A Rotational Modulator (RM) gamma ray imager, consisting of a single grid of lead slats rotating above an array
of detectors with diameter equal to the slat spacing, has the capability of providing angular resolution significantly
better than the geometric resolution (i.e., the ratio of detector diameter to mask/detector separation). The
sensitivity, weight, and angular resolution are comparable to that of a coded aperture device, but with significantly
less complexity. As the grid rotates, the transmission from a source is modulated on each detector between 0 and
100%. The count profile is cross-correlated with precalculated modulation profiles to produce an approximate
source image. Deconvolution of this image with the known imager response can accurately resolve point sources
and complex emissions. The appropriate deconvolution technique can achieve angular resolution better than
the basic geometrical resolution of the instrument. A prototype RM developed at Louisiana State University
features high sensitivity and energy resolution, functional angular resolution of 15, and a simple readout system.
The detector array consists of 19 1.5 × 1 thick cerium-doped lanthanum bromide (LaBr<sub>3</sub>:Ce) crystals. LaBr3
produces significantly more light than other common scintillators, offering < 3% FWHM energy resolution at 662
keV. A grid spaced ~1.2 m from the detection plane with slat width 1.5 offers a 13.8° field of view. We present
our reconstruction technique, deconvolution algorithms, and simulated and experimental imaging results.
The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10-600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology.
Inorganic scintillators such as NaI(Tl) and CsI(Na) have been used extensively in hard x-ray and low-energy gamma-ray imaging systems. Recently, a new generation of scintillators has been developed with properties that could greatly enhance the performance of such imaging systems. In particular, the lanthanum halides show great promise with increased light yield and peak emission at shorter wavelengths compared to NaI or CsI. Since these scintillators emit at relatively short wavelengths, wavelength-shifting fibers can be used which re-emit at wavelengths around 420 nm, providing a good match to bialkali photocathode response. Multi-anode photomultiplier tubes can be used to read out individual fibers from orthogonal layers to provide x-y position information, while energy measurements can be made by large area photomultiplier tubes. Such an arrangement potentially provides improved overall position and energy resolution and lower thresholds compared to imaging systems configured as standard NaI or CsI gamma cameras. We present measurements of the energy resolution obtained from lanthanum chloride (LaCl<sub>3</sub>) and lanthanum bromide (LaBr<sub>3</sub>) scintillators viewed both perpendicular to the axis and down the length of square multi-clad wavelength-shifting fibers. These results are compared to a standard NaI detector with wavelength-shifting fibers. The implications of these results for gamma-ray imaging will then be discussed.