Experimental study of cluster effects in binary and ternary decays of low excited actinides

[摘要] ENGLISH ABSTRACT : The experimental study of rare decay modes of ternary fission performedby the FOBOS group at the Joint Institute for Nuclear Research (JINR)in Dubna, Russia, revealed a new ternary decay mode of low excitedheavy nuclei. This new ternary decay mode is referred to as CollinearCluster Tri-partition (CCT). The distinct features of the CCT include thecentre of mass of one of the decay fragments and the centre of mass ofthe two other decay fragments moving in opposite directions relative toeach other. Another experimental signature for identifying the CCT isthat one of the three decay fragments have a magic nucleon numberconfiguration.The first experimental observation of the CCT was revealed in a studyof spontaneous decay of 252Cf performed using a so-called modified FOBOS spectrometer facility installed at the JINR. This phenomenonwas observed using the missing-mass method, where two fragmentswere detected, the third one being missing. The missing mass wasaccounted for by the difference in the masses of the detected fragmentsand the mass of the initial nucleus. The CCT manifested itself througha bump in the mass-mass distribution of fission fragments from thedecay of 252Cf. Further confirmation of the CCT using the same missingmass method was obtained from the reaction 235%(nth,) where anexperiment was performed using a spectrometer referred to as miniFOBOS. This experiment made use of a neutron beam delivered by theIBR-2 Reactor from the Frank Laboratory of Neutron Physics at theJINR. In both experiments the CCT was revealed as a bump in the massmass distribution which was linked to the magic nickel cluster. Thebump was referred to as the Ni-bump.It quickly became clear from the early experiments that a directdetection of all the decay products of the CCT will be a more convincingexperimental approach. In an effort to detect all decay products, a new double arm time-of-flight (TOF) spectrometer, based on a mosaicdetection system, aimed at detecting all the CCT products was designedand is presented in this work. This spectrometer is referred to asCOMETA (Correlation Mosaic Energy – Time Array). The COMETAspectrometer registers the energy and time signals of heavy ions usingmosaics of PIN diode detectors in each arm.This new spectrometer required a new parametrization procedure tocalculate the masses of the registered fragments of the CCT. Theparametrization procedure also forms part of this work. This proceduretakes into consideration the well-known experimental challenges whenregistering heavy ions using semiconductor-based detectors such as PINdiodes. The first challenge is the so-called pulse-height defect, which ismanifested when registering the energy of heavy ions. The otherchallenge is the so-called plasma delay, which occurs when registeringthe time signal for the heavy ions.To test this procedure a special experimental setup called Light IonSpectrometer for South Africa (LIS-SA) was put together. In this work,an experiment performed with the LIS-SA setup, that tested thisprocedure in the mass reconstruction of the fission fragments of theCCT, is also presented.The results from the COMETA spectrometer that confirmed theexistence of the CCT are also presented. In the mass-mass distributionof fission fragments from the COMETA, the CCT reveals itself asrectangular structures bounded by known deformed magic clusters suchas 123Mo, 63Sr and also magic clusters such as 128Sn. These structuresappeared in the same vicinity where the Ni-bump was observed earlier.Further analysis of these structures revealed that they are linked to theNi-bump.This work did not only provide a more convincing approach in the studyof multi-body decay of low excited nuclei (the CCT), the existence of theCCT phenomenon was successfully confirmed through the direct detection of all the decay products of the CCT. The CCT has beenconfirmed as a decay process that takes place as a two-stage break-up ofthe initial three body chain-like nuclear configuration with an elongatedcentral cluster.

[发布机构] Stellenbosch University