Answer to Question #610 Submitted to "Ask the Experts"

The term krad is short for kilorad or thousands of rad. This is a unit of absorbed dose, which in turn is the amount of energy absorbed per unit mass by the irradiated object (in this case, your sample of seeds).

1 rad = 0.01 joules/kilogram; 1 krad = 10 J/kg.

The term mas (more commonly written as mAs) stands for milliampere-seconds. It is the product of the current through the x-ray tube (mA) and the time the tube was energized (s). For a given x-ray machine, operated at a given kilovoltage potential, the radiation output rate is proportional to the current (mA) and the total radiation output for an exposure episode is proportional to the mAs. The absorbed dose (krad) to the sample (seeds) is the important factor for your experiment. The mAs is how the machine operator describes the particular exposure.

How do we reconcile the two? First of all, if all other factors are held constant (particularly machine kilovoltage, distance from the x-ray tube focal spot to the irradiated sample, and the size and shape of the sample), then the absorbed dose (krad) for each of the various irradiated samples is proportional to the mAs delivered. For example, if the mAs was doubled from sample 1 to sample 2, the absorbed dose to sample 2 was twice that to sample 1; if the mAs was increased by 4 times, the absorbed dose increased by four times, etc.

This tells you the relative dose delivered to your various samples but doesn't tell you the actual quantitative values. The relationship between the mAs delivered by the machine and the absorbed dose (krad) received by the sample depends upon a number of factors which you did not specify with your question. Some of the more important factors are:

1. The potential (kilovoltage or kVp) at which the machine was operated—radiation output increases strongly with potential.
    
2. "Filtration" in the beam.
    
3. Distance from the focal point of the x-ray tube to the irradiated sample. The radiation intensity varies approximately inversely with the square of the distance. For example, if the dose rate was specified for a 1-meter distance, the dose rate would be 1/4 as great if the sample were irradiated at a 2-meter distance and would be four times as high if the sample were placed at a 0.5-meter distance.
    
4. Other factors such as the type of power supplied to the machine (single phase or 3-phase), materials of the x-ray tube target, etc., also affect the radiation output for a given mAs.

The best way to assign an absorbed dose value to your irradiated seeds is to measure the absorbed dose per mAs using a suitable radiation instrument. Alternatively, if the x-ray machine has been calibrated recently by a radiation physicist, the calibration report should indicate radiation output per mAs. Note that it is important that the measurement be for the same kilovoltage (possibly expressed as kVp) as used for the seed irradiation.

If the distances were different between the calibration and your seed irradiation, you can correct using the inverse square law mentioned above. In the absence of measured radiation output values for the machine and kilovoltage, the next best thing is to look for published literature values for a similar x-ray machine operated at the same kilovoltage. For example, National Council on Radiation Protection and Measurements (NCRP) Report No. 102, Table B.3, gives average air kerma rates produced by diagnostic x-ray equipment. (Don't let the term "air kerma" throw you, it is essentially the absorbed dose to air.)

For a machine with total filtration equivalent to 2.5 mm aluminum, a three-phase generator, and operation at 100 kVp, the air kerma rate at 1 meter is 1.1 rad per 100 mAs. This is 0.000011 krad/mAs. What does this mean in terms of absorbed dose to the seeds? The seeds will have a slightly higher mass absorption coefficient for x rays than does air and the absorbed dose to the seeds will be slightly higher than the air kerma. I don't know the absorption coefficient for seeds but it should be close to that for water.

A good approximation would be an increase of 10%. Thus the value of absorbed dose for this hypothetical x-ray source would be on the order of 0.000012 krad/mAs. Of course, if you go this route to determine the doses for your experiment, you need to use factors corresponding to the x-ray generator type, tube potential (kVp), beam filtration, and irradiation distance actually used in your experiment. If you cannot get an actual measurement of the dose, send me all of these four factors and I will look up an estimate of dose for you. Good luck with your project.

Charles E. Roessler, CHP, Ph.D.