A 24 detector extensive air shower array is being operated at Ootacamund (2300 m altitude, 11.4 deg N latitude) in southern India for a study of arrival directions of showers of energies greater than 5 x 10 to the 13th power eV. Various configurations of the array of detectors have been used to estimate the accuracy in determination of arrival angle of showers with such an array. These studies show that it is possible to achieve an angular resolution of better than 2 deg with the Ooty array for search for point sources of Cosmic gamma rays at energies above 5 x 10 to the 13th power eV

Apte, A. R.

Gopalakrishnan, N. V.

Tonwar, S. C.

Uma, V.

English

NASA Technical Reports Server

4 6 g 03 9.5-8
ANGULAR RESOLUTION OF AN EAS ARRAY FOR
GA_ RAY ASTRONOMY AT ENERGIES>5 x 1013 eV
Apte, A.R., Gopalakrishnan,N.V., Tonwar, S.C., and Uma, V.
TQta Institute of Fundamental Research
Homi Bhabha Marg, Colaba, Bombay-400 005
India
ABSTRACT
A 24 detector extensive air shower array is being operated
at Ootacamund (2300 m altitude, II.4°N latitude) in southern India for,a
study of arrival directions of showers of energies greater than 5 x 10'_
eV. Various configurationsof the array of detectors have been used to
estimate the accuracy in determination of arrival angle of showers with
such an array. These studies show that it is possible to achieve an
angular resolution of better than 2° with the Ooty array for search for
point sources of Cosmic gamma rays at energies above 5 x 1013 eV.
1. INTRODUCTION
Interstellarmagnetic field causes charged particle
trajectories to be tortuous and uncertain for all but the most energetic
particles. Therefore information on the highest energy processes
occurring in astrophysics can be obtained only from studies of high
energy gamma ray photons which travel unscattered and practically unabso
-rbed through great lengths of galact_R and intergalactic space. Observa
-tions at energies less than about 10TM eV have been carried out with
satellite borne instruments which can discriminate efficiently between
photons from a source and the isotropic charged particle background.
However, studies of photons of energies above 1011 eV require very large
effective collection areas which are possible with only ground based
techniques. Atmospher_ Cerenkov radiation technique has been successful
for energies above 10l/ eV and studies of extensive air showers have
been carried out to detect sources of very high energy photons of energi
-es above 1014 eV. Since most of the Cerenkov photons are produced in the
atmosphere near the cascade maximum and suffer negligible scattering
before arriving at the detectors placed at sea level or mountain altitude
relative timing techniques using detectors spaced few tens of meters
apart enable the determination of arrival direction of individual showers
to an accuracy1 of better than about 0.5 e. Such good angular resolution
of an atmospheric Cerenkov radiation (ACR) telescope allows observation
of a small angular region around the suspected source of high energy
photons enabling achievement of a large value For the signal to backgrou
-nd (S/B) ratio. Studies of extensive air showers (EAS), on the other
_ hand, involve detection of electrons which suffer considerable scattering
in the atmosphere. It is therefore expected that angular resolution of an
EAS telescope would be much poorer compared to an ACR telescope. However
no detailed and systematic studies exist on the posslble angular
https://ntrs.nasa.gov/search.jsp?R=19850026672 2020-03-20T16:46:25+00:00Z
4 7 0 C_ 9.5-8
resolution that can be achieved with an air shower array at various
energies abo_e 1014 eV. In Fact, various angular size bins have been
used in different experiments2,3,4 based on approximate estimates of
angular resolution of respective arrays, which leads to widely varying
values of the S/B ratio causing uncertainitiesin the estimates of flux
of high energy photons from a suspected source.
Since June '84, we have operated an EAS array at Ootacamund
(Ooty For short_ 2200 m altitude, 11.4 ° N latitude) in southern India
For study of sources of high energy Cosmic ray photons. Wlth data from
this experiment we have studied in various ways the time resolution of
air shower detectors and the angular resolution achievable with this
array. In the next section are presented the details of the array,
shower selection and recording system. The observed time resolution of
EAS detectors is discussed in section 3 and in section 4 are presented
the results on angular resolution achieved with Ooty array for showers
of energies)5 x 10"o eV. The conclusions drawn from _his study are
given in the last section.
2. EXPERIMENTALSYSTEM
The EAS array at Ooty consists of 24 scintillation detect
-ors of various sizes spread over an area of radius of about 40 meters.
20 detectors are plastic scintillators, each 5 an thick and the other
four called N,E,W, and S,are liquid scintillators using 10 cm thick
column of mineral oil. These four scintillation detectors are only used
for selection of showers through a 4-fold coincidence called NEWS.
These four detectors are located at the corners of an approximate
square around the centre of side length of about 10 m ( see figure 1 of
paper 03 2.6-8 ). All the scintillators are viewed by 5 cm diameter fast
photomultipliers (RCA 8575) placed some distance above the scintillator
giving about 10% uniformity in signal over the area of the scintillator.
Showers are first selected with a four fold coincidence of 100 ns wide
pulses obtained from discriminators (LRS 623B) for N,E,W, andS. The
thresholds for the discriminators have been set at 30mV while the mean
pulse amplitude for near vertical muans is about 100 mV for these
detectors. However, this selection with an observed shower rate of
about 30 per minute selects many very small size showers which do not
trigger many of the other 20 detectors. Since the angular resolution is
expected to depend on the number of detectors available for determinat-
-ion of shower arrival direction, showers which trigger only Few
detectors are not considered to be useful far studies of Cosmic gamma
ray sources. In an attempt to optimise the selection of showers which
trigger a large number of detectors but at the same time keeping the
shower size threshold lower, an additional requirement of at least3
particles in one of the four selection detectors is included in the
Final shower trigger. Observed shower rate with this final trigger is
about 7 per minute. Each such trigger causes the digitization of the
charge collected at the anode for each photomultlpller using fast ADC's
(LRS 2248A) and the digitization of the relative delay between the
trigger and the output from discriminators for each detector with an
accuracy of 0.25 ns by using fast TDC's (LRS 2228A). Pulses from the
photomultipllers going to the discriminators are amplified with gain 10
(LRS 612A) to keep the effective threshold low while obtaining a good
4 7 1 OG 9.5-8
charge measurement for single particles with the ADC's. For each shower
information from all the ADCand TDC channels is transferred to a memory
buffer preceding a magnetic tape recording system such that the system
can record all the details of showers coming as close as 1.5 ms relative
to each other. Real time information is read for each shower from a
clock running on a crystal with stability of about I part in 107per day
which is corrected periodically with ATA time signals broadcast from
New Delhi. Data collected on magtape at Ooty are then analysed at Bombay
using the CYBER 170/730 computer system at the Tara Institute.
3. TIME RESOLUTIONOF EAS DETECTORS
The inherent time resolution of particle detection system
used in the present experiment is contributed by the rise time and time
jitter in the photomultiplier, amplifier, discriminator and the TOC
apart from the scintillator and photon reflections inthe hood on which
the photomultiplier is mounted. This resolution has been measured
experimentally by mounting two photomultipliers on the same scintillator.
The relative time measured between these two channels for showers has a
distribution with standard deviation (_) of about 1 ns which seems to
depend only weakly en the particle density over the detector. However
the distribution of the relative time between two detectors ( N and 6)
which are located within 2 m of each other is much broader reflecting
the thickness of the shower disk. It is this distribution which is
relevant for estimating the accuracy in the determination of arrival
angle of showers. The standard deviation for this _istribution is 5.2 ns
for particle densities of less than 5 particles m-_ and 3.8 ns for
larger densities. Correcting for the angular distribution of showers
and the distance between these two detectors, it is clear that the
minimum uncertainity in time measurement by individual detectors, at
least for low size showers is about 3 ns. Therefore an EAS array with
. largest distance span of abou_ 20 m between timing detectors, for
example the EAS array at Kiel _, can hardly achieve an angular resolution
better than about 3° for small size showers.
4. ANGULARRESOLUTIONOFOOTY EAS ARRAY
Sine timing information from 24 detectors is available for
getting a best fit to a plane shower front passing through all these
detectors, it is possible to reduce the effect of timing fluctuations
discussed above and achieve a better abgular resolution. In an attempt
to estimate ti'le ongular resol6tion of the array, data have been analysed
using different sets of detectors and the values of right ascension and
declination are compared. In the first instance, the array is split into
two arrays, one with odd numbered,detectors and the other with even
numbered detectors. RA and _ are computed independently with the two
arrays. The distribution of the differences between the two RAvalues
as well as the distribution of the differences between the two'values,
show a broad symmetric distribution with O" of about 3°. This suggests
a value for g" of about 2° for each sub-array. Another study has been
made by comparing the RA and _ values for two arrays, one with all the
20 detectors ( excluding the timing selection and reference detectors
N,E,W, and S) and the other with only inner 9 detectors. The value of
for the distribution of the difference in RAvalues as well as the
distribution of the difference in _ values is about 2°. This result
suggests that the angular resolution for the array using all the 20
472 OG 9.5-8
detectors is much better, assuming that the array with only inner 9
detectors has a resolution of about 2°.
These results have been obtained using only those showers
which have triggered all the 24 detectors, evidently a sample of some
what larger size showers. However similar results have been obtained
using showers which have triggered only inner 15 detectors. It should
also be noted that showers selected in the manner discussed earlier
have on the average 20 out of 24 detectors triggered and nearly 80%
of the showers have timing information available from at least 15 out
of the 20 detectors, ignoring the selection detectors. The energy
threshold for showers triggering at least 15 out of 20 detectors is
estimated to be about 5 x 1013 eV.
4. CONCLUSIONS
Ooty air shower array with 24 scintillation detectors, has
been used to measure the arrival angles af showers using the relative
timing technique. From the computed differences in the values of the
right ascension and the declination for showers for various configura
-tions of detectors used for calculation of arrival directions, it is
estimated that the angular resolution of Ooty array is better than
about 2° for showers of energies greater than about 5 x 1013 eV. Results
from the search for sources of very high energy gammarays using data
collected with the Ooty array since June '84 is reported elsewhere
(03 2.6-8) in this Conference using the values of angular resolution
obtained here.
ACKNOWLEDGE_AENTS
It is a pleasure to thank Prof. B.V. Sreekantan for his
encouragement and fruitful discussions.
References:
1. Gupta, S.K. etal., preprint (1785).
2. Samorski, M. and Stamm, W., Astrophys. J,(Letters) 268, L17(1783)
3. Lloyd-Evans, J., et al.,Nature 305, 784 (1983)
4. Morello, M.et al.,in18th Intern-_ional Cosmic Ray Conference,
Conference Papers, Bangalore, 1983, Vgl.1, p.127


Angular Resolution of an EAS Array for Gamma Ray Astronomy at Energies Greater Than 5 x 10 (13) Ev

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