Neutron Capture Cross Sections from 1 MeV to 2 MeV by Activation Measurements

G.D.Kim, T.K.Yang, Y.S.Kim, H.J.Woo, H.W.Choi, W. Hong and J.H.Chang*
Korea Institute of Geoscience and Mineral Resources
* Korea Atomic Energy Research Institute

Historically, activation with nearly mono-energetic neutrons was the first means of measurement of neutron capture cross sections in the MeV range. Although the method gives neither the sensitivity nor the neutron energy resolution of the time of flight method used with large liquid scintillators, it is completely selective for a given nuclide in a mixture of target isotopes, and the experiments are relatively simple to carry out. In KIGAM's neutron facility, mono-energetic neutrons were produced with 3T(p,n)3He reaction. Neutron cross section data at neutron energies from 1 to 2 MeV, which are interested in the fusion reactor materials research. are being measured. As a start, fast neutron captured cross sections on 63Cu and 186W are measured relative to a well known 197Au neutron capture cross section. The thin TiT target for this reaction was provided by the CEA company in France. By the analytical results of our RBS and ERD-TOF, the areal density of T found to be a good agreement to that of CEA within 10 %. But the result of Ti was quite different from that of CEA. 2" x 2" BC-501 and NE-213 liquid scintillator were used to measure the energies of produced neutrons and to monitor neutron yields, respectively. The pulse shape discrimination method for both detectors was applied for separating neutron signals from gamma ray signals in neutron-gamma mixed field. A 2" x 2" HPGe detector was used for detecting gamma rays emitted from fast neutron activation samples. The equivalent geometrical efficiency of this detector was obtained both by the measurement with a mixed source and MCNP code. The absolute efficiency of the HPGe were calculated for activated samples respectively with MCNP code and equivalent geometry of this detector. The calculated absolute efficiency of HP Ge for 411 keV, which was emitted from the activated Au sample, 511 keV ,which was emitted from the activated 63Cu, and 636 keV, which was emitted from the activated 186W, are 0.0287, 0.0290 and 0.0184, respectively. In the calculation of neutron captured cross section, compensation for the decreases of the activated nuclei during irradiation time and measuring time was carried out. The neutron capture cross sections of 63Cu and 186W, which had been measured by the reference comparison method, were compared with that of ENDF-

4rd workshop on nuclear data production and evaluation, 24-25, Aug. 2001, Pohang Univ., Pohang

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