Measurements of Ionizing Radiation Doses Induced by High Irradiance Laser on Targets in LCLS MEC Instrument - Paper 75

High-power laser interaction with targets in vacuum at high laser irradiance levels can generate plasma and, consequently, create ionizing radiation hazards. The theory for ionizing radiation generation from laser-plasma interaction is complicated. The basics of theory and phenomenology are available qualitatively. However, the ionizing radiation yields associated with laser-plasma interaction have not been well quantified. Based on the theoretical analysis and review of the published reports and measurement data, SLAC National Accelerator Laboratory (SLAC) developed an analytic model to estimate conservatively the electron temperature and photon radiation yield (Sv/J) of a laser pulse hitting a solid target at 1 m as a function of laser irradiance between 10¹⁵ and 10²¹ W/cm². Ionizing radiation measurements from high-intensity laser-induced plasma interactions on solid targets inside the Al-walled vacuum chamber of the LCLS MEC instrument at SLAC were conducted using passive and active detectors at irradiance between 3x10¹⁶ and 6x10¹⁷ W/cm². Each run of the more than 20 runs had a unique set of laser, optic and target conditions with laser pulses rastering over the target surface mainly perpendicularly at a fixed irradiance. Significant effort was made in characterizing the laser irradiance values for each configuration of laser, focusing mirror, and target; four irradiances with targets of Au foils and copper plate were found to produce good measurement results. Electron doses inside the target chamber were highly anisotropic and peaked at backward direction by several orders of magnitude. Photon doses outside the target chamber were nearly isotropic (a factor of ∼2 higher in backward direction). When considering the large uncertainties of the irradiance (a factor of 2-3) and the potential shielding effects from detectors and target chamber, the measured photon dose rates at 1 m (150 mJ and 10 Hz) outside the target chamber (when averaged over different types of detectors) were in good agreement (a factor of ∼3) with the calculated values of the SLAC model, except at 3.0x10¹⁶ W/cm², which has an electron temperature of only 14 keV. At 6x10¹⁷ W/cm², a low level of ∼0.1 mrem/h from fast neutrons was measured (at 1 m, 150 mJ and 10 Hz) with a neutron-to-photon dose ratio of 0.2%. This is equivalent to a neutron dose yield of 2.8x10^-8 mSv/J (to be compared to the LULI measurements of 1x10^-4 mSv/J at 3x10¹⁹ W/cm²). Both active and passive types of detectors performed well and the dose results were consistent, which is the same as the dose measurements in the Titan laser facility. Active detectors were not affected by the electromagnetic pulses associated with the MEC laser system; this was different from the Titan radiation measurements, in which all active detectors failed to respond, likely due to the high EMP in the Titan's high laser intensity fields. (authors)