Nuclear Instrumentation

A detector is usable so long as it remains sensitive to neutrons and capable of transmitting sufficient signal to be used by the electronics. The sensitivity of the detector will gradually decrease over time as it is exposed to thermal neutron flux, and the detector will remain viable as long as the electronics allow for the decrease in sensitivity. For B-10 lined proportional counters the gas internal to the detector is vital to linear performance of the detector and is subject to degradation during operation in high radiation environments. Photonis proportional counters may include an additional reservoir of proportional gas which extends detector life (up to ~5 x 1018 n/cm2) substantially, making total lifetime greater than the life of similar detectors without this reservoir and significantly longer than BF3 counters.

Detector operation can be significantly affected as the ambient temperature they are exposed to increases above 400 C, by several different mechanisms. The internal gases used to ensure linear operation of the detector must be carefully chosen to ensure they can remain chemically unchanged at these high temperatures. Further, as temperature rises above 400 C the internal resistance of the detector may decrease, causing a rise in leakage current which could limit the functional range or decrease the accuracy of the detector. For these concerns Photonis has designed high temperature detectors, utilizing the appropriate gases, materials, and guard ring construction to allow accurate and reliable operation up to 600 C.

‘thermal’ and ‘fast’ refer to the energy level of the neutron incident on a detector. Thermal neutrons are typically defined as having energy <.025 eV. These lower energy neutrons are more likely to be absorbed by neutron sensitive materials, meaning thermal neutron detectors tend to have higher overall sensitivity. Thermal neutron detectors are designed to generate the majority of their usable signal from interactions with thermal neutrons, where fast neutron detectors are primarily capable of measuring high energy neutrons (>1 MeV). This has applications for research, where it may be important to use multiple detectors to differentiate between neutron energies, or in reactor applications where measurable neutron flux is primarily in high energy neutrons.

Each detector type offers unique characteristics which suit it to a particular application. B-10 lined proportional counters offer excellent sensitivity to thermal neutrons, allowing them to achieve sufficient count rates in low neutron fluxes. For example, this feature makes the use of B-10 proportional counters attractive as monitors during initial reactor criticality, where the increased detector sensitivity ensures that smaller, less expensive, start-up sources can be installed in the reactor core. Fission chambers, in contrast, have lower sensitivity to thermal neutrons but have superior resistance to gamma ray interference and offer the capability to operate over many decades of neutron flux. This feature makes fission chambers the ideal choice as wide range monitors of reactor power. The Photonis technical team is happy to work with customers to determine which detector type makes the most sense in each customer application.

Integrating a long mineral insulated cable with the body of the detector is ideal for the severe operating environments (high radiation level, humidity, vibration, high temperature) to maximize reliability and life of the instrument. Photonis recommend the use of integral cable for nuclear power applications to ensure trouble-free long-term operation. In mild environments, or in applications which otherwise do not permit integral cabling, Photonis is able to offer detectors with connection directly at the connector body.

Triaxial cables provide additional screening against unwanted sources of noise compared to coaxial cables- EMI/RFI interference and potential ground loops for small signals can be minimized, although this comes at a trade-off more difficult mechanical construction, lead-time, and cost. Typically, coaxial cables are sufficient for most nuclear applications but unusual, demanding, or unique applications may require the use of a triaxial cable solution. The Photonis Nuclear Instrumentation team has experience both working with our clients in determining the correct cable solution and qualifying the solution for use in industry.