Brent S. Budden

Angular resolution obtained with a LaBr3-based rotational modulator

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 (LaBr3: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.

Lanthanum bromide-based rotational modulation gamma ray imager

A time modulation imaging device uses a periodic structure which translates or rotates above one or few position-insensitive detectors. One common design, a Rotational Modulation Collimator (RMC) uses a bi-grid collimator which rotates above a single detector, and is able to attain very good angular resolution. The two grids cost sensitivity and weight, however, making the RMC unattractive for certain applications. A Rotational Modulator (RM) consists of a single grid of transparent and opaque slats of width a, above an array of several detectors with diameter d, subject to the constraint a = d. The sensitivity, weight, and angular resolution can be comparable to that of a coded aperture device. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. This count profile is cross-correlated with pre-calculated modulation profiles to produce an initial source image. Further processing of the image with a “cleaning” technique that incorporates information from the point-spread function can accurately resolve point sources. In an RMC imager recently constructed at LSU, LaBr3:Ce detectors are used, which produce significantly more light than other common scintillators, offering ≪ 3% FWHM energy resolution at 662 keV. The instrument features high sensitivity and energy resolution, angular resolution of 0.8° (1σ), and a simple readout system. The detector array consists of 19 1:5″×1″ thick LaBr3:Ce detectors in a concentric circular layout. A grid spaced ∼1.2 m from the detection plane with slat width 1:5″ offers a field of view radius of 6:9°. We present our reconstruction technique, cleaning algorithms, and imaging results for the RM prototype.

A High Sensitivity Gamma Ray Imager (HiSGRI) Based on Wavelength-Shifting Fiber Readout of LaBr3 Scintillators

Cerium-doped lanthanum bromide offers increased light yield and peak emission at shorter wavelengths compared to standard inorganic scintillators such as NaI or CsI. As a result, lanthanum bromide scintillators can be used as the basis of large scale hard X-ray/gamma ray imaging systems. By coupling a Ce:LaBr3 scintillator with orthogonal layers of wavelength-shifting fibers viewed by multianode photomultiplier tubes to provide x-y position together with an Anger camera array of large area photomultiplier tubes to measure energy, a detector can be constructed with millimeter-scale position resolution and energy resolution only somewhat less than that obtained with bulk scintillators. We present measurements of the light yields and position resolution achievable with such a system.

Long duration balloon flight exposure of a Ce:LaBr 3 crystal

Cerium-doped lanthanum bromide is an attractive scintillator for use in hard x-ray/low energy gamma ray astronomy instruments. Its high light output and correspondingly excellent energy resolution, fast decay time, and relatively high density offer advantages, but the relatively high internal background due to the presence of the naturally occurring radioactive isotope 138La is a disadvantage. It is desirable to expose LaBr3 to a near-space environment in order to investigate the effects of space exposure on the internal background of the crystal. We have flown an unshielded 1 inch diameter cylindrical LaBr3 and an identical NaI(Tl) scintillator on the ATIC long duration balloon flight from Antarctica for a total of 16 days exposure. We present results from this test, including the background rates at float altitude and changes in the measured spectra over time. No indication is seen of any increase in rate due to activation. The results indicate that LaBr3 appears to be satisfactory as a balloon- or satellite-borne x-ray/gamma ray detector.

Design concept for a high altitude rotating modulator gamma-ray imager

A rotating modulator (RM) is capable of imaging hard x-rays and gamma rays by the temporal modulation of incident photons. It combines a single mask of equally-wide slats and slits, rotating above an array of detectors with diameter equal to the slats. Since the RM works in the temporal domain, appropriate analysis of the measured data enables super-resolution (resolution better than the geometric resolution), which consequently allows for larger detector elements, better efficiency at higher energies, and a simplified detector design. We describe the RM concept and discuss briefly a novel image reconstruction technique to achieve super-resolution and suppress fluctuations arising from noise. We present experimental results from a laboratory prototype and the concept and expected results for a high-altitude balloon flight of an RM. The High-Altitude Rotating Modulator for Energetic Radiation Imaging (HARMEnI) is a prototype balloon-borne instrument that offers a 20° field-of-view and 1.9° intrinsic resolution with a sensitivity in the range of 30–700 keV. At its intrinsic resolution, HARMEnI will obtain a 20σ observation of the Crab Nebula and Cygnus X-1 in approximately one hour. For a 1-day flight from Ft. Sumner, New Mexico, exposure times for these sources will exceed 3 hours, enabling enhancement of image resolution.