As a result of the NBSR upgrade to 20 MW, an extensive program of recharacterization of the irradiation facilities was undertaken. These measurements were made to (1) evaluate the neutron flux density and its variability over the irradiation volume, and (2) help establish safe irradiation conditions for samples in the NBSR. Techniques used to measure the following parameters are described: thermal neutron flux density; flux variations within an irradiation volume and between irradiations; relative fast neutron flux density; maximum temperatures generated by different sample matrices in various irradiation facilities; and pressures generated in sealed containers by radiolytic decomposition.
The availability of natural matrix reference materials evaluated for trace element content has resulted in their widespread use as standards (i.e., calibration materials; comparators) for instrumental neutron activation analysis (INAA). Due to the uncertainties associated with their certified values, the limited number available, and the relative matrix independence of INAA, these reference materials are more properly utilized as quality assessment materials, after calibration of the INAA analytical system with true primary standards. Terminology is defined, the use of matrix reference materials to evaluate the analytical system is discussed, techniques for the accurate preparation of primary standards for trace element analyses are reviewed, and necessary precautions in the accurate comparison of samples to standards are presented.
The NIST nuclear reactor irradiation facilities used for neutron activation analysis have been well characterized for absolute neutron flux measurements, variations within each irradiation facility, neutron energy measurements of each facility, and temperature and pressure measurements. This has been accomplished with various sample materials and under a range of conditions including biological samples inside of sealed quartz ampoules, and for radiation damage measurements on the polyethylene rabbits used for all pneumatic tube irradiations. As a result of all these measurements, initiated in 1969 and still continuing, the NBSR irradiation facilities have become the most highly characterized NAA irradiation facilities in the world. This paper describes the methods and techniques developed and used for characterization of the NBSR irradiation facilities.
Recently a new rapid pneumatic tube facility was inserted into a long unused location in the NIST 20 MW nuclear reactor. This
facility was designed and constructed specifically for rapid INAA using short lived activation products. Included is a computer
controlled console which uses fast sensors to accurately measure the irradiation capsule flight time, and a loss-free counting
system connected to a 32% efficient PHGe detector with a transistor reset preamplifier. Measurement of travel, times from
end-of-irradiation to detector were 473±8 ms. Measurement of the thermal neutron fluence rate was 5.0·1013 n·cm−2·s−1. The other three pneumatic tubes in the NIST reactor have transfer times of 3 to 15 seconds, and no timing capability more
accurate than human response. This new facility substantially improves our ability to accurately determine activation products
with half-lives from 1 to 100 seconds. Characterization information reported on this new irradiation facility includes absolute
fluence measurements, fluence rate variations within the capsule and variations with time, and determination of analytical
sensitivities for fluorine-20 selenium-77m, and silver-110g.
The ASTM Task Group on Nuclear Methods of Chemical Analysis (E10.05.12) has conducted a trace element intercomparison among some of its members over the past two years. Eight non-NIST laboratories submitted data using nuclear techniques, with a total of 111 values for the apple leaves and 116 values for the peach leaves, on 46 and 50 elements, respectively. This intercomparison provided a unique opportunity for the analytical laboratories, because the analytical values submitted could be later compared to the NIST certified values. For the seven elements which were certified by NIST and had three or more intercomparison values, the results showed that: 1) 61% of all 56 intercomparison values submitted had results whose stated uncertainty overlapped the uncertainty limits of the NIST certified values, and 2) less than 6% of the intercomparison values had means which fell outside ±20% of the NIST values. In general, the intercomparison values submitted showed excellent agreement with the NIST values. However, many reported uncertainties accompanying intercomparison values appeared overly optimistic.
During the author's 39 years with the National Bureau of Standards (NBS)/National Institute for Standards and Technology (NIST)
as an employee, and since then as Guest Researcher, he has been intimately involved with biological Standard Reference Material
(SRM) production and analyses. His involvement with biological reference materials started with the very first biological
certified reference material (CRM), the SRM 1571, Orchard Leaves, initiated in 1968 and issued in 1971, through the latest
material (SRM 1575a, Pine Needles - renewal), issued in 2003. In addition, for more than 20 years he was Technical Coordinator
for botanical SRMs for the NBS/NIST Analytical Chemistry Division. This paper contains his historical reflections and highlights
from those years, and includes the techniques used to obtain and process these materials, new developments and procedures
that resulted in vastly improved reference materials, the application of high accuracy neutron activation analysis to the
certification of these standards, and the trace element quality assurance vital to the accuracy of these standards.
Ouelques caractéristiques du réacteur nucléaire NBSR utilisé pour l'analyse par activation neutronique ont été évaluées; les
techniques de mesure développées sont décrites. La discussion porte sur les caractéristiques suivantes: (1) le flux de neutrons
thermiques absorbés par le cadmium, (2) la distribution énergétique des neutrons mesurée grâce aux rapports cadmium et à des
détecteurs à seuil. L'excès de pression dans les échantillons engendre l'irradiation.
A radiochemical neutron activation analysis (RNAA) combustion method coupled with a neutron exposure normalization technique was used to determine low g/kg mercury levels in three National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs). Two coals (sub-bituminous and bituminous) and a diet material were analyzed. The results obtained provided recommended values of approximately 5 g/kg for SRM 1548a Typical Diet, 24 g/kg for SRM 1635 Trace Elements in Coal (sub-bituminous), and 100 g/kg for SRM 1632b Trace Elements in Coal (bituminous).
The National Institute of Standards and Technology (NIST) recently released the second renewal of its Trace Elements in Coal Fly Ash Standard Reference Material (SRM 1633b). This new material is currently certified for 23 major, minor and trace elements, and concentrations of an additional 24 elements are provided for information only purposes. Current plans are to certify the concentrations of a number of rare earths upon completion of additional analytical work now in progress. Instrumental neutron activation analysis (INAA) has played a major role in the certification of this new material in view of its potential for accuracy, multielemental capability, ability to assess homogeneity, high sensitivity for many elements, and essentially blank-free nature. For an element to be certified in a NIST SRM its concentration is usually determined by at least two independent analytical techniques. INAA has provided analytical information for 15 of the 23 elements certified, as well as for 22 of the 24 elements listed for information only. In addition, INAA has provided much of the homogeneity information for this SRM. This paper will describe these analytical procedures, and highlight those designed to optimize and assess the accuracy of the INAA measurements.
Neutron activation analysis is one of many analytical techniques used at the National Institute of Standards and Technology (NIST) for the certification of NIST Standard Reference Materials (SRMs). NAA competes favorably with all other techniques because of it's unique capabilities for high accuracy even at very low concentrations for many elements. In this paper, instrumental and radiochemical NAA results are described for 25 elements in two new NIST SRMs, SRM 1515 (Apple Leaves) and SRM 1547 (Peach Leaves), and are compared to the certified values for 19 elements in these two new botanical reference materials.