N-type High-purity Germanium Detector Research Articles

Modeling of N and P-Type of Coaxial Ge Detectors Using MCNPX and the Effect of Dead Layer Variation on Its Response Function

Abstract This study assessed the response function of a p-type and n-type coaxial high-purity germanium (HPGe) detector via Monte Carlo N-Particle eXtended (MCNPX). MCNPX was employed to model the Coaxial Ge detectors, and for a precise simulation, the dimensions of the dead layer of germanium crystals were added. The dead layer was separated into front and lateral surfaces, and the thickness of each dead layer was modeled. In this work, the simulated detectors have been performed at different energy lines using a radioactive source Eu-152 to study the response function of each with dead layer variations for the front dead layer and study the range of relative deviation of the Monte Carlo simulation outputs from the manufactured declared data. The results proved that the n-type coaxial HPGe detector is more sensitive to the dead layer change than the p-type with a thick change of 0.01 mm. This research has significant effects on the efficiencies of the radiation detection systems in the energy range ∼ (120–1410) keV.

Estimating thickness of the inner dead-layer of n-type HPGe detector

In this study, a procedure to estimate thickness of the inner dead-layer of an n-type coaxial HPGe detector is described. Experimental measurements are carried out with standard point sources: 54Mn, 57Co, 60Co, 88Y, 109Cd, 134Cs, 137Cs, and 152Eu at distances of 5 and 10cm from source to detector. Shape and dimensions of a high-purity germanium (HPGe) detector are determined by radiography to characterize the geometry accurately for Monte Carlo simulation. The role of thickness of the inner dead-layer on full energy peak efficiency is illustrated by MCNP5 code, and it is observed that slope coefficient of efficiency curve has a linear relationship with thickness of the dead-layer. The adjustment of dead-layer yields good agreement, with relative deviation of ≤3%, between experimental efficiency and simulated efficiency in the energy range of 88–1836keV.

Validation of an efficiency calibration procedure for a coaxial n-type and a well-type HPGe detector used for the measurement of environmental radioactivity

To obtain reliable measurements of the environmental radionuclide activity using HPGe (High Purity Germanium) detectors, the knowledge of the absolute peak efficiency is required. This work presents a practical procedure for efficiency calibration of a coaxial n-type and a well-type HPGe detector using experimental and Monte Carlo simulations methods. The method was performed in an energy range from 40 to 1460keV and it can be used for both, solid and liquid environmental samples. The calibration was initially verified measuring several reference materials provided by the IAEA (International Atomic Energy Agency). Finally, through the participation in two Proficiency Tests organized by IAEA for the members of the ALMERA network (Analytical Laboratories for the Measurement of Environmental Radioactivity) the validity of the developed procedure was confirmed. The validation also showed that measurement of 226Ra should be conducted using coaxial n-type HPGe detector in order to minimize the true coincidence summing effect.

Comparison of digital signal processing modules in gamma-ray spectrometry

Commercial digital signal-processing modules have been tested for their applicability to gamma-ray spectrometry. The tests were based on the same n-type high purity germanium detector. The spectrum quality was studied in terms of energy resolution and peak area versus shaping parameters, using a Eu-152 point source. The stability of a reference peak count rate versus the total count rate was also examined. The reliability of the quantitative results is discussed for their use in measurement at the metrological level.

Gamma spectrometer measurements of microgram quantities of plutonium using uranium L X-rays in the 13–21 keV region

Nineteen groups of Marple series cascade impact filters were used to collect aerosol particles associated with various operations in plutonium glove boxes. The cascade impactor separated particles based upon size, while the total mass of plutonium deposited on the filters ranged from 10−4 to 10−8 g. Gamma spectrometer measurements with an n-type high-purity germanium detector (HPGe) were performed with the goal of accurately quantifying the 239Pu mass on each filter using counts from the uranium L X-rays in the 13–21 keV region. However, the mass was great enough in some of the measurements to also quantify using the 239Pu gamma ray at 129.29 keV. One-half portions of each filter were also analyzed using thermal ionization mass spectroscopy (TIMS) and comparisons between the two methods were performed. A significant challenge was to find a calibration source with very low self-absorption characteristics so that the detector response to the low-energy X-rays could be properly characterized. This was accomplished by doping an actual Marple filter with a small NIST traceable quantity of 239Pu (7.3 μg). The comparison of gamma spectrometry to TIMS results yielded largely favorable results. Some notable sources of measurement bias beyond counting statistics were uncovered in the process. The particle distributions on the filters were not perfectly symmetrical, and since the TIMS analyses were performed on just one-half of a filter, a significant difference could result. Replicate filter gamma measurements revealed potential biases in the reproducibility of sample geometry, and high concentrations of 241Am appeared to cause L-shell X-ray fluorescence effects in two of the filter sets.

Performance of HPGe gamma spectrometry system for the measurement of low level radioactivity

Comprehensive quality assurance/quality control procedure is very much necessary to obtain accurate and precise analytical measurement result. This paper discusses the quality control aspects of the High-Purity Germanium (HPGe) based gamma spectrometry system, which has been used for the measurement of low-level radioactivity in environmental samples. The gamma spectrometry system consisting of coaxial n-type HPGe detector having 50% relative efficiency with respect to 7.62 cm x 7.62 cm NaI (Tl), Nuclear Instrumentation Module (NIM) based pulse processing electronic accessories and 8 k MCA. To reduce the background contribution, 7.5 cm thick lead has been placed surrounding the detector. The minimum detectable activities (MDA) with 95% confidence level (for 300 g soil sample and 100,000 s counting time) for important radionuclides such as 238U, 226Ra, 232Th, 40K, and 137Cs are 10.4, 4.3, 4.1, 16.9 and 0.1 Bq kg−1, respectively. The Quality control (X bar R) charts were plotted using 137Cs and 40K background counts observed periodically, which showed that the fluctuation is well within the confidence limit and confirms the stability of the system. The laboratory has been participating in the proficiency tests (PTs) of the International Atomic Energy Agency (IAEA). In recently concluded PTs, the samples include soil, spiked standard solution, spinach, phosphogypsum and spiked air filter were analysed for the natural, fission and activation products radionuclides. The performance evaluation of the IAEA PTs showed that the laboratory results were in good agreement with the target value, which confirms the reliability and traceability of the gamma spectrometric measurement result of the laboratory.

Solar proton damage in high-purity germanium detectors

High-purity germanium (HPGe) detectors used in space for gamma-ray spectroscopy in astrophysics and planetary exploration are known to be damaged by energetic particles. For interplanetary missions close to the Sun such as Messenger or BepiColombo to explore planet Mercury, solar protons represent an important source of damage. In this work, irradiation tests were performed on two large-volume coaxial n-type HPGe detectors with mono-energetic beams of 50–60MeV protons. One of the detectors, designed for spatial applications, was incrementally exposed to a proton fluence up to 7.5×1010p/cm2 and the other to a unique fluence of 1010p/cm2. The results showed that the degradation of the energy resolution appeared for fluences higher than 5×108p/cm2. Moreover, a loss in detection efficiency was observed for fluences above 1010p/cm2. Annealings above 80°C allowed the recovery of the initial resolution but not the initial efficiency. By extrapolating the results beyond the experimental conditions, this study also establishes the limits for the use of spaceborne HPGe detectors in harsh low-energy proton environment.

An empirical expression for the full energy peak efficiency of an N-type high purity germanium detector

An empirical expression for the full energy peak efficiency (e) in terms of the gamma-ray energy (E) and the vertical distance from the detector surface (z) (i.e.,e=e(z,E)) has been obtained for an N-type high purity germanium (HPGE) detector using an extended mixed standard radionuclide solution. Comparison of the calculated efficiencies and the experimentally measured values for the energy range of 59.5–1332.5 keV and a source-to-detector distance of 1.2–7.2 cm showed that the theoretical values agree with that of the experiment within 1.5–3.7% standard deviations. This shows a good agreement between the theory and the experiment.

The assessment of the natural radiation dose committed to the Hong Kong people

The natural radionuclide ( 238U, 226Ra, 232Th and 40K) contents of soil samples at various locations in Hong Kong, building materials commonly used in Hong Kong and construction materials for roads have been determined by low background gamma-ray spectroscopy using an n-type high purity germanium detector. From the measured radionuclide contents, estimations have been made of the absorbed gamma dose rate in air and the indoor radon concentration in Hong Kong. Both are in good agreement with in-situ measurements. Finally, calculations have been made of the annual individual effective dose equivalent contributed by all kinds of natural background radiations. The total value is 3·2 mSv which is about 60% higher than the global average. Of this total value, 80% comes from the radiation from building materials. The present work suggests that building materials are the primary source of natural background radiation in Hong Kong. Therefore, more extensive studies and perhaps limitation of the radionuclide concentration of building materials in the near future seems necessary.

Proton-induced radiation damage in germanium detectors

High-purity germanium (HPGe) detectors will be used in future space missions for gamma-ray measurements and will be subject to interactions with energetic particles. To simulate this process, several large-volume n-type HPGe detectors were incrementally exposed to a particle fluence of up to 10/sup 8/ protons cm/sup -2/ (proton energy: 1.5 GeV) at different operating temperatures (90 to 120 K) to induce radiation damage. Basic scientific and engineering data on detector performance were collected. During the incremental irradiation, the peak shape produced by the detectors showed a significant change from a Gaussian shape to a broad complex structure. After the irradiation, all detectors were thoroughly characterized by measuring many parameters. To remove the accumulated radiation damage, the detectors were stepwise-annealed at temperatures of T >