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 quantity for the total accumulated dose by using background subtracted PMT counts, OSL sensitivity, and calibration factor. It is this calibration factor that is determined by the pre-dosed OSLs and can see the relationship between the PMT counts and exposed dose. By creating a custom calibration, the calibration factor has no affect as a direct relationship is found between PMT counts and exposed dose. Which would require a conversion from Roentgen to Rad. By converting units, it was found that 1.14554 Roentgen = 1 Rad. Using this conversation allowed 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.

Add formula for OSL reader (maybe better in OSL reader section?)

Add linear fit

High dose and low dose

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Click here for calibration data

Thesis