Coaxial Ge Research Articles

Effect of prolonged annealing on the performance of coaxial Ge gamma-ray detectors

The effects of prolonged annealing at elevated temperatures have been investigated in a 53cm3 closed-end coaxial high purity germanium detector in the reverse electrode configuration. The detector was multiply annealed at 100°C in block periods of 7 days. After each anneal cycle it was cooled to 77 K and the relative efficiency, peak channel location and FWHM energy resolution measured at 6 gamma-ray energies. At the present time, the detector has completed 16 anneal cycles. It was found that above ∼ 662 keV the photopeak efficiency decreased almost linearly at a rate of ∼ 1.5% per anneal cycle, although the energy resolution and centroid (and therefore charge collection efficiency) remained unchanged. The change in detection efficiency is attributed to the expansion of the inner n+ contact due to the thermal drive-in of Li ions into the bulk. The rate is found to follow a power-law dependence in agreement with that expected from Fick's diffusion laws. Using these data, we have derived a simple 1-D phenomenological model in which the n+ contact thickness is simply related to the Li diffusion length. For annealing at 100°C, the thickness of the n+ contact, d, as a function of the annealing time, t, can be described semi-empirically by d(mm) = 0.231 × t(days)1/2.

Optimized background reduction in low-level gamma-ray spectrometry at a surface laboratory

The background of a coaxial Ge detector placed at a surface laboratory has been reduced by means of a background reduction setup consisting of a passive shield of low-activity lead, a simple radon suppression system and an active shield with a plastic scintillation plate. In particular, we have devoted our efforts to in-depth optimization of each parameter associated with different anticoincidence setups and to their subsequent intercomparison. The overall performance of the active shield was improved by using the optimum time parameters for each setup. The final objective is to decrease the cosmic-ray background and, by this way, to reduce the detection limits of gamma-ray spectrometers at conventional laboratories, and consequently make them competitive for different measurements like 210Pb dating.

A semi-empirical approach for determination of low-energy gamma-emmiters in sediment samples with coaxial Ge-detectors

A simple method for the determination in sediment samples of low-energy gamma-emitters with coaxial Ge detectors and using a cylindrical sample geometry is outlined. This method allows an easy estimation of self-absorption effects by knowing the apparent densities and the composition of the investigated samples. As calibration matrixes solid samples enriched in known amounts of natural radionuclides emitting low-energy gamma rays were used. In this way, the difficult homogenisation steps needed when liquid spikes are added to a solid blank for manufacturing the calibration sample are avoided. The method has been carefully checked and validated by applying it to sediment samples with known activities of some low-energy gamma-ray emitters. Additionally, these sediment measurements have allowed to evaluate the magnitude and importance of the self-absorption corrections.

CdZnTe background measurements at balloon altitudes with PoRTIA

Measurements of the CdZnTe internal background at balloon altitudes are essential to determine which physical processes make the most important background contributions. We present results from CdZnTe background measurements made by PoRTIA, a small CdZnTe balloon instrument that was flown three times in three different shielding configurations. PoRTIA was passively shielded during its first flight from Palestine, Texas and actively shielded as a piggyback instrument on the GRIS balloon experiment during its second and third flights from Alice Springs, Australia, using the thick GRIS NaI anticoincidence shield. A significant CdZnTe background reduction was achieved during the third flight with PoRTIA placed completely inside the GRIS shield and blocking crystal, and thus completely surrounded by 15 cm of NaI. A unique balloon altitude background data set is provided by CdZnTe and Ge detectors simultaneously surrounded by the same thick anticoincidence shield; the presence of a single coaxial Ge detector inside the shield next to PoRTIA allowed a measurement of the ambient neutron flux inside the shield throughout the flight. These neutrons interact with the detector material to produce isomeric states of the Cd, Zn, and Te nuclei that radiatively decay; calculations are presented to determine the relative contribution of these decays to the fully shielded CdZnTe background measured by PoRTIA.

Determination of multielements in a typical Japanese diet certified reference material by instrumental neutron activation analysis.

Multielements in a typical Japanese diet certified reference material prepared at the National Institute for Environmental Studies (NIES) of Japan, in collaboration with the National Institute of Radiological Sciences (NIRS) of Japan were determined by instrumental neutron activation analysis (INAA). Five samples (ca. 510-1000 mg) and comparative standards were irradiated for a short time (10 s) at a thermal neutron flux of 1.5 x 10(12) n cm(-2) s(-1) (pneumatic transfer) and for a long time (6 h) at a thermal neutron flux of 3.7 x 10(12) n cm(-2) s(-1) (central thimble) in the Rikkyo University Research Reactor (TRIGA Mark-II, 100 kW). The irradiated samples were measured by conventional gama-ray spectrometry using a coaxial Ge detector, and by anti-coincidence and coincidence gamma-ray spectrometry with a coaxial Ge detector and a well-type NaI(Tl) detector. The concentrations of 38 elements were determined by these methods.

機器中性子放射化分析法による純鉄標準物質中のトランプ元素を中心とした微量元素の定量

Trace tramp elements (Zn, As, Sn and Sb) and other trace elements in pure iron certified standard materials, JSS003-4, JSS168-7 and JSS172-7, prepared by the Iron and Steel Institute of Japan were determined by instrumental neutron activation analysis (INAA). Four aliquot samples of each certified standard material (ca. 100-450 mg) were irradiated for 6 h at a thermal neutron flux of 3.7 × 1012 n cm-2 s-1 in the Rikkyo University Research Reactor. The irradiated samples were measured by γ-ray spectrometry using a coaxial Ge detector. The concentration of 3 elements (Co, Ni and As) in the JSS003-4, the concentration of 7 elements (Cr, Co, Ni, As, Mo, Sn and Sb) in JSS168-7 and the concentration of 7 elements (Cr, Co, Ni, As, Mo, Sb and Ta) in JSS172-7 were determined. The determined values were in good or nearly good agreement with the certified values or the reported values. The lower limit of determination values for the tramp elements were 0.01-0.1, μg/g Sb and As and 10-100 μg/g Zn and Sn respectively.

210Pb determination by gamma spectrometry in voluminal samples (cylindrical geometry)

In this paper a simple method for the direct analysis of 210Pb ( T 1/2=22.3 years) by gamma-ray spectrometry in sediment samples with coaxial Ge detectors is outlined. This method, developed for a cylindrical sample geometry, accounts for variable sample heights and provides a fundamental advantage: individual self-absorption corrections can easily be determined knowing the apparent densities of the samples. The results obtained with the proposed method are in good agreement with those given by other techniques.

Balloon flight test of pulse shape discrimination (PSD) electronics and background model performance on the HIREGS payload

We flew a prototype of the INTEGRAL/SPI PSD electronics with a narrow-bore coaxial Ge γ-ray detector on the January 1998 Antarctic balloon flight of the HIREGS payload. The first goal for this test flight was to demonstrate the performance of the PSD electronics in a radiation background and space environment similar to that in which SPI will operate, which was successfully demonstrated. The second goal was to measure the fraction of the background due to localized β −-decays, and to validate the model used to predict SPI's background and the sensitivity improvement due to the PSD background rejection. Our background measurements show good agreement with the models developed for SPI, and strongly support the expected improvement in sensitivity due to PSD on SPI.

Three-dimensional position sensitivity in two-dimensionally segmented HP-Ge detectors

Measured- and simulated-pulse shapes in electrically segmented coaxial Ge detectors have been investigated. Three-dimensional position sensitivities have been determined experimentally and theoretically in a 36-fold segmented Ge detector. By using the two-dimensional segmentation in conjunction with pulse-shape analysis, a position sensitivity of better than 1 mm can be obtained in three dimensions at an energy of 374 keV. This is achieved by analyzing the shape of net charge signals of segments containing interactions and of transient image charge signals of neighboring segments. The ability to locate interactions in three-dimensions is one of the crucial properties in the proposed γ-ray energy tracking array (GRETA). The concept of γ-ray tracking will not only increase the efficiency in detecting γ radiation but also enables the localization and characterization of unknown γ-ray sources with much higher accuracy than is possible with current instruments.

The influence of anisotropic electron drift velocity on the signal shapes of closed-end HPGe detectors

This study is concerned with the anisotropy of the electron drift velocity in germanium crystals at high electric fields and low temperature, and its influence on the charge collection process in n-type, high-purity germanium (HPGe) detectors of closed-end, coaxial geometry. The electron trajectories inside HPGe detectors are simulated using a phenomenological model to calculate the dependence of the drift velocity on the angle between the electric field and the crystal orientation. The resulting induced currents and pulse shapes for a given detector geometry and preamplifier bandwidth are compared to experiment. Experimentally, the dependence of the pulse shapes on the conductivity anisotropy in closed-end HPGe detectors was observed. The experimental data on pulse shapes were obtained by sampling preamplifier signals of an encapsulated, hexaconical EUROBALL detector, which was irradiated by collimated 22 Na and 241 Am sources. The crystal orientation was measured by neutron reflection. Qualitative agreement between the simulated and experimental pulse shapes was found. A variation in the charge collection time of up to 50 ns was observed for different drift directions of the carriers relative to the crystal orientation. Furthermore, a deflection of the trajectories from a straight radial drift direction of about 20° was predicted by the simulations. These two main effects of charge carrier drift velocity anisotropy in coaxial Ge detectors are expected to play an important role in the development of γ-ray tracking detectors.

Standardization of 169Yb and γ-ray emission probability data

Within the framework of EUROMET project No. 410, standardization of a 169Yb solution has been carried out and the gamma-ray emission probabilities at its decay have been measured. 169Yb activity was measured by the 4πβ–γ-coincidence extrapolation method. Special attention was paid to the measurement conditions associated with the existence of 316 keV metastable level of 169Tm ( T≈0.66 μs ). The 169Yb gamma-ray emission probabilities have been determined from the results of absolute activity measurements and data obtained using a coaxial Ge(Li) spectrometer.

A Semiempirical Approach for the Determination of HPGe Detector Photopeak Efficiency Aiming at k0-INAA

The experimental work involved the efficiency determination of three coaxial hyperpure Ge detectors, using point-like sources placed at various distances from the top cover of the detectors. Curves were fitted to the experimental data and a microcomputer routine was developed to calculate the efficiencies of cylindrical sources, including self-attenuation correction. Tested at various source and distance conditions, the procedure showed results sufficiently consistent for routine k0-INAA.

Measurements of Neutron Photoproduction Cross Sections for 232Th and 238U Using Capture Gamma Rays

Neutron photoproduction studies for 232Th and 238U were carried out from 5.61 to 10.83 MeV, by using up to 30 neutron capture gamma rays with high resolution in energy (4 to 20 eV), produced in an experimental arrangement at the IPEN-IEA-R1 2-MW research reactor. Samples of U3O8 depleted to 0.34% in 235U and natural ThO2 were irradiated inside a 4π sr long-counter neutron detector system, 520.5 cm away from the capture target. The gamma-ray flux was determined by means of a coaxial solid state Ge(Li) detector (EG&G ORTEC, 25 cm3, 5%) previously calibrated with capture gamma rays from a standard target of nitrogen (melamine). The compound neutron photoproduction cross section was measured for the gamma-ray spectrum produced by each capture target. Two methods to unfold the set of experimental data were proposed in order to obtain the differential cross sections at the main gamma line energies: the iterative and the least-squares methods. The calculated neutron photoproduction cross sections for 232Th and 238U were compared with experimental data reported by other authors who have employed different gamma-ray sources. A good overall agreement was observed among the experimental data, however, marked discrepancies were identified for some data points, indicating the possibility of narrow structures showing up at these excitation energies.

Comparative technique in measurements of Ge detectors effective volumes

A simple and quick procedure was proposed for measurements of large coaxial Ge detectors effective volumes. It included a comparison of background spectra collected with several detectors without any shielding in an underground laboratory. Such measurements were performed in Homestake (USA) and Canfranc (Spain) laboratories with several 1 kg and 2 kg detectors. Monte-Carlo calculations confirmed that ratios of numbers of events in continua of the spectra should be either equal or very close to the ratios of the detectors effective volumes.

Determination of multipolarities and mixing ratios of some γ transitions following 125Sb decay by γ– γ Angular Correlation

125Sb decays by β − emission to 125Te with a half-life of 2.76 years. The angular correlations of some γ-ray transitions of this decay have been studied in order to determine their multipolarities and mixing ratios. The experimental set-up included a planar-Ge detector to measure the low-energy γ-rays, a coaxial Ge detector and a movable device to determine precisely the angle between the axes of the detectors. Results obtained in the experiments and other available data were processed to obtain the best values for the mixing ratios.

(α,3pxn) or (α,αpxn) reactions on natural dysprosium

Excitation functions for the production of155Tb,156(ml+m2+g)Tb,160Tb and161Tb by (α,3pxn) reactions on natural dysprosium were experimentally determined by the method of activation of stacked metallic foils. The irradiations were performed in the extemal beam of the isochronous cyclotron at Karlsruhe (Germany) with α-particles up to 88.5 MeV. Gamma-rays spectrometry with an n-type intrinsic coaxial Ge detector was used to determine the reaction product activities. The data are compared with theoretical predictions provided by equilibrium and pre-equilibrium reaction models. Thick target yields for the production of155Tb,156(ml+m2)+g)Tb,160Tb and161Tb are presented.

Radioisotope yields from 1.85-GeV protons on Mo and 1.85- and 5.0-GeV protons on Te

Radioisotope yields from 1.85-GeV proton interactions in a natural isotopic composition Mo target and those from 1.85- and 5.0-GeV protons in natural Te targets were measured at Lawrence Berkeley National Laboratory{close_quote}s Bevatron. The radioisotope yields were determined by {gamma}-counting the targets using 100-cm{sup 3} coaxial Ge detectors following the irradiations. Cross sections were determined for the production of 36 radioactive nuclides, ranging from Z=35, A=74 to Z=43, A=97, from the Mo target and for 43 radioactive nuclides, ranging from Z=35, A=75 to Z=53, A=130 from the Te targets. The average deviations of the experimental cross sections from those predicted by the semiempirical isotopic cross sections of Silberberg and Tsao were 53{percent} for p+Mo at 1.85 GeV, 66{percent} for p+Te at 1.85 GeV, and 35{percent} for p+Te at 5.0 GeV. These deviations are higher than those found previously for medium and heavy targets and for elemental cross sections. The minimum production cross section of {sup 91}Nb, which may be of interest as a cosmic-ray chronometer, was found to be 18{plus_minus}3 mb for the p+Mo reaction. {copyright} {ital 1997} {ital The American Physical Society}

Cosmic-ray induced background reduction by means of an anticoincidence shield

The cosmic-ray induced background of a spectrometer is reduced by means of an active veto shielding. Two such spectrometers have been developed: one equipped with a well type Ge detector and the other with a 50% relative efficiency coaxial Ge detector. In both cases, the active veto shielding consists of plastic scintillating plates and an anticoincidence electronic system. Although these devices have lower performances that those placed in an underground laboratory, their advantage is that they can be used in a normal laboratory environment. The background is reduced by 4 or 5 in the whole 30 keV to 2.7 MeV energy range and 5–10 in the energy range below 500 keV. Under these conditions, the count rates are smaller than 10 −4 counts s −1 keV −1.

An evaluation of the influence of γ—γ angular correlation on the coincidence-summing corrections in γ-ray spectrometry

The effect of angular correlation on coincidence-summing corrections has been evaluated for three detectors (planar Ge, coaxial Ge and well NaI), at different measurement geometries and for some selected radionuclides. For the detectors and nuclides studied, it can be concluded that the effect of angular correlation on coincidence-summing corrections can be neglected if the radioactive source is measured either in contact with the detector or at large distances.

A new low background gamma spectrometer equipped with an anti-cosmic device

A well type Ge detector and a 48 percent relative efficiency coaxial Ge detector are used with cosmic ray anticoincidence shielding (plastic scintillating plates). Although these devices have lower performance than those placed in an underground laboratory, their advantage is that they can be used in a normal laboratory environment. The background is reduced by 4 or 5 in the whole 30 keV to 2.7 MeV energy range and by 5 to 10 in the energy range below 500 keV. Under these conditions, the count rates are smaller than 10 −4 count s −1 keV −1.