A crucial charactristic of air shower arrays in the field of high energy gamma-ray astronomy is their angular resolving power, the arrival directions being obtained by the time of flight measurements. A small air shower array-telescope is used to study the resolution in the definition of the shower front as a function of the shower size

Morello, C.

Navarra, G.

Periale, L.

Vallania, P.

English

NASA Technical Reports Server

OG 9.5-14
489
THE ANGULAR RESOLUTION OF AIR SHOWER GAMMA RAY TELESCOPES
(_)
C.Morello, G.Navarra , L.Periale and P.Vallania
Istituto di Comogeofisica del CNR and
(*) Istituto di Fisica Generale dell'Universit& di Torino, Italia
A crucial characteristic of air shower arrays in the field of high
energy gamma-ray Astronomy is their angular resolving power, the arri-
val directions being obtained by the time of flight measurements. An
approach to the optimization of "air shower array-telescopes" has been
recently discussed by J.Linsley (i). By using the small installation
operating at the Plateau Ros& station (3500 m a.s.l.) we have studied
the resolution in the definition of the shower front as a function of
the shower size.
The apparatus (2) consists of four liquid scintillator detectors posi-
tioned at the corners of a rhombus ( 7 x i% m diagonals, 8 m side). The
detectors dimensions are i00 x i00 x 30 cm , each of them is viewed by
two photomultipliers placed at two opposite corners.
The traversal of a particle is defined by the coincidence of the two
photomultipliers and its arrival time by the mean of the two detection
times ( the discrimination is done in leading edge mode, at a 0.25
particle Level).An EAS is defined by a four-fold coincidence.
The timing resolution is obtained by measuring AT = T1 - T2 (gT = 50
ps step ), where T1 and T2 are the time of flights between the detec-
tors placed at the vertex of two parallel sides.
Different shower sizes are selected by _easuring the particle density
on a central plastic scintillator (0.5 m area, 15 cm thick), see Fig.
I. For given thresholds in the number of particles (N)2on the central
detector: the mean number _ of particles on each i m detector, the
mean distance _ from the shower core, and the mean primary energy E ,
o
are calculated on the hypotesis of pure electromagnetic cascades.
Due to the tails in the timing distributions we have defined our errors
as the half widths of the distributions at half maxima (HWHM). The
measured HWHM of the response of an isolated detector to the trasversal
of a single particle is Sid = 1.25 ns and that of the sole electronic
system (including the rise time fluctuations of the PM s) is S. = 0.59le
ns. Since we don't expect the temporal response of the detector being
constant as a function of the number of particles on it, we have used
for the instrumental uncertainties (S.) the two extreme values Sid and
Sie. After subtracting from the HWHM (_) of the distribution of T1 -T2
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OO 9.5-i&
490
600
a•tt
_00o
O :'.
200 " ".'.
E
__ 4e •
• oQ,% ..e
e_e"'e°•°°'e_•% • •%, _e,, e,,•%%%,, e, ,••o _% e, °I I I
1 5 1o N
Fig.l: Pulse height spectrum of thecentral detector triggered by
the fourfold coincidence. The cut on the left side is due
to the ADC threshold•
such instrumental resolution, we obtain the physical limit to the
accuracy in the definition of the shower front as a function of N.
Fig. 2 shows such accuracies:
a) measured (VI/(SN/2) 2 - SI.2 );
b) deduced from the resolution measured at the 1 particle level,
by I/V-_ (sv /V-_,sv:I/2 ( So/2 )2_si 2);scaling
c) obtained by using the Linsley's formula ( 2.6 (5/30 + I) 1.5/_--
ns, 5 in m )
agaist N, n, r, -E- •
o
It can be seen that the trend of the measured resolutions as 1 /_
is verified inside the dncertainty of the instrumental dispersion and
also the absolute values do not move much away from those obtained
by using the Linsley's formula.
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0
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q_
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: }
0 5 lO 40 I00 N
z. 19 l.,O ! 57 363 g
36 26 22 16 13 _'(m)
1.8"1014 5-1014 8"I0 IL_ 2"10 5 3.2 loISFc_eV)
Fig.2: Accuracies in the definition of the shower front: a- continuous
line; b- dotted line; c- full dots (see text).
At our atmosperic depth and by using detectors of 1 m2 area an accuracy
in the definition of the _ower front St < 0.5 ns can be reached at
primary energies-E e_ 2.10 eV at a typical core distance _ = 16 m.
O
References
I. Linsley J. (1984), Research Note UNML - 7/6/84, University of New
Mexico, Albunquerque.
2. Morello C. and Navarra G., (1981), N.I.M., 187,533


The angular resolution of air shower gamma ray telescopes

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