Neutron-induced background in charge-coupled device detectors

The inertial confinement fusion (ICF) community must become more cognizant of the neutron-induced background levels in charge-coupled device (CCD) det ectors that are replacing film as the recording medium in many ICF diagnost ics. This background degrades the signal-to-noise ratio (SNR) of the record ed signals and for the highest-yield shots comprises a substantial fraction of the pixel's full well capacity. CCD detectors located anywhere in the O MEGA Target Bay are precluded from recording high precision signals (SNR > 30) for deuterium-tritium neutron yields greater than 10(13). CCDs make exc ellent calibrated neutron detectors. The average CCD background level is pr oportional to the neutron yield, and we have measured a linear response ove r four decades. The spectrum of deposited energy per pixel is heavily weigh ted to low energies, < 50 keV, with a few isolated saturated pixels. Most o f the background recorded by the CCDs is due to secondary radiation produce d by interactions of the primary neutrons with all the materials in the Tar get Bay as well as the shield walls and the floor. Since the noise source c omes from all directions it is very difficult to shield. The fallback posit ion of using film instead of CCD cameras for high-neutron-yield target shot s is flawed, as we have observed substantially increased fog levels on our x-ray recording film as a function of the neutron yield. (C) 2001 American Institute of Physics.