- Email: [email protected]
Studies of the knee in the electron and muon components of extensive air showers at EAS-TOP
[!(-[EAg Pl !'~';Ic :3
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 75A (1999) 251-255
Studies of the knee in the electron and muon c o m p o n e n t s of Extensive Air Showers a,t E A S - T O P M.Aglietta t,'-', B . A l e s s a n d r 0 2 P.Antonioli a, F.Arneodo 4, L . B e r g a m a s c o =,5, M . B e r t a i n a 2,~, (1.C!astagnoli l''e, A.Castellina 1'", A.Chiavassa. 2,s, G.Cini Castagnoli 2,~, B . D ' E t t o r r e Piazzoli 6, G.Di Seiascio Is, W. Fulgione 1,2, p.Galeottiu,5 P . L . G h i a 1'2 M.Iacovacci 6, A. L i m a de Godoi r, G . M a n n o c c h i 1'~, ( '..Morello 1,e, G . N a v a r r a 2,5, O . S a a v e d r a "~,a, G.C.TrincheroLe, P.Vallania 1,2, S.Vernetto 1,'~ , C.Vigorito~, 5. l l~tit.uto di (:osmo-Geofisica del CNR, Torino, Italy, 2Istituto Nazionale di Fisica Nucleate, Torino, Italy, :"tstituto Nazionale di Fisica Nucleare, Bologna, Italy, 4INFN, Laboratorio Nazionale del Gran Sasso, L' Aquila, Italy, 5Dipart.iinento di Fisica (]enerale dell' Universitg, Torino, Italy, 6Dipaxtimento di Seienze Fisiche dell' Universitg and tNFN, Napoli, ltaly, r Universidade de Sgo Paulo, $5.o Paulo, Brasil T'he region of the "knee" of the cosmic ray primary spectrum (10 L~ < E0 < 1016eV) is studied in the electromagnetic and muon components of Extensive Air Showers by means of the EAS-TOP array. Independent and eorrelated analysis of the two measurements are presented as a function of zenith angle (i.e. atmospheric depth).
1. I n t r o d u c t i o n M e a s u r e m e n t s of" the electromagnetic and ninon c o m p o n e n t s of Extensive Air Showers and of their behaviour versus a t m o s p h e r i c depth (i.e. zenith angle) provide i n f o r m a t i o n s on the cosmic ray p r i m a r y co, nposition combining the complem e n t a r y aspects of the Ne-N> relation and of the cascade d e v e l o p m e n t in the atmosphere. Such m e a s u r e m e n t s are of particular interest in the region of t h(~ '" knee" of the p r i m a r y s p e c t r u m ( E . ~ l/) ~' - It)~%V) since: a) coml~osit.ion d a t a are crucial tbr its interpretation; b) the energy range exceeds the one reached with accelerator da.t a (mostly on N-N interactions); ~) the signa.ture of the break which could identi~* a p r i m a r y energy and mass can be exploited to get iuformations on the p r i m a r y nuclei d o m i n a t i n g at t,h,' knee.
2. T h e E x p e r i m e n t T h e E A S - T O P array is located at C a m p o Iraperatore, 2005 m a.s.l. (above the underground G r a n Sa.sso laboratories), at 820 g crn -2 atnlospheric depth. Its electromagnetic detector [1] is m a d e of 35 scintillator modules, 10 m'-' each. T h e dist~uwe 1)el.ween detectors ranges from ~ 20 m
t.o ~ 80 m. Event selection for the present analysis requires at least 6 (or 7) modules fired and the highest particle density recorded by an inner detector. T h e core location, the slope (s) of the lateral distribution (ldJ) function and the shower size (Ne) are m e a s u r e d by m e a n s of a m i n i m u m ?(~ fit to the theoretical N K G ldf [2]. T h e ninon detector [3] (:overs a surface of 144 m ~ and consists of 9 identical planes. Each plane is m a d e of two layers of s t r e a m e r (for m u o n tracking) and one layer of quasi p r o p o r t i o n a l tubes (for hadron calorimetry). Planes are separated by 13 cm thick iron absorbers. On each plane the x coordinate of m u o n track is obtained from the signals of the anode wires (368 in a layer),.the y one is measured from the induced signals on strips orthogonal to the wires. T h e distance a m o n g the wires and the width of the strips is 3 crn. A m u o n track is defined from the alignment of at least 6 hits (wires on) in different layers of tubes. T h e m u o n energy threshold for verticM events is I Get/. T h e m u o n size ( N # ) is o b t a i n e d from the average measured m u o n ldf, (r0 = 300 rn):
= p(,.)
+
where p(#) is the detected m u o n s density.
0920-5632/99/5 - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0920-5632(99)00257-1
M. dglietta et al./Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 251-255
252
3. E l e c t r o m a g n e t i c D a t a Events with zenith angle 0 < 40 ° and core inside a fiducial area, in which the detection efficiency is ~ > 95% for Ne > 1052, are used. A full analysis is presented in [4]. Figure 1 shows the differential size spectra measured in six different intervals of zenith angle; the spectra are multiplied by Ne 25 to emphasize the change of slope at the knee. - ~ 31HP * ¢ ' ~ ' ~ 7: "7
~
_
~
,
~,..~.
depth (Nek(x) = Nek(xo) e - ( x - x ° ) / i k ) we obtain (dashed lines in figure 1): I ( > Nek) = (S.1 ± 0.7) × l O - S r n - 2 s - l s r -1, logNek(0 °) = 6.15 ± 0.02, and Ak ----222 ± 3 gcrn -2. The all particle energy spectrum is obtained (see figure 2), from all events with 0 < 40 °, converting every shower size to 810 gcrn -2 using the measured, from constant intensity cut, absorption length (Ae -- 219 ± 3 gcrn-2). The conversion from primary energy (E0, in T e V ) and mass (A) to shower size has been obtained by means of complete simulations of cascades in atmosphere using the CORSIKA-HDPM code [6]:
•7 : 2 0 0
Ne(Eo, A) : a(A)EZo (A)
g
(3)
where a(A) = 197.5A -°521, and /3(A) = 1.107A °°35. The values of A = Aef] used [4] range from Ae]! = 7.1 for Ne = 105.2 , Aeff = 8.9 for Ne = 106.0 to A~H = 10.9 for Nc = 1065.
09 Z {00 • ~) Z
8O 70 c '~ 61{!
sol
2
I
40 bl
5.2
5.4
5.6
5.8
6
6.2
6.4
6.6 6.8 Log(Ne)
Figure 1. Differential shower size spectra measured at different atmospheric depths. Solid lines: independent fit, dashed lines: fit requiring constant integral flux above the knee.
~ 0.9 N, 0.8 ~0.7 '~ 0.6 .4 0.5
~
0.4
Ne-~(e)
.
•
MSU
• TIBET
OD
[3
•
•
AV
AA
•
© JACEE
I
0.1
10 3
I
10 4
E (TeV)
(2)
71(= 2.56 + 0.02 constant[4], see tablel) below and 72 above the knee (figure 1, solid lines, and table 1). As already reported [5], the size at the knee (Nc~.) decreases with increasing atmospheric depth, while the integral intensity ( I ( > Nek)) is costa.nt inside the experimental uncertainties. Requiring a constant integral flux at Nek (I(> :V~;:) = (lkNek)/(72 -- 1)), with shower size attenuating exponentially with atmospheric
•
EAS-TOP
[] GRIGOROV
Results are obtained fitting each spectrum independently with expression:
I(X~, O) = t~(e)
AKENO
0.3
0.2
Ne )--a,~(o)
•
Figure 2. The all particle energy spectrum obtained from the EAS-TOP shower size data, compared with the results of other experiments operating outside the atmosphere or at ground level.
4.
Muon
Data
The muon number spectra are measured in two different intervals of zenith angles: around the
253
M. Aglietta et al./Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 251-255 Table 1 Results obtained from the fits to the A sec 0 7i
Ne and N// spectra measured in different bins of zenith angle. I ( > Nek, Npk) x 10 7 Log(Net:, N#,~) 72 m-28-lsr-1
e 1.00 - 1.05 1.05 - 1.10 1.10 - 1.15 1.15 - 1.20 1.20 - 1.25 1.25 - 1.30 p, 1.00 ~ 1.05 1.10-1.15
2.56 4- 0.02 2.55 + 0.02 2.55 4. 0.03 2.56 4- 0.03 2.59 4- 0.03 2.55 4- 0.07 3.12-1- 0.1 3.07+0.1
2.99 -4- 0.09 2.93 -4- 0.11 2.85 4. 0.12 2.81 4- 0.16 2.91 4- 0.26 2.80 4- 0.11 3.52 + 0.1 3.52+0.1
vertical direction (1.00 < sec0 < 1.05) and at about 25 ° (1.10 < sec0 < 1.15). In order to avoid large poissonian fluctuations and saturation problems, events with core position located inside a raage of distances from the # detector 120 < r < 230 m have been selected. The two spectra are shown in figure 3 and the change of slope at Np ~ [0 4'9 is visible in both of them. 7F.... and Nt~a, are obtained by a minimum X2 procedure comparing the experimental data to a trial spectrum taking into account the poissonian fluctnations of the number of detected muons. I ( > Npk) is obtained from the total number of events with N/t > Npk.
0.99 + 0.2 1.01 4- 0.3 0.93 4- 0.4 0.804- 0.4 0.524- 0.3 1.30-I- 0.6 1.0 -4- 0.2 0.7 =1:0.2
5. E l e c t r o m a g n e t i c
6.09 4- 0.05 6.02 4- 0.07 5.97 4- 0.08 5.934- 0.14 5.954- 0.ii 5.63+ 0.12 4.72 4. 0.1 4.774.0.1
and muon data
From the comparison of the electromagnetic and muon data we obtain: a) tile integral fluxes above the knee I ( > Nek) and I ( > Npa) are compatible at both atmospheric depths, as expected for a feature occuring at fixed primary energy (see table 1).
~23.2 3,1 3
- - 7 - ~
~l.00
~//~///
1.10
2.9
11)'~
, •
•
e
+ '
÷ ÷ ,
*
T±
+ i *
1.00
Figure 4. Comparison of the slopes of the Ne and Np spectra measured below and above the knee
• 1.10
4.6
'
'
'
'
417
4.11
5.1
5.2
Log(Nta)
Figure 3. Nt, spectra measured in two different intervals of zenith angles.
b) The slopes of muon and electron number spectra below and above the knee, reported in figure 4, show that a value of a ~-- 0.75 (concerning the relation Ne cx N# ~) holds in both angular bins,
254
M. Xglietta et at/Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 251-255
thus indicating that no sudden changes in secondary production rates occurs at primary energies around the knee. c) The location of the knee (Nek, Npk) on the scatter plot of the N p - N e data, at both zenith angles (see figures 5 and 9), is far from both the lower and upper {corresponding to proton and iron primaries respectively) edges of the distriI)ution, indicating the preference for the identificatiou of an "intermediate" primary mass. This is confirmed by simulations performed through the (IORSIKA-HDPM code [6]. Figures 6,7, 8 obta.ined in vertical direction for different primari,,s. and figure 10, obtained around 25 ° for helium primaries, show the expectations for different experimental conditions. Such anlysis leads thus to the preliminary conclusion that at both angles He (or "helium like") nuclei are favoured as bending componellt. The knee energy (E0k), obtained from expression (3) and Ne~. = 10 ~'09, is E0k = 2.7 P e V for A = i, E,~. = 3.4 P e V (A = 4), E0k = 4.1 P e V (.t = 14) , Euk = 4.9 P e V (A = 56). Nek and Ntea. provide thus, as best identification of the kime energy, E0k = (3.5 + 4.0) PeV. On the other side a "normal" atmospheric absorption of EAS at the knee, as resulting from the constancy of the integral fluxes (above both Nek and .'V#,. at different zenith angles), the attennat,ion length Ak of Nel~ and the consistent location of A'~a, and Npk (at different zenith angles), is obtmned.
:m 6 Z
Experimental data 1.00
~5.75 5.5 5.25 S 4.75 4.S 4.2.~ 4
Log(Ne)
Figure 5. Scatter plot of experimental Ne - N # data for vertical direction. The knee location error box is shown.
Proton l.O0
REFERENCES
1. Aglietta M. et al., N.I.M. A, 336, (1993) 310. 2. l(a.mataK, et al., Suppl. Progr. Theor. Phys., (5, (]~5~)93. 3. Aglietta M. et al., N.I.M. A, in press {1998). 4. Aglietta M. et al., Astrop. Phys., in press (1998). 5. EAS-TOP Coll., Proc 24th ICRC, Roma, 2, (1995) 732; EAS-TOP Coll., Proc. 25th I(:FIC, Durban, 4, (1997) 125; Navarra G. et a].. Nuclear Physics B (Proc. Suppl.), 60B, ( 1 ~gs) I05. 6. (:a.pedevielle J.N. et al., Kernforschungszentc~,m Korlshrue, KfK 4998, (1992)
D D
• °, *, ~ , "
• -,~..~ 4,7~ I 4.S
t
[ 5.2
£4
5.6
5.8
6
a.2
~.4
6.6
6.S 7 Log(Ne)
Figure 6. Same as figure 5 for simulated vertical primary protons. Notice the good fit of simulated points to the lower edge of the experimental distribution (as expected for primary protons)
M. Aglietta et aL/Nuclear Physics B (Proc. SuppL) 75,4 (1999) 251-255
Z
.•575
Itelium l.lllkst~:0< I AI5
Experimental data 1 . 1 l l < s e c 0 < l . 1 5
255
J/
5.5
525 I
I
•
5.25
~r• % .
5
4.75
4,75
4.5
4.25
4
Log(Ne)
Log(Ne)
Figure 7. Same as figure 5 for simulated vertical primary helium.
Figure 9. Same as figure 5 for experimental data for 24.60 < 0 < 29.60 .
I r o n I.I)O
Helium I . l O < s e c O < l . 1 5
o
.
-~#" t o " . 5.25
i
, -.~, .t i'..'~: "''" 4.75
25
,, 4.5
4.25
Log(Ne)
Figure 8. Same as figure 5 for simulated vertical primary iron. Notice the good fit of simulated points to the upper edge of the experimental distribution (as expected for primary iron nuclei)
4
Log(Ne)
Figure 10. Same as figure 5 for simulated helium primaries at 24.60 < 0 < 29.6°..
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 75A (1999) 251-255
Studies of the knee in the electron and muon c o m p o n e n t s of Extensive Air Showers a,t E A S - T O P M.Aglietta t,'-', B . A l e s s a n d r 0 2 P.Antonioli a, F.Arneodo 4, L . B e r g a m a s c o =,5, M . B e r t a i n a 2,~, (1.C!astagnoli l''e, A.Castellina 1'", A.Chiavassa. 2,s, G.Cini Castagnoli 2,~, B . D ' E t t o r r e Piazzoli 6, G.Di Seiascio Is, W. Fulgione 1,2, p.Galeottiu,5 P . L . G h i a 1'2 M.Iacovacci 6, A. L i m a de Godoi r, G . M a n n o c c h i 1'~, ( '..Morello 1,e, G . N a v a r r a 2,5, O . S a a v e d r a "~,a, G.C.TrincheroLe, P.Vallania 1,2, S.Vernetto 1,'~ , C.Vigorito~, 5. l l~tit.uto di (:osmo-Geofisica del CNR, Torino, Italy, 2Istituto Nazionale di Fisica Nucleate, Torino, Italy, :"tstituto Nazionale di Fisica Nucleare, Bologna, Italy, 4INFN, Laboratorio Nazionale del Gran Sasso, L' Aquila, Italy, 5Dipart.iinento di Fisica (]enerale dell' Universitg, Torino, Italy, 6Dipaxtimento di Seienze Fisiche dell' Universitg and tNFN, Napoli, ltaly, r Universidade de Sgo Paulo, $5.o Paulo, Brasil T'he region of the "knee" of the cosmic ray primary spectrum (10 L~ < E0 < 1016eV) is studied in the electromagnetic and muon components of Extensive Air Showers by means of the EAS-TOP array. Independent and eorrelated analysis of the two measurements are presented as a function of zenith angle (i.e. atmospheric depth).
1. I n t r o d u c t i o n M e a s u r e m e n t s of" the electromagnetic and ninon c o m p o n e n t s of Extensive Air Showers and of their behaviour versus a t m o s p h e r i c depth (i.e. zenith angle) provide i n f o r m a t i o n s on the cosmic ray p r i m a r y co, nposition combining the complem e n t a r y aspects of the Ne-N> relation and of the cascade d e v e l o p m e n t in the atmosphere. Such m e a s u r e m e n t s are of particular interest in the region of t h(~ '" knee" of the p r i m a r y s p e c t r u m ( E . ~ l/) ~' - It)~%V) since: a) coml~osit.ion d a t a are crucial tbr its interpretation; b) the energy range exceeds the one reached with accelerator da.t a (mostly on N-N interactions); ~) the signa.ture of the break which could identi~* a p r i m a r y energy and mass can be exploited to get iuformations on the p r i m a r y nuclei d o m i n a t i n g at t,h,' knee.
2. T h e E x p e r i m e n t T h e E A S - T O P array is located at C a m p o Iraperatore, 2005 m a.s.l. (above the underground G r a n Sa.sso laboratories), at 820 g crn -2 atnlospheric depth. Its electromagnetic detector [1] is m a d e of 35 scintillator modules, 10 m'-' each. T h e dist~uwe 1)el.ween detectors ranges from ~ 20 m
t.o ~ 80 m. Event selection for the present analysis requires at least 6 (or 7) modules fired and the highest particle density recorded by an inner detector. T h e core location, the slope (s) of the lateral distribution (ldJ) function and the shower size (Ne) are m e a s u r e d by m e a n s of a m i n i m u m ?(~ fit to the theoretical N K G ldf [2]. T h e ninon detector [3] (:overs a surface of 144 m ~ and consists of 9 identical planes. Each plane is m a d e of two layers of s t r e a m e r (for m u o n tracking) and one layer of quasi p r o p o r t i o n a l tubes (for hadron calorimetry). Planes are separated by 13 cm thick iron absorbers. On each plane the x coordinate of m u o n track is obtained from the signals of the anode wires (368 in a layer),.the y one is measured from the induced signals on strips orthogonal to the wires. T h e distance a m o n g the wires and the width of the strips is 3 crn. A m u o n track is defined from the alignment of at least 6 hits (wires on) in different layers of tubes. T h e m u o n energy threshold for verticM events is I Get/. T h e m u o n size ( N # ) is o b t a i n e d from the average measured m u o n ldf, (r0 = 300 rn):
= p(,.)
+
where p(#) is the detected m u o n s density.
0920-5632/99/5 - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0920-5632(99)00257-1
M. dglietta et al./Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 251-255
252
3. E l e c t r o m a g n e t i c D a t a Events with zenith angle 0 < 40 ° and core inside a fiducial area, in which the detection efficiency is ~ > 95% for Ne > 1052, are used. A full analysis is presented in [4]. Figure 1 shows the differential size spectra measured in six different intervals of zenith angle; the spectra are multiplied by Ne 25 to emphasize the change of slope at the knee. - ~ 31HP * ¢ ' ~ ' ~ 7: "7
~
_
~
,
~,..~.
depth (Nek(x) = Nek(xo) e - ( x - x ° ) / i k ) we obtain (dashed lines in figure 1): I ( > Nek) = (S.1 ± 0.7) × l O - S r n - 2 s - l s r -1, logNek(0 °) = 6.15 ± 0.02, and Ak ----222 ± 3 gcrn -2. The all particle energy spectrum is obtained (see figure 2), from all events with 0 < 40 °, converting every shower size to 810 gcrn -2 using the measured, from constant intensity cut, absorption length (Ae -- 219 ± 3 gcrn-2). The conversion from primary energy (E0, in T e V ) and mass (A) to shower size has been obtained by means of complete simulations of cascades in atmosphere using the CORSIKA-HDPM code [6]:
•7 : 2 0 0
Ne(Eo, A) : a(A)EZo (A)
g
(3)
where a(A) = 197.5A -°521, and /3(A) = 1.107A °°35. The values of A = Aef] used [4] range from Ae]! = 7.1 for Ne = 105.2 , Aeff = 8.9 for Ne = 106.0 to A~H = 10.9 for Nc = 1065.
09 Z {00 • ~) Z
8O 70 c '~ 61{!
sol
2
I
40 bl
5.2
5.4
5.6
5.8
6
6.2
6.4
6.6 6.8 Log(Ne)
Figure 1. Differential shower size spectra measured at different atmospheric depths. Solid lines: independent fit, dashed lines: fit requiring constant integral flux above the knee.
~ 0.9 N, 0.8 ~0.7 '~ 0.6 .4 0.5
~
0.4
Ne-~(e)
.
•
MSU
• TIBET
OD
[3
•
•
AV
AA
•
© JACEE
I
0.1
10 3
I
10 4
E (TeV)
(2)
71(= 2.56 + 0.02 constant[4], see tablel) below and 72 above the knee (figure 1, solid lines, and table 1). As already reported [5], the size at the knee (Nc~.) decreases with increasing atmospheric depth, while the integral intensity ( I ( > Nek)) is costa.nt inside the experimental uncertainties. Requiring a constant integral flux at Nek (I(> :V~;:) = (lkNek)/(72 -- 1)), with shower size attenuating exponentially with atmospheric
•
EAS-TOP
[] GRIGOROV
Results are obtained fitting each spectrum independently with expression:
I(X~, O) = t~(e)
AKENO
0.3
0.2
Ne )--a,~(o)
•
Figure 2. The all particle energy spectrum obtained from the EAS-TOP shower size data, compared with the results of other experiments operating outside the atmosphere or at ground level.
4.
Muon
Data
The muon number spectra are measured in two different intervals of zenith angles: around the
253
M. Aglietta et al./Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 251-255 Table 1 Results obtained from the fits to the A sec 0 7i
Ne and N// spectra measured in different bins of zenith angle. I ( > Nek, Npk) x 10 7 Log(Net:, N#,~) 72 m-28-lsr-1
e 1.00 - 1.05 1.05 - 1.10 1.10 - 1.15 1.15 - 1.20 1.20 - 1.25 1.25 - 1.30 p, 1.00 ~ 1.05 1.10-1.15
2.56 4- 0.02 2.55 + 0.02 2.55 4. 0.03 2.56 4- 0.03 2.59 4- 0.03 2.55 4- 0.07 3.12-1- 0.1 3.07+0.1
2.99 -4- 0.09 2.93 -4- 0.11 2.85 4. 0.12 2.81 4- 0.16 2.91 4- 0.26 2.80 4- 0.11 3.52 + 0.1 3.52+0.1
vertical direction (1.00 < sec0 < 1.05) and at about 25 ° (1.10 < sec0 < 1.15). In order to avoid large poissonian fluctuations and saturation problems, events with core position located inside a raage of distances from the # detector 120 < r < 230 m have been selected. The two spectra are shown in figure 3 and the change of slope at Np ~ [0 4'9 is visible in both of them. 7F.... and Nt~a, are obtained by a minimum X2 procedure comparing the experimental data to a trial spectrum taking into account the poissonian fluctnations of the number of detected muons. I ( > Npk) is obtained from the total number of events with N/t > Npk.
0.99 + 0.2 1.01 4- 0.3 0.93 4- 0.4 0.804- 0.4 0.524- 0.3 1.30-I- 0.6 1.0 -4- 0.2 0.7 =1:0.2
5. E l e c t r o m a g n e t i c
6.09 4- 0.05 6.02 4- 0.07 5.97 4- 0.08 5.934- 0.14 5.954- 0.ii 5.63+ 0.12 4.72 4. 0.1 4.774.0.1
and muon data
From the comparison of the electromagnetic and muon data we obtain: a) tile integral fluxes above the knee I ( > Nek) and I ( > Npa) are compatible at both atmospheric depths, as expected for a feature occuring at fixed primary energy (see table 1).
~23.2 3,1 3
- - 7 - ~
~l.00
~//~///
1.10
2.9
11)'~
, •
•
e
+ '
÷ ÷ ,
*
T±
+ i *
1.00
Figure 4. Comparison of the slopes of the Ne and Np spectra measured below and above the knee
• 1.10
4.6
'
'
'
'
417
4.11
5.1
5.2
Log(Nta)
Figure 3. Nt, spectra measured in two different intervals of zenith angles.
b) The slopes of muon and electron number spectra below and above the knee, reported in figure 4, show that a value of a ~-- 0.75 (concerning the relation Ne cx N# ~) holds in both angular bins,
254
M. Xglietta et at/Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 251-255
thus indicating that no sudden changes in secondary production rates occurs at primary energies around the knee. c) The location of the knee (Nek, Npk) on the scatter plot of the N p - N e data, at both zenith angles (see figures 5 and 9), is far from both the lower and upper {corresponding to proton and iron primaries respectively) edges of the distriI)ution, indicating the preference for the identificatiou of an "intermediate" primary mass. This is confirmed by simulations performed through the (IORSIKA-HDPM code [6]. Figures 6,7, 8 obta.ined in vertical direction for different primari,,s. and figure 10, obtained around 25 ° for helium primaries, show the expectations for different experimental conditions. Such anlysis leads thus to the preliminary conclusion that at both angles He (or "helium like") nuclei are favoured as bending componellt. The knee energy (E0k), obtained from expression (3) and Ne~. = 10 ~'09, is E0k = 2.7 P e V for A = i, E,~. = 3.4 P e V (A = 4), E0k = 4.1 P e V (.t = 14) , Euk = 4.9 P e V (A = 56). Nek and Ntea. provide thus, as best identification of the kime energy, E0k = (3.5 + 4.0) PeV. On the other side a "normal" atmospheric absorption of EAS at the knee, as resulting from the constancy of the integral fluxes (above both Nek and .'V#,. at different zenith angles), the attennat,ion length Ak of Nel~ and the consistent location of A'~a, and Npk (at different zenith angles), is obtmned.
:m 6 Z
Experimental data 1.00
~5.75 5.5 5.25 S 4.75 4.S 4.2.~ 4
Log(Ne)
Figure 5. Scatter plot of experimental Ne - N # data for vertical direction. The knee location error box is shown.
Proton l.O0
REFERENCES
1. Aglietta M. et al., N.I.M. A, 336, (1993) 310. 2. l(a.mataK, et al., Suppl. Progr. Theor. Phys., (5, (]~5~)93. 3. Aglietta M. et al., N.I.M. A, in press {1998). 4. Aglietta M. et al., Astrop. Phys., in press (1998). 5. EAS-TOP Coll., Proc 24th ICRC, Roma, 2, (1995) 732; EAS-TOP Coll., Proc. 25th I(:FIC, Durban, 4, (1997) 125; Navarra G. et a].. Nuclear Physics B (Proc. Suppl.), 60B, ( 1 ~gs) I05. 6. (:a.pedevielle J.N. et al., Kernforschungszentc~,m Korlshrue, KfK 4998, (1992)
D D
• °, *, ~ , "
• -,~..~ 4,7~ I 4.S
t
[ 5.2
£4
5.6
5.8
6
a.2
~.4
6.6
6.S 7 Log(Ne)
Figure 6. Same as figure 5 for simulated vertical primary protons. Notice the good fit of simulated points to the lower edge of the experimental distribution (as expected for primary protons)
M. Aglietta et aL/Nuclear Physics B (Proc. SuppL) 75,4 (1999) 251-255
Z
.•575
Itelium l.lllkst~:0< I AI5
Experimental data 1 . 1 l l < s e c 0 < l . 1 5
255
J/
5.5
525 I
I
•
5.25
~r• % .
5
4.75
4,75
4.5
4.25
4
Log(Ne)
Log(Ne)
Figure 7. Same as figure 5 for simulated vertical primary helium.
Figure 9. Same as figure 5 for experimental data for 24.60 < 0 < 29.60 .
I r o n I.I)O
Helium I . l O < s e c O < l . 1 5
o
.
-~#" t o " . 5.25
i
, -.~, .t i'..'~: "''" 4.75
25
,, 4.5
4.25
Log(Ne)
Figure 8. Same as figure 5 for simulated vertical primary iron. Notice the good fit of simulated points to the upper edge of the experimental distribution (as expected for primary iron nuclei)
4
Log(Ne)
Figure 10. Same as figure 5 for simulated helium primaries at 24.60 < 0 < 29.6°..