Synthesis and Characterization of Phosphors for TL Dosimetry

H.S. Virk1 and S.K. Omanwar2

1Department of Nanotechnology, SGGS World University, Fatehgarh Sahib (Pb.)-140406

2Department of Physics, SGB Amravati University, Amravati (MS) 444602

TL Dosimetry

• TL dosimetry is used in many scientific and applied fields such as radiation protection, radiotherapy, industry, and environmental and space research, using many different materials known as phosphors. The basic demands of a thermoluminescent dosimeter (TLD) are good reproducibility, low hygroscopicity, and high sensitivity for very low dose measurements and good response at high doses in radiotherapy and in mixed radiation fields.

TLD Phosphors

• A phosphor is a substance that exhibits the phenomenon of luminescence. Phosphors are often transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights. Phosphors are usually made from a suitable host material with an added activator. The host materials are typically oxides, nitrides and oxy-nitrides, sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminium, silicon, or various rare earth metals. The activators prolong the emission time (afterglow).

CaSO4: Dy Phosphor

• CaSO4 is one of the most sensitive TLD products used in dosimetry. CaSO4: Dy and CaSO4: Tm are the most interesting phosphors in CaSO4 series, since their response is considerably stable. The CaSO4: Dy phosphor has been used as a dosimeter in radiation dosimetry particularly for low-dose measurements. Considerable work has been done on CaSO4 singly doped phosphors in the last few decades, and Dy or Tm doped CaSO4 thermoluminescent phosphors are highly attractive due to their desired dosimetric properties.

Precipitation Method used for the Synthesis of CaSO4: Dy

• Ingle et al. (Nagpur) have described the simplest method for preparing CaSO4: Dy phosphor by precipitation from aqueous solution of CaCl2 with dilute sulphuric acid. For incorporating Dy, appropriate amount of aqueous solution of DyCl3 was added prior to precipitation. Ratio of Ca2+:Dy3+ in solution was 1000:1. The precipitate was repeatedly washed by distilled water to remove the traces of acid, dried and then annealed at 975 K for 1 h followed by quenching to room temperature (RT). This sample will be designated as CaSO4: Dy–P.

Thermoluminescence glow curves of CaSO4:Dy phosphors

Typical PL of Dy3+ in CaSO4:Dy

TL glow curve of gamma irradiated CaSO4:Dy

Dose response of CaSO4: Dy as a function of gamma absorbed dose

Heating Rate Effect

• CaSO4:Dy phosphor obtained was annealed at different temperatures in the range of 773 to 1173 K for 1 h. The samples remained 24 h in the oven and then switched off in order to reach the room temperature. The glow curve of sensitized CaSO4: Dy samples by giving different heat treatments, in the temperature range (773–1173 K), and irradiated at the gamma dose (0.1 Gy), is recorded. It is observed that the intensity of TL glow curve increases with temperature.

TL intensity of CaSO4: Dy as a function of annealing temperature

LiF: MCP Phosphor

• LiF: MCP material is being considered to be a standard detector in TL dosimetry. LiF: MCP is highly sensitive thermoluminescence detector, as it is able to measure doses over a wide range (generally from µGy to kGy). LiF:Mg,Ti was one of the sensitive LiF based phosphor used in personnel dosimetry. It was made commercially available by Harshaw chemical company in 1967 (US Patent). Various laboratories developed LiF: MCP phosphor with different dopants.

LiF: MCP Synthesis

• 5ml of Hydrofluoric acid (HF) was taken in a Teflon beaker. Lithium carbonate was added smoothly to it to form LiF. The precipitate so formed has been dried under blowing air. To the dried precipitate, CuCl2 solution was sprinkled and dried again. This precipitate was then heated in a test tube at 500°C with NH4Cl to remove OH- ion traces in LiF. Reactive Atmosphere Process was used in which powder was then poured in graphite crucible containing MgF2 and NH4H2PO4. MgF2 is prepared freshly by adding the MgCO3 in the HF. Crucible was heated at 870°C to melt LiF powder. Finally melted LiF: MCP is rapidly quenched to room temperature.

TL glow curve of LiF: MCP compared with commercially available phosphors

Effect of Dopant Concentration

• The glow curve structure and TL intensity of LiF: MCP is mainly dependent on the impurities doped in the host. In order to obtain maximum TL output from LiF: MCP, concentration of Mg, Cu and P must be properly adjusted. Slight variation in this can alter the dosimetric properties of the phosphor. We varied the concentration of these dopants as follows: Mg from (0.15% to 0.3%), Cu from (20 ppm to 500ppm) and P from (0.6% to 2%). The concentrations mentioned here are the concentrations of MgF2, CuCl2 and NH4H2PO4 added to LiF powder.

Effect of Mg concentration on TL intensity of LiF:Mg,Cu,P Phosphor

Effect of P concentration on the TL Intensity of LiF:MCP Phosphor

TL Emission Spectra of LiF: MCP Phosphor prepared by RAP technique

Grain Size Effect on the TL of LiF: MCP

Effect of annealing temperature on the TL of LiF: MCP phosphor

Linearity of Dose Response of LiF: MCP

Photon Energy Response of LiF: MCP

The Heating rate Effect on LiF: MCP

Reproducibility of LiF: MCP samples with same Sensitivity

• In order to check the reproducibility of samples with same sensitivity, a batch of 10 samples each of 10g weight was prepared. Variation in the TL intensity of sample in the batch was found to be around + 5% to – 5%. The TL intensity of each of the sample is around 30 % of LiF: MCP TLD-100H obtained from Harshaw chemical company. No fading was observed in the LiF: MCP sample prepared by us over a period of 15 days.

Reproducibility of LiF: MCP

LiCaAlF6:Eu Phosphor

• The scintillation properties of LiCaAlF6:Eu scintillator and its capability for thermal neutron imaging application has been investigated. LiCaAlF6 rare-earth-ion-doped scintillators of neutron detectors with high efficiency have been developed for the neutron gamma discrimination by using the pulse shape discrimination in the LiCAF:Ce scintillator, which is frequently used in organic liquid scintillators, to selectively detect neutrons with the LiCAF: Ce scintillator under an intense high-energy gamma-ray field.

TL excitation and emission peaks of LiCaAlF6:Eu(0.1%)

Variation in TL intensity of LiCaAlF6: Eu with quenching temperature

TL emission spectra of the LiCaAlF6:Eu

Effect of Eu concentration on PL and TL of LiCaAlF6: Eu Phosphor

Dose Response and Linearity for LiCaAlF6: Eu and CaSO4: Dy

Comparison of TL glow curves of some Commericial Phosphors

Energy dependence of LiCaAlF6:Eu (0.1%) material

Effect of storage on the TL of LiCaAlF6: Eu (0.1%)

Conclusions

• In the last three decades, consecutive studies on CaSO4 and LiF standard materials gives a special attendance to their use in TL and TL related phenomena. Because of good chemical stability and applications, above said phosphors are prominently and consistently used in research as well as in applied sciences. The LiF: MCP and LiCaAlF6: Eu samples developed by our group show a comparable TL intensity with LiF:MCP TLD-100H obtained from Harshaw chemical company and other commercial phosphors.