-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. Figure <• shows a typical ternary a pulse-height spectra with a detection level of 15 MeV. — 151-
TREATMENT OF DATA, RESULTS AND COMPARISON WITH OTHER T/B MEASUREMENTS
1. Energy region above 1 eV
The analysis of the data is rather straightforward. After correction for background and underlying cross-sections, the areas of corresponding resonances in ternary CT] and binary CB) fission are derived. Their ratio yields the T/B value for the resonance considered.
Table 2 shows these T/B values for measurements I and II ; these results are normalized to T/B = 100 for the 10.95 eV resonance. In order to obtain more conclusive values we calculated the weighted mean of the T/B values from both measurements. With these values we tried to classify the resonances in a "high" £H), a "low" EL) and an "uncertain" tU) group, resp., based on the following criteria :
H : T/B > Ю5 ; U : 106 > T/B > 103 ; L : 103 >* T/B
Brackets are used for the resonances far which the statistical error does not allow an unambigeous classification. We find that there is a significant difference between the 15.5 eV resonance EH] and most other resonances CD, especially the strong 7.B5 , 10.95 and 11.Э0 eV resonances. This higher a counting-rate cannot be caused by a-particles from the 239PuCn,a) 236u reaction since the highest energy a-line generated in this way is 11.46 MeV, which is well below the detection level in our experiment.
We performed some statistical tests on these data. We calculated the weighted mean Xw for the different measurements, taking into account all the data resp. L + (L) and H + EH]. These results are summarized in table 4, where we also calculated the hx-values of Birge :
Xi - X2 hx = 2 2 {••2 Coj + a2 )
X] and X2 represent the weighted mean of the "low group" L + CU and the "high group" H + (H) and a\ and 02 the corresponding statistical errors. Taking into account that the probability that Xj and Xg are compatible with a unique value is smaller than 0.01 for hx > 1.83, we conclude from this table that there is a significant difference between the "high" and the "low" T/B values.
It is difficult to compare our results with those of llelkonian et al.5), who performed the only other T/B measurement in this energy region, since they did not -152-
Dubl:^- f- г", ^•jr'sriJal values for the different resonance? nsza'-ss cf their rather poor .statistical accuracy. ;jevertneiess thay found that Г--5 .-.=ss significantly larger for two resonances with ." = 0, dhicn is in qualitative agreement with our results.
2. Energy region below I e" Th<2 third measurement i? analysed in a somewhat different vay. 3y cutting cadmium filters in tre bea~ -,e ;nec^.ej -he DacKgrounJ in the Energy region D9low 0.2 eV and found it nsgligiLls compared to the counting rate in that region. Then we divided The ternary and binary fission t.o.f. spectra in several intervals in the energy region frorr, 20 msV to 1 eV [thus including the strong 0.3 sV resonance] and calculated the ratios of the areas of the corresponding intervals. These results, which are normalized to the first two measurements, are represented graphically in fig. 5 and given numerically in table 3. We find that there is no significant difference between these T/3 values. Moreover, if we consider the weighted averages given in the fourth column of this table, we deduce that the D.2S5 eV resonance is very probably a "low" one and that also the thermal T/B value is predominantly low. Within the experimental errors our results agree with those of Panov et al.i6) [10 \ exp.err.) and Schroder17] [2 % exp.err.].
Finally we examined whether our low energy data are compatible with a uniaue distribution or not. Therefore we calculated x2 = 5.16 (with 7 degrees of freedom) 2 > 2 = with the corresponding probability P(x X D) 70 4. : Furthermore we obtained for the Birge ratio 8)'9j R = 0 / 0 a yaiue 0f 0.869 and a probable deviation of R from unity of 0.076. From these tests we deduce that the data considered are compatible with a unique distribution, although not very pronounced.
So we fitted a straight line through these data (fig. 5i. Although this fit is in agreement with the statistical laws, we were not completely satisfied since we felt that a weak structure might be present. So we tried to fit an interference curve through our d^ta, which is also represented in fig. 6. This curve follows better the experi^Rrtal points and tne fit results in a better Rvalue than in the stiaight-line case.
Sur.h a possible interference effect is, however, not so easy to explain. In 1967 Mostovaya -°J rr-easured the r/3 ratio for 235U. She found a very pronounced inter ference effect in the neighbourhood of the 2 eV resonance, which she explained by a different number of open fission channels in binary and in ternary fission. -153-
This assumption ;з hnwpvsr nnt so evident. In ojr ;:азе the e- fe: t if ei f'Л '. there is - is much smaller and lass convincing. So ws consider t'is -?it snown in fig. 6 rather as a scientific guess.
DISCUSSION
In table 5 we г.отелг? our proposed clasai ricatian от' the resonances t?ased an trsir Г/Б value with the direct determination of the resonance spin J and .vir.h core other fission characteristics. This table clearly demonstrates that there is a complete agreement between our unambiguously classified resonances and the direct spin- determinations of Sauter and Bowman 22), Asghar 23J, Trochon et al.2"1} and Simpson st al. 5). Compared with the spinvalues of Fraser and Schwart?. ^'5 one of the five common resonances disagrees. However it must be stressed that none of ther- determined the spin of the irrportant 0.296 eV and 15-5 eV resonances. This has been done for the 0.296 eV resonance by Chrien et al.^63 via the detection nf high-energy capture утауз. They assigned J^ = 1f for this resonance, which agrees with our result. Moreover Farrell 27) and Becker "8) assignee both J11 = 0* For the 15.5 eV resonance based on the "interference method" tindirect determination).
When comparing our results with the average kinetic energy of the fission fragments measured by Plelkonian and Hehta b) we find a difference for the important 1'ï.'3 eV resonance. The same may be said for the fission symmetry measurements of Cw.'iav et al. 'M in the 15.5 eV resonance. However our J^ = 1+ result for the O.V9fi eV resonance agrees with Cowan's propor?1 for this resonance, which is based on tfv; results of Regier et al. 29b Here nowever it must be mentioned that Regier's result that the asymmetric-to symmetric fission ratio for the thermal region is about three times smaller than that for the 0.29S eV resonance, is nut reflscted in our Г/В values, which are not significantly different in both regions.
Л rather difficult criterium for comparison is the fission neutron multiplicity v. If we examine the various measurements we observe a rather clear biats dun to the rJs Lent ion method. (n) Weston st al.e) and I rochon et al.^i find no variation in \>, Thay both detectnu the fission neutrons dircci.ly. (b) Weinstein et al.b), Ryaoov et al.7) and Phackleton p.t al.10) find e variation of v" from resonance to resonance. They all detected the fission neutrons after moderation.
If we compare the absolute data of Weinstein and Ryabov shown in ta'iie b, wr чпе that they are generally anticorrelated. Nevertheless this in not sn fnr thnir -154- spin assignments, which are almost in agreement. This is especially the case for the 0.296 eV and 15.5 eV resonances, which are also in agreement with our results.
CONCLUSIONS
In the energy region from 20 meV tD 50 eV we observe only one resonance with a significantly higher T/B valut- : i.e. the 15.5 eV resonance. We assign J~ - С for resonances with a high Т/Б value and Jv = 1+ for resonances with a low T/B v/alue. This higher value for 0+ resonances cannot be due to an eventual loss of binary fission counts for one spin state - thus a 5-value which would be too small - since the difference between the Kinetic energies of the 0+ and the 1+ states is only about 0.7 % according to flelkonian and Hehta 5).
As we have shown in the introduction, this higher ternary ot-yield for О* resonance: is in the sense expected according to Bohr 2) and Wheeler 3), since the 0+ states are more symmetric than the 1+ states. More symmetric fission corresponds in gene to a lower total Kinetic energy of the fission fragments, thus to a higher excitat: energy of the fragments and a higher probability for ternary a-emission.
We can reach the same conclusions based on energetic considerations. From several analysis of experimental data we Know that the average fission width for 0+ states is much larger than that for 1 + states : < Pf > g+ >> < Tf > ^+ . This implies that the number of open fission channels for 0+ states N(0+] is larger than IMC1+), thus that the 0* channel lies below the 1+ channel in the transition channel spectrum. From this we deduce that the remaining excitation energy via a D+ channel is larger than for fission via a 1+ channel. This implies a higher probability for ternary fission via a 0+ channel than via a 1+ channel.
ACKNOWLEDGEMENTS
The authors wish to thank Dr. M. Nève de Mévergnies für fruitful discussions. They are indebted to many colleagues from CBNPI, Geel for the preparation of the samples and the operation of the linac. They especially acknowledge the skilful assistance of Пг. R. Barthélémy, G. Le Dez and J. Van Gils during these experiment; -155-
REFERENCES
1) Т. Krogulski. J- Chwaszczewska, П. DakowsKi, E. Piasecki, M. Sowinski and J. Tys, Nucl. Phys- A 12Ö (1969), 219
2) A. Bohr, Proc. Int. Conf. on the peaceful uses of atomic energy, Geneva [195:•, vol. 2, p. 151
3) J. Wheeler, Proc. Int. Conf. on nuclear reactions, Amsterdam (1956), p. 1103
4) G. Cowan, B- Bayhurst, R. Prestwood, J. Gilmore and G. Knobeloch, Phys. Rev., 144 (1965), p. 979-983
5) E. Melkonian and G. Mehta, 1st Symp. on physics and chemistry of fission, Salzburg, 1965 (IAEA, Vienna) p. 355
6) S. Weinstein, R. Reed and R. Block, 2nd Symp. on physics and chemistry of fissior IAEA, Vienna (1969), p. 477
7) Y. Ryabov, So Don Sik, N. Chikov and N. Yaneva, Sov. Journ. of Nucl. Phys., vol. 14, 5 (1972), p. 519
B) L. Weston and J. Todd, Proc. Conf. on Neutron Cross Sections and Techn., Knoxville (1971), CONF. 710301, p. 661
9) J. Trochon, B. Lucas and A. Michaudon, Proc. of the European Nucl. Phys. Conf., Aix-en-Provence (1972), vol. 2, 23
10) D. Shackleton, J. Trochon, J. Frehaut and M. Le Bars, Phys. Lett., vol. **2 8, 3 (1972), 344
11) C. Wagemans and A.J. Deruytter, Nucl. Phys. A 194 (1972), p. 657-672
12) V.M. Strutinsky, Nucl. Phys. A 95, (1967), p. 420
13) J.E. Lynn, Theory of neutron resonance reactions. Clarendon press, Oxford (1968) p. 463
14) H. Weigmann, Zeit. Phys., 214 (1968), 7
15) S. Bjzirnholm, European Nucl. Phys. Conf., Aix-en-Provsnce (1972), p. C5-33
16) A. Panov, Sov. Phys. JETP, 16, 5 (1963), 140B
17) I.G. Schroder, Nucl. Phys. A 195 (1972), 257
18) R.T. Birge, Phys. Rev. 40 (1932), 207
19) B. Taylor, W. Parker and D. Langenberg, The fundamental constants and quantum electrodynamics. Academic Press* New York (1969) -156-
üj T.A. Mostovaya and G. Jakoviev, Report IAE-1439 (1967)
1] J. Fraser and R. Schwartz, Nucl. Phys. 3D [1962], 269
2) G. Sauter and D. Bowman, Phys. Rev. Lett., vol. 15, 19 (1965], 761
3] M. Asghar. Proc. 2nd Conf. on Nucl. Data for Reactors, Paris (1965), vol. II, 185
4] J. Trochon, H. Derrien, B. Lucas and A. Michaudon, Proc. of the 2nd Canf. en Nucl. Data for Reactors, Helsinki С19701, 49S
5] F. Simpson. L. Miller, M. Moore, R. Hockenburv and T. King, Nucl. Phys, А 164 (1971], 34
6] R. Chrien, 0. Wasson, S. Dritsa, S. Sokharse and J. Garg, Proc. of the 2no Canf. on Nucl. Data for Reactors, Helsinki 11970], 377
73 J. Farrell, Phys. Rev., vol. 165, 4 (1968), 1371
3) W. Becker, Doctorsthesis, Philipps-Univ. Marburg/Lahn [1973]
3) R. Regier, W. Burgus, R. Tromp and B. Sorensen, Phys. Rev., vol. 119, 5 {19601, 2017 TABLE 1: Experimental conditions for the different measurements
Measurement Energy region Linac parameters Overlap filter number
I 0.1-50 eV 24 ns, 600 Hz Cd E . = 65 MeV, i = 45 ,uA el av / P = 3 kW 70 ns, 400 Hz E . = 65 MeV, i = 50 ,uA el av / P = 3. 25 kW
II 0. 1 - 50 eV 33 ns, 400 Hz Cd E , = 67 MeV, i = 44 ,uA el av / ' P = 3 kW
III 0.01 - 15 eV 1. 2 ,us, 100 Hz none E , 4 53 MeV, i = 45 ,uA el av / P = 2.4kW TABLE 2: Results normalized to T/B = 100 for the 10.95 eV resonance
Resonance Measurement I Measurement II Weighted mean (Meas. I and II) energy Class. T/B t Stat, err. T/B i Stat. err. T/B Stat. err. (eV)
7.85 98.7 ] 2.8 99.3 ! 3.1 99 2. 1 L 10.95 100 | 2.1 100 1 2.4 100 1.6 L 11.90 101.5 | 3.9 9B. 1 [ 4,5 100 3 L 14.30 ) 107.5 ; 4.5 106.2 [ 5 106.9 3.4 (H)a> 14.68 ) 103.7 ' 3.4 96.9 | 3.8 100.7 2.6 L 15. 50 ) 112.4 ! 4.3 108.1 ; 5 110.6 3.3 H 15b) 106.9 ! 2.3 102.2 1 2.6 104.9 1.8 17.7 104.8 ! 4 99.5 ! 4.6 102.5 3 L 22.2 105.6 [ 3.7 101.4 ! 4.2 103.8 2.8 U 26.2 106.4 J 5.6 106.8 ! 6.7 106.6 4.3 (H) 47.8 99.5 J 7.Й 96.6 ! 9 98.3 5.9 (L) 50.1 104.4 J 7.5 97.2 | 8.4 101.2 5.6 ,(ь)
Classification: H: T/B г 106 U: 106 > T/B > 103 L: 103 г Т/В Brackets indicate that the statistical error is too large to be conclusive. an unambiguous classification is impossible due to the influence of the underlying tail of the 15. 5 eV resonance. b) this stands for the group of resonances at 14. 30, 14. 68 and 15. 50 eV. -159-
TABLE 3: Results from Measurement III normalized to the weighted mean of the 7.85, 10.95 and 11.90 eV resonance/ as ob tained from Measurements I and II (T/B = 99. 7 + 1. Z)
E(eV) т/в Stat. err. Weighted mean
0.29 6 102. 2 0.65 7.85 97 4.6 ) 10.95 102.2 3.7 ) 99.7 + 2.7 U 90 97 6.7 ) - aя\ 0.886 ' 102.4 2.9 0.465 102. 0 2. 2 0.374 103.9 2.9 0. 345 99.9 2.2 0.319 100.9 1.8 ) 0.29 6 102. 2 1.6 ) 101. 4 + 1 0.275 101 1.7 ) 0.257 102.7 2.0 0.240 102.3 2.3 0.225 105.5 2.6 0.205 101.8 2. 1 0. 182 106.2 2. 5 0.163 105.9 2.6 0. 146 104. 0 2.6 0. 132 101.1 2.6 0. 120 103.8 " 2.6 0. 110 102. 5 2.6 0, 101 98.7 2.6 0.09 25 101.4 2.6 0.08 53 105.4 2.6 0.0790 103.4 2.6 0.0733 102.9 2.6 0.0683 104.0 2.6 0.0637 103.7 2. 6 0.0596 102.8 2.7 0.0558 102.0 2. 7 0. 0524 98.7 2.9 0. 049 3 100.3 2.9 0.0465 97.6 3.0 0.0439 103. 1 3.1 0. 0404 98.9 2.3 0.0364 102.7 2.4 0.0329 104. 3 2.6 0.0299 99.1 2.7 ) 0.0273 106.1 3.0 ) 0., 0250 100.3 3. 0 ) 102. 3 ± 1. 3 0. 0230 104.2 3.3 ) 0.0212 100. 5 3. 5 ) 0.0197 105.0 3.8 )
the energies indicated below correspond to the middle of the energy- interval considered. TABLE 4: Weighted average values and hx-values of Birge
Measurement number Data used X (weighted average) hx
I all data 102.81 + 1. 11 L + (L) 101. 03 i 1.29 ) H + (H) 109.20 4 2.72 ( 1.92
II all data 100.51 + 1. 26 L + (L) 98.96 + 1.46 ) H + (H) 107. 08 +_ 3.13 | 1,66
weighted mean of all data 101.82 + 0.84 I and II L+ (L) 100.14 + 0.98 ) H+ (H) 108.29+2,1 | 2. 49
III all data 102.32 + 0.40 Table 5: Comparison of our classification Of the resonances via their T/B values with other classification methods
Kinetic Fission Fission neutron Our classi energy of symmetry ("v ) measurements fication via via neutron scatter ing vla у -ray via interference fragm. a} T/B values E(eV) detection method
12 .I.»» r 24) ,15) 5 4 2 6 7 rei. * ~f. > rei. ref. ' ref. »t»> ref. „f. > «t. » " ret. » «f. >
+ .+ + о. г? ь i A(l ) M^) L(l+) 7.85 1+ н(Г) L H(l*) L 10.95 •t :+ ,+ н L H L. 11.90 !• н H 14.30 \ ;; н L H (i-.)c) 14.68 0 i 1 H J. 15.50 н Ac) L(0+) H(0ь +,) 17.7 :+ t н A L H ъ гг. г * t !; Jï i+ н Л L H и гб. г i; <н) A L L (И) 47.8 0+ о: 0+ A H
The notation L means lower average kinetic energy of ttus fission fragmenteand H higher average kinetic energy. The notation A means predominant asymmetric fission. ' Influenced by other resonances, ' The notation L means hete lower v v*lue and ti higher v value. -162-
FIGURE CAPTIONS
Fig. 1: Lay-out of the detection and data collection system 239 Fig. 2: Upper part: The Pu ternary alpha time-of-flight spectrum (Run I) Lower part: Corresponding binary fission time-of-flight spectrum 239 Fig. 3: Upper part: The Pu ternary alpha time-of-flight spectrum (Run III) Lower part: Corresponding binary fission time-of-flight spectrum
Fig. 4: Pulse-height spectrum of ternary or -particles (discrimination level 1 5 MeV) Fig. 5: T/B-ratios in the energy region from 0. 02 eV - 1 eV (Run III)
Fig. 6: Interference curve fitted through the data below 1 eV. 1 63-
239 Pu foil
removable Al screens
detectors
TOF ANALYSER
4096 CH. MEMORY
INTERFACE
IBM 2311 DISK
IBM 1800 BINARY FISSION COUNTS TERNARY FtSSION COUNTS UI 01 о о UI о (Л о о о о о о
*-*+ 26.2 eV
ел о + * * 22.2 eV
"^ + +* » +• + + 17.7 eV ** О п х > z + •* + + 15* eV *** z I m t~ z с 4 vr- 11.90 eV ш гп -л* 10.95 eV
*> +* 7.85 eV *»»* s-* **•+ to "0 С -165-
375
500 1000 1500 2000 CHANNEL NUMBER
10 000 •
1Л
3 о
й 5000 >- ос < со
0 •*- 1000 CHANNEL NUMBER
Fig. 3. •166-
13NNVH0/SlNnOO E,eV
Fig. 5. -168-
Д = 1 meV
ff = Га= 100 meV
Fig. 6.