On the fission interference correction and its dependence on the epithermal to thermal neutron flux ratio in thermal naa of molybdenum

The present work aims at the following: (1) analyzing the experimental fission interference factor for molybdenum, F_mo, obtained by the authors, who have described the irradiation conditions used as concerns the epithermal to thermal neutron flux ratio, Φ_epi/Φ₀; (2) establishing a simple calculation model that describes the dependence ofF _mo on Φ_epi/Φ₀ in an adequate way, to provide a satisfactory basis to explain the scatter found in the existing experimental data; and (3) clearly indicating the basic recommendations to take into account in order to obtain with high accuracy the concentration of molybdenum in samples containing uranium.     

An evaluation of thermal and epithermal neutron activation analysis compton suppression methods for biological reference materials

For neutron activation analysis (NAA), the usual matrix problems of sodium, chlorine, and bromine are well known to give rise to high backgrounds that inhibit the determination of several trace elements for short-lived or medium-lived NAA. For long counting times in long-lived NAA, very low backgrounds are required to achieve good sensitivities. We have investigated the use of thermal and epithermal NAA in conjunction with Compton suppression to determine several elements such as arsenic, antimony, cadmium, and mercury, at the level of a few nanograms. The values of these techniques are discussed in contrast to the standard radiochemical methods.     

Chromium,nickel, and arsenic determinations in human samples by thermal and epithermal neutron activation analyses

Both thermal and epithermal neutron activation analyses have been employed to determine chromium and nickel in lung tissue and arsenic in urine. Based on accuracy, precision, and detection limits, these techniques have been successfully used to analyze lung tissue from a deceased welder, who died, from cancer, and to paratake in an interlaboratory toxicological urine program.     

RBE of the MIT epithermal neutron beam for crypt cell regeneration in mice

The RBE of the new MIT fission converter epithermal neutron capture therapy (NCT) beam has been determined using intestinal crypt regeneration in mice as the reference biological system. Female BALB/c mice were positioned separately at depths of 2.5 and 9.7 cm in a Lucite phantom where the measured total absorbed dose rates were 0.45 and 0.17 Gy/ min, respectively, and irradiated to the whole body with no boron present. The gamma-ray (low-LET) contributions to the total absorbed dose (low- + high-LET dose components) were 77% (2.5 cm) and 90% (9.7 cm), respectively. Control irradiations were performed with the same batch of animals using 6 MV photons at a dose rate of 0.83 Gy/min as the reference radiation. The data were consistent with there being a single RBE for each NCT beam relative to the reference 6 MV photon beam. Fitting the data according to the LQ model, the RBEs of the NCT beams were estimated as 1.50 +/- 0.04 and 1.03 +/- 0.03 at depths of 2.5 and 9.7 cm, respectively. An alternative parameterization of the LQ model considering the proportion of the high- and low-LET dose components yielded RBE values at a survival level corresponding to 20 crypts (16.7%) of 5.2 +/- 0.6 and 4.0 +/- 0.7 for the high-LET component (neutrons) at 2.5 and 9.7 cm, respectively. The two estimates are significantly different (P = 0.016). There was also some evidence to suggest that the shapes of the curves do differ somewhat for the different radiation sources. These discrepancies could be ascribed to differences in the mechanism of action, to dose-rate effects, or, more likely, to differential sampling of a more complex dose-response relationship.     

Nondestructive determination of arsenic in urine by epithermal neutron activation analysis and Compton suppression

Biological Trace Element Research - Epithermal neutron activation analysis, in conjunction with Compton suppression, has been employed to determine arsenic levels in artificially doped urine...     

Measurements of neutron energy using a recoil-proton telescope and a high-pressure ionization chamber

Two very different techniques for measuring the energy of neutrons in the energy range 0.1-10 MeV are presented and compared. A recoil-proton spectrometer is used to determine the energy spectra of neutrons produced by the d(4)-Be and p(4)-Be reactions down to the low-energy threshold of 0.7 MeV. The same radiation fields are also measured with a recently developed method using a high-pressure ionization chamber that can be used to determine the mean energy of the neutrons in a mixed neutron-gamma radiation field provided the gamma-ray absorbed dose fraction is determined independently. An intercomparison of the two methods shows that the high-pressure ionization chamber compares well and supplements the established recoil-proton spectrometer technique. The almost isotropic response of the chamber has enabled measurements to be made of the variation of mean neutron energy with depth in water for the two radiation fields.     

Molecular structural analysis of HPRT mutations induced by thermal and epithermal neutrons in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells were exposed to thermal and epithermal neutrons, and the occurrence of mutations at the HPRT locus was investigated. The Kyoto University Research Reactor (KUR), which has been improved for use in neutron capture therapy, was the neutron source. Neutron energy spectra ranging from nearly pure thermal to epithermal can be chosen using the spectrum shifters and thermal neutron filters. To determine mutant frequency and cell survival, cells were irradiated with thermal and epithermal neutrons under three conditions: thermal neutron mode, mixed mode with thermal and epithermal neutrons, and epithermal neutron mode. The mutagenicity was different among the three irradiation modes, with the epithermal neutrons showing a mutation frequency about 5-fold that of the thermal neutrons and about 1.5-fold that of the mixed mode. In the thermal neutron and mixed mode, boron did not significantly increase the frequency of the mutants at the same dose. Therefore, the effect of boron as used in boron neutron capture therapy (BNCT) is quantitatively minimal in terms of mutation induction. Over 300 independent neutron-induced mutant clones were isolated from 12 experiments. The molecular structure of HPRT mutations was determined by analysis of all nine exons by multiplex polymerase chain reaction. In the thermal neutron and mixed modes, total and partial deletions were dominant and the fraction of total deletions was increased in the presence of boron. In the epithermal neutron mode, more than half of the mutations observed were total deletions. Our results suggest that there are clear differences between thermal and epithermal neutron beams in their mutagenicity and in the structural pattern of the mutants that they induce. Mapping of deletion breakpoints of 173 partial-deletion mutants showed that regions of introns 3-4, 7/8-9 and 9-0 are sensitive to the induction of mutants by neutron irradiation.     

Determination of molecular weight by neutron scattering

Theoretical and practical advantages of neutron scattering for the determination of molecular weights of particles in solution are discussed. The method presented does not use known particles for calibration and is applicable to a wide molecular weight range (10⁴–10⁹); it is not sensitive to assumptions about the partial specific volume of the particles, and the experiments are performed quite easily in standard spectrophotometer cells using 100–500 μg of material. The method is nondestructive and the sample can be recovered totally. The use of D₂O in solvents has particular advantages especially for multicomponent particles, for which the in situ molecular weight of each component can be determined separately.     

Monitoring the atmosphere in an anaerobic chamber

The Couloximeter, a fuel cell designed to measure trace amounts of oxygen, was used to monitor the atmosphere in an anaerobic chamber. The device, easy to operate and to maintain, allowed both major and minor fluctuations in oxygen concentration to be measured. Using a hose attached to the outlet within the box, defective (ruptured) gloves were consistently distinguishable from intact gloves.     

Determination of precious metals using the neutron activation analysis technique

Samples of residual sands were analyzed for precious metals content. These samples were collected from one of the largest silver mines in the world with the purpose of determining the existence and quantities of precious metals. The collected samples were approximately 5 kg, taken from different sites within the deposit grounds. Each sample was passed through a sieve in order to select only the smallest particles and about 100 g per sample were obtained. Twenty samples were prepared that produced thin foils from which the samples were cut in the shape of disks. The neutron activation analysis method (NAA) was used in this work, using a nuclear reactor (Triga Mark III) as a neutron source and a high-purity germanium detector as spectrometer. The results from the analysis of this study are satisfactory; the main precious metals found were Au, Ru, Os, and Ir. A commercial study is necessary to determine if the exploitation of these sands is financially feasible.     

Differences in thermal sensitivities of the regulatory-catalytic and catalytic forms of cyclic AMP-dependent protein kinases from various tissues.

The cAMP-dependent protein kinase from various tissues was more thermally sensitive when activated by cAMP than the non-activated enzyme. For example, when the activity ratio (the activity of protein kinase assayed -cAMP/+cAMP) was 0.40, 80% and 76% of total hepatic cAMP dependent protein kinase activity was recoverable after incubations at 45 degrees C for 15 and 30 minutes, respectively. However, when the activity ratio was elevated to about 0.80 - 0.90 by increasing cAMP levels in vivo or adding exogenous cAMP to soluble liver extracts, the total protein kinase activity recoverable after incubations at 45 degrees C for 15 minutes was 34-44% and 19-22%, respectively. This observation was used to estimate the degree of activation of the enzyme in vivo and in vitro, since the loss of enzyme activity at 45 degrees C was directly related to the degree of activation of the enzyme in tissue extracts. The regulatory-catalytic form of cAMP-dependent protein kinase was thermally resistant at 45 degrees C unless activated by incubation with exogenous cAMP, histones or NaCl, while the catalytic form of the enzyme was highly thermally sensitive at this same temperature. These data describe a new property of the cAMP-dependent protein kinase and suggest an alternative method which measure the degree of activation of the enzyme.     

Sublethal and potentially lethal damage repair on thermal neutron capture therapy

Tonicity shock or caffeine postirradiation treatment makes evident fast-type potentially lethal damage (PLD). Caffeine expresses fast-type PLD more efficiently than tonicity shock in X-irradiated B-16 mouse melanoma cells, compared with V79 Chinese hamster cells. The survival curves of thermal neutrons for either V79 or B-16 cells exhibit no shoulder. Neither V79 nor B-16 cells show the sublethal damage (SLD) repair of thermal neutrons. Caffeine-sensitive fast-type PLD repairs exist in X-irradiated B-16 cells, as well as V79 cells. The fast-type PLD repair of B-16 cells exposed to thermal neutrons alone is rather less than that of X-irradiated cells. Furthermore, an extremely low level of fast-type PLD repair of B-16 cells with 10B1-paraboronophenylalanine (BPA) preincubation (20 hours) followed by thermal neutron irradiation indicated that 10B(n,alpha)7Li reaction effectively eradicates actively growing melanoma cells. The plateau-phase B-16 cells are well able to repair the slow-type PLD of X-rays. However, cells can not repair the slow-type PLD induced by thermal neutron irradiation with or without 10B1-BPA preincubation. These results suggest that thermal neutron capture therapy can effectively kill radioresistant melanoma cells in both proliferating and quiescent phases.     

Determination of the permeability of human lymphocytes with a microscope diffusion chamber

A diffusion chamber similar to that proposed by J.J. McGrath (J. Microsc., in press) was constructed which allows microscopic observation of osmotically induced volume changes of individual cells in small (microliter) sample volumes. The cells are kept fixed in position in the upper compartment of the chamber by means of a highly permeable membrane and exposed to a step-like change in concentration generated in the lower compartment. An electrical conductivity probe in the upper compartment was used to monitor the temporal change of salt concentration as experienced by the cells. The rise from isotonic to hypertonic can be approximated by an exponential function. Its time constant of tau = 2.08 sec seems to be mainly determined by the change in flushing solution as tau = 1.48 sec was measured with no membrane installed. With human lymphocytes, no loss of cell volume was detected before 5 sec, i.e., when 95% of the final concentration was reached extracellularly. A step change can hence be assumed when modeling exosmosis for determining the lymphocyte membrane permeability. The equations for coupled transport of water and salt were solved numerically and fitted to the experimental data. The results were also compared to various other transport models described in the literature. Human lymphocytes are almost ideally semipermeable with a hydraulic reference permeability of Lp = 4.23 X 10(-4) cm/sec (3.13 X 10(-3) micron X atm-1 X sec-1) at T = 23 degrees C. The temperature and concentration dependence are described by an activation energy Ea = 14.3 kJ/mol and a concentration coefficient alpha 2 = 0.261 osmol/kg. An osmotically inactive volume fraction of 36.9% was determined from the final cell volumes reached asymptotically after shrinkage.     

Construction of a thermotaxis chamber providing spatial or temporal thermal gradients monitored by an infrared video camera system.

A thermotaxis chamber was constructed to quantitatively study thermotaxis in eukaryotic amoeboid cells. The apparatus provided either spatial or temporal temperature gradients in an observation chamber set in an inverted microscope. With an infrared video camera system, spatial thermal gradients were monitored directly and the temperature at the actual location of the cells could be estimated accurately. This enabled a precise determination of the strength of thermal stimuli. With this apparatus, we were able to simultaneously measure temperature and observe cellular behavior directly. This feature permits quantitative studies on stimulus-response relationships. The utility of the apparatus was demonstrated by thermotaxis assay under a spatial thermal gradient in polymorphonuclear leukocytes. Since this apparatus can also provide temporal thermal gradients, it may have several applications in studies of temperature-dependent phenomena in cell biology.     

A source with a 1013 DT neutron yield on the basis of a spherical plasma focus chamber

Results from preliminary experimental research of neutron emission generated by a spherical plasma focus chamber filled with an equal-component deuterium-tritium mixture are presented. At a maximum current amplitude in the discharge chamber of ～1.5 MA, neutron pulses with a full width at half-maximum of 75–80 ns and an integral yield of ～1.3 × 10¹³ DT neutrons have been recorded.