Calibration Info

Initial attempts to calibrate the Nanodot OSLs were to follow the manufacturers guidelines, through which two different calibrations are created; a high dose, and a low dose. The low dose calibration is used by the OSL reader when the measured dose is less than 10000mRad, and the high dose is used for any values exceeding 10000mRad. These calibrations are created by reading in pre-dosed OSLs provided by Landauer so the OSL reader software can create a linear fit between PMT Counts and dose. Using these calibrations assumes that the pre-dosed OSLs have not been subjected to any other sort of exposure during the shipping and storing before use. It was this thought that lead to the creation of a custom calibration using a Cs-137 source. Using a source of known activity allows for calculated exposure rates and a well known and understood calibration as there is less uncertainty in the dose on the OSL. The calibration factor that allows for the conversion from PMT counts to dose is the slope of the linear fit between dose and PMT counts.

It is the equation of this line 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 response of OSLs to electrons and photons can be quantified in terms of dose through calibration. 

The setup for the custom calibration positions the source 30cm from 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 using 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] dose (mRad) = \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 measurements 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 conversion 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. - rewrite

Add linear fit <- this will be at the end

High dose and low dose <- put this at the beginning

Calibration factor is not directly involved as one can expose the OSLs and then read in the pmt counts. Then using the linear fit, get the know dose using basic y = mx+b

Looking at the linear fits, it may best to separate into low dose and high dose fits. Will look at this later.

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Thesis