Research
(Main themes which are based on my dissertations)
(Main themes which are based on my dissertations)
[Updating]
In 2013, new low-background gamma spectroscopy was established at the Nuclear Technique Laboratory, University of Science Ho Chi Minh City. This gamma spectroscopy uses a coaxial n-type HPGe detector called GMX35P4-70 which is manufactured by Ortec. The very thin beryllium window (0.5 mm in thickness) enables this detector to extend the lower range of useful energies to around 3 keV.
It motivates us to investigate the efficiency of the GMX35P4-70 detector and set up a procedure to analyze radioactivity of environmental samples via the correction self-absorption effect. The method used in this research is Monte Carlo simulation by using MCNP5.
In this thesis, an investigation into the efficiency of the GMX35P4-70 detector is carried out with standard point sources as well as volume sources. Firstly, the gamma spectroscopy including GMX35P4-70, shielding, and a point source is modeled by MCNP5. All parameters for the MCNP5 input file are based on the manufacturer’s information. In order to check the validation of simulated modeling, some main properties of the detector such as energy resolution (FWHM), peak to Compton ratio (P/C), and relative efficiency at 1332.5 keV of 60Co placed at 25 cm from detector window are calculated from simulated results and then compared with values from the manufacturer.
The simulated FWHM (1.85 keV) is adequate. However, the simulated P/C and relative efficiency are significantly different from the values of the manufacturer. Particularly, the discrepancy between simulated relative efficiency (42 %) and manufacturer’s relative efficiency (38 %) is 10.52 %; the discrepancy between simulated P/C (74:1) and manufacturer’s P/C (61:1) is 21.31 %.
In addition, full-energy peak efficiencies at gamma rays (81.0 keV, 88.0 keV, 122.1 keV, 136.5 keV, 276.4 keV, 302.9 keV, 356.0 keV, 661.7 keV, 834.8 keV, 1115.5 keV, 1173.2 keV, 1274.5 keV and 1332.5 keV) of standard point sources 57Co, 60Co, 22Na, 109Cd, 133Ba, 65Zn, 54Mn, and 137Cs placed at 25.55 cm from detector window are investigated by experiment and simulation. The discrepancy between experimental and simulated efficiency is up to 17.30 %.
The difference between simulated and experimental values proposes a problem that the nominal values of detector parameters supplied by the manufacturer are different from the real parameters. Two revised modelings are proposed to obtain an adequate agreement between experimental and simulated efficiency.
In revised modeling 1, some geometrical parameters such as length and radius of germanium crystal, crystal edge rounding radius, and crystal-window distance are changed from initial values to optimized values by using MCNP5. In detail, crystal length is decreased 0.84 mm, crystal radius is decreased 0.8 mm, crystal edge rounding radius is decreased 1.0 mm and crystal-window distance is increased 0.5 mm. After calculating from simulated results, the simulated FWHM is 1.85 keV; simulated relative efficiency is 40 % and the discrepancy between simulated relative efficiency and manufacturer’s relative efficiency is 5.26 %; simulated P/C is 72:1 and discrepancy between simulated P/C and manufacturer’s P/C is 18.03 %. The discrepancy between experimental and simulated full-energy peak efficiency of standard point sources placed at 25.55 cm from the detector window is up to 11.81 %. These results indicate that revised modeling 1 is not suited to the real spectroscopy. In revised modeling 2, only the thickness of the dead layer (lithium layer covering the crystal hole) is changed from 0.7 mm to 2.0 mm by using MCNP5, while the rest of the geometrical parameters are unchanged. After calculating from simulated results, the simulated FWHM is 1.84 keV; simulated relative efficiency is 39 % and the discrepancy between simulated relative efficiency and manufacturer’s relative efficiency is 2.63 %; simulated P/C is 68:1 and discrepancy between simulated P/C and manufacturer’s P/C is 11.48 %. The discrepancies between experimental and simulated full-energy peak efficiency of standards point sources placed at 25.55 cm from the detector window are under 10 % at all observed gamma rays. According to these results, revised modeling 2 is more suited to the real spectroscopy and is used in the next study of this thesis.
Full-energy peak efficiencies at gamma rays (59.5 keV, 63.3 keV, 88.0 keV, 122.1 keV, 295.2 keV, 351.9 keV, 661.7 keV, 834.8 keV, 1173.2 keV and 1332.5 keV) of cylindrical samples which have different matrices (dirt, dry dirt, grass powder and milk powder) with observed density (0.5, 0.8, 1.0, 1.2 and 2.0 g/cm3) are investigated by using MCNP5. In the case of samples having the same matrix, full-energy peak efficiency decreases significantly when the sample’s density increases gradually from 0.5 g/cm3 to 2.0 g/cm3. In the case of samples having the same density, the effect of the matrix on full-energy peak efficiency is clear at low energy regions (less than 100 keV) and can be negligible at higher energy regions. As the density of samples increases, the full-energy peak efficiency discrepancy at low energy regions between samples having different matrices is more and more important. Particularly, at sample’s density 2.0 g/cm3, the full-energy peak efficiency discrepancy between dry dirt sample and grass powder sample is up to 31.39 % at 59.5 keV and the full-energy peak efficiency discrepancy between dry dirt sample and milk powder sample is up to 31.50 % at 59.5 keV.
Additionally, in order to study the self-absorption effect correction for analyzing radioactivity of environmental samples, a coefficient of self-absorption effect correction f is calculated from MCNP5 simulated results. At defined energy of initial gamma-ray, coefficient f is the ratio of full-energy peak efficiency of observed sample to full-energy peak efficiency of standard sample. This coefficient depends on the energy of the initial gamma-ray and the density of samples. In this research, a coefficient of self-absorption effect correction f of dry dirt sample is evaluated based on full-energy peak efficiencies of water samples (standard sample with density = 1.0 g/cm3) and full-energy peak efficiencies of dry dirt samples (observed sample) with different sample’s densities (0.5 – 2.0 g/cm3). Afterward, because the main chemical compositions of dry dirt are suited to these of soil standard RGU1-IAEA, coefficient f of dry dirt is applied to evaluate the full-energy peak efficiencies at observed gamma rays and then the activity concentration of 238U of RGU1-IAEA sample via 63.3 keV regarding self-absorption effect correction.
In conclusion, initial modeling and two revised modelings of gamma spectroscopy are established by MCNP5. Full-energy peak efficiencies at observed gamma rays of point sources are investigated and full-energy peak efficiencies at observed gamma rays of cylindrical samples with different matrices at different densities (0.5 – 2.0 g/cm3) are evaluated as well. Activity concentration of 238U of RGU1-IAEA via 63.3 keV is determined regarding the correction of the self-absorption effect. At 95 % confidence interval, the activity concentration of 238U of RGU1-IAEA from computation is 4835 – 4899 Bq/kg and from IAEA is 4910 – 4970 Bq/kg.
May 31st has been chosen to be “The day without cigarettes” and we still celebrate this event each year in order to inform and warn the public about the harm of smoking cigarettes. Although there are many banners preventing people from smoking and also many dangerous warnings on cigarette packages such as “Smoking is harmful to your health” or “Smoking increases the risk of causing lung cancer”, smokers still cannot quit their bad habit. According to surveys and researches, about 47.4% of males and 1.4% of females aged 15 and above in Vietnam smoke cigarettes. Moreover, Vietnamese can purchase cigarettes easily because of the very low tax and price and the government does not have any strict methods to limit the consumption of cigarettes as well. This leads to the fact that Vietnam is one of the countries consuming the most cigarettes in the world and there are about 40000 deaths caused by diseases that relate to smoking cigarettes each year (this amount is four times higher than deaths caused by traffic accidents).
Cigarette smoke is not only harmful to smokers but also to others. Non-smokers are exposed to second-hand smoke which is comprised of smoke released from the burning tip of a cigarette between puffs and the smoke exhaled by the smokers. Cigarette smoke contains over 3500 different chemicals, and about 60 of them have been classified as carcinogenic to human beings such as nicotine, tar, carbon monoxide… Besides, there are also many toxic components in cigarette smoke that can surprise us; for example, ammonia is used in household cleaner, arsenic is used in rat poison, benzene is used for making dye, formaldehyde is used for preserving things, and so on. However, not many people know and believe that there are also very dangerous components in cigarette smoke, and they are radioisotope polonium-210 (210Po) which emits alpha particles and its precursor lead-210 (210Pb). In general, there are many radioisotopes existing in cigarettes due to environmental radiation, but 210Po is considerable because it is a volatile substance at the temperature of a burning cigarette (about 500 – 700oC).
Besides, the presence of 210Pb in tobacco is also of interest because it is a long-lived precursor (22.3 years) that supports the radioisotope 210Po. 210Pb is not sublimated at the temperature of a burning cigarette but is a component of the resulting smoke and ash. Therefore, 210Po and 210Pb easily go into the human body via inhaling cigarette smoke. Additionally, alpha particles are heavy charged particles, they can get stuck in the bronchial tubes and lungs when people inhale cigarette smoke. Due to their ionization ability, our lungs seriously suffer from internal exposure. Therefore, 210Po and 210Pb take a rather essential role to increase the risk of causing lung cancer and bronchial disorders, and many serious diseases in humans. As a result, it is important to know the content of the dangerous radioisotopes 210Po and 210Pb in cigarettes, and their adverse impact on human health.
Eighteen random cigarette brands produced in Vietnam were chosen for analysis. The purpose of the sample preparation procedure is to render 210Po into a suitable form for analysis. The experiments were conducted by following a simple radiochemical separation and spontaneous deposition of 210Po on a copper disc. Two grams of tobacco taken from cigarettes were put into a Teflon container. This sample was dissolved using a mixture of 20ml concentrated HNO3 and 5ml concentrated HF. After 30 minutes, the digestion was performed on a hot plate at about 80oC in temperature. To completely digest organic materials and remove the trace of HF, the sample residue was treated carefully with 8ml concentrated HClO4, then it was evaporated to near dryness. Then, 5 mL concentrated HCl was added to the residue and the sample was evaporated to near dryness at the temperature below 100oC. The final residue was dissolved in 100ml distilled water and 100ml HCl 0.5M. In order to eliminate the interference of Fe3+, 0.5g ascorbic acid powder was also added to the solution. Some portions of NH4OH 25% were used to adjust the pH of the sample solution to 1. Then, the copper disc was fixed in the Teflon disc holder after polishing the surface with acetone. The glass beaker containing the sample solution was placed in a water bath heated by the magnetic stirrer, the Teflon disc holder was mounted in the solution and 210Po was spontaneously plated on the copper disc at 80oC for 2 hours. Afterward, the copper disc was removed from the solution, rinsed with distilled water, and dried at room temperature. The sample disc was mounted in the 0.01torr vacuum chamber and measured for 24 hours using a passivated implanted planar silicon (PIPS) detector. The data were analyzed by the Genie 2000 Alpha Acquisition & Analyst Software (AAS).
It is well known that the determination of 210Pb through the ingrowth of 210Po is much more sensitive and accurate. After the deposition of 210Po, the sample solutions were stored for about 6 months to wait for the growth of 210Po from 210Pb. The second deposition of 210Po on copper disc, then, was carried out following the same steps at the same experimental conditions as the first one. The data of 210Po measured for the second time was used to evaluate activity concentrations of 210Pb.
The deposition efficiency of 210Po on a copper disc was estimated to be 0.94. The evaluation of the ratio between activity concentrations of 210Po and 210Pb in cigarettes was approximately 0.87. The activity concentrations of 210Po and 210Pb in cigarettes ranged from 13.8 to 82.6mBq/cigarette (mean value is 26.4mBq/cigarette), and from 13.9 to 78.8mBq/cigarette (mean value is 25.8mBq/cigarette), respectively. As a result, cigarette smoking increases the internal intake of 210Po and 210Pb with high concentrations. Besides, the average annual committed effective dose via inhaling 210Po and 210Pb in cigarette smoke that smokers have suffered is 295.4µSv/year assuming that an individual smokes 20 cigarettes per day during one year (223.0µSv/year and 72.4µSv/year from 210Po and 210Pb, respectively). This figure indicates that the risk of developing lung cancer, respiratory diseases, and other serious illnesses is approximately 60 times higher for smokers than for non-smokers.