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Gamma-Multiplicity at Nuclear Fission

#3543


Measurement of gamma-multiplicity at nuclear fission of californium–252, uranium–235 and plutonium–239

Tech Area / Field

  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
3 Approved without Funding

Registration date
16.06.2006

Leading Institute
VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Collaborators

  • Oak Ridge National Laboratory, USA, TN, Oak Ridge

Project summary

The aim of the Project is to determine by experimentation gamma-multiplicity at nuclear fission of 252Cf, 235U and 239Pu. Gamma-multiplicity is a probability distribution of emitting a definite amount of gamma-quanta at heavy nucleus fission. The average amount of gamma-quanta (the first moment of gamma-multiplicity) emitted at fission has long been measured practically for all heavy nuclei. But not until recently the probability of emission of the definite amount of gamma-quanta at fission was measured only for 252Cf (by Brunson, LANL). Such the state of affairs is conditioned, on the one hand, by very complicated staging of such experiments and, on the other hand, by the absence of practical interest to this problem from the perspective of its using in nuclear weapon or nuclear energy generation. At the same time the information on multiplicity of gamma-quanta emitted at fission of heavy nuclei is of keen interest as regards the nuclear fission physics.

The procedure of the proposed nuclear-physical experiment is as follows. The fission chamber with the fissile material being tested (for example, 235U) is placed in the center of the measuring device. Several (eight) fast scintillation plastic detectors intended for prompt gamma-quanta monitoring are placed around the fission chamber at equal distance from it. The input windows of these detectors close the major portion of the spherical surface that surrounds the fission chamber. Fissions in the substance being tested (for instance, 235U) are induced by neutrons of the reactor EBR-L placed in the adjacent room behind the shielding (the concrete wall 2 m thick) and operating in the static mode. Neutrons leave the reactor, pass through the collimator in the shielding and incidence, in the form of a pencil beam, on the fission chamber thus initiating the nuclear fission (for example, 235U).

The registration circuit operates in the counting mode, i.e., the mode of alternate counting of gamma-quanta from inpidual nuclear fission events. The time window in the circuit is opened for recording a signal from the fission chamber; the signal is generated as a result of recording a nuclear fission fragment (for instance, 235U). It is closed within about 10 ns after monitoring of all gamma-quanta of this fission prior to the moment of prompt fission neutrons’ coming. Hence neutrons are excluded from the recording process reasoning from the flight time. Following the processing of data from one time window, the recording cycle is repeated until statistically significant data array is acquired.

The set of experiments is proposed to be performed for three fissile materials at several values of discrimination threshold in order to obtain the relation of gamma-multiplicity to energy of gamma-quanta emitted at fission.


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