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Abstract  

Because of the short half life of Al-28, the determination of aluminum by neutron activation is subject to many inaccuracies: variation of irradiation conditions between sample and standard, uncertainties in timing, and the effects of high and varying count rate, in addition to other sources of error that must be controlled even in work with long-lived nuclides. These errors can all be made smaller than the fundamental limit set by counting statistics, even when that limit is below 0.5 percent. The transfer function from the observed number of net counts to the counting rate at the end of irradiation is modeled as a product of three processes: radioactive decay and extending and nonextending dead time.The procedure has been applied to the analysis of NBS SRM 1633a Fly Ash. The mean concentration measured was 14.085% Al, with a standard deviation of the mean 0.023% Al for four determinations. The final results showed no significant imprecision beyond counting statistics. The accuracy of the method is shown by the analysis of high-purity single-crystal sapphire.

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This paper presents the results of laser-induced-pressure-pulse (LIPP) measurements of space charge accumulation in 0.5 mm thick planar XLPE samples with aluminium electrodes, over the temperature range 30–90°C. The applied field strength was 10 kV mm−1. The increase in transit time of the pressure pulse across the sample with increasing temperature indicated that Young's modulus decreased by 50% from 30 to 90°C. It was concluded that electrons injected at the cathode are transported to the anode at temperatures above 40°C, and that electron traps cause an accumulation of negative charge immediately adjacent to both cathode and anode. It appears that a concentration of impurity molecules accumulates close to the cathode, and field-assisted thermal ionisation of these molecules generates an immobile positive space charge adjoining the electronic space charge adjacent to the cathode.

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This paper introduces the thermal column or the cold neutron guide beam of the 20 MW NBSR at the National Institute of Standards and Technology as a directk o measurement facility. Measurement ofk o at this facility not only produces accurate values, but avoids the additional correction factors needed in other measurement methods. Thek o of Sb, Ag and Cr with respect to Sc as monitor have been measured and their values are comparable to values based on tabulated nuclear constants and to those measured by other researchers.

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In the course of making high-accuracy measurements of arsenic, we found that the most recently published and compiled half-life of 76As did not agree with our data as well as the earlier accepted value. To redetermine this parameter, 76As sources were measured on four Ge gamma detector systems, and an exponential function was fitted to the decay data by two different nonlinear least-squares methods. We obtained T 1/2 = 1.09379 days with a standard uncertainty of 0.00045 days. This result is 1.5% higher than the most recent value, but is in agreement with the older, less precise, consensus value.

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Abstract  

One of the most recent developments in equipment for high and/or varying count rates, as encountered in the gamma-ray spectrometry needed for INAA via short-lived radionuclides, is the advent of EG&G Ortec's DSPECplus. This all-in-one apparatus provides digital signal processing and zero-dead-time counting. One such DSPEC unit was tested at IRI with respect to performance as a function of count rate. Optimum parameter settings for different applications were established. It is concluded that this spectrometer allows for measurements with count rates up to at least 105 cps with an accuracy of a few percent (depending on the calibration approach), a peak position stability of 5.10-3%. and a peak width stability of 5%. It is also concluded that the normal spectrum provides a good estimate for the uncertainties in the zero-dead-time spectrum acquired with this spectrometer.

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Abstract  

Using a low-energy neutron beam as an isotopic probe, neutron depth profiling (NDP) provides quantitative depth profiles in nearly all solid matrix materials. Several of the light elements, such as He, Li, B, and N can be nondestructively analyzed by NDP. The information obtained using NDP is difficult if not impossible to determine by non-nuclear techniques. As a result, NDP is used collaboratively with techniques such as SIMS, RBS, FTIR, PGAA, and AES. Profiles measured by NDP are given for semiconductor and optical processing materials, and light weight alloys. Improvements in the technique are discussed with emphasis on the use of intense cold neutron beams.

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Thermally stimulated luminescence (TSL) and thermally stimulated depolarization current (TSD) measurements were made simultaneously on low density polyethylene samples 0.175 mm thick, carrying evaporated gold electrodes. The samples were dc-polarized at room temperature at a field strength of 90 kV/cm, cooled to −190°C and X-irradiated with the field still applied, and then heated in short-circuit at 3 deg/min. Comparing the TSL and TSD data, particularly the effects of oxidizing the samples and immersing them in fuming nitric acid, it was shown that (1) the space-charge transport component of the TSD current between −190 and 30°C is negligible, (2) charge injection begins around −10°C (for evaporated gold electrodes) and increases rapidly with increasing temperature, and (3) all the injected charge is transported through the sample to the opposite electrode. Such transport takes place only through the amorphous regions of the sample.

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Abstract  

Skin as a manageable organ can provide direct or indirect information of tissue iron overload resulting from inherited disorders as hemochromatosis. Patients with hemochromatosis were evaluated at three consecutive phases along the therapy programme. Nuclear microprobe techniques were used to assess skin iron and Total Reflection X-ray Fluorescence to determine the plasma iron concentrations. Results showed that iron pools were differently correlated at the three therapy phases. These variations highlighted the value of skin iron content to assess organ iron deposition and therapy efficacy. Skin iron content can be used for a better management of patients with iron overload pathologies.

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