The analysis and attribution of the time-dependent neutron background resultant from sample irradiation in a SLOWPOKE-2 reactor
- 198 Downloads
The Royal Military College of Canada (RMCC) has commissioned a Delayed Neutron Counting (DNC) system for the analysis of special nuclear materials. A significant, time-dependent neutron background with an initial maximum count rate, more than 50 times that of the time-independent background, was characterised during the validation of this system. This time-dependent background was found to be dependent on the presence of the polyethylene (PE) vials used to transport the fissile samples, yet was not an activation product of vial impurities. The magnitude of the time-dependent background was found to be irradiation site specific and independent of the mass of PE. The capability of RMCC’s DNC system to analyze the neutron count rates in time intervals <1 s facilitated a more detailed data analysis than that obtained in previous DNC systems recording cumulative neutron counts. An analysis of the time-dependent background behaviour suggested that an equivalent of 120 ng of 235U contamination was present on each irradiated vial. However, Inductively Coupled Plasma—Mass Spectroscopy measurements of material leached from the outer vial surfaces after their irradiations found only trace amounts of uranium, 0.118 ± 0.048 ng of 235U derived from natural uranium. These quantities are insufficient to account for the time-independent background, and in fact could not be discriminated from the noise associated with time-independent background. It is suggested that delayed neutron emitters are deposited in the vial surface following fission recoil, leaving the main body of uranium within the irradiation site. This hypothesis is supported by the physical cleaning of the site with materials soaked in distilled water and HNO3, which lowered the background from a nominal 235U mass equivalent of 120 to 50 ng per vial.
KeywordsDelayed neutron counting Neutron background attribution Uranium Nuclear forensics
The thoughtful input from Dr. Gregory Kennedy from École Polytechnique on the source of contamination in the SLOWPOKE-2 irradiation site is appreciated. The authors would also like to thank Dave Ferguson, Kristine Mattson, Kathy Nielsen and Steve White for their assistance in experimentation. Funding for this project was provided by the Director General of Nuclear Safety and the Natural Sciences and Engineering Research Council of Canada.
- 1.Duke MJM, Taylor CAJ, Fernando A, (2000) Uranium analysis of ores and tailings: a comparison of delayed neutron counting, natural gamma-ray spectrometry, INAA and XRF, 3rd international conference on isotopes: isotope production and applications in the 21st century. doi: 10.1142/9789812793867_0025
- 3.Musilek A, Buchtela K, Grass F (1996) Trace Microprobe Tech 14(1):29–36Google Scholar
- 8.Lindstorm RM, Glasgow DC, Downing RG, (2006) Trace fissile measurement by delayed neutron activation analysis at NIST, American Chemistry Society Meeting, September 10–14Google Scholar
- 9.Rosenstock W, Köble T, Risse M, Berky W, (2009) Detection of concealed fissionable material by delayed neutron counting, international topical meeting on nuclear research applications and utilization of accelerators, May 4–8Google Scholar
- 10.Ni B, Xiao C, Huang D et al (2012) Radioanal Nucl Chem 291(2):313–319Google Scholar
- 14.Keepin GR, Wimett TF, Zeigler RK (1957) Phys Rev 107(4):1044–1049Google Scholar
- 15.Handbook of Nuclear Data for Safeguards (2008) International Atomic Energy Agency, Vienna. http://www-nds.iaea.org/sgnucdata/safeg2008.pdf. Accessed 16 Feb 2011
- 17.Andrews WS (1989) Thermal neutron flux mapping around the reactor core of the SLOWPOKE-2 at RMC, MASC Thesis, Royal Military College of CanadaGoogle Scholar