A Comprehensive Study of the Neutron Activation Analysis of Uranium by Delayed-Neutron Counting

Abstract

DISCLAIMERGindler'') lists eleven methods and reports on the range of quantity of uranium in micrograms for which each method is applicable. These methods and the lower limits of analysis are listed in Table I. An alternate approach to the neutron activat'ion analysis of uranium is to count the delayed neutrons emitted by the fission products of U-235. Echo and Turk(6) first described the application of this technique to the determination of By a systematic irradiation and counting procedure, they made determinations of u~~~ in a number of liquid samples containing known q~antities of which -2 varied from 10 to. lom3 micrograms. Three synthetic ore samples containing known amounts of $35 were also analyzed. They reported 0.05 microgram of u~~~ as a lower limit for a determination. They found the method to be rapid, and accurate, to require very little sample preparation, and to be free of any interferences except from those materials which have high neutron fission cross-sections (such as ?u 239 and $33) or high-neutron capture cross-sections (such as mie el'^) has made a more extensive study of delayed neutron emission for the determination of fissionable materials. He applied the method.to the determination of uranium in a large number of geolog'ical'~samp1es and to samples of pure uranium reagents, as well as the determination of the isotopic composition of a uranium sample. from the fission of $35, $33, and Pu239 by both thermal and fast neutrons and from the fission of U238 and d32 by fast neutrons.It was found that the delayed-neutron activity resulting from the fission of each of these nuclides could be resolved into six groups with fair agreement in the half-lives of corresponding groups.Slight differences in the half-lives of corresponding groups were obtained when-different nuclides were fissioned as well as when the method of fission was different, as for example, when U-235 was fissioned first with thermal neutrons and then with fast neutrons.Quite.large differences between corresponding groups for absolute group yields were obtained when different nuclides were fissioned.The data obtained, by Keepin, et a1, (9) for thermal neutron fission of 335, $33, and l?u239 is given in Table 111 Table IV lists An analysis for uranium is made by irradiating a test sample in a nuclear reactor for a period of time ranging from seconds to minutes, rapidly removing the sample to a neutron counting facility, allowilig b~b s o l u t e group y i e l d (dp) means t h e number of delayed neutrons per 100 f i s sians .t h e counter t o c o l l e c t counts f o r a period of time and recording t h e ' c o l l e c t e d count, A blank sample and a comparator sample a r e then t r e a t e d p r e c i s e l y a s was t h e t e s t sample. The blank sample u s u a l l y c o n s i s t s of an empty r a b b i t . In those cases i n which a uranium a n a l y s i s i s desired on a material t o which uranium has been added, t h e o r i g i n a l matrix material t o which no uranium has been added can serve a s the blank along wi...

Correction methods for uranium analysis by DNAA

“…The 236 U atom undergoes spontaneous fission to produce a number of prompt decay products, some of which remain in highly excited states and decay further by negatron (b -) emission [2]. These nuclei are known as delayed neutron precursors, since the negatron decay is followed by the instantaneous emission of a delayed neutron for which the half-life coincides with that of its precursor [2]. Table 1 lists the main properties of the six known delayed neutron groups [14] with uncertainties adopted from evaluation of delayed neutron data for the major actinides [15].…”

“…DNAA is highly sensitive [1,2] and very precise [2], and uses a relatively large sample size compared with other analytical techniques.…”

“…DNAA is also non-destructive [1] and can be used routinely to analyse large numbers of samples without requiring sample pre-treatment [2].…”

“…However, methods are not available in DNAA for samples that contain high uranium contents or for those containing high contents of REE. Consequently, in this work, a method has been developed to correct for the delayed neutron counting losses in materials with high uranium, using the delayed neutron emission calculation proposed by Dyer et al [2]. In addition, the thermal neutron self-shielding correction method developed by Chilian et al [11,12] was extended to uranium analysis by DNAA for samples containing REE.…”

Development of the DNAA System for Screening Environmental Samples to Measure fissile Material

“…Therefore, DNAA is virtually immune to the "matrix" effects that complicate other methods. 2) In this work, the DNAA system made up of pneumatic transport system, neutron detector assembly, and signal processing equipment was installed in HANARO under the DOE-MOST PCG (Permanent Coordinating Group) safeguards arrangement. This research reactor at KAERI has 30 MW maximum thermal power and sufficient neutron flux to detect less than one nanogram of the total fissile content.…”