Segmented HPGe Research Articles

Multi-parameter optimization of gamma emission tomography instruments for irradiated nuclear fuel examination

Material test reactors have an extended use in irradiation testing of novel nuclear fuel materials and the fuel behavior in off-normal conditions. The performance of the nuclear fuel is examined in in-pile and out-of-pile post-irradiation examinations (PIEs), e.g., using Gamma Emission Tomography (GET). GET is a nondestructive assay that images the internal spatial distribution of gamma-emitting nuclides built up in the fuel due to irradiation. Since GET can be performed close to the reactor and without intrusion in the fuel object, it can potentially speed up the data generation from PIE in irradiation testing.The performance metrics of GET devices can be identified regarding time requirements, noise in the reconstructed image, signal-to-background ratio, and spatial resolution. However, these are complicated to determine, partly due to inherent trade-offs between the metrics themselves, partly because they depend on the fuel activity and its spectrum (i.e., object dependent), and, finally, on the GET setup and its configuration.This work proposes a structured methodology for optimizing the collimator design for a new generation of GET tomography setups, intending to improve spatial resolution by one order of magnitude: from the millimeter scale to the hundred-micron scale. The conflicting performance metrics are determined based on the controllable parameters of the GET setup and the uncontrollable parameters of an anticipated fuel object, able to provide a signal-to-background ratio above a customized limit, chosen based on the specific application. The trade-off between the performance remaining metrics is then visualized by a Pareto approach, where dominated solutions are rejected. Finally, constraints on noise level and measurement time are used to find the optimal spatial resolution, without applying noise suppression filters.Two GET setups are presented using the outlined method. Firstly, to upgrade the tomography test bench BETTAN at Uppsala University, a new segmented HPGe detector is planned to be used with low-activity fuel rod mock-ups. Secondly, a GET system for investigating high-activity nuclear fuel rods of representative burnup. For a nuclear fuel inspection, the results showed that a spatial resolution of about 300 μm is possible with reasonable noise and measurement time constraints.

Agata characterisation and pulse shape analysis

The AGATA and GRETA spectrometers are large arrays of highly segmented HPGe detectors that use the technique of gamma ray tracking to reconstruct the scattering path of gamma rays interacting within the active material. A basic requirement is a precise reconstruction of the individual interaction locations within the detectors. This is possible through the use of pulse shape analysis which has to be conducted in real time due to the high data rates generated by the spectrometer. The methodologies that have been evaluated to perform this for AGATA are discussed along with the approaches used to calculate the pulse shape databases required by these algorithms. Finally, the performance and limitations of the existing approaches are reviewed.

First Experimental Demonstration of the Use of a Novel Planar Segmented HPGe Detector for Gamma Emission Tomography of Mockup Fuel Rods

Postirradiation examination of nuclear fuel is routinely performed to characterize the important properties of current and future fuel. Gamma emission tomography is a proven noninvasive technique for this purpose. Among various measurement elements of the technique, a gamma-ray detector is an important element whose spectroscopic abilities and detection efficiency affect the overall results. Finding a combination of high detection efficiency and excellent energy resolution in a single detector is often a challenge. We have designed a novel planar segmented high-purity germanium detector that offers simultaneous measurement in six lines of sight with excellent energy resolution. The simultaneous detection ability enables faster data acquisition in a tomographic measurement, which may facilitate achieving higher spatial resolution. In this work, we have demonstrated the first use of the detector by performing a full tomographic measurement of mockup fuel rods. Two methods of detector data analysis were used to make spectra, and the images (tomograms) were reconstructed using the filtered back projection algorithm. The reconstructed images validate the successful use of the detector for tomographic measurement. The use of the detector for real fuel measurement is being planned and will be performed in the near future.

Experimental evaluation of the performance of a novel planar segmented HPGe detector for use in gamma emission tomography

The use of segmented HPGe detectors for gamma-ray tracking applications is well established. The spectroscopic capabilities of these detectors make them most suitable for such applications. For similar reasons, the use of such detectors in the tomographic measurement of irradiated nuclear fuel has also been envisioned. Especially, these detectors can facilitate faster fuel examination with excellent energy resolution. We have proposed and designed a novel planar segmented HPGe detector for use in gamma emission tomography. The design of the detector segmentation and the mode of operation is unique and offers six simultaneous detection channels for tomographic measurements. This work reports the first experimental evaluation of the performance of the detector. Important characteristics of the detector have been obtained, such as energy resolution of the segments in 1-fold (one segment) and 2-fold (two segments) modes, throughput curves, crosstalk energy corrections, and mislocalisation rate. Collimated source tests have been performed and the results have been compared with the MCNP simulations results. The obtained results are as expected and in good agreement with the simulation results, and it is estimated that using this detector can speed up the data collection by 3.2 times in comparison to an unsegmented detector of the same overall dimensions, in a tomographic application. Further improvements are foreseeable if scaling up to a larger detector with greater segmentation.

Locating events in a segment of a segmented HPGe detector at KRISS

Abstract A specific segment of the 24 fold segmented HPGe detector was studied for improving the interaction position resolution from several cm to mm. The segmented HPGe detector was modeled by CAD program with their electrodes which was isolated by the Boolean function. The Shockley-Ramo theorem changed the Gauss law to Laplace equation problem for calculating the induced charge on each electrode. Finite Element Method solved the Poisson and Laplace equation with their boundary conditions for Electric field and potential. The drift velocities of charge carriers were compared between both along the 100 > and one between 100 > and 110 > Ge crystal axis at a specific segment and used to simulate the pulse shapes with 1 mm position resolution. Coincidence system between a specific segment and 3 × 3 HPGe detector verified the simulated pulse shapes by comparing these with the experimental ones which are generated at the specific target region with a fixed scattering angle. The experimental and simulated ones were χ 2 fitted to find the least χ 2 value and locate the interaction positions. The interaction positions of experimental pulse shapes were analyzed statistically after the rise time correction along 100 > and 110 > Ge crystal axis. The position resolution as FWHM on x, y, and z axis is the 3.12, 4.38 and 8.69 mm with the collimator size 3, 6, and 6 mm, respectively.

Tracking $ \gamma$ rays in highly segmented HPGe detectors: A review of AGATA and GRETINA

The AGATA and GRETINA/GRETA tracking spectrometers allow for the highest sensitivity $ \gamma$ -ray spectroscopy achievable today. Central to these instruments is the task of tracking $ \gamma$ rays, the process by which the interaction points observed in a tracking array are assembled into actual $ \gamma$ rays. Furthermore, the process evaluates the confidence with which individual photons are assembled; i.e., whether the track reconstruction results in a full-energy $ \gamma$ ray or rather in an event associated with Compton scattering, absorption in dead layers or other effects requiring rejection in order to produce spectra with a good peak-to-total ratio. The physics capabilities provided by these spectrometers as well as the different methods of tracking that have been developed are reviewed. The methods used to determine the interaction points from the charge collection signals obtained from the central contact and the segments of HPGe detectors, are presented as well. The performances of the existing tracking arrays, together with possible improvements, are summarized and discussed.

Pulse-Shape Analysis and position resolution in highly segmented HPGe AGATA detectors

The performance of the Pulse-Shape Analysis (PSA) in AGATA HPGe detectors was investigated and improved employing a $\gamma$ -ray source measurement based on $ e^{+}e^{-}$ annihilation radiation after decays of$^{22}$Na by $ \beta^{+}$ decay. The first interaction positions of the two 511keV $\gamma$ rays were determined and the connecting line of these two positions was compared to the known source position as a measure for the PSA performance. The position resolution and its dependence on the PSA parameters were investigated by varying most relevant input quantities: the charge carrier mobility of the holes, the response of the employed measuring electronics especially the preamplifier rise time. The relative statistical weight of charge signals and transient signals was scrutinized. The optimal distance metric of the grid-search algorithm and its impact on the position resolution were determined.

Experimental determination of reference pulses for highly segmented HPGe detectors and application to Pulse Shape Analysis used in $\gamma$γ-ray tracking arrays

For the first time, bases of signals delivered by highly segmented HPGe detectors, for identified hit locations, have been determined in situ, that is in the actual accelerator-target-detection system conditions corresponding to data acquisition during a physics experiment. As a consequence, these bases include all the genuine features and alterations of the signals induced by the experimental setup, e.g. diaphony, electronic response, specificity of individual crystals. The present pulse shape bases were constructed using calibration source data taken at the beginning of the AGATA campaign at GANIL. An experiment performed at GANIL using the AGATA $\gamma$-ray detector together with the VAMOS spectrometer was used to validate the bases. The performance of the bases when used for pulse-shape analysis has been compared to the performance of the standard bases, composed of pulse shapes generated by a computer simulation used for AGATA. This is done by comparing the Doppler correction capability. The so-called Jacobian method used to generate the in situ bases also produces correlations that can be applied to locate in a direct way (no search algorithm) the location where a $\gamma$-ray interacted given that only one segment is hit. As about 50% of all pulse-shape analysis is performed on crystals with only one segment hit this will allow for a large reduction in the needed computer power. Different ways to improve the results of this prospective work are discussed.

Performance of a segmented HPGe detector at KRISS

A 24 segmented HPGe coaxial detector was set up with a digitized data acquisition system (DAQ). The DAQ was composed of a digitizer (5 × 107sampling/s), a Field-Programmable Gate Array (FPGA), and a real time operating system. The Full Width Half Maximum (FWHM), rise time, signal characteristics, and spectra of a 137Cs source were evaluated. The data were processed using an in-house developed gamma-ray tracking system.

Energy reconstruction of an n-type segmented inverted coaxial point-contact HPGe detector

We have characterized, for the first time, an n-type segmented inverted coaxial point-contact detector. This novel detector technology relies on a large variation in drift time of the majority charge carriers, as well as image and net charges observed on the segments, to achieve a potential γ-ray interaction position resolution of better than 1 mm. However, the intrinsic energy resolution in this detector is poor (more than 20 keV at 1332 keV) because of charge (electron) trapping effects. We propose an algorithm that enables restoration of the resolution to a value of 3.44 ± 0.03 keV at 1332 keV for events with a single interaction. The algorithm is based on a measurement of the azimuthal angle and the electron drift time of a given event; the energy of the event is corrected as a function of these two values.

Alpha-event and surface characterisation in segmented true-coaxial HPGe detectors

A detailed study of alpha interactions on the passivation layer on the end-plate of a true-coaxial high-purity germanium detector is presented. The observation of alpha events on such a surface indicates an unexpectedly thin so-called “effective dead layer” of less than 20µm thickness. In addition, the influence of the metalisation close to the end-plate on the time evolution of the output pulses is discussed. The results indicate that alpha contamination can result in events which could be mistaken as signals for neutrinoless double beta decay and provide some guidance on how to prevent this.

Lifetime measurement of neutron-rich even-even molybdenum isotopes

Background: In the neutron-rich $A\ensuremath{\approx}100$ mass region, rapid shape changes as a function of nucleon number as well as coexistence of prolate, oblate, and triaxial shapes are predicted by various theoretical models. Lifetime measurements of excited levels in the molybdenum isotopes allow the determination of transitional quadrupole moments, which in turn provides structural information regarding the predicted shape change.Purpose: The present paper reports on the experimental setup, the method that allowed one to measure the lifetimes of excited states in even-even molybdenum isotopes from mass $A=100$ up to mass $A=108$, and the results that were obtained.Method: The isotopes of interest were populated by secondary knock-out reaction of neutron-rich nuclei separated and identified by the GSI fragment separator at relativistic beam energies and detected by the sensitive PreSPEC-AGATA experimental setup. The latter included the Lund-York-Cologne calorimeter for identification, tracking, and velocity measurement of ejectiles, and AGATA, an array of position sensitive segmented HPGe detectors, used to determine the interaction positions of the $\ensuremath{\gamma}$ ray enabling a precise Doppler correction. The lifetimes were determined with a relativistic version of the Doppler-shift-attenuation method using the systematic shift of the energy after Doppler correction of a $\ensuremath{\gamma}$-ray transition with a known energy. This relativistic Doppler-shift-attenuation method allowed the determination of mean lifetimes from 2 to 250 ps.Results: Even-even molybdenum isotopes from mass $A=100$ to $A=108$ were studied. The decays of the low-lying states in the ground-state band were observed. In particular, two mean lifetimes were measured for the first time: $\ensuremath{\tau}=29.{7}_{\ensuremath{-}9.1}^{+11.3}$ ps for the ${4}^{+}$ state of $^{108}\mathrm{Mo}$ and $\ensuremath{\tau}=3.{2}_{\ensuremath{-}0.7}^{+0.7}$ ps for the ${6}^{+}$ state of $^{102}\mathrm{Mo}$.Conclusions: The reduced transition strengths $B(E2)$, calculated from lifetimes measured in this experiment, compared to beyond-mean-field calculations, indicate a gradual shape transition in the chain of molybdenum isotopes when going from $A=100$ to $A=108$ with a maximum reached at $N=64$. The transition probabilities decrease for $^{108}\mathrm{Mo}$ which may be related to its well-pronounced triaxial shape indicated by the calculations.

Search for E1 strength in 62,64Fe around the threshold

The structure and nature of the pygmy dipole resonance (PDR) states below and above the neutron threshold is a recent open problem, particularly in exotic, neutron rich nuclei. Present experimental observations give only limited information on this subject. New experiments using different methods are needed. A recent measurement at the GSI laboratories on 62,64Fe with the PRESPEC (2014) setup, following a past experiment with the RISING (2005) setup on 68Ni, will contribute to solve the open questions. The setup located at GSI consists of the segmented HPGe detector array AGATA, scintillators (HECTOR), an Energy Loss / Total Energy time of flight measuring detector system called LYCCA and the fragment separator (FRS) apparatus. The experiment is based on relativistic Coulomb excitation together with the detection of the incoming and outgoing particles event by event. The detection of the produced y-rays in the reactions, provides insight into the problem of the electric dipole response and E1 strength distribution around particle separation threshold.

Study of theγdecay of high-lying states in208Pb via inelastic scattering of 17O ions

A measurement of the high-lying states in 208Pb has been made using 17O beams at 20 MeV/u. The gamma decay following inelastic excitation was measured with the detector system AGATA Demonstrator based on segmented HPGe detectors, coupled to an array of large volume LaBr3:Ce scintillators and to an array of Si detectors. Preliminary results in comparison with (γ,γ’) data, for states in the 5-8 MeV energy interval, are presented.

Direct determination of the hit locations from experimental HPGe pulses

The gamma-tracking technique optimises the determination of the energy and emission angle of gamma-rays detected by modern segmented HPGe detectors. This entails the determination, using the delivered pulse shapes, of the interaction points of the gamma-ray within the crystal. The direct method presented here allows the localisation of the hits using only a large sample of pulses detected in the actual operating conditions. No external crystal scanning system or pulse shape simulation code is needed. In order to validate this method, it is applied to sets of pulses obtained using the University of Liverpool scanning system. The hit locations are determined by the method with good precision.

Improved energy resolution of highly segmented HPGe detectors by noise reduction

Built-in redundancies in highly segmented high-purity Ge detectors are exploited to increase the energy resolution of these semiconductor devices for detection of electromagnetic radiation in the X-ray and γ-ray regime. The information of the two electronically decoupled independent measurements, the cathode and the anode electrodes, provides an improved signal-to-noise ratio through a combination of the individually measured signals performed on an event-by-event basis. The average energy resolution values of the AGATA triple cluster detector for an energy deposition of 60keV was measured to be 1.1 keV (FWHM) for the 36 segments and 1.2 keV for the core. The averaged signals of the core and the segments show an improved resolution value of 0.87 keV which is close to the expected theoretical limit. At higher γ-ray energy the averaging technique allows for an enhanced energy resolution with a FWHM of 2.15keV at 1.3MeV. By means of the position sensitive operation of AGATA a new value for the Fano factor was determined and the noise contributions to the FWHM of a γ-ray peak separated.

Gamma-ray spectroscopy at TRIUMF-ISAC: recent highlights and future plans

High-resolution gamma-ray spectroscopy is essential to fully exploit the unique scientific opportunities at the next generation radioactive beam facilities such as TRIUMF Isotope Separator and Accelerator (ISAC). At ISAC the 8π spectrometer and its associated auxiliary detectors is optimized for β-decay studies while TIGRESS, an array of segmented clover HPGe detectors has been designed for reaction studies with accelerated radioactive ion beams. This paper gives a brief overview of these facilities, recent results from the diverse program of nuclear structure and fundamental interaction studies they support and future plans.

Axes determination for segmented true-coaxial HPGe detectors

A fast method to determine the crystallographic axes of segmented true-coaxial high-purity germanium detectors is presented. It is based on the analysis of segment-occupancy patterns obtained by irradiation with radioactive sources. The measured patterns are compared to predictions for different axes orientations. The predictions require a simulation of the trajectories of the charge carriers taking the transverse anisotropy of their drift into account.

Pulse shape analysis for γ-ray tracking (Part I): Pulse shape simulation with JASS

Next-generation \( \gamma\) -ray spectrometers based on highly segmented HPGe detectors are using the recent technique of \( \gamma\) -ray tracking to significantly improve on efficiency and Doppler correction capabilities. A precise reconstruction of the individual interaction locations within the active material is possible through the use of pulse shape analysis (PSA) which, in turn, demands an accurate knowledge of the detector response. We developed JASS, a Java-based simulation software package to generate pulse shapes for the AGATA detectors from physics constraints and basic material parameters. For verifying the simulation experimental data from a coincidence scan with known interaction locations was used. The achieved position resolution, in the order of a few millimeters, is within the requirements of the \( \gamma\) -ray tracking array.

Pulse shape analysis for γ-ray tracking (Part II): Fully informed particle swarm algorithm applied to AGATA

AGATA is an array of highly segmented HPGe detectors using the technique of \( \gamma\) -ray tracking to reconstruct the scattering path of \( \gamma\) -rays interacting within the active material. A basic requirement therefore is a precise reconstruction of the individual interaction locations within the detectors. This is possible through the use of pulse shape analysis (PSA) which, however, has to be conducted in real time due to the high data rates expected. We report on the use of a fully informed particle swarm (FIPS) algorithm for PSA. The achieved position resolution, in the order of a few millimeters, is within the requirements of a \( \gamma\) -ray tracking array. The energy resolutions in Doppler-corrected spectra of a simulated in-beam experiment are comparable to those of a non-real-time extensive PSA based on a full grid search.