Implications of Power and X-Ray Production cont'd

DOE-HDBK-1109-97

Radiological Safety Training for Radiation-Producing (X-Ray) Devices

Student's Guide

The percentage of the power transformed to X-rays can be estimated by the following

relationship:

Fraction of incident electron energy transformed into X-ray energy equals:

(7 X 10 -4) x Z x E

Where Z is the atomic number of the element (74 for tungsten) and E is the maximum

energy of the incoming electrons in MeV.

In this case, the fraction would be:

(7 X 10 -4) x 74 x 0.150 = 0.008

In the above example P is 7500 W. So the electron energy incident upon the anode is:

7500 W = 7500 J/s

Then the energy transformed into X-rays would be 0.008 [7500] = 60 J/s.

1 J = 107 ergs, and 100 ergs/g = 1 rad.

So this X-ray energy represents:

6.0 x 108 ergs/sec.

If all this X-ray energy were deposited in 1 g of tissue, the dose would be:

6.0 x 108 ergs/sec [1 rad/100ergs/g] =

6.0 x 106 rad/sec.

However, in practical applications X-ray beams are filtered to remove softer X-rays not

useful in radiology, the X-ray pulse is much less than 1 second, and the useful beam

region is several cm away from the anode target. These design features lower the dose

rates of the useful X-ray beam significantly. The dose rate in a typical X-ray beam is

estimated in Module 103 section E iii.