Analysis of the optical coupling of wavelength-shifting fibers to organic liquid scintillator filled fluoropolymer tubes for industrial and nuclear security applications

[摘要] Industrial and nuclear security applications continue to push radiation detection development into new and exciting frontiers. In this work, an innovative detection module is developed and tested for use in a cosmic ray imaging (CRI) system designed for oil field characterization and is evaluated for its potential use in a fast neutron detection system for nuclear security applications. By measuring density changes in the reservoir, the CRI system will provide real-time information about steam chamber development during the enhanced oil field recovery process known as steam assisted gravity drainage (SAGD). The ability to monitor the development of the steam chamber region has the potential to provide important information, which could be used to optimize the growth and uniformity of the underground steam chamber and minimize costs. The organic liquid scintillator based detection modules also detect fast neutrons. During the initial characterization of an unidentified radioactive source, it is important to have the capability to determine if special nuclear material (SNM) is present and if it is configured to produce a nuclear yield. The emission of multiple neutrons during a single fission makes it possible to use this unique timing characteristic to identify SNM. The number of specialists trained to handle nuclear devices is limited making this determination a critical step in properly responding to the situation. The detector module consists of a 5 mm diameter by 2-meter long fluoropolymer tube filled with organic liquid scintillator (OLS), optically read-out using wavelength shifting (WLS) fibers. The 1:400 ratio of diameter to length makes light collection from the organic scintillator very challenging. Over ten configurations of OLS, fluoropolymer tubes, and WLS fibers were tested. The final configuration consisted of two 2mm BCF-91A WLS fibers optically coupled to the outside of an optically transparent fluorinated ethylene propylene (FEP) tube filled with a commercial OLS (EJ-309). Cosmic ray muons produce large light pulses in the OLS of which a portion reaches the external WLS fibers. The WLS fibers re-emits the light at longer wavelengths and acts as a multi-mode light guide channeling the signal to photomultiplier tubes located at each end of the WLS fibers. This module demonstrated excellent detection efficiency with less than 5% signal reduction, at any point along the module, due to optical attenuation. Timing analysis of the WLS fiber signals also provided coarse position determination, 40 cm, which opens design options not previously available. An important characteristic required of neutron detectors for nuclear security applications is the ability to discriminate fast neutron and gamma ray events. Initial tests have demonstrated the capability of our module to discriminate neutron and gamma rays by applying the rise time pulse shape discrimination (PSD) method to the WLS fiber signals. EJ-309 is well known for its PSD capabilities. Coupling this desirable characteristic with loss free, low attenuation optical read-out through a WLS fiber has the potential to broaden significantly liquid scintillator applications.

[发布机构] Massachusetts Institute of Technology