KAHIBARO
5.3. Energy Units
Table of Contents
`eV`
In GATE, all energies are expressed using explicit units from the Geant4 units system. The base energy unit is the electronvolt, written in code as eV. Conceptually, 1 electronvolt is the kinetic energy gained by an electron when it passes through a potential difference of 1 volt.
Numerically, the joule equivalent is
$$1 \,\text{eV} \approx 1.602 \times 10^{-19} \,\text{J}.$$
In practice, you rarely work at the eV scale in medical physics, because common clinical photon and particle energies are much higher. However, it is useful to understand that all higher energy units used in GATE are multiples of eV.
In OpenGATE Python scripts, you always multiply numerical values by the unit symbol so that GATE can interpret the quantity correctly. For example, to define a very low energy:
import opengate as gate
sim = gate.Simulation()
low_energy = 100 * gate.g4_units.eV
Here the number 100 by itself has no physical meaning until you multiply it by gate.g4_units.eV. This combination tells GATE that the energy is 100 eV.
You will mostly encounter eV indirectly, for example as part of internal physics thresholds or when the simulation reports very small secondary energies. For most imaging and therapy use cases, you will use keV and MeV instead.
Always specify energy in GATE as value unit, for example 140 keV or 6 * MeV. Never pass bare numbers without units.
`keV`
The kiloelectronvolt, written as keV, is equal to one thousand electronvolts:
$$1 \,\text{keV} = 10^3 \,\text{eV}.$$
In medical physics simulations, keV is a natural unit for most diagnostic X rays and many gamma emitters. For example, CT X ray spectra range from tens to about 150 keV. Common SPECT and PET radionuclides are also described in keV.
Typical values include about 140 keV for technetium 99m photons in SPECT, and 511 keV for annihilation photons in PET. These are written in GATE code as
E_tc99m = 140 * gate.g4_units.keV
E_511 = 511 * gate.g4_units.keV
You can freely mix keV and MeV in the same script, but be careful to keep the physical magnitude correct. For instance, 0.511 MeV and 511 keV represent the same energy:
$$0.511 \,\text{MeV} = 511 \,\text{keV} = 511{,}000 \,\text{eV}.$$
GATE and Geant4 will handle the conversion automatically based on the unit you provide.
A common source of error is confusing keV with MeV. For example, specifying a 511 MeV gamma instead of 511 keV introduces a factor of 1000 error in the energy and makes the simulated physics completely unrealistic.
Check all source energies and energy windows for the correct scale.
511 keV is 511 keV, not 511 MeV.
140 keV is 140 keV, not 140 MeV.
`MeV`
The megaelectronvolt, MeV, is one million electronvolts:
$$1 \,\text{MeV} = 10^6 \,\text{eV}.$$
MeV is the standard unit for higher energy photons, electrons, and protons used in radiotherapy, as well as for describing many nuclear level transitions. In medical applications, external photon therapy beams are typically in the range of several MeV, for example 6 MeV or 10 MeV nominal photon beams from linear accelerators. Proton therapy beam energies are often between about 70 and 250 MeV.
In a GATE script, you would specify such energies as
photon_beam_energy = 6 * gate.g4_units.MeV
proton_beam_energy = 150 * gate.g4_units.MeVNote that, conceptually, 1 MeV is equal to 1000 keV:
$$1 \,\text{MeV} = 10^3 \,\text{keV}.$$
This relation is useful when translating between nuclear data tables or published spectra that might switch between keV and MeV notation.
In diagnostic imaging simulations, you will occasionally use MeV when configuring physics lists or when interpreting high energy secondaries, but most primary photon sources there remain in keV. In contrast, in radiotherapy or proton therapy simulations, defining beam energies in MeV is the norm.
Before running a simulation, verify that high energy beams and source definitions use realistic MeV values, not accidentally scaled keV values. For example, 6 MeV is 6 MeV, while 6 keV is 6 keV. They differ by a factor of 1000.
`GeV`
The gigaelectronvolt, GeV, is equal to one billion electronvolts:
$$1 \,\text{GeV} = 10^9 \,\text{eV} = 10^3 \,\text{MeV}.$$
In typical clinical medical physics simulations performed with GATE, you will rarely need GeV energies, because imaging and therapy beams lie mostly in the keV to few hundred MeV range. However, GeV is part of the same energy system and is fully supported by Geant4 and GATE.
You may encounter GeV units in example codes coming from high energy physics, in generic Geant4 documentation, or if you use GATE for more energetic accelerator studies that go beyond standard medical energies.
In code, defining a GeV scale energy looks like
high_energy_proton = 1 * gate.g4_units.GeVInternally, GATE converts all these units into a consistent base energy. You do not need to perform any manual conversion as long as you attach the correct unit symbol to the numerical value.
Although GeV is not central to typical PET, SPECT, CT, or radiotherapy simulations, it is important to recognize that the same unit system spans a very broad energy range from eV up to GeV and beyond. This consistency is what allows GATE to reuse the underlying Geant4 physics engine across many energy scales.
Views: 11