Effect of Annealing Temperature on Effect of Annealing Temperature on Doped CaSO₄

Effect of Annealing Temperature on Effect of Annealing Temperature on Doped CaSO₄

Resmi Nair1*, Dr. K. Madhukumar1 , C. M. K Nair1 and Jayasudha. S1

Dept. of Physics Mahatma Gandhi College

Thiruvananthapuram-695004. India

1. S. Elias2 2Regional Cancer Centre Thiruvananthapuram, India

   Abstract Nanoparticles of rare earth doped CaSO4 with size around 28 nm were prepared for high temperature radiation dosimetry applications, via Wet Precipitation Method. Samples were characterised by XRD, UV-Vis and TL techniques. Phosphors annealed at different temperatures were exposed to 6MeV and 15MeV energy photon beam and the TL responses were recorded. Samples annealed at 900o C for 2 hours gave maximum TL response. The peak emission temperature observed for the prepared phosphors was 356 o C which is very high compared with the standard phosphor CaSO4: Dy which is around 240oC. The XRD pattern showed orthorhombic structure.

   Keywords: Thermoluminescence, Dosimeter

   1. INTRODUCTION

Thermoluminescence has applications in Archaeology, Geology, Biology, Forensic Science, Space Science etc and its most striking application is in Radiation Dosimetry [1]. The development of thermoluminescent dosimeters using CaSO4 has a very long history due to its high sensitivity, simple structure, and good chemical and thermal stability. A large number of variants have been proposed from time to time. Rare earth doped CaSO4 are excellent thermoluminescent phosphors and CaSO4: Dy and CaSO4: Tm are important among them due to high sensitivity and very low fading when stored under standard environmental conditions. They are very useful in monitoring radiation levels from various sources [2, 3, 4]. Co doping in very small quantities plays an important role in the luminescence efficiency of the phosphor. It may enhance or subside, the thermoluminescence efficiency depending on the host dopant lattice matching. Attempts to improve the thermoluminescence characteristics of CaSO4: Dy and CaSO4: Tm, are sustained till date. The present work reports a new phosphor with a very high TL peak temperature. The phosphor is prepared by co-doping CaSO4: Dy with suitable co-dopant. [3, 4]. The phosphor can be used for high temperature TL applications.

II EXPERIMENTAL

Wet precipitation method was used for phosphor synthesis [8, 9]. All reactants were of analytical grade,

Merck. The wet precipitation of CaSO4 occurs through the chemical reaction.

Ca (NO3)2 + (NH4)2SO4 CaSO4

Dy2O3 and Sodium Meta Silicate (SMS) were used as dopants. Samples were prepared by varying dopant concentrations.

0.1M solutions of Ca (NO3)2 and (NH4)2SO4 were prepared. Dopant solutions were added and the solutions were mixed under constant stirring using a magnetic stirrer and kept undisturbed for 24 hours. The precipitate was collected, washed with double distilled water and dried. The sample was calcinated at 500°C for 3 hours. The resultant phosphor was finely powdered, annealed at temperatures 700°C, 800°C and 900°C for two hours. The samples so obtained were subjected to different studies.

1. RESULTS AND DISCUSSION

   1. X-Ray Diffraction

      The X-ray diffraction studies were carried out using a Cu-Ka target on Bruker AXS D8 Advance X ray Diffractometer. Fig 1 shows the X-ray Diffraction pattern of the phosphor. The pattern matches with JCPDS File number 37-1496 that of CaSO4. The phosphor is having orthorhombic structure with Bmmb (63) space group. The lattice parameters calculated were in good agreement with the standards reported.

      Lattice parameters reported	Lattice parameters calculated
      a = 6.993A°	a = 6.985 A°
      b= 7.001 A°	b = 6.987 A°
      c = 6.241A°	c = 6.234 A°

      CaSO4:.2Dy,.1SMS

      CaSO4:.2Dy,.1SMS

      5000

      4000

      intensity

      intensity

      3000

      2000

      1000

      0

      111

      020

      002

      012

      220

      202

      212

      131

      103

      222

      032

      400

      232

      331

      240

      402

      412

      024

      214

      224

      432

      234

      111

      020

      002

      012

      220

      202

      212

      131

      103

      222

      032

      400

      232

      331

      240

      402

      412

      024

      214

      224

      432

      234

      20 30 40 50 60 70 80 90

      2-thetao

      As annealing temperature increases, the temperature of emission shifts from 1870C to 3560C. The intensity of emission also increases. This implicates an increase in trap depth as well as effective trap filling. The phosphor annealed at 9000C gave maximum TL response. The very high emission temperature indicates the effectiveness of this phosphor to be used in high temperature radiation dosimetry applications. Fig: 3 shows the TL response for sample S2 annealed at different temperatures and exposed to 6MeV. As in the case of sample S1, the peak emission temperature is 3560C whereas the intensity of emission increases. This may be due to the increase in doping concentration of Dy.

      Fig 1: XRD pattern of the synthesized phosphor

      The particle size was also calculated using Scherers formula,

      d = 0.9/cos

      assuming the particles are stress free [5, 6, 9]. The average grain size of the phosphor is approximately 28 nm.

   2. Thermoluminescence Study

   1)TL Variation with Annealing Temperature

   The TL glow curve of the phosphors annealed at

   2600

   2400

   2200

   2000

   TL Intensity (a.u)

   TL Intensity (a.u)

   1800

   1600

   1400

   1200

   1000

   800

   600

   400

   200

   0

   o

   o

   900 C (2hr

   900 C (2hr

   2

   2

   o

   o

   o

   o

   s)

   S

   S

   800 C (2hrs)

   800 C (2hrs)

   700 C (2hrs)

   700 C (2hrs)

   0 50 100 150 200 250 300 350 400

   Temperature oC

   temperatures 700°C, 800°C and 900°C were recorded using a TL analyzer TL1007 NUCLEONIX at a heating rate of 10°C/sec after subjecting the phosphor to 6MeV and 15 MeV X-ray photon beam. Samples with two different dopant concentrations were prepared. CaSO4:0.2Dy, 0.1SMS (S1) and CaSO4:0.3Dy, 0.1SMS (S2). Figure 2

   shows the TL glow curve of sample S1 annealed at different temperatures and exposed to 6MeV photons.

   o

   Fig 3: TL response for phosphor S2 (6MeV)

   The TL peak temperature of both S1 and S2 is very high compared with the standard phosphor CaSO4: Dy used for dosimetric applications, which is around 2400C and intensity of emission is low. Low emission intensity may be due to the quenching effect of sodium ions present in the phosphor. Co- doping with SMS incerases the trap depth but gave less effective trap filling compared to standard. The main advantage of the emission peak at 3560 is its applications in high temperature radiation dosimetry [7, 8].

   2000

   TL Intensity (a.u)

   TL Intensity (a.u)

   1500

   1000

   500

   0

   1

   S

   S

   C

   C

   700 o (2hrs)

   o

   800 C (2hrs)

   900 C (2hrs)

   The energy dependence of a dosimeter decides its suitability in medical dosimetry. The phosphors S1 and S2 were subjected to photon radiation of 15 MeV energy and the TL responses were recorded. Fig: 4 show the TL response for sample S1 and S 2 at energy 15MeV. From the figure it is clear that the glow curv intensity decreases in comparison with the phosphor irradiated at energy 6MeV. The peak emission temperature also shifted to the low temperature side. Interaction of X-radiation at 6MeV energy with the phosphor is predominantly photoelectric than the radiation at energy 15 MeV. The probability for a photoelectric phenomenon to occur is inversely proportional to the cube of radiation energy [8].

   0 50 100 150 200 250 300 350 400

   Temperature oC

   Fig 2: TL glow curve of sample S1 (6MeV)

   1000

   TL Intensity (a.u)

   TL Intensity (a.u)

   800

   600

   400

   200

   0

   900 C

   900 C

   S

   S

   800 C

   800 C

   700 C

   700 C

   o

   o

   1

   1

   o

   o

   o

   o

   0 50 100 150 200 250 300 350

   Temperature oC

   1.8

   1.6

   1.4

   1.2

   (F(R) .h)2

   (F(R) .h)2

   1.0

   0.8

   0.6

   0.4

   0.2

   0.0

   -0.2

   S

   1

   Eg=4.61eV

   S

   1

   Eg=4.61eV

   0 1 2 3 4 5 6

   2

   2

   h(eV)

   1800

   1700

   1600

   1500

   1400

   TL Intensity (a.u)

   TL Intensity (a.u)

   1300

   1200

   1100

   1000

   900

   800

   700

   600

   500

   400

   300

   200

   100

   0

   o 900 C

   S

   2

   o 800 C

   o 700 C

   o 900 C

   S

   2

   o 800 C

   o 700 C

   0 50 100 150 200 250 300 350

   Temperature0 C

   Fig

   100

   80

   (F(R).h

   (F(R).h

   60

   40

   20

   0

   S

   S

   Eg=3.24eV

   Eg=3.24eV

   0 1 2 3 4 5 6

   h(eV)

   Fig:5 Kubelka-Munk plot for S1 and S2

   4: TL responses for S1 and S 2 (15 MeV energy)

   B) UV-Visible Analysis

   The sample obtained after heat treatment was subjected to UV-VISIBLE analysis using Varian, Cary 5000 Spectrophotometer over a spectral range of 237nm to 1963nm. The KubelkaMunk transformation of the measured reflectance can be given by the following equation;

   K= (1-R) 2/2R=F(R)

   where K is reflectence transformed according to Kubelka- Munk, R is the reflectancy (%) and F(R) is the Kubelka Munk function. The band gap Eg and the absorption coefficient are related as

   h = A(h-Eg)1/2

   If the compound scatters in perfectly diffuse manner, K becomes equal to 2. So by using the equation

   (F(R) h) 2=A (h-Eg)

   band gap energy of the phosphor can be calculated by plotting (F(R) h) 2 versus h. X intercept of the linear region of the plot will give the band gap energy of the prepared phosphor [6, 10]. Figure 5 shows the Kubelka- Munk plot for samples S1 and S2 annealed at 9000C.

   The band gap for S1 is Eg=4.61eV and for S2 is Eg=3.24eV.

2. CONCLUSIONS

The samples were synthesized successfully. The prepared samples CaSO4:0.2Dy,0.1SMS(S1) and CaSO4:0.3Dy,0.1SMS (S2) gave a very high TL emission temperature which makes it applicable in high temperature radiation dosimetry. The energy dependence on TL phenomena was also studied. Co-doping resulted in a huge shift in peak emission temperature from 240C to 356C compared with the standard phosphor used for dosimetry applications. Other properties like surface morphology, reusability, dose dependence, fading etc are to be investigated further.

ACKNOWLEDGEMENT

The authors are thankful to the Staff, SAIF, STIC, CUSAT for the technical support. The financial assistance provided by KSCSTE, Government of Kerala, for this work is gratefully acknowledged. The support from staff, Department of Physics, Mahatma Gandhi College Thiruvananthapuram is greatly acknowledged.

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