Coaxial HPGe Detector Research Articles

Cross section measurements for 75As isotope at neutron energies from 13.5 to 14.8 MeV

Cross sections which is relevant in research of neutron transmutation doping of some important semiconducting materials were measured at neutron energies from 13.5 to 14.8 MeV for the reactions 75As(n, 2n) 74As, 75As(n, p) 75m+gGe, 75As(n, α) 72m+gGa by activation relative to the 27Al(n, α) 24Na reaction. Measurements were carried out by γ-detection using a coaxial HPGe detector. Natural realgar (As 2S 2) powder of 99.9% purity was used as samples. Fast neutrons were produced by the T(d, n) 4He reaction. The results obtained are compared with existing data.

Development of integrated whole body counter

Instead of conventional human counter using NaI(Tl) scintillator in scanning bed geometry, integrated whole body counter was developed. This is constituted by 10 detectors that are located just above the objective organs in order to improve the identification of nuclides. Two sets of p-type arrayed planar HPGe detectors composed of two crystals are used for lungs, and two sets of p-type high efficiency coaxial HPGe detectors are used for gastrointestinal tract. Similarly n-type HPGe is used for chest or thyroid or skull and the other p-types are used for the liver and kidney, respectively. An electric cooling system by adiabatic expansion was adapted as cooling apparatus for all detectors with the object of asphyxia prevention and continuous operation. In the efficiency calibration, Japan Atomic Energy Research Institute (JAERI) phantom with newly designed tissue equivalent lungs, which contain 241Am homogeneously distributed, was applied to lung detector, and Bottle Manikin Absorbtion (BOMAB) phantom regulated by American National Standards Institute (ANSI) was also applied to detectors for trunk.

Evaluation of radioactive background rejection in 76Ge neutrino-less double-beta decay experiments using a highly segmented HPGe detector

A highly segmented coaxial HPGe detector was operated in a low background counting facility for over 1 year to experimentally evaluate possible segmentation strategies for the proposed Majorana neutrino-less double-beta decay experiment. Segmentation schemes were evaluated on their ability to reject multi-segment events while retaining single-segment events. To quantify a segmentation scheme's acceptance efficiency the percentage of peak area due to single segment events was calculated for peaks located in the energy region 911-2614 keV. Single interaction site events were represented by the double-escape peak from the 2614 keV decay in {sup 208}Tl located at 1592 keV. In spite of its prototypical nature, the detector performed well under realistic operating conditions and required only minimal human interaction. Though the energy resolution for events with interactions in multiple segments was impacted by inter-segment cross-talk, the implementation of a cross-talk correlation matrix restored acceptable resolution. Additionally, simulations utilizing the MaGe simulation package were performed and found to be in good agreement with experimental observations verifying the external nature of the background radiation.

Characterization of the virtual point detector effect for coaxial HPGe detectors using Monte Carlo simulation

The scaling of detector efficiency with source distance is generally a complex function of the detector shape and measurement geometry. The concept of a virtual point detector has been developed and validated in the past for Ge (Li) and HPGe detectors to deal with this problem. Characterizing the virtual point detector effect has traditionally involved tedious and time-consuming measurements at multiple energies and source distances. The situation is therefore well suited to a Monte Carlo approach. In this paper, the results of using Monte Carlo simulation to characterize the virtual point detector effect for large-volume coaxial HPGe detectors are presented.

Analysis of radiocaesium in the Lebanese soil one decade after the Chernobyl accident

Fallout from the Chernobyl reactor accident due to the transport of a radioactive cloud over Lebanon in the beginning of May 1986 was studied 12 years after the accident for determining the level of 137Cs concentration in soil. Gamma spectroscopy measurements were performed by using coaxial high sensitivity HPGe detectors. More than 90 soil samples were collected from points uniformly distributed throughout the land of Lebanon in order to evaluate their radioactivity. The data obtained showed a relatively high 137Cs activity per surface area contamination, up to 6545 Bq m −2 in the top soil layer 0–3 cm. The average activity of 137Cs in the top soil layer 0–3 cm in depth was 59.7 Bq kg −1 dry soil ranging from 15 to 119 Bq kg −1 dry soil. The horizontal variability was found to be about 45% between the sampling sites. The depth distribution of total 137Cs activity in soil showed an exponential decrease. Estimation of the annual effective dose due to external radiation from 137Cs contaminated soil for selected sites gave values ranging from 19.3 to 91.6 μSv y −1.

Cross-section measurements for the reactions of 14 MeV neutrons on indium isotopes

The cross sections for the reactions 115In(n, p) 115gCd, 115In(n, α) 112Ag, 115In(n, 2n) 114mIn, 113In(n, 2n) 112mIn, 115In(n, n′) 115mIn, and 113In(n, n′) 113mIn induced by 14 MeV neutrons have been measured by activation relative to the 27Al(n, α) 24Na. Measurements were carried out by γ-detection using a coaxial HPGe detector. As samples, natural indium has been used. The fast neutrons were produced by the T(d, n) 4He reaction. The results obtained are compared with existing data.

Application of PENELOPE code to the efficiency calibration of coaxial germanium detectors

The code PENELOPE is applied here to the efficiency calibration exercise described in the EUROMET project 428 [Lépy et al., 2001. Appl. Radiat. Isot. 55, 493]. We evaluated the peak efficiencies for a coaxial HPGe detector in the range 60–2000 keV, for point sources located at various distances from the detector and for a cylindrical box containing two different matrixes: a low-density silica and a hydrochloric acid solution. After an optimization of several detector parameters, the results were in good agreement with the experimental values.

Determination of the 209Bi(n,γ) capture cross section at a cold neutron beam

The total capture cross section of 209Bi was determined at the cold neutron beam PGAA-NIPS facilities at the Budapest Neutron Centre. The measurements were performed using a coaxial HPGe detector with Compton suppression. The total and partial gamma-ray production cross sections were deduced relative to the 14N(n,γ) partial gamma-ray production cross section. By using a bismuth nitrate stoichiometric compound as the sample, we excluded various systematic uncertainties. The total capture cross section is in very good agreement with the compilation of Mughaghab, but is slightly lower than the most recent value determined at the high flux reactor of the ILL in Grenoble, France. We also performed measurements using a 0.5 mm thin Bi metal disc. The relative intensities determined from the Bi disc and the compound samples are in good agreement.

X-ray fluorescence analysis in environmental radiological surveillance using HPGe detectors

X-ray fluorescence (XRF) has been proven to be a valuable tool for determining trace quantities of heavy metals, such as uranium and lead, in different types of samples. The present paper demonstrates the applicability of XRF spectrometry to measure the concentrations of these heavy metals in samples from natural ore and soil. The values of uranium concentrations in rock from the Peña Blanca uranium ore, in Chihuahua, México, were calculated for the purpose of precertifying the rock powders samples. The comparison with other techniques, such as inductively coupled plasma atomic emission spectrometry, atomic absorption spectrometry, alpha spectrometry and electron microscopy, was used to complete the precertification process, so that the sample powders may be used as secondary standards. The source-sample-detector geometry and the incident angle are the most important factors for obtaining low detection limits. The selected system uses a 57Co source of about 0.1 mCi to excite the K X-rays from uranium and lead. X-rays were recorded on a CANBERRA HPGe coaxial detector. The comparative results for two incident angles (90° and 180°) performed previously by other authors show that the best geometry is the backscattering geometry. In the present paper, using EGS4 code system with Monte Carlo simulation, it was possible to determine the location and distribution of background produced by the Compton edge in the optimized geometry. This procedure allowed to find the minimum detectable concentration of uranium and lead, which was experimentally calculated using standards. The possibility of performing in vivo measurements rapidly and easily, as well as the factors affecting accuracy and the minimum detectable concentration in several samples are also discussed.

A neutron induced prompt gamma-ray spectroscopy system using a252Cf neutron source for quantitative analysis of aqueous samples

A neutron induced prompt γ -ray spectrometry (NIPS) facility has been developed at the Nuclear Chemistry Research Division, of the Korea Atomic Energy Research Institute (KAERI) with the aim of analyzing the major components of various elements in aqueous samples. The facility is equipped with a 252Cf neutron source and a γ-γ coincidence setup with two n-type coaxial HPGe detectors based on NIM spectrometric modules in association with data acquisition and spectral analysis systems. The development of the system, its set-up and the calibration of detection efficiency up to 8 MeV using a set of radionuclides and the (n,γ) reactions of chlorine are described in the paper.

Resolution and sensitivity as a function of energy and incident geometry for germanium detectors

The use of modeling programs such as MCNP to predict the response of HPGe detectors is increasing in importance. Accurate simulation of germanium detectors to incident gamma rays relies on knowledge of the performance of the detector in different detector–source geometries. Two important performance parameters are the resolution and sensitivity. The resolution is the FWHM and FW.1M/FWHM ratio. The IEEE 325-1996 standard only specifies the FWHM measurement at one geometry and two energies. Nearly all measurements are made in a different geometry and at other energies. Other investigators [Specifications for Today’s Coaxial HPGe Detectors, 2001 ANS Annual Meeting, Milwaukee, WI; Metzger, private communication, see also: Radionuclide Depth Distribution by Collimated Spectroscopy, 2002 ANS Topical Meeting, Santa Fe, NM], have shown that the sensitivity and resolution change with position of the incident gamma ray on the front of the detector. Such variability has possible implications for the accuracy of peak shape and area determination, since the calibration is potentially a function of angle of incidence. To quantify the sensitivity and resolution variation as a function of energy and point of incidence, measurements have been made on several coaxial detectors of various crystal types and sizes in different source–detector geometries. The full-energy peaks from 59 keV to 2.6 MeV were used. The detectors were placed in a low-background shield to reduce any contribution from external sources. None of the detectors tested was a low-background type. The sources used were an 241Am source, 60Co source and a natural thorium oxide sample. The 241Am 59 keV gamma rays were collimated by a 2 cm thick, 1 mm diameter lead collimator. Several gamma rays from the thorium source were used and collimated by a 10 cm thick and 2 mm diameter tungsten collimator. These collimated sources were used to collect spectra for the incident beam on the front and sides of the detectors. The peak widths were calculated using the methods outlined in IEEE 325-1996. Data are presented to show that the peak shape and sensitivity change with incident beam position and full peak energy.

Efficiency transfer in the calibration of a coaxial p-type HpGe detector using the Monte Carlo method

Monte Carlo simulation was applied to the efficiency transfer exercise described in the EUROMET428 project (Appl. Radiat. Isot. 55 (2001) 493), evaluating the peak efficiencies in the energy range 60–2000 keV for a typical coaxial p-type HpGe detector and several types of source configuration: point sources located at various distances from the detector and a cylindrical box containing three matrices. The efficiency values were derived in two ways: (a) by direct calculation taking into account the physical dimensions of the detector provided by the supplier, and (b) by means of relative computation (efficiency transfer) taking also into consideration the known efficiency values for a reference point source. As expected, some significant discrepancies between the calculated and experimental values were found when a direct computation was made using the data provided by the supplier. On the contrary, the results for the peak efficiency derived by relative calculation by means of an efficiency transfer were in good agreement with the experimental values. The deviations found with this last procedure were generally below 5% for all the geometries considered, which is entirely satisfactory for the purposes of routine measurements.

A dual purpose Compton suppression spectrometer

A gamma-ray spectrometer with a passive and an active shield is described. It consists of a HPGe coaxial detector of 42% efficiency and 4 NaI(Tl) detectors. The energy output pulses of the Ge detector are delivered into the 3 spectrometry chains giving the normal, anti- and coincidence spectra. From the spectra of a number of 137Cs and 60Co sources a Compton suppression factor, SF and a Compton reduction factor, RF, as the parameters characterizing the system performance, were calculated as a function of energy and source activity and compared with those given in literature. The natural background is reduced about 8 times in the anticoincidence mode of operation, compared to the normal spectrum which results in decreasing the detection limits for non-coincident gamma-rays up to a factor of 3. In the presence of other gamma-ray activities, in the range from 5 to 11 kBq, non- and coincident, the detection limits can be decreased for some nuclides by a factor of 3 to 5.7.

Low energy gamma- and X-ray measurements by means of Compton suppression technique for safeguards environmental samples

For analyzing ultra-trace amounts of nuclear materials in safeguards environmental samples, JAERI constructed a clean room facility. To eliminate the samples with excessive amounts of radioactive materials from the facility, that is the screening, the authors need to roughly estimate actinides and radioisotopes by non-destructive analysis. In this estimation, it is very inefficient to precisely measure the actinides by using HPGe detector because the intensities of the actinides' gamma-rays are generally low and their photopeaks (mostly below 200 keV) are severely interfered by Compton continuum caused by gamma-rays of fission products etc. within the samples. For suppressing Compton scattering signals with the energy less than several hundreds of keV, the authors have examined the feasibility of planar HPGe-LEPS and “short” coaxial low-energy HPGe detectors which were symmetrically surrounded with Nal(Tl) Compton suppressor detectors.In preliminary examination by measuring 137Cs standard source, background level was reduced by a factor of about 2 in the energy range from 50 to 400 keV.

Monte Carlo simulation of the self-absorption corrections for natural samples in gamma-ray spectrometry

Gamma-ray self-attenuation corrections in the energy range 60–2000 keV were evaluated by means of Monte Carlo calculations for environmental samples in a cylindrical measuring geometry. The dependence of the full-energy peak efficiency on the sample density was obtained for some particular photon energies and, as a result, the corresponding self-attenuation correction factors were obtained. The calculations were performed by assuming that natural materials have mass attenuation coefficients very similar to those of water in the energy range studied. Three different HpGe coaxial detectors were considered: an n-type detector with 44.3% relative efficiency and two p-type detectors of relative efficiencies 20.0% and 30.5%. Our calculations were in very good agreement with the self-attenuation correction factors obtained experimentally by other workers for environmental samples of different densities. This work demonstrates the reliability of Monte Carlo calculations for correcting photon self-attenuation in natural samples. The results also show that the corresponding correction factors are essentially unaffected by the specific coaxial detector used.

Influence of the geometrical characteristics of an HpGe detector on its efficiency

Computer codes based on Monte Carlo calculations have been extensively developed for the computation of the efficiency in gamma-ray spectrometry. The errors in the specific parameters of the detector due to the lack of precise knowledge of its characteristics usually represent one of the most important sources of inaccuracy in this simulation technique. We present here the influence of several detector parameters on the efficiency for a typical coaxial n-type HpGe detector. Calculations of the full-energy peak efficiencies were performed by means of a Monte Carlo code in the range 122-1836 keV for several types of source configuration: point source, cellulose filter, and two different cylindrical boxes containing a solid matrix of SiO 2 . The detector parameters varied were the crystal diameter, crystal height, diameter of the internal core, and the position of the crystal with respect to the beryllium window. The study shows that significant deviations in the efficiency, depending on the source geometry and the photon energy, can be produced by varying only slightly some of the detector parameters.

Ultra low-level gamma ray spectrometry of thorium in human bone samples.

Following the use of in vivo measurements of 210Pb to estimate retrospectively radon exposure, interest has been expressed in the use of in vivo measurements of 208Tl to estimate thorium intake. To aid with calibration and to determine the optimum part of the body on which in vivo measurements should be made, the distribution of 208Tl and 228Ac amongst different human bones was measured in the underground laboratory HADES. The 208Tl activity was determined by the 2614.5 keV and the 583.2 keV gamma ray lines. The 225Ac activity was determined by the 911.2 keV and the 969.0 keV gamma ray lines. The background under those peaks when measured on the 106% relative efficiency coaxial HPGe detector in HADES is of the order of 1 d(-1), resulting in detection limits in the order of 1 mBq for both radionuclides for a typical 10 g bone sample and for a measuring time of I week.

Efficiency of segmented HPGe detectors: design criteria for pulse shape analysis

The problems with reconstructing the trajectory of /spl gamma/-rays and identifying their interaction points in segmented coaxial HPGe detectors are discussed here in view of their importance in the design of the next generation of detector arrays for /spl gamma/-ray spectroscopy studies. In fact, the tracking quality of the /spl gamma/-ray interactions in the medium controls the overall detection efficiency. This paper focuses on two main points. One is the study of how the tracking reconstruction efficiency is affected by the ability of disentangling multiple interactions per segment. The other is the study of the consequences that different types of segmentation have on the overall efficiency. Two geometries have been chosen: that of a single cylindrical detector and that of a spherical shell.

HPGe detector photopeak efficiency calculation including self-absorption and coincidence corrections for cylindrical sources using compact analytical expressions

Total and full-energy peak efficiencies, coincidence correction factors and the source self-absorption of a p-type coaxial HPGe detector for cylindrical sources have been calculated using direct analytical expressions. In the experiments gamma aqueous sources containing several radionuclides covering the energy range from 60 to 1836 keV were used. By comparison, the theoretical and experimental full-energy peak efficiency values are in good agreement.

Absolute measurements of photon emission probabilities of 169Yb

A solution of 169Yb was absolutely standardized by the 4π(EC,X)-γ coincidence counting method and the result was used to obtain direct measurements of gamma-ray emission probabilities with a coaxial HPGe detector. The empirical relation proposed by (Vaño, F., Gonzalez, L., Gaeta R., Gonzalez, J.A., 1975. An empirical function which relates the slope of the Ge efficiency curves and the active volume Nucl. Instr. Meth. 123, 573) was tested using the gamma spectral response above 200 keV. The half-life of 169Yb was also measured with a 4πγ ionization chamber.