Intrinsic Detector Research Articles

Remote thermal neutron die-away measurements to improve Differential Die-Away Analysis

Differential Die-away Analysis (DDAA) is a very effective detection technique of special nuclear material (SNM). It is based on the detection of fast fission neutrons that decay with a time constant characteristic of the thermal neutron that created them. The presence of fast neutrons with this slow die-away time is a positive, sensitive indication of the presence of SNM in the inspected cargo. The thermal decay time of an inspected medium is not known before a DDAA measurement is taken. The choice of when, from the end of a neutron pulse, data are to be acquired must be taken conservatively. If the thermal decay time of an inspected medium can be determined, a priori, then a better estimate of the expected time of separation between the intrinsic detector signal and the fission signal can be achieved. With a better estimate of this time, the choice of times after the end of the pulse to perform signal integration can be optimized. A more optimal choice for this time will result in higher detection sensitivity. This paper describes a method of remotely assessing the thermal die-away properties of an interrogated cargo, and grossly characterizes the class of material being inspected in terms of neutron attenuation and moderation properties.

Compton imager using room temperature silicon detectors

We have been developing a multi-layer Compton Gamma Ray Imager using position-sensitive, intrinsic silicon detectors. Advantages of this approach include room temperature operation, reduced Doppler broadening, and use of conventional silicon fabrication technologies. We have obtained results on the imaging performance of a multi-layer instrument where each layer consists of a 2×2 array of double-sided strip detectors. Each detector is 63 mm×63 mm×2 mm thick and has 64 strips providing a strip pitch of approximately 0.9 mm. The detectors were fabricated by SINTEF ICT (Oslo Norway) from 100 mm diameter wafers. The use of large arrays of silicon detectors appears especially advantageous for applications that require excellent sensitivity, spectral resolution and imaging such as gamma ray astrophysics, detection of special nuclear materials, and medical imaging. The multiple Compton interactions (three or more) in the low- Z silicon enable the energy and direction of the incident gamma ray to be determined without full deposition of the incident gamma-ray energy in the detector. The performance of large volume instruments for various applications are presented, including an instrument under consideration for NASA's Advanced Compton Telescope (ACT) mission and applications to Homeland Security. Technology developments that could further extend the sensitivity and performance of silicon Compton Imagers are presented, including the use of low-energy (few hundred keV) electron tracking within novel silicon detectors and the potential for a wafer-bonding approach to produce thicker, position-sensitive silicon detectors with an associated reduction of required electronics and instrument cost.

Medipix 2 in X-ray diffraction

The Medipix 2 detector is a detector with a high dynamic range and, in principle, no intrinsic detector noise. This makes the detector perfectly suitable for use in X-ray diffraction experiments. For X-ray diffraction systems used in materials science, it is common to use X-rays with an energy of 8 keV. When the Medipix 2 read-out chip is bump-bonded to a 300 μ m silicon sensor it is able to detect these X-rays with a high efficiency. The Medipix 2 detector has been integrated into a PANalytical X’Pert PRO X-ray diffraction system. Details about this integration will be discussed and measurements performed with this system will be shown.

Charge transport properties of single crystal CVD-diamond particle detectors

The charge transport properties of virgin and heavily irradiated intrinsic Single-Crystal CVD-Diamond Detectors (SC-CVD-DD) are discussed by means of the Transient-Current Technique (TCT), which proved to be a sensitive tool to detect the first changes of the detector's performance. Charge-carrier mobilities μ e = 1300–3100 cm 2/V s, μ h = 2400 cm 2/V s and effective deep-trapping lifetimes τ eff-h = 300–900 ns, τ eff-e = 160–360 ns were measured in the non-irradiated state, decreasing slightly after irradiation by a fluency F > 10 13 p 26MeV cm − 2 . The carriers drift velocity remained almost unchanged after irradiation, indicating the creation of mainly neutral defects. After exposure to fluency above 10 16 p cm − 2 however, TC signals were not detectable with the available broadband electronics. A significant improvement was observed after annealing at 1000 °C, and almost complete restoration was obtained by priming with 90Sr-electrons. Optical absorption (OA) spectra confirmed the degradation of the detector performance after exposure to increasing particle fluencies, showing deviations from the intrinsic ‘edge absorption’ shape around 5.3 eV that was observed before irradiation.

Towards a continuous crystal APD-based PET detector design

Monolithic scintillator blocks provide an efficient way to significantly increase the sensitivity of high-resolution positron emission tomography (PET) systems. Using statistical or neural network-based positioning schemes, the measured intrinsic detector resolution for perpendicular incident photons is 1.7 mm full-width at half-maximum (FWHM) for 10 mm thick LSO blocks and 2.0 mm FWHM for 20 mm thick LSO blocks. Because the positioning algorithms determine the incidence position as opposed to the combination of interaction position and interaction depth, a very good parallax correction can be achieved. Energy and detector time resolution were 11.5% FWHM and 1.6 ns FWHM, respectively.

Leggett-Garg Inequality with a Kicked Quantum Pump

A kicked quantum nondemolition measurement is introduced, where a qubit is weakly measured by pumping current. Measurement statistics are derived for weak measurements combined with single-qubit unitary operations. These results are applied to violate a generalization of the Leggett-Garg inequality. The violation is related to the failure of the noninvasive detector assumption, and may be interpreted as either intrinsic detector backaction, or the qubit entangling the microscopic detector excitations. The results are discussed in terms of a quantum point contact kicked by a pulse generator, measuring a double quantum dot.

Simulating the charge dispersion phenomena in Micro Pattern Gas Detectors with a resistive anode

The Time Projection Chamber (TPC) for the International Linear Collider (ILC) will need to measure about 200 track points with a transverse resolution close to 100 μm. The resolution goal is beyond the capability of the conventional proportional wire/cathode pad TPC and Micro-Pattern Gas Detectors (MPGD) are being developed to meet the challenge. The standard MPGD readout techniques will, however, have difficulty in achieving the ILC–TPC resolution goal with the 2×6 mm 2 wide pads as was initially envisioned. Proposals for smaller width pads will improve the resolution but will require a larger number of readout channels and increase the TPC detector cost and complexity. The new MPGD readout concept of charge dispersion has the potential to achieve the ILC–TPC resolution goal without resorting to narrower pads. This was recently demonstrated in cosmic ray tests of a small prototype TPC read out with MPGDs using the charge dispersion technique. Here we describe the simulation of the charge dispersion phenomena for the MPGD–TPC. The detailed simulation includes initial ionization clustering, electron drift, diffusion effects, the intrinsic detector pulse-shape and electronics effects. The simulation is in excellent agreement with the experimental data and can be used to optimize the MPGD charge dispersion readout for the TPC.

Incorporation of system resolution compensation (RC) in the ordered-subset transmission (OSTR) algorithm for transmission imaging in SPECT

In order to reconstruct attenuation maps with improved spatial resolution and quantitative accuracy, we developed an approximate method of incorporating system resolution compensation (RC) in the ordered-subset transmission (OSTR) algorithm for transmission reconstruction. Our method approximately models the blur caused by the finite intrinsic detector resolution, the nonideal source collimation and detector collimation. We derived the formulation using the optimization transfer principle as in the derivation of the OSTR algorithm. The formulation includes one forward-blur step and one back-blur step, which do not severely slow down reconstruction. The formulation could be applicable to various transmission geometries, such as point-source, line-source, and sheet-source systems. Through computer simulations of the MCAT phantom and transmission measurements of the air-filled Data Spectrum Deluxe single photo emission computed tomography (SPECT) Phantom on a system which employed a cone-beam geometry and a system which employed a scanning-line-source geometry, we showed that incorporation of RC increased spatial resolution and improved the quantitative accuracy of reconstruction. In simulation studies, attenuation maps reconstructed with RC correction improved the quantitative accuracy of emission reconstruction.

The Majorana project

The objective of the Majorana experiment is to study neutrinoless double beta decay ββ(0ν) with an effective Majorana-neutrino mass sensitivity near 100 meV in order to characterize theMajorana nature of the neutrino, the Majorana mass spectrum, and the absolute mass scale. An experimental study of the neutrino mass scale implied by neutrino oscillation results is now technically within our grasp. This exciting physics goal is best pursued using the well-established technique of searching for ββ(0ν) of 76Ge, augmented with recent advances in signal processing and detector design. The Majorana experiment will consist of a large mass of 76Ge in the form of high-resolution intrinsic germanium detectors located deep underground within a low-background shielding environment. Observation of a sharp peak at the ββ endpoint will quantify the ββ(0ν) half-life and thus the effective Majorana mass of the electron neutrino.

System characteristics of SPECT with a slat collimated strip detector

In classical SPECT with parallel hole collimation, the sensitivity is constant over the field of view (FOV). This is no longer the case if a rotating slat collimator with planar photon collection is used: there will be a significant variation of the sensitivity within the FOV. Since not compensating for this inhomogeneous sensitivity distribution would result in non-quantitative images, an accurate knowledge of the sensitivity is mandatory to account for it during reconstruction. On the other hand, the spatial resolution versus distance dependency remains unaltered compared to parallel hole collimation. For deriving the sensitivity, different factors have to be taken into account: a first factor concerns the intrinsic detector properties and will be incorporated into the calculations as a detection efficiency term depending on the incident angle. The calculations are based on a second and more pronounced factor: the collimator and detector geometry. Several assumptions will be made for the calculation of the sensitivity formulae and it will be proven that these calculations deliver a valid prediction of the sensitivity at points far enough from the collimator. To derive a close field model which also accounts for points close to the collimator surface, a modified calculation method is used. After calculating the sensitivity in one plane it is easy to obtain the tomographic sensitivity. This is done by rotating the sensitivity maps for spin and camera rotation. The results derived from the calculations are then compared to simulation results and both show good agreement after including the aforementioned detection efficiency term. The validity of the calculations is also proven by measuring the sensitivity in the FOV of a prototype rotating slat gamma camera. An expression for the resolution of these planar collimation systems is obtained. It is shown that for equal collimator dimensions the same resolution–distance relationship is obtained as for parallel hole collimators. Although, a better spatial resolution can be obtained with our prototype camera due to the smaller pitch of the slats. This can be achieved without a major drop in system sensitivity due to the fact that the slats consist of less collimator material compared to a parallel hole collimator. The accuracy of the calculated resolution is proven by comparison with Monte Carlo simulation and measurement resolution values.

DETERMINATION OF GROSS ALPHA, 224Ra, 226Ra, AND 228Ra ACTIVITIES IN DRINKING WATER USING A SINGLE SAMPLE PREPARATION PROCEDURE

The current federal and New Jersey State regulations have greatly increased the number of gross alpha and radium tests for public and private drinking water supplies. The determination of radium isotopes in water generally involves lengthy and complicated processes. In this study, a new approach is presented for the determination of gross alpha, 224Ra, 226Ra, and 228Ra activities in water samples. The method includes a single sample preparation procedure followed by alpha counting and gamma-ray spectroscopy. The sample preparation technique incorporates an EPA-approved co-precipitation methodology for gross alpha determination with a few alterations and improvements. Using 3-L aliquots of sample, spiked with 133Ba tracer, the alpha-emitting radionuclides are isolated by a BaSO4 and Fe(OH)3 co-precipitation scheme. First the gross alpha-particle activity of the sample is measured with a low-background gas-flow proportional counter, followed by radium isotopes assay by gamma-ray spectroscopy, using the same prepared sample. Gamma-ray determination of 133Ba tracer is used to assess the radium chemical recovery. The 224Ra, 226Ra, and 228Ra activities in the sample are measured through their gamma-ray-emitting decay products, 212Pb, 214Pb/214Bi, and 228Ac, respectively. In cases where 224Ra determination is required, the gamma-ray counting should be performed within 2-4 d from sample collection. To measure 226Ra activity in the sample, the gamma-ray spectroscopy can be repeated 21 d after sample preparation to ensure that 226Ra and its progeny have reached the equilibrium state. At this point, the 228Ac equilibration with parent 228Ra is already established. Analysis of aliquots of de-ionized water spiked with NIST-traceable 230Th, 224Ra, 226Ra, and 228Ra standards demonstrated the accuracy and precision of this method. Various performance evaluation samples were also assayed for gross alpha as well as radium isotope activity determination using this procedure and the results were in close agreement with the assigned values. In addition, method comparison results of actual sample analyses agreed well with the ones performed using EPA-approved procedures. With a 3-L sample aliquot and 1,000-min counting time, the average gross alpha minimum detectable concentration (MDC) was about 0.002 Bq L(-1). The average MDC's for 224Ra, 226Ra, and 228Ra were 0.034 Bq L(-1), 0.017 Bq L(-1), and 0.036 Bq L(-1), respectively, based on a 3-L sample aliquot, 85% chemical yield, 40% intrinsic Ge detector, and 1,000-min count time. This method combines and simplifies the analytical procedures and reduces labor while achieving the precision, accuracy, and minimum detection limit requirements of EPA regulations.

A 2-D MSGC-based imaging detector for neutrons

The development and testing of a two-dimensional (2-D) prototype detector based on a microstrip gas chamber (MSGC) is reported using a gas mixture of 2.5 bar /sup 3/He and 2.5 bar CF/sub 4/. The second coordinate is obtained by utilising a plane of wires as pick up electrodes. The detector is operated with the wire plane at such a potential so as not to induce any gain around the wires. This means that the high tolerances normally associated with wire planes in multi wire proportional counters are not mandatory. The detector comprises of 48 individually instrumented channels in both X (MSGC strips) and Y (orthogonal wire plane). A specially designed encoding module has been constructed which feeds digital addresses for each event to the ISIS Data Acquisition Electronics (DAE) system. An intrinsic detector resolution of /spl sim/1 mm full-width at half-maximum has been measured for both dimensions (in experimental exposures on the ROTAX beamline at ISIS) which is degraded slightly by the digital resolution for the overall system. This readout method is shown to be very tolerant of a poor signal to noise ratio in the readout channels (unlike traditional analogue wire chamber readout systems) and permits the operation of the MSGC at low avalanche gains (/spl sim/10) which helps to maximize the rate and lifetime performance of the detector as well as permitting data capture rates in the megahertz range. The event timing resolution is comfortably submicrosecond and is, therefore, suitable for applications on spallation neutron sources.

A 2-dimensional MSGC-based imaging detector for X-rays

The development and testing of a two-dimensional (2-D) prototype detector based on a microstrip gas chamber (MSGC) are reported. The second coordinate is obtained by utilising a plane of wires as pick up electrodes. The detector is operated with the wire plane at such a potential so as not to induce any gain around the wires. This means that the high tolerances normally associated with wire planes in multiwire proportional counters are not necessary, making the manufacture and repair of such a device relatively easier. The detector comprises of 48 individually instrumented channels in both X (MSGC strips) and Y (transverse wire plane). A specially designed encoding module has been constructed which feeds digital addresses for each event to the ISIS data acquisition electronics (DAE) system. (ISIS is the spallation neutron source at the Rutherford Appleton Laboratory.) An intrinsic detector resolution of /spl sim/0.5 mm full-width at half-maximum (FWHM) has been measured for both dimensions which is degraded slightly by the digital resolution for the overall system. This readout method is shown to be very tolerant of a poor signal-to-noise ratio in the readout channels (unlike traditional analogue wire chamber readout systems) and permits the operation of the MSGC at moderate avalanche gains (/spl sim/1000) which helps to maximize the rate and lifetime performance of the detector as well as permitting data capture rates in the MHz range.

High Z and medium Z scintillators in ultra-high-resolution small animal PET

As small animal positron emission tomography (PET) scanners are continuously improving in their performances, one is lead to the question of how far can spatial resolution go. In this paper, we address the limiting effects to the intrinsic detector spatial resolution and whether the photoelectric interaction, and therefore high Z materials, outperform dense medium Z scintillators. In particular, with the EGSnrc Monte Carlo simulation, we compare the ultimate performances, in spatial resolution, of three scintillators: BGO, NaI(Tl), and YAP:Ce. BGO is the PET scintillator that has the highest photofraction, whereas YAP:Ce has the lowest. NaI(Tl), instead, is a relatively high Z but low-density scintillator. There are three principle contributions to the degradation of the intrinsic detector spatial resolution: multiple Compton scattering, electron range after a gamma interaction, and K-shell fluorescence emission. We present the results of simulations of crystals with different thicknesses, with and without K-shell fluorescence emission and electron transport. We conclude that the effects of multiple scattering, electron range, and fluorescence emission to the spatial resolution are smallest for medium Z, high-density materials like YAP:Ce. The fraction of misplaced events, which are defined here as F=N/sub Wrong//N/sub Tot/, is F/sub 0.5mm/ = 41 % for BGO in the case of 0.5-mm binning, increasing to F/sub 0.1mm/ = 80% for the 0.1-mm binning. In the case of YAP:Ce, the misplaced events are, respectively, F/sub 0.5mm/ = 32% and F/sub 0.1mm/ = 47%. We conclude that for ultra-high-resolution PET detectors, dense medium Z scintillators, such as YAP:Ce, may outperform high Z materials.

A current mode detector array for [formula omitted]-ray asymmetry measurements

We have built a CsI(Tl) γ -ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV γ -rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion–nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode γ -ray detection with vacuum photodiodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of γ -rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of 10 - 9 in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.

The proposed Majorana 76Ge double-beta decay experiment

The proposed Majorana experiment is based on an array of segmented intrinsic Ge detectors with a total mass of 500 kg of Ge isotopically enriched to 86% in 76 Ge. Background reduction will be accomplished by: material selection, detector segmentation, pulse shape analysis, electro-formation of copper parts, and granularity of detector spacing. The predicted experimental sensitivity for measurement of the neutrinoless double-beta decay mode of 76 Ge, over a data acquisition period of 5000 kg·y, is T 1 / 2 0 ν ∼ 4 × 10 27 y .

High-resolution PET detector design: modelling components of intrinsic spatial resolution

The development of dedicated small animal PET (positron emission tomography) scanners has led to significantly higher spatial resolution and comparable sensitivity to clinical scanners. However, it is not clear whether we are approaching the fundamental limit of spatial resolution. This work aims to understand what is currently limiting spatial resolution during data formation and collection and how to apply that knowledge to obtain the best possible resolution for small animal PET without sacrificing sensitivity. Monte Carlo simulations were performed of the interactions of a 511 keV photon in a variety of detector materials to evaluate the modulation transfer function of the materials. Positron range, non-colinearity and pixel size were modelled to determine the contribution of additional components of data formation and collection on the complete modulation transfer function of a PET system. These simulations are shown to predict the intrinsic detector resolution of current high resolution systems very well. They also show that current detectors are not limited by inter-crystal scatter. An intrinsic resolution of 0.5 mm can be achieved, but would require a detector with a pixel size of around 250 µm that can be read out unambiguously. It is shown that a range of different detector materials, both scintillators and semiconductors, can be used in these high-resolution detectors. While this design relies on thin (∼3 mm) pieces of material, stacks of the material are shown to simultaneously provide spatial resolution near 0.5 mm and 60% efficiency. This work has shown that detectors with significantly better resolution and sensitivity can be developed for small animal PET applications.

Bioturbation in near-surface sediments from the COMRA Polymetallic Nodule Area: Evidence from excess210Pb measurements

In order to evaluate bioturbation in sediments from the COMRA Polymetallic Nodule Area in the northeast tropical Pacific, excess210Pb profiles in sediments cores collected with multiple corers during R/V DAYANGYIHAO Environmental Program Cruise in 1998 were measured by direct gamma assay using Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detectors. A steady-state diffusion model of excess210Pb profiles suggests that bioturbation mixing depths and biodiffusion coefficients are 16 cm and 2.75 cm2/a in East Zone, and 6 cm and 0.26 cm2/a in West Zone, respectively. Furthermore, the observations of macrofauna and measurements of total organic carbon (TOC) content in sediments suggest that bioturbation is directly controlled by species and abundance of benthic fauna, such aspolychaete, and the bioturbation mixing depth and intensity are positively correlated with the organic matter content.

First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip

We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm-thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, and radiation monitoring in nuclear facilities. The Medipix2 + CdTe hybrid detector features 256 /spl times/ 256 square pixels, a pitch of 55 /spl mu/m, a sensitive area of 14/spl times/14 mm/sup 2/. We analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt ohmic contacts, showed an ohmic response when negatively biased up to less than 60 V (electrons collection mode). Tests were also performed in holes collection mode, where a nonresistive behavior was observed above +15 V. We performed a series of imaging tests at low voltage bias with gamma radioactive sources and with an X-ray tube. Under uniform irradiation, we observed for all detectors the presence of numerous, stable structures in the form of small circles of about 200 /spl mu/m diameter, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime). Also long filament structures have been observed. Further investigations will reveal whether they are due to an intrinsic detector response (e.g., due to Te inclusions) or to the bump-bonding process.

Design and simulation of a high-resolution stationary SPECT system for small animals

Exciting new SPECT systems can be created by combining pinhole imaging with compact high-resolution gamma cameras. These new systems are able to solve the problem of the limited sensitivity-resolution trade-off that hampers contemporary small animal SPECT. The design presented here (U-SPECT-III) uses a set of detectors placed in a polygonal configuration and a cylindrical collimator that contains 135 pinholes arranged in nine rings. Each ring contains 15 gold pinhole apertures that focus on the centre of the cylinder. A non-overlapping projection is acquired via each pinhole. Consequently, when a mouse brain is placed in the central field-of-view, each voxel in the cerebrum can be observed via 130 to 135 different pinholes simultaneously. A method for high-resolution scintillation detection is described that eliminates the depth-of-interaction problem encountered with pinhole cameras, and is expected to provide intrinsic detector resolutions better than 150 µm. By means of simulations U-SPECT-III is compared to a simulated dual pinhole SPECT (DP-SPECT) system with a pixelated array consisting of 2.0 × 2.0 mm NaI crystals. Analytic calculations indicate that the proposed U-SPECT-III system yields an almost four times higher linear and about sixty times higher volumetric system resolution than DP-SPECT, when the systems are compared at matching system sensitivity. In addition, it should be possible to achieve a 15 up to 30 times higher sensitivity with U-SPECT-III when the systems are compared at equal resolution. Simulated images of a digital mouse-brain phantom show much more detail with U-SPECT-III than with DP-SPECT. In a resolution phantom, 0.3 mm diameter cold rods are clearly visible with U-SPECT-III, whereas with DP-SPECT the smallest visible rods are about 0.6–0.8 mm. Furthermore, with U-SPECT-III, the image deformations outside the central plane of reconstruction that hamper conventional pinhole SPECT are strongly suppressed. Simulation results indicate that future pinhole SPECT systems are likely to bring about significant improvements in radio-molecular imaging of small animals.