Measurement of delayed-neutron yield from 237 Np fission induced by thermal neutrons
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CLEI Experiment
Measurement of Delayed-Neutron Yield from 237 Np Fission Induced by Thermal Neutrons N. A. Gundorin, K. V. Zhdanova, V. E. Zhuchko, L. B. Pikelner* , N. V. Rebrova, I. M. Salamatin, V. I. Smirnov, and V. I. Furman Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Received August 9, 2006; in final form, September 26, 2006
Abstract—The delayed-neutron yield from thermal-neutron-induced fission of the 237 Np nucleus was measured using a sample periodically exposed to a pulsed neutron beam with subsequent detection of neutrons during the time intervals between pulses. The experiment was realized on an Isomer-M setup mounted in the IBR-2 pulsed reactor channel equipped with a mirror neutron guide. The setup and the experimental procedure are described, the background sources are thoroughly analyzed, and the experimental data are presented. The total delayed-neutron yield from 237 Np fission induced by thermal neutrons is νd = 0.0110 ± 0.0009. This study was performed at the Frank Laboratory of Neutron Physics (JINR, Dubna). PACS numbers: 28.20.-v, 25.85.Ec DOI: 10.1134/S1063778807060014
INTRODUCTION Neutron-induced fission of nuclei is known to be accompanied by the emission of several prompt fission neutrons. Additionally, neutrons can be emitted from the fission fragments after their β decay, provided that the excitation energy is sufficiently large. The delayed neutron (DN) yield from heavy-nucleus fission has been studied from the very beginning of nuclear energy utilization because DN emission is of basic importance for the realization of a controlled chain reaction of nuclear fission. The yields and the temporal characteristics of DNs from neutron-induced fission of the main and minor reactor isotopes are important nuclear constants used in calculations of the nuclear reactor kinetics. For high burn-up reactors intended in particular for the transmutation of minor actinides such as Np and Am, the exact knowledge on the DN yields is important for improving reactor dynamics and the conditions of safe and economic operation. Unfortunately, the DN characteristics of these nuclei are still insufficiently studied, especially in the case of thermal-neutron excitation. Most experimental procedures used to measure the DN yield involve the transfer of a sample from the site of irradiation to a detector for DN monitoring. However, these procedures are not universal since they do not allow the characteristics of DNs emitted *
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within the first second after fission to be determined with good precision. In 1993–1995, the Isomer setup was created at the Frank Laboratory of Neutron Physics (JINR, Dubna) for studying the DN yield from fission of a series of actinides under the action of thermal neutrons. This setup was used to realize an experimental procedure [1] based on a periodic exposure of a sample to a pulsed neutron beam and the DN yield measurements between pulses. In this case, prompt and delayed neutrons are separated by means of temporal analysis and, hence, the sample t
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