The arrival time distributions of muons with energies above 1.0GeV and 0.5GeV have been measured in the Akeno air-shower array to study the longitudinal development of muons in air showers with primary energies in the range 10 to the 17th power to 10 to the 18th power ev. The average rise times of muons with energies above 1.0GeV at large core distances are consistent with those expected from very high multiplicity models and, on the contrary, with those expected from the low multiplicity models at small core distances. This implies that the longitudinal development at atmospheric depth smaller than 500 cm square is very fast and that at larger atmospheric depths is rather slow

Enoki, T.

Kakimoto, F.

Nishi, K.

Suga, K.

Tsuchimoto, I.

English

NASA Technical Reports Server

324 HE 4.7-4
Longitudinal development of muons in large air showers
studied from the arrival time distributions measured
at 900m above sea level
Kakimoto, F., Tsuchimoto, I., Enoki, T. and Suga, K.*
Department of Physics, Tokyo Institute of Technology,
Meguro, Tokyo 152, Japan
Nishi, K.
The Institute of Physical and Chemical Research,
Wako, Saitama 351, Japan
ABSTRACT
The arrival time distributions of muons with energies above I.OGeV
and 0.5GeV have been measured in the Akeno air-shower array to study the
longitudinal development of muons in air showers with primary energies in
the range 1017eV to lOl8ev. The average rise times of muons with ener-
gies above I.OGeV at large core distances are consistent with those ex-
pected from very high multiplicity models and, on the contrary, with
those expected from the low multiplicity models at small core distances.
This implies that the longitudinal development at atmospheric depths
smaller than 50Ogcm -2 is very fast and that at larger atmospheric depths
is rather slow.
Comparison of the mean arrival times of muons with energies above
0.5GeV and the calculation made by McComb et al suggests no serious con-
tribution of muons arising from photon-nucleus collisions to the arrival
time distributions of muons with energies above 0.5GeV.
i. Introduction
The Bolivian Air Shower Joint Experiment (BASJE) group and the Tokyo
Institute of Technology group carried out aseries of experiments to study
the longitudinal development of electrons and that of muons by measuring
the arrival time distributions both of atmospheric Cerenkov light and of
muons, at widely separated atmospheric depths of 55Ogcm-2(Mt. Chacaltaya)
and 93Ogcm-2(Akeno). The longitudinal development of muons derived from
the experiment at Chacaltaya and that of electrons derived from both ex-
periments atChacaltaya and atAkeno have already been published(1),(2),(3).
In the present paper, we report on the arrival time distributions of
muons with energies above I.OGeV in air showers with primary energies in
the range lOlTev to iOl8eV observed in the Akeno air-shower array. Also
reported in the paper are arrival time distributions of muons, with a
l_nver threshold energy of O.5GeV, in air showers around iO18eV. On this
basis an estimate is made of the contribution of muons arising from
photon-nucleus collisions (photoproduced muons) to the arrival time dis-
tributions of muons with energies above 0.5Gey_ by comparing the mean ar-
rival times of muons calculated byMcComb et al_) with those measued in the
resent experiment.
• Experimental
The first run (May 1981-December 1982) was made to measure the arri-
val time distributions of muons with energies above l.OGeV in air showers
with primary energies around 1017eV. The resul_ of this run was already
reported(4).
*Present address: Department of Physics, Meisei University,
Hodokubo 337, Hino-shi, Tokyo 191, Japan
https://ntrs.nasa.gov/search.jsp?R=19850027742 2020-03-20T17:04:57+00:00Z
325 HE 4.7-4
In the second run (January 1983-October 1984) the measurement was
made both at the M4 muon station (Ep>I.OGeV) and at the ME3 muon station
(Ep>O.5GeV), which were separated by 100m from each other. Four 4m 2
shielded scintillation detectors were operated along with a4m 2 unshielded
scintillation detector at each station (hereafter called muon detector
and electron detector, respectively). The inside wall of muon detector
was covered with black paper while that of electron detector was painted
white, and a 5in fast photomultiplier with 14 stages (Hamamatsu Photonics
R1250) viewed a scintillator in the detector. Signals from four muon de-
tectors were added in a circuit through a coaxial cable (IIC4AF). A
s_gnal from the electron detector was delayed by'l.8ps through a coaxial
cable (8C4AF) and then added in the circuit. This combined signal was
fed to a waveform recorder (Biomation 8100) with a sampling interval of
20ns. When the local trigger pulse generated by the passage of at least
one particle through the electron detector was coincident with a master
pulse from the Akeno 4km 2 air-shower array (5), which observed an air
shower with electron size larger than 107.5 , the combined signal was
stored in the recorder.
The time response of the whole system was 16.2ns for rise time be-
tween 20% and 20% of the integrated full signal from the muon detector
(T20_?O).
3. Arrival time distributions of muons with energies above I.OGeV
In figure i average values of T20_7ofor muons with _nergies above 1.0
GeV in the second run are presented against core distances, where the
data are free from biases arising from the requirement for the local
trigger and the limited dynamic range of the pulse height in the recording
system. Also shown in the figure are values of T20T708 calculated fromfour models for primary protons with energies of i0 eV and sece of i.i.
As is seen in the figure, the average values of T20_70 at small core dis-
tances are consistent with those calculated from low multiplicity models,
while high multiplicity models reproduce the present T20_70 consistently
at large core distances, indicating that the observed values of T20-70 at
all core distances seem not to be explained by any single model as in the
case of lower primary energies(4). Since the showers observed in the
present experiment were classified by their electron sizes, the sample of
showers with a given electron size observed in the Akeno array contains
- showers initiated at the top of the atmosphere with a particular primary
energy as well as those from lower primary energies starting at the
deeper atmosphere. This fact was already confirmed from the measurement
of atmospheric Cerenkov light at Akeno(3). Considering this fluctuation,
a model with an enhanced EI/2 multiplicity law reproduces the observed
values of T20_70 better than a model with an EI/2 multiplicity law at
2argest core distances.
W
4. Comparison of both arrival time distributions of muons above 0.5GeV
and I.OGeV
The average values of T20_70 for muons with energies above 0.5GeV
" are longer than those with energies above l.OGeV by about 50% for showers
with electron sizes in the range 108.0 to 108.5 and sec8 in the range 1.0
to 1.2, as shown in figure 2. It is worth noting that the average values
of T20_70 for muons above 0.5GeV are almost consistent with those above
0.42GeV obtained by the Nottingham group in the Haverah Park array[ 6), if
the differences of the system time response and the threshold energy of
326
HE4.7-4
muons in the two experiments are allowed for.
In figure 3 the mean arrival times of muons above 0.5GeV and those
above I.OGeV were calculated from observed muon signals and are compared
with those estimated from the mean arrival times and lateral distribu-
tions of photoproduced and normal muons above 0.3GeV at sea level calcu-
lated by McComb et al (?) using a scaling model for proton and iron-initi-
ated showers. After allowing for differences of the threshold energy of
muons and the atmospheric depth between the calculation and the present
experiment, we would conclude that the longitudinal development of muons
in air showers with primary energies around lOl8eV is faster than that
calculated by McComb et al using a scaling model and primary iron, fur-
thermore the proportion of photoproduced muons to all muons above 0.5GeV
must be much smaller than that calculated by these authors.
5. Discussion and conclusions
The observed values of T20_?O at small core distances are consistent
with those calculated from low multiplicity models while at large core
distances these observed values of T20_70 are well explained by high mul-
tiplicity models. According to the present calculation, the average pro-
duction depth of muons arriving at the time intervals of T20_?O decreases
with increasing core distance. We may conclude, therefore, that the
longitudinal development of muons in air showers with primary energies in
the range lOlTeV to lOl8eV is as fast as expected from an enhanced EI/2
multiplicity law from 200gcm -2 to 500gem -2 and is rather slow beyond that
atmospheric depth. This conclusion is consistent with that derived from
the arrival time distributions of muons in air showers with primary ener-
gies above 1017eV observed at Chacaltaya (I) and of atmospheric Cerenkov
light from air showers with primary energies in the range 5×lOlseV to 2×
lO_TeV observed at Akeno (3) as well as at Chacaltaya(2).
The fact that photoproduced muons appear to make no significant con-
tribution to the arrival time distributions of muons with energies above
0.5GeV means that the character of pa?ticle interactions may be conven-
iently studied by measuring the arrival time distributions of muons above
0.5GeV, instead of I.OGeV, and comparing them with calculations for normal
muons. This way is important to reduce the cost of the shielding materi-
al of muon stations with large area in a large air-shower array.
References
(i) F. Kakimoto et al: J. Phys. G: Nucl. Phys. 5(1983)339.
(2) N. Inoue et al: J. Phys. G: Nucl. Phys. 11(1985)669.
(3) N. Inoue et al: J. Phys. G: Nucl. Phys. 11(1985)657.
(4) T. Tamura et al: Proc. of 18th ICRC, Bangalore, 6(1983)133.
(5) T. Hara et al: Proc. of 18th ICRC, Bangalore, 11(1983)276.
(6) P.R. Blake et al: J. Phys. G: Nucl. Phys. 8(1982)1605.
(7) T.J. McComb et al: J. Phys. G: Nucl. Phys.-5(1979)1613.
327
HE4.7-4
i I I i , i 500 i i i i i I II
10Ol- A a c'c
_u _ 50
4o
_ 20, I I ' i I f m , , 20 _, I I I I I I I
200 LI00 600 800: 200 400 600 800
Core distance (m) Core distance (m)
Figure i. The average T20_70 of Figure 2. The average T20_70 of
the arrival time distributions of the arrival time distributions of
muons with energies above I.OGeV muons with energies above 0.5GeV
for showers with Ne oflO8"O-lO 8"5 (_) and those above 1.OGeV(_).
and sece of 1.O-1.2 compared with
those calculated from A: scaling
model, B: a model with an E I/4
multiplicity law, C: a model with
an EI12 multiplicity law and D: a 8oo
with an enhanced E I/2 mul- //
model
tiplicity law and with first- _ 700
interaction depths of 40gem -2 for _ /
A-D and 120gcm-2 for C'-D'. 600 //
•H /40
soo /
Figure 3. Mean arrival times of _> /
" muons with energies above 0 5GeV .,4 / T• ;_ 400
(_) and those above I.OGeV(_) for _ /
showers- with Ne of 108. 0-108" 5 and / _r I
, sece of 1.0-1.2 against core dis- _• 300 /
?
tances compared with those of _ /
muons above 0.3GeV estimated for 200 / Y\
primary protons (_) and for //_ m "_
primary irons (- - -) with ener-
gies of 1018eV and sece of 1.0. I00
0
300 500 700
Core distance (m)


Longitudinal development of muons in large air showers studies from the arrival time distributions measured at 900m above sea level

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