Identifying the sources of beam transmission loss of carbon isotopes
Dr Gurazada Prasad1, Dr Alexander Cherkinsky1
1University Of Georgia, Athens, United States
The continuous decrease of 12C beam transmission through the accelerator with increasing injection current is well known. For high enough beam currents, the post acceleration 13C+/12C+ ratio of a sample is no longer a constant due to a greater loss of 12C+ beam current, which is detrimental for online d13C estimation, as reported by some NEC 250 kV SSAMS
facilities including ours. Factors such as beam emittance, space charge at the ion source and scattering at the gas stripping column of the accelerator are thought to contribute to the changes in beam transmission. In this study we attempt to identify the sources of transmission-loss by looking at all three isotopes of carbon.
The two extreme cases are 1) very high beam currents of 12C and 2) practically no beam case of 14C through the accelerator. Our earlier analyses of SSAMS data showed that 12C transmission steadily decreased until 85μA of injected current, and then dropped rapidly for higher currents. We also found that even the 13C+/12C- ratio decreased steadily where – and + signs refer to pre and post-acceleration currents respectively. The 14C+/13C+ ratio of a sample did not change with the beam current and therefore did not affect the accuracy of pMC measurements.
In the current study, we examined the 14C+/12C- ratios on SSAMS. The 14C data of OXII standards from several runs were analyzed. We have indeed found that 14C+/12C- ratio is not a constant for a given sample, but decreases linearly with the 12C- beam. The rate of decrease varies across runs due to operating conditions, but typically ~1% for a 10μA change in 12C-. This is very interesting because 14C being a rare isotope, its transmission is not expected to be affected during acceleration. This observation implies that for low to moderately high beam currents, steady decline in transmission is not due to scattering effects at the stripping column but likely due to ion-source effects. The beam emittance of the ion-source, which is a measure of beam width and divergence may have a decisive effect on beam transmission that needs further investigation.
On the other hand, the sudden drop in the beam transmission of 12C above 85μA appears to occur only during acceleration. While scattering due to argon gas is the most likely cause, any effects of space charge due to charge exchange collisions might also need consideration.
In effect, we observed a steady decrease in beam transmission for all three isotopes with increasing 12C beam. The important end note is that even though both 13C and 14C showed the beam dependence, they both underwent the same rate of decrease and the ratio 14C+/13C+ remained unchanged over the range of beam currents observed (70-100 μA of 12C-).
Ravi Prasad is a senior research scientist at the Center for Applied Isotope Studies at the University of Georgia. He manages the 500 kV CAMS and 250 kV SSAMS units at CAIS.