Understanding fade correction in LiF:Mg,Ti (TLD-100, 600, 700) dosimetry systems. Have you ever wondered why Harshaw® TLD dose algorithms based on LiF:Mg,Ti incorporate 8 days pre- and 8 days post-irradiation into their calculations? If you are using Harshaw TLD dose calculations with TLD-100, TLD-600, or TLD-700 (LiF:Mg,Ti), the system implements a built-in fade correction calculation. This applies to whole body, environmental, and extremity algorithms. Important: For dosimetry systems using TLD-100H, 600H, or 700H (LiF:Mg,Cu,P), fade is minimal (approximately 5% over one year) when using the factory-recommended Time Temperature Profile (TTP), so this 8-day consideration is not required in the same way. The Fade Correction Built into the Algorithm The Harshaw TLD dose algorithm includes a fade factor designed to eliminate fade effects during the calibration process. The fade equation is: F = exp[-0.00331 × (t1 – 8)] Where: t1 = T / 2 T = elapsed time between preparation and readout When the algorithms were developed, consideration was given to the total time between annealing calibration dosimeters, irradiating them at the calibration laboratory, and reading them out. Since users may not control transit times or scheduling delays at calibration facilities, the algorithm standardizes this process with the 8-day pre- and post-irradiation structure. Why 8 Days Before and After Irradiation? LiF:Mg,Ti exhibits both pre-fade and post-fade. Pre-fade results from short-term sensitivity loss, while post-fade results from signal loss. Although they occur simultaneously, separating them conceptually helps explain the correction model. During calibration—whether for Cs-137 relative response factors or beta and neutron calibration— the 8-day pre- and post-irradiation timing becomes critical. All Harshaw algorithm calculations are based on: 8 days between anneal and irradiation 8 days between irradiation and readout In other words, day 16 is considered time zero for the algorithm. When you substitute 16 for T in the equation (8 days pre + 8 days post), the fade factor equals 1 — effectively “no fade.” This timing structure is not just part of the final fade calculation. All dose calculations assume this 16-day framework. Best Practices for Calibration When annealing TLD cards or extremity elements for calibration: Do not irradiate the dosimeters until 8 days after annealing. After irradiation, wait 8 days before readout. Use the same Time Temperature Profile (TTP) for readout as used for annealing. Create Element Correction Coefficients (ECCs) under the same controlled conditions. Even if your secondary or tertiary calibration source is on-site, eliminating transit delays does not eliminate the need to adhere to the 8-day timing rule. Understanding Fade Improves Accuracy Fade in LiF:Mg,Ti is a predictable physical behavior. The Harshaw algorithm accounts for it through a structured, time-based correction model built around the 8-day pre- and post-irradiation framework. Following this timing structure ensures accurate calibration, consistent dose calculations, and reliable long-term dosimetry performance. Stay tuned for more information on the differences between pre-fade and post-fade in future posts.
