A reliable, real-time supernova monitoring system was devised using the IMB neutrino detector to serve as an 'early-warning' system for EGRET and other instruments on GRO. New methods and software were developed to allow the IMB monitoring computer in Cleveland to: recognize that a trigger burst had occurred; make a judgement on whether the burst was spurrious or an actual supernova; prepare brief summary files and 'quick-look' data so that a final disposition could be made by a trained scientist; and contact the 'watch' scientist via personal beeper in Baton Rouge. This system ran from Dec. 1990 to Apr. 1991, when the neutrino detector failed for unrelated reasons. In addition to the supernova system, high-energy neutrino data was prepared and formatted for comparison with EGRET gamma-ray data

Svoboda, Robert C.

English

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NASA-CR-194367
,_ OL./I z_I CLflE? _-/ C( -7 // _ _7
A Program of Correlated Observations Using the EGRET Instrument
on GRO and the IMB Neutrino Detector
NAG 5-1520
Final Technical Report
/
/_,?,,
Louisiana State University
(NASA-CR-194367) A PROGRAM OF
CORRELATED OBSERVATIONS USING THE
EGRET INSTRUMENT ON GRO AND THE IMB
NEUTRINO DETECTOR Final Technical
Report (louisiana State Univ.)
9 p
N94-I3856
Unclas
G3/72 0185610
i
https://ntrs.nasa.gov/search.jsp?R=19940009383 2020-06-16T21:29:39+00:00Z
Abstract
A reliable, real-time supernova monitoring system was devised using the IMB neu-
trino detector to serve as an "early-warning" system for EGRET and other instru-
ments on GRO. New methods and software were developed to allow the IMB moni-
toring computer in Cleveland to: 1) recognize that a trigger burst had occurred, 2)
make a judgement on whether the burst was spurrious or an actual supernova, 3)
prepare brief summary files and "quick-look" data so that a final disposition could be
made by a trained scientist, and 4) contact the "watch" scientist via personal beeper
in Baton Rouge. This system ran from December 1990 to April 1991, when the
neutrino detector failed for unrelated reasons. In addition to the supernova system,
high-energy neutrino data was prepared and formatted for comparison with EGRET
gamma-ray data.
2
I. Supernova Watch
The Problems
Neutrinos can provide an "early-warning" system for light telescopes because the
explosion takes several hours to move through the mantle of the exploding star, while
the neutrinos exit the core and pass through the star with only a few seconds delay
due to trapping. Thus in December 1990, work began under the auspices of this
proposal to make the IMB detector in Cleveland capable of responding to a detected
neutrino burst on a time scale of less than one hour. This required solving several
problems:
1. the detector normally operates unattended. Thus there is normally no operator
to respond to a detected neutrino burst. All procedures for handling a burst
would have to be pre-programmed into a computer.
.
data reduction to find the neutrino bust from SN1987a took about two weeks
(for both IMB and Kamiokande). The success of this project required reducing
this analysis cycle to less than 1 hour.
3. There must be virtually no "false alarms", which could lead to uneccessary
spacecraft manuevers and/or wasted personnel effort.
. A detected neutrino burst must be validated at a central site with access to
IAU telegrams, network connections, and people experienced in the detector
operation.
The Solutions - Cleveland
At the IMB mine site, major sections of the real-time software for the VAX3200
monitoring computer was re-written to monitor the incoming data stream (20 Kbytes/sec)
for coincidences in the arrival times of low-energy events. Virtually all of this data is
due to the cosmic-ray muon flux which manages to penetrate the 600m of rock over-
burden to reach the detector (2.7 per second). Background neutrinos from cosmic
ray air showers occur about twice a day, and in the energy range of supernova (SN)
neutrinos, only once every three days. It is finding these neutrino "needles" in the
cosmic ray muon "haystack" that consumes many days of computer processing and
manual scanning.
In order to reduce the analysis time, it was decided early on to not try and sep-
arate the neutrinos from the muons in the monitoring process, but rather to make
use of the time structure and energy range information gained from SN1987a. SN
neutrinos should arrive in bursts lasting on the order of ten seconds and fire less
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Figure 1: Distribution of the number of firing PMT's in the events in the raw IMB
data stream. The hatched region indicates the region containing all the SN1987a
neutrino events.
than 70 photomultiplier tubes (PMT's) of the 2048 availablein the IMB detector.
On the other hand, figurei shows the distributionof the number of firingPMT's
in an event for a typicaldata tape. The hatched region indicatesthe region of SN
ncutrlno energy. It can be seen that much of the muon data can bc excluded by only
looking at low PMT events.
The rate of such low energy events isabout 0.2 events/second. Thus the proba-
bility(P) of getting n such events in a ten-second window iseasilycalculated using
Poisson statistics.Unfortunately,a_tcrmuch late-nighttrialsand tribulations,itwas
discovered that the data stream of eventswith lessthan 30 PMT's was non-random
and so Poisson statisticsdid not work. The non-random correlationsbetween events
was determined to be due to two sources: I) cross-talkbetween some PMT chan-
nelsthrough theircommon high-voltagesources,and 2) low-levelarcing in the PMT
dynodc for some tubes. Figure 2 shows the distributionof low-energy events in the
raw data. A lower cut made at 30 PMT's elimlnates the non-Poisson "bump" and
4
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O
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z
4O
3O
2O
10
9
0 20 40 60 80 100
Number of Firing PMTs
Figure 2: Distribution of the number of firing PMT's in the events in the raw IMB
data stream. The bumb is due to non-random correlations. The hatched region was
selected as the target region.
still keeps almost all neutrino events expected from a galactic SN. Figure 3 shows
the number of events into equal time bins. The histogram is data and the X's are the
expected values based on the Poisson formula. The agreement was now quite satis-
factory to allow the calculation of the operator alarm rate (from random alignments
in time) versus the threshold number of events in a time bin required to generate an
alarm. The results of these calculations are shown in Table 1.
Thus a threshold of 9 was selected in order to give the lowest possible threshold
without an undue number of operator alarms. Figure 4 shows the results of calcu-
lations done by B..MiUer (the graduate student supported by this proposal) for the
SN distance versus the number of actual neutrino interactions expected. As can be
seen, the monitor setpoint allows sensitivity out to 50 kpc, effectively encompassing
the whole galaxy.
5
2OOO
1500
0
-_ i000
_L
500
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0 I .'_'. I - I .
---__. 0 2 4 6 8 10
Number of Events w/in 2v
Figure 3: Distribution of the number of in a time bin for the target region events in
the raw IMB data stream. The solid lines are the data and the X's are the predictions
from Poisson statistics.
Table 1: Operator Alarm Rate versus Threshold Number of PMT's
number of PMT's
6
7
8
9
I0
probability per 10 sec. bin
5.9 x 10 -4
8.3 x 10 -5
1.0 x 10 -5
1.1 x 10 -s
1.1 x 10 -_
alarms/year
1862
262
32
3.4
0.35
q
0_
q)
>
:'%
10 4
103
i01
• Fast
Real-Time
Buffer Capacity
Monitor Setpoint
I ! ! " ! '
i0 20 - 30 40 50
Distance to SN (kpc)
Figure 4: Distance of a supernova versus number of detected neutrino interactions
in the IMB detector. The monitor setpoint is low enough to extend the range out to
50 kpc.
In determining the time standard to use, it was found that neither the internal
clock of the computer or the WWVB system achieved the reliability required to keep
the number of Operator Alarms to a minimum. The most reliable "clock" was found
to be the high-energy cosmic ray muon flux itself. Using this method 27 total events
corresponded to 10 + 2 seconds in real time. Thus at the mine, the computer would
monitor the data stream in the targeted region to look for 9-fold or higher coinci-
dences of low energy events out of 27 total events.
When a coincidence was detected, the computer would take steps to alert a local
operator (if present) or a remote site (LSU). The detected "burst" would then be
subjected to the standard event validation algorithms to determine whether a true
neutrino burst had occured, or just a random alignment of cosmic-ray muon events
or other problems (such as an arcing PMT).
The Solutions - Baton Rouge
Most of the operators in Cleveland were not scientists and were not trained in per-
forming neutrino event validation. It was decided early on that a Operator Alarms
would be validated independently by two member scientists of the IMB team before
an actual Neutrino Burst was declared. Since this all has to take place in less than
an hour, it was necessary to have the real-time monitor in Cleveland prepare "quick-
look" data files and to contact the lead project cience team at LSU in Baton Rouge,
Louisiana. This was done by running the raw data stream through a software FIFO
buffer such that the events that caused the alarm would still be in memory when
the alarm was generated. In practice, the buffer was made several minutes long so
that not only the alarm-generating events could be written to a special file, but also
a sample of data before and after the alarm. This proved very useful in catching
electronics anomalies and other "glitches".
Software was written for the real-time computer to call out on a modem to a
commercial nationwide "beeper" company. Physicists in Baton Rouge manned the
"SN Alert Beeper" 24 hours a day, each taking a one-week shift. Modems and com-
puter terminals were installed at the residence of the Watch Scientists so that they
would be able to respond to alarms while not in the vicinity of the university. If the
beeper went of, the scientist would call out and connect up with the mine modem.
Upon connection, custom software would given a quick summary of what caused
the alarm, what time it occured, how many events were involved, etc. The custom
software would also send the raw event data over the modem line to Baton Rouge
where event validation could be done.
In order to test the system, custom software was written to allow the on-site
operators to enter false Operator Alarms once a week on a random basis. This not
only ensured that the hardware and software was working, but allowed for realistic
measurementsof the response time (since Watch Scientists would not know the time
of the weekly test).
This system was run successfully for several months, and in practice worked quite
well. Response times were typically 15-45 minutes (depending on where the Watch
Scientist was when the beeper went off!). No "false alarms" were generated to the
GRO team (though there were several Operator Alarms due to electronics problems).
A request was then made for connection of the IMB detector directly into SPAN to
cut out the 5-10 minute transfer time associated with moving the data through tele-
phone lines. This request was approved, but installtion was not carried through due
to a catastrophic water leak which permanently damaged the neutrino detector in
April, 1991. Nevertheless, it was shown that a reliable, fast neutrino watch system
could be achieved at a rather low cost. These results were presented to other neu-
trino groups at several conferences (listed below).
II. High Energy Neutrino/Gamma-Ray Correlations
In addition to establishing a real-time supernova watch, the proposal called for
the correlation the IMB high-energy neutrino data (1 GeV and above) with the
EGRET gamma-ray data. The goal was to investigate "bumps" in the relatively
smooth distribution of "albedo" neutrinos detected by IMB during its ten years of
operation (presented at the International Conf. on High Energy Physics, Singapore
in 1990). This work is still in progess and will be published upon its completion.
III. Talks Given
Talks on the supernova watch and high-energy neutrinos were given at:
1. The 22nd International Cosmic Ray Conference, HE 5.4, 11-23 August, 1991,
Dublin, Ireland (by R.Svoboda).
2. First EGRET/OSSE Workshop, 5-6 February, 1991, Greenbelt, Maryland (by
R.Svoboda).
3. Weekly Astrophysics Colloquium, Goddard Space Flight Center, 6 August,
1991, (by R.Svoboda).
4. IMB Collaboration Meeting, 9 March, 1991, Univ. of Hawaii (by R.Miller).


A program of correlated observations using the EGRET instrument on GRO and the IMB neutrino detector

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