There are two factors that we thought might affect the throughput of particles. The throughput is the percentage of particles that are completely identified out of the total particles that enter the machine. These two factors are the voltage of the center plate and the energy of the particles. Many particles are not identified because they crash into a ceramic plate within the instrument. A higher voltage should deflect the particles away from the ceramic, while a less energetic particle should allow for more deflection.
To test these hypotheses, we ran the data files of tests for which we knew the energy of the particles and the voltage of the center plate.
To determine how voltage affects throughput, we compared:
20 MeV Carbon at -0kV Throughput=9.5%
20 MeV Iron at -0kV Throughput=11.1%
Figure A: This is the footprint of the beam in the instrument after going through the collimator. The purple trace is undeflected and the green trace has been shifted to the left, further on to the Solid State Detector surface. The purpose of the deflection is to determine the charge to energy ratio (Q/E) of the particle. We used the SSD to measure the energy (E) of the particle. With these two pieces of information, we can calculate the charge (Q) of the particle.
Figure B: Throughput vs. Deflection Voltage for carbon and iron beams.
Figure C: Throughput vs. Beam Deflection (mm) for carbon and iron beams. 70% is the maximum expected throughput for the particles because of necessary supporting structures inside the intrument.
25 MeV Iron at 11.92 kV Charge State=12+ Throughput=69.0%
60 MeV Iron at 11.92 kV Charge State=12+ Throughput=49.5%
Conclusion : For lower energy per charge particles, such as 20 MeV carbon and iron, a higher voltage produces a higher deflection, allowing the particles to avoid the ceramic support, and thus producing a higher throughput. For these two beams, we had to move them about their footprint's width (Figure A) to get maximum throughput.
According to the iron data, it seems that for higher energy per charge particles, the deflection is lower, resulting in more particles colliding with the ceramic SSD support, and therefore a lower throughput.
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