Traceability for High Energy Photon Dosimetry for Non-Intrusive Inspection Systems
Objectives for MPD E.4.0
- Clear and definitive standards are required to measure the performance of cargo inspection systems employing multi-energy high-energy X-ray photons to detect the presence of explosive materials in large target cargo containers.
- Standardize measurement methodologies are needed for detection rates for specific quantities of explosive materials.
Background: While standards exist for identifying and mitigating the threat posed by explosives that may be hidden inside of small targets such as check and carry-on baggage, significant standards development is required to assure the detection of explosive threats potentially hidden inside of large targets such as ISO cargo containers. Next generation baggage inspection systems will employ multi-energy low-energy X-ray photons (100 – 180 KV) generated using traditional X-ray tubes to discriminate explosive threat materials. Cargo inspection systems require much higher energy photons (4 – 10 MeV) to penetrate large cargo containers and multiple pulsed energies to effectively discriminate explosive materials. These multi-energy, high-energy X-ray photons are generated by bremsstrahlung conversion using electron accelerators which are capable of producing multi-energy pulses of high-energy electrons that are then directed onto high-Z targets creating bremsstrahlung photons proportional to the energy of the electron beam. Next generation and current x-ray inspection systems use density and/or atomic composition as primary indicators to discriminate explosive threats from most benign materials. When both of these attributes are known, they can provide a reasonable identifier of the presence of explosives. The density of potential explosive threat materials inside of a object to be inspected can be calculated based upon attenuated photon flux. However, density alone has limited value because observations correspond to averages over regions of space where a mix of low-, medium-, and high-density non-threat materials may coexist. The effective atomic number (Zeff) or the weighted mean of the atomic numbers of the elements in a compound. A surrogate material used in the performance standards for the next generation x-ray scanners should possesses the same attenuation properties as its parent compound over the photon energy region of interest. This can only be accomplished with a compound whose density and Zeff are nearly identical. As with density, measurements that average over a large region of space utilizing poorly selected or created surrogates may dilute the explosive signature with surrounding materials.
Low energy inspection systems are widely deployed with well-established performance standards for multiple energy material discrimination of explosive materials. While X-ray photon signatures required to determine the density and Zeff of explosive materials can be confidently measured at low energies, these signatures are relatively subtle and difficult to confidently measure at the high energies required to penetrate large targets such as a cargo container. New multi-energy, high-energy LINAC designs and detector materials are being sought to enhance the material discrimination capability in non-destructive imaging air, land and sea cargo inspection systems. The current generation of LINACS have high pulse-to-pulse variability which affects the ability to detect the subtle signature changes that occur when X-rays interact with explosive materials at high photon energies. The needed next generation of imaging interrogation LINACs must have minimal radiation leakage and high pulse-to-pulse reproducibility (low variability in end point energy and integrated pulse energy) to improve the detectability of explosive materials. This next generation of LINACs needs to have high beam on to beam off ratios with very little dark current for high fidelity measurements. Photon detectors used in these high-energy active interrogation techniques are count-rate limited, often relying on detectors that have fast recovery times and provide poor or no energy resolution. Advancements in detector technologies must facilitate higher count-rates. In addition to measuring precisely the quantity of photons, these new detectors should improve on one or more traditional detector parameters such as efficiency, energy resolution (or energy thresholding), size, cost, and photon/neutron discrimination. Clear and definitive standards are required to measure the performance of cargo inspection systems for the detection of explosives.
1 – Standards, such as those being developed for high energy imaging equipment through the American National Standards Institute (ANSI-N45.46) for measuring the performance of imaging X-ray systems for cargo security screening have to be established for explosive detection. Such standards should include the use of NIST traceable detector materials and explosive surrogates that provide X-ray and composition correct signatures for sensitivity analysis.
1 – A minimum of 2 NIST person-years per year over the next three year time period is required to launch into these objectives.
2 – Access to linear accelerators capable of producing multi-energy high-energy X-ray photons for development and verifications of detector materials and explosive surrogates.
3 - Partnerships between NIST and DHS are essential in this area.