neutron pair and

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80

85

911

95

100

IO5

110

115

I”0

M, (amu) for a fission probability pr = (2X 10-‘-5X Fig. 1. The Q,,(M,_), and the Ek,max (M,) distribution f)), (2 X 10e5-5 X 10-5)/MeV (232U(n,h, f)), (1 X 10m6-5 X 10K6)/MeV lo-‘)/MeV (229Th(n,,, (239Pu(n,,,, fj). The continuous lines represent E,(M,) for certain fixed values of the light fragment kinetic energy E,.

producing odd-N fragments starting from an even 2 and even N fissioning nucleus 230Th* is much higher than for a proton pair, and, as a result, most of the neutron pairing energy 24, is used up in neutron pair breaking and the amplitude of oscillations on Ek,max(M,), that correspond to the even-N fragmentations should be very much smaller than for the even-2 fragmentations. To check out this possibility, we present in Fig. 2a, the Q,,,
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200 t

a

22gTh + nth

.a

SO

1110

90

I,l,

110

1211

(3mf_l)

Fig. 2. The same as Fig. 1, but here neutron pairing energy has been removed from the Q,,,(ML) curve.

as shown in ref. [19] and, therefore, for these fragmentations the neutron pairing energy should remain substantial. We found that when both the curve, there neutron and proton pairing energies are removed from the Q,,(ML) is little resemblance between this curve and that of the Ek,max(ML) distribution. Fig. 3a shows the final total excitation energy E*(M,) = (Q,,,(M,)-E,,,(M,)) for the 230Th* fissioning system. The estimated mean uncertainty on Ekmax is = kO.4 MeV. Before we go further, we note that we ignore the precision kinetic energy if any (one can argue [lo,151 that it should go to zero as E, 2 Q> and identify Ekmaxwith the Coulomb repulsion energy EC between the nascent binary fragments at scission. If we assume (a little arbitrarily) that cold fission is realised for E * G 5 MeV we that fission present all fragmentations with M, = 80-99. However, colder fission is realised for 96, where the complementary heavy fragments contain N = 82 and Z = 50 M,spherical shells and they enjoy an increased stability. For these fragmentations a 6, and 6,

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80

85

90

95 IA,

100

105

110

Iarm’

Fig. 3. The total available final excitation energy E*(M,)= (Q,,,(M,jEk,,&ML) for 22yTh(n,h, f), 232U(n,,, f) and 23yPu(n,h, f). For 239Pu(n,,, f), the E * values of Simon [6] are included.

gain in reaction energy Q of N 8-10 MeV is such that, as discussed in ref. [lo], the compact scission configurations consist of nascent fragments in (or close to) their ground states connected by a neck simulated by a distance d Q 3 fm between the sharp (liquid drop) surfaces of the fragments. Fig. 4a presents the ground-state deformations of fragments in the form of quadrupole moments QzO as calculated by Moller and Nix [16]. One notices that around ML/M, = 96/134, the heavy fragment is spherical; but the light fragment has QzO= 3 b. The fragments with N < 58 are soft and not rigidly deformed (see below). However, as Fig. 3a shows that for 229Th(n,,, f), the coldest fission shows up around M, = 84-86. Here the spherical M, is in the middle of the N = 50 spherical shell and the reaction energy Q goes UP by = 4-5 MeV. As to the heavy fragment Mu, apart from a deformation Q20 = 3b C/3= 0.21, there seems to be nothing special about it. Then, how does the coldest fission occur for this region of fragmentations? It seems that for these fragmentations, the scission configurations consist of nascent heavy fragments with

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120

130

140

150

22gTh + nth

6

80

85

90

95

IflO

105

a

110

115

M, (amu)

Fig. 4. The calculated ground-state quadrup-ole deformations [16] Q2,, of fragments for “9Th(n,,,, f), 232U(n,,, f) and 239Pu(n,,, 0. The empty and full circles represent the Qr,, values of the light and heavy fragments, respectively. In the case of 239Pu(n,t,, f) the empty and full triangles represent the Qr,, values that, for a given fragmentation M, /MH, correspond to the fragment charge ratio Z, /Z, that leads to the second highest reaction energy Q.

MH=

144 that are not created in their ground states, but in a highly deformed state with p = 0.65 due to a very strong, deformed N = 88 shell [ll]; for a quadrupole deformation, this corresponds to a semi-major axis to semi-minor axis ratio of = 2 - the same as for the fission shape isomers. This shell energy compensates almost completely the liquid drop model energy needed to deform the Mu from p = 0.2 to p = 0.65, and, as a result, the reaction energy Q does not change. For such configurations Ek,max= E, * Q, but, here, d < 3 fm. Thus, here is a case of cold fission, where the scission configurations contain the nascent Mu in a “shape isomeric state”. After scission, as the binary fragments fly apart and their mutual influence decreases, the Mr.., should jump back to its ground-state configuration.

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4.2. 232U(nth, f) Fig. lb shows the Q,,(M& and E,,(M,) at a level of pr = (2-5)10-5/MeV for this reaction. Here one observes again that the E&M,) lines for E, = 112 and 115 MeV are parallel to the Q,,,(Mr) curve for M, = 75-100. Unlike the even-even fissioning nuclear such as 230Th* and 240Pu*, where both the even and odd-Z values contribute to Q,,,(Mr), the Q,,,(Mr) for 233U* results only from even-Z values. As the fissioning nucleus 233U* is odd-N and even-Z in nature, the Q,,,(Mr> for any fragmentation contains only about half the neutron pairing energy. Moreover, for this fissioning nucleus, the intrinsic neutron odd-even effect Sn should, as a rule, be close to zero. It can be shown that the experimental value of Sn 2.73% (ref. [9]) results mainly from prompt neutron evaporation [20]. The number of oscillations on Q,,,(M,_) is much less than for 230Th* and 240Pu* (see below) and they are quite well in phase with the oscillations on Ek,max(M,_) indicating again that the proton odd-even effect Sp is quite important for high E, or E, values: Sp = 30% for E, = 109 MeV (ref. [9]), and an important part of the proton pairing energy (- 1 MeV on the average) is added to Ek,,,JML). Fig. 2b presents Q,,W,_> where the remaining neutron pairing energy has been removed, along with Ek,,,= 04,) The phase relationship is a little better than before. The 233U* data confirm indirectly the low mean Sn values for 230Th* and 240Pu* (see sect. 4.3). Fig. 3b shows the E *CM,) distribution for this nucleus. Again with the criterion of E * < 5 MeV for cold fission, one observes that cold fission is realised for ML = 84-107 as suggested by the tip model of cold fission [lo]. The coldest fission shows up for M, = 100-105, where, as Fig. 4b indicates, the light fragments with N > 58 have large (Qzo = 4 b) and rigid ground-state deformations [17,18], and the complementary heavy fragments contain the N = 82 and Z = 50 spherical shells. For M, = 85-90, relatively colder fission is present, because of the simultaneous presence of the N = 50 spherical shell in the light fragments and the N = 88 deformed shell in the complementary heavy fragments. However now, these shells do not coincide or overlap so well as in the case of 229Th(n,,, f). Hence, it seems that, here, again “the shape isomeric state” of the heavy fragments with N = 88 plays a role for the realisation of cold fission. 4.3. 239Pu(n,h, f) Fig. lc presents, for this fissioning nucleus, the Qmax(M,_) curve along with the E k,m&t4L) distribution obtained for a fission probability pf = (1-5)10-6/MeV. Unlike the 230Th* and 233U* fissioning nuclei, the E,JM,_) obtained for E, = 116 and 118 MeV, and the E k,max(ML) itself, start to diverge from the Q,,W,_) curve for M, < 100. Here again the number of oscillations on Q,,(M,_> is much higher than on E k,m&4,_). In Fig. 2c, we present the Q,,(Mr) curve stripped off

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neutron pairing energy. Here again the remaining oscillations on the Q,,, 58 have large (Fig. 4c), rigid deformations as discussed above in the case of 232U(n,,, f). However, the absence of cold fission around M, = 85, shows that the influence of the N = 50 shell in conjunction with the N = 88 deformed shell has completely disappeared, because they do not overlap anymore. According to the tip model of cold fission [lo], it should exist for M, = SO-100 for 239Pu(n,,, f); if one looks at the ground-state deformation Qzo values, Fig. 4c, one observes that in this case, the nascent light and heavy deformed fragments should play an identical role. However, the presence of cold fission for Mr_ = 95-112 only does not back up this assumption. The absence of cold fission for M, < 95 seems to indicate that the light-fragment ground-state deformation and not that of the heavy fragment, may be the deciding factor. This idea was suggested in ref. [5] in the context of 235U(nth, f). However, it is difficult to accept this idea as such. We feel that the groundstate deformation values do play some role, but the deciding factor seems to be the presence of shells, deformed and spherical, in the nascent complementary fragments. In the case of 229Th(n,i,, f) and 232U(n,,, f) we saw the role of the presence of the non-ground state N = 88 deformed (p = 0.65) shell in the heavy fragment overlapping the N = 50 spherical shell in the complementary light fragment. When this overlapping disappears as in the case of 239Pu(n,,, f), cold fission is not favoured here anymore. In the case of the N = 82 and Z = 50 spherical shells in the heavy fragment, the fissioning system gains so much energy that it is not necessary that the complementary light fragment, should have a strong shell, too. However, as discussed before, when these light fragments have N > 58, suddenly, they start to have large, rigid ground-state deformation 117,181,because of the presence of a potential mountain (anti-shell) for p = O-O.2 and a = 2 MeV deep-deformed neutron shell for p = 0.3 (ref. [111X This seems to be the reason that for the fissioning systems such as 233u* 234ue , 7 236U* and 240P~, the coldest fission is realised for these fragmentations. In the case of 248Cm(s, f), the coldest fission is realised [S] for fragmenta-

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tions, where the heavy fragments contain the N = 88 deformed shell and the light fragments are with N > 58.

5. Conclusions

The Cosi fan tutte spectrometer installed at the Grenoble high-flux reactor has been used to measure the light-fragment-group mass-energy correlations for 229Th(n,,, f), 232U(n,,, f> and 239Pu(n,,, f). From these data, the probability of cold fission has been determined for these three fissioning systems. These are the first results on cold fission for 229Th(n,,, f) and 232U(nth, f). The present results on cold fission for 239Pu(n,,, f) are consistent with the existing data. For 229Th(n,,,, f), cold fission is realised for the whole mass range M, = 80-99 present in the spectrum; but the coldest fission shows up for M, = 85. For 232U(n,,, f) cold fission is present for M, = 84-107, with the coldest fission manifesting for M, = 100-105. In the case of 239Pu, cold fission exists for M, = 95-102, and here the coldest fission is produced for M, = 105-110. This systematic study of cold fission shows that shells in the nascent fission fragments seem to play a decisive role in the realisation of this phenomenon. This work was done in collaboration with the University of Tiibingen (F. Gonnenwein and J. Kaufmann). We are grateful to Mme F. Parisot for typing the text.

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