Exposure Dose and Radiation Dose Measurement

Exposure Dose

Exposure describes the ionization produced by X-rays or gamma rays in air. It is historically expressed using units such as roentgen (R). Modern radiation protection practice generally uses absorbed, equivalent and effective dose quantities rather than exposure alone.

Important Dose Quantities

1. Absorbed Dose

Energy deposited per unit mass:

$$D=\frac{d\bar{\epsilon}}{dm}$$

SI unit: gray (Gy), where $1\,Gy=1\,J/kg$.

2. Equivalent Dose

Accounts for the radiation type using a radiation weighting factor:

$$H_T=\sum_R w_R D_{T,R}$$

Unit: sievert (Sv).

3. Effective Dose

Accounts for different tissue sensitivities:

$$E=\sum_T w_T H_T$$

Unit: sievert (Sv).

How Radiation Dose is Measured

Different instruments are used according to the application:

• Ionization chambers: accurate measurement of exposure/air kerma and output of X-ray equipment.
• Geiger-Müller counters: radiation detection and survey applications.
• Scintillation detectors: convert radiation into light and electrical pulses.
• Semiconductor detectors: compact, sensitive detectors used in dosimetry and spectroscopy.
• Personal dosimeters: monitor occupational exposure over time.
• Thermoluminescent dosimeters (TLDs): store radiation energy and release light when heated during readout.
• OSL dosimeters: use optically stimulated luminescence for dose readout.

X-ray Equipment Output

In diagnostic radiology, output can be measured using an ionization chamber or solid-state dosimeter. Patient dose may be estimated/measured using quantities such as entrance-surface air kerma or dose-area product, depending on the procedure.

Conclusion

Radiation dose measurement quantifies radiation exposure so that equipment performance can be controlled and patient and occupational doses can be kept appropriately low under the ALARA principle.