Kil-Oung Choi

Neutron spectrum at the underground laboratory for the ultra low background experiment

We measured the background neutron spectrum using a Bonner sphere spectrometer at the YangYang underground laboratory, where the vertical earth overburden is approximately 700 m. The neutron fluence rate and the energy spectrum were determined through the unfolding procedure. The measured neutron fluence rate was (0.242 ± 0.008) n cm(-2)h(-1) for k=1, and most of the neutrons were below 20 MeV.

Correction equations of coincidence summing using75Se radionuclide in the efficiency of HpGe detector

Correction equations of the coincidence-summing effect for efficiencies of HpGe detector based on the decay scheme were developed by considering the summing up to triple coincidence. The correction equations which do not dependent on the kind of the Ge detector are very useful for efficiency calibrations of a Ge detector in the energy region from 60 to 400 keV by using75Se radionuclide even with very short source-to-detector distances.

Neutron Spectrum Measurement at the Workplace of Nuclear Power Plant with Bonner Sphere Spectrometer

With BSS system, the neutron spectral fluences at the workplace of nuclear power plant at Wolsong were measured. The 9 points at the 4th and 5th floor of the reactor building were selected for the spectrum measurement, where the radiation workers access for the ordinary check of the operation. The spectra show the distribution from the thermal to several MeV, which are composed of the original fission spectrum, scattered fast neutrons and thermal neutrons. The spectral shapes for the places at the same floor are similar to each other. Although the number of data points is limited, we may determine the standard neutron spectrum for each floor and this would help improving the uncertainties of neutron doses. For the comparison, we measured the neutron ambient dose with neutron survey meter (EG&G Ortec, LB6411) and personal dose with TLD on PMMA phantom.

Impurity correction factor of MnSO4 compound for the determination of neutron emission rate on the manganese bath method

The manganese sulphate bath method is widely used for measurements of neutron source strength. In this study, the analytical chemistry method based on the Inductively Coupled Plasma (ICP) spectrometry was used for examining the impurity contents of MnSO4·H2O, to induce55Mn(n,γ)56Mn reactions. From the analytical results, mainly K, Co, and Zn as well as trace amounts of Cd, Li, etc., have turned out to be the relevant impurities absorbing the neutrons and the fraction of neutron absorbed by the total impurities was determined to be 1.37%.

Measurement of the 250Cf component in a 252Cf neutron source at KRISS

Neutron emission rate measurements have been carried out at the Korea Research Institute of Standards and Science using a manganese sulphate bath system for (252)Cf and (241)Am-Be sources since 2004. The relative measurement method was chosen in 2012, and the neutron emission rates agreed with those by the absolute measurement method within uncertainties. The neutron emission rate of an old (252)Cf source has been measured three times: in 2004, 2009 and 2012. The (250)Cf component was fitted to a double-exponential function of (252)Cf+(250)Cf, and the ratio of the (250)Cf component to the (252)Cf component was estimated to be 7.8 % in 2004 and 46.8 % in 2012. Underestimation of the neutron emission rates of old (252)Cf sources can be corrected if the neutron emission rate of the (250)Cf component is taken into account.

The Determination of Neutron Dose for Accelerator-Produced Neutrons by Using a Bonner Sphere Spectrometer

The spectral fluences of the neutrons from the 18O(p,n)18F reaction were measured at Korea Institute of Radiological & Medical Sciences MC-50 cyclotron. Neutrons were measured with the Bonner sphere spectrometer of Korea Research Institute of Standards and Science. The neutron spectral fluences including the scattered neutrons were evaluated by unfolding together with the initial guess spectrum. The variation of the spectral fluence and the ambient dose equivalent rates were investigated by changing thermal-, intermediate-, and high-energy ratios of the initial guess spectrum. The results show that the spectral fluences and the ambient dose equivalent rates are stable within 2% even though the initial guess spectrum is highly changed. Therefore, if the spectral shape of the initial guess spectrum is chosen properly for the measurement, the spectral fluence of the accelerator-produced neutron can be determined by the unfolding process

Collisional processes in muon catalyzed fusion as studied by the classical collision theory

Collisional processes involving a negative muon in the deuterium and tritium system were studied using the classical binary encounter theory. The time needed for slowing down of a 10 keV muon was found to be of the order of 10−8s to 10−12s, depending on the density of the system. The “Sticking Probabilities” for the d-t and d-d fusions were obtained to be 0.48% and 10.2%, respectively. The usefulness of the classical model for understanding fundamental processes in muon catalyzed fusion is suggested.

Determination of the ratio of the hydrogen and manganese absorption cross sections by the manganese bath technique

The ratio of the hydrogen and manganese neutron absorption cross sections, σH/σMn, is a most important parameter in the determination of radioactive neutron source strength by the manganese bath technique. The ratio is well measured by observing the change in56Mn activity induced in the manganese bath by a fixed neutron source as the manganese concentration of the bath is changed. In the present study, the neutron source was a Maxwellian beam from252Cf. Concentrations were determined by the two methods: volumetric and gravimetric. The cross section ratio has turned out to be σH/σMn=0.02506.