J.L. Kim

Recent developments of optically stimulated luminescence materials and techniques for radiation dosimetry and clinical applications

During the last 10 years, optically stimulated luminescence (OSL) has emerged as a formidable competitor not only to thermoluminescence dosimetry (TLD) but also to several other dosimetry systems. Though a large number of materials have been synthesized and studied for OSL, Al2O3:C continues to dominate the dosimetric applications. Re-investigations of OSL in BeOindicate that this material might provide an alternative to Al2O3:C. Study of OSL of electronic components of mobile phones and ID cards appears to have opened up a feasibility of dosimetry and dose reconstruction using the electronic components of gadgets of everyday use in the events of unforeseen situations of radiological accidents, including the event of a dirty bomb by terrorist groups. Among the newly reported materials, a very recent development of NaMgF3:Eu2+ appears fascinating because of its high OSL sensitivity and tolerable tissue equivalence. In clinical dosimetry, an OSL as a passive dosimeter could do all that TLD can do, much faster with a better or at least the same efficiency; and in addition, it provides a possibility of repeated readout unlike TLD, in which all the dose information is lost in a single readout. Of late, OSL has also emerged as a practical real-time dosimeter for in vivo measurements in radiation therapy (for both external beams and brachytherapy) and in various diagnostic radiological examinations including mammography and CT dosimetry. For in vivo measurements, a probe of Al2O3:C of size of a fraction of a millimeter provides the information on both the dose rate and the total dose from the readout of radioluminescence and OSL signals respectively, from the same probe. The availability of OSL dosimeters in various sizes and shapes and their performance characteristics as compared to established dosimeters such as plastic scintillation dosimeters, diode detectors, MOSFET detectors, radiochromic films, etc., shows that OSL may soon become the first choice for point dose measurements in clinical applications. A brief review of the recent developments is presented.During the last 10 years, optically stimulated luminescence (OSL) has emerged as a formidable competitor not only to thermoluminescence dosimetry (TLD) but also to several other dosimetry systems. Though a large number of materials have been synthesized and studied for OSL, Al 2 O 3 :C continues to dominate the dosimetric applications. Re-investigations of OSL in BeOindicate that this material might provide an alternative to Al 2 O 3 :C. Study of OSL of electronic components of mobile phones and ID cards appears to have opened up a feasibility of dosimetry and dose reconstruction using the electronic components of gadgets of everyday use in the events of unforeseen situations of radiological accidents, including the event of a dirty bomb by terrorist groups. Among the newly reported materials, a very recent development of NaMgF 3 :Eu 2+ appears fascinating because of its high OSL sensitivity and tolerable tissue equivalence. In clinical dosimetry, an OSL as a passive dosimeter could do all that TLD can do, much faster with a better or at least the same efficiency; and in addition, it provides a possibility of repeated readout unlike TLD, in which all the dose information is lost in a single readout. Of late, OSL has also emerged as a practical real-time dosimeter for in vivo measurements in radiation therapy (for both external beams and brachytherapy) and in various diagnostic radiological examinations including mammography and CT dosimetry. For in vivo measurements, a probe of Al 2 O 3 :C of size of a fraction of a millimeter provides the information on both the dose rate and the total dose from the readout of radioluminescence and OSL signals respectively, from the same probe. The availability of OSL dosimeters in various sizes and shapes and their performance characteristics as compared to established dosimeters such as plastic scintillation dosimeters, diode detectors, MOSFET detectors, radiochromic films, etc., shows that OSL may soon become the first choice for point dose measurements in clinical applications. A brief review of the recent developments is presented.

Dosimetric characteristics of LiF:Mg,Cu,Si thermoluminescent materials

Dosimetric characteristics of LiF:Mg,Cu,Si thermoluminescent (TL) material developed at KAERI have been investigated and compared with those of commercially available LiF:Mg,Cu,P (GR-200A). LiF:Mg,Cu,Si thermoluminescence dosimeter (TLD) can be heated up to 573K without any loss of TL sensitivity or any change in the glow curve structure. High-temperature glow peak in LiF:Mg,Cu,Si is significantly lower than that in GR-200A, consequently the residual signal is only 0.025%, which is about 35 times less than that of GR-200A. The TL sensitivity of the LiF:Mg,Cu,Si TLD is about 55 and 1.1 times higher than those of the LiF:Mg,Ti (TLD-100) and GR-200A, respectively.

Further studies on higher temperature TL glow peaks of 7LiF:Mg,Ti

Thermoluminescence (TL) characteristics of the higher temperature peaks of LiF:Mg,Ti, which exhibit a unique property of high relative response to high LET radiation, were studied in view of some recent findings and discussions. By making separate readouts of dosimetric peaks (mainly peak 5) and higher temperature peaks (mainly peak 7), the precision in TL measurements, reusability and, fading were found comparable for the TL readouts in the two regions. However, the intensity of higher temperature peaks was found to be susceptible to the annealing treatments. In the standard annealing treatment of LiF:Mg,Ti, namely, 400 degrees C for 1 h followed by 100 degrees C for 2 h, the intensity of the higher temperature glow peaks was significantly affected when the second step of lower temperature annealing treatment at 100 degrees C for 2 h was not used. The dose response function f(D) of higher temperature peaks to gamma rays in the range from 30 to 150 mGy was found to be within about 10% (-4 and +9%) but above 200 mGy it increased sharply. The intricacies of dosimetry of mixed fields of low and high LET radiation are discussed.

Dosimetric properties of sintered LiF:Mg,Cu,Na,Si TL detectors

Abstract Selected dosimetric properties of sintered LiF : Mg , Cu , Na , Si detectors were studied. Solid LiF : Mg , Cu , Na , Si pellets of diameters 4.5 mm and thickness 0.9 mm , dark blue in colour, were obtained by cold pressing and sintering the powder at 820°C. The dosimetric properties of the newly developed detectors were studied and compared with the properties of LiF : Mg , Cu , P pellets (MCP-N). The annealing conditions were the same as those used for MCP-N detectors. X-ray exposures were performed at the KAERI,Taejon,Korea, while other irradiation and readout were carried out at the INP in Krakow, Poland. The glow-curve structure of LiF : Mg , Cu , Na , Si pellets is found to be comparable to that of MCP-N ( LiF : Mg , Cu , P ) detectors but the absolute sensitivity is about 50% lower. The photon energy response after doses of X-rays of energy about 100 keV shows a decrease, similar to that in LiF : Mg , Cu , P . For lower energies the response is higher than that for LiF : Mg , Cu , P due to the presence of high- Z elements (Na,Cu, and Si). The relative TL efficiency after doses of alpha particles from an 214 Am source of the sintered LiF : Mg , Cu , Na , Si detectors is similar to that of MCP-N ( LiF : Mg , Cu , P ) .

A personal thermoluminescence dosimeter using LiF:Mg,Cu,Na,Si detectors for photon fields.

A new personal thermoluminescence (TL) dosimeter for photon fields using LiF:Mg,Cu,Na,Si TL detector was developed by taking advantage of its dosimetric properties including energy dependencies. Solid pellet type LiF:Mg,Cu,Na,Si detector was developed and fabricated at Korea Atomic Energy Research Institute (KAERI) and has been studied on its dosimetric properties such as TL grow curve, dose response, energy response and reusability. Its dosimetric properties show the feasibility of application of LiF:Mg,Cu,Na,Si TL detector to personal dosimetry fields. A new dosimeter using LiF:Mg,Cu,Na,Si TL detector was designed and tested through irradiation experiments. This multi-element TL dosimeter allows the measurement of a personal dose equivalent Hp(d) in photon fields. Based on the experimental results of the proposed dosimeter, it was demonstrated that a personal TL dosimeter using sintered LiF:Mg,Cu,Na,Si TL detector is appropriate to estimate personal dose equivalent for wide range energy of photon fields.

Response of six neutron survey meters in mixed fields of fast and thermal neutrons

Calibration neutron fields have been developed at KAERI (Korea Atomic Energy Research Institute) to study the responses of commonly used neutron survey meters in the presence of fast neutrons of energy around 10 MeV. The neutron fields were produced by using neutrons from the (241)Am-Be sources held in a graphite pile and a DT neutron generator. The spectral details and the ambient dose equivalent rates of the calibration fields were established, and the responses of six neutron survey meters were evaluated. Four single-moderator-based survey meters exhibited an under-responses ranging from ∼9 to 55 %. DINEUTRUN, commonly used in fields around nuclear reactors, exhibited an over-response by a factor of three in the thermal neutron field and an under-response of ∼85 % in the mixed fields. REM-500 (tissue-equivalent proportional counter) exhibited a response close to 1.0 in the fast neutron fields and an under-response of ∼50 % in the thermal neutron field.

EVALUATION OF NEUTRON SCATTERING CORRECTION USING THE SEMI-EMPIRICAL METHOD AND THE SHADOW-CONE METHOD FOR THE NEUTRON FIELD OF THE KOREA ATOMIC ENERGY RESEARCH INSTITUTE

When neutron survey metres are calibrated in neutron fields, the results for room- and air-scattered neutrons vary according to the distance from the source and the size, shape and construction of the neutron calibration room. ISO 8529-2 recommends four approaches for correcting these effects: the shadow-cone method, semi-empirical method, generalised fit method and reduced-fitting method. In this study, neutron scattering effects are evaluated and compared using the shadow-cone and semi-empirical methods for the neutron field of the Korea Atomic Energy Research Institute (KAERI). The neutron field is constructed using a 252Cf neutron source positioned in the centre of the neutron calibration room. To compare the neutron scattering effects using the two correction methods, measurements and simulations are performed using respectively KAERIs Bonner sphere spectrometer (BBS) and Monte Carlo N-Particle code at twenty different positions. Neutron spectra are measured by a europium-activated lithium iodide [6LiI(Eu)] scintillator in combination with the BBS. The calibration factors obtained using each methods show good agreement within 1.1%.

Development of KCT-300 TL Pellet with High Sensitivity and High Mechanical Strength

KCT-300 (KAERI CaS04:Dy TL detector with a P-compound of 7.8 wt%) developed at KAERI, appeared to have a higher radiation sensitivity than a commercialized Teflon embedded CaS04:Dy TL pellet. However, it was mechanically weak and fragile. Therefore, we try to improve its mechanical strength by increasing the P-compound concentration. It was difficult because of surface ruggedness problem in the TL pellet after sintering process. We could finally succeed in improving the mechanical strength of KCT-300 with high P-compound concentration by adding a thermal treatment process in the TL pellet fabrication process. This paper presents the method and result to improve the mechanical strength of KCT, which can be effectively used in the personal and environmental radiation monitoring.

Development of a TL Personal Dosimeter Identifiable PA Exposure, a Comparison with Commercial TL Dosimeters

A single-dosimeter worn on the anterior surface of the body of a worker was found to significantly underestimate the effective dose to the worker when the radiation comes from the back. Several researchers suggested that this sort of underestimation can be corrected to a certain extent by using an extra dosimeter on the back. However, use of multiple dosimeters also has disadvantages such as complication in control or incurrence of extra cost. Instead of the common multi-dosimeter approach, in this study, a single dosimeter introducing asymmetric filters which enabled to identify PA exposure was designed, and its dose evaluation algorithm for AP-PA mixed radiation fields was established. A prototype TL personal dosimeter was designed and constructed. The Monte Carlo simulations were utilized in the design process and verified by experiments. The dosimeter and algorithm were applicable to photon radiation having an effective energy beyond 100 keV in AP-PA mixed radiation fields. A simplified performance test based on ANSI N13.ll showed satisfactory results. Considering that the requirements of the International Electrotechnical Commission(IEC) and the American National Standards Institute(ANSI) with regard to the dosimeter on angular dependency is reinforced, the dosimeter and the dose evaluation algorithm developed in this study provides a useful approach in practical personal dosimetry against inhomogeneous high energy radiation fields.

Calibration of Spherical Neutron Measuring Devices with a 252Cf Source in the Neutron Calibration Room of the KAERI

ISO recommends that the neutron scattering correction procedures, such as the shadow-cone method, the polynomial fit method and the semi-empirical method, be used for a calibration of neutron measuring devices. This study compared three methods using the Bonner Multisphere Spectrometer (BMS) and neutron survey meters based on a 252Cf source in the neutron calibration room of the Korea Atomic Energy Research Institute (KAERI). As for the results, the average correction factors and calibration factors of the shadow-cone method compared with the semi-empirical method using the BMS and neutron survey meters. The results were consistent to within ± 0.7 % and ± 2.6 % respectively. These averaged values for the three methods by the BMS and the neutron survey meters were then varied within ± 5 %, respectively. It is then noted that the results based on the BMS and the neutron survey meters agreed well with each other. From the results, it is suggested that the semi-empirical method be useful for the neutron calibration room of the KAERI.