The High Energy Gamma Ray Balloon Instrument was built in part to verify certain subsystems' performance for the Energetic Gamma Ray Experiment Telescope (EGRET) instrument, the high energy telescope to be carried on the Gamma Ray Observatory. This paper describes the instrument, the performance of some subsystems, and some relevant results

Baker, R. G.

Bertsch, D. L.

Chesney, J. R.

Derdeyn, S. M.

Ehrmann, C. H.

Fichtel, C. E.

Hunter, S. D.

Jacques, J. S.

Laubenthal, N. A.

Thompson, D. J.

English

NASA Technical Reports Server

S36
oG9.2-12
HIGH ENERGYGAMMARAY BALLOONINSTRUMENT
D.J. Thompson, R.G. Baker, D.L. Bertsch, J.R. Chesney,
S.M. Derdeyn, C.H. Ehrmann, C.E. Fichtel, SoD. Hunter,
J.S. Jacques, N.A. Laubenthal, R.W. Ross
Code 662, NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771 U.S.A
' i. Introduction. The High Energy Gamma Ray Balloon Instrument was
built in part to verify certain subsystems' performance for the
Energetic GammaRay Experiment Telescope (EGRET) instrument, the high
energy telescope to be carried on the GammaRay Observatory (I). This
paper describes the instrument, the performance of some subsystems, and
some relevant results.
2. Instrument Description. The instrument is similar in size to EGRET
and fias the basic elements of a conventional high energy y-ray
telescope. However, being a test vehicle and constrained by the weight
and cost limitations of a balloon program, it is much more austere than
EGRET. It has, for example, only 12 spark chamber modules (Figure I),
compared to the 36 on EGRET. A y-ray entering from above produces no
signal in the anticoincidence scintillator "A", but may convert to an
electron-positron pair in one of the plates interleaved with the upper
I0 spark chamber modules. The electron and positron trigger the
coincidence system consisting of scintillators "B", "C", and "D" with
the proper time of flight signature between "B" and "C". The inclusion
.of the "D" signal was optional and commandable. The coincidence signal
is used to initiate the high voltage pulse to the spark chambers and the
readout of the %-ray event. The basic information about the _-ray,
arrival direction and estimated energy, is derived from the
reconstructed picture of the electron and positron trajectories in the
spark chamber. Each of the eight pair production plates is a sheet of
0.08 radiation length lead, supported on a grid of stretched high-
strength steel wires. The spark chambers have an active area of 81 cm
by 81 cm, are of the wire grid design with magnetic core readout and are
essentially identical to those being used on the EGRETinstrument.
The anticoincidence counter
consists of three pieces of ANT,-CmNC,_E m MOOU,ESPARKCOUNTER _ / CHAMBER
scintillator which form a five "a"
sided box around the upper
spark chamber assembly. The _
placement of the anti -
coincidence system inside the _PRESSUREYESSEL
pressure vessel reduces weight "B'T-----_
and is far less costly than T,MEOFLIGHT
COINCIDENCE i 2 MODULE SPARK
the machined and polished dome sYSTEM _ CHAMBER
surrounding the entire upper ..c-L-_ °
porti on of the EGRET wD"SCINTILLATOR
instrument. The shorter,
modular design of the balloon
anticoincidence is not as Figure I -- Schematic dlagram of the
effective as the monolithic High Energy Gamma Ray Balloon Instrumentdome and it therefore
https://ntrs.nasa.gov/search.jsp?R=19850026638 2020-03-20T16:47:26+00:00Z
339
0G9.2-12
represents a more severe test of its ability to screen out unwanted
events.
The "B" and "C" parts of the triggering system each consist of an array
of 16 scintillator tiles in a 4 by 4 pattern. Each tile has its own
light pipe and phototube. This coincidence system serves two
purposes: (I) The allowed combinations of "B" and "C" tiles define the
instrument aperture. (2) These scintillator planes are operated in a
time-of-flight arrangement which requires that the particles pass
through the "B" plane before the "C" plane, i.e., they must be downward
moving. The performance of this coincidence system is discussed in
Section 3. The EGRET coincidence system uses the same 4 by 4
scintillator arrays and an almost identical time-of-flight electronics
system. The "D" detector is a single unit of plastic scintillator which
may be used to verify that at least one of the particles penetrates the
entire detector.
The active detectors are contained in a pressure vessel made of aluminum
honeycomb with Kevlar face sheets. This compositR vessel is lightweight
and low density (area density less than 0.5 g cm-_) yet capable of being
evacuated to remove contaminants. The upper portion of the vessel and
the lower spark chamber area are filled with a spark chamber gas mixture
(98.5% neon, 0.75% ethane, 0.75% argon), while the region containing the
coincidence scintillator, phototubes, and electronics is filled with
air. The active area iE approximately 6560 cm2 with an area efficiency
factor of about 1800 cmz (E> 400 MeV) The instrument size is 3 m by
1.6 m diameter. With its gondola, it weighs 1300 kg. Balloon flights
which provide the results reported here were launched from Palestine,
Texas.
3. Relevant Subsystems.
a. Spark Chamber. The individual modules have 992 wires in each plane,
giving a positional resolution of about 0.4 mm for spark location. The
modules are made entirely of low-outgassing materials with the beams
themselves being made of Macor. An extensive effort was made in
developing techniques for stacking, holding, and determining the
alignment of the spark chambers, so that the absolute pointing direction
of the assembly could be determined with high precision. The use of a
set of optical refererences allowed the absolute pointing direction to
be determined to an arcmin. The techniques which have been used in the
EGRETdevelopment are a direct successor. The performance of the spark
chambers is seen in Fig. 2, which shows the electron-positron pair
resulting from a high energy y ray interaction. The vertical scale has
been compressed in the figure by a factor of 4.7.
b. Time-of-flight coincidence system. The time'of-flight measurement
is an important discriminator against unwanted triggers. Each of the 32
tile signals from the "B" and "C" arrays is sent to a constant fraction
discriminator. The discriminator signals are summed for each array
(with propagation times matched). The time difference between the total
"B" signal and the total "C" signal is then digitized using a circuit
similar to previous experiments (2). With a 75 cm separation, the time
difference between upward-moving and downward-moving particles is 5
340
0G9.2-12
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Figure 2 - Orthogonal views of Figure 3 - Time of flight histogram
a y-ray pair production event for neutral events during one
in the spark chamber, balloon flight.
nsec. The acceptance threshold for the time difference is adjustable by
command. The discrimination, summation,digitization, and comparison is
completed in about 400 nsec and is included in the trigger signal to the
spark chamber. A histogram of the time-of-flight measurements is
accumulated in the data system and regularly telemetered to the ground
station. A sample histogram for neutral events, taken directly from the
Ground Support Equipment display, is shown in Figure 3. The peak to the
right represents downward-movingX rays and is well separated from the
upward-moving peak to the left.
c. Anticoincidence System. As noted in Section 2, the anticoincidence
scintillator on the High Energy Gamma Ray Balloon Instrument does not
surround the upper layer of the time-of-flight coincidence system, in
contrast to other instruments such as SAS-2, COS-B, and EGRET; This
short anticoincidence system was recognized as a negative design aspect,
because horizontal or even partially downward moving particles could
, interact above the upper layer of the coincidence system and produce a
downward moving particle within the instrument aperture.
An approximate model of predicted event rates was developed from the
data of SAS-2 and tested on previous balloon instruments. When applied
to the geometry and material factors of this instrument, this model gave
the predicted rates which are shown in Table 1, which also shows the
observed event rates during a balloon flight. The model did not take
into account the different characteristicsof this anticoincidence
system from all the others.
The overall agreement between the predicted and observed event rate is
an indication that the short anticoincidence system was not a major
limitation of the instrument. The differences in the types of events
seen is probably in large part the result of the anticoincidence
configuration, for which no correction was made in the model. The
fraction of events which are recognizabley rays is also high compared
341
0G9.2-12
Table I
Type of Event Predicted Events/s Observed Events/s
Gamma Ray pairs (1) 1.1 0.9
Upper Wall Events (2) 0.9 0.6
Scattered single tracks (3) 0.8 ,0.8
Other (4) 0.6 1.0
(i) Recognizable pairs. The prediction includes atmosphericy rays and
y rays produced in the outer instrument shell.
(2) Tracks originating in the walls of the upper chamber.
(3) This number is genera|ly consistent with the expected number of low
energy Compton y-rays plus pair production y-rays for which one
track is too short to meet the acceptance criteria.
(4) This category includes short single tracks, multiple single tracks,
and events with little information.
to earlier balloon instruments. This favorable rate of useful events
was predicted by the model based on the better active volume to wall
ratio and to the improved directional recognition of the time-of-flight
system. The similarity of this balloon instrument to the EGRET
instrument and the known superior aspects of EGRET strongly suggest that
EGRET will also have a high fraction of useful events.
d. Automatic Data Processing. The majority of the data for a high
energy y-ray telescope are the spark chamber pictures. A set of
programs, originally developed for the SAS-2 instrument and since
refined, analyze the event pictures by pattern recognition. These
programs efficiently screen out pictures which do not contain useful
information and identify the track structure of potential y-ray events,
such as the one shown in Fig. 2. Even though the informationcontent of
this instrument is low compared to other y-ray telescopes, the results
of the automatic analysis showed that this program works very well even
here, both in selecting desired events and structuring them correctly.
4. Summary. The High Energy Gamma Ray Balloon Instrumenthas provided
a flight test of an austere version of the EGRET telescope. The results
have proven the instrument subsystems and approaches and have assisted
in the development of assembly procedures used for EGRET.
5. Acknowledgements. We thank George Simpson and Bill Cruickshank for
their valuable assistance in the early stages of the program. Our
thanks go also to the National Scientific Balloon Facility, Palestine,
Texas, for their flight support.
6. References
1. Fichtel, C.E., et al. 1983, ICRC, Paper T1-10, p. 19.
2. Ross, R.W., and Chesney, J.R., 1979, IEEE Trans. Nuc. Sci, NS-27,
p.370.


High energy gamma ray balloon instrument

High energy gamma ray balloon instrument

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