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 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.