HPGe detectors for low-temperature nuclear orientation

Using the Low-Temperature Nuclear Orientation (LTNO) technique one can study various interesting properties of atomic nuclei and nuclear decay which can be deduced from the measurements of the angular distributions of charged particles emitted during the decay. However, the use of particle detectors working in conditions of LTNO devices (which are generally not available commercially) is a necessary precondition for the realization of these experiments.

Planar HPGe detectors for detection of charged particles at “liquid helium” temperatures were developed and produced at NPI e . Relatively simple technology using vacuum evaporation and diffusion was employed. The performance of detectors at low temperatures was tested and their characteristics measured in a testing cryostat before using them in real experiments.

The HPGe detectors were extensively used in a whole range of LTNO experiments with various physical objectives — in offline (IKS Leuven) as well as online (CERN-ISOLDE, Louvain-la-Neuve — LISOL) experiments. In frame of the project “Meson-Exchange Enhancement of First-Forbidden Beta Transitions in the Lead Region”, the measurements of angular distribution of emitted β-particles allowed to determine experimentally the “meson-exchange currents” contribution to the β-decay. In the project “Isospin Mixing in NZ nuclei”, the isospin-forbidden β-transitions of the nuclei in region (A=50–100) were studied in order to obtain information on the isospin structure of the nuclear states. A new project looking for the possible presence of the tensor currents contribution to the β-decay is being prepared for the CERN-ISOLDE facility.     

Studies on a pulse shaping system for fast coincidence with very large volume HPGe detectors

A variant of the leading edge timing (LET) has been proposed which compensates the “walk” due to risetime spread in very large volume (∼ 100 cm³) HPGe detectors. The method - shape compensated leading edge timing (SCLET) — can be used over a wide dynamic range of energies with 100% efficiency and has been compared with the LET and ARC methods. A time resolution of 10 ns fwhm and 21 ns fwtm has been obtained with ²²Na gamma rays and two HPGe detectors of 96 and 114 cm³ volume. This circuit is easy to duplicate and use and can be a low cost alternative to commercial circuits in experiments requiring a large number of detectors.     

A pulse shape analysis algorithm for HPGe detectors

A pulse shape analysis (PSA) algorithm for segmented HPGe detectors is discussed and some test cases are analysed. The procedure allows the identification of the number of interactions inside a segment and their radial positions. An important characteristic of the algorithm is that it does not need the induced signals of the neighbouring segments as it is based on the comparison between the current pulse from the net charge collecting segment and a basis of reference signals. The calculation time that is required to analyse one event scales linearly with the number of interactions to disentangle. The efficiency of the algorithm for the correct identification of the number of interaction points and their radial position calculated for 600 keV γ-rays ranges from 65% to 95% depending on the complexity of the analysed event.     

Fast neutron-induced damage in INTEGRAL n-type HPGe detectors

Several INTEGRAL n-type HPGe detectors have been irradiated by fast neutrons and their degradation studied through the analysis of line shapes. The availability of three different fast neutron beams (5, 16 and 6–70 MeV) allowed a quantitative analysis of the importance of the neutron energy on the amount of damage. A comparison is made with the degradation induced by high-energy proton irradiations. Transient effects on the measured resolution are reported after high voltage cut-off on degraded detectors.     

Development of an anti-Compton veto for HPGe detectors operated in liquid argon using silicon photo-multipliers

A proof of concept detector is presented for scintillation light detection in liquid argon using silicon photo-multipliers. The aim of the work is to build an anti-Compton veto for germanium detectors operated directly in liquid argon as in the GERDA experiment. Wavelength shifting fibers are used to collect the scintillation light and to guide it to Multi-Pixel Photon Counters (MPPC). Sufficient light yield was achieved to realize an effective anti-Compton veto. Properties of the MPPC were studied at cryogenic temperatures and are additionally reported.     

Calculation of the volume of an HPGe crystal

An important characteristic of a high-purity germanium detector for gamma spectrometry is the volume of the crystal, which relates to the detection efficiency and to the measured background level. The crystal dimensions provided by the detector manufacturer were used to calculate the crystal volume. The calculation method accounts for the various sub-volumes of the crystal, in particular the contact hole in the back face and the rounding of the 90° right edge in the front face. The calculation was corroborated using the INVENTOR(?) 3D software. The crystal volume was calculated to be 419.7 cm(3) compared with the 413 cm(3) stated by the manufacturer.     

Absolute efficiency and time-response measurement of single-photon detectors

Using correlated photons from spontaneous parametric downconversion, we have measured both the absolute quantum efficiencies and the time responses of four single-photon detectors. Efficiencies as high as (76.4 ± 2.3)% (at 702 nm) were seen, which to our knowledge are the highest reported single-photon detection efficiencies. An auxiliary retroreflection mirror was found to increase the net detection efficiency by as much as a factor of 1.19. The narrowest time profile for coincidences between two detectors displays a peak with 300 ps FWHM. We also investigated the presence of afterpulses and the effects of saturation and varying device parameters.     

Computer interactive graphics fitting of efficiency curves for Ge detectors

A method of fitting the experimental data for a Ge detector efficiency curve using interactive computer graphics is described that allows the user to adjust the analytical fit to correct the undesired structure in the fitted curve sometimes introduced by the method of least squares. Special types of plots of the efficiency data are described that enhance the visual resolution and allow correction of distortions in the fitted efficiency curve that are not possible from the traditional log/log graph of the efficiency vs energy.     

Inhomogeneity effects on HPGe gamma spectrometry detection efficiency using Monte Carlo technique

This work deals with the inhomogeneity effects on the full energy peak efficiency determination with HPGe gamma spectrometry. The inhomogeneity of the sample was defined in this work as being the fraction of its volume, which does not contain gamma emitters. We applied the Monte Carlo technique based on the GEANT4 code of CERN to study these effects for soil samples for the case of high density (1.54 g/cm³) where the attenuation effects are expected to increase the errors on the activity measurement due to the inhomogeneity. The correction of the efficiency against these effects using Monte Carlo method has been applied in two cases. The first is between samples with different homogeneity values and a calibration standard, which is perfectly homogeneous, and the second is between a sample and a calibration standard with different imperfect homogeneities.     

On the photopeak efficiency of germanium gamma-ray detectors

An empirical equation describing the photopeak efficiency of Ge(Li) and high purity germanium (HPGe) photon detectors in the energy range from 60 keV to the onset of escape peak losses is presented. The functional parameters are fitted to calibration points obtained from long-lived commercially available standards (²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹³⁷Cs, ⁵⁴Mn, ⁶⁰Co, ²²Na). The fitted function is shown to describe 61 efficiency functions of 60 different detectors with an accuracy better than the quoted uncertainty of the calibration sources, including those energy regions where long-lived calibration standards are difficult to obtain. In contrast to other proposed functions, this equation has functional restrictions with respect to the general shape of the photopeak efficiency curve and, therefore, it is well suited to describe the photopeak efficiency, even when fitted only to a few calibration points.     

Efficiency calibration of large volume liquid scintillation detectors

An array of seven large (10 cm thick by 20 cm diameter) NE213 scintillation detectors for use in neutron time-of-flight experiments has been calibrated to determine a matrix of relative detection efficiency as a function of incident neutron energy and detector bias. This matrix is then used to determine the relative efficiency for neutron groups in time-of-flight spectra to which an efficiency-optimizing dynamic bias has been applied.     

A study of light collection efficiency in scintillation detectors

A group of computer programs was set up to study the light of collection efficiency in scintillation detectors with rectangular cross sections. The input conditions can be chosen arbitrarily, including: the size, light attenuation length and refraction index of the scintillator and light guide, the refraction index of the coupling medium, the reflection characteristics of the walls, and the position of light sources. A few examples are discussed in this article.     

高纯锗γ谱仪活度测量标准装置

介绍高纯度γ谱仪活度测量标准装置的效率刻度、性能检验和对测量误差的详细分析。在在５９－１４０８ｋｅＶ能量范围为３．７＊１０＾３－３．７８１０＾５Ｂｑ活度范围内，本标准装置测量３０多种常用放射性核素活度的扩展不确定度为１．４％－５．０％。     

Empirical relationships between some manufacturing practices and performance

Intense global competition, rapid technological changes, advances in manufacturing and information technology and discerning customers are forcing manufacturers to adopt manufacturing practices and competitive priorities that enable them to deliver high quality products in a short period of time. Identifying manufacturers’ competitive priorities and effective manufacturing practices has long been considered one of the key elements in manufacturing strategy research. This paper presents the results of a study conducted to identify some of the effective manufacturing practices that have a significant influence on manufacturing performance. This study also identifies the main competitive objectives of manufacturing industries that participated in the study. The results reported in this paper are based on data collected from a survey using a standard questionnaire administered to 1000 manufacturers in Australia. Evidence indicates that product quality and reliability are the main competitive factors for manufacturers and price has become surprisingly a relatively less important factor. Results show that simultaneous pursuit of advanced quality practices can neutralize the potential negative impacts of manufacturing difficulties and significantly improve product quality and manufacturing performance. Failure mode and effect analysis (FMEA) is shown to be an important tool for improving product quality and on time delivery performance. FMEA practice driven by the intention to improve customer satisfaction is more effective than that practised to fulfil customer requirements. Effective supplier relationships are shown to contribute positively to the manufacturing performance. The results also suggest that maintaining a supplier rating system and product data management and regularly updating them with field failure and warranty data are important manufacturing practices.     

Evaluation of geometric efficiency and radiation exposure in z-axis for volume scan

The International Electrotechnical Commission (IEC) recommended that a display be provided on the operating console of medical computed tomography (CT) scanners when the geometric efficiency (GE) in the z-axis is 70% or less. Taking into account the increase in the number of detector rows and the changes in the scanning method, the present study was conducted to review the GE and to evaluate the exposure dose in the z-axis direction. The GE for a single-rotation scan was calculated in accordance with IEC-606024 Ed. 2. The findings of the present study suggest that 320-row area detector computed tomography volume scan (single axial scan with no table movement in which it just happened to use a very large beam width) extends 18.8 mm beyond the nominal beam collimation, even when the GE is >70%. However, compared with the helical scan, which is the present mainstream of CT examinations, the dose beyond the scan length is narrower in volume scan for a limited scan length of 160 mm or less.     

Determination of the efficiency of gamma detectors from particle-gamma angular correlations

Particle-γ angular correlation technique can be used to determine the efficiency of γ-detectors. This method is demonstrated for 4.44 MeV γ-rays from the reaction ¹²C(α, α′γ).     

Empirical relationships between extinction coefficient and visibility in fog

Relationships between visibility and an extinction coefficient that is due to fog in optical windows that are free from molecular absorption are derived. The extinction coefficients in the visible (0.55 microm), the near IR (1.2 microm), and the mid IR (3.7 microm) are comparable to and roughly twice as much as that in the far IR (10.6 microm) when visibility is less than a few hundred meters. The advantage of far-IR radiation compared with shorter wavelengths grows as visibility exceeds 500 m. Correspondingly, the relationship between extinction coefficient and visibility becomes more sensitive to variations in the particle-size distribution of fog.