Presented at the XVI International Workshop on Physics of ... - fobos

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refs therein). Nowadays we achieved a certain progress in treating the CCT data [2] and in order to ... literature which seems to be reproduced excellently (fig. 3). 70. 80 ... Comparison of the mass spectrum of the fission fragments of 252Cf(sf) obtained ... important for drawing the physical conclusions on the CCT mechanism.
EXPERIMENTAL APPROACH TO THE SEARCH OF COLLINEAR CLUSTER TRIPARTITION WITH THE FOBOS DETECTOR D.V. Kamanin1, Yu.V. Pyatkov2, A.A. Alexandrov1, I.A. Alexandrova1, E.A. Sokol1,S.V. Mitrofanov1, V.G. Tishchenko1, E.N. Kuznetsova1, S.V. Khlebnikov3, T.E. Kuzmina3, Yu.V.Ryabov4, S.R. Yamaletdinov1, S.N. Denisov1 1

Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia 2 Moscow Engineering Physics Institute, 115409 Moscow, Russia 3 Khlopin-Radium-lnsntute, 194021 St. Petersburg, Russia 4 Institute for Nuclear Research RAN, 117312 Moscow, Russia

This paper is devoted mostly to the accuracy and correct handling of the data in the experimental search for indications of the process called by us “collinear cluster tripartition” (CCT). The experiment has been performed at the 4π-spectrometer FOBOS installed at the Flerov laboratory of nuclear reactions of the JINR [1]. During last five years we intensively study and discuss this phenomenon (see [2, 3] and refs therein). Nowadays we achieved a certain progress in treating the CCT data [2] and in order to increase the convincing power of our arguments we would like to discuss in details the procedure of mass calibration, which is the most critical task, and also some important points of our data analysis.

Fig.1. The first observation the structures formed by the unusual fission events with the deficit both of the total mass and TKE of complimentary fragments observed at the FOBOS spectrometer in the reaction 248Cm(sf).

The manifestation of CCT in experimental data is easily seen, in particular, in the structures like that shown in fig.1. Our analysis of the previous data obtained at the FOBOS spectrometer in the reaction 248 Cm(sf) and 252Cf(sf) showed that the structures under discussion are very unlikely the result of mishandling the data or due to some disturbance like scattering of FF on the supporting grids or on the wires of coordinate planes. However, the obvious asymmetry in the mass-mass correlations enforced the symmetrization of the detector arms by developing the special three-electrodes start PAC with the internal Cf source [4] and also improvement of the calibration procedure described below. Due to the extreme importance of the latter in the discussion of the reliability of our data we described its accuracy in details below. The TOF calibration is performed by means of fit of the mass distribution of 252Cf which is wellknown from the literature. The free parameters of fit were the effective thickness of the adsorbing matter on the particle flight pass constrained by the assumed uncertainty of the thickness of the Mylar foils. The

Presented at the XVIth International Workshop on Physics of Nuclear Fission, October 7-10, 2003 IPPE, Obninsk, Russia

obtained fit shown in fig. 2 reproduces quit well not only positions of the centers of gravity of the light and heavy peaks but also the widths of their mass distribution. Additional checking of the mass calibration has been performed by comparing the velocity distribution also with the data known from the literature which seems to be reproduced excellently (fig. 3).

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Fig.2. Comparison of the mass spectrum of the fission fragments of 252Cf(sf) obtained by the TOF-TOF analysis with that taken from the literature [5].

experiment: = 1.360 σV = 0.0705 literature: = 1.375 σV = 0.0665

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Fig.3. The experimental spectrum of the velocity of the 252Cf(sf) fission fragments. The literature data for comparison are from ref. [5]

In order to improve also the mass calibration in the TOF-E method we have improved the procedure to restore the fragment energy. This is the task of special difficulty for slow CCT fragments since they loose up to 70% of their initial energy. The final mass spectrum obtained by the TOF-E analysis is given in fig. 4. Of course, the resolution of the TOF-E mass is not such a good as that from the TOF-TOF analysis but in the case of incomplete kinematics, e.g. in CCT events, the TOF-TOF method become unusable. Taking into account that the middle non-observable fragment deposits almost the third part of the mass detecting a large mass deficit doesn’t require a perfect mass resolution. On the other hand, the correct determination of the mean TOF-E mass value is extremely important for drawing the physical conclusions on the CCT mechanism. Our efforts in improving the transport calculations resulted in the perfect correlation (fig. 4) between the measured energy in Bragg

Presented at the XVIth International Workshop on Physics of Nuclear Fission, October 7-10, 2003 IPPE, Obninsk, Russia

chambers and the energy obtained from the TOF-TOF analysis (corrected to the post-fission neutrons and entrance foils). This linear dependence means that our calibration is unambiguous, linear and nonshifted. The data analysis presented in the ref. [2] (during this conference) already exploited the above described improved energy calibration.

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Fig.4. Comparison of the mass distribution of the fission fragments obtained by the TOF-E analysis with the literature data from Ref. [6].

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Fig.5. The obtained correlation of the FF energy calculated from the TOF-TOF analysis corrected to the post-fission neutrons and entrance foils with the raw energy signals in the Bragg chambers.

Taking into account comparatively poor resolution of the TOF-E mass reconstruction method one could doubt in the possibility to recognize any structures like those shown in fig 1. However, one should take into account that at low statistics only the most probable values of a variable (e.g. mass) should occur. The criterion of recognition of the linear structure is that its length should be few times larger than

Presented at the XVIth International Workshop on Physics of Nuclear Fission, October 7-10, 2003 IPPE, Obninsk, Russia

its width. Therefore, the very simple simulation of the range (i.e. difference between the largest and smallest values) of a normally distributed variable can help in understanding that the structures are really observable. Indeed, the simulation shown in fig. 6 demonstrates that if the typical size of the structure amounted to 4-6 points, it’s width should be approximately 1.5σ to 3σ taking into account the dispersion of the average shown at the right axis. This means, that with the given mass resolution the structure formed, for example, by Ni-cluster could be already recognized from that originated by Se-cluster (see ref. [2]).

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Fig.6. The average difference between the largest and smallest values of a normally distributed variable (range) and its dispersion.

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Fig.7. The comparison of the simulated neutron multiplicity with the experimentally measured one. The difference of the 30% between the probability of 3-fold neutron event with the actual source compared to that in the background-less ideal case means high reliability of the measured high-fold neutron data in searching for the true high-multiplicity events.

The independent criterion of the CCT should be a high multiplicity of neutrons accompanying the CCT events. Therefore, the FOBOS spectrometer has been coupled with the “neutron belt” consisting of 140 3He counters in the PE moderator [4, 7]. The legitimacy of using the observed high multiplicity as

Presented at the XVIth International Workshop on Physics of Nuclear Fission, October 7-10, 2003 IPPE, Obninsk, Russia

the indication of the true high multiplicity under the certain experimental conditions by means of such parameters as the registration efficiency, background of any type and the decay rate of the source has been exhaustively studied in [8]. The very important conclusion is that the part of the events with the true multiplicity higher than 3 among the observed 3- and more-fold neutron events amounted to 70%. The illustration is given in fig.7.

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Fig.8 The drift-time plot. The unusual events could be found far from main loci along the diagonal, even some structures are recognizable.

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Fig.9.The same as in fig. 8but under the condition of at least two neutrons coincided.

Presented at the XVIth International Workshop on Physics of Nuclear Fission, October 7-10, 2003 IPPE, Obninsk, Russia

The additional independent variable sensitive by the definition to the CCT events is the total charge of the complementary fragments, namely, a large deficit of the total charge is expected. Moreover, charge is measured physically independent and should not be affected by scattering on detector materials, on the contrary to the fragment energy. Unfortunately, the Bragg-spectroscopy is out of rule for low energies of particles and one should measure the drift time in the Bragg chamber for extracting the particle charge. This well known technique has been already tried with the FOBOS modules [2]. Although drift time depends not only on the particle charge but also on its energy, the plot shown in fig. 8 represents the physical information similar to the mass-plot given in fig. 1. The unusual events located along the diagonal far from the main loci are the most interesting from the CCT point of view. Indeed, under the coincidence with two neutrons some number of these events survived, so they are unlikely just a random seed. The preliminary analysis of the drift-time allowed us to suspect the deficit of the total charge in the events ascribed to CCT [3]. However, the more detailed analysis of the charge data is planned in more careful measurement with the modified mini-FOBOS setup [9] which is under preparation. Summarizing we would stress again what we observe in the experiment. We extract some number (10 -10-5 from the whole data bulk) of the correlating pairs of FF (one of them seems to be a cluster [2]) whose velocities are perfectly collinear with the precision of 1°. These FF pairs exhibit a large deficit of the total mass (at least 30% is lost) and also a lowered TKE [10]. These FF pairs can be accompanied by an increased number of isotropically emitted neutrons (for sure more than 3). We have obtained the first experimental indications that their total charge is notably lower than the charge of 252Cf. Taking into account that we observe the indications of CCT in few different measurements at FOBOS with 248Cm and 252Cf we think that there is the strong experimental evidence of CCT and there is already a base for the systematical study of this process. -6

This work is supported in part by CRDF, grant MO-011-0. References 1. H.-G. Ortlepp et al., Nucl. Instr. and Meth. A 403 (1998) 65 2. Yu.V. Pyatkov et al., Contribution to this conference 3. Yu. V. Pyatkov et al., Physics of Atomic Nuclei, v. 66 (2003)1631 4. D.V. Kamanin et al., Physics of Atomic Nuclei, v. 66 ( 2003) 1655 5. H.V Shmitt et al., Phys. Rev. 141 (1966) 1146 6. A.C.Wahl et al., Phys. Rev. 126 (1962) 1112 7. E.A.Kuznetsova et al., Contribution to this conference 8. A.N.Tjukavkin et al., Contribution to this conference 9. D.V.Kamanin et al., in FLNR Scientific report 2001-2002, Dubna, 2003 10. Yu. V. Pyatkov et al., Int. Symp. On Exotic Nuclei (EXON-2001), Baikal Lake, July 24-28, 2001, 181

Presented at the XVIth International Workshop on Physics of Nuclear Fission, October 7-10, 2003 IPPE, Obninsk, Russia