X-ray phase-contrast (XPC) images of mouse lungs were acquired in vivo with a benchtop XPC system employing a conventional microfocus source. A strong speckled intensity pattern was present in lung regions of the XPC radiographs, previously only observed in synchroton experiments and in situ benchtop studies. We showed how the texture characteristics of the speckle is influenced by the amount of air present in the lungs at different points in the breathing cycle.
We present the results from a benchtop X-ray phase-contrast (XPC) method for lung imaging that represents a paradigm shift in the way small animal lung imaging is performed. In our method, information regarding airway microstructure that is encoded within speckle texture of a single XPC radiograph is decoded to spatially resolve changes in lung properties such as microstructure sizes, air volumes, and compliance, to name a few. Such functional information cannot be derived from conventional lung radiography or any other 2D imaging modality. By computing these images at different time points within a breathing cycle, dynamic functional imaging can be potentially achieved without the need for tomography.
X-ray phase-contrast (XPC) imaging methods are well-suited for lung imaging applications due to the weakly absorbing nature of lung tissue and the strong refractive effects associated with tissue-air interfaces. Until recently, XPC lung imaging had only been accomplished at synchrotron facilities. In this work, we investigate the manifestation of speckle in propagation-based XPC images of mouse lungs acquired in situ by use of a benchtop imager. The key contributions of the work are: a) the demonstration that lung speckle can be observed by use of a benchtop XPC imaging system employing a polychromatic tube-source; and b) a systematic experimental investigation of how the texture of the speckle pattern depends on the parameters of the imaging system. Our analyses consists of image texture characterization based on the statistical properties of pixel intensity distributions. Results show how image texture measures of lung regions are strongly dependent on imaging system parameters associated with XPC sensitivity.
In-line x-ray phase-contrast (XPC) tomosynthesis combines the concepts of tomosynthesis and in-line XPC imaging to utilize the advantages of both for biological imaging applications. Tomosynthesis permits reductions in acquisition times compared with conventional tomography scans while in-line XPC imaging provides high contrast and resolution in images of weakly absorbing materials. In this work, we develop an advanced iterative algorithm as an approach for dealing with the incomplete (and often noisy) data inherent to XPC tomosynthesis. We also investigate the depth resolution properties of XPC tomosynthesis and demonstrate that the z-resolution properties of XPC tomosynthesis is superior to that of conventional absorption-based tomosynthesis. More specifically, we find in-plane structures display strong boundary-enhancement while out-of-plane structures do not. This effect can facilitate the identification of in-plane structures.
X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such
as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and
tested a hard X-ray polarimeter, X-Calibur, to be used in the focal plane of the InFOCμS grazing incidence hard
X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the
polarization of 20−60 keV X-rays making use of the fact that polarized photons Compton scatter preferentially
perpendicular to the electric field orientation; in principal, a similar space-borne experiment could be operated
in the 5 − 100 keV regime. X-Calibur achieves a high detection efficiency of order unity.
Cadmium Zinc Telluride (CZT) is the detector material of choice for the detection of X-rays in the 10 keV-1MeV
energy band with excellent spatial and energy resolutions and without cryogenic cooling. In this contribution,
we report on recent results of the CZT detector development program and several astrophysical experiments
which make use of CZT detectors. In the first part of the paper, we discuss the performance of pixel and
cross-strip CZT detectors read out with an ASIC developed at the Brookhaven National Laboratory. Our pixel
detectors achieve some of the best energy resolutions reported in the literature. Cross-strip detectors are found
to give an inferior performance and we investigate the reason for this performance difference. We also present
results from a precision measurement of the effect of a steering grid on multi-pixel events obtained with a
200 micrometer collimator. In the second part of the paper, we describe the design and performance of the
hard X-ray polarimeter X-Calibur. The polarimeter uses a 14 cm long scintillator scatterer, surrounded by an
assembly of 32 2-5 mm thick CZT detectors. We discuss the sensitivity of the polarimeter to measure the linear
polarization of 10 keV-80 keV X-rays on short and long balloon flights and results from testing the polarimeter
in the laboratory.
X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such
as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and
tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCμS grazing incidence hard
X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the
polarization of 10-80 keV X-rays making use of the fact that polarized photons Compton scatter preferentially
perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity.
The hard X-ray sky now being studied by INTEGRAL and Swift and soon by NuSTAR is rich with energetic phenomena
and highly variable non-thermal phenomena on a broad range of timescales. The High Energy Telescope (HET) on the
proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission will repeatedly survey the full sky for rare and
luminous hard X-ray phenomena at unprecedented sensitivities. It will detect and localize (<20", at 5σ threshold) X-ray
sources quickly for immediate followup identification by two other onboard telescopes - the Soft X-ray imager (SXI)
and Optical/Infrared Telescope (IRT). The large array (4.5 m2) of imaging (0.6 mm pixel) CZT detectors in the HET, a
coded-aperture telescope, will provide unprecedented high sensitivity (~0.06 mCrab Full Sky in a 2 year continuous
scanning survey) in the 5 - 600 keV band. The large field of view (90° × 70°) and zenith scanning with alternating-orbital
nodding motion planned for the first 2 years of the mission will enable nearly continuous monitoring of the full
sky. A 3y followup pointed mission phase provides deep UV-Optical-IR-Soft X-ray and Hard X-ray imaging and
spectroscopy for thousands of sources discovered in the Survey. We review the HET design concept and report the
recent progress of the CZT detector development, which is underway through a series of balloon-borne wide-field hard
X-ray telescope experiments, ProtoEXIST. We carried out a successful flight of the first generation of fine pixel large
area CZT detectors (ProtoEXIST1) on Oct 9, 2009. We also summarize our future plan (ProtoEXIST2 & 3) for the
technology development needed for the HET.
ProtoEXIST1 is a pathfinder for the EXIST-HET, a coded aperture hard X-ray telescope with a 4.5 m2 CZT
detector plane a 90x70 degree field of view to be flown as the primary instrument on the EXIST mission and
is intended to monitor the full sky every 3 h in an effort to locate GRBs and other high energy transients.
ProtoEXIST1 consists of a 256 cm2 tiled CZT detector plane containing 4096 pixels composed of an 8x8 array
of individual 1.95 cm x 1.95 cm x 0.5 cm CZT detector modules each with a 8 x 8 pixilated anode configured
as a coded aperture telescope with a fully coded 10° x 10° field of view employing passive side shielding and
an active CsI anti-coincidence rear shield, recently completed its maiden flight out of Ft. Sumner, NM on the
9th of October 2009. During the duration of its 6 hour flight on-board calibration of the detector plane was
carried out utilizing a single tagged 198.8 nCi Am-241 source along with the simultaneous measurement of the
background spectrum and an observation of Cygnus X-1. Here we recount the events of the flight and report
on the detector performance in a near space environment. We also briefly discuss ProtoEXIST2: the next
stage of detector development which employs the NuSTAR ASIC enabling finer (32×32) anode pixilation. When
completed ProtoEXIST2 will consist of a 256 cm2 tiled array and be flown simultaneously with the ProtoEXIST1
telescope.
X-ray polarimetry offers a unique vantage to investigate particle acceleration from compact objects and relativistic
outflows. The HX-POL concept uses a combination of Si and Cadmium Zinc Telluride (CZT) detectors to measure
the polarization of 50 keV - 500 keV X-rays from cosmic sources through the azimuthal distribution of Compton
scattered events. HX-POL would allow us to measure the polarization degrees of Crab-like sources well below
10% for a one day balloon flight. A longer (15-30 day) flight would improve the polarization degree sensitivity
to a few percent. In this contribution, we discuss the sensitivity of a space-borne HX-POL payload, and present
new results from laboratory tests of the HX-POL Si and CZT detectors.
Cadmium Zinc Telluride (CZT) continues to progress in quality and cost as a material for the detection of
hard X-ray and gamma-ray photons with excellent spatial and energy resolutions. We are developing large-volume
(0.5×3.9×3.9 cm3) cross-strip CZT detectors with the objective to combine the excellent performance
achieved so far only with pixelated CZT detectors with a reduced number of readout channels. In this contribution,
we discuss the spectroscopic performance of large volume CZT detectors from the company Orbotech
when contacted as pixelated detectors. Subsequently, we present results obtained when the same substrates
where contacted with cross-strip contacts. Finally, we use the results from a simulation study to discuss the
optimization of the design of the strip contacts and the readout electronics.
We report on the continued development and testing of unique types of Cadmium Zinc Telluride (CZT) detectors.
Using large volume (10×20×20 mm3) CZT crystals, we contact various "dual anode" detector designs. We
incorporate a segmented cathode with five regions so that the charge on all seven contacts can be used to
determine the energy and the 3-D interaction location of detected X-ray and gamma-ray photons. We describe
the status of the detector development program, emphasize strengths and weaknesses of the different contact
configurations, and discuss possible applications of Dual Anode Detectors in radiation detection applications.
We report our progress on the development of pixellated imaging CZT detector arrays for our first-generation balloon-borne
wide-field hard X-ray (20 - 600 keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the
High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope (EXIST), a proposed implementation of
the Black Hole Finder Probe. ProtoEXIST1 consists of four independent coded-aperture telescopes with close-tiled (~0.4
mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple shielding/field-of-view configurations are
planned to identify optimal geometry for the HET in EXIST. The primary technical challenge in ProtoEXIST is the
development of large area, close-tiled modules of imaging CZT detectors (1000 cm2 for ProtoEXIST1), with all readout
and control systems for the ASIC readout vertically stacked. We describe the overall telescope configuration of
ProtoEXIST1 and review the current development status of the CZT detectors, from individual detector crystal units
(DCUs) to a full detector module (DM). We have built the first units of each component for the detector plane and have
completed a few Rev2 DCUs (2x2 cm2), which are under a series of tests. Bare DCUs (pre-crystal bonding) show high,
uniform ASIC yield (~70%) and ~30% reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU
already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial radiation tests on a Rev2 DCU shows
~ 4 keV FWHM at 60 keV (vs. 4.7 keV for Rev1). We therefore expect about ≤1% FWHM at 662 keV with the Rev2 detectors.
One of the key aspects of a detector material for space-borne hard X-ray and gamma-ray telescopes is the rate of
prompt and delayed background events generated inside the material by charged and neutral particles striking the detector. These particles are Cosmic Rays, particles trapped in Earth's magnetic field, and secondaries
from Cosmic Ray interacting with the atmosphere and the spacecraft. Here, we present a preliminary study of
Cadmium Zinc Telluride (CZT) and its behaviour in space environments. We have used the simulation package
MGGPOD to estimate the background of the CZT detectors in the proposed Energetic X-ray Imaging Survey
Telescope (EXIST) for possible orbital parameters. The EXIST mission will make use of ~6 m2 of >0.5 cm
thick CZT detectors to record cosmic X-rays in the energy range from 10 keV to 600 keV. The detectors will be
shielded by a fully or partly active shield. For the specific detector and shielding geometry considered here and
an orbit with a low (7°) inclination, the background rate is dominated by diffuse extragalactic photons below
~200 keV. Prompt and delayed hadronic backgrounds grow increasingly important above this energy, becoming
the main contributors to the total background above ~1 MeV. A fully active shield performs slightly better than
a half active/half passive shield.
We report here on the optimization of 0.5 cm thick pixelated Orbotech CZT detectors with regards to the
best contacting materials and the use of steering grids. We evaluated the performance of different contacting
materials. Our study differs from earlier ones in that we investigated the performance of different anode and
cathode materials separately. We obtain the best performance with Au cathodes. For different anode materials
Ti and In give the best energy resolutions. The detector (2.0×2.0×0.5 cm3, 8×8 pixels) shows excellent 59 keV,
122 keV and 662 keV energy resolutions of 1.4 keV, 1.9 keV, and 7.4 keV, respectively. Furthermore, we report
on using steering grids to improve on the performance of the pixelated detectors. Previously, the benefit of
steering grids had been limited by additional electronic noise associated with currents between the negatively
biased steering grids and the anode pixels. We are currently exploring the possibility to isolate the steering grid
from the CZT substrates by a thin layer of Al2O3. We performed a series of measurements to determine by
how much the isolation layer reduces the grid-pixel currents. Comparing the currents between two Au contacts
before and after isolating one of the two contacts from the CZT with a 700 nm thick layer of Al2O3, we measure
that the isolation layer reduces the currents by a factor of about 10 at 500 V. We present some results from
a detector before and after deposition of an isolated steering grid. The grid indeed improves on the detectors
energy resolution and detection efficiency. We show that simulations can be used to model the anode to cathode
charge correlation in excellent agreement with the experimental results.
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