X-ray dose distribution measurements for electron-beam optimization on the Mercury inductive voltage adder

Summary form only given. An intense bremsstrahlung x-ray pulse is generated by the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder.¹ A study of the diode configuration was undertaken to optimize the forward-directed radiation. To this end, the diode AK gap was varied between 23 and 43 cm and an ID-reducing insert in the vacuum chamber wall was added to adjust the incidence angle and the electron charge at the tantalum anode converter.². Anode current monitors measure the portion of load-region current reaching the converter. Arrays of CaF₂ TLDs, x-ray pin diodes and an x-ray pinhole camera are used to measure the x-ray dose distributions. Anode current data are presented which show that electron losses to the insert and to the outer-conductor wall increase with AK gap, in agreement with pinhole camera measurements. Pin diode signals are analyzed to determine beam dynamics during the pulse. TLD data are presented which show that, as the AK gap increases, the angular dose distribution narrows and that the on-axis dose increases, until the 43-cm AK gap configuration. Here, axial dose decreases as electron losses to the insert and vacuum chamber wall override any further benefit due to smaller incidence angle for electrons striking the anode. Bremsstrahlung produced by electrons at large radius impacting the walls is removed from the x-ray beam by a thick steel collimator. Though wall losses with a small, 23-cm gap are low, LSP/ITS simulations predict large electron impact angles, which would reduce the on-axis dose downstream of the diode. Research has begun to modify such smaller-gap diodes to magnetically steer electrons to converter impact angles closer to the normal³.