Radiation Lab Simulator

Radiation Output Lab

A dosimeter measures exposure (output) by collecting and measuring electrons liberated from air molecules when x-ray photons ionize air. Exposure is measured in units of Roentgens (R). The Roentgen represents a certain quantity of charge liberated per mass of air.

Measured output for an x-ray beam depends upon the following parameters.

mA (milliamps)

X-rays are produced when fast-moving electrons accelerated by the positive charge of the target collide with the giving up their kinetic energy as heat and radiation.
Milliamperage or mA is the rate of electron flow (millicoulomb/sec) from cathode to anode.

Exposure Time

Exposure time is the time duration during which high voltage is applied across the x-ray tube and x-rays are produced.
Doubling the exposure time with no other technique change will double the total number of electrons striking the target and thus double the amount of radiation produced.

mAs (milliAmp-seconds)

mAs (pronounced “mass”) is the product of mA and exposure time. mAs is indicative of the total number of electrons striking the x-ray tube target during an exposure.
As long as mAs and distance are kept constant, the same exposure should be measured. For example, doubling the mA (rate) while cutting the exposure time in half will result in the same mAs and thus the same amount of radiation.

kVp (kilovoltage)

Kilovoltage is the voltage (measured in thousands or kilo's of Volts) applied to the x-ray tube target. The higher the voltage applied to the x-ray tube, the faster electrons are traveling when they strike the target. Kilovoltage determines the energy mix or spectrum of the x-ray beam.
Output is proportional somewhere between the square and cube of kilovoltage.
- R α kVpⁿ
  - R = Exposure
  - kVp = kilovoltage
  - n is a constant between 2 and 3

Distance from focal spot

Solid geometry tells us intensity is inversely proportional to the square of distance for a point source
R α 1/D²

R = Exposure
D = Distance between focal spot and dosimeter

Focal Spot

Measured output does not depend upon which focal spot is used. The focal spot only determines the area of the target from which x-rays are emitted. As long as the mA, exposure time, and kVp are not changed, selection of focal spot will not affect output.

This lab simulation allows you to set up exposure factors, make a simulated exposure, and measure the output. There are two ways you may perform this lab.

Free selection of exposure factors

OR
Make series of exposures changing a single parameter (note how output (mR) and normalized output (mR/mAs) varies with that parameter)

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