R. Gijbels

An oxygen standard for the determination of oxygen in steel by 14-MeV neutron activation analysis

Abstract A relative method for the determination of oxygen in steel via the 16O(n,p)16N reaction by means of 14-MeV neutrons is described. A standard is irradiated immediately behind the sample and the induced activities are counted simultaneously with two separate but identical detector systems. The standard mixture (ca. 5 g of graphite plus iron oxide containing 80 mg of oxygen per g) is compressed into a steel capsule of the same external dimensions as the samples (26 mm diameter, 9 mm thick). Dimensional tolerances, choice and purity control of the oxygen compound and preparation of the standard mixture are discussed. Fast neutron shielding, absorption of fast neutrons, self-absorption of the 16N /gg-rays and the average neutron flux in sample and standards are considered and a total correction factor is established. Flux inhomogeneities and differences in counting geometry and discriminator setting can be determined by irradiation and counting of two identical standards. The accuracy of this method was checked by comparison of the results with those of the reducing fusion method; satisfactory agreement was observed, although the activation results tended to be slightly higher. The mean long-term standard deviation for analysis of a given sample over a period of 6 months was found to be ±3%.

Practical aspects of neutron activation determination of the platinum metals

Triple gamma coincidence counting of192Ir allowed the determination of Ir by instrumental neutron activation analysis down to 1 ppb in ultrabasic rocks and down to ca. 20 ppb in some high-furnace slags; the limiting factor for the latter matrix was the presence of124Sb. Radiochemical neutron activation analysis of the USGS standard rocks revealed that the Ir contents are up to three orders of magnitude lower than previously reported, except for the ultrabasic rocks. The factor of merit of several scintillation and semiconductor, gamma-ray detectors was determined for the neutron activation determination of Pd, Pt and Os. In the case of radiochemically pure sources, a NaI(Tl) wafer was preferred; in the presence of high-energy gamma-emitters, a Ge(Li) low-energy photon detector was superior.

The determination of traces of osmium in ruthenium sponge by neutron activation analysis

Abstract Tracer techniques confirmed that a quantitative separation of osmium and ruthenium is possible by distillation from a hydrogen peroxide-sulphuric acid solution. Osmium distils quantitatively as OsO4 at a temperature of 105 ± 5° in about 30 min. The ruthenium contamination is approximately 0.01 %. In the present work a neutron activation analysis is described for the determination of traces of osmium in ruthenium sponge. When quantities of osmium below 30 p.p.m. are determined, the ruthenium contamination of the distillate must be taken into account, when the measurement is made with a 3” x 3” NaI(Tl) crystal. This can easily be achieved by measurement in two energy regions with a γ-spectrometer or with a multichannel pulse-height analyzer. With a NaI(T1) wafer as detector, the correction for ruthenium can be omitted for osmium concentrations above 10 p.p.m. With the addition method of analysis, 10–2000 p.p.m. in 10-mg samples of ruthenium sponge can be determined by neutron activation analysis. Chemical separation is necessary but no carriers are required. The lowest limit of determination is about 3 p.p.m. for a 3” x 3” crystal; for the wafer, about 1 p.p.m. can be determined.

The simultaneous determination of ruthenium and iridium in osmium sponge by neutron activation analysis

Abstract A radiochemica1 separation procedure for Os-Ru-Ir is decribed based upon selective distillation. In sulfuric acid-hydrogen peroxide 99.999–99.9999% of the osmium can be distilled if reoxidation of the osmium with permanganate is applied. From sulfuric acid-sodium bromate solution more than 99% of the ruthenium activity can be recovered. More than 99.995% of the iridium remains in the residue. The method was applied to neutron activation analysis of Io-mg samples of osmium sponge and allows the determination of traces of ruthenium down to approximately ro p.p.m. and of iridium down to 0.5 p,p.m. For lower iridium contents, second-order interference of the osmium must be taken into account, the error being approximately 0.05 p.p.m. after an irradiation period of 5 days at a flux of 4–1011 n/sec/cm2.

The determination of silicon in steel by 14-mev neutron activation analysis

Abstract A fast (2–5 min) non-destructive determination of silicon in steel by 14-MeV neutron activation is described. The 1.78-MeV 28Al activity, induced by the reaction 28Si(n,p)28Al, is counted on a NaI(Tl) detector. An oxygen flux monitor is used to normalise to the same neutron flux. Two methods are described to correct for the 56Mn activity (2.58 h), induced into the iron matrix via 56Fe(n,p)56Mn. Nuclear interferences of phosphorus and aluminium have been examined. Special attention has been paid to stainless steels. A sensitivity of 0.02 to 0.05% of silicon is obtained. The precision is 2 to 3% for steels containing above 1% silicon, and 7% for 0.1% of silicon.

Experimental comparison of multi-channel analyzer. Dead-time corrections for short-lived radionuclides

The accuracy of the live-time circuit of a 400-channel analyzer was studied in detail, and was found to be unsatisfactory even for long-lived radionuclides. It was found that automatic live-time correction with the multi-channel analyzer gave rise to increasing positive errors with increasing count rate; this overall positive error was composed of a positive error due to the slowness of the electronic circuitry, and a smaller negative error due to the finite pulse-width. Adequate correction could be performed by feeding the information from the dead-time output of the multi-channel analyzer to an external live-time circuit with variable oscillator frequency and pulse-width. Four methods for dead-time correction were compared experimentally in the case of short-lived radionuclides (T as low as 7 sec): the method of Bartošek et al., the method of Schonfeld, the use of a sufficiently short counting time as compared to the half-life, and the live-time mode of counting without additional correction. These four methods were applied to the determination of oxygen and silicon in rocks by 14 MeV neutron activation analysis. Results are given for USGS standard rock G-2.

The determination of traces of iridium in rhodium by neutron activation analysis

Abstract Iridium traces were determined in rhodium by thermal neutron activation for 2 days at a flux of 4.1011 n·cm-2·sec-1. After cooling for at least 1 week, the samples were analysed by γ-spectrometry. No radiochemical separations were required. Special attention was paid to self-absorption phenomena in the rhodium samples.

Systematic errors in 14-MeV neutron activation analysis for oxygen : Part II. A general standardization method for the determination of oxygen

Abstract A general standardization method is described for the determination of oxygen in solid samples via the 16 O(n,p) 16 N reaction. Two systems of flux monitoring are considered: the sample versus standard comparator method and BF 3 monitoring. The average flux in sample and standard, fast neutron shielding, fast neutron scattering, absorption of fast neutrons, absorption of 16 N γ-rays and counting efficiency of sample and standard are considered. The influence of the target diameter on the obtained correction factors has also been studied. Total achievable accuracy is believed to be about 1%.

INTERNAL STANDARD ACTIVATION ANALYSIS OF SILICON IN STEEL.

Abstract Non-destructive 14-MeV neutron activation analysis for silicon in steel has been applied with 56Mn as internal standard.56Mn is formed from the iron matrix via the 56Fe(n,p)56Mn reaction. Several methods of internal standardisation via56Mn are discussed. The 0.84-MeV photopeak of 56Mn is recommended if steel samples of about the same composition are to be analysed. Chemically analysed steel samples are used as silicon standards. A precision of 0.7% was obtained for an analysis plus standardisation time of 13 min. Special attention was paid to interferences produced by concentration changes of impurity elements. Several possible sources of errors were investigated.

Systematic errors in 14-MeV neutron activation analysis for oxygen: Part I. Neutron and γ-ray attenuation effects

Abstract A detailed account is given of neutron and γ-ray attenuation effects in 14-MeV neutron activation analysis of oxygen. Appropriate neutron cross-section values have been determined in two different ways and compared with literature values. It appears that the attenuation process is best described in terms of nonelastic scattering cross-sections. It is also shown that the narrow beam total γ-ray attenuation coefficients at 6 MeV, given in the literature are suitable for correction purposes if 16N γ-rays are counted with a window of 4.5–6.5 MeV. Attention was paid to the contribution of β-rays when the 16N activity is counted in this energy interval with a NaI(Tl) detector.