A mini-array that utilizes the Linsley effect is proposed for the measurement of large air showers. An estimate of the detectable shower rates for various shower sizes is made. Details of the detection and data collection systems are also described

Chan, S. K.

Hazen, E. S.

Hazen, W. E.

Ng, L. K.

English

NASA Technical Reports Server

347
HE 4.7 - 10
A Mini-array for Large Air Showers
L.K. Ng and S.K. Chan
Physics Department, University of Hong Kong
Hong Kong
W. Hazen and E. Hazen
Physics Department, University of Michigan
U.S.A.
Abstract
A mini-array that utilizes the Linsley effect is
proposed for the measurement of large air showers. An
estimate of the detectable shower rates for various shower
sizes is made. Details of the detection and data collection
systems are also described.
i. Introduction. Interest in the measurement of large air
showers has been enhanced by the conflicting result reported
in 1982 (Yakutskl). However, since intensities of large air
showers are extremely low and only a few large centres are
able to do the measurement, it was proposed by J. Linsley 2
that more independent measurements could be done with
rooftop mini-arrays by making use of the particle arrival
time distributions (Linsley effect). This idea has been
pursued by us, and the present proposed system is the result
after looking into various problems associated with such
mini-arrays.
2. Theorectical estimation. To be able to measure the
arrival time distribution, at least two particles per event
are required. With a mini-array of 2 m 2, a threshold density
of the shower particles arriving at this mini-array should
. be pl = 1 m -2. Using the differential intensity expression,
j(N,@) = j(N) cos_e ,
• with _ = 8.6 at sea level, the shower rate for large showers
with size N to N+6N should be
_(N)6N 2_j(N)A(N)6N _/2 le= cos _+ sin_d0
u
2_
- e+2 j(N)A(N)_N,
where A(N) is the area containing the cores of all the air
showers detected by the mini-array. Hence the integral
shower rate is
2_ _ j (N)A (N) tiN.
¢(>Nmin) = _+---_SN-inm
https://ntrs.nasa.gov/search.jsp?R=19850027748 2020-03-20T17:05:01+00:00Z
348
HE 4.7 - 10
The area A(N) is an annular ring with its outer radius
determined by the density threshold Pl = 1 m-2and its inner
radius determined by the minimum time spread oi selected.
Therefore, it has the form,
(CN) 2/n 01 2/B
A(N) = 7[ 71 - (_-) ]
Hillas' integral intensity spectrum 3 ,
J(>N) = 8.10 12 (N/108) -1"7 m-2sr-ls -I
is used to determine j(N), and Linsley's empirical formula 2,
o(ns) = 2 6 ( r + 1) 1"5 with r in m
" 3--6
is used to relate the time spread o to the core distance r
from the mini-array. The lateral distribution of electrons
at Aken_ for large showers,
p = 5.10 -5 N (r/ro) -3"8 with ro = 79 m
is used to relate Q to r.
The final expression for the shower rate is given by
(>oi, >Pl) = 9 07 106pi-1"7o -2"97day-i• . I ,
and the values are tabulated below.
pi(m-2) ot(ns) Nmi n Shower rate (day -1 )
1 50 9.07.105 81.6
1 100 6 25.106 10.4
1 200 3[04.107 1.33
1 300 8.49.107 0.40
1 400 1.76.108 0.17
Table I
3. Detection system. From the estimation given in Table I,
it is clear that a mini-array so constructed should be able
to pick out the very few large air shower events from a
swarm of irrelevant events including the counter noises, the
background soft radiations and the small air showers. Fast
electronics is also required for the determination and
collection of the arrival time spread.
In order to eliminate the large number of small air
showers, a minimum time spread oi has to be assigned.
According to Table I, for a mini-array of 2 m 2, a minimum
acceptable shower size of 9.07.105 requires a minimum time
spread o l equal to 50 ns.
34g HE 4.7 - 10
A mini-array of 2 m2 is then assembled which consists
of _ pieces of plastic scintillators each with dimensions
G.Sm x 0.5m x 0.05m. In view of the small particle density
encountered, each scintillator is not expected to receive
more than one particle at a time from a shower.
Each scintillator is viewed by two 5" photomultipliers,
the signal coincidence of which practically eliminates all
the counter noises. In order to exclude the background soft
radiations, the array is shielded by thin sheets of
galvanized iron (0.5mm thick). Particle track identification
may be found necessary at a later stage to acertain genuine
large shower events. This can be done by sandwiching the
array by 4 layers of small diameter proportional counters.
The mini-array is placed in another small air shower
array system, which may provide additional information on
the nature of showers received.
4. Fast electronics. Precise measurement of the particle
arrival time is essential in this experiment, and hence an
anti-jitter preamplifier per counter is employed to remove
the time jitter between the particle arrival time and the
signal output time. The preamplifier as shown in figure 1 is
a slightly modified version of the circuit described in
another paper (HE 4.7 - 9), and the modification provides
another coincidence input from a second photomultiplier.
r
D_lington___ Level ] Delay cable
_PMT_ stage _ Level _ _ _ _:::_A
Fig. 1 Co_ter pre_plifier.
Outputs from the eight counter preamplifiers are
individually shaped to narrow pulses of 20 ns each, and then
" combined together to give a train of narrow pulses per
shower. The time delay in each of the 8 channels from the
particle arrival to the pulse output after combination has
. been equalized by adjusting the length of the delay cable in
each preamplifier. The number of pulses in the train depends
on the number of counters giving out particle signals, and
the relative positions of the pulses in the train depend on
their particle arrival time. The circuit block diagram is
given in figure 2.
350 HE 4.7 - 10
20 ns
_pez8
I _ I Inte_ated
, _ > I _s_
Fig. 2 _e electro_c _stem. I I I _igg,z
RW _2_ -_ '
I I _e_ister I
(Ap. II) _ ("64 Bits) I
The pulse train is temporarily stored in a set of four
shift registers. The shift registers have 4 x 64 bits in
total and hence can readily store 2 ps of information, the
clock rate for accepting the pulse train is equivalent to
100 MHz. Consequently, it provides a time resolution of
10 ns in the measurement of particle arrival time.
The criteria for recording a pulse train temporarily
stored in the shift registers are (i) at least two pulses
are found in the train, and (ii) the time spread of the
pulses should not be less than ol. Criterion (i) is dealt
with by means an integrated pulse-width trigger circuit. Its
basic element is given in figure 3. The charge accumulated
in the capacitor C is proportional to the number of pulses
in the train. A trigger pulse can be produced from the
circuit at a pre-selected discrimination level. The trigger
pulse then transfers data to the microcomputer shown in
figure 2. Criterion (ii) is conveniently handled by software
means after the data is read into the microcomputer.
2L/I--
_Is_ R _685
C
@
The above system has been assembled and thoroughly
tested.
Reference.
i. Diminstein O Set al, 8th Eur.Cosmic-ray Symp. (Rome),1982
2. Linsley J, (preprint).
3. Hillas A Met al, Phys. Reports, 20C (1975) 79.
4. Hara T et al, Proc. 18th ICRC (Bangalore), ii (7983) 276.


A mini-array for large air showers

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