First application of calorimetric low-temperature detectors in accelerator mass spectrometry

For the first time, calorimetric low-temperature detectors were applied in accelerator mass spectrometry, a well-known method for determination of very small isotope ratios with high sensitivity. The aim of the experiment was to determine with high accuracy the isotope ratio of ²³⁶U/²³⁸U for several samples of natural uranium, ²³⁶U being known as a sensitive monitor for neutron flux. Measurements were performed at the VERA tandem accelerator at Vienna, Austria. The detectors consist of sapphire absorbers and superconducting transition edge thermometers operated at T≈ 1.5 K. The relative energy resolution obtained for 17.39 MeV ²³⁸U is ΔE/E=4–9×10⁻³, depending on the experimental conditions. This performance enabled to substantially reduce background from neighbouring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of ²³⁶U/²³⁸U=6.5×10⁻¹² could be obtained, representing the smallest ²³⁶U/²³⁸U ratio measured until now.     

Artificial neural network modelling of uncertainty in gamma-ray spectrometry

An artificial neural network (ANN) model for the prediction of measuring uncertainties in gamma-ray spectrometry was developed and optimized. A three-layer feed-forward ANN with back-propagation learning algorithm was used to model uncertainties of measurement of activity levels of eight radionuclides (²²⁶Ra, ²³⁸U, ²³⁵U, ⁴⁰K, ²³²Th, ¹³⁴Cs, ¹³⁷Cs and ⁷Be) in soil samples as a function of measurement time. It was shown that the neural network provides useful data even from small experimental databases. The performance of the optimized neural network was found to be very good, with correlation coefficients (R²) between measured and predicted uncertainties ranging from 0.9050 to 0.9915. The correlation coefficients did not significantly deteriorate when the network was tested on samples with greatly different uranium-to-thorium (²³⁸U/²³²Th) ratios. The differences between measured and predicted uncertainties were not influenced by the absolute values of uncertainties of measured radionuclide activities. Once the ANN is trained, it could be employed in analyzing soil samples regardless of the ²³⁸U/²³²Th ratio. It was concluded that a considerable saving in time could be obtained using the trained neural network model for predicting the measurement times needed to attain the desired statistical accuracy.     

The water purification system for the low background counting test facility of the Borexino experiment at Gran Sasso

The Borexino experiment, for the study of solar neutrino physics, requires radiopurity at the level of 5 × 10⁻¹⁶ g/g ²³⁸U equivalent (or 6 × 10⁻⁹ Bq/kg) on a detector mass of many tons of scintillator. Feasibility studies are performed in a counting test facility now operating at LNGS, which consists of 4 t of liquid scintillator viewed by 100 photomultipliers and shielded by 100 t of water. The accomplishment of this goal requires the shielding liquid, water, to be at the 10⁻¹³ g/g contamination level (1.2 × 10⁻⁶ Bq/kg) or better. This paper describes the water purification system; it consists of a combination of several purification processes to remove particulate, radioactive ions, dissolved gases and other impurities. Residual contaminations are measured by analytical or direct-counting techniques. For radon measurement, particularly challenging at this low activity levels, a low background counting method has been developed.     

A fission-fragment-sensitive target for X-ray spectroscopy in neutron-induced fission

A fission-fragment-sensitive detector built for low-energy photon spectroscopy applications at the WNR “white” neutron source at Los Alamos is described. The detector consists of eight layers of thin photovoltaic cells, onto which 1 mg/cm² of pure ²³⁸U is deposited. The detector serves as an active target to select fission events from background and other reaction channels. The fairly small thickness of the detector with respect to transmission of 20–50 keV photons permits the measurement of prompt fission-fragment X-rays. Results with the GEANIE photon spectrometer are presented.     

Measurements of thick target fission yields as a probe of fission excitation functions for deep sub-barrier energies

A close-geometry experimental technique has been developed and used to measure thick target fission yields for protons on ²³⁸U from 4.2 to 12 MeV. Fission fragments were detected at back angles by an annular photo-voltaic detector in coincidence with γ-ray registered in large NaI(Tl) detector. The neutron induced fission background was monitored by a detector on the downstream side of the thick target. Measured yields compare well with those calculated from published thin target cross sections. This method has the required sensitivity to carry out measurements at still lower energies where recent measurements give conflicting findings.     

Functional equation for the fading correction of imaging plates

Several experiments were performed to investigate the effects of temperature on the fading of the commercial imaging plate, IP (BAS-UR). The fading characteristics were measured under controlled temperatures between 0°C and 60°C after irradiation with ²³⁸U alpha-ray and ⁶⁰Co gamma-ray sources. Applying Arrhenius' equation to the experimental results, we calculated a universal functional equation that includes two variables: elapsed time (t) and temperature (K). The photo-stimulated luminescence (PSL) calculated by this equation showed good agreement with the results of our experimental ones between 0°C and 40°C. From this equation, the activation energies of the fading are estimated to be about 0.9 eV for both ²³⁸U alpha-ray and ⁶⁰Co gamma-ray sources.     

Verification of U mass content in nuclear fuel plates by an absolute method

Nuclear Safeguards is referred to a verification System by which a State can control all nuclear materials (NM) and nuclear activities under its authority. An effective and efficient Safeguards System must include a system of measurements with capabilities sufficient to verify such NM. Measurements of NM using absolute methods could eliminate the dependency on NM Standards, which are necessary for other relative or semi-absolute methods. In this work, an absolute method has been investigated to verify the ²³⁵U mass content in nuclear fuel plates of Material Testing Reactor (MTR) type. The most intense gamma-ray signature at 185.7 keV emitted after -decay of the ²³⁵U nuclei was employed in the method. The measuring system (an HPGe-spectrometer) was mathematically calibrated for efficiency using the general Monte Carlo transport code MCNP-4B. The calibration results and the measured net count rate were used to estimate the ²³⁵U mass content in fuel plates at different detector-to-fuel plate distances. Two sets of fuel plates, containing natural and low enriched uranium, were measured at the Fuel Fabrication Facility. Average accuracies for the estimated ²³⁵U masses of about 2.62% and 0.3% are obtained for the fuel plates containing natural and low enriched uranium; respectively, with a precision of about 3%.     

A large solid-angle array for heavy ions from APEX

A large solid-angle array of Low Pressure Multi-Wire Proportional Counters (LPMWPCs) has been constructed as part of the ATLAS Positron Experiment (APEX). Eight three-element trapezoidal counters span 360° in φ and 20–68° in laboratory θ. Angle sensitivity in θ is provided by a transmission-line delay cathode, while the 24-fold segmentation provides angle sensitivity in φ. Details are given on the design of the counters and associated hardware. Performance of the counters is described, based on tests with scattered heavy ions ranging from ⁵⁸Ni to ²³⁸U.     

Measurement of high momentum transfer reactions by recoil detection

A method is described for studying reactions at medium energies by measuring the magnetic rigidity, time of flight, and energy loss of the resulting recoil products. The method works best for reactions with two-body final states where one of the outgoing particles is relatively light. A magnetic spectrometer with a special focal plane detector has been used. The detector consists of two position sensitive parallel plate avalanche counters and two proportional counters. Differential cross sections for the ¹²C(p, π⁺¹³C reaction at recoil angles which correspond to the pion being emitted at a backward angle in the center of mass have been measured at bombarding energies of 166 and 186 MeV. Data were also obtained for previously unobserved ¹²C(p, π⁰)¹³N reaction. Several advantages of the method are discussed.     

Trace uranium analysis by isotope dilution alpha and mass spectrometry and comparison with other techniques

In this paper uranium determination by isotope dilution mass (MSID) and alpha spectrometry (ASID) using ²³³U as an isotope diluent is discussed. In addition, a new form of MSID employing two tracers (²³⁵U and ²³³U) is developed, where two independent uranium values are obtained for single dilution. The precision and accuracy of each technique are evaluated by comparison with other techniques like X-ray fluorescence and instrumental neutron activation analysis. The techniques, employed here for geological samples, can easily be adapted for environmental materials.     

A contribution to improvement of the nuclear data concerning alpha decay of U

Compared to other alpha-particle emitting nuclides, for those with long half-lives (T_1/2>10⁶ years), there is generally some inconsistency and inaccuracy in the associated nuclear data. However, these radionuclides play a major role in modern society and, for a number of reasons, are the subject of widespread interest. The study of their alpha decay, which is more difficult than in the case of the other nuclides, enables knowledge of their nuclear data to be increased. In the present paper, we accordingly consider one of the most well known ones: ²³⁵U. Using a quasi mono-isotopic deposit of ²³⁵U obtained by electromagnetic separation, we have studied the alpha-particle decay of ²³⁵U using a high-energy resolution spectrometer developed in our laboratory. The stability of the spectrometer made it possible to perform long measurements which is crucial in the case of ²³⁵U. The Colégram program, with a new fit criterion to better take into account peaks of low statistical significance, was used to extract the data (alpha-particle emission probabilities and energies). Monte-Carlo simulations carried out in the context of this work were also used to corroborate the data obtained. Finally, thirteen alpha-particle emissions from ²³⁵U were measured, from which only ten were previously observed and measured. This work has substantially reduced the uncertainty concerning alpha-particle emission probability and energy values and has resolved some of the ambiguities concerning the ²³⁵U disintegration scheme to ²³¹Th.     

Preparation of a Pu standard source II. Source preparation and standardization

A standard source of ²³⁸Pu was prepared for calibrating the counting efficiency of alpha-ray detector. The plutonium was electrodeposited on a platinum or tantalum disk using isopropyl alcohol-hydrochloric acid solution as an electrolyte. The absolute activity was certified by isotope dilution alpha-ray spectrometry. Several types of the source, whose areas ²³⁸Pu-deposited are from 2.0 to 25.0 mm in diameter, were also prepared by the method. The overall uncertainties of the certified values for the standard sources prepared are estimated to be within 0.15 to 0.25% (1σ).     

CeO2修饰Mn-Fe-O复合材料及其NH3-SCR脱硝催化性能

氨选择性催化还原(NH3-SCR)技术需要进一步研发在相对较低温度(<300℃)下具有良好催化活性、高稳定性及环境友好的脱硝催化材料。本工作采用草酸共沉淀法制备Mn-Fe-O催化材料,并对其进行不同含量CeO2修饰,用于低温NH3-SCR脱硝催化反应。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、氮气吸附-脱附、X射线光电子能谱(XPS)、程序升温还原或脱附(H2-TPR、NH3-TPD)等手段对催化剂进行了表征。催化结果表明,在相同反应条件下适量CeO2修饰后的Mn-Fe-O样品比纯Mn-Fe-O表现出更优异的NH3-SCR脱硝催化性能,在80℃时NO转化率在95%以上,且具有较高的N2选择性。CeO2修饰提高了Mn-Fe-O氧化物表面的Fe^3+、Mn^3+和Mn4+含量及表面酸性位点数量,从而有助于NH3的吸附及催化反应的进行,并且Fe^2+/Fe^3+、Mn^2+/Mn^3+/Mn^4+以及Ce^3+/Ce^4+电子对之间的相互氧化还原反应提高了催化剂的氧化还原能力及稳定性。     

Detector for Rn measurements in air at the 1 mBq/m level

The emission of and β radiation in the decay of long lived (22.3 yr) ²¹⁰Pb to ²⁰⁶Pb gives rise to background events in the fiducial volume of the Borexino solar neutrino detector that fall in the energy window of the ⁷Be neutrino signal. Consequently, ²²²Rn as the predecessor of ²¹⁰Pb must be controlled in the central part (inner vessel) of the Borexino detector and in the air of all rooms where parts of the inner vessel are prepared or handled. The Rn monitor designed for Borexino uses electrostatic collection of the ²²²Rn daughter ²¹⁸Po on an detector. The detector (418 l active volume) operates at 30 kV. The detection limit for ²²²Rn in air is below 1 mBq/m³. The method of electrostatic collection has the principal disadvantage that the collection efficiency of the daughter ions depends on the degree of contamination of the air sample with water, hydrocarbons or other components. This causes an additional systematic error if the degree of pollution is unknown. A method to correct for this is outlined.     

Prompt neutron spectrum measurement of U fission induced by 10.17 and 12.12 MeV neutrons

The prompt neutron spectra of ²³⁸U fission induced by 10.17 and 12.12 MeV neutrons have been measured using a multi-segment fission chamber and two ST-451 liquid scintillator neutron detectors in conjunction with a double-TOF technique. Use of coincidence between the fission fragments and neutrons enabled the fission neutrons to be distinguished from other secondary ones. The detection efficiencies of the ST-451 liquid scintillators were determined by combining Monte Carlo calculation using a standard code NEFF4 [1] with experimental calibration using a low-mass ²⁵²Cf ionization chamber. The measured spectra can be roughly described by Maxwellian distributions. The Maxwellian temperatures are obtained by fitting equal 1.40±0.04 MeV for 10.17 MeV incident and 1.47±0.04 MeV for 12.12 MeV, respectively. The experimental spectra are compared with theoretical calculated ones by code FINESSE [2] and the evaluation by ENDF/B-VI [3].     

A coincidence system for radionuclide standardization using surface barrier detectors

A system for the standardization of alpha-gamma or electron-X radionuclide emitters is described. The system consists of one or two surface barrier detectors for alpha or electron detection which are coupled to thin-window NaI(Tl) crystals suitable for low-energy X- or gamma-ray detection. The performance of the system has been verified by the standardization of ²⁴¹Am, ¹³⁷Cs and ¹⁰⁹Cd solutions. The activity has been obtained using the extrapolation method applied to the 4π -γ and 2π e-X coincidence techniques. The surface barrier detection efficiency was varied by placing absorbers over the radioactive source or by changing the source-to-detector distance. The results were compared to those obtained using conventional absolute systems based on gas-flow and pressurized 4π proportional counters, or using radioactive solutions standardized in international comparisons sponsored by the Bureau International des Poids et Mesures, France. The expected and measured activities agreed within the experimental uncertainties, which were: 0.2% for ²⁴¹Am, 0.7% for ¹³⁷Cs and 0.6% for ¹⁰⁹Cd.     

Detection of particle unbound states using a single Gas–Si–CsI hybrid telescope

A Gas–Si–CsI hybrid detector telescope is described and its performance assessed in measuring breakup reactions leading to particle unbound states. The telescope consists of an ionisation chamber, a silicon strip detector and a CsI(Tl) scintillator-photodiode detector. The segmentation of the strip detector allows the simultaneous detection, identification and reconstruction of heavy and light ions within a single hybrid telescope. For double hits in the strip detector, crosstalk between the two active strips modifies the measured energies requiring a correction to the energy calibrations. The technique has been extended to the detection of energetic -particles from the decay of ⁸Be_gs fragments. Using two telescopes, a measurement of the ¹²C(²⁴Mg, ¹⁶O ¹²C) reaction at 170 MeV has identified final states where the ¹²C ion and -particle originate from unbound states in ¹⁶O. The sensitivity to small relative energies between the fragments makes the telescope particularly suitable for the study of near-threshold states of astrophysical interest.     

Preparation of a Pu standard source: I. Isotope dilution alpha-ray spectrometry used for standardization

A ²³⁸Pu source was prepared by electrodeposition method. For the standardization of the source, isotope dilution alpha-ray spectrometry (IDAS) was investigated: The content of ²³⁸Pu in a sample solution was determined both by IDAS and by isotope dilution mass spectrometry (IDMS) using a standard reference material of ²³⁹Pu as a spike. The estimated uncertainty in the determination of ²³⁸Pu is 0.13% for IDAS, and the factor of IDAS/IDMS is calculated to be 1.0000±0.0015.     

The production of monoenergetic fast neutrons of known flux: An improved method to identify the associated particle

The performance of a deuterium gas target to produce neutrons from the D(d, n)³He reaction is reported. The associated particle method is used. The associated particle is unequivocally identified with a telescope employing microchannel-plate electron multipliers and a silicon surface barrier detector, providing both time-of-flight (TOF) and energy (E) information. The system was set up at the 7 MV Van de Graaff CN accelerator of the laboratori Nazionali di Legnaro (Padova), and has been tested with deuteron energies from 2.5 to 6.5 MeV. About 300 tagged neutrons per s sr nA atm are produced. The TOF-E identification method to discriminate the associated particle will be used in the future in measurements at higher deuteron energies and with the reaction T(d, n)⁴He.     

The kinematic separator VASSILISSA performance and experimental results

Within the last five years the kinematic separator VASSILISSA [A.V. Yeremin et al., Nucl. Instr. and Meth. A 274 (1989) 528; and A.V. Yeremin et al., Preprint JINR E15-90-347, Dubna 1990] has been used for investigations of neutron deficient evaporation residues (ER) produced in heavy ion fusion reactions. In the course of this work optimization of the ion-optical and other systems of the separator including improvements of the focal plane detector system have been made. As a result, the separation efficiency values ranging from 3 to 30% were achieved for ERs produced in the reactions with heavy ions ranging from ¹⁶O to ⁴⁰Ar. The suppression factors >10¹⁹ for the full energy beam particles and >10⁴ for multinucleon transfer reaction products were achieved. The results of the performed experiments illuminating the separator performance are briefly presented.