Electron-beam optimization for bremsstrahlung-induced photofission

There is ongoing interest in the use of an intense bremsstrahlung pulse to induce photofission in fissionable material.^{1, 2} To optimize the radiation for inducing fissions in the forward direction, electrons should approach normal incidence and their charge on the anode converter should be maximum. Here, these are optimized on the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder³ by varying the diode AK gap between 23 and 43 cm and adding an ID-reducing insert in the outer-conductor wall.² Current monitors at the anode determine the load-region current fraction reaching the converter. TLDs and an x-ray pinhole camera measure the angular and radial x-ray dose distributions⁴, and ³ He detectors measure fissions induced in a 30×30×2.5-cm³ depleted uranium (DU) plate. Measurements are in broad agreement with LSP/ITS simulations of the electron dynamics and radiation transport. Current traces show that electron losses to the insert and outer-conductor wall increase with AK gap, confirming LSP predictions and pinhole camera measurements. With increasing AK gap, measured angular dose distributions narrow, on-axis dose and DU fission-neutrons increase, indicating that electron angles are approaching the converter normal. With the 43-cm gap, axial dose and fission neutrons decrease. For 40-cm gaps or larger, LSP electron impact angles on the converter are within 5 degrees of normal. For 8-MeV electrons, x-ray divergence does not narrow significantly for smaller electron angles. However, electron losses to the insert and wall continue to increase for larger gaps, explaining reduced performance at 43 cm. Bremsstrahlung produced by these electrons at large radius is collimated out of the x-ray beam reaching the DU. If the current lost to the insert and walls could reach the converter, LSP/ITS predicts a fission yield 1.75-times higher than that achieved with a 40-cm gap. Though wall losses wi- h a 23-cm gap are small, the predicted 20-deg electron impact angles produce only about 1/3 of the on-axis fissions of the 40-cm gap. Therefore, research has begun to modify such smaller-gap diodes to magnetically steer electrons to converter impact angles closer to the normal.⁵