Single HPGe Detector Research Articles

Sensitivity analysis towards trace-uranium detection with γ-γ coincidence NAA

We present an improved approach for detection of trace amounts of 238U by means of neutron activation analysis (NAA). The analysis enhancement is obtained by utilizing γ-γ coincidence counting. An empirical method for evaluating the sensitivity gain in the presence of a large source-related background is presented. A comparison of detection limits is made between two counting schemes; namely, counting single gammas using one HPGe detector versus counting coincident gammas using two HPGe detectors placed face-to-face. A data-validated radiation transport model, created in GEANT4 for a single HPGe detector, is extended to handle the two-detector setup. In this counting scheme, a 238U-detection sensitivity enhancement of about a factor of 8 is predicted for a sample of Saint-Gobain G3 Sapphire when compared to the simpler singles counting approach.

Calibration of HPGe–HPGe coincidence spectrometer through performing standardisation of 125I activity by X-ray-gamma coincidence spectrometry using two HPGe detectors

An X-ray-gamma coincidence measurement method for efficiency calibration of a HPGe–HPGe system, using the methodology for activity standardisation of 125I, has been developed. By taking one list-mode time-stamped measurement of the 125I source, six spectra were generated in post-processing: total spectra, coincidence spectra and energy gated coincidence spectra for each of the two detectors. The method provides enough observables for source activity to be determined without a prior knowledge of the detector efficiencies. In addition, once the source is calibrated in this way the same spectra can also be used to perform efficiency calibration of the individual detectors in the low energy range. This new methodology for source activity determination is an alternative to the already established X-ray-(X-ray, gamma) coincidence counting method; with two NaI(Tl) detectors and the sum-peak method using a single HPGe detector. When compared to the coincidence counting method using two NaI(Tl) detectors, the newly developed method displays improved energy resolution of HPGe detectors combined with measurement of only full peak areas, without the need for total efficiency determination. This enables activity determination even in presence of other gamma emitters in the sample. Standard coincidence counting with NaI(Tl) detectors provides lower uncertainties. The method has been used for calibration of a coincidence HPGe spectrometer in the low energy range of 125I and fine adjustments of a Monte Carlo model of the coincidence system.

Mazinger, a γ-ray spectrometry system of high efficiency and very low background for paloeoclimate applications

The study of changes in climate on the scale of the entire history of Earth requires absolute dating methods that cover the total time period. 230Th and 234U disequilibria, as well as 231Pa and 235U, have shown as good candidates to provide ages older than 50ky. Therefore, they are the basis for a method that allows extending the 14C dating up to 500ky or even further. The wide variety of samples, their high number and the fact that several radionuclides are needed to obtain samples’ chronology make γ spectrometry to be a suitable technique in this task. However, its low efficiency and high background, therefore, high detection limits in comparison with α spectrometry, need to be improved to become a real alternative in samples with very low activity levels. With this purpose, Mazinger, a new γ spectrometry system based on two HPGe detectors, which are surrounded by active and passive shieldings and placed 12m underground, has been set up in the Ionizing Radiation Laboratory of the University of Salamanca.In this work, the Mazinger's performance is studied at the full energy range useful for natural radionuclides. In composite detectors, energy resolution is a consequence of the electronics adjustments, which have to enable the raw spectra to be summed without resolution worsening. Therefore, the method used to set up electronics and add spectra is described. Detector background is also analysed and compared with the one of a single low-background HPGe detector as a function of the different shielding elements, obtaining gross values of order 10−4cps. Efficiency and figure of merit are obtained for the main natural radionuclides in marine sediments belonging to the Mediterranean MD99-2348 core. Special focus is made on the 230Th determination whose results are compared with the ones provided by α spectrometry.Results show that Mazinger´s performance, in terms of figure of merit, is four times better than the one of a low-level background HPGe detector, specially suited to detect natural radionuclides, being comparable to the performance of α spectrometry in the 230Th determination, which is the radionuclide of lower γ intensity, and doing Mazinger a first class in radiometry of natural radionuclides.

Application of modern anticoincidence (AC) system in HPGe γ-spectrometry for the detection limit lowering of the radionuclides in air filters

The use of active and passive shields can substantially reduce the Minimum Detectable Activity (MDA) of the γ-ray counting systems, rejecting events induced by cosmic-rays or by environmental radioactivity. However, the size and geometry of the samples lead to limitations in the background reduction in routine measurements. The Minimum Detectable Activity (MDA) values for low energy of γ-ray emitting radionuclides (<200 keV) deposited in three typical air filter geometries have been compared for anticoincidence and single HPGe detector mode of γ-spectrometry systems. The relative increase in the Figure Of Merit (FOM) values from to 10–37% has been achieved for AC counting mode for radionuclides of 210Pb, 234Th, 235U and 226Ra deposited on the three kinds of air filters.

Comparison of two spectrometric counting modes for fast analysis of selected radionuclides activity

The two counting modes: a normal with a single HPGE detector and second with the additional anti-Compton shield of the annular NaI(Tl) detector have been compared for fast determination of the activity concentration of thorium 232Th in the building materials. The 232Th activity concentration was calculated by measurement of its decay products: 212Pb, 212Bi and 208Tl as well 228Ac content. Although the Compton suppression mode applied in gamma spectrometry systems in general increase sensitivity of the analysis, but in case of 583 keV the most abundant 208Tl γ-line, the significant reduction of photon counting rate was observed.

Photonuclear reactions with zinc: A case for clinical linacs

The use of bremsstrahlung photons produced by a linac to induce photonuclear reactions is wide spread. However, using a clinical linac to produce the photons is a new concept. We aimed to induce photonuclear reactions on zinc isotopes and measure the subsequent transition energies and half-lives. For this purpose, a bremsstrahlung photon beam of 18MeV endpoint energy produced by the Philips SLI-25 linac has been used. The subsequent decay has been measured with a well-shielded single HPGe detector. The results obtained for transition energies are in good agreement with the literature data and in many cases surpass these in accuracy. For the half-lives, we are in agreement with the literature data, but do not achieve their precision. The obtained accuracy for the transition energies show what is achievable in an experiment such as ours. We demonstrate the usefulness and benefits of employing clinical linacs for nuclear physics experiments.

Low-level radioactive waste measurement comparisons of three gamma-ray counting systems

Three different gamma-ray counting systems constructed by 1–3 HPGe detectors were used in this study to compare their system performance. One measurement scheme involved positioning a single HPGe detector on a movable cart with a 90° collimation angle to the observed item. The other two waste assay systems were configured with two or three HPGe detectors towards the sample drums, while the three-HPGe-detector counting system was in a shielded counter cavity. The measurement consistency of 38 low-level waste drums, system operating costs and acquisition times to achieve the same MDA for these counting systems were compared and discussed in this study.

SU-F-BRCD-02: Activity Assay of a Y-90 Microsphere Sample Using a Coincidence Detection System.

This work takes advantage of a newly-determined, low-uncertainty branching ratio of the internal pair production component of Y-90 decay to spectroscopically assay the activity of a Y-90 microsphere sample with a coincidence detection system (CDS). The CDS pairs a HPGe detector with a large NaI detector. The system is able to electronically filter the bremsstrahlung continuum from the photon spectrum by gating the energy signal from the HPGe with the coincidence signal. This reduces the uncertainty in the spectra measurement compared to measurements with a single HPGe detector. A series of pulsers were used to correct for counting losses. A geometric characterization was completed to find the optimal source position for measurements. An efficiency calibration of the CDS was completed using a Na-22 standard source. To validate the measurement accuracy of the CDS, the activity of a Y-90 standard activity solution from the NIST SRM program was determined and compared to the value given by NIST. The CDS was then used to determine the activity of a Y-90 microsphere sample. This value was compared to the 3.0 GBq value given by the manufacturer. The activity determined with the CDS was within 2.6% from the given activity of the NIST SRM source, which is within the expanded uncertainty associated with the CDS measurement. The activity of the Y-90 microsphere sample was determined to be 3.72 GBq +/- 1.9%. This is 19% higher than the manufacturer-stated activity of 3 GBq. This is outside the manufacturer-stated uncertainty of +/-10%. The ability of CDS to determine the activity of a Y-90 source has been validated with comparison to a NIST Y-90 standard activity solution. The use of the CDS has been extended to determine the activity of a Y-90 microsphere sample.

The performance evaluation of a movable gamma-ray counting system for radwaste measurement

Performance tests of a movable gamma-ray waste drum scanning system were carried out in this study. The scanner consists of a single HPGe detector on a movable cart with a 90° collimation angle to the observed item. The detection efficiency for 137Cs was determined using the ISOTOPIC software. The activities of 35 low-level radioactive waste drums were measured using this system and compared with the results of two other measurement systems. This movable scanning system gave the same result as the other two systems with a relative spread (one standard deviation) of about 7.2%.

On the absolute source activity measurement with a single detector: The 133Ba case

The activity of a 133Ba point source is measured with a single HPGe detector by placing the source on the detector end-cap. Activity is obtained using net peak areas and total spectrum area, as well as true and random (pile-up) peaks. For data analysis, two different methods are applied. In one, the method for activity observation that implies an exact treatment of the decay scheme and all possible outcomes introduced by Novković et al. [9] (D. Novković, A. Kandić, M. Đurašević, I. Vukanac, Z. Milošević, L. Nađđerđ, Nucl. Instr. and Meth. A 582 (2007) 592.) is extended to pile-up events of first order, which predicts 2375 different peaks in the spectrum of 133Ba. In the second method, since the 133Ba decay is followed by emission of one dominant cascade, a simple method was developed for activity measurement of 133Ba source where the summing probability of X-rays with gamma rays can be neglected. It is shown that by taking into account the pile-up events in the spectrum, via the methods described here, accurate direct activity measurements of 133Ba source are possible even at higher count rates. The methods are applied to the spectra with count rates up to 36×10 3 s −1, and the measured activity values agree with the reference values within 1%.

Development of a prompt gamma activation analysis facility using diffracted polychromatic neutron beam

A prompt gamma activation analysis facility has recently been developed at Hanaro, the 24 MW research reactor in the Korea Atomic Energy Research Institute. Polychromatic thermal neutrons are extracted by setting pyrolytic graphite crystals at a Bragg angle of 45°. The detection system comprises a large single n-type HPGe detector, signal electronics and a fast ADC. Neutron beam characterization was performed both theoretically and experimentally. The neutron flux was measured to be 7.9×10 7 n/cm 2 s in a 1×1 cm 2 beam area at the sample position with a uniformity of 12%. The corresponding Cd-ratio for gold was found to be 266. The beam quality was compared with other representative thermal neutron prompt gamma activation analysis. The detection efficiency was calibrated up to 11 MeV using a set of radionuclides and the (n,γ) reactions of N and Cl. Finally, the sensitivities and the detection limits were obtained for several elements.

Distinguishing between electrons and γ-rays in experiment TGV using a pulse rise time

The TGV (Telescope Germanium Vertical) collaboration is interested in the measurement of double-beta decay of 48Ca (TGV I) and 106Cd (TGV II). The background in the experiment TGV I has been suppressed by several methods. One of them was based on distinguishing between electrons and γ-rays due to the different rise time of the signals obtained from semiconductor detector. Two experimental setups have been tested, the first one used a charge-sensitive analog-to-digital converter, while the second one utilized digital oscilloscope. The reduction of the background counting rate (due to γ-rays) in single HPGe detector by a factor of (1.4-3.8) was reached.

Study of nuclear quadrupole interactions in different environments of decaying atoms of 75Se by sum peak method

The sum peak intensity I 400 (279 + 121 keV) relative to its singles peak intensity I 279 or I 121 has been determined in various environments of 75Se by gamma–gamma summing technique in a single HPGe detector. The change in intensity has been used to determine nuclear quadrupole interaction frequencies and electric field gradient components in different environments, e.g. Se-glycerol, Se-hydrochloric acid (HCl), Se-perchloric acid (HClO 4), Se-blood samples with and without anti-coagulant, Se-alanine and Se-cysteine at different pH values. The electric field gradient is found to vary with chemical complex formation.

Two-phonon J = 1 states in even-mass Te isotopes with A = 122–130

Excited states of the nuclei 122,126,130Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E γ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1 − state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1 + state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

Beta-Decay Energies of121,123,125Ba and121–124,126Cs

The β + -ray maximum energies of neutron-deficient 121,123,125 Ba and 121–124,126 Cs isotopes were measured using single HPGe-detector method, which does not need an annihilation-photon detection system. The radioactive sources were produced by bombarding Mo targets with 32 S 10+ and 35 Cl 11+ ions, followed by on-line mass-separation. The analysis of the experimental β + -ray spectra using response functions to monoenergetic positrons yielded β + -ray maximum energies and Q -values of orbital-electron capture ( Q EC -values) in the decay of these nuclides. The present experiment has extended the Q EC determination to 121,123 Ba isotopes in addition to improving the accuracy for the other nuclides.

Automated system for neutron activation analysis determination of short lived isotopes at The DOW Chemical Company's TRIGA research reactor

An automated neutron activation analysis (NAA) system for the determination of short lived isotopes was constructed at The DOW Chemical Company's TRIGA Research Reactor in 1993. The NAA group of the Analytical Sciences Laboratory uses the reactor for thousands of analyses each year and therefore automation is important to achieve and maintain high throughput and precision (productivity). This project is complementary to automation of the long-lived counting facilities (see Romick et al., these Proceedings). Canberra/Nuclear Data Systems DEC-based software and electronics modules and an I/O mounting board are the basic commercial components. A Fortran program on a VAX computer controls I/O via ethernet to an Acquisition Interface Module (AIM). The AIM controls the γ spectrometer modules and is interfaced to a Remote Parallel Interface (RPI) module which controls the pneumatic transfer apparatus with TTL signals to the I/O mounting board. Near-infrared sensors are used to monitor key points in the transfer system. Spectra are acquired by a single HPGe detector mounted on a sliding rail to allow flexible and more reproducible counting geometries than with manual sample handling. The maximum sample size is 8 ml in a heat-sealed two dram vial. The sample vial is nested into a “rabbit” vial for irradiation which can be automatically removed prior to spectrum collection. The system was designed to be used by the reactor operator at the control console without the aid of an additional experimenter. Applications include the determination of selenium and silver in coal and water, fluorine in tetra-fluoro ethylene (TFE) coated membranes, aluminum and titanium in composite materials and trace fluorine in non-chlorinated cleaning solvents. Variable dead time software allows analysis for 77mSe despite high dead times from 16N encountered in samples.

Determination of perturbed angular correlation parameters in some compounds of133Ba using the sum peak method

The sum peak intensity relative to its single peak was determined in various compounds of133Ba by the technique of gamma-ramma-ray sum peaks observed in a single HPGe detector. The change in intensity ratio was used to determine the time integral perturbed angular correlation coefficients for BaCl2, Ba(NO3)2, BaSO4, Ba-EDTA, Ba-MES and Ba-BSA compounds.

K-Capture in the Decay of 191Pt

Relative K-capture probabilities to three levels of 1911r are determined from the decay of 191 Pt using the sum coincidence method based on summing of gamma rays and K X-rays in a single HPGe detector. The results agree with theoretical values. The electron-capture decay energy calculated using j3-decay properties to different levels belonging to the same rotational family is in agreement with the experimental value.

K-electron capture probability to the 136 keV level in the decay of 57Co

The K-electron capture probability to 136 keV in the decay of 57Co has been determined by K x-ray—γ-ray sum peaks observed in single HPGe (28.27 mm 2 × 5.5 mm) detector. The results are independent of fluorescence yields and detector efficiencies.