A coded aperture imaging system was designed for the Gamma-Ray imaging spectrometer (GRIS). The system is optimized for imaging 511 keV positron-annihilation photons. For a galactic center 511-keV source strength of 0.001 sq/s, the source location accuracy is expected to be + or - 0.2 deg

Cline, T. L.

Gehrels, N.

Huters, A. F.

Leventhal, M.

Maccallum, C. J.

Reber, J. D.

Stang, P. D.

Teegarden, B. J.

Tueller, J.

English

NASA Technical Reports Server

OG 9.2-3
3O3
A CODED APERTURE IMAGING SYSTEM
OPTIMIZED FOR HARD X-RAY AND GAMMA RAY ASTRONOMY
N. GehrelsI, T. L. ClineI, A. F. Huters3,
M. Leventhal2, C. J. MacCallum3, J. D. Reber1'4
P. D. Stang3, B. J. TeegardenI, and J. TuellerI
1. NASA/Goddard Space Flight Center, Greenbelt, MD 20771
2. AT&T Bell Laboratories,Murray Hill, NJ 07974
3. Sandia National Laboratories,Albuquerque, NM 87185
4. SUNY/Geneseo, NY 14454
ABSTRACT
A coded aperture imaging system has been designed for the Gamma-Ray
Imaging Spectrometer (GRIS). The system is optimized for imaging 511-
keV positron-annihilationphotons. For a galactic center 511-keV source
strength of 10-3 cm-2s-I, the source location accuracy is expected to be
±0.2 °.
I. Introduction Current gamma-ray spectrometers observing in the 70
keV to i0 MeV nuclear-line energy range have poor source localization
capabilities. They typically have broad fields of view (~ I0 ° FWHM)and
no imaging systems. Recent discoveries of a number of gamma-ray sources
with lines in their spectra (1,2,3) have increased interest in building
spectrometers capable of mapping the source regions. An example is the
positron annihilation line at 511 keV from the direction of the galactic
center, whose location is known only to within ±4° (4). This large error
circles several galactic-center X-ray sources in addition to the compact
radio source Sgr A near the dynamic center of the galaxy. Localizing
the line emission is currently one of the major goals in gamma-ray line
astronomy.
The most promising technique for mapping in this energy range is coded
aperture imaging using Uniformly Redundant Arrays (URAS)o The basic
idea is to place an array of blocking elments in the instrument aperture
whose pattern is chosen such that the shadows cast on the detectors by
sources at different locations in the field of view (FOV) give linearly
independent sets of detector signals (5). The signals measured during
an observation can then be uniquely deconvolved to give maps of the
sky. Several papers have been written (6-10) discussing the application
of coded aperture imaging to low-energy gamma-ray astronomy.
We are currently building a balloon-borne spectrometer to be flown in
fall 1986 called the Gamma-Ray Imaging Spectrometer (GRIS) that will
have a URA imaging system. In this paper we describe the imaging system
and present the results of laboratory and computer simulations of its
performance. A general description of the GRIS instrument is given in a
companion paper (11).
2. Description of the Imagin 9 System The Ge detectors in the GRIS
instrument are located in every other cell of a 7.5 cm lattice as shown
in Figure I. The mask is a URA with a 3x5 element unit cell. It is
located 1.5 m above the detectors and is arranged on a 7.5 cm grid. The
https://ntrs.nasa.gov/search.jsp?R=19850026629 2020-03-20T16:48:05+00:00Z
304 OG 9.2-3
opaque elements are individual Nal blocks 6.8 cm square by 7.6 cm thick,
each viewed by Its own photomultlplier tube. Calculations and simul-
ations we performed during the instrument design phase showed that it
was essential for the mask to be active in order to achieve low levels
for the instrumental line background at 511 keV and the continuum
background over the entire spectrum.
Another feature of the GRIS imaging system that is essential for high-
background observations is the ability to alternate between mask and
antimask (see, e.g., ref. 7). The antimask is the complement of the
mask/having opaque elements where the mask is open and vice versa. The
incident gamma-ray flux is therefore square-wave modulated by the alter-
nation, which is the optimum modulation for background subtraction.
Normal]y mask-antimask systems have a separate structure for both the
mask and antimask, each of which move in and out of the field of view.
By a novel technique that reduces instrument weight and size, the GRIS
mask is itself converted into its own antimask as illustrated in Figure
I. The mask divides into halves while two independent elements rotate
into the center.
Both the mask and antimask also move as units left and right one cell so
that a detector is located under each of the 15 elements in the mask
unit cell at least once. In this way the 7-detector GRIS array is made
to act like a 15-detector array in terms of its imaging capability. To
produce a map, the mask system moves through 6 positions (3 mask, 3
antimask).
The maps produced by the imaging system have 15 sky bins arranged in a
3x5 matrix. The bins are each 3° square as determined by the mask cell
size and the detector-mask separation. The region mapped is therefore
9°x15°. As will be shown below, this bin size and mapping region are a
good match for the hard X-ray source distribution in the galactic center
direction.
3. Imagin9 Sensitivity and Resolution There is a decrease in sensi-
tivity that all imaging systems suffer compared with non-imaging wide
FOV systems which occurs when a source is not in the center of a sky
bin. This is due to the fact that the source signal i_ the detectors is
not fully modulated. The effect can be as large as a factor of 2.2 in
sensitivity if the source is at the corner of four bins. To reduce this
variation in sensitivity across the map and also increase the number of
effective pixels in the final map, we will use an offset pointing tech-
nique. The basic idea is that the pointing direction of the instrument
is changed between imaging cycles so that a given source appears at
OG 9.2-3
305
different locations in the different sky maps. By changing the pointing
I° at a time in a 3x3 grid, the 3° sky bins are divided into 1° pixels.
The final map then has an approximately uniform sensitivitythat turns
out for a single point source to be ~1.4 times poorer than a wide FOV
instrument. The 3° bin size combined with the finite detector size
gives the imaging system a point-spread function FWHM of ~4°.
The localization accuracy of the centroid of a point source image
depends on the source strength and the number of sources in the field,
but is in general much better than the 4° beam width. For the GRIS mask
and detector arrangement operated in the offset pointing mode, the
accuracy to which the position of a point source of strength S (photons
cm-2s-1) can be determined is given by
0s
source location accuracy (lo) - 1.4 e-_-- (I)
where _ is the bin size of the mask (3°), and o_ is the uncertainty in
the flux measurement that would occur for a wideTOV observation. For a
galactic center 511-keV line flux of 10-3 ph cm'2s-I observed by GRIS
for 8 hours at 3.5 g cm-2 atmosphere depth from Alice Springs, Australia,
the value of o /S is better than 1/20. Hence, GRIS will be able to
localize the source position to within ±0.2°, which will be more than an
order of magnitude improvementover the present best position (4).
4. Laboratory Tests and Computer Simulations A special laboratory test
setup of the GRIS imaging system was assembled using Nal detectors in
place of the Ge array and Fe and Al mask elements in place of the Nal
elements. The detector and mask geometries were similar to those of
GRIS except that the laboratory detector-mask separation was 45 cm in-
stead of 1.5 m. This gives 9° sky bins, and was done to reduce the
effects of the nonparallel gamma-ray beam produced by the radioactive
source used in the laboratory. An 241Am source was located 10 m from
the detectors on a large, precise x-y positioning system. Figure 2
shows a map generated in the laboratory of a single on-axis source. The
9 offset pointing maps were combined into a final map with 3° pixel size
(three times GRIS). The laboratory data were taken with very high sta-
tistical significance (~.1%) and then noise was added to the detector
signals by computer. The noise level was chosen such that each un-
blocked detector signal was a 1.7o measurement. This corresponds to a
measurement of total significance_s/S = 0.06 in Equation (1).
To better understand the mapping capabilities of the GRIS imaging
system, a computer simulation of the galactic center was performed. The
computer simulation assumed a full 8-hour observation with GRIS of the
20-80 keV X-ray range from Alice Springs. Included in the galactic
center direction were the 3 strongest sources (GXI+4, GCX-1, and GX5-1),
with intensities assumed to be those measured by HEAO A4. In order to
make the simulation as realistic as possible the data were "taken" using
offset pointing, rotation of the image in the FOV, different length
mask-antimask intervals during transit, and the expected instrumental
background (12). Figure 3 shows the results, with the three sources
clearly resolved.
306 OG 9.2-3
COMPUTER SIMULATION
I,AIOlU_TOP.YSlINLA1'ION ALACTIC CENTER (20-80 keV)
27 v , , I ' ' I
18 _ -20" GXt*4
i ° "
-, ( -3o"
_35o 6
-27 I I I I I A [
-18 -9 o 9 "18 IOhO0m ITh_,0m
DEGREES RIGHT ASCENSION
, Fig. 2 Fig. 3
References
I. M. Leventhal, et al., Ap. J. 225, Lli, 1978.
2. R. C. Lamb, et_ Nature 30_J-_-_7,1983.
3. W. A. Mahoney, e--t--al.,Ap..-J_--286,578, 1984.
4. G. R. Riegler, et a--1.,Ap. J. 247, L13, 1981.
5. E. E. Fenimore_-M. Canno-K_-,Applied Optics 17, 337, 1978.
6. G. Di Cocco, et al., IEEE Trans. Nucl. Sci. NS-3_No. 1, 771, 1984.
7. M. L. McConne_Fl,et at., IEEE Trans. on Nucl.-_-ciNS-29, No. 1, 155,
1982.
8. R. Kroeger and D. Muller, 18th ICRC, Vol. T, 1, 1983.
9. J.-P. Roques and G. Debousy, 14th Inter. Symp. on Space Tech. and
Sci., Tokyo, p. 1523, 1984.
10. W. R. Cook, et al., IEEE Trans. Nuc1. Sci. NS-31, No. I, 771, 1984.
11. B. J. Teegare-den_,et at., 19th ICRC, 1985.
12. N. Gehrels, to be_shed in Nucl. Inst. Meth., 1985,


A coded aperture imaging system optimized for hard X-ray and gamma ray astronomy

A coded aperture imaging system optimized for hard X-ray and gamma ray astronomy

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