Parameterization of detector efficiency for the standardization of NAa with stable low flux reactors

With SLOWPOKE and MNS reactors which have reproducible neutron fluxes, the standardization of multielement NAA can be reduced to measuring activation constants once for all elements and then determining relative detection efficiencies for new detectors and counting geometries. In this work, a method has been developed for the parameterization of the efficiency of gemanium detectors. The gamma-ray detection efficiency was measured as a function of energy and distance for three detectors. The variation with distance was found to follow a modified EID law, within 1%, for point sources 1 mm to 250 mm from the detector. A model, including coincidence summing corrections, was developed to calculate efficiency for NAA samples; it requires 16 measured parameters. Tests showed that the calculated relative detection efficiencies are accurate to better than 3% for close counting geometries and sample volumes up to a few millilitres. Areas of possible improvement to the accurarcy of the method are suggested.     

Characterization of HPGe gamma spectrometers by geant4 Monte Carlo simulations

Two coaxial and a low-energy HPGe detector were characterized with Monte Carlo simulations, using the geant4 toolkit. The geometry of the detectors, including the dimensions of the crystal and the internal structural parts, were initially taken from the factory specifications and from X-ray radiographies, and were later fine-tuned. The detector response functions, with special emphasis on the absolute full-energy peak efficiencies and peak-to-total ratios, were calculated and compared to experimental data taken at different measurement geometries. Between 150 keV and 11 MeV an agreement within 1–2 standard deviation has been achieved, whereas systematic deviations were experienced at lower energies.     

True coincidence summing corrections in point and extended sources

The true coincidence summing (TCS) effect on the full energy peak (FEP) efficiency calibration of an HPGe detector has been studied as a function of sample-to-detector distance using multi-gamma sources. Analytical method has been used to calculate coincidence correction factors for ¹⁵²Eu, ¹³³Ba, ¹³⁴Cs and ⁶⁰Co for point and extended source geometry at close sample-to-detector distance. Peak and total efficiencies required for this method have been obtained by using MCNP code by using the optimized detector geometry. The correction factors have also been obtained experimentally. The analytical and the experimental correction factors have been found to match within 1–5%. The method has been applied to obtain the activity of the radionuclides (¹⁰⁶Ru, ¹²⁵Sb, ¹³⁴Cs and ¹⁴⁴Ce) present in a fission product sample.     

Characteristic absolute efficiency response curves of a high purity germanium detector in the energy range 50–1500 keV

The characteristic absolute efficiency response curves of a high purity germanium detector (HPGe) for different counting geometries have been established in the energy range 50–1500 keV by measuring the absolute efficiencies using both mono-energetic and multi-gamma emitting radionuclide point calibrated sources supplied by IAEA. Several fitting functions proposed in the literature were assessed for interpolation within the intermediate energy range of interest. The values of the function parameters have been determined by using the linear least square methods. The problems associated with the measurements of experimental efficiency data at small source–detector distances and the importance of the correlation matrix in the estimation of precise uncertainties have been shown. It was found that the inclusion of correlation matrices in the propagation of error formulae plays a significant role up to 450 keV gamma-ray energy and results in a drastic reduction of errors associated with the predicted efficiencies. The discrepancy at closer counting geometries in the absence of true gamma-gamma coincidence corrections is found to reach to about 30%.     

Assessment of the suitability of Monte Carlo simulation for activity measurements of extended sources

Monte Carlo simulations can be a powerful tool in calibrating high-resolution gamma-ray spectrometry based on high pure germanium (HPGe) detectors. The purpose of this work is to examine the applicability of Monte Carlo simulations for the computation of the efficiency transfer in various measurement geometries on the basis of the detected efficiency for point source geometry. For this, GEANT4 code was applied for the computation of the detection efficiency for incident gamma energy of radionuclide placed at different distances from HPGe detector from 50 to 2,000 keV in addition for volume sources of different compositions and densities. The experimental efficiency curves were compared with the prediction of the GEANT4 code. Efficiency is computed at discrete values of point and volume sources in different distances to derive new efficiencies values for other distances.     

Mathematical efficiency calibration methods for high quality laboratory based gamma spectrometry systems

The efficiency calibration of laboratory based gamma spectrometry systems typically involves the purchase or construction of calibration samples that are supposed to represent the geometries of the unknown samples to be measured. For complete and correct calibrations, these sample containers must span the operational range of the system, which at times can include difficult configurations of size, density, matrix, and source distribution. The efficiency calibration of a system is dependent not only on the detector, but on the radiation attenuation factors in the detector–source configuration, and therefore is invalid unless all parameters of the sample assay condition are identical to the calibration condition. An alternative to source-based calibrations is to mathematically model the efficiency response of a given detector–sample configuration. In this approach, the measurement system is calibrated using physically accurate models whose parameters can generally be easily measured. Using modeled efficiencies, systems can be quickly adapted to changing sample containers and detector configurations. This paper explores the advantages of using mathematically computed efficiencies in place of traditional source-based measured efficiencies for laboratory samples, focusing specifically on the possibility of sample optimization for a given detector, uncertainty estimation, and cascade summing corrections.     

Linear relations for efficiency calibration of γ-spectrometric measurements of bulk samples

The self-attenuation correction factor is used to relate the efficiency for a sample with a given matrix to the efficiency for an ideal sample with identical geometry but negligible photon attenuation. A certain linear relation for the efficiency for a given sample as a function of the efficiencies for a number of subsamples into which the original sample can be decomposed is established and experimentally validated. This relation can be used also in the case when the sample and the subsamples have different matrices. In this way the efficiency for volume samples with arbitrary compositions and densities can be constructed on the basis of the efficiencies (independently measured) for a number of basic geometries. Also a possibility to check the consistency of efficiency calibrations carried out with different standard sources (with different matrices) is provided.     

Semi-empirical determination of detector absolute efficiency in INAA of voluminous samples

When neutron activation analysis of voluminous samples is performed using the absolute method, the detector absolute efficiency for -ray emiting distributed sources must be known. In this study, a Monte-Carlo program was developed to include the calculation of the effective solid angle subtended by a collimated detector from irregularly shaped voluminous samples. The program cna cope with dififerent sample shapes and geometries provided that the sample covers the view of the detector. Data such as the source and detector dimensions, the source-detector distance, the detector view at a cartain distance, the thickness and the composition of any intervening materials, the -ray energies of interest and the corresponding attenuation coefficients for each material are required. The method adopted for calculating the detector absolute efficiency of the voluminous sample in a certain geometry takes into account the effect by -rays baing emited from different position within the sample and also considers their attenuation in the sample material as well as any intervening materials between the sample and the detector and is compared with a reference point source. The alculations are varified experimentally using a distributed source of 75 mm diameter and 100 mm thickness and two semiconductor detectors. The difference between the calculated and measured absolute efficiencies did not exceed 4%.     

Monte Carlo characterisation of a Compton suppressed broad-energy HPGe detector

GEANT4 Monte Carlo simulations have been successfully utilised to characterise a Compton suppressed broad-energy HPGe detector. The detector setup has been fully recreated in the simulation, which has been optimised to consistently reproduce the detector response. The peak efficiencies for both the primary BEGe detector and NaI(Tl) guard detectors agree with the simulated values for multiple test sources within 3 %. Compton suppression has also been simulated, with good agreement seen between the simulated and actual CSF values (<10 %) for multiple radionuclides. A secondary reference source was also simulated, which contained up to 30 radionuclides in a different geometry to that of the previous source. This showed excellent agreement with experimental data in both unsuppressed and suppressed modes of operation.     

Stability of metallofullerenes following neutron capture reaction on the metal ion

To achieve the highest possible sensitivity of analysis for environmental samples it is common practice to use both a high efficiency detector and a close measurement geometry with a large sample size (e.g. Marinelli beaker). Under such conditions, the typical efficiency calibration procedure results in a biased activity value for many nuclides due to the true coincidence summing effect. While there are a few methods to correct for this effect with special calibration standards, such calibrations can be both time consuming and expensive. Due to these calibration difficulties, the true coincidence summing effect is often simply ignored. Recently, it has been demonstrated that the coincidence summing correction can be performed mathematically even for voluminous sources. This new method consists of an integration of the coincidence correction factor over the sample volume while taking into account its chemical composition and the container. In this paper, we will discuss the latest approaches for establishing the peak efficiency and peak-to-total efficiency curves, which are required for this method. These approaches have been tested for HPGe detectors of two different relative efficiencies.     

Quantitative portable gamma-spectroscopy sample analysis for non-standard sample geometries

Utilizing a portable spectroscopy system, a quantitative method for analysis of samples containing a mixture of fission and activation products in nonstandard geometries was developed. This method was not developed to replace other methods such as Monte Carlo or Discrete Ordinates but rather to offer an alternative rapid solution. The method can be used with various sample and shielding configurations where analysis on a laboratory based gamma-spectroscopy system is impractical. The portalle gamma-spectroscopy method involves calibration of the detector and modeling of the sample and shielding to identify and quantify the radionuclides present in the sample. The method utilizes the intrinsic efficiency of the detector and the unattenuated gamma fluence rate at the detector surface per unit activity from the sample to calculate the nuclide activity and Minimum Detectable Activity (MDA). For a complex geometry, a computer code written for shielding applications (MICROSHIELD) is utilized to determine the unattenuated gamma fluence rate per unit activity at the detector surface. Lastly, the method is only applicable to nuclides which emit gamma-rays and cannot be used for pure beta or alpha emitters. In addition, if sample self absorption and shielding is significant, the attenuation will result in high MDA's for nuclides which solely emit low energy gamma-rays. The following presents the analysis technique and presents verification results using actual experimental data, rather than comparisons to other approximations such as Monte Carlo techniques, to demonstrate the accuracy of the method given a known geometry and source term.     

Advances in HPGe Detectors for Real-World Applications

Germanium detector use and crystal production has progressed to such a degree that the IEEE standard for the performance of the detector is no longer adequate to predict the efficacy for a different situation. The specifications of the standard do not predict how a detector will perform at other energies or geometries. One such geometry that is poorly predicted is extended sources. Many parts of the detector and electronics that can be changed to make the total system perform better for a specific case, but such changes can worsen the performance of the detector at the IEEE specification. Examples will be given for the efficiency, MDA, throughput, and resolution for different source—detector and crystal configurations.     

Using Monte Carlo methods to estimate efficiencies of gamma-ray emitters with complex geometries

In the event of a radioactive disaster, one of the biggest tasks is to estimate the radiation dosage received by people to determine the actions of emergency response teams. The first and the most rapid screening method of internally contaminated people in case of an emergency response is to perform in-vivo measurements for gamma-emitters. Development of virtual gamma-ray calibration techniques will be critical for emergency invivo measurements because there are inadequate numbers of phantom types to approximate all body shapes and sizes. The purpose of this project was to find a reliable way to estimate the efficiency of gamma-systems using Monte Carlo computations, and to validate that efficiency by making measurements of a standard geometry. Two geometries, a 5-ml ampoule and a Bottle Manikin Absorption (BOMAB) phantom head, spiked with ⁶⁷Ga were used as standard geometries. The radioactive objects are measured at a number of distances from a high purity germanium (HPGe) detector, and the experimental efficiency for our gamma-spectrometry system is determined. The same set of experiments was then modeled using the Monte Carlo N-Particle Transport Code (MCNP). The conclusion of this project is that computationally derived detector efficiency calibrations can be comparable to those derived experimentally from physical standards.     

Design and construction of an ultra-low-background 14-crystal germanium array for high efficiency and coincidence measurements

Physics experiments, environmental surveillance, and treaty verification techniques continue to require increased sensitivity for detecting and quantifying radionuclides of interest. This can be done by detecting a greater fraction of gamma emissions from a sample (higher detection efficiency) and reducing instrument backgrounds. A current effort for increased sensitivity in high resolution gamma spectroscopy will produce an intrinsic germanium (HPGe) array designed for high detection efficiency, ultra-low-background performance, and useful coincidence efficiencies. The system design is optimized to accommodate filter paper samples, e.g. samples collected by the Radionuclide Aerosol Sampler/Analyzer (RASA). The system will provide high sensitivity for weak collections on atmospheric filter samples, as well as offering the potential to gather additional information from more active filters using gamma cascade coincidence detection. The current effort is constructing an ultra-low-background HPGe crystal array consisting of two vacuum cryostats, each housing a hexagonal array of 7 crystals on the order of 70% relative efficiency per crystal. Traditional methods for constructing ultra-low-background detectors are used, including use of materials known to be low in radioactive contaminants, use of ultra pure reagents, clean room assembly, etc. The cryostat will be constructed mainly from copper electroformed into near-final geometry at PNNL. Details of the detector design, simulation of efficiency and coincidence performance, HPGe crystal testing, and progress on cryostat construction are presented.     

Ab initio study of hydrogen bonding and proton transfer in 3:1 FH:NH3 and FH:collidine complexes: structures and one- and two-bond coupling constants across hydrogen bonds

Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH3 complexes at their own optimized MP2/6-31+G(d,p) geometries and at the optimized geometries in the hydrogen-bonding regions of corresponding 3:1 FH:collidine complexes. The isolated gas-phase equilibrium 3:1 FH:NH3 complex has an open structure with a proton-shared Fa-Ha-N hydrogen bond, while the isolated equilibrium 3:1 FH:collidine complex has a perpendicular structure with an Fa-Ha-N hydrogen bond that is on the ion-pair side of proton-shared. The Fa-N coupling constant ((2h)J(Fa-N)) for the equilibrium 3:1 FH:NH3 complex is large and negative, consistent with a proton-shared Fa-Ha-N hydrogen bond; (2h)JFb-Fa is positive, reflecting a short Fb-Fa distance and partial proton transfer from Fb to Fa across the Fb-Hb-Fa hydrogen bond. In contrast, (2h)JFa-N has a smaller absolute value and (2h)JFb-Fa is greater for the 3:1 FH:NH3 complex at the equilibrium 3:1 FH:collidine geometry, consistent with the structural characteristics of the Fa-Ha-N and Fb-Hb-Fa hydrogen bonds. Coupling constants computed at proton-transferred 3:1 FH:collidine perpendicular geometries are consistent with experimental coupling constants for the 3:1 FH:collidine complex in solution and indicate that the role of the solvent is to promote further proton transfer from Fa to N across the Fa-Ha-N hydrogen bond, and from Fb to Fa across the two equivalent Fb-Hb-Fa hydrogen bonds. The best correlations between experimental and computed coupling constants are found for complexes with perpendicular proton-transferred structures, one having the optimized geometry of a 3:1 FH:collidine complex at an Fa-Ha distance of 1.80 A, and the other at the optimized 3:1 FH:collidine geometry with distances derived from the experimental coupling constants. These calculations provide support for the proposed perpendicular structure of the 3:1 FH:collidine complex as the structure which exists in solution.     

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.     

Spectroscopic investigations and ab initio computations of the dye Chromotrope 2R

Detailed analysis of the NIR FT-Raman, FT-IR and UV–visible spectra of the dye Chromotrope 2R (C2R) has been performed. The optimized geometry of the dye is theoretically computed with the HF and DFT levels using the standard 6-31G(d) and LANL2DZ basis sets. Optimized geometry and vibrational spectra indicate that the major species in the solid state are the trans form of hydrogen bonded hydrazone tautomer. The effect of H-bonding in stabilizing a particular type of structure is also discussed. The most preferred trans-configuration for its photochemical activity has been demonstrated on the basis of torsional potential energy surface (PES) scan studies. The optimized geometries and calculated vibrational wavenumbers are evaluated via comparison with experimental values. Electronic spectra are in accordance with the nature of the electronic transitions predicted by time-dependent B3LYP/DZ calculations.     

The molecular, electronic structures and vibrational spectra of metal-free, N,N'-dideuterio and magnesium tetra-2,3-pyridino-porphyrazines: Density functional calculations

A theoretical investigation of the fully optimized geometries and electronic structures of the metal-free (TPdPzH(2)), N,N'-dideuterio (TPdPzD(2)), and magnesium (TPdPzMg) tetra-2,3-pyridino-porphyrazine has been conducted based on density functional theory. The optimized geometries at density functional theory level for these compounds are reported here for the first time. A comparison between the different molecules for the geometry, molecular orbital, and atomic charge is made. The substituent effect of the N atoms on the molecular structures of these compounds is discussed. The IR and Raman spectra for these three compounds have also been calculated at density functional B3LYP level using the 6-31G(d) basis set. Detailed assignments of the NH, NM, and pyridine ring vibrational bands in the IR and Raman spectra have been made based on assistance of animated pictures. The simulated IR spectra of TPdPzH(2) are compared with the experimental absorption spectra, and very good consistency has been found. The isotope effect on the IR and Raman spectra is also discussed.     

Improved detector response characterization method in ISOCS and LabSOCS

The In-Situ Object Calibration Software (ISOCS) and the Laboratory Sourceless Calibration Software (LabSOCS) developed and patented by Canberra Industries have found widespread use in the gamma-spectrometry community. Using the ISOCS methodology, one can determine the full energy peak efficiencies of a germanium detector in the 45 keV-7 MeV energy range, for practically any source matrix and geometry. The underlying mathematical techniques used in ISOCS (and LabSOCS) have undergone significant improvements and enhancements since their first release in 1996. One of these improvements is  a spatial response characterization technique that is capable of handling the large variations in efficiency that occurs within a small region. The technique has been in use in ISOCS and LabSOCS releases since 1999, and has significantly improved the overall quality of the close-in and off-axis response characterization for HPGe detectors, especially for Canberra’s Broad Energy Germanium (BEGe) detectors. In this method, the detector response is characterized by creating a set of fine spatial efficiency grids at 15 energies in the 45 keV-7 MeV range. The spatial grids are created in (r,&odash;) space about the detector, with the radius r varying from 0 to 500 meters, and the angle &odash; varying from 0 to π. The reference efficiencies for creating the spatial grids are determined from MCNP calculations using a validated detector model. Once the efficiency grids are created, the detector response can be determined at any arbitrary point within a sphere of 500-meter radius, and at any arbitrary energy within the specified range. Results are presented highlighting the improved performance achieved using the gridding methodology.