Optimization of HANARO cold neutron induced prompt gamma activation analysis system by using Monte Carlo code

Prompt gamma activation analysis (PGAA) is a nuclear analytical technique for non-destructive determination of elemental and isotopic compositions. The principle of PGAA technique is based on detection of captured gamma-ray emitted from an analytical sample while being irradiated with neutrons. Use of a cold neutron beam guide greatly reduces the gamma-ray background at the analytical sample while maintaining a neutron capture rate is comparable to that of standard thermal neutron PGAA. A new cold neutron induced prompt gamma activation analysis (CN-PGAA) system has been under construction since April of 2009 at the HANARO Cold Neutron Building (KAERI, Republic of KOREA). In this study, the Compton suppression factor of the CN-PGAA system was estimated to be 5.5 using a ⁶⁰Co radioactive source in conjunction with the MCNPX simulations. Several parameters of the CN-PGAA system were studied to estimate and optimize the performance of the system: scintillation material in the guarded detector of a Compton suppression spectrometer (CSS); the relative positions of the HPGe detector and annular detector; and the distance between the HPGe detector and back catcher BGO detectors of the CSS. In addition, the neutron ray-trace simulation package, McStas, was adopted to predict the neutron flux and wavelength distribution at the end of the cold neutron beam guide. These results served as input for the MCNPX simulation of the CN-PGAA system.     

A comparison of neutron-activation analysis and hot extraction analysis of the oxygen content of steel

A system developed for the fast neutron-activation analysis of the oxygen content of metals has been tested comparatively with the conventional vacuum fusion and carrier-gas fusion techniques. The results of these tests indicate that neutron-activation analysis is much faster (the total analysis takes only 2 min or less), and more reliable than vacuum fusion and carrier-gas fusion methods because all oxygen present is analysed. Samples can be much larger than the 0.2-3 g commonly used for the fusion methods. Furthermore, the analysis is non-destructive-the same samples can be re-analysed as often as desired. The fast neutron-analysis system includes a 14-MeV neutron generator producing 10(11) neutrons/sec, a dual-tube pneumatic transfer system, a 5 x 5 inch NaI(T1)crystal, a single-channel analyser, two scalers, and timers and switch-gear. A sample, in a polyethylene bottle, and a Lucite reference are irradiated simultaneously, after which the sample is returned to a detector for counting the (16)N gammas from the (16)O(n,p)(16) N reaction. The reference is then counted in a second detector; the ratio of the sample counts to the reference counts is proportional to the oxygen content of the sample. Samples with oxygen contents from 0.002 to 0.1 % of oxygen have been analysed by neutron activation, then cut in several pieces for hot extraction analysis of the total sample.     

Track etching characteristics of polyester track detector and its application to uranium estimation in seawater samples

Garware Polyester Film, an indigenously available material has been evaluated systematically as a nuclear track detector for the detection of fission fragments. The relative fission track detection efficiency of this film was found to be (86.0±4.0)%. The bulk etch rate, determined by the gravimetric method, was found to be 0.75±0.05 μm/h. The track etch rate was determined as 15.0±1.5 μm/h. This detector was employed for the estimation of uranium in seawater samples and the results obtained were compared with the results obtained by using the commonly used Lexan detector. Uranium fractions after chemical separation from seawater samples were also analyzed by alpha-spectrometry and neutron activation analysis techniques and the results were compared with that obtained by the fission track method. Fission track method has the advantage, as it does not require any chemical separation. The indigenously available polyester film (polyethylene terphthalate) appears to be a good substitute of Lexan as nuclear track detector.     

Determination of rare-earth elements in rock samples by neutron activation analysis with a lithium-drifted germanium detector after chemical group-separation

A lithium-drifted germanium detector combined with chemical group-separation has been utilized for the determination of rare-earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb and Lu) in rock samples by neutron activation. This procedure has the advantage of a low background level which cannot be attained in the non-destructive method. The combination of the Ge(Li) detector and chemical group-separation also offers a distinct simplification in the correction of contributions from other nuclides. For optimum utility of a Ge(Li) detector in neutron activation analysis, chemical group-separations are recommended.     

NAAPRO: A code for predicting results and performance of neutron activation analysis

Summary A code is described for predicting the results and main characteristics of neutron activation analysis (NAA) on the basis of a simulated gamma-ray spectrum of activation products, calculated for the specified analysis conditions. These are analysis time mode, analyzed sample mass and elemental composition, characteristics of the irradiating neutron flux and irradiation conditions, gamma-spectrometry measurement geometry and background conditions, as well as detector and spectrometry system parameters. Gamma-ray dose rates for different points of time after sample irradiation and input count rate of the spectrometry system are also predicted.     

On-line analysis of coal by neutron induced gamma spectrometry

Thermal neutron prompt gamma activation analysis (PGNAA) has proven to be a useful tool for the multi-elemental characterization of coal. The use of isotopic neutron sources allows the construction of relatively small irradiation facilities for the on-line analysis of large volume samples. As a first step to an on-line analysis system for process control in a coal blending plant we have developed a facility using a Cf-252 neutron source with an actual strength of 1 mg. This source, which is shielded by layers of high purity lead and paraffin, can be moved to the center of cylindrical sample holders containing up to 150 kg of coal. To avoid background radiation the sample holders are made from polyethylene. The prompt gamma rays emitted from the sample are measured with a 22% high purity germanium detector housed in a special horizontally extended cryostate assembly. The detector is positioned on top of the sample; it is surrounded by a multi-layer decreasing Z-absorber to reduce low energy gamma background. Paraffin loaded with 95% enriched lithium-6-carbonate is used between sample and detector as a gamma window with high absorption for thermal neutrons. Moreover the detector is shielded against background radiation by lead and natural lithiumcarbonate. The data processing system consists of high count-rate analogue electronics including pileup-suppression and a fast data acquisition system, which permits on-line analysis of the collected data. Analysis of several samples of simulated coals as well as Western Germany coals shows a good spectral response of our system for the elements H, C, Al, Si, S, Cl, Ca, Ti and Fe, whereas the sensitivity for N, K and Na is somewhat lower. This means, that the quantitative determination of the above elements will be possible within 20 minutes. Optimization of the system is continued with a stress on analysis time and detection sensitivity. Supported by Bundesministerium für Forschung und Technologie D-5300 Bonn 1, Fed. Rep. of Germany.     

An automated activation analysis data acquisition system

An automated neutron activation analysis data acquisition system has been assembled from commercially available equipment. The modifications of the components needed to make this into a working system are described in the text. The main components of the data acquisition system are a sample changer, a Ge(Li) detector, a magnetic tape deck and a minicomputer based multichannel analyzer. The sample changer has a 200-sample capacity and can handle both solid and liquid samples. Software for controlling the data acquisition system is flexible, yet simple to use. The system has operated reliably for a year and has sharply reduced the effort needed for data acquisition.     

Analysis of boron in biological reference materials using prompt gamma activation analysis

Summary A prompt gamma activation analysis facility has been constructed on the ST1 horizontal beam port at the HANARO research reactor, KAERI in 2003. The detector system consists of a high-purity Ge detector surrounded by BGO/NaI(Tl) scintillators as an annulus type to reject the Compton scattered photons. Detection sensitivity for boron was obtained from the prompt gamma-ray spectra of boric acid, B(OH)₃, containing 0.1-65 μg boron. The net peak for the calculation of the boron concentration was obtained by eliminating the sodium 472 keV peak, involved in the boron 478 keV peak. The biological samples used are NIST SRMs such as Peach Leaves, Apple Leaves, Tomato Leaves, Spinach Leaves, Total Diet, Typical Diet, Oyster Tissue and Corn Bran, etc. The measured values for high boron concentration showed up to a 3% of the relative, but in a low concentration below 5 ppm, present values were higher than the certified ones.     

Comparison of neutron capture spectra taken by a constellation Xe-110 detector and a 1″×1″ NaI(Tl) crystal

Constellation Technology Corporation has developed a spectroscopic nuclear detector called the Xe-110 that is well suited for downhole use in oil and gas well logging. The advantages possessed by this detector over traditional technologies such as NaI(Tl) scintillators are summarized. We discuss an analysis of gamma-ray spectra taken by the Xe-110, which suggests that the superior energy resolution and low background level of this detector enables identification of important features in neutron activation spectra of hydrocarbons that might not be identifiable using an NaI(Tl) crystal.     

Multicommutated flow analysis system for determination of creatinine in physiological fluids by Jaffe method

For the needs of photometric determination of creatinine according to Jaffe protocol a dedicated paired emitter detector diode (PEDD) detector has been developed. This PEDD device has been constructed in the compact form of flow-through cell (30 μL total volume and 7 mm optical pathlength) integrated with 505 nm LED-based emitter and 525 nm LED-based detector compatible with multicommutated flow analysis (MCFA) system. This fully mechanized MCFA system configured of microsolenoid valves and pumps is operating under microprocessor control. The developed analytical system offers determination of creatinine in the submillimolar range of concentrations with detection limit at ppm level. The throughput offered by the system operating according to multi-point fixed-time procedure for kinetic measurements is 15–40 samples per hour depending on the mode of measurements. The developed PEDD-based MCFA system has been successfully applied for the determination of creatinine in real samples of human urine as well as serum. The developed sampling unit used the system is free from effects caused by differences in sample viscosity.     

Usefulness of thermal and epithermal cyclic activation analysis with a reactor system

A new cyclic activation system (CAS) has recently been installed at the University of London Reactor and is briefly described. It has the unique feature that it allows choice as to whether irradiations are to be carried out under cadmium or in a bare tube and thus both epithermal and thermal cyclic activation analysis can be performed. Measurements in this work, have been carried out using a Ge(Li) detector but the facility allows other detectors to be used and an additional return line from the central distributive mechanism of the system (not yet installed) will allow samples to be cycled to and from a neutron detector thus providing the possibility of measuring radionuclides emitting delayed neutrons. By way of illustration of the theory of cyclic activation analysis, since the paper is to serve as an introduction to other papers in the conference using the technique, the results of a calculation for the detection of uranium under thermal and epithermal conditions are presented and compared to conventional activation. The usefulness of cyclic neutron activation analysis (CNAA) is also demonstrated in cases where identification of an isotope through its gamma-ray energy alone is not sufficient and conformation is achieved through determination of the half-life of the activity of interest, without resort to further experimental work or additional instrumentation. This leads on the thorny problem as to whether Poisson statistics should continue to be used in cases where the period of observation is of the same order as the decay constant of the activity of interest and it is pointed out how CNAA can be employed to extract the information required if a binomial-type statistical density function is used. Finally in order to prove the system and the cyclic method and as a preliminary to a larger project, prepared single element standards and standard reference materials [NBS Orchard Leaves, NBS Bovine Liver, IAEA Animal Blood, IAEA Hair standard and Bowen's Kale] are analysed under the two cyclic irradiation conditions. Sensitivities and cadmium ratios are obtained for the twelve elements from the elemental standards (Na, Al, Sc, Ge, Se, Rh, Pd, Ag, Ce, Er, Hf and Pb) and detection limits for a number of elements of interest presented for the standard reference materials.     

Efficiency calibration of an HPGe detector in the 0.1–2.5 MeV energy range for Am-Be neutron source-based PGAA applications

A 740 GBq ²⁴¹Am-Be neutron source based prompt gamma-ray activation analysis (PGAA) setup in combination with a typical coaxial n-type HPGe detector (REGe) system was used to analyze light elements like H, B, C, N, etc. The absolute full energy peak (FEP) efficiencies of the shielded REGe detector for irradiation and counting geometries and for sources with different sizes (point, ampoule and cylindrical) were measured in the 0.1–2.5 MeV energy range by utilizing calibrated sources (point, liquid and solid). 4th order polynomials were fitted to the experimental data. Efficiencies in far irradiation and counting geometries are compared.     

Experimental Evaluation of a Portable Neutron-Based Gamma-Spectroscopy System for Chloride Measurements in Reinforced Concrete

A portable prompt gamma neutron activation (PGNA) spectroscopy system has been developed to analyze the elemental composition (Ca, Si, Al, etc.) of reinforced concrete and to measure chloride contamination. The portable PGNA system consists of a high purity germanium (HPGe) gamma detector with a 70% relative efficiency, a ²⁵²Cf neutron source and moderator subsystem, and a portable multichannel analyzer system integrated with a laptop computer. Two types of activation experiments were performed to evaluate the device: first, a detector calibration using a Cl gamma standard provided by a PGNA facility; second, an evaluation of the actual performance of the complete system with the ²⁵²Cf source using full scale test slabs containing known amounts of chloride. Both methods indicate that it is feasible to use this device to measure the chloride content of reinforced concrete in the field. The chloride level for the corrosion threshold can be measured with a precision of 10% for a counting time of roughly 6 minutes. This makes the PGNA method competitive with the conventional destructive method.     

Laser-induced fluorescence microscopic system using an optical parametric oscillator for tunable detection in microchip analysis

A laser-induced fluorescence microscopic system based on optical parametric oscillation has been constructed as a tunable detector for microchip analysis. The detection limit of sulforhodamine B (Ex. 520 nm, Em. 570 nm) was 0.2 mol, which was approximately eight orders of magnitude better than with a conventional fluorophotometer. The system was applied to the determination of fluorescence-labeled DNA (Ex. 494 nm, Em. 519 nm) in a microchannel and the detection limit reached a single molecule. These results showed the feasibility of this system as a highly sensitive and tunable fluorescence detector for microchip analysis.     

A new NaI detector arrangement for efficient detection of high energy gamma-rays

Summary Prompt gamma-ray neutron activation analysis (PGNAA) and neutron inelastic scattering (NIS) techniques have been widely used for measuring elemental composition in bulk samples. The neutrons and gamma-rays used in this technique are highly penetrating, which allows the analysis of large sample volumes. In the oil well logging industry, there are limitations on the size of detectors used -particularly the detector diameter. This limitation can lead to a low detection probability for the high energy gamma-rays. A new NaI detector arrangement (patent pending) has been designedto deal with this problem. The arrangement consists of two NaI detectors, one of which is a well type. The first detector is 1"×5" and the second is a well type 5" long with a wall thickness of 0.35" and a hole diameter of one inch. The first detector is placed inside of the well detector for use. Feasibility studies were performed with this arrangement using a ²⁴Na radioactive source and a sulfur sample or prompt gamma-rays. An enhancement in the signal-to-noise ratio (SNR) was observed in both cases based on the peak height to continuum height in the resulting prompt gamma-ray spectra.     

A new highly efficient set-up for cyclic and pseudocyclic short time activation analysis with high count rates

A new counting geometry with a simple sample changer was constructed to enable cyclic and pseudocyclic short-time activation analysis. With the new system it is possible to cycle a sample, or successively an indefinite number of samples up to 20 times. The sample changer acts at the same time as sample catcher for two n-type HPGe detectors and can release the sample into a well-type HPGe detector. The new system enables the simultaneous counting of the irradiated samples by means of two endcap HPGe detectors, and subsequent counting by means of the well HPGe detector or both detector types. A well detector ensures a high counting efficiency which improves the sensitivity of a large number of short lived nuclides. Some standard reference materials (i.e., BCR-176, NIST SRM 1633b, IAEA-336, 335b, 335c) were prepared and analysed in replicates. The results indicate that up to 46 nuclides can be determined in BCR-176 if the samples are irradiated with and without the⁶LiD converter. An automatic evaluation programme was developed that determines the FWHM calibration parameters for each spectrum for accurate peak-area estimation at high count rates.     

Determination of titanium in bauxite: A comparison of neutron activation analysis with the X-ray fluorescence method

The determination of the titanium content of bauxite samples of various origin was studied by thermal neutron activation and X-ray analysis. A²⁵²Cf-fission neutron source and a Ge(Li) detector as well as a³H exciting source and a Si(Li) detector were used in the investigations. Within equal measuring times and with a sample weight of 8 g the sensitivity of the activation method is 0.35 w% Ti with an absolute statistical error less than 10%, while that of the X-ray method is 0.06 w% Ti and the absolute statistical error does not exceed 5%.     

Optimized automated activation analysis

A combination of special techniques has been developed for optimization of experimental conditions in order to improve the analytical capability, to facilitate automation and to broaden the applicability of instrumental neutron activation analysis. The techniques used are: (1) compensation for the rapid radioactive decay of short-lived nuclides with the increase of the counting efficiency by automatic source movement to the detector during the counting period, to minimize count rate variations and to prolong the counting period, (2) repeated cyclic and cumulative activation to improve the counting statistics, (3) instrumental correction of counting losses at high and varying count rates by a loss-free counting system and (4) differentiation of the reactor neutron spectrum to enhance the counts from the nuclides of interest by reducing matrix interferences. By optimized combination and automation of these techniques significant improvement of the capability of instrumental neutron activation analysis can be achieved.     

Simultaneous determination of zirconium and hafnium in standard rocks by neutron activation analysis

A sensitive analytical technique has been developed to determine zirconium and hafnium at the 1 ppm and 0.01 ppm levels, respectively, in natural silicate matrices. The technique is based on the use of a Ge(Li) detector for high-resolution γ-ray spectrometry following irradiation with thermal neutrons. Simultaneous separation of both elements is achieved by addition of only zirconium carrier and use of a strongly basic anion-exchange resin. Hafnium was shown to follow zirconium throughout the chemical procedures. The chemical yield of the separation procedure was determined for each sample by use of an automated fast-neutron activation analysis system, which obviated the need to convert the separated zirconium to a conventional gravimetric weighing form. The method has been applied to the analysis of a variety of standard rocks and related natural materials.     

Rapid determination of antimony in biological and environmental samples using instrumental neutron activation analysis

A rapid non-destructive activation analysis method has been developed for the determination of antimony. A high resolution low energy Ge detector is used to measure the 61.6 keV γ-ray from^122mSb (T=4.2 min). Sensitivities and detection limits for biological and environmental samples activated with thermal and epithermal neutrons are listed. The time required for the anlaysis is about 12 min per sample using thermal activation and 22 minutes using epithermal activation analysis.