Beta Transmission and Ionization
GEANT4 simulated Beta particles in the drift region with a 90/10 Ar/CO2 gas. U-233 emits Beta particles with a range extends up to 600 keV, Although Beta particles are low in rates as compared to Alpha rates; they contribute in the detector total charge. GEANT4 is used for simulating the charge for a Beta particle that passed through the drift region, and estimated the primary and secondary electrons for each Beta particle energy. Additionally, GEANT4 helps studying the penetration of beta particles through 1mm FR4 shutter.
Beta particles energy rates
Beta particles are emitted from U-233 radioactive isotope, their energy spectrum vs the percentage of the emitted Beta is shown in figure XX. Based on the figure, the rates are lower than 0.1 percent, with only two specific energies reach up to 6 percent for 10 keV for one of the energies, and 30 keV for the other.
Primary and secondary ionization
A simulation for Beta ionization was performed by using GEANT4. GEANT4 simulated the interaction of a beta particle in the drift region that contained 90/10 Ar/CO2 gas; the simulation estimated the ionization primary and secondary electrons. Figure XX illustrates that the number of primary and secondary electrons decreased when the incident beta particle increased (indirectly proportional); such a relationship agrees with Bethe-Bloch equation for estimating the energy loss. The figure also shows that Betas with energy of 400 keV and above contribute with the least number of free electrons in the drift region, this number of electrons is 10-17 electrons.
Beta Ionization with FR4 shutter
The FR4 shutter stops the low energy betas from U-233, and passes the high energy ones to ionize the gas in the drift region. U-233 beta emission relative rate is as low as 0.1 for most beta energies, as mentioned previously, the highest beta rates are for those of kinetic energy of 10 keV and 30 keV, GEANT4 simulation shows in figure yy that 80% of Beta particles of energy of 1 MeV penetrate the shutter, and close to 100% penetration is for beta energies equal or larger than 1.3 MeV, but those low energy betas have the major contribution to the total number of primary and secondary electrons in the drift region, therefore, when the shutter is closed, only beta particles with energy more 400keV contribute to the number of primary and secondary electrons, which have a negligible effect on the detector total charge due to the their low emission rates and the amount of charge they create in the drift region.
Combined Gamma
Electron multiple scattering causes Gamma particles to appear through beta transmission in the FR4. Electron scattering is one of the interactions of beta particles with FR4, photons are produced through this process depending on beta's energy.
It is noticed from the figure above that the maximum scattering cross section is for 1100 keV beta particles, the number of photons reaches to 1800 for each beta particle that transport through 1cm of FR4.