The longitudinal shower development of extensive air showers (EAS) observed in the fly's eye is used to determine the distribution of X sub max, the depth in the atmosphere of the EAS maximum. Data and Monte Carlo simulations of proton and iron primaries are compared. A substantial contribution from light primaries is noted

Baltrusaitis, R. M.

Cassiday, G. L.

Cooper, R.

Elbert, J. W.

Gaissner, T. K.

Gerhardy, P. R.

Loh, E. C.

Mizumoto, Y.

Sokolsky, P.

Steck, O.

English

NASA Technical Reports Server

' 1 66 OG 5.1-7
STUDY OF THE C_4POSITION OF CO_IIC RAYS WITH ENERGY .7<E<3.EeV
Baltrusaitis, R.M., Cassiday, G.L., Cooper, R., Elbert, J.W.,
Gerhardy, P.R., Loh, E.C., Mizumoto, Y., Sokolsky, P., & Steck, D.
Department of Physics, University of Utah, Salt Lake City, UT 84112
Gaisser, T.K., & Stanev, T.
Bartol Research Foundation of the Franklin Institute
University of Delaware, Newark, Delaware
ABSTRACT
The longitudinal shower development of EAS observed
in the _y's Eye is used to determine the distribution of
Xmax, the depth in the atmosphere of the EAS maximum.
Work in progress to compare data and Monte Carlo simulations
of proton and iron primaries is described. Preliminary
evidence is in favor of a substantial contribution from
light primaries.
1. Introduction. The overall longitudinaldevelopment of the EAS
detected by the Ny's Eye(l) can be used to determine the depth in the
atmosphere in gm/cm2 of the EAS maximum X_ax" The distribution in Xmax
is in principle sensitive to the composizlon of the primary particles
since iron nuclei and protons will give rise to Xmax distributions that
peak at shallower and deeper depths and have narrower and wider widths,
respectively. It also follows that a mixed composition will have a
broader Xmax distribution than any single source.
In what follows, we discuss the reconstructionof longitudinal
shower profiles and the systematics of determining Xmax distributions
and then discuss work in progress on Monte Carlo simulations,which
include the details of the _y's Eye acceptance, for pure protons,
iron, and a mixed composition.
2. Shower Size Measurement. A fit to the relative time of arrival i
of light to succeeding phototubes in the event-detector plane yields
Rp, the impact parameter of the shower to the detector, and the zenith
and azimuthal angles of the EAS. Measured values of optical gathering
power, efficiencies and electronic gains and pedestals are used to
convert photoelectron yields into apparent brightness, i.e., numbers of
photons arriving at the detector from the source. This can be converted
into intrinsic source fluorescent brightness after correcting for: (a)
directly produced Cerenkov light beamed in the direction of the detector;
(b) Cerenkov light scattered in the direction of the detector due to
Rayleigh and Mie scattering; and (c) atmospheric attenuation of light.
The details of these corrections are described in reference 1. The
intrinsic fluorescence brightness can be translated directly into a
shower size using the known nitrogen fluorescence efficiency.
J
The Cerenkov light production model, and in particular, the
dependence of the Cerenkov light intensity on emission angle has been
checked by using a sample of events seen by both Ny's Eye I and Ny's °
Eye II, a smaller station situated 3.3 km from _y's Eye I. We find
good consistency in size estimates of sections of EAS viewed
166  1-7 
UDY   Dr4P SI ION  Sf>lI   I H RGY 7<E<3.  
altr saitis, . ., as i , . ., ooper, ., l rt, . ., 
er , . ., , . ., i ot , ., l ky, .,  t , . 
e artme t f ysics, ni ersity f ta , alt ake it , T 84112 
ais r, . .,  t , . 
art  es ch ion   r in tute 
niver ity f el ar , e ar , el re 
T 
he lo itudinal sho er e elopme t f E S served 
i  t  Fly's ye is s  to termine t  istribution f 
' the depth in the at osphere of the EAS axi u . 
ork i  r r ss t  p r  t  a  onte arl  si l tions 
f r ton  iron ri aries i  ribed. r liminary 
d e  n    ial t ibution rom 
1 i t ri ri s • 
. Introduction. he overall longitudinal development of the E S 
detected by the Fly's ye(1) can be used to determine the depth in the 
atmosphere in gm/cm 2 of the EAS maximum X~ax' The distribution in Xmax 
i  in ri i le itive t  t  positlon f t  rim ry arti les 
since iron nuclei and protons will give rise to Xmax distributions that 
eak t s llow r a  r t s a   rrow r a  i r i t s, 
r s tiv ly. It lso f llows t t  i  position ill   
broader Xmax distribution than any single source. 
In hat follows, e discuss the reconstruction of longitudinal 
sho er profiles and the syste atics of determining Xmax distributions 
 t n i  r  i  r r s  ont  arlo i l tions, hi  
i l e t  t  il s f th  Fly' s ye tan e, f r r  r t s, 
iro , and a ixed c positi . 
.  i  easur ent    o h  tive im  f ival 
f li t t  s ccee ing t tu es in the e e t- etector la e iel s 
Rp, the impact parameter of the shower to the detector, and the zenith 
and azi uthal angles of the E S. easured values of optical gathering 
er, fficien ies  l tronic i s  est ls r  s d t  
ert ot l tron i l  i to t i t , . ., bers f 
t s arriving at t e etect r from t e s rce. is can be c erted 
i t  i trinsic r e flu resce t rig tness ft r rr ting f r: ( ) 
directly produced erenkov light beamed in the direction of the detector; 
) er ov ig  tered n  i ion   t   o 
a l igh  i  ttering;  ( ) t s ric ttenu tion f lig t. 
 t i    r ions  ibed n erence   
i trinsic fluorescence ri tn ss  e translated ir tly i to  
r iz  ing t  own itrogen luorescence iciency. 
he erenkov lig t r ction odel, and in artic lar, t e 
e f t  er nkov ~  i t ity  i ion le  n 
ed  ing  sam l  f t  n  t  Fly' s  I  Fly' s 
Eye II, a s aller station situated 3.3 km from Fly's Eye I. e find 
 si tency in i  tim t   tions   iewed 
• 
https://ntrs.nasa.gov/search.jsp?R=19850025736 2020-03-20T18:01:30+00:00Z
167 OG 5.1-7
simultaneously at different emission angles by the two eyes.
The size versus depth distributions are fitted with a Gaussian
form and the Xmax and energy of the event determined(i). The Gaussian
form fits most showers well. Figure 1 shows a typical shower profile.
3. Systematics of
_.,zc,e,,_._F,,,_ Shower Maximum Distri-
_-.....,,,,,, ,:_-,._'." _,._'- butions The ab ute
l..'_:_,_l 7.F.F'407 O,F.q THFT¢_: Pt'_,6 _-e'- _,1 P,'_
7.,E,_:.,._.o,_ depth of Xmax is-7._Cg¢)1 I. ¢-_.-_,?
¢........ the parameter most
_-_"_-_'._''_ sensitive to
{ systematic errors of
__,.\ any that we measure.
._,_ Symmetric random
L errors in the zenith
'!'i\.i,,..,i_ angle, Oz, result in; I_ non-symmetric errors
-'_'_ in Xmax: Symmetric
, \'.... errors in Rp also
_,__ _ '.,."'., yield non-symmetric
.S"gt _ "', "_'- errors in Xmax•_o_._ ....... _'_'I ............. __ .... "...........
o. _. '_. _. _. _' because of the
exponential atmo-
Figure 1. Typical Longitudinal Shower spheric density
Profile distribution.
There is also an
intrinsic correlation
between Xmax and the shower energy. Showers whose reconstruction
err to smaller Rp and larger oz will have systematically deeper
Xmax and smaller estimated energy while errors that lead to larger
TR_and smaller 0z yield smaller Xmax and larger energy estimates.is is a direct consequence of random erro s and the exponen ial
nature of the atmosphere. Any additional systematic bias in 0z or
Rp will shift the Xmax distributions accordingly.
To reduce these effects to a minimum, we consider events
that are very well reconstructed,with Rp>2.0 km, projected track length
>50°, SRD/RD<.I, SOz<IO° and relative uncertainties in Gaussian
" width, X_ax',and energy of <.4. These cuts also have the effect of
reducing"'theCerenkov subtractions to a level where 50% variation in
the Cerenkov light model parameters do not significantly affect the
Xmax distributions. Since any residual reconstructionbias will
affect data near the tails of the energy distribution,we cut on .7<E<3
EeV, around the maximum of our energy acceptance. We believe residual
systematic effects in this data sample will produce less than a + 50
gm/cm2 shift in the average Xmax. We note that the width of the
Xmax distribution is much less sensitive to systematic errors. We
- estimate the systematic error in the width to be +_10 gm/cm2. The
resultant distribution in Xmax is shown in Figure 2. The average
Xmax for this sample is 730+60 gm/cm2 while the width (standard
deviation) is 120+40 gm/cm2.
1/ 
167  . 7 
si ultaneously at different e ission angles by the two eyes. 
The size versus depth distributions are fitted ith a aussian 
form and the Xmax and energy of the event determined(1). The Gaussian 
f rm fit  ost s ers ell. i re  s s a t ical s er r fil . 
~IZI:.-------------------
10/1? ... R'1 FVI o.I::r.:-al 
A ~I<.".,,'\ <.HI~t.P. 
C.'I1f~-'"II", 
I. !If:t{~ 7. F.F+n," 
7."E'~~ 4.:!c..\U. 
-7.tCIIH 1 .... :Cu),:~ 
("H'lu:J"II'n 
.G'4f:..H0 • 1.:'~.'1~ ~.~U.".l 
7. r.x u)..:! 4.ZCH)t 
2.1F.+02 3. IF.+01 
.~.oo 
." 
1.;"~ 
l:.:"lf'.A-J:...k.<, 2.3Q + .... -
"1'= I 4,1] f,·· 
llFT(t:;: rl;.6 +,.-
I 
Figure 1. Typical Longitudinal Sho er 
r fi e 
(1.11 Et:"V 
fl. J ~'" 
f:. t rllr~ 
. yst at s  
Sh er a lmum lstr
t o   sol t  
depth of Xmax is 
t  r et r ost 
si ive  
s ste atic err rs f 
any that we measure. 
metr c andom 
errors in the zenith 
angle, az' result in 
non-symmetric errors 
in X a ' Symmetric 
errors in Rp also 
i l  y et ic 
errors in Xmax 
e f  
t al tm
spheric density 
ribution. 
e  so  
rinsic r tion 
between Xmax and the shower energy. Showers whose reconstruction 
err to smaller Rp and larger 8z will have systematically deeper 
Xmax and smaller estimated energy while errors that lead to larger 
Rp and smaller 8z yield smaller Xmax and larger energy estimates. his is a direct consequence of random erro s and the exponential 
nature of the at osphere. Any additional systematic bias in 8  or 
Rp will shift the Xmax distributions accordingly. 
 duce e s   i imu    s 
that are very well reconstructed, with Rp>2.0 km, projected track length 
>50°, SRp/Rp<.1, S8 <10o and relative uncertainties in Gaussian 
width, Xm ' and energy of <.4. These cuts also have the effect of 
reducing the Cerenkov subtractions to a level here 50% variation in 
the erenkov light odel para eters do not significantly affect the 
Xmax distributions. Since any residual reconstruction bias will 
affect data near the tails of the energy istribution, e cut on .7<E<3 
EeV, around the maximum of our energy acceptance. We believe residual 
systematic effects in this data sample will produce less than a ± 50 
gm/cm 2 shift in the average X ' We note that the width of the 
Xmax distribution is much less sensitive to systematic errors. We 
estimate the systematic error in the idth to be ± 10 g /cm 2 • The 
resultant distribution in Xmax is shown in Figure 2. The average 
Xmax for this sample is 730±60 gm/cm 2 while the width (standard 
deviation) is 120±40 gm/cm 2 • 
1 68 OG 5.1-7
4. Monte Carlo
i_ ' ' ' ' ' ' ' _ ' ' ' ' ' ' ' Generation
<X>=(72E_3 +- S6)E-I SD=(1163 +-- 39)E-I
Prol;onand
140- iron induced showers
12_- _ are generated in a
Monte Carlo program
I_ - ! _ with the following
• _ input. Protons
i - are assumed to obey
_ T Hillas pure scaling
_ _ - with the only scalev ola ing effects
- _ being rising cross-44_
_ _ sections as
parameterized by
f _ _ , , Gaisser and
0 _' _ ! " I ' I ' I ' I ' ! , X! , -
.4 .5 .6 .7 .8 ._ _._ _._ _ _ Yodh(2) The mean
xt_5
_x inelasticity is
assumed to be .5.
Figure 2. Distributionsof Xmax in For the case of
gm/cm2. iron primaries a
superposition model
is assumed. The
Monte Carlo follows hardons and electromagneticparticles down to
1/30th of the primary energy after which parameterizationsof shower
development are used. We use these showers to predict the number of
photoelectrons and relative time delays observed in the detector and
generate fake events which are then passed thru the same reconstruction
and analysis programs as the real data. This work is in progress but
preliminary indications are that the depth of maximum distribution for
heavies is substrantially narrower than that for protons. It is
important to note that comparing the observed Xmax distributions to
theoretical predictions without taking into account the details of
detector response can lead to misleading conclusions.
5. Conclusions. Although we are not yet ready to quote quantitative .
comparisions between the data and Monte Carlo simulations, preliminary
evidence based on comparisons of the widths of the Xm@x distribution
to Monte Carlo is in favor of a substantial contributlon from light
primaries to the cosmic ray composition at these energies. Detailed
comparisons will be forthcoming.
6. Acknowledgements. We gratefully acknowledge the United States
National' Science Foundation for its generous support of this work under
grant PHY8201089.
References.
1. Ba'ltrusaitis,R.M., et al., Nuclear Instruments Methods (to be
published).
2. Gaisser, T.K., and Yodh, G.B., Ann., Rev. of Nucl. and Particle
Science, 30 (1980), p.475.
u
8 
160 
)=(7288 - 56)E-l O=( 163 - 8)E-1 
40 
20 
100 
B0 
60 
0 
t t t 1 t t t 
20 
t t 
0 
4 5 6 7 8 3 
GItJS »nx 
Figure 2. Distributions of Xmax in 
g /cm 2 • 
1.0- 1.1 {eS 
 . 7 
. ont  ar  
enerat on 
r ton  
ron nd ced e  
are generated in a 
ont  arl  ram 
ith the following 
input. Protons 
r  ssum  t  e  
illas pure scaling 
with the only scale 
i l ing ff t  
being rising cross-
ions  
et i   
ai   
odh( ).  e  
l city  
sum d    
For the case of 
iron pri aries a 
s er sitio  odel 
 u   
ont  arlo llows   lectrom tic arti les  t  
1/30th of the primary energy after hich para eterizations of sho er 
development are used. We use these showers to predict the number of 
photoelectrons and relative time delays observed in the detector and 
generate fake events hich are then passed thru the same reconstruction 
and analysis programs as the real data. his ork is in progress but 
preliminary indications are that the depth of maximum distribution for 
i s i  s str tially rrow r than t t f r r t s. It i  
i portant to note that co paring the observed Xmax distributions to 
theoretical predictions ithout taking into account the details of 
detector response can lead to isleading conclusions. 
. oncl si s. lthough e r  t t r ady to t  antit tive 
co parisions between the data and onte arlo si ulations, preliminary 
evidence based on comparisons of the widths of the Xm~x distribution 
to onte arlo is in favor of a substantial contributlon from lig t 
pri aries to the cos ic ray co position at these energies. Detailed 
co parisons ill be forthcoming. 
6. ckno ledge ents. We gratefully ackno ledge the nited States 
ational cience oundation for its generous support of t is ork under 
grant PHY8201089. 
ef . 
 altrusaitis, . .,  l  ucl  trum s et  o  
publ i shed) • 
. ai , . .,  o , . ., n ., e .  ucl.  rticle 
cie ce, 30 ( ), P . . 
.. 
• 


Study of composition of cosmic rays with energy .7  E  3 Ee

Study of composition of cosmic rays with energy .7 E 3 Ee

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