Relative efficiency calibration for determining isotopic composition and age of HEU items by passive non-destructive gamma spectrometry.

The observation of the 239 keV gamma line from 232U decay in high-enriched uranium (HEU) samples prompted us to utilize 232U (with main gamma energies of its daughters at 239, 583, 763, and 860 keV) as a mediator isotope for performing relative (intrinsic) efficiency calibration among 232U, 234U, 235U, 238U, and 214Bi isotopes. By this way, the isotopic composition and age of HEU samples can be derived from activity ratios constituted between pairs of these isotopes on the basis of a common relative efficiency calibration curve. The method proved to be a useful tool for γ-spectrometric characterization of HEU items, especially of weapon grade, shielded uranium material.

Gamma spectrometry in the ITWG CMX-4 exercise.

Low enriched uranium samples of unknown origin were analyzed by 16 laboratories in the context of a Collaborative Materials Exercise (CMX), organized by the Nuclear Forensics International Technical Working Group (ITWG). The purpose was to compare and prioritize nuclear forensic methods and techniques, and to evaluate attribution capabilities among participants. This paper gives a snapshot of the gamma spectrometric capabilities of the participating laboratories and summarizes the results achieved by gamma spectrometry.

Quantitative NDA of isotopic neutron sources.

A non-destructive method for assaying transuranic neutron sources was developed, using a combination of gamma-spectrometry and neutron correlation technique. Source strength or actinide content of a number of PuBe, AmBe, AmLi, (244)Cm, and (252)Cf sources was assessed, both as a safety issue and with respect to combating illicit trafficking. A passive neutron coincidence collar was designed with (3)He counters embedded in a polyethylene moderator (lined with Cd) surrounding the sources to be measured. The electronics consist of independent channels of pulse amplifiers and discriminators as well as a shift register for coincidence counting. The neutron output of the sources was determined by gross neutron counting, and the actinide content was found out by adopting specific spontaneous fission and (alpha,n) reaction yields of individual isotopes from the literature. Identification of an unknown source type and constituents can be made by gamma-spectrometry. The coincidences are due to spontaneous fission in the case of Cm and Cf sources, while they are mostly due to neutron-induced fission of the Pu isotopes (i.e. self-multiplication) and the (9)Be(n,2n)(8)Be reaction in Be-containing sources. Recording coincidence rate offers a potential for calibration, exploiting a correlation between the Pu amount and the coincidence-to-total ratio. The method and the equipment were tested in an in-field demonstration exercise, with participation of national public authorities and foreign observers. Seizure of the illicit transport of a PuBe source was simulated in the exercise, and the Pu content of the source was determined. It is expected that the method could be used for identification and assay of illicit, found, or not documented neutron sources.

Revealing smuggled nuclear material covered by a legitimate radioisotope shipment using CdTe-based gamma-ray spectrometry.

Illicit trade of nuclear materials (NM) represents a serious challenge to radiation monitoring upon scenarios, when legitimate radioisotope shipments are used to obscure the weak radiation of NM. Planar and hemispherical Cd(Zn)Te detectors with a portable mini-multichannel analyzer were proven to be suitable, in measuring times of 10min order, for revealing the presence of low-enriched or natural U-bearing reactor fuel pellets in amounts of kg order, placed beside transport containers of lead or depleted uranium, which contain high activity 60Co (10GBq range) or 192Ir (TBq range) radioisotope sources. Such a hand-held or portable device may help authorities combating illicit trafficking of nuclear materials.

Monitoring bremsstrahlung from 4 MeV electrons by nuclear excitation and Al2O3 thermoluminescence dosemeters.

Isomer excitation by gamma,gamma' reactions and aluminium oxide thermoluminescence dosemeters (TLDs) have been used to monitor bremsstrahlung from the 4 MeV electron beam of a linear accelerator type LPR4 produced on a 0.9 mm Pt converter foil. Natural indium and osmium as well as TLDs have been irradiated at different distances (2-11 cm) and angles (0 degrees-90 degrees). Dose rates measured by TLDs were 5-110 kGy x h(-1). Isomer excitation of 115In (half-life 4.5 h) was used for monitoring bremsstrahlung of energies above 1 MeV, while that of 189Os (half-life 5.8 h) extended the available range down to 200 keV. Isomer production yields measured by gamma spectrometry and found to be about 10(-19)-10(-18) Bq per nucleus were calibrated against dose rate. A graphical method based on a semiempirical formula was used to evaluate the bremsstrahlung flux as well as the dose rate from the activity of isomeric monitors with uncertainties below 20%. The method is simple, of linear response in a large scale, independent of temperature, and able to monitor extremely high gamma intensities.

Verification of 235U enrichment of fresh VVER-440 fuel assemblies.

Enrichment of uniformly and non-uniformly enriched ("profiled") fuel assemblies in a range of 1.6-4.4% was verified by gamma-ray spectrometry at a nuclear power plant (NPP). HPGe detectors and a CdZnTe (CZT) detector, the latter fitting into the central tube of the assemblies, were used for obtaining information from outer and inner fuel rods. A procedure which has minimal impact on the NPP work was developed for verifying freshly arrived assemblies under normal operational conditions, and is now in routine use.

Neutron interrogation of shielded/unshielded uranium by a 4 MeV linac.

A non-destructive active assay method was developed for revealing illicit trafficking of uranium. Photoneutrons produced in beryllium or heavy water by bremsstrahlung from a linac induced fission in the samples. Delayed fission neutrons were detected by a neutron collar built up of (3)He counters embedded in polyethylene moderator. High-enriched uranium samples shielded and unshielded by lead up to 14 mm thickness were detected, with a performance practically unaltered. 25 mg (235)U can be revealed in a 1 min interrogation time.