Calibration Info

It is this calibration factor, found by creating a calibration with the provided pre-dosed OSLs that is brought into question. By subjecting the OSLs to a known total exposure, it is possible to eliminate the effect of any unknown factors in the listed dose on the provided OSLs. Therefore to fully understand the relationship between background subtracted PMT (photo multiplier tube) Counts and total exposure for the Nanodot OSLs, a custom calibration is needed. This calibration is used in lieu of the calibration given by the OSL reader, which is created using the pre-dosed OSLs. To begin the calibration, a set of fifteen previously unexposed Nanodot OSLs is chosen at random and exposed to a 9.3Ci Cesium-137 source.

The setup for the custom calibration uses a distance of 30cm from the faceplate of the source to the OSLs in an attempt to achieve some uniformity of the radiation field across the subjected OSLs. The distance from the faceplate to the surface of the source needs to be taken into account given the [math]\frac{1}{D^{2}}[/math] dependency of the exposure rate. The total distance D is then, D = (Distance to faceplate +11.2cm) {as provided by the Technical Safety Office at Idaho State University}. The exposure rate for any given distance is calculated in units of Roentgen using the equation [math] \dot R = \frac {\Gamma A }{ D^2} [/math]. A gamma factor of, [math] {\Gamma} = 0.33 \frac {(m^2)(R)}{(Ci)(hr)} [/math] and activity [math]{A} = 9.3 Ci [/math] is used in these calculations. With the exposure rate found with these known variables, it is possible to find the total exposure given to the Nanodot OSLs by integrating the exposure rate over the time the OSL was exposed to the source, [math]R_{tot}=\int\limits_{t_0}^{t_f}\dot R\ dt [/math].

The Microstar reader used to analyze the OSLs produces a numeric quantity in units of milliRad (mRad) for the total accumulated dose by reading the PMT counts, which is then input into [math] accumulated\ dose = \frac{PMT Counts}{(Cal. Factor)(OSL Sensitivity)}[/math] along with the OSL sensitivity and calibration factor. By creating a custom calibration, the calibration factor has no effect on subsequent OSL analyzation as a direct relationship is found between PMT counts and total exposure. It is necessary when working with exposure to be able to convert to the units supplied by the OSL reader as the two quantities are directly related. Through unit analysis, it was found that 1.14554 Roentgen = 1 Rad. Using this conversation allows for a linear fit calibration to be created to visualize the relationship between background subtracted PMT counts and calculated dose. This calibration is then used to get well understood measurements during experiments involving the Nanodot OSLs.

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High dose and low dose

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Thesis