An advanced gamma-ray spectrometer that is currently in development is described. It will obtain a sensitivity of  0.0001 ph/sq cm./sec in a 6 hour balloon observation and uses innovative techniques for background reduction and source imaging

Anderson, K. A.

Durouchoux, P.

Hurley, K. C.

Lin, R. P.

Matteson, J. L.

Pehl, R. H.

Pelling, M. R.

Peterson, L. E.

Sniel, M.

Vedrenne, G.

English

NASA Technical Reports Server

3 2 6 oGg. 2-9
ADVANCED TECHNIQUES FOR HIGH RESOLUTION SPECTROSCOPIC
OBSERVATIONS OF COSMIC GAMMA-RAY SOURCES
J.L. Matteson, M.R. Pelling and L.E. Peterson
Center for Astrophysics and Space Sciences
University of California, San Diego
LaJolla, CA 92093 USA
R.P. Lin and K.A. Anderson
Space Sciences Laboratory,
University of California, Berkeley
Berkeley, CA 94720 USA
R.H. Pehl
Lawrence Berkeley Laboratory
University of California, Berkeley
Berkeley, CA 94720 USA
K.C. Hurley, G. Vedrenne and M. Niel
Centre d'Etude Spatiale des Rayonnements
31029 Toulouse, Cedex, France
Ph. Durouchoux
Service d'Astrophysique
Centre d'Etudes Nucleaires de Saclay
91190 Gif-Sur-Yvette Cedex, France
ABSTP_ACT
We describe an advanced gamma-ray spectrometer that is
currently in development. It will obtain a sensitivity of <
lO-4ph/cm2-sec in a 6 hour balloon observation and uses
innovative techniques for background reduction and source
imaging.
!. Introduction. Over the past decade gamma-ray spectrometers have
achieved sensitivities of 10 -4 to lO-3ph/cm2-sec to steady sources. This
has lead to the discovery of gamma-ray line emission and narrow band
continuum structure in the spectra of a wide variety of objects and
phenomena [1], e.g. the galactic center, X-ray pulsators, the Crab pulsar,
gamma-ray bursts and transients, and solar flares, the interstellar medium
[2] and SS433 [3]. lh spite of the impressive observational progress, it
is generally true that line emission has been detected from only the
brightest sources. The discovery of fainter sources and detailed study of
the brighter ones requires factors of i0 to 100 sensitivity gain [4]. This
is only practical with much lower background instruments than are
available today. The goal of the collaborative program described here is
to develop an instrument that has much lower background per unit area and
is an order of magnitude more sensitive per unit observing time than
present instruments. This will be used for observations of isolated
sources and complex source fields from balloons at a sensitivity of •
lO-4ph/cm2-sec, and its techniques will be applicable to future
instruments carried in space, where • lO-Sph/cm2-sec could be achieved.
https://ntrs.nasa.gov/search.jsp?R=19850026635 2020-03-20T16:47:14+00:00Z
3 2 7 OG9.2-9
Instrumentation. The instrument, shown in Figure i, contains an aray of
twelve 5.5 x 5.5 cm, coaxial, n-type Ge detectors. These have an energy
resolution of 1.4 (<0.6) keY at 511 (<i00) keY, a total area of ~ 300 cm2
and operate from i0 keV to i0 MeV. They are contained in a single cryostat
that has very high thermal efficiency, allowing 80 hours of operation with
a i0 liter LN 2 Dewar. This is surrounded by anticoincidence shield made of
5 cm thick BGO at the sides and rear and i0 cm thick CsI at the front.
Apertures in the latter define a 19 ° FWHM field of view. The instrument
has 3 operating modes, or configurations, which result from the use of
additional collimation components. (a) The "Source Mode" uses a 3° FWHM
passive collimator which is optimized for observations of discrete sources
at known positions. (b) The "Image Mode" uses an optimum coded modulator
to observe complex source regions by imaging a 12 ° FWHM field with 2.5"
angular resolution (c) In the "Diffuse Mode" the 19 ° FWHM aperture is left
clear in order to maximize the sensitivity to diffuse sources. The
instrument mass is 450 kg.
The detectors use dual electrical segments and pulse shape
discrimination to distinguished between Compton scattered gamma-rays,
which are multiple site interactions, and _-decays which are essentially
single site [5]. The latter are the dominant background component from a
few hundred keV to • 1 MeV in heavily shielded Ge instruments [6,7,8] and
represent a fundamental obstacle to sensitivity improvements. Their
elimination is a major goal of this program. Gamma-rays _ 200 keY are
mostly detected in the 1.5 cm thick front segment. Higher energy gamma-
rays are mostly detected as a coincidence between the two segments or a
multiple site event in the rear, 4.0 cm thick, segment. The latter is
possible because the detector's finite charge drift velocity results in a
sharp (broad) charge pulse for single (multiple) site events which is
sensed by a pulse shape discriminator. This rejects > 95 percent of the
•single site events while accepting >80 percent of the multiple site
events.
Imaging is achieved through the use of the optimum coded modulator.
This produces temporal aperture modulation that is mathematically
equivalent to the better known spatial aperture modulations [9]. It
consists of 63 opaque and transparent moving elements which are located ~
200 cm above the detectors and modulate the flux from gamma-ray sources.
The resulting moving shadowgram produces temporally modulated counting
rates in the detectors, from which a 2-dimensional, sidelobe-free gamma-
ray image is deconvolved. The modulator elements, made of BGO with PIN
photodiodes, operate in anticoincidence with the detectors.
The anticoincidenCe shield is primarily made of BGO, which offers
greater attenuation per unit shield mass than CsI or NaI and also results
in a more compact shield due to its high density, 2 times that of NaI.
However, the low light output of BGO necessitates good PMT light
collection in order to achieve a low anticoincidence threshold, < I00 keV.
Therefore the shield will use ~ 50 separate BGO elements, based on a
nominal 5 x 7 x 21 cm size, each with its own PMT.
3. Backqround and Sensitivity. The background is shown in Figure 2 and the
sensitivity for a 6 hour balloon observation is shown in Figure 3. At 1 MeV
the background is 0.14 and the detector volume is 11.5 times that of the
328 o(;9.2-9
OPTIMUM COOED
-- (Imoge Mode)
MOOULATOR
ELEMENT LATOR ELEMENT
MOTION
X-RAY
SHAOOWGRAM 1 1
SHADOWGRAM
MOTIOH
240cm
3' PASSWE COLLIMATOR
Mode)
SHtELO APERTURE
(]9 ° FWHMI
Cs[ FRONT SHIELD
SEGMENTEOGe OETECTOR
6om diD. x 5cm, 12 TOTAL
DEWAR BGO SIDE SHIELD
5crn 8GO REAR SHIELOPREAMP 8=H.V. FILTER
COLO FINGER 3' PHOTOMULTIPLtER
0 5rOom
Figure 1. The high reduction gamma-ray spectrometer contains an
array of 12 coaxial Ge detectors surrounded by a BGO and CsI
anticoincidence shield. A cross section is shown of the optimum
coded modulator, which is located 200 cm above the detector and used
for imaging observations.
most sensitive contemporary instrument [10,11] resulting in a sensitivity
improvement per unit observing time that is a factor of 9 (- 11.5/0.14).
The detectors' superior energy resolution results in an additional
sensitivity gain of a factor of ~ 1.5 below ~ 500 keY.
4. Program Status and Plans. The detector, shield and modulator concepts
have been proven in extensive laboratory tests. In the fall of 1985 a test
balloon flight of a single, heavily shielded detector will be Performed.
The 8 cm thick CsI shield will reduce the gamma-ray leakage to a point
where rejection of detector radioactivity can be clearly seen. It is
planned to conduct the first scientific observations with the complete
instrument in the fall of 1987.
5. Acknowledgement This work is supported at the University of California
by NASA Grant, NAGW-449. *
3 2 9 0c9.2-9
JO"_
SOURCE MODE BACKGROUND
IO-S '1 ' ' , .... I ' ' ' _ .... I ' ' ' _ ....
L-LEAKAGE
_ R- RADIOACTIVITY _ /
' T T- TOTAL
, TE
"-, lO'e T
o9I-.- xZ :D
,o
ll.I _ • I •
,,'-3, \ I
o FRONTfREAR  O
o 10-7 SEGMENT_ _ +
FRONT-REAR _ IMAGEMODE
COINCIDENCE _ SOURCEMODE _ _(
...... DIFFUSEMODE0 1]
I0 I0" , , , i .... I , , , , .... I , , ,t,,,,,, i0= I0 3 I0 '_
I0' tO= tO_ I0" ENERG'f _keV)
ENERGY (keV)
Figure 2. The Source Mode Figure 3. The sensitivity to
background components are shown narrow lines is shown for a 6 hour
for the different gamma-ray balloon observation. The increase
acceptance modes of the detectors, below ~ 50 key is primarily due to
The 3° collimator results in a decreasing atmospheric
small aperture flux which is ~ 20 transmission. Differences among
times greater below 300 keV in the the observing modes are due to
Image Mode. A 98 percent rejection different backgrounds and source
of detector radioactivity at 1 MeV _odulation efficiencies. Carried
is assumed. Such a reduction would in space with a i0 cm BGO shield,
allow a lO times further reduction this instrument could achieve an
in the total background of future Image Mode sensitivity of
instruments through the use of a lO-Sph/cm2-sec over most of its
thicker anticoincidence shield, energy range in a 7-day
Re ferences observation.
I. Matteson, J.L., (1983), Ad___vv.S_pace Res., 3, 135.
2. Mahoney, W.A., et al., (1984)AD.J. 286, 578.
3. Lamb, R.C., e% al.. (1983), Nature 305, 37.
4. Matteson, J.L., (1981), in X-Rav Astronomy in the 1980's, Holt, S.S.,
ed., NASATM 83848, p. 549.
5. Roth, J., et al., (1984), IEEE Transactions on Nuclear Science, NS-31,
367.
6. Matteson, J.L., et al., (1982), UCSD proposal-9006, _ Resolution
Gamma-Ray Spectroscopy of Cosmic Sources.
7. Gehrels, N., et al., (1984), IEEE Transactions on Nuclear Science, NS-
31, 307.
8. Gehrels, N., (1985), NASA TM 86162.
9. Proctor, R.J., (1981), in x-Ray Astronomy in the 1980's, Holt, S.S.,
ed., NASA TM 83848, p. 335.
10. Leventhal, M., et al., (1977), A__.J., 216, 491.
II. Leventhal, M., et al., (1977), AD._J. (Letters, 22___5,LII.


Advanced techniques for high resolution spectroscopic observations of cosmic gamma-ray sources

Advanced techniques for high resolution spectroscopic observations of cosmic gamma-ray sources

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