Se-Hwan Park

Performance and operation modes of the Duke FEL storage ring

We present the recent status and performance of the Duke Free Electron Laser (FEL) storage ring. We report a large transverse and energy aperture observed in the storage ring. We describe the consequences of this large aperture on the operation of the storage ring. Several unusual phenomena are reported in this paper, including large amplitude transverse multibunch coherent oscillations (Saturn rings) and beam capture from outside the RF separatrix. We also present the established operation modes for the storage ring, including energy ramping, working point tuning, and different bunch-mode operations to optimize the ring as a synchrotron light source, an FEL, and a gamma-ray source. Finally, we summarize the achieved storage ring parameters since its first operation in November 1994.

Surface Passivation Effect on CZT-Metal Contact

The process of cadmium zinc telluride (CZT) surface passivation is very important to reduce the leakage current of the detector and to improve the detector performance. NH4/H2O2 solution was identified as an effective passivation agent for CZT. We fabricated a CZT planar detector and measured the detector performances before and after the NH4/H2O2 passivation. For the first time, the passivation effect on CZT-metal contact was measured. The depth profile of the chemical composition of metal contact was measured with AES to understand the change of the CZT detector performance with passivation.

Polarity effect of the thimble-type ionization chamber at a low dose rate

It is known that the current collected from an ionization chamber exposed to a constant radiation intensity changes in magnitude when the polarity of the collecting potential is reversed. It is called the polarity effect of the ionization chamber. There are many possible causes that induce the polarity effect and one of them can be a field distortion due to a potential difference between the guard electrode and the collector. We studied how much the polarity effect depends on the design of the electrodes in the thimble-type ionization chamber. Two thimble-type ionization chambers, which had different electrode structures, were designed and fabricated at KAERI. We calculated the field distortions due to the potential difference between the guard electrode and the collector for the two ionization chambers. MAXWELL and Garfield were employed to calculate the electron drift lines inside the chamber. The polarity effects of the two ionization chambers were measured, and they were consistent with the field calculation. We could conclude that the polarity effect is mostly induced from the field distortion due to the potential difference between the guard electrode and the collector in our experiment and it depends significantly on the design of the electrodes.

Characteristics of Fabricated Neutron Detectors Based on a SiC Semiconductor

Wide-band-gap semiconductors such as SiC, AlN, and GaN are promising materials for harsh environment applications due to their high-temperature operation capability. Two types of PIN-type semiconductor neutron detectors based on SiC were designed and fabricated for nuclear power plant (NPP) applications such an in-core reactor neutron flux monitoring and safeguarding nuclear materials. One is for fast neutron detection and the other, which was evaporated with 6LiF, is for thermal neutron detection. In this study, preliminary tests, such as the determination of I-V and alpha responses, were performed. Reaction probabilities with respect to neutron energies were also calculated by using an MCNPX code for comparison with the experimental results. Responses of the neutrons were measured at the Ex-core Neutron irradiation Facility (ENF) of the High-flux Advanced Neutron Application Reactor (HANARO) research reactor at the Korea Atomic Energy Research Institute (KAERI). Pulse height spectra and count rates were measured with respect to the neutron fluxes from 1:6 × 106 n/cm2·s to 1:9 × 107 n/cm2·s. Also, a 0.99 root-mean-square value of linearity against the fluxes to the count rates was obtained with the fabricated neutron detectors. For a thermal neutron detector, a 3.3% detection efficiency was obtained.

New concept of CZT-based hand-held radioisotope identifier

Radiation dose can be determined with the radiation dosimeter. Recently, hand-held radioisotope identifier is being developed by many researchers to measure the radiation dose and to determine the type of the radioisotope. New design concept of the radioisotope identifier is introduced in this work. In this design, one can measure the radiation dose and the type of the radioisotope, and one can also determine the direction where the radiation comes. The hand-held radioisotope identifier is based on cadmium zinc telluride (CZT) detector. The detection principle is from the change of the energy spectrum as the direction of the incident radiation is varied. EGSnrc Monte Carlo simulation is done to calculate the energy spectrum of the CZT detector, and the result is compared with the measured one. The energy spectra are calculated when the gamma-ray is incident on the detection at different directions. The simulation results show the change of the spectrum shape as the incident radiation comes from different directions. The calculation is also done with different radiation energies and detector sizes. A prototype radioisotope identifier is being developed at Korea Atomic Energy Research Institute (KAERI). This work would be helpful to develop next generation radiation dosimeter.

Optimization of hybrid-type instrumentation for Pu accountancy of U/TRU ingot in pyroprocessing.

One of the final products of pyroprocessing for spent nuclear fuel recycling is a U/TRU ingot consisting of rare earth (RE), uranium (U), and transuranic (TRU) elements. The amounts of nuclear materials in a U/TRU ingot must be measured as precisely as possible in order to secure the safeguardability of a pyroprocessing facility, as it contains the most amount of Pu among spent nuclear fuels. In this paper, we propose a new nuclear material accountancy method for measurement of Pu mass in a U/TRU ingot. This is a hybrid system combining two techniques, based on measurement of neutrons from both (1) fast- and (2) thermal-neutron-induced fission events. In technique #1, the change in the average neutron energy is a signature that is determined using the so-called ring ratio method, according to which two detector rings are positioned close to and far from the sample, respectively, to measure the increase of the average neutron energy due to the increased number of fast-neutron-induced fission events and, in turn, the Pu mass in the ingot. We call this technique, fast-neutron energy multiplication (FNEM). In technique #2, which is well known as Passive Neutron Albedo Reactivity (PNAR), a neutron populations changes resulting from thermal-neutron-induced fission events due to the presence or absence of a cadmium (Cd) liner in the samples cavity wall, and reflected in the Cd ratio, is the signature that is measured. In the present study, it was considered that the use of a hybrid, FNEM×PNAR technique would significantly enhance the signature of a Pu mass. Therefore, the performance of such a system was investigated for different detector parameters in order to determine the optimal geometry. The performance was additionally evaluated by MCNP6 Monte Carlo simulations for different U/TRU compositions reflecting different burnups (BU), initial enrichments (IE), and cooling times (CT) to estimate its performance in real situations.

Development of SiC detector for the harsh environment applications

Silicon carbide (SiC) radiation detector was developed at Korea Atomic Energy Research Institute (KAERI) for the harsh environment applications, which include power monitor of the reactor core of the nuclear power plant. The wide band gap of SiC semiconductor makes the SiC attractive candidate for use in high temperature and high radiation environment. The effects of radiation irradiation on the detector performance were studied. Especially, the effect of metal electrode on the radiation tolerance of the SiC detector was investigated. The leakage currents and the alpha energy spectra measured with the SiC detectors were compared before and after the neutron and gamma-ray irradiation. The operation properties of the SiC detector were also measured as the temperature of the detector was increased. A water-proof SiC detector assembly was fabricated as a power monitor of the power plant. The reactor power was successfully monitored in the reactor core of High-flux Advanced Neutron Application ReactOr (HANARO) of KAERI with the detector assembly.

Burnup measurement of spent fuel assembly by CZT-based gamma-ray spectroscopy for input nuclear material accountancy of pyroprocessing

Input nuclear material accountancy is crucial for a pyroprocessing facility safeguards. Until a direct Pu measurement technique is established, an indirect method based on code calculations with burnup measurement and neutron counting for 244Cm could be a practical option. Burnup can be determined by destructive analysis (DA) for final dispositive accuracy or by nondestructive assay (NDA) for near-real time accountancy. In the present study, an underwater burnup measurement system based on gamma-ray spectroscopy with the CZT detector was developed and tested on a spent fuel assembly. Burnup was determined according to the 134Cs/137Cs activity ratio with efficiency correction by Geant4 Monte Carlo simulations. The activity ratio as a function of burnup was obtained by ORIGEN calculations. The measured burnup error was 8.6%, which was within the measurement uncertainty. It is expected that the underwater burnup measurement system could fulfill an important role as a means of near-real time accountancy at a future pyroprocessing facility.

Property of a CZT Semiconductor Detector for Radionuclide Identification

Compound semiconductors of high Z value material have been studied intensively for X-ray and γ -ray spectroscopy at room temperature. CdZnTe has wide band gap energy as 1.6 eV and can provide high quantum efficiency with reasonably good energy resolution at room temperature. This study is aimed at determining radionuclide analysis ability by measuring energy resolution of CZT detector which will be applied at nuclear material identification purpose. For experiment we used a CZT detector (5 × 5 × 5 mm3) which is manufactured by eV Products. We have performed our measurement at varied temperatures similar to the outdoor environment for the investigation about temperature dependence of energy resolution and peak centroid fluctuation of CZT detector by using gas cooling and Peltier cooling methods. In order to test radionuclide identification we used various radionuclide samples; plutonium, europium and other standard sources. Pulse height spectra were obtained by standard electronics which consists of a preamplifier, a shaping amplifier, and a multi-channel analyzer.

Critical systems for high peak power storage ring FEL

The Duke storage ring is a facility dedicated for the deep UV free electron laser (FEL) development. It is especially designed for high peak power applications, having a long optical cavity and large energy acceptance. In this paper we describe a FEL gain modulator and optical cavity control system, which are essential for obtaining maximal peak power.