Why the Harshaw TLD Dose Algorithms use 8 days for Fade

Posted by Joe Rotunda on

Have you ever wondered why the Harshaw TLD dose algorithms based on LiF:Mg,Ti have 8 days pre and post irradiation built into their calculations?

If you are using the Harshaw TLD dose calculations with TLD-100, 600 or 700 (LiF:Mg,Ti) the calculations implement a fade correction calculation. This applies to whole body, environmental and extremity algorithms. Please note that for dosimetry systems using TLD-100H, 600H or 700H (LiF:Mg,Cu,P) this is not a concern when using the factory recommended time temperature profile (TTP) as there is only about 5% fade in one year.

Algorithm Fade Calculation

Built into the Harshaw TLD dose algorithm is a fade calculation used to eliminate fade as part of the calibration process. The algorithm is shown below:

F = exp[-0.00331 x (t1 – 8)]
Where:
t1 = T / 2
T = the elapsed time between preparation and readout.

When the algorithms were developed, consideration was given for the time between annealing the calibration dosimeters, exposing the dosimeters at the calibration laboratory, and reading the calibration dosimeters. Since the user may not have control over the time it takes for the dosimeters to transit to and from the calibration laboratory as well as scheduling at the calibration laboratory. For some dosimetry laboratories the secondary or tertiary calibration facilities are on-site. This may save the transit time or the scheduling with the irradiation sources, but you still must adhere to the 8-day rule to maintain accurate readouts.

First, from a previous blog on Fade, you will know that LiF:Mg,Ti has fade. This fade is classified as pre- and post-fade. The pre-fade is due to short term sensitivity loss while the post-fade is due to signal loss. Actually, pre- and post-fade occur together, but it is easier to explain them separately to gain understanding of the concept.

During the process of calibrating the dose algorithm you need to prepare dosimeters for the “Cs137 Relative Response” factors and possibly beta and neutron calibration factors if you also need them calibrated. This is the time where the 8 days of pre- and post-irradiation come into consideration. All the algorithm calculations are based on the 8-day pre- and post-irradiation, so day 16 is considered time zero for the algorithm. Notice that when you use 16 for T in the above equation (8 for pre-irradiation + 8 for post-irradiation) you get a fade factor of 1, otherwise known as “no fade.”

To reemphasize the above point, all the dose calculations are based on 16 days between anneal and readout with irradiation at day 8, not just the final fade calculation.

When you anneal the TLD cards or extremity elements and put them in the holders for irradiation at the calibration laboratory, ensure the dosimeters are not irradiated until 8 days after they were annealed. Have the calibration laboratory return the dosimeters back to the dosimetry laboratory and 8 days after the dosimeters were irradiated read them out as normal using the same Time Temperature Profile (TTP) as they were annealed as well as creating their Element Correction Coefficients (ECCs).

Come back soon for more information on the difference between pre and post fade.

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