Earticle

When the PDD (percentage depth dose) in the megavoltage beams is measured in the water phantom, the polarity and ionrecombination effects of ionization chambers with depth in water are not usually taken into consideration. We try toinvestigate if those variations with depth should be taken into consideration or could be ignored for the thimble type semi-flex ionization chamber (PTW 31010TM, SN 1551). According to the recommendation of IAEA TRS-398, the 4 representativedepths of ds, dmax, d90and d50were used for the electron beams. For the photon beams, the 4 depths were arbitrarily chosenfor the photon beams, which were ds, dmax, d10and d20. For the high energy photon beam both polarity and ion recombinationfactors of the chamber with depth in water gives the good agreements within the maximum ±0.2%, while the Cpols withdepth came within the maximum ± 0.4% and the CIRs within the maximum ±0.6% in every electron beam used. This studyshows that PDI (percentage depth ionization) could be a good approximation to PDD for the chamber used.

Mitochondrial DNA (mtDNA) deletion is a well-known marker for oxidative stress and aging and also contributes to theirunfavorable effects in cultured cells and animal tissues. This study was conducted to investigate the effect of ionizingradiation (IR) on mtDNA deletion and the involvement of reactive oxygen species (ROS) in this process in human lungfibroblast (IMR-90) cells. Young IMR-90 cells at population doubling (PD) 39 were irradiated with 137Cs -rays and theintracellular ROS level was determined by 2',7'-dichlorofluorescein diacetate (DCFH-DA) and mtDNA common deletion(4977bp) was detected by nested PCR. Old cells at PD 55 and H2O2-treated young cells were compared as the positivecontrol. IR increased the intracellular ROS level and mtDNA 4977 bp deletion in IMR-90 cells dose-dependently.Theincreases of ROS level and mtDNA deletion were also observed in old cells and H2O2-treated young cells. To confirmtheincreased ROS level is essential for mtDNA deletion in irradiated cells, the effects of N-acetylcysteine (NAC) on IR-induced ROS and mtDNA deletion were examined. 5 mM NAC significantly attenuated the IR-induced ROS increaseand mtDNA deletion. These results suggest that IR induces the mtDNA deletion and this process is mediated by ROS inIMR-90 cells.

In a pressurized heavy water reactor (PHWR), radiation workers who have access to radiation controlled areas submittheir urine samples to health physicists periodically; internal radiation exposure is evaluated by the monitoring of theseurine samples. Internal radiation exposure at PHWRs accounts for approximately 20 ~ 40% of total radiation exposure;most internal radiation exposure is attributed to tritium. Carbon-14 is not a dominant nuclide in the radiation exposure ofworkers, but it is one potential nuclide to be necessarily monitored. Carbon-14 is a low energy beta emitter and passesrelatively easily into the body of workers by inhalation because its dominant chemical form is radioactive carbon dioxide(14CO2). Most inhaled carbon-14 is rapidly exhaled from the worker’s body, but a small amount of carbon-14 remainsinside the body and is excreted by urine. In this study, a method for dual analysis of tritium and carbon-14 in urine samplesof workers at nuclear power plants is developed and a method for internal dose assessment using its excretion rate resultis established. As a result of the developed dual analysis of tritium and carbon-14 in urine samples of radiation workerswho entered the high radiation field area at a PHWR, it was found that internal exposure to carbon-14 is unlikely to occur.Inaddition, through the urine counting results of radiation workers who participated in the open process of steam generators,it was found that the likelihood of internal exposure to either tritium or carbon-14 is extremely low at pressurized waterreactors (PWRs).

Metal-semiconductor contact is very important for the operating property of semiconductor detector. Cd0.96Zn0.04Tesemiconductor crystal was grown with Bridgman method, and the crystal was cut and polished. EPMA (Electron ProbeMicro Analyzer) and ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis were done to obtain the chemicalcomposition and impurity of the crystal. Metal contact was deposited with thermal evaporator on both sides of the crystal.Detectors with Au/CZT/Au and In/CZT/Au structure were made, and I-V curve and the energy spectrum were measuredwith the detectors. It could be seen that the detector with the In/CZT/Au structure has superior property than the detectorwith Au/CZT/Au structure when the crystal resistivity was low. However, the metal contact structure effect becomes lowwhen the crystal resistivity was high.

To obtain precise information about characteristics of gamma ray detector system responses, we developed new GUIcomputer program to analize full-energy absorption peak using our developed Delphi computer code for educational purpose. By use ofthe well known 4 nonlinear peak shaping functions, peaks were fitted with least square fit method in this code. In this paper, wedescribed the methods to search for 12 coefficients in above 4 nonlinear peak shaping functions by use of our developed code in details.The computer code was tested for 1 µCi 137Cs 661 keV gamma ray peak spectrum detected by 25 % relative efficiency HPGe detectorwith 5.35 cm (D)5.5 cm (L) size.

An Area Radiation Monitoring System (ARMS) ionization chamber, which had an 11.8 L active volume, was fabricatedand performance-tested at KAERI. Low leakage currents, linearities at low and high dose rates were achieved fromperformance tests. The correlation coefficients between the ionization currents and the dose rates are 1 at high dose rateand 0.99 at low dose rate. In this study, an integration-type ARMS ionization chamber was tested over a year for anevaluation of its long-term stability at a radioisotope (RI) repository of the Young-gwang nuclear power plant. The standarddeviation of dose rate of 1 day data and over a 100-days mean value were 6.2 µR/h and 2.9 µR/h, respectively. Thefabricated ARMS ionization chamber showed stable performance from the results of the long-term tests. Design andperformance characteristics of the fabricated ionization chamber for the ARMS from performance-tests are also addressed.

The single crystal scintillator of bismuth germinate (Bi4Ge3O12:BGO) was successfully grown by the conventionalCzochraski technique. The characteristics of the grown BGO were evaluated and presented on the excitation, emissionresponses and energy spectra of the -rays from 241Am, 133Ba, 57Co, 22Na, 137Cs and 54Mn radio-isotopes. The energyresolution of grown BGO, ∆E/E, was estimated to be 12.1% at 662 keV of -ray for 137Cs nuclide. Compared to thecommercial BGO crystal, we confirmed that the grown BGO has a good performance and is comparable to reference one.

Tritium is the one of the dominant contributors to the internal radiation exposure of workers at pressurized heavy waterreactors (PHWRs). This nuclide is likely to release to work places as tritiated water vapor (HTO) from a nuclear reactor and getsrelatively easily into the body of workers by inhalation. Inhaled tritium usually reaches the equilibrium of concentration afterapproximately 2 hours inside the body and then is excreted from the body with a half-life of 10 days. Because tritium inside the bodytransports with body fluids, a whole body receives radiation exposure. Internal radiation exposure at PHWRs accounts forapproximately 20-40% of total radiation exposure; most internal radiation exposure is attributed to tritium. Thus, tritium is an importantnuclide to be necessarily monitored for the radiation management safety. In this paper, metabolism for tritium is established using itsexcretion rate results in urine samples of workers at PHWRs and an effective half-life, a key parameter to estimate the radiationexposure, was derived from these results. As a result, it was found that the effective half-life for workers at Korean nuclear power plantsis shorter than that of International Commission on Radiological Protection guides, a half-life of 10 days.