Calibration Info

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 eq.1 along with the OSL sensitivity and calibration factor.

Eq.1 [math] \frac{PMT Counts}{(Cal. Factor)(OSL Sensitivity)}=dose (mRad)[/math]

The response of OSLs to electrons and photons can be quantified in terms of dose through calibration. 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 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 understood calibration as there is less uncertainty in the dose on the OSL.

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.

By creating a custom calibration, the initial calibration factor has no effect on subsequent OSL measurements as a direct relationship is found between PMT counts and total exposure. 

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]. 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. Use of this conversion when creating the calibration fit leads to meaningful data when using the Nanodot OSLs in an experimental setting.

Will create new images for low dose and high dose calibrations as well due to possible issues with Y intercept. More comparable to calibrations creating using the OSL reader this way

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 known dose using basic y = mx+b

---

Click here for calibration data

Thesis