Measured- and simulated-pulse shapes in electrically segmented coaxial Ge detectors have been investigated. Three-dimensional position sensitivities have been determined experimentally and theoretically in a 36-fold segmented Ge detector. By using the two-dimensional segmentation in conjunction with pulse-shape analysis, a position sensitivity of better than 1 mm can be obtained in three dimensions at an energy of 374 keV. This is achieved by analyzing the shape of net charge signals of segments containing interactions and of transient image charge signals of neighboring segments. The ability to locate interactions in three-dimensions is one of the crucial properties in the proposed γ-ray energy tracking array (GRETA). The concept of γ-ray tracking will not only increase the efficiency in detecting γ radiation but also enables the localization and characterization of unknown γ-ray sources with much higher accuracy than is possible with current instruments.

/pdf/three-dimensional-position-sensitivity-in-two-dimensionally-1dla7lq9sj.pdf

Three-dimensional position sensitivity in two-dimensionally segmented HP-Ge detectors

Compton suppression (CS) has increased the sensitivity of gamma spectroscopy systems tenfold, and is routinely used in laboratories for environmental analysis and the monitoring of the CTBT. There are several different techniques available, and many more variables to consider when designing or optimising a CS system. An overview and discussion of these is presented here.

Compton suppression systems for environmental radiological analysis

The performance of gamma-ray spectrometers at high energies (several MeV) can be greatly improved through intelligent spectroscopic analysis if spatial information is obtained for each energy deposition. In position-sensitive detectors, the energy and three-dimensional (3-D) position of each interaction in the detector are determined. Recognizing the signatures of multiple interactions in the detector can help to reconstruct the energies of the initial gamma-rays even when the full energies are not deposited. Experimental work by our research group has demonstrated the feasibility of carrying out spatially resolved measurements of individual gamma-ray interactions throughout the volume of a CdZnTe spectrometer. We present the results of a simulation study for gamma-rays incident upon a 6-cm/sup 3/ CdZnTe detector using two reconstruction methods: high-efficiency intelligent spectroscopy (HEIS) in which the peak-to-total ratio is greatly improved relative to traditional spectroscopy while maintaining almost the same intrinsic peak efficiency; and peak-only intelligent spectroscopy (POIS) in which the peak-to-total ratio can approach 0.9, assuming realistic values for energy resolution. Although POIS reduces the intrinsic peak efficiency, it will significantly improve the signal-to-noise ratio for many measurements. The predicted performance is unprecedented for a detector of such small volume and illustrates the gains that can be expected by exploiting 3-D information.

/pdf/intelligent-gamma-ray-spectroscopy-using-3-d-position-1vaccss4be.pdf

Intelligent gamma-ray spectroscopy using 3-D position-sensitive detectors

A method for measuring weak capture-γ-ray resonances via γγ-coincidence counting techniques is described. The coincidence apparatus consisted of a large-volume germanium detector and an annular NaI(Tl) crystal. The setup was tested by measuring the weak ER=227 keV resonance in 26Mg(p,γ)27Al. Absolute germanium and NaI(Tl) counting efficiencies for a range of γ-ray energies and for different detector–target geometries are presented. Studies of the γ-ray background in our spectra are described. Compared to previous work, our method improves the detection sensitivity for weak capture-γ-ray resonances by a factor of ≈100. The usefulness of the present technique for investigations of interest to nuclear astrophysics is discussed.

Studies of weak capture-γ-ray resonances via coincidence techniques

There are several applications for which it is desirable to calculate the locations and energies of individual gamma-ray interactions within a high purity germanium (HPGe) detector. These include gamma-ray imaging and Compton suppression. With a segmented detector this can be accomplished by analyzing the pulse shapes of the signals from the various segments. We examine the fundamental limits to the spatial resolution attainable with this approach. The primary source of error is the series noise of the field effect transistors (FETs) at the inputs of the charge amplifiers. We show how to calculate the noise spectral density at the output of the charge amplifiers due to an optimally selected FET. This calculation is based only on the detector capacitance and a noise constant for the FET technology. We show how to use this spectral density to calculate the uncertainties in parameters, such as interaction locations and energies, that are derived from pulse shape analysis using maximum likelihood estimation (MLE) applied to filtered and digitized recordings of the charge signals. Example calculations are given to illustrate our approach. Experimental results are given that demonstrate that one can construct complete systems, from detector through data analysis, that come near the theoretical limits.

/pdf/spatial-resolution-attainable-in-germanium-detectors-by-1g1q13ohc5.pdf

Spatial resolution attainable in germanium detectors by pulse shape analysis

We present a new technique for High Purity Germanium (HPGe) Compton suppression using pulse shape analysis (PSA). The novel aspect of our approach involves a complete unfolding of the charge pulse shape into a discrete sum of component y-ray interactions. Using the energy and position information obtained from such an unfolding, an algorithm is then applied which favorably rejects Compton escape events. The advantage of the current PSA approach, as compared with other recent approaches, is the potential to reject not only single-site escape events, but also multiple site escape events. Here we discuss the details of our algorithm, and present experimental results from a real-time implementation on a 5 cm X 5 cm HPGe. An experimental comparison with a standard BGO suppresser is shown. We also discuss the possible improvements to the current PSA approach that could be obtained if the HPGe could be highly segmented on the outer contact.

/pdf/hpge-compton-suppression-using-pulse-shape-analysis-wn9ne82twd.pdf

HPGe compton suppression using pulse shape analysis


 Despite its value and importance to oilfield development and reservoir management, carbon/oxygen (CO) logs are commonly associated with significant challenges that are either related to the wellbore logging environment and/or the physics of the measurement. Shallow depth of investigation is considered the greatest challenge related to the nature of the pulsed-neutron (PN) measurement. It can imply a high degree of uncertainty on the measurement and consequently the calculated water saturation, affecting the true assessment of the reservoir fluids’ saturations, especially in challenging logging environments. In this paper we introduce and prove an innovative approach to increase the depth of investigation of the PN measurement.
 Currently, all PN logging tools use an electric pulsed neutron generator (PNG), or "particle accelerator" or Minitron, to probe downhole formations with 14 MeV neutrons and record the returning gamma ray signal at a shallow depth of investigation (DOI), which is generally in the range of 7 inches for C/O measurement and 12 inches for sigma measurement. In this new approach, we introduce the idea of increasing DOI of the measured gamma rays through increasing the energy level of the neutrons emitted by a PNG. To prove the concept, a computer modeling and simulation study was conducted using Monte Carlo N-Particle (MCNP) for a pulsed-neutron logging tool to determine DOI for neutron energies higher than 14 MeV.
 The study involved five different combinations of borehole and formation fluids. Each involved a "block" of 24 MCNP calculations. The 24 calculations inside each block represented the 24 possible combinations of 3 neutron energies (14, 20, 40 MeV), two gamma ray spectral types (inelastic, capture), and four detectors. Data simulation shows that the DOI rises substantially with energy for all tested detectors. Where the enhancement in DOI with the increase in neutron energy is more prolific in case of the inelastic measurement compared to the capture measurement. And of course the deeper the detector (further from the source) the better the DOI, although this can compromise the precision of the measurement. Yet with the recent technology advancements mainly in PNG (producing more neutron population) and GR detector technology (higher and faster count rates), this shall enhance the precision of the measurement and enable us to acquire both accurate and precise measurements at deeper detectors.
 This patented, innovative approach shall significantly reduce and possibly eliminate one of the main reasons behind the uncertainty of reservoir saturation monitoring using PN logs, which is shallow depth of investigation of the measurement. Having a PNG that can produce neutrons at higher energy levels compared to current industry standard shall allow a deeper, more accurate and a representative evaluation of the reservoir.

A Novel System for Large Depth-of-Investigation Pulsed Neutron Measurements and Enhanced Reservoir Saturation Evaluation

We are developing a technique to correct for the resolution-degrading ballistic deficit in cadmium-zinc-telluride (CZT) gamma-ray detectors using our patent-pending pulse-shape analysis methodology. The pulse-shape-analysis methodology has already been demonstrated by LLNL for application in Compton rejection (technology being licensed) and possible application to gamma-ray imaging with Ge-based systems. If the extension of our technique to CZT detectors is successful, this approach will yield a truly high-resolution CZT detector without the limitations associated with co-planar grids or other designs. Current indications are that we will achieve dramatic improvement to the resolution characteristics of CZT and greatly increase the utility of such devices for radiation detection and assessment applications. The current project status including simulations and experimental measurements are discussed.

Dramatic resolution enhancement of gamma-ray spectra acquired with cadmium-zinc-telluride detectors via a patented pulse-shape-analysis methodology

In this report we have discussed two applications for digital pulse shape analysis in Ge detectors: Compton suppression and {gamma}-ray imaging. The Compton suppression aspect has been thoroughly studied during the past few years, and a real-time, laboratory-prototype system has been fielded. A summary of results from that set up have been discussed here. The {gamma}-ray imaging aspect, while not yet developed experimentally, looks very promising theoretically as the simulations presented here have shown. Experimental work currently underway at Berkeley (as discussed in section 4.3) should help further guide us towards the proper developmental path.

/pdf/innovative-uses-for-conventional-radiation-detectors-via-17jtnb6sdn.pdf

Innovative uses for conventional radiation detectors via pulse shape analysis


 Most of the mature South American oil and natural gas fields are represented by formations with moderate porosity, low resistivity, heavy oil, and general anisotropic behavior of other petrophysical properties. This leads to complex decision making when attempting to identify zones of interest with good reservoir qualities in addition to hydrocarbon saturation. These mature fields offer a unique challenge in that the petrophysical analysis needs to rely on data from newly drilled cased hole wellbores, necessitating numerous corrections. In this challenging environment, very few technologies can provide the data required for a precise petrophysical assessment. For this reason, a geochemical spectroscopy tool was employed with measurements that included elemental concentrations, the sigma formation cross section, and a direct measurement of the carbon concentration in the surrounding formation. The resulting data and innovative interpretation provided an accurate assessment of formation mineralogy including clay types, porosity, and hydrocarbon saturation.
 These reservoirs, which have been in production for decades, contain relatively fresh water (approximately 2-5 kppm NaCl), moderate porosities (9 to 20 p.u.), and lithologies dominated by sand or sand/shale. The primary objective of the logging program was to incorporate nuclear spectroscopy applications to evaluate and characterize the zones of interest aligned with historical production data. In addition to the geochemical spectroscopy tool, the logging program also included gamma ray, spectral gamma ray, neutron porosity, and density. This suite of tools supplied the measurements required to characterize the formation through the determination of mineralogy, porosity, and hydrocarbon saturation.
 The mineralogical model was based upon quartz, feldspar, pyrite, a highly conductive mineral, and numerous clays including illite, kaolinite, chlorite, and smectite. Open hole information from nearby wells was also incorporated into the petrophysical interpretation using normalization procedures and prediction analytics. Since this was a cased hole logging program, and calcium was a significant component of the cement, a cement-mimicking mineral was constructed based upon calcium oxide. This provided important quality control information regarding the condition of the borehole and cement placement, as very little calcite was present in the subsurface formations of the field.
 Once the formation mineralogy, porosity, and matrix density were computed, the hydrocarbon saturation was calculated using two approaches: excess carbon and the ratio of carbon to oxygen elemental yields. The final interpretation provided key information, not only for the drilling campaign, but also for the workover planning and quantification of OOIP (original oil in place). Field examples are provided to demonstrate the complete workflow from the design of the logging program to the specialized interpretation methods and final deliverables.

Using Nuclear Spectroscopy in Cased Hole Wells to Estimate Petrophysical Properties and Hydrocarbon Saturation in South American Freshwater Sands

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