c. Quality Factor (Q) - The quality factor is an artificial factor dependent on the linear energy transfer of the radiation by means of which biological effects resulting from absorbed doses of different types of radiation may be related to X- and gamma-radiation doses. Figure 18.104.22.168.3.1-1 (below) presents current Q factors for a number of different types of radiation.
|Type of radiation||Quality factor, Q|
|Gamma rays and bremsstrahlung||1|
|Beta particles, electrons, 1.0 MeV||1|
|Beta particles, 1.0 MeV||1|
|Neutrons, thermal energy||2.8|
|Neutrons, 0.0001 MeV||2.2|
|Neutrons, 0.005 MeV||2.4|
|Neutrons, 0.02 MeV||5|
|Neutrons, 0.5 MeV||10.2|
|Neutrons, 1.0 MeV||10.5|
|Neutrons, 10.0 MeV||6.4|
|Protons, greater than 100 MeV||1-2|
|Protons, 1.0 MeV||8.5|
|Protons, 0.1 MeV||0|
|Alpha particles (helium nuclei), 5 MeV||15|
|Alpha particles, 1 MeV||20|
Reference: 92, Table 9-1; NASA-STD-3000 165
As a given particle degrades in tissue, the Q will rise as its energy transfer per micrometer (see definition of LET below) rises. For a beam of protons having a wide range of energies, the average Q tends to drop with increasing depth in tissue as the lower energy component tends to be removed with increasing depth and the high-energy component continues its traversal.
The standard Q values are based on the most detrimental chronic biological effect (e.g., carcinogenesis by neutrons) for continuous low-dose exposure that might be met in industrial situations. However, the Q for many acute high dose rate exposures may be very much lower.
For this reason, recognized committees of radiation experts are currently reevaluating the data upon which the Q values are based and are likely to revise these Q factors.