The goal of this work is to elucidate critical points of failure of the Front-End Digitizer (FrEnD) under neutron irradiation. These data will inform the design of future iterations of the FrEnD system to improve its tolerance to ionizing and neutron radiation. Circuitry that is hardened to ionizing radiation (gamma, protons, energetic heavy ions) has undergone significant study in the commercial sector, mostly for space flight applications. However, the effects of neutron irradiation on electronics is not well understood, although it is critically important for further developments in nuclear and high-energy physics applications. The proposed work will generate significant data for the performance of commercial-off-the-shelf (COTS) circuit components to neutron irradiation.



The current version of the FrEnD acquisition system is designed to multiplex and transmit signals from up to seven sensors located in a high-neutron, ionizing radiation environment over a single optical fiber. The FrEnD system is designed to improve signal fidelity while minimizing the need for penetrations into nuclear containment structures. In its present design, FrEnD uses low-cost COTS components, including junction-gate field effect transistors (JFETs), which are inherently resistant to ionizing (gamma) radiation. However, these JFETs' resistance to neutron radiation is unknown. To characterize the tolerance of the FrEnD system to neutron radiation, three JFET-based printed circuit boards (PCBs), each 12x15 cm, will be placed in an irradiation standpipe at the North Carolina State University (NCSU) PULSTAR reactor. During irradiation, the PCBs will be powered, input signals will be provided to the circuitry, and relevant test points will be monitored through wired (twisted pair) connections to a Saleae data acquisition system. Irradiation standpipes at the PULSTAR have customize-able neutron fluxes ranging from 10^7-10^12 n/cm^2/s and a diameter of 16 cm. To reach a total fluence of 10^14-10^15 n/cm^2, the experiment will run for eight hours per day over the course of five days for a total of 36 hours. Reactor downtime each day is desired to measure any possible annealing effects that may occur in our circuitry and the behavior produced by that recovery.

The goal of the Front-End Digitizer (FrEnD) project is to design neutron and ionizing radiation-hardened (rad-hard) electronics to perform signal conditioning for an entire suite of nuclear sensors located in a high-radiation area and to transmit digitized sensor data over a single optical fiber. When successfully implemented, FrEnD will produce significant gains in measurement fidelity while decreasing the need for penetrations into containment structures. This makes FrEnD---which is currently funded by the Nuclear Energy Enabling Technologies (NEET) Advanced Sensors and Instrumentation (ASI) program---applicable across a broad range of US Department of Energy (DOE) Office of Nuclear Energy (NE) thrust areas. These areas range from enabling deployment of advanced nuclear reactors to enabling the continuing operation of existing US nuclear reactors through supporting modernization of the current fleet. For example, in microreactors, rad-hard electronics like FrEnD will be essential because microreactors will necessarily have less shielding. Therefore acquisition systems for nuclear sensors in microreactors must function much closer to the operating core. In its present design, FrEnD consists of ionizing rad-hard JFETs which have been shown to withstand >100 Mrad (Si) total ionizing dose, making FrEnD directly applicable to use for acquiring data from instrumentation in spent fuel casks, which is useful for both safety and safeguards applications. However, the JFETs neutron response is not so well defined. Therefore, analysis of neutron irradiation using FrEnD is an essential step to ensuring its ability to operate in and around nuclear reactors.