S. N. Dmitriev

Versatile ultrathin nanoporous silicon nitride membranes

Single- and multiple-nanopore membranes are both highly interesting for biosensing and separation processes, as well as their ability to mimic biological membranes. The density of pores, their shape, and their surface chemistry are the key factors that determine membrane transport and separation capabilities. Here, we report silicon nitride (SiN) membranes with fully controlled porosity, pore geometry, and pore surface chemistry. An ultrathin freestanding SiN platform is described with conical or double-conical nanopores of diameters as small as several nanometers, prepared by the track-etching technique. This technique allows the membrane porosity to be tuned from one to billions of pores per square centimeter. We demonstrate the separation capabilities of these membranes by discrimination of dye and protein molecules based on their charge and size. This separation process is based on an electrostatic mechanism and operates in physiological electrolyte conditions. As we have also shown, the separation capabilities can be tuned by chemically modifying the pore walls. Compared with typical membranes with cylindrical pores, the conical and double-conical pores reported here allow for higher fluxes, a critical advantage in separation applications. In addition, the conical pore shape results in a shorter effective length, which gives advantages for single biomolecule detection applications such as nanopore-based DNA analysis.

Preparation of 99Mo and 99mTc by 100Mo(γ,n) photonuclear reaction on an electron accelerator, MT-25 microtron

Molybdenum-99 was prepared by 100Mo(γ,n) photonuclear reaction on an electron accelerator, MT-25 microtron (FLNR). Natural Mo foil (9.6% 100Mo) and 100MoO3 (100% 100Mo) were used as the targets. The radiation yield of 99Mo was 3.2 kBq/(μA h mg 100Mo). Technetium-99m was isolated from the target material and concomitant radioactive impurities by an ion-exchange procedure. The possibility of preparing 99mTc generator from molybdenum carbonyl was studied. The content of radioactive impurities in the final 99mTc preparations was no higher than 10−5 Bq Bq−1.

Ultra-pure 236Pu and 237Pu for environmental and biomedical research

Abstract Isotopically pure 236Pu (>99.999% by activity) has been produced by irradiating U (95% 235U) with 4He ions and refining the product by electromagnetic mass separation. Further developments in the production of 237Pu are also reported, giving material containing, by activity, only 3 × 10−6% 236Pu and probably only ∼ 10−5% 238Pu.

Preparation of 225Ac by 226Ra(γ, n) photonuclear reaction on an electron accelerator, MT-25 microtron

Abstract237U was prepared by 238U(γ,n) reaction on an electron accelerator (MT-25 microtron) at FLNR. The radiation yield of 237U was 1 Bq/(μA h mg 238U). 237U was recovered from the target by collection of recoil nuclei in the solid:solid system. Group I–III element fluorides were used as the 237U acceptor. 237U was separated from the fission products by extraction and ion-exchange procedures. The specific activity of 237U in the sample was 49 500 kBq/mg 238U. The content of radioactive impurities in the preparation was no higher than 10−5. The chemical yield of 237U was 70%.

Production of 237Pu with high radiochemical purity

Abstract 237 Pu (half-life 45.3 d) is a valid metabolic tracer for the long-lived isotopes of plutonium, but its use in human subjects has previously been restricted by difficulties in producing it with acceptable isotopic purity. These difficulties have been overcome by applying mass-separation techniques to the impure product of irradiating 235 U with 25-MeV 4 He + ions.

Determination of gold in natural waters by neutron activation and γ-spectrometry after preconcentration with tributyl phosphate as solid extractant

Abstract A method for the preconcentration of gold in natural waters at the sampling site using tributyl phosphate as a solid extractant [Se(TBP)] was developed as a preliminary step prior to the determination of gold by neutron activation and γ-spectrometry. The SE(TBP) was saturated with gaseous chlorine for extracting all gold species. In batch experiments gold was quantitatively retained on the SE(TBP) in 10 min. After extraction and washing, the SE(TBP) was ashed or back-extracted. Gold was quantitatively eluted with hot, neutral 0.025 M thiourea. The gold content of residues of ashing or eluents after evaporation was determined by neutron activation and γ-spectrometry. The detection limit for the overall procedure was 0.2 ng 1 −1 . The efficiency was tested on ‘equilibrated’ solutions prepared from river water and tracer solutions of gold. For comparison, the gold content of natural water samples was determined using preconcentration on activated charcoal.

THE HALF-LIFE OF 237PU

Abstract Serial measurements of K x-ray emissions from 10 sources of 237 Pu, over periods up to 483 d, indicate a half-life of 45.66 ± 0.04 d. This result supports an early (1957) report of 45.6 d, but significantly exceeds currently recommended values of 45.2–45.3 d.

Low level measurements of thorium and neptunium in environmental samples using the (γ, f) reaction

A new highly sensitive track method for the determination of thorium (232Th) and neptunium (237Np) has been developed. The technique includes the radiochemical separation procedure of the isotopes followed by the irradiation of the resultant samples on the MT-25 microtron. The detection limit is équivalent up to 3·10−13 g of232Th and 7·10−14 of237Np. The method was used to determine232Th and237Np isotopes in water samples.

Detecting system of the setup for studying chemical properties of superheavy elements using the gas chemistry techniques

A setup for studying the physicochemical properties of superheavy elements in experiments involving gas transport systems is described. The setup is composed of four detecting modules with semiconductor detectors; systems of data acquisition, storage, and processing; a cooling system for semiconductor detectors; and a vacuum system. In each detecting module, there are two oppositely located four-strip semiconductor detectors. The detecting system is capable of detecting with a high efficiency α particles and spontaneous fission fragments produced in decays of superheavy elements.

Cation-exchange separation of Group V elements: Model experiments on isolation and chemical identification of Db

A cation-exchange procedure was developed for separating Nb from Ta, Pa from Ta, and Nb, Pa, and Ta from Zr, Hf, and lanthanides in dilute HC1/HF solutions. The stability of the fluoride complexes of Group IV and V elements decreases in the following order: Nb ∼ Pa > Zr > Hf > Ta. This procedure can be used in experiments on synthesis of superheavy nuclei for isolation of Db from the reaction products and for its chemical identification.