Neutron ‘thunder’ accompanying an extensive air shower Erlykin A.D. P.N.Lebedev Physical Institute, Moscow, Russia

PeV energy region Findings ( Antonova V.A. et al., 2002, J.Phys.G, 28, 251 ): 1. There are a lot of neutrons delayed by hundreds of ms after the main shower front (‘the neutron thunder’). Their temporal distribution is different from the standard one in neutron monitors. 2. Distortions of the temporal distributions seem to have a threshold and begin in the PeV (‘knee’) region. 3. Multiplicity of such neutrons is very high compared with EAS model expectations. 4. These neutrons are concentrated in the EAS core region. 5. Delayed neutrons are accompanied by delayed gamma- quanta and electrons.

Structure of the neutron monitor

Delay  s I,  s -1 0 Neutron Monitor counting rate, I

Delay  s I,  s -1 Neutron monitor counting rate, II

Delay  s I,  s -1 Neutron monitor counting rate, III

Saturation level I max ≈ N/τ ≈ 6/2μs = 3μs -1

Threshold of the distortions Standard temporal distribution in the neutron monitor: I max  N/  Threshold multiplicity :

 s Distribution of the attenuation coefficient  I ~ exp(- )

Layout of monitor and e  modules Concentration of neutrons in the EAS core region

Simulations Primary proton E 0 = 1 PeV Zenith angle Θ = 0 o Observation level: 3340 m a.s.l. ( 687 gcm -2 ) Interaction model QGSJET-II + Gheisha 2002d Electromagnetic component: NKG Energy thresholds: 50 MeV for h,   MeV for e, 

Lateral distribution of protons, pions and neutrons Neutrons are the most abundant in total and at large distances from the core among all EAS hadrons

Energy spectrum of neutrons LogE, GeV total inside monitor Most of neutrons have low energies, but in the core there are TeV and tens TeV neutrons. The energy spectrum in the whole shower~E -2, In the core ~E -1.

Hadrons in EAS and in the monitor Total in EAS Monitor at R = 0 Monitor at R=4m NhNh EhEh NnNn EnEn , GeV

Application of the calibration results M = 35 E h 0.5 applied for E h = 40 TeV gives M ≈ 0.7 ×10 4

Electron counting rate ee n n Delayed gamma-quanta and electrons

Conclusions for PeV energies 1. The bulk of observed neutrons are not born in the shower. They are produced inside the neutron monitor. 2. Their temporal distribution is the standard monitor distribution, distorted by the saturation of the counting rate at high neutron fluxes. 3. The distortions start at the threshold when the counting rate reaches the saturation level. 4. The very high multiplicity of produced neutrons and their concentration near the EAS core are due to the narrow lateral distribution of EAS hadrons and their energy around the core. 5. Delayed gamma-quanta and electrons have also a secondary origin – they are produced by delayed neutrons in the detector environment.

This interpretation of ‘the neutron thunder’, based on the analysis of the Chubenko et al. experiment at Tien-Shan and Monte Carlo simulations, coincides with that of Stenkin et al., based on their own experimental data in Mexico City and Baksan.

EeV energy region Problems: 1. Delayed signals in large EAS were observed long ago ( scintillators, neutron monitors ) and their possible origin from neutrons was discussed by Greisen K., Linsley J., Watson A. et al. 2. Previous simulations showed that low energy neutrons spread out up to km-long distances from the EAS core. Could these neutrons contribute to the signals in water cherenkov detectors ?

Simulations Primary proton E 0 = 1 EeV Zenith angle Θ = 0 o Observation level: 1400 m a.s.l. ( 875 gcm -2 ) Interaction model QGSJET-II + Gheisha 2002d Electromagnetic component: EGS4, thinning: Energy thresholds: 50 MeV for h,   MeV for e, 

Lateral distribution of e ,  and n particles energy Neutrons can contribute up to 10% of the signal at km-long distances from the EAS core

 -R T-R E-RT-E Correlations between different characteristics of EAS neutrons highest energy neutrons Interestingly EAS neutrons appear as two well separated groups of low (recoil) and high energy (secondary) neutrons

Temporal distribution of e     and n at different EAS core distances Time,  s or ms R<10m R=100m R=1000m After 5ms at the core distance of 1km neutrons are the dominant component of the shower

Conclusions for EeV energies 1. Neutrons are a dominant EAS component at Km-long distances from the core and at  s-delays after the main shower front. These distances and times are typical for Pierre Auger experiment. 2. However, neutrons are neutral and at these distances – non-relativistic, therefore they cannot give signals in the water cherenkov detectors. 3. Experiments in PeV region showed that neutrons are accompanied by gamma-quanta and electrons, which in principle could give cherenkov light. Sensitivity of water cherenkov detectors to neutrons should be tested.

General remarks 1.The discovery of ‘the neutron thunder’ by Chubenko A.P. with his colleagues is an outstanding achievement. If our interpretation of it is correct, this phenomenon extends our understanding of the EAS development and its interaction with detectors and their environment. The study of EAS neutrons is complementary to the study of other EAS components by Geiger counters, scintillators, ionization calorimeters, gamma-telescopes, X-ray films etc. and all together they could give the full picture of atmospheric shower.

2.The phenomenon of ‘the neutron thunder’ emphasizes the role of detectors and their environment in the observed signals. The surrounding materials containing water could increase the neutron scattering, moderation, and production of secondary gamma-quanta and electrons. In this aspect, mountain studies can be particularly vulnerable. Mind that the EAS core at mountains as the neutron generator carries much bigger energy than at sea level. As for the environment, a good part of the year mountain stations ( viz. Tien-Shan, Aragats, Antarctic etc.) are covered by snow sometimes of meters thick. As for the Tien-Shan station, there might be an additional factor emphasizing the role of neutrons – its ground is a permafrost containing a good fraction of ice. There might be effects at shallow depths underground connected with the propagation of neutrons produced when the EAS core strikes the ground.

EAS-TOP in winter

Aragats in the spring

Aragats, May 2006

3. The water cherenkov detectors are particularly worth of attention. First of all water as hydrogen containing stuff is a moderator like a polyethilene of the neutron monitor. Secondly it might be sensitive to secondary electrons and gamma-quanta, produced by neutrons interacting with water ( mind the discovery of cherenkov radiation itself ). Since water and ice cherenkov detectors are wide spread all over the world ( MILAGRO, NEVOD, ICE-TOP ) and in particular used in Pierre Auger Observatory, the contribution of neutrons at large core distances and  s delays might be noticable and needs a special study. 4. The same remark is relevant for large EAS, based on hydrogen containing plastic scintillators ( Yakutsk, Telescope Array ).

In any case the phenomenon of ‘neutron thunder’ complements our knowledge of the EAS development and it is certainly worth of further experimental and theoretical study