Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: R. Clark x
  • Refine by Access: All Content x
Clear All Modify Search

A graphG ismaximally nonhamiltonian iffG is not hamiltonian butG + e is hamiltonian for each edgee inG c, i.e., any two non-adjacent vertices ofG are ends of a hamiltonian path. Bollobás posed the problem of finding the least number of edges,f(n), possible in a maximally nonhamiltonian graph of ordern. Results of Bondy show thatf(n)3/2 n forn ≤ 7. We exhibit graphs of even ordern ≥ 36 for which the bound is attained. These graphs are the “snarks”,J k, of Isaacs and mild variations of them. For oddn ≥ 55 we construct graphs from the graphsJ k showing that in this case,f(n) = 3n + 1/2 or 3n + 3/2 and leave the determination of which is correct as an open problem. Finally we note that the graphsJ k, k ≤ 7 are hypohamiltonian cubics with girth 6.

Restricted access

Abstract  

A 14 MeV neutron activation analysis system is described that is being used primarily to determine trace levels of oxygen in metals. No sample container is required for relatively inert solid metal samples thus increasing the sensitivity of the method by a factor of five. Under these conditions the sensitivity and precision foroxygen is about 6±3.3 μg in a single determination, resulting from collecting 1.216N counts per μg of oxygen. A method is described for calibrating the system for the analyses of large metal samples. The preparation and use of sample containers for the analyses of particulate and/or reactive samples is also presented.

Restricted access

Abstract  

The simultaneous determination of multiple actinide isotopes in samples where total quantity is limited can sometimes present a unique challenge for radioanalytical chemists. In this study, re-determination of 238Pu, 239+240Pu, and 241Am for soils collected and analyzed approximately three decades ago was the goal, along with direct determination of 241Pu. The soils had been collected in the early 1970’s from a shallow land burial site for radioactive wastes called the Subsurface Disposal Area (SDA) at the Idaho National Lab (INL), analyzed for 238Pu, 239+240Pu, and 241Am, and any remaining soils after analysis had been archived and stored. We designed an approach to reanalyze the 238Pu, 239+240Pu, and 241Am and determine for the first time 241Pu using a combination of traditional and new radioanalytical methodologies. The methods used are described, along with estimates of the limits of detection for gamma-and alpha-spectrometry, and liquid scintillation counting. Comparison of our results to the earlier work documents the ingrowth of 241Am from 241Pu, and demonstrates that the total amount of 241Am activity in these soil samples is greater than would be expected due to ingrowth from 241Pu decay.

Restricted access

Abstract  

The accurate and precise determination of232Th in biological samples is very important for the development of biokinetic models for thorium and for improving our knowledge on its distribution in human tissues. Radiochemical neutron activation analysis has long been one of the most sensitive methods for the determination of232Th. However, these determinations suffer in reliability because recovery information following the separation is not typically available. This information is particularly important for difficult matrices such as human bone where recoveries may be significantly less than unity. Also, the separation of difficult matrices following neutron activation may involve relatively high personal dose from the co-activated matrix. A novel approach for the determination of radiochemical yield has been developed which employs the use of a readily available, gamma-emitting isotope of thorium,227Th.227Th, obtained by radiochemical separation from227Ac, is added to each, dissolved sample prior to separation and the chemical yield determined by gamma-ray spectrometry following the separation. This pre-concentration step is then followed by neutron activation and the232Th determined via233Pa using gamma-ray spectrometry. Detection limits were approximately an order of magnitude lower than obtained by alpha-spectrometry.

Restricted access

Abstract  

The determination of isotopic thorium by alpha-spectrometric methods is a routine practice for bioassay and environmental measurement programs. Alpha-spectrometry has excellent detection limits (by mass) for all isotopes of thorium except232Th due to its extremely long half-life. This paper reports a pre-concentration neutron activation analysis (PCNAA) method for232Th that may be performed following alpha-spectrometry if a suitable source preparation material is utilized. Human tissues and other samples were spiked with229Th and the thorium was isolated from the sample using ion exchange chromatography. The thorium was then electrodeposited from a sulfate-based medium onto a vanadium planchet, counted by alpha-spectrometry, and then analyzed for232Th by neutron activation analysis. The radiochemical yield was determined from the alpha-spectrometric method. Detection limits for232Th by this PCNAA method are approximately 50 times lower than achieved by alphaspectrometry.

Restricted access

Abstract  

Laser-induced kinetic phosphorimetry is an accurate, sensitive and rapid alternative to radiometric determination of natural and depleted uranium in aqueous solutions. This method offers detection limits below 10 ng/l U (2.5·10−4 Bq/l natural U) and a broad analytical range to 5 mg/l U (130 Bq/l natural U). For many samples, dilution is the only sample preparation required. However, because this technique infers uranyl concentrations from time-resolved phosphorescence intensities, results are dependent upon sample matrix constituents that affect the phosphorescence of the uranyl cation. This study examines the influence of cations, anions and ligands common to natural water, process and bioassay samples on the quenching of uranyl phosphorescence and the consequences for lower limits of detection and accuracy of measurements.

Restricted access

Abstract  

Kinetic dissolution studies were conducted on four prominent U-Ca-PO4 minerals (metaschoepite, becquerelite, chernikovite and metaautunite). Synthetic samples were contacted with four extractants (acetic acid, deionized water, EDTA and sodium bicarbonate) at room temperature at two concentrations, 100 mM and 1 mM. Dissolution progress was monitored by periodic sampling for dissolved U, and dissolution rates were obtained from fits to a three term exponential model. Significant variations were observed in the rate and extent of dissolution among the mineralsexamined. The uranyl phosphates chernikovite and metaautunite proved resistant to dissolution in non-carbonate systems, with dissolution half-times of days to weeks in 100 mM systems and weeks to years in 1 mM systems. In contrast, the uranyl oxide hydrates schoepite and becquerelite were solubilized over much shorter time scales. While 100 mM bicarbonate was successful in dissolving U in all forms, dissolution rates varied among the four minerals. Overall, EDTA was the least sensitive to a 100 to 1 mM drop in its concentration in its solubilization of all four mineral phases, underscoring the importance of organic complexation for the environmental mobility of uranium.

Restricted access

Abstract  

Although potentially very useful as a screening tool, currently-applied “gross” alpha/beta analyses are often considered unreliable by the environmental monitoring community. We describe here an alternative approach based on direct counting of pressed soil “wafers” to estimate total alpha/beta activities. The system was calibrated using a series of natural soils and sediment standards with total α/β activities estimated by use of a combination of available certified values, equilibrium assumptions, and our own measurements. A set of 10 such standards that span a wide range of activities were prepared by thoroughly mixing several grams of selected NIST and IAEA natural matrix standards together with reagent grade cellulose in a 4∶1 sample-to-binder ratio and pressing in a 40-mm stainles steel die. The resulting wafers, assumed to be at infinite thickness for expected radionuclides, were counted in a gas flow proportional counter set for simultaneous α/β counting. Both the alpha and beta count rates increased in a linear, systematic manner with increasing total estimated activities. This technique should prove to be an inexpensive, simple, and waste-free approach for screening total radioactivity in soil samples.

Restricted access

Abstract  

Alpha-spectrometric measurements using Si detectors is the standard method for the determination of alpha emitting actinide elements. This method requires the preparation of sources for analysis which do not degrade the energy spectrum of the emitted alpha particles via sample self-absorption. A variety of methods for the electrodeposition of actinides have been reported in the literature, many of which require long deposition times and lack reproducibility. A sulfate based method has been evaluated for the preparation of these sources using chemometric analysis to optimize the method and evaluate several variables and their interactions with the goal to achieve high yield source preparation in 1 hour or less. Typical resolution for this method is 30 keV or less with recoveries approaching unity.

Restricted access