Miscellaneous Detectors

1. Miscellaneous Detectors November 11, 2003 Prof. Tsi-chian Chao

2. Thermoluminescent Dosimeters (TLD) One measurement A set of measurementsOne measurement A set of measurements

3. Glow Curve

4. Readout Cycle Pre-heat period Without light integration to discriminate against unstable low-temperature traps Read period Spanning the emission of the part of the glow curve to be read as a measure of the dose Annealing period During which the remainder of the stored energy is dumped without light integration Cool-down period After the heater-pan power is turned off

5. Trap Stability Annealing Apply after TLD signals have been �read� To avoid trap configuration change TL phosphors give best performance as dosimeters if they receive uniform, reproducible, and optimal heat treatment before and after use Ex. LiF (TLD-100) 400 0C for 1 h, quick cooling, then 80 0C for 24 h

6. Advantages Wide useful dose range From a few mrad to ?103 rad linearly Dose-rate independence 0-1011 rad/s Small size Chips, rods, powder Commercial availability Reusability Can be reused many times

7. Advantages Economy Reusability reduces cost Availability of different types with different sensitivities to thermal neutrons TLD-700 (7LiF) Sensitive to photons only TLD-100 (93% 7LiF + 7% 6LiF) Sensitive to both neutrons and photons TLD-600 (96% 6LiF) Sensitive to neutrons only

8. Advantages Readout convenience Readout rapidly (<30 s) Automation compatibility Automatic reader for mass amount of TLDs Accuracy and precision Reproducibility of 1-2 % 1-2 % accuracy through individual calibration and averaging of several dosimeters in a cluster

9. Disadvantages Lack of uniformity Sensitivity varies from batch to batch, even from dosimeter to dosimeter of the same batch Storage instability Sensitivity varies with time Fading Gradual loss of the latent TLD signal

10. Disadvantages Light sensitivity Sensitive to light�especially to UV, sunlight, or fluorescent light Spurious TL Scraping, chipping, or surface contamination by dirt or humidity can cause spurious TL readings Loss of a reading No second chance at getting a reading

11. Disadvantages Memory of radiation and thermal history Sensitivity increased or decreased after receiving a large dose of radiation Reader instability Reader constancy is difficult to maintain over long time periods

12. Photographic Dosimetry Exposure Radiation hits photographic emulsion and generate ion pairs near AgBr grains then converting Ag+ ions to Ag atoms Chemical processing Developing in the chemical process all of the Ag+ converted to Ag atoms, leaving behind an opaque microscopic grain of silver Stop bath Hypo

13. Optical Density of Film

14. Energy Dependence Increased response for energy < 200 keV due to photoelectric effect Use of filter to eliminate this over response

15. Advantages Spatial resolution Unrivaled in spatial resolution Reading Permanence The record is permanent Commercial availability Geometry Thin and flat shape allow simple use Can approach B-G cavity dimensions Linearity vs. dose Dose-rate independence

16. Disadvantages Wet chemical processing Require careful control of wet-chemical development process Energy dependence of X rays Over-response for energy below 300 keV due to photoelectric interactions with silver bromide grains sensitivity to hostile environments Double-valued response functions Over-saturate of film response cause double-valued response Blindness to low-energy neutrons

17. Chemical Dosimetry Chemical Dosimetry Basic Principles Radiation interacts with water produce chemically active primary products (free radicals, such as H2 & H2O2) in about 10-10 s or less initially distributed heterogeneously, close to the charged-particle tracks by 10-6 s, diffuse to become more homogeneous, simultaneous with their chemical interactions with the solutes present

18. Chemical Dosimetry Radiation chemical yield (G-value) Defined as the number of chemical entities produced, destroyed, or changed by the expenditure of 100 eV of radiation energy in moles/J Calculation of absorbed dose ?M (mole/liter) the change in molar concentration of product X due to the irradiation ? (g/cm3 or kg/liter): solution density

19. Chemical Dosimetry Popular example Fricke Ferrous Sulfate Dosimeter Fe2+ ? Fe3+ oxidation reaction Composition 0.001 M FeSO4 or Fe(NH4)2(SO4)2 and 0.8 N H2SO4

20. Chemical Dosimetry Advantages Z, ?en/? & ? similar to water Liquid dosimeters can be made similar in shape and volume to the studied object Absolute dosimetry possible Different chemical dosimeters can be used to cover various dose ranges: 10-1010 rad Linear dose response vs. dose in useful ranges

21. Chemical Dosimetry Disadvantages Lack of storage stability prevents commercial availability, requiring wet chemistry in the user�s lab Useful dose ranges too high for personnel monitoring or small source measurements Some degree of dose-rate and LET dependence Dependence on the temperature of the solution during irradiation and during the readout procedure

22. Calorimetric Dosimetry Calorimetric Dosimetry Direct measurement of the full energy imparted to matter by radiation Closest of any method for absolute dose measurement ?T: temperature change h: thermal capacity (cal/g 0C or J/kg 0C) ?: thermal defect The fraction of E that dose not appear as hear, due to competing chemical reactions

23. Calorimetric Dosimetry Advantages Absolute dosimetry Closest of any method being a direct measurement of the energy involved in the absorbed dose Almost any material can be employed in the sensitive volume Dose-rate independent No LET dependence Relatively stable against radiation damage

24. Calorimetric Dosimetry Disadvantages Temperature rise small, limiting measurement to relatively large doses Apparatus bulky, difficult to transport and set up For low dose rates, thermal leakage limits the accuracy and precision achievable Thermal defect problem