Calibration Info

To fully understand the relationship between 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 pre-dosed OSLs provided by the manufacturer, Landauer. 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. To begin the development of a custom calibration, a distance of 30cm from the faceplate of the source to the OSLs is used, and an adjustment distance from the faceplate to the surface of the source needs to be taken into account given the 1/rsquared dependency of the exposure rate. The total distance D is then, D = (Distance to faceplate +11.2cm). 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 of [math]{A} = 9.3 Ci [/math] is used in these calculations. Using the exposure rate found in this calculation, it is possible to find the total exposure of the Nanodot OSL 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 OSLs.

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Thesis