Development of a TL detector for neutron measurement by CaSO4:Dy phosphors

Personal neutron dosimetry is quite a difficult area because a neutron is always accompanied with gamma radiation, which is required of a capability for mixed field dosimetry. CaSO4:Dy phosphor is known to have a very high sensitivity to gamma radiation, but the neutron capture cross section of the constituents of CaSO4:Dy are so small that the interactions between the thermal neutron and the phosphor are rare. One method to improve the neutron interaction is by introducing an impurity ion with a large thermal neutron captures cross section into the phosphor to act as a neutron target centre such as 6Li. In neutron-gamma mixed radiation fields, if two detectors for the 6Li-7Li compounds embedded CaSO4:Dy thermoluminescent (TL) pellets are used, a 6Li-compound embedded pellet can detect the neutron and gamma radiations together, and the other pellet can only detect the gamma radiation. Recently, the Korea Atomic Energy Research Institute (KAERI) has developed a new type of CaSO4:Dy TL materials embedded with phosphorous (KCT-300) to detect beta and gamma radiation with a very high sensitivity. This paper presents the development of CaSO4:Dy TL pellets embedded with 6Li compound for a thermal neutron measurement, and the detection method of the neutron and gamma dose in mixed fields with CaSO4:Dy TL pellets embedded with a 6Li compound (KCT-306) and CaSO4:Dy TL pellets embedded with a 7Li compound (KCT-307) is introduced. The net neutron sensitivity of CaSO4:Dy TL pellets embedded with 6Li compound developed in this study is about two times higher than that of the TLD-600 (Harshaw Chemical) dosemeter which is available commercially.     

Thermoluminescence characteristics of teflon embedded CaSO4:Dy TLD

The Korea Atomic Energy Research Institute (KAERI) is manufacturing CaSO4:Dy Teflon TL pellets which have more sensitivity and stability than commercial TLD. A method is presented of preparing the CaSO4:Dy phosphor-embedded Teflon powder, which is then compressed to a thin pellet form used as the TLD element. Investigations are made to determine optimum preparation conditions and dosimetric characteristics of the CaSO4:Dy Teflon pellet such as the sensitivity, energy response, dose response, fading, re-usability, and lowest level of detection. The results show that the sensitivity of the CaSO4:Dy pellet is 2 times higher than that of the commercial Teledyne CaSO4:Dy pellet. A dose-response was observed to be linear in the range from 10(-5) to 10 Gy. The relative energy response in the low energy region was 9.6 (normalised to the 137Cs gamma source), and the fading rate was about 10% for five months. The re-usability was estimated to be more than 60 cycles, and the low level of detection dose was 22 microGy. From the results, the CaSO4:Dy pellet developed in KAERI can be successfully used in personal dosemeters through appropriate filter design for compensating the energy response.     

Batch-to-batch variation in the TL glow peaks and sensitivity in the production of CaSO4:Dy TLD phosphor

The thermoluminescence (TL) glow peak height ratio of the main dosimetric peak (at about 240 degrees C) and the lower temperature satellite peak (at about 140 degrees C) of CaSO4:Dy was found to vary between 1.4 and 11.5 in more than 100 batches of CaSO4:Dy TLD tested in the last 5 years. Efforts were made to minimise the batch-to-batch variation. In most of the batches (90% of cases), the peak height ratio was more than 5 and the variation in the TL sensitivity (with respect to the reference batch) rarely exceeded 10%. The study of the grain size dependence of the peak height ratio of ground and unground and with and without acid wash of phosphor grains indicated that the surface effects contribute significantly to the batch-to-batch variation. Crystallisation of phosphor grains was found to be affected considerably by the interruptions in the process of evaporation of acid during the preparation. Phosphor grain size was found to be an important parameter for maintaining the quality of production of CaSO4:Dy TLD phosphor.     

Response of CaSO4:Dy based TL dosemeter system to high-energy photon beams: theoretical simulation

Response of thermoluminescence (TL) discs under different filter regions of a CaSO(4):Dy based TL dosemeter system was simulated to high-energy photon beams in the energy range of 1.25 MeV to 24 MV ( approximately 9 MeV). This was done using FLUKA Monte Carlo code and also experimentally verified for some energy points. Response of disc D1 under metal filter combination was found to increase with photon energy, whereas that for the discs under polystyrene filter and open window regions of the dosemeter decreases continuously. The changes in the response of the discs under polystyrene filter and open window were attributed to the lack of build-up material. The increase in the response of disc D1 was due to the contribution from secondary electrons produced through Compton and pair production processes mainly arising out from the metal filter combination. The knowledge of the change in the response of individual discs and the ratio of discs' responses under different filter regions of the dosemeter system could be used for the measurement of energy of bremsstrahlung radiation that exists in and around high-energy electron accelerator and could be used for accurate evaluation of personal dose equivalent in high-energy photon field.     

Estimation of bremsstrahlung photon energy in the environment of high-energy electron accelerator using CaSO4:Dy based TL dosemeter

High-energy bremsstrahlung X rays constitute the major radiation hazard to working personnel around the high-energy electron accelerators. Thermoluminescent (TL) dosemeter system based on CaSO4:Dy Teflon disc used in the routine individual monitoring was used to estimate the bremsstruhlung photon energy at different locations of the experimental hall of 450-MeV synchrotron accelerator. The response of TL discs under different filter regions of the dosemeter system undergo change with photon energy due to the lack of build up and interaction of photon in the metal filters. This change in the response of the discs used to estimate the energy of the bremsstrauhlung photon in conjunction with suitable calibration curve generated using known photon energy from medical linear accelerator. The photon energies estimated were in the range 1-4 MeV, depending on the locations.     

Mechano and thermoluminescence of gamma-irradiated CaSO4:Dy phosphor

In order to have a better idea of the interaction of the defect centres produced by y-irradiation with dislocation in the processes of deformation destruction, mechanoluminescence (ML) and thermoluminescence (TL) of gamma-irradiated CaSO4:Dy, these phosphors have been investigated. CaSO4:Dy phosphors were prepared by dissolving CaSO4.2H2O in sulphuric acid and evaporating the excess acid around 300 degrees C. ML was excited impulsively by dropping a load on to the sample. Two peaks have been observed in the ML intensity against time curve. The total light output, i.e. integrated ML intensity, increases with concentration of dopant, strain rate and with irradiation doses. The TL glow curves of CaSO4:Dy phosphors at different concentrations of dopant and irradiation doses were also recorded. Studies of the influence of post-irradiation annealing on the ML of CaSO4:Dy show that with the removal of the TL dosemetric peak (approximately 210 degrees C) the ML intensity decreases markedly. A spectroscopic study of ML and TL has also been carried out to elucidate the mechanism of ML. Correlation between ML and TL has also been found.     

Studies of UV induced phototransferred thermoluminescence in CaSO4:Dy pellets

Single event spectra of a clinical carbon beam have been measured by an ultra-miniature tissue-equivalent proportional counter (UMC). In order to cover the energy range of the Bragg peak, the incident energy of the carbon beam was degraded by aluminium plates. Single event spectra for carbon-events incident to the UMC were analysed and selected at several carbon energies using thin scintillation counters. It was found that the dose weighted lineal energy distributions have a doublet peak structure due to incident carbon beam and fragment contributions.     

Defect centres and thermoluminescence in CaSO4:Dy,Ag phosphor

The defect centres formed in the TL phosphor CaSO4:Dy,Ag are studied using the technique of Electron Spin Resonance. The Ag co-doped phosphor exhibits three glow peaks around 130, 220 and 375 degrees C in contrast with the two glow peaks observed in the CaSO4:Dy phosphor at 130 and 220 degrees C at a gamma ray dose of 1Gy. ESR studies show that the additional peak at 375 degrees C correlates with a Ag2+ centre formed due to gamma irradiation and observable only below -170 degrees C. The Ag2+ centre is characterised by an axial g-tensor with principal values g(parallel) = 2.38 and g(perpendicular) = 2.41. ESR studies further indicate that the precursor to a centre observable at low temperature (-170 degrees C) appears to act as the recombination centre for the TL peak at 375 degrees C; this radical is characterised by the g-values g(parallel) = 2.0023 and g(perpendicular) = 2.0038 and is assigned to SO3- radical. It is observed that there is more incorporation of Ag in the CaSO4:Dy system as compared with that in pure CaSO4 system.     

Evaluation of the dosimetric response for CaSO4:Dy at low temperature

Homemade solid state CaSO4:Dy detectors were tested to evaluate their response to gamma radiation at liquid nitrogen temperature (77 K). The dosemeters were irradiated with doses between 12 and 1071 Gy. For this study these dosemeters were exposed to gamma rays with a dose rate of 1.19 Gy.min(-1). The analysis for these crystals was made by thermoluminiscence. The dose response at liquid nitrogen temperature was linear in the dose range studied and it is about 20% lower with respect to the response at room temperature. The response is reproducible with the same geometric set-up.