Large Volume HPGe Detectors Research Articles

Time characterization of cosmic-ray induced events in HPGe detector by Monte Carlo simulations

Detailed studies of background events provide information necessary for the planning of active and passive shielding of the detector in all experiments, especially ones searching for rare nuclear events. An effective approach in understanding the contribution of cosmic rays to the background of the HPGe detector is the use of Monte Carlo simulations. In this study, the Monte Carlo simulations were performed in order to obtain the time spectrum of the events induced by cosmic-ray muons and neutrons in the materials in the vicinity of the detector system and the detector itself. In order to reject background events originating from environmental radionuclides (from 238U and 232Th series, 40K, etc.), a coincidence system based on a large-volume HPGe detector and plastic scintillation detector was developed and the same setup was constructed in simulations. The experimental spectrum was compared with the simulated one. The possibility to distinguish these events based on the time was presented and illustrated through the selection of several time intervals of the time spectrum obtained in the simulation and corresponding spectra.Furthermore, the contribution of the muon-induced neutrons and cosmic-ray neutron-induced events to the background spectrum of the large-volume HPGe detector was analyzed. It was found that neutrons produced in the interaction of cosmic-muons with detector shielding and the detector itself are relatively small contributors to the overall neutron-induced events (i.e. low-energy region up to 100 keV originating dominantly from recoils and gamma lines originating from inelastic neutron scattering) registered in coincidence spectrum of HPGe detector compared to the contribution of cosmic-neutron themselves. The share of neutron components in this low-energy region of the coincidence spectrum of the HPGe detector was estimated to be ≈88%. In experiments searching for hypothetical dark matter particles, expected signals are in this energy region, thus in this type of experiment, it is essential to perform rejection of events induced by neutron interaction, in order to avoid misidentification of registered signals.

Ultra-sensitive radionuclide analyses: new frontiers in radioanalytics

The most sensitive technologies for ultra-low-level analyses of long-lived radionuclides have been accelerator mass spectrometry and inductively coupled plasma mass spectrometry, reaching detection limits about 1 nBq/g. Together with underground operation of large volume HPGe detectors they have had great impact on underground physics experiments, approaching new frontiers in radioanalytics—a single atom counting.

Ballistic deficit compensation method for a large-volume HPGe detector at high count rates

A large-volume HPGe detector normally has a severe ballistic deficit due to its long rise time of the output signals. Despite the trapezoidal shaping algorithm adopted as a remedy to deal with the signals, the algorithm cannot fully eliminate the ballistic deficit in the case of a high counting rate. To resolve the problem, we propose a ballistic deficit compensation method that is based on the measurement of rise time of the signals before shaping. We find that the ballistic deficit after trapezoidal shaping has little relation to the time constant, but shows a quadratic relationship with the rise time and has a negative correlation with the shaping time. In the case of high count rates, the high resolution is handled by fitting the curve to the rise time and the amplitude deficit of the signal after shaping and by compensating for the signal amplitude after trapezoidal shaping. Tests indicate that when the count rate is about 100 kcps, the resolution of Co-60 improves from traditional 2.32 up to 1.91 keV, thus reaching a higher level.

Excited states above the proton threshold in 26Si

The level scheme above the proton threshold in 26Si is crucial for evaluating the 25Al(p, γ)26Si stellar reaction, which is important for understanding the astrophysical origin of the long-lived cosmic radioactivity 26Al(T 1/2 = 7.17 × 105 y) in the Galaxy. The excited states in 26Si have been studied using an in-beam γ-ray spectroscopy technique with the 24Mg(3He, nγ)26Si reaction. γ-rays with energies up to 4.6 MeV emitted from excited states in 26Si have been measured using large volume HPGe detectors. The spin-parity of one of the most important states reported recently at 5890.0keV has been assigned as 0+ by γ-γ angular correlation measurements in this work.

Conversion electron measurements and determination of rotational parameters in 177Lu and 177Hf

Conversion electron and gamma spectroscopic measurements were carried out on the 160.44d 177mLu decay with a mini-orange electron transporter coupled to a Si(Li) detector and a large-volume HPGe detector. Conversion electron intensities and internal conversion coefficients of 32 transitions in the two nuclei were obtained by the Normalized Peak to Gamma method. Of these, many are being reported for the first time. Mixing ratios were determined for the M1 + E2 transitions of the rotational bands 7/2+[404] in 177Lu , 7/2−[514] and 9/2+[624] in 177Hf populated by the isomer’s decay. The value of the rotational parameter |(gK − gR)/Q0| was calculated for all the three bands. A revised decay scheme is proposed for 177mLu .

Large-volume HPGe detectors for rare events with a low deposited energy

The manufacturing technique and the main characteristics of large-volume HPGe detectors with a capacitance of ∼0.5 pF are described. Based on the developed technique, it is possible to produce a detector such that it is comparable in mass to a coaxial HPGe detector, but its energy resolution and detection threshold are close to the values characteristic of small X-ray HPGe detectors.

Space charge reconstruction in highly segmented HPGe detectors through capacitance-voltage measurements

The space charge distribution inside highly segmented large volume HPGe detectors determined from capacitance–voltage (CV) measurements is investigated. A computer code was developed to understand the impact of impurity concentrations on the resulting capacitances between core contact and outer contact for HPGe detectors biased at different high voltages. The code is intended as a tool for the reconstruction of the doping profile within irregularly shaped detector crystals. The results are validated by comparison with the exact solution of a true coaxial detector. Existing methods for space charge parameter extraction are briefly reviewed. The space charge reconstruction under cylindrical symmetry is derived. An extension of this scheme to higher dimensions for inhomogeneous space charge distributions is illustrated for a two-dimensional segmented coaxial detector.

Determination of space charge distributions in highly segmented large volume HPGe detectors from capacitance–voltage measurements

The space charge distribution of a large volume highly segmented HPGe detector was determined by a non-destructive capacitance–voltage measurement. The capacitances between the 36 segments and the core were measured simultaneously with a precision pulser which was implemented in the core preamplifier. The pulser measurement was compared to and validated by direct capacitance measurements. The three-dimensional doping profile was reconstructed using analytical and numerical methods. Consistent values for the impurity concentration in the range of 0.5 and 1.5×10 10 cm −3 were obtained.

Efficiency Inter-Comparison Measurements of Medium and Large Volume HPGe Detectors in Far and Close Geometries

Inter-comparisons of the absolute full-energy peak efficiencies of two high-purity-germanium (HPGe-50 and HPGe-20) Ortec-model photon detectors were carried out with efficiencies previously measured from a HPGe-20 Tennelec detector in the energy range from 40 to 1,500 keV. The functional parameters are fitted to calibration points from 11 long-lived standard sources (129I, 241Am, 109Cd, 57Co, 137Cs, 60Co,22Na, 133Ba, 152Eu, 154Eu and 166Ho) to an accuracy better than the quoted uncertainties of the calibration sources. The efficiencies in far and close geometries for both detectors are compared.

124Te AND THE E(5) CRITICAL POINT SYMMETRY

We present a new study of the low-lying states in 124 Te nucleus by γ-ray spectroscopy following 124 I β+/ε decay. The β radioactive sources were produced in the 124 Te ( p, n )124 I reaction induced by 11 MeV protons, delivered by the Bucharest FN Tandem Accelerator. The γ-rays were measured in a low background area with three large volume HPGe detectors. A total number of 276 milion double coincidence events were recorded in a six-day run. Most of the gamma line intensities previously measured were confirmed with improved accuracy and several gamma lines were obtained for the first time. Our results, combined with those from a recent ( n , γ) study are compared with the predictions of the E(5) critical point symmetry model and numerical IBA-1 model calculations at the critical point of the U(5)–O(6) phase transition.

Low-level gamma-ray spectrometry for environmental samples

Low-level gamma-ray spectrometry with large volume HPGe detectors has been widely used in analysis of environmental radionuclides. The reasons are excellent energy resolution and high efficiency that permits selective and non-destructive analyses of several radionuclides in composite samples. Although the most effective way of increasing the sensitivity of a gamma-ray spectrometer is to increase counting efficiency and the amount of the sample, very often the only possible way is to decrease the detector’s background. The typical background components of a low-level HPGe detector, not situated deep underground, are cosmic radiation (cosmic muons, neutrons and activation products), radioactivity of construction materials, radon and its progenies. A review of Monte Carlo simulations of background components of HPGe detectors, and their characteristics in coincidence and anti-Compton mode of operation are presented and discussed.

Low-level single and coincidence gamma-ray spectrometry

Analyses of anthropogenic and natural gamma-ray emitters in the environment require high sensitive detector systems operating in coincidence-anticoincidence modes. Thanks to an excellent energy resolution and a high efficiency, large volume HPGe detectors have been widely used in low-level gamma-ray spectrometry. In the present paper we discuss the characteristics of single and coincidence (HPGe-NaI(Tl)) arrangements suitable for analysis of environmental samples containing cascade gamma-ray emitters (e.g., 60Co), positron emitters (e.g., 22Na) and single gamma-ray emitters (e.g., 137Cs). The detectors were placed in a large volume shields consisting of iron, lead and copper layers. The reduction of background for the single gamma-ray spectrometer is between 60 and 250, depending on the gamma-ray energy. As an improvement of the apparatus, low detection limits for analysis of 137Cs (0.3 Bq·kg−1) and 60Co (0.1 Bq·kg−1) in environmental samples, respectively, have been obtained.

High resolution measurement of neutron inelastic scattering and [formula omitted] cross-sections for 209Bi

Gamma production cross-sections were measured for the neutron inelastic scattering and ( n , 2 n γ ) reactions on 209Bi using the white neutron spectrum of GELINA with the time-of-flight technique at the 200 m flight-path station. The full energy range, from the inelastic threshold up to 20 MeV was covered in one experiment with an unprecedented energy resolution of 1.1 keV at 1 MeV and 35 keV at 10 MeV. The gamma-rays were detected with large volume HPGe detectors. The flux was determined with a 235U fission chamber based on the 235U( n , f ) standard cross-section. A metallic Bi sample was used. Inelastic gamma production cross-sections were measured for 39 transitions up to an excitation energy of 3.8 MeV. Based on the adopted level scheme of 209Bi, the total inelastic and the level cross-sections were constructed. A total uncertainty smaller than 5% was obtained for the total inelastic cross-section up to 10 MeV. The ( n , 2 n ) gamma production cross-section was measured for the 8 transitions from 208Bi nucleus up to 1.09 MeV excitation energy. The results are compared with earlier experimental works and with model calculations performed with the TALYS code, version 0.72. Calculations with the TALYS model code show good agreement with the deduced total inelastic cross-section in the entire incident energy range and for deduced level cross-sections below 3 MeV. Discrepancies between calculation and experiment that occur for gamma production cross-sections above 3 MeV are discussed.

The DGF Pixie-4 spectrometer – compact digital readout electronics for HPGe clover detectors

Large volume HPGe detectors are commonly used in applications that require good energy resolution and high detection efficiency, but are expensive and difficult to grow. Clover detectors consisting of four smaller crystals in a common cryostat are a possible alternative, but traditionally require complex readout electronics. In contrast, the DGF Pixie-4 is a compact, digital spectrometer providing on a single 3U CompactPCI/PXI card all the electronics required for clover detectors, including computation of addback spectra. This paper describes the DGF Pixie-4 system architecture, characterizes its energy resolution and throughput, and presents results of test measurements with a clover detector.

High resolution measurement of neutron inelastic scattering and ( [formula omitted]) cross-sections for 52Cr

Cross-sections were measured for gammas produced by neutron inelastic scattering and ( n , 2 n ) reactions on 52Cr using the white neutron spectrum of GELINA with the time-of-flight technique at the 200 m flight-path station. The full energy range, from the inelastic threshold up to 18 MeV was covered in one experiment with an unprecedented neutron energy resolution of 1.1 keV at 1 MeV and 35 keV at 10 MeV. The gamma rays were detected with large volume HPGe detectors. The flux was determined with a 235U fission chamber based on the 235U( n , F ) standard cross-section. A Cr 2O 3 sample was used with a 52Cr/Cr concentration of 99.85% wt. Inelastic gamma production cross-sections were measured for 12 transitions, at least one transition from each level up to an excitation energy of 3.77 MeV. Based on the adopted level scheme of 52Cr, the total inelastic and the level cross-sections were constructed. A total uncertainty smaller than 5% was obtained for the total inelastic cross-section up to 10 MeV. The ( n , 2 n ) gamma production cross-section was measured for the 749.06 keV and 1164.4 keV transitions from 51Cr. Results are compared with earlier experimental works and model calculations performed with the TALYS code. Calculations with the default parameters of TALYS show in general rather good agreement with the present data.

Characterization of large volume HPGe detectors. Part I: Electron and hole mobility parameterization

An analytical model for the hole mobility in a Ge crystal lattice was developed to describe the hole drift anisotropy with experimental velocity values along the crystal axis as parameters. The new model is based on the drifted Maxwellian hole distribution in Ge. It is verified by reproducing experimental longitudinal hole anisotropy data with high accuracy. A comparison between electron and hole mobility shows large differences for the longitudinal and tangential velocity anisotropy as a function of the electrical field orientation with respect to the crystal orientation. The anisotropic mobility causes measurable differences on rise times and pulse shapes which vary with the location where the charge carriers are created in the Ge crystal. These effects are relevant for position determination and γ -ray tracking.

Characterization of large volume HPGe detectors. Part II: Experimental results

Measurements on a 12-fold segmented, n-type, large volume, irregular shaped HPGe detector were performed in order to determine the parameters of anisotropic mobility for electrons and holes as charge carriers created by γ -ray interactions. To characterize the electron mobility the complete outer detector surface was scanned in small steps employing photopeak interactions at 60 keV. A precise measurement of the hole drift anisotropy was performed with 356 keV γ -rays. The drift velocity anisotropy and crystal geometry cause considerable rise time differences in pulse shapes depending on the position of the spatial charge carrier creation. Pulse shapes of direct and transient signals are reproduced by weighting potential calculations with high precision. The measured angular dependence of rise times is caused by the anisotropic mobility, crystal geometry, changing field strength and space charge effects. Preamplified signals were processed employing digital spectroscopy electronics. Response functions, crosstalk contributions and averaging procedures were taken into account implying novel methods due to the segmentation of the Ge-crystal and digital signal processing electronics.

Measurement of neutron inelastic scattering cross sections for 52Cr from threshold up to 18 MeV

The powerful white neutron spectrum from Gelina is used for the investigation of gamma ray production cross sections in the reaction 52Cr(n,n'gamma)52Cr. The gamma rays from the inelastic process are detected with two large volume HPGe detectors placed at 110 degrees and 150 degrees with respect to the beam direction. The neutron flux has been measured with a 235U fission chamber. For the main transitions the inelastic cross section is given with an incident neutron energy resolution ranging from 1.12 keV at 1 MeV to 35.7 keV at 10 MeV, the overall statistical error being <5%. Based on the known 52Cr level scheme, the level and total inelastic cross sections have been obtained up to 3.77 MeV excitation energy. Progress on the implementation of a new acquisition system based on a fast digitiser is presented in this paper.

IAEA-MEL's underground counting laboratory (CAVE) for the analysis of radionuclides in the environment at very low-levels

An underground laboratory for low-level gamma- and beta-spectrometry has been constructed at IAEA-MEL, Monaco, for the analysis of environmental radionuclides. The laboratory is situated at a depth of 35 m water equivalent underground and equipped with 4, large volume HPGe detectors placed in a common lead shield with anti-cosmic plastic scintillator shielding. There is also an anti-Compton gamma-spectrometer, comprized of an HPGe detector and NaI(Tl) shielding, and finally, a Quantulus liquid scintillation spectrometer. The performance of the gammaspectrometers with different shielding settings/adjustments are discussed, as well as their possible applications in the analysis of natural and anthropogenic radionuclides in the marine environment.

IAEA-MEL's underground counting laboratory in Monaco—background characteristics of HPGe detectors with anti-cosmic shielding

The background characteristics of large volume HPGe detectors installed in IAEA-MEL's underground counting laboratory situated at a depth of 35 m water equivalent have been investigated. Both single gamma-ray spectra as well as gamma-ray spectra gated by anti-cosmic shielding made of plastic scintillators are presented. The advantages of anti-cosmic shielding for HPGe detectors, especially in underground counting laboratories operating at shallow depths, are discussed. The authors present several examples of analysis of radionuclides in marine samples by single gamma-ray spectrometers with anti-cosmic shielding, an anti-Compton spectrometer and a Ge–Ge telescope designed for the analysis of low energy gamma-emitters.