-147- TERNARY TO BINARY FISSION RATIO FOR NEUTRON INDUCED FISSION IN «'Pu IN THE ENERGY REGION FROM 0.02 eV TO 50 eV С WAGEMANS* S.C.K./C.E.N.. B-2400 Mol. Belgium and A.J. DERUYTTER C.B.N.M., EURATOM, 8-2440 Geel, Belgium Abstract The measurements were performed at a 8.1 m flight-path of the C.B.N.M.-Linac at Geel. Binary and ternary fission fragments were consecutively counted with banks of sur­ face - barrier detectors on both sides of a double-faced "9Pu sample. After correction for background and underlying cross-sections, the areas of corresponding resonances in ternary (T) and binary (B) fission and derived ; their ratio yields the T/B value for the resonance considered. Moreover the energy region through the first large resonance from 0.02 eV to 1 eV was subdivided into several zones for which T/B values were also calculated. The T/B ratio seems not to vary significantly in the energy region from 0.02 eV to 50 eV, except for the 15.5 eV resonance where we find about 10 % more ternary fission. Correlations of T/B with the resonance spin J and with other fission characteristics are discussed. *N F WO , University of Ghent and S.C.K./C.E.N. -1Д8- INTRODÜCTIOM Wnen a iLicls_= Missions - spci-ta^eix.sly er ina-ced пу se'"e energetic partiele or j-sciatijn - it generally brsa\s .• i-to twe nes.-. -rag~er.t£ ar,- two or three neu­ trons. This is the so-called Dinary fission. ;- n.v to Q.3 % of the cases these twj- nea-.y fragments are accompanied by a very energetic light particle, mostly an a-particls. This is the ternary or LRA -is- sion. The energy distribution •- these a-particles is compatible with a Gaussian shape, with a peak at 15 MeV a;-j a ^e'i^.jT energy of about 30 ™eV -or the ^^"j Missioning cQ~tpoLir.G r'-Jcla-5. : ,• In the measurements which we are going to cescribe the zl+03u compound nucleus is obtained by bombarding а гз9Ри nucleus iJ"' = 1/2 J with s-wave neutrons ££ = 0, IS + s = 1/2). So compound states with J = D and 1 will be excited. In terms of the classical channel theory of 8ohrz) and Wheeler^) these conpound 0 states cor­ respond either to the ground state or to the 0 quadruools vibration of the nucleus at the saddle point, which are essentially symmetric states. This is not so for the compound 1 state, which is considered to be formed oy the coupling of the octupole vibrations К = G and K. - 1 which are both asymmetric states. These different properties of the 0 ano 1 states will be reflected in the correspon­ ding fission exit channels, which are thus expected to be very different in na­ ture. This implies that also the fission properties are expected to be very dif­ ferent. Baseo on this idea several attempts have been made to classify the neutron resonan- ces into two groups corresponding to J =0 resp. J = 1 by measuring the mass- asymmetry1*) , the total kinetic energy of the heavy fission fragments5), the average nurr-ber of neutrons emitted in fission6) 1(^) and the relative ternary fission yield^). We were especially interested in this last item since recently we were able to clas­ sify the neutron resonances in гз5и neutron induced fission intc two groups corres­ ponding to "low" and "high" ternary to binary fission ratios11). Moreover these T/B-values were correlated with the most reliable direct spin-determinations and with some other fission characteristics. Sir.ce 2ii5U with J ' = 7/2 forms compouno 3 and 4 states under Dompardment with s-wave neutrons, states whicn, according to the channel theory, are thought to have rather similar properties, we expect a priori to obserle an even stronger ef­ fect in the Z39PL neutror. I-^cec fissic . Untii now we have only thought in terms of the Bohr-wheeler picture of transition states on the top of a simple inverted oscillator potential barrier. Although their theory has a lot of merits, actually this picture is e little bit oversim­ plified in view of the discovery of sub-barrier- ana isomeric tission and its ex­ planation on the basis of a "double-humped" fission parrier12j'*)lüS. However -149- ;cr-i-g to Bjdr-Kclm- 5 асе .-. - ". -,f the nuclei deexcite into the first well and only 0.01 % intD the second, whereas about 96 \ lead to normal fission. Moreover for neutron induced fission of 23<3Pu the energy available аооиэ the lowest fission threshold is about 1.6 MeV. In view of these considerations ;t is logical to expect that the channel theory of fission will still oe applicable in resonance neutron fission of 239Pu, although the subsequent interactions ta^inf, place between the saddle- and the scission point should attenuate the chanrel effects taking place at the saddle point. In the measurements which we will describe, we tried to classify the resonances according to their ternary to binary fission ratio T/B. Ir, the energy region from 1 eV to 50 eV we calculated the T/B values for the strongest resolved resonances and in the region from 20 meV to 1 eV we calculated T/B for 40 energy zones. EXPERIMENTAL ARRANGEMENTS The measurements were performed at a 8.1 m flightpath of the C8NM linac. By bombarding a mercury cooled uranium target with the 70 MeV electron Deam of the linac, fast neutrons were produced which were then slowed down by a polyethylene slab. Figure 1 gives the lay-out of the detection and data collection system. The detection chamber is an evacuated cylindrical chamber with thin aluminium entrance and exit windows j its inner diameter is 50 cm. In the center of this chamber a double faced 239Pu layer (99.956 at.% ; 1 mg/cm2 thick) is mounted, viewed on each side by a bank of four large Si(Au) surfsea barrier detectors. These aetectors were calibrated daily based on the pulse-ri^:!.;,ht of the natural plutonium ct-particles. Moreover, aluminium screens of different thicknesses can be inserted (ternary fission) or withdrawn (binary fissicn) from between the layers and the detectors. This detection technique allows rather good resolution of the time-of-flight spectra in the low-energy region, since only one back-to-back deposit of 239Pu is used. Moreover, in this way less scattering material is introduced into the neutron beam than with a multi-plate ionization chamber. Pairs of detectors are connected in parallel and the signals from each pair are amplified by a charge-sensitive preamplifier ?nd a DDL main amplifier. The amplified signals are sent into a fast timing single channel analyser (T3CA). After mixing all the fast timing signals are fed into a 4096-channel time-of-flight analyser with an "accordeon" system. Fronn the analyser memory the data are transferred via an interface unit to an IBM 2311 disk for storage. The data handling is performed afterwards with an IBM 1800 system. -1 50- MEASURING PROCEDURE With the apparatus described before three measurements of the ratio of the ternary to binary fission cross-sections vs. neutron energy were performed. Table 1 gives earns details of the different exDerimental conditions. The first and the second measurement cover the energy region from 0.1 eV to 50 eV ; the third covers the region from 10 meV to 15 eV. The total discrimination level for ths ternary ct-particlss was fixed at about 15 JleV for ail the measurements. Each measurement consists of several binary, ternary and background runs. - In a binary run we register the time-of-flight spectrum of the fission fragments in the analyser memory ; in this case there are no aluminium screens between the detectors and the plutonium layers and the discriminators are set to detect only the fission fragments. - In a ternary run we register the t.o.f. spectrum of the light ternary fission fragments (LRA] in the analyser memory. The ternary a-particles are separated from the heavy fission fragments and from the natural a-particles emitted by the plutonium isotopes in the target by inserting a 20 um thick aluminium screen between the detectors and the target. By appropriate discriminator settings the detection levels were adjusted to 15 MeV. - Figure 2 shows the ternary [upper part] and binary [lower part] fission t.o.f. spectra from measurement I and fig. 3 shows the same from measurement III. - For both binary and ternary runs, background runs were performed by putting appropriate neutron filters into the beam. Moreover, for measurement III runs were also performed with a cadmium filter in the beam to evaluate background due to un-timed epicadmium neutrons in the beam and room neutron background (in the energy region below 0.1 eV). In all the measurements a thorough search for the various possible background sources has been done. A detailed description of this background study is given in i !). As a conclusion of all these background measurements we found that the different background contributions were extremely small, especially in measurement III where the repetition frequency was only 100 Hz. The same detection system as described before was moved to the BR2 high flux reactoi of S.CK./C.E.N., Mol, where it was installed at a beam tube and connected to a pulse-height analyser. With this apparatus several pulse-height spectra of binary fission fragments and of ternary a-particles were performed to control the quality of these spectra and to verify the exact position uf the detection levels.