On OSL ( Optically Stimulated Luminescence)

1. On OSL(Optically Stimulated Luminescence) Thursday January 27, 2011 Richard R. Brey, Ph.D., C.H.P. Professor of Physics Director, ISU Health Physics Program

2. Radiation Dosimetry • A broad system of techniques intended to quantify the energy deposited by ionizing radiation into some object with the ultimately goal providing information for assessment purposes • Includes traditional external dosimetry • Includes internal dosimetry procedures and processes

3. External Dosimetry • Traditionally accomplished using among many approaches: • Integrated ion chambers • Radiographic film • Thermo luminescence • And now accomplished using Optically Stimulated Luminescence

4. To understand OSL one may contrast it with TL • The basic TL system:

5. The key to a functional TL material is developing crystalline structures with well controlled metastable states that trap excited electrons/hole pairs at energy states that require the addition of appropriate quantities of energy if release is to occur. The TL Process

6. Light output is plotted versus heat input to produce a glow curve. The integrated area under a glow curve is correlated to the energy absorbed by the TL material. The input of heat is rather crude and has been accomplished using: Resistive heating elements Hot gas LASER light Light output although typically blue-green is rather broad in spectrum Light output

7. Many materials may be made to undergo luminescence, including ice. Materials with “good” Thermoluminescent properties have high concentrations of uniform traps between the valence band and the conduction band. Crystals are often intentionally doped with impurities such as Mn, and Dy (dysprosium) to achieve this condition. Other Properties of useful thermoluminescent material include: 1. Retention of trapped carriers for long periods of time at temperatures encountered during occupational or environmental exposure. 2. Large amount of light output. 3. Linear response over a large dose range. 4. Perfect annealing to enable repetitive use. Annealing is the process of heating a crystal to very high temperatures and maintaining it at those temperatures for an extended period of time in order to drive all excited electrons and the counterpart holes back into the valence band. 5. TLDs show very little angular dependence. They also have a much “flatter” energy response than film. The four most popular materials are: 1. Lithium Fluoride (LiF) A) TLD - 100: Contains natural Li (92.6% 7Li, 7.4% 6Li) Responds to gamma and thermal neutrons thermal neutrons are detected through an (n, α) reaction at 950 b on 6Li Zeff = 8.2 Range = mR to 1 x 104R B) TLD - 600 Contains enriched 6Li (7.38% 7Li, 92.62% 6Li) Extremely sensitive to thermal neutrons Zeff = 8.2 Range = mR to 1 x 105R C) TLD – 700 Contains excess Li-7 (99.993% 7Li, 0.007% 6Li) Sensitive to gamma only because the 7Li cross section is so small about 0.033b. Zeff = 8.2 Range = mR to 1 x 105R Commonly combinations of either TLD-100 and TLD-700 or TLD-600 and TLD-700 are used to obtain a value for the neutron dose received. The doping of LiF with either Mg, Cu, or P is reported to enhance LiF sensitivity by as much as 23 times over that experienced by TLD-600 chips. 2) Calcium Fluoride (CaF2) Used naturally without doping or when doped with Mn. Natural Zeff = 16.3 Range = mR to 1 x 103R Mn doped i.e. CaF2: Mn Zeff = 16.3. Range = mR to 1 x 105R 3) Calcium Sulfate (CaSO4) Often doped with Mn i.e. CaSO4: Mn Zeff = 15.3 Range = μR to 1 x 103R 4) Lithium Borate (Li2B4O7) The response of Li2B4Ois said to be essentially equivalent to human tissue. Unfortunately, its dose response is reported to be linear only up to 6000 rad while its sensitivity is about 10 to 15 rad. TL Materials

8. The quantity of energy deposition (i.e. radiation exposure) is determined by quantifying the blue-light output of Al2O3:C (crystals) after precisely controlled stimulation with green-light from either a LASER or light emitting diode on a small well defined but arbitrary location of the luminescent material. Reference: Image from http://www.osldosimetry.com/introduction/technology.asp See Landauer Inc. home page Used with Permission – Personal communication Craig Yoder Optically Stimulated Luminescence (OSL)

9. Figure Reference: Journal of Nuclear Science and Technology, Vol. 39, No. 2, p 211-213, February 2002. Decaying Patterns of Optically Stimulated Luminescence from Al2O3:C for different Quality Radiations; Yasuda, Kobayashi, Morishima Basic OSL operating System

10. OSL versus TL

11. Limited ISU Experience With OSL • ISU EAL investigated operational parameters of a microstar OSL reader for a period of about one week. • Preliminary investigated precision • Preliminary Considered linearity • Considered a 6-week field exposure • ISU EML student currently completing thesis project looking at robust nature of chip, comparison to E-perm and PIC data

12. Preliminary Considered linearity

13. Preliminary investigated precision

14. Considered a 6-week field exposure

15. Data Appropriate for an Engineering Design Base Justification for a New Type of Environmental Dosimeter Sensitivity Precision, Accuracy, Bias Energy Response To be representative of ambient dose this device must be filtered, techniques for filtration and their appropriateness must be investigated. Temperature Humidity Tribo (Drop Test) Response characteristics when various potentials interferences are present including: Microwave Radiofrequency ELF-EMF backgrounds

16. Recommendation Perhaps time to consider upgrading TL technology with OSL dosimetry