Dose Rate from a Radioactive Planar Source Calculator

Technical background for radioactive line sources

The Ionactive website already features some excellent resource on this : try this if you want to delve into the physics and maths behind this calculator: When 1/d²
breaks down - part 2: area (planar) source. 

Release notes

Version 1.0  The Dose Rate from a Radioactive Planar Source Calculator has two operational modes:

• Forward (Total Activity  → Dose Rate): This mode allows you to calculate the dose rate from a Planar source at a total activity (of an area that you specify), or as an activity per unit area (e.g. Bq/m²).
• Reverse (Dose Rate → Activity density): Here you may enter known, measured or assumed dose rate data and infer the potential activity of the Planar source (assuming linear activity per unit area). 

The parameters, choices and inputs will vary slightly between the two modes selected. In almost all cases, if you make a change to the variables then you need to click calculate to see the changes. 

We first consider input choices for the forward calculation. 

• Radionuclide: Select from a popular list of beta / gamma radionuclides covering medicine, industry and of historical significance.  
• Unit system: Choose SI or Non Si units (you cannot mix between the two).
• Output format: Choose normal or scientific.
• Mode: Here you select Total activity → Dose Rate (forward) or Dose Rate → Activity density (reverse).
• Planar shape: You can select either a circle (where you later specify diameter) or a rectangle (where you later specify the X and Y dimensions.
• Activity input type (forward): The default choice is Total activity on plane (A), but you can alternatively choose activity per unit area (e.g. Bq/m² etc).
• Activity input (A): Depending on your choice above, either enter a total activity or an activity per unit area.
• Perpendicular distance (H): Enter the distance between the calculation point and the centre of the planar source
• Position: Use this to choose between calculating from the centre of the source, or an offset. If you choose offset you can enter a negative or positive value for X and Y coordinates to position the calculation point as you like.  
• Perp distance (p): This is the perpendicular distance between the mid point of the source and the calculation point (unit selectable). Use this, in conjunction with the offset (if required) to define the actual desired calculation point.
• Area of planer source: If a circular source has been selected, enter the diameter here, if a rectangular source is prefered then enter the X and Y dimensions.
• Add shielding: choose to add either lead, steel, concrete (several densities given) and water. If this setting is used then the shielding thickness input fields will appear where the thickness and desired units can be entered.
• Dose scaling output: Choose base for Gy / Sv / R etc, or micro, milli, kilo etc as desired. 

At the bottom of the calculator is the calculate and reset buttons. Use the calculate button each time you make a change to the desired inputs. The reset button will revert the calculator to standard settings as if the page has been reloaded. 

The outputs in the forward (Total Activity  → Dose Rate) direction are as follows.

• A summary of the calculation case.
• Planar source absorbed dose rate in air.
• Planar source effective dose rate.
• A point source equivalent absorbed and effective dose rate (for comparison with the planar source calculation).
• The ratio of the planar to point source calculation for effective dose rate. This is useful  for investigating how the inverse square law may apply, and if planar source calculations are actually needed.   

We now consider the input choices for reverse calculations. Only those different from the forward calculation will be featured below. 

• Measured dose rate: This is the dose rate that has been measured (or determined in some other way) which you wish to use in the calculation of planar source activity. You can choose to specify this as effective dose rate or absorbed dose rate in air. 

The outputs in the reverse calculation (Dose Rate → Activity density) are as follows:

• A summary of the calculation case.
• Calculated activity density (A/Area) - e.g.  Bq/m².
• Calculated total activity on the planar source (A) - e.g. Bq.
• The planar source dose rate (same as input). 

Have a play with the calculator and see what you come up with.