Experimental time resolved electron beam temperature measurements using Bremsstrahlung diagnostics

Abstract

Electron beam temperature, PL (= vL/v), is important to control for the development of high dose flash radiographic bremsstrahlung sources. At high voltage (> 5 MV) increasing electron beam temperature has a serious deleterious effect on dose production. The average and time resolved behavior of beam temperature was measured during radio~mphic experiments on the HERMES III accelerator (10 MV, 50 kA, 70 ns).A linear array of thermoluminescent dosimeters (TLDs) were used to estimate the time integrated average of beam temperature.On and off-axis photoconducting diamond (PCD) detectors were used to measure the time resolved bremsstrahlung dose rate, which is dependent on beam energy and temperature.The beam temperature can be determined by correlating PCD response with accelerator voltage and current and also by analyzing the ratio of PCD amplitudes on and off axis. This ratio is insensitive to voltage and current and thus, is more reliable than utilizing absolute dose rate. The data is unfolded using comparisons with Monte Carlo simulations to obtain absolute beam temperatures. The data taken on HERMES III show abrupt increases in !3~midway through the pulse indicating rapid onset of beam instability. I. EFFECT OF 9L ON X-RAY DOSEThe successful application of Inductive Voltage Adders (IVAS) to the development of high dose (= kRad at 1. m) flash x-ray radiography requires the production of high current, high voltage electron beams while maintaining low divergence (low beam temperature) [1]. The effect of beam temperature on dose can be seen in figure 1. This graph shows that at 10 MV the on-axis dose is reduced by 60% for an e-beam going from cold tõ L= 0.3 ( 17°divergence).These curves were compiled from several Monte Carlo [2] runs varying the electron beam temperature at 5 and 10 MV. It is, therefore, important to know the beam temperature in experiment in order to compare expected and measured dose, verify physical understanding of the radiographic diode, and examine possible instabilities of which increased PL is a symptom.