Determination of trace impurities in gallium arsenide by NAA

GaAs is not an ideal matrix for INAA because elements yielding activation products with half-lives up to about 5 d cannot be measured due to the interference by⁷²Ga and⁷⁶As (t_1/2=14.1 h and 26.4 h, respectively). The measurement of radionuclides with longer half-lives is interfered with by⁷⁴As (t_1/2=17.7 d), generated by fast neutrons. However, using an irradiation facility with a very low flux of fast neutrons, in which the generation of⁷⁴As is minimal, five elements could be determined in GaAs (Cr, Co, Zn, Ag, and Hg). For 27 elements the detection limits were below 1 g/g and for ten of them below 10 ng/g. The determination of nitrogen in GaAs has been carried out using the (n, p)-reaction on¹⁴N, which is induced by thermal neutrons. The activation product,¹⁴C, can be effectively separated and purified via¹⁴CO₂ and counted with high efficiency in a liquid scintillation counter, and nitrogen can be determined with fairly low detection limits if sufficiently high neutron fluxes and long irradiation times are applied. The procedure described is based on a reactor irradiation with a thermal flux of 2·10¹⁴ n·cm^–2·s^–1 for 51 days. 0.16±0.09 g/g N in GaAs were determined and the detection limit was about 3 ng/g.     

Estimation of the possibilities of a neutron generator-based source of thermalized neutrons for activation analysis

Characteristics of a source of thermal neutrons based on an evacuated NG-400 neutron generator with the maximum flux (Φ_f) 2 × 10¹¹ neutron/s for 14 MeV neutrons and 2 × 10⁹ neutrons/s for 3 meV neutrons have been investigated. The possibilities of its application for neutron activation analysis have been estimated. The distribution, composition, and density (φ_T) values of the thermal neutron flux have been measured in the inner cavity of the moderator using activation detectors. φ_T was 2 × 10⁸ and 2 × 10⁶ neutrons/cm² s for thermalized neutrons with energies of 14 and 3 MeV, respectively. The possibilities of the apparatus have been estimated theoretically and experimentally for the cases of thermalized neutrons of 14 MeV and 3 MeV.     

Self-shielding and burn-out effects in the irradiation of strongly-neutron-absorbing material

Self-shielding and burn-out effects are discussed in the evaluation of radioisotopes formed by neutron irradiation of a strongly-neutron-absorbing material. A method of the evaluation of such effects is developed both for thermal and epithermal neutrons. Results of the calculation are discussed in comparison with the observed yields of¹⁵³Gd,¹⁶⁰Tb, and¹⁶¹Tb for the case of neutron irradiation of disc-shaped targets of gadolinium oxide.     

Reactor neutrons irradiation effect on the photocatalytic activity of SnO<Subscript>2</Subscript> thin films

SnO₂ thin films synthesized by sol-gel are irradiated with reactor neutrons up to fast neutron fluence of 9.6 × 10¹⁷ neutrons cm^–2 at 40°C. The influence of defects generated by neutrons irradiation, through the properties modification, on the photo-catalytic activity of SnO₂ films is investigated. It is found that the photoactivity of the irradiated films is enhanced after reactor neutrons irradiation. An improvement of 41% is observed for the sample irradiated at a neutron fluence of 9.6 × 10¹⁷ neutrons cm^–2. This is attributed to several parameters modified by the reactor neutron irradiation principally the crystallite size and space charge region which are closely related to the photocatalytic performance.     

Second-order interference in the neutron activation analysis of arsenic in a germanium matrix

The second-order interference ⁷⁴Ge(n, γ; β^-, n, γ)⁷⁶ As can occur in the activation analysis of arsenic in a germanium matrix, using thermal neutrons. As the literature data show poor agreement, this interference was determined experimentally. A practical formula was derived, for irradiation times longer than 2 h, which showed that the interference, expressed as an apparent arsenic concentration, is proportional to the neutron flux. Experiments were performed for irradiation times of 10, 15 and 20 h at a neutron flux of 10¹⁴ n/cm²/sec, yielding apparent arsenic concentrations in the germanium matrix of respectively 223, 408 and 597 p.p.b. From these results a value of 0.48 ±0.06 barn could be calculated for the activation cross-section of ⁷⁴Ge for neutron capture.     

Fast neutron activation analysis of silicon in aluminum alloys

The silicon content in an aluminum-silicon alloy was measured by nondestructive fast neutron activation analysis with fission spectrum neutrons. A boron nitride irradiation container reduced the flux of thermal and epithermal neutrons at the sample position, enhancing the²⁹Si (n, p)²⁹Al reaction. A detection limit of 0.4% silicon in a 0.5 g alloy sample was obtained.     

Extraction of the 131Cs from neutron irradiated barium oxide under microwave radiation

In this work, the solubility dependence of radioactive baric samples under influence of microwave radiation (MWR) has been investigated at various concentrations of HCl after irradiation of BaO target at a flux of thermal neutrons. It is established that the use of MWR for dissolution of a sparingly-soluble barium oxide will accelerate a dissolution process at almost an order of magnitude in comparison with usual heating. Each time after separation of the ¹³¹Cs from radioactive barium solution analyses on radionuclide purity of the ¹³¹Cs product solutions were carried out.     

Determination of nitrogen by neutron activation in meteorites and rocks

The results of determination of nitrogen content in meteorites by neutron activation are reported. The method is based on the¹⁴N(n,p)¹⁴C reaction, which occurs upon irradiation of the samples by neutrons. The use of proportional gas-filled countes for the recording of¹⁴C made it possible to obtain the low nitrogen detection limit of 0.001 g.     

Leakage neutrons and X-rays in a therapeutic microtron facility

Measurements of dose equivalent rate distribution in a medical microtron facility were done for neutrons and X-rays originated from 14 MV or 8 MV X-ray irradiation by the microtron. Measured data are described with empirical formulas for convenience of quantitative reconstruction of the data. A formula consisting of a simple power function agrees well with measured data except for thermal neutrons, and is understood to describe the dose rate separating into the scattered and unscattered components in the case of the point source. The thermal neutron distribution in the maze is described with another formula of an exponential function. A discrepancy was observed between the X-ray distribution in the maze from 14 MV X-ray irradiation and that from 8 MV irradiation. This is estimated to be caused by a contribution of capture gamma-rays increasingly emitted in the case of 14 MV X-ray irradiation.     

Design and construction of a facility for neutron activation analysis using the 14 MeV neutron generator at HMS Sultan

The potential for using a small, sealed tube, DT neutron generator for neutron activation analysis has been well documented but not well demonstrated, except for 14 MeV activation analysis. This paper describes the design, construction and characterization of a neutron irradiation facility incorporating a small sealed tube DT neutron generator producing 14 MeV neutrons with fluence rates of 2·10⁸ s⁻¹ in 4π (steady state) and 10¹¹ s⁻¹ in 4π (pulsed). Monte Carlo modeling using MCNP4c and McBend9 has been used to optimize the design of this facility, including the location of a thermal irradiation facility for conventional neutron activation analysis. A significant factor in designing the facility has been the requirement to conform with Ionising Radiation Regulations and the design has been optimized to keep potential radiation doses to less that 1 μSv/h at the external walls of the facility. Activation of gold foils has been used for flux characterization and the experimental results agree well with the modeling.     

Sulfur determination in coal by 14 MeV neutron activation analysis

A procedure involving the irradiation of coal samples with 14 MeV neutrons and subsequent gamma-ray spectrometry of the irradiated sample for the estimation of solfur in coal, has been outlined. The samples were irradiated with 14MeV neutrons from a Cockroft-Walton type generator for one minute and then subjected to gamma-ray spectrometry for another minute using an automated transfer cyclic system. Ten such cycles were repeated for accumulating events under the 2130 keV gamma ray photopeak belonging to³⁴P (T=12.4 s) produced by the³⁴S(n, p)³⁴P reaction for assessing the lower level of detection, LLD, of Sulfur. Interferences due to the presence of other elements in coal were also determined. Sulfur can be determined at LLD of 0.25% in coal provided a 5 g sample of the coal is irradiated with a neutron flux of 5·10⁹ n·cm⁻²·sec⁻¹ assayed with a gamma ray spectrometer having a large hollow core Ge(Li) detector and an anti-Compton shield.     

Contribution a l’etude du dosage par radioactivation du soufre et du phosphore en tres faibles concentrations dans les metaux de tres haute purete (Aluminium,magnesium, cuivre,fer, nickel)

The analytical procedure for the determination of sulfur in copper by activation with thermal neutrons is given. The purifications necessary to obtain a radiochemically pure precipitate of baryum sulphate are described. The occurrence of some discrepant values in the determinations led to the observation, for the first time, of abnormally high contents of³⁵S^* at a relatively important depth in the samples of irradiated copper (as deep as about 300 μ). Therefore it is necessary to etch the samples to a depth of 300 μ at least on each surface, before doing the chemical separations. Owing to the great influence of the reaction³⁵Cl(n,p)³⁵S^*, the determination of sulfur by thermal neutrons is only possible at contents higher than 10⁻⁶, even if the chlorine concentration is very low (2·10⁻⁸ Cl introduces a correction in terms of sulfur of the order of 10⁻⁶). For sulfur contents lower than 10⁻⁶, the determination is made by irradiation in fast neutrons. The analytical procedure is described and the corrections in the presence of phosphorus and chlorine are discussed. At contents of the order of 10⁻⁸, the corrections are very small and the determination of some 10⁻⁷ of sulfur is easy. Finally, the determination of phosphorus in copper by activation in thermal neutrons is given. The chemical separations are the same as in the preceding case. Contents of the order of 10⁻⁸ phosphorus are determined without difficulty. The results of the determination of sulfur and phosphorus in many samples of copper are indicated: OFHC copper, High Purity copper (99.999%) and different samples of zone refined copper prepared at the Vitry’s Laboratory.      

Measurements of nuclear data of minor actinides for transmutation of high-level waste

For nuclear transmutation of minor actinides, delayed neutron emission measurement for²⁴¹Am was carried out in thermal neutron irradiation location. The neutron capture cross sections of²⁴¹Am were also measured radiochemically. The transmutation process of²⁴¹Am in reactor is discussed by calculating the yields of minor actinides with the nuclear data measured in this study and the evaluated values. The accelerator driven transmutation of minor actinides by high-flux neutrons from spallation reactions is also presented.     

A neutron activation procedure for the determination of dissolved silver in sea water

A neutron activation procedure has been developed for the determination of silver in sea water. The element is preconcentrated by an anion-exchange procedure. The concentrate is submitted to irradiation with thermal neutrons. Silver-11Om is separated from other radionuclides by means of a conventional radiochemical separation scheme. The method gave a coefficient of variation of ± 10% at a silver level of 40 ng l^?1.     

A method for bulk hydrogen analysis based on transmission and back scattering of fast neutrons

A method was proposed for bulk hydrogen analysis. It is based on simultaneous detection of transmitted fast neutrons and back scattered thermal neutrons from the investigated samples by ³He detectors. The fast neutron beams were obtained from ²⁵²Cf and Pu–Be neutron sources. The experimental set-up as well as samples preparation were described. Incident thermal neutrons beams obtained from either ²⁵²Cf or Pu–Be sources, were used to investigate the samples by neutron backscattering. The results obtained from transmission and backscattering of fast neutrons were compared and discussed. The advantage and capabilities of the proposed method were presented. The results obtained using fast neutron beams are more sensitive than those obtained using thermal neutron beams. Validation procedures were proposed to credit the results.     

Production of87Br for studying neutron decay

An improved gas flow system is described for the production of⁸⁷Br through a fast radiochemical separation of fission products from a solution of²³⁵U irradiated with thermal neutrons. It can produce⁸⁷Br with a higher yield relative to⁸⁸Br than was previously possible for measurement of neutron energy distribution. Contrary to previous indication, large amounts of iodine fission products have been found to come out of the irradiation cell in the gas stream necessitating their isolation.     

Szilard-Chalmers reaction in praseodymium compounds

The β-emitting isotope^142gPr (T _1/2=19.13 h, E_(β)max=2.16 MeV) is a suitable radionuclide for internal radiotherapy with tumor-seeking biomolecules.^142gPr can be prepared from natural¹⁴¹Pr by irradiation with thermal neutrons. We investigated the Szilard-Chalmers reaction in well-defined complexes with various ligands and a salt in order to measure the enrichment factors and extracted yields to find a route for the production^142gPr of high specific radioactivity, ionic^142gPr³⁺ was separated from the complex by cation exchange and high performance liquid chromatography (HPLC).     

Contribution a l'etude du dosage par radioactivation du sourfre et du phosphore en tres faibles concentrations dans les metaux de tres haute purete (Aluminium,magnesium, cuivre,fer, nickel)

The chemical separation of sulphur and phosphorus from pure iron and nickel, is described. Sulphur has been determined first by irradiation in fast neutrons, then by irradiation in mixed fast and thermal neutrons. The two methods gave two different results; so, we were led to point out the same phenomenon asJ. Le Hericy had already pointed out for the determination of sulphur in copper. We found that the abnormal amounts of sulphur came from external contamination with chlorine since sulphur is then produced by the³⁵Cl(n, p)³⁵S reaction. The diffusion of³⁵S atoms (produced on the edge of the samples) explains that the concentration of sulphur decreases from the edge to the middle of the sample. We prepared samples of iron and nickel, artificially covered with ammonium chloride, and we studied the apparent concentration of sulphur as a function of the depth in these samples. Our experiences pointed out that the contamination in sulphur yielded by the (n, p) reaction, reaches very quickly the middle of the samples. This phenomenon limits the determination of sulphur by the³⁴S(n, γ)³⁵S reaction, in nickel and iron.      

Evaluation of imaging plate technique coupled with activation detector as the passive neutron monitor

The spatial distribution of neutrons was measured at the muon science laboratory of KEK by the activation detector method using an imaging plate for the radioactivity measurement. It was confirmed that this method is highly sensitive to detect the average neutron dose of 10 μSv/h. The distribution of thermal and epithermal neutrons was also measured in the experimental room. The cadmium ratio inside the experimental room is one except for the neutron leakage point. The spatial distribution of neutrons inside the concrete shield of KENS was measured by the same method. Aluminum and gold foils were used for the measurement of fast and thermal neutrons, respectively. Two dimensional change of the reaction rate of the ²⁷Al(n,α)²⁴Na reaction shows a good agreement with the results calculated by the Monte Carlo simulation using MARS14 code. Thermal and epithermal neutron flux ratio on the beam axis was measured by the cadmium ratio method. The flux ratios were about 30 and almost constant for every slot except for the surface of the shield, because the cadmium ratio is 2. This method was very useful to measure the activity of many pieces of detector simultaneously without any efficiency and decay correction. Wide dynamic range and high sensitivity are also the merit of this method.