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High energy gamma-families with halo and mass composition of primary cosmic rays in energy region above 10 PeV
PROCEEDINGS SUPPLEMENTS Nuclear
ELSEVIER
Physics
B (Proc.
Suppl.)
97 (2001)
113-115
www.elsevier.nlbcatc/npe
High energy gamma-families with halo and mass composition of primary cosmic rays in energy region above 10 PeV A.S.Borisov”, Z.M.Gusevaa, S.A.Slavatinskya aLebedev Leninskii
S.A.Karpovaa,
J.Kempab,
A.Krysb,
V.M.Maximenkoa,
V.S.Puchkova,
Physical Institute pr. 53, Moscow 117924, Russia
bLodz University 90-236 Lodz ZFWE,
Pomorska
149/153,
Poland
The experimental spectrum of halo areas for gamma-families with c E-, 1 500 TeV is obtained and compared with simulations in quark-gluon string model. The comparison suggests a N 2 times decrease of the proton fraction in primary cosmic ray (PCR) composition in the energy region 1016 + 10” eV. However at an energy above 10” eV, the experimental flux of halo events with area exceeding 300 + 400 mm2 is 5 + 10 times larger than the simulated one. Such a difference may be explained by the assumption that in this region the fraction of protons increases as much as 3 times, while the slope of the spectrum decreases (y = 2.6 + 2.7).
1. Introduction
The aim of the present work is the further analysis of Pamir experimental data with respect to a possible decrease of the light components of PCR at the energy region 10 + 100 PeV [1,2]. The Pamir experimental data on spectrum of halo areas are compared with calculations based on the quark-&ion string model MQ [3], which reproduces the main features of gamma-hadron families with C ET = 100 i 400 TeV ( that corresponds to Ec = 1015 + 10’s eV) under the assumption that mass composition of PCR is normal ( p+a: 50% and subiron+iron: 20% ). This nuclear composition is near to the extrapolation of direct measurements from the lower energy region. The halo is a large diffuse dark spot, recorded in X-ray emulsion chamber along with a highenergy gamma-hadron family. The area of a halo, i.e., the region where optical density of darkness exceeds overall background of the X-ray film, amounts to several centimeters squared. The halo formation accompanying superfamilies with C E7 2 500 TeV is connected usually with a narrow bundle of high energy particles from the atmosphere. A halo may arise from a
pure electromagnetic cascade in the atmosphere initiated by a neutral pion or may be related to a nuclear interaction produced not so far from the observation level if the created flow of energy density is > 20 TeV.mmm2. Therefore halo events are most efficiently produced by primary protons due to their high penetrating power.
2. Experimental
Data
The experimental data used in the present analysis were recorded by a thin X-ray emulsion chamber with lead absorber (P-block) exposed at the Pamir’s (4370 m as1 or 600 gcmM2) either as the separate unit or as a part of thick hadron chamber with carbon absorber. The total thickness of the F-block is 6 cm of Pb with 2 or 3 recording layers of X-ray film. The halo measurements were performed employing an automatic scanning densitometer. Densitograms of halo events were measured on X-ray films, exposed at the depth 9 - 11 C.U. (4 t 6 cm of Pb). The following phenomenological quantitative criterion of halo creation is accepted : the area S bounded by the isodense with an optical darkness density of D = 0.5 at a depth of 10 C.U. for
0920-5632/01/% - see front matter 0 2001 Elsevier Science B.V: All rights reserved. PII SO920-5632(01)01207-S
114
A.S. Borisov
one side emulsion
Physics B (Prvc. Suppl.)
97 (2001) 1 I3-115
plate is
dS 14 mm2
S=
et al. /Nuclear
I
(1)
D20.5 or,
for multi-core
“halo”, dS 2 4 mm2,
S’i 1 1 mm2 .(2)
The halo darkness D = 0.5 corresponds to the particle density 0.04 /lm-2. The total exposure of P-block chambers and the total number of superfamilies with halo areas larger than 4 mm2 are 2600 m2.year and 64, respectively. Besides, 8 events with halo area S > 300 mm2 obtained during total exposure ST = 10000 m2.year were used in the present analysis. 3. Model Calculations Model calculations of the halo spectrum were carried out for four mass compositions of PCR from normal to heavy one, for the primary particles in the energy region 2 . 1015 -z- 5 . 10” eV. Monte-Carlo sampling procedure [4] taking into account the isotropic angular distribution of the primary particles was used. The calculations show that the events with halo area 4 + 100 mm2 are produced by the primary particles in the energy region 2 . 1015 4 1017 eV and superfamilies with halo size larger than 300 mm2 correspond to the highest energy region (> 1017 eV). 4. Results Figure 1 presents the experimental integral spectrum of halo areas along with calculated spectra for two mass compositions specified as normal ( MQl ) = (p - 40%, LY- lo%, M+H - 30%, VH - 20%) and heavy ( MQ2 ) = (p - lo%, Q - lo%, MS-H - 20%, VH - 60%). As is seen from Fig. 1, the gradual decreasing of the fraction of protons and o-particles in the energy range 2.10i5t 1017 eV will make it possible to fit experimental data with simulated ones in the halo area range 4 - 100 mm2. The best fit is attained when the fraction of p+a is M 30%. But at the halo area larger than 300 mm2, the integral spectrum of simulated halo
St
mm* *2
Figure 1. Intensity of Halo events geometric halo size S (D 2 0.5).
in terms
of
areas S strongly deviates from the experimental values. The difference between calculated data and the experimental values at S > 1000 mm2 is about 20 times. Up to now 5 events are known in the world statistics with visible energy (E,, Eh,Ehalo) 2 1016 eV and halo areas S> 1000 mm2 (total effective exposure for the Pamir level is about 13000 m2.year). The primary energies of these events are estimated as lOi eV and more. In order to analyse which primary particles are responsible for the halo with areas greater than 300 mm2, the probability of halo creation was calculated for protons, CNO nuclei and Fe at the energy E = 10” eV. As is seen in Fig.2, protons and may be light nuclei can produce halo areas S 2 1000 mm2 but the probability is not more than 5 + 10%. Nearly the same result was
A.S. Borisov
et al. /Nuclear
Physics B (Proc. Suppl.)
0.35
&
0.30
Y
c 0.25 \ -
M
HP *
CNO
q
FE
0.20 •(
0. IL *
0. IO.. 0.05~-
.
Y
*
CLOO-, ,),, 1
H... I . . ,,,, n
10’
102
*
CI.. *
.
q
*
. .
I ,,,, f
.
103
S, mm**2
115
ier. In any case the assumption of essential softening of the secondary particle spectra in fragmentation region [7] does not favor the halo formation. The third hypothesis of halo creation is connected with long-flying particles which interact or decay not far from the observation level. This idea can explain many unusual phenomena observed in cosmic rays at the energy around 1016 eV (for example, the phenomenon of coplanar emission of high energy secondary particles). This work was supported by Russian Foundation for Basic Research, project N 00 - 02 -17851.
9.40 o-l -lJ \
97 (2001) 113-115
... LO&
REFERENCES 1. 2.
Figure 2. The probability of the halo formation for different primary particles (p, CNO, Fe) at the energy of 10” TeV.
3. 4.
obtained in calculations [5] based on CORSIKA code, employing the QGSJet model for nuclear interaction. The deviation of our experimental results from calculated ones strongly suggests the change of primary mass composition around 1017 eV. An agreement between the experiment and the simulations may be attained if the contribution of the proton primaries is close to 100% and the real air-shower intensity around lo’* eV is 2 + 3 times higher than generally accepted [6]. But this conclusion is valid if the efficiency of the halo formation depends only on primary energy and mass composition. An alternative interpretation is also possible, namely that nuclear composition remains unchanged but the properties of nuclear interaction at this energy change. The inelasticity coefficient in MQ-model strongly increases up to 0.8 at 1017 eV. It seems that the decreasing of this value may reduce the discrepancy between the experiment and calculations in the energy region lOi i 1018 eV, but in the energy region 1016 + 1017 eV the disagreement will increase and, therefore, primary mass composition will be heav-
5.
6. 7.
Borisov A.S. et al., Proc. 24th Int. Cosmic Ray Conference, Roma, V. 1, (1995), p. 182. Borisov A.S., Guseva Z.M., Karpova S.A. et al., Bull.Russ.Acad.Sci. (Physics), V. 61,N. 3, p. 449. Dunaevsky A.M. et aZ., Proc. 5th ISVHECRI, Lodz, (1988) p. 59. Borisov A.S., Guseva Z.M., Karpova S.A. et al., Nuclear Physics B, (Proc. Suppl.), 52B (1997) p. 185. Puchkov V.S., Guseva Z.M., Karpova S.A. et al,Proc. 26th Int.Cosmic Ray Conference, Salt Lake City, USA, (1999), V. 1, p. 104. Nesterova N.M. et al., Proc. 24th Int. Cosmic Ray Conference, Roma, (1995), V. 2, p. 748. Nikolsky S.I., Proc. 26th Int. Cosmic Ray Conference, Salt Lake City, USA, (1999), V. 1, p. 159.
ELSEVIER
Physics
B (Proc.
Suppl.)
97 (2001)
113-115
www.elsevier.nlbcatc/npe
High energy gamma-families with halo and mass composition of primary cosmic rays in energy region above 10 PeV A.S.Borisov”, Z.M.Gusevaa, S.A.Slavatinskya aLebedev Leninskii
S.A.Karpovaa,
J.Kempab,
A.Krysb,
V.M.Maximenkoa,
V.S.Puchkova,
Physical Institute pr. 53, Moscow 117924, Russia
bLodz University 90-236 Lodz ZFWE,
Pomorska
149/153,
Poland
The experimental spectrum of halo areas for gamma-families with c E-, 1 500 TeV is obtained and compared with simulations in quark-gluon string model. The comparison suggests a N 2 times decrease of the proton fraction in primary cosmic ray (PCR) composition in the energy region 1016 + 10” eV. However at an energy above 10” eV, the experimental flux of halo events with area exceeding 300 + 400 mm2 is 5 + 10 times larger than the simulated one. Such a difference may be explained by the assumption that in this region the fraction of protons increases as much as 3 times, while the slope of the spectrum decreases (y = 2.6 + 2.7).
1. Introduction
The aim of the present work is the further analysis of Pamir experimental data with respect to a possible decrease of the light components of PCR at the energy region 10 + 100 PeV [1,2]. The Pamir experimental data on spectrum of halo areas are compared with calculations based on the quark-&ion string model MQ [3], which reproduces the main features of gamma-hadron families with C ET = 100 i 400 TeV ( that corresponds to Ec = 1015 + 10’s eV) under the assumption that mass composition of PCR is normal ( p+a: 50% and subiron+iron: 20% ). This nuclear composition is near to the extrapolation of direct measurements from the lower energy region. The halo is a large diffuse dark spot, recorded in X-ray emulsion chamber along with a highenergy gamma-hadron family. The area of a halo, i.e., the region where optical density of darkness exceeds overall background of the X-ray film, amounts to several centimeters squared. The halo formation accompanying superfamilies with C E7 2 500 TeV is connected usually with a narrow bundle of high energy particles from the atmosphere. A halo may arise from a
pure electromagnetic cascade in the atmosphere initiated by a neutral pion or may be related to a nuclear interaction produced not so far from the observation level if the created flow of energy density is > 20 TeV.mmm2. Therefore halo events are most efficiently produced by primary protons due to their high penetrating power.
2. Experimental
Data
The experimental data used in the present analysis were recorded by a thin X-ray emulsion chamber with lead absorber (P-block) exposed at the Pamir’s (4370 m as1 or 600 gcmM2) either as the separate unit or as a part of thick hadron chamber with carbon absorber. The total thickness of the F-block is 6 cm of Pb with 2 or 3 recording layers of X-ray film. The halo measurements were performed employing an automatic scanning densitometer. Densitograms of halo events were measured on X-ray films, exposed at the depth 9 - 11 C.U. (4 t 6 cm of Pb). The following phenomenological quantitative criterion of halo creation is accepted : the area S bounded by the isodense with an optical darkness density of D = 0.5 at a depth of 10 C.U. for
0920-5632/01/% - see front matter 0 2001 Elsevier Science B.V: All rights reserved. PII SO920-5632(01)01207-S
114
A.S. Borisov
one side emulsion
Physics B (Prvc. Suppl.)
97 (2001) 1 I3-115
plate is
dS 14 mm2
S=
et al. /Nuclear
I
(1)
D20.5 or,
for multi-core
“halo”, dS 2 4 mm2,
S’i 1 1 mm2 .(2)
The halo darkness D = 0.5 corresponds to the particle density 0.04 /lm-2. The total exposure of P-block chambers and the total number of superfamilies with halo areas larger than 4 mm2 are 2600 m2.year and 64, respectively. Besides, 8 events with halo area S > 300 mm2 obtained during total exposure ST = 10000 m2.year were used in the present analysis. 3. Model Calculations Model calculations of the halo spectrum were carried out for four mass compositions of PCR from normal to heavy one, for the primary particles in the energy region 2 . 1015 -z- 5 . 10” eV. Monte-Carlo sampling procedure [4] taking into account the isotropic angular distribution of the primary particles was used. The calculations show that the events with halo area 4 + 100 mm2 are produced by the primary particles in the energy region 2 . 1015 4 1017 eV and superfamilies with halo size larger than 300 mm2 correspond to the highest energy region (> 1017 eV). 4. Results Figure 1 presents the experimental integral spectrum of halo areas along with calculated spectra for two mass compositions specified as normal ( MQl ) = (p - 40%, LY- lo%, M+H - 30%, VH - 20%) and heavy ( MQ2 ) = (p - lo%, Q - lo%, MS-H - 20%, VH - 60%). As is seen from Fig. 1, the gradual decreasing of the fraction of protons and o-particles in the energy range 2.10i5t 1017 eV will make it possible to fit experimental data with simulated ones in the halo area range 4 - 100 mm2. The best fit is attained when the fraction of p+a is M 30%. But at the halo area larger than 300 mm2, the integral spectrum of simulated halo
St
mm* *2
Figure 1. Intensity of Halo events geometric halo size S (D 2 0.5).
in terms
of
areas S strongly deviates from the experimental values. The difference between calculated data and the experimental values at S > 1000 mm2 is about 20 times. Up to now 5 events are known in the world statistics with visible energy (E,, Eh,Ehalo) 2 1016 eV and halo areas S> 1000 mm2 (total effective exposure for the Pamir level is about 13000 m2.year). The primary energies of these events are estimated as lOi eV and more. In order to analyse which primary particles are responsible for the halo with areas greater than 300 mm2, the probability of halo creation was calculated for protons, CNO nuclei and Fe at the energy E = 10” eV. As is seen in Fig.2, protons and may be light nuclei can produce halo areas S 2 1000 mm2 but the probability is not more than 5 + 10%. Nearly the same result was
A.S. Borisov
et al. /Nuclear
Physics B (Proc. Suppl.)
0.35
&
0.30
Y
c 0.25 \ -
M
HP *
CNO
q
FE
0.20 •(
0. IL *
0. IO.. 0.05~-
.
Y
*
CLOO-, ,),, 1
H... I . . ,,,, n
10’
102
*
CI.. *
.
q
*
. .
I ,,,, f
.
103
S, mm**2
115
ier. In any case the assumption of essential softening of the secondary particle spectra in fragmentation region [7] does not favor the halo formation. The third hypothesis of halo creation is connected with long-flying particles which interact or decay not far from the observation level. This idea can explain many unusual phenomena observed in cosmic rays at the energy around 1016 eV (for example, the phenomenon of coplanar emission of high energy secondary particles). This work was supported by Russian Foundation for Basic Research, project N 00 - 02 -17851.
9.40 o-l -lJ \
97 (2001) 113-115
... LO&
REFERENCES 1. 2.
Figure 2. The probability of the halo formation for different primary particles (p, CNO, Fe) at the energy of 10” TeV.
3. 4.
obtained in calculations [5] based on CORSIKA code, employing the QGSJet model for nuclear interaction. The deviation of our experimental results from calculated ones strongly suggests the change of primary mass composition around 1017 eV. An agreement between the experiment and the simulations may be attained if the contribution of the proton primaries is close to 100% and the real air-shower intensity around lo’* eV is 2 + 3 times higher than generally accepted [6]. But this conclusion is valid if the efficiency of the halo formation depends only on primary energy and mass composition. An alternative interpretation is also possible, namely that nuclear composition remains unchanged but the properties of nuclear interaction at this energy change. The inelasticity coefficient in MQ-model strongly increases up to 0.8 at 1017 eV. It seems that the decreasing of this value may reduce the discrepancy between the experiment and calculations in the energy region lOi i 1018 eV, but in the energy region 1016 + 1017 eV the disagreement will increase and, therefore, primary mass composition will be heav-
5.
6. 7.
Borisov A.S. et al., Proc. 24th Int. Cosmic Ray Conference, Roma, V. 1, (1995), p. 182. Borisov A.S., Guseva Z.M., Karpova S.A. et al., Bull.Russ.Acad.Sci. (Physics), V. 61,N. 3, p. 449. Dunaevsky A.M. et aZ., Proc. 5th ISVHECRI, Lodz, (1988) p. 59. Borisov A.S., Guseva Z.M., Karpova S.A. et al., Nuclear Physics B, (Proc. Suppl.), 52B (1997) p. 185. Puchkov V.S., Guseva Z.M., Karpova S.A. et al,Proc. 26th Int.Cosmic Ray Conference, Salt Lake City, USA, (1999), V. 1, p. 104. Nesterova N.M. et al., Proc. 24th Int. Cosmic Ray Conference, Roma, (1995), V. 2, p. 748. Nikolsky S.I., Proc. 26th Int. Cosmic Ray Conference, Salt Lake City, USA, (1999), V. 1, p. 159.