Basics of B3 dosimetry

1 - B3 Radiochromic Film Dosimeter

B3 radiochromic film dosimeters are used for absorbed dose measurements in radiation processing. When irradiated, a chemical reaction occurs within the film causing a change in the visible light spectrum; this change is directly related to the quantity of absorbed dose the film receives and is consistent and quantifiable. See Figure 1.

Due to the film’s small thickness and low mass, it is an excellent dosimeter for measuring dose to products irradiated by electron beam, X-ray, and gamma technologies. Examples of end uses of such technologies include medical device sterilization, curing of inks and coatings and the decontamination of food or food packaging. Dosimeters are commonly used for mapping of dose distribution during qualification of these irradiation processes and routinely monitoring the process output, dose magnitude, for conformity assessment and process control purposes.

Intended Use

B3 film dosimeters are a part of a radiochromic film dosimetry system, as defined in ASTM 52628. B3 dosimeter products are intended to be used a part of a dosimeter measurement system that includes:

• B3 film dosimeters

• Reader (spectrophotometer)

• Heat treatment device (incubator)

• Dosimeter measurement process and procedures.

GEX B3 dosimeter products

GEX manufactures a variety of B3 dosimeter products, ideal for routine dosimeter measurements, research and testing of Electron Beam, X-Ray and Gamma irradiation processes. GEX B3 dosimeter products utilize the B3 radiochromic film and are designed to meet current industry guidelines.

2 - Calculating Dose

The response (R) of the dosimeter to ionizing radiation is defined in terms of optical absorbance (Ai) per unit thickness (t), also referred to as the “specific absorbance”,

R = A_i/t

Users may occasionally subtract the unirradiated, original absorbance (A_o), sometimes called “background Absorbance”, from the A_i to derive the “net specific absorbance”, such that R = (A_i-A_o)/t

For B3 dosimeters, the A_o is typically ignored for the following reasons:

• A_o is stable (unchanging) over a long period of time.

• B3 dosimeter thickness is both thin and uniform and the measurement of the A_o cannot help the user to resolve the small variances in thickness between dosimeters in the same batch and average thickness group.

• Since there is no technical gain in subtracting A_o, GEX recommends that the user ignore the A_o in any calculation of R, and to use the simple response calculation of R = A_i/t

The relationship of R with absorbed dose (D) is expressed as A_i/t = R = ƒ(D) where the dosimeter response is a function of the quantity of absorbed dose received by the dosimeter. To calculate absorbed dose from the dosimeter’s measured Absorbance, 

A_i/t = R = f(D)

The GEX DoseControl Dosimetry System automatically calculates absorbed dose (D) for each dosimeter measurement.

3 - Response

The response (R) has a slope of 2.0 at approximately 2.0 kGy, reducing to a slope of 1.0 near 25 kGy, and a slope near 0.6 at approximately 100 kGy. 

Response Measurement Reproducibility – The user should be able to achieve the following percentage coefficient of variation (C.V.%) for repeated measurement of the same dosimeter:

1.5% or better at 1.0 kGy

1.0% or better at 10.0 kGy

0.5% or better at 20.0 kGy and higher

4 - Resolution

B3 has a calculated dose resolution of approximately 0.1 kGy at 25 kGy. Every 0.001 A_i change results in a change of approximately 0.1 kGy.

5 - Practical Dose Range

The dose range of the B3 film is approximately <1 kGy to 160 kGy. Some users have been successful in using the product above and below these values. Saturation of response begins near 150 kGy with full response saturation occurring at a dose of approximately 165 kGy or a specific absorbance of approximately 90.0 A*mm-1 (approximately 1.700 A) under normal temperature conditions.

3The low dose range may be extended by special techniques while the high dose range is situationally dependent on the irradiation conditions. The user should consider measurement uncertainty when deciding on the practical range. Some users have been successful in using the product above and below these values.

6 - Dosimeter Batch Calibration (also called ‘dosimetry system calibration’)

The dosimetry system must be calibrated to achieve dose values traceable to an international standard (per ASTM 52628 Standard Practice for Dosimetry in Radiation Processing).

More information:

• Complete information, guidance, technical reports: Dosimeter Batch Calibration (Dosimetry System Calibration).

• Specific B3 dosimeter batch calibration overview: B3 Dosimeter Batch Calibration

7 - Dosimeter Measurement Uncertainty

All measurements of absorbed dose (D) must be accompanied by an estimate of uncertainty. All components of uncertainty should be included in the estimated uncertainty, including those arising from calibration (dosimeter batch calibration), dosimeter reproducibility, instrument stability and the effect of influence quantities. See complete information here: Dosimetry System Calibration - Developing and Using Uncertainty Statements.

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