FLASH Memory for Detection of U235, and Pu239 Fissile Isotopes

Richard Blish and Tim Hossain
Advanced Micro Devices, Inc.
Tuesday, March 25, 2003

Abstract:

Widely used radiation detectors such Geiger-Counters cannot detect important fissionable materials such as U235 and Pu239. Alpha radiation and low energy gamma rays emitted from these isotopes are easily stopped in the packaging, so detection relying on these emissions is not very effective, particularly for inspection purposes. An alternative method would utilize the emission of neutrons from spontaneous fission. Penetrating neutrons (common sources might be Am-Be, Pu-Be or Cf252 can be picked up outside the packaging with a sensitive neutron counter. A new and innovative neutron detector based on a FLASH Memory cell has been patented by AMD (6,075,261 is available for license). A special manufacturing process is utilized to modify one of the dielectric layers so that "Bit Flip" in the memory cell is induced by an incoming neutron. The inherent BPTEOS layer is deposited using B10 isotopes instead of natural Boron. The neutron capture cross-section of the B10 is very large, thus a neutron is readily absorbed by the BPTEOS layer. The result of the neutron capture leads to the break up of the boron nucleus into two particles, Li7, and He4 (alpha particle). Alpha particles generated in-situ are injected into the nearby floating gate of the Memory cell. This causes the "Bit Flip" and indicates a neutron event. A very sensitive U235, and Pu239 detector can be manufactured using a modified FLASH memory technology.