The nature of pulses delayed by 5 mcs in scintillation detectors from showers with the energy above 1E17 eV
aa r X i v : . [ a s t r o - ph . I M ] O c t The nature of pulses delayed by 5 µ s in scintillation detectorsfrom showers with the energy above eV S. P. Knurenko and A. Sabourov
Yu. G. Shafer Institute of cosmophysical research and aeronomy ∗ Here we consider EAS events with energy above 10 eV with recorded pulsesdelayed by τ ≥ µ s in scintillation detectors with different thresholds: 10, 5 and1 . Keywords: extensive air showers, time-related measurements
I. INTRODUCTION
Measurements of time-related parameters of various detectors response have been pro-vided by many experiments. In the field of ultra-high energy they are large arrays, such asVolcano Ranch, Haverah Park, Yakutsk and AGASA. In recent years similar measurementsare provided at giant array Pierre Auger Observatory (PAO) with the use of water-filledCherenkov tanks. All these surveys share one peculiarity: in some large extensive air show-ers (EAS) there are registered pulses from particles delayed relatively to registration of theinitial particle by τ d >> µ s, while according to model calculations all main particles ofEAS disk must arrive compactly within ∼ − µ s. In works [1, 2] we have shown thatin showers with E ≥ eV under certain circumstances in the time-base of scintillationdetector response there pulses are observed delayed by time exceeding 5 µ s. Accordingto [3], one can speak of observing a nucleon component at the observation level at largedistances from shower axis, namely — neutrons. In order to interpret the obtained results ∗ s.p.knurenko@ikfia.ysn.ru we have performed calculations of the neutrons arrival time distribution in showers with E ≥ eV for zenith angles θ from 0 ◦ to 60 ◦ . We used CORSIKA code (version 6.900)compiled with QGSJET01d model [4]. II. THE SETUP FOR TIME-RELATED MEASUREMENTS
A brief description of the setup for registering EAS particles with detectors of differenttypes is given in [1, 2]. The setup includes three measurement points separated bu 300 −
500 m from each other. Synchronization is provided via GPS system. The ADC system,which records the pulses, is controlled by triggering signals from the main array and the smallsetup. After the registering program is started, the ADC continuously converts signals fromdetectors and cyclically records counts into the buffer memory called “pre-history”. I.e. the“pre-history” stores the most recent signals. The interval between closest counts amountsto 4 −
10 ns. When triggering signal arrives, it is passed to all ADC boards, and the otherarea of buffer memory if filled, called the “history”. The “history” stores codes of “coloring”of the master signal — was it from the main array or from the small Cherenkov setup —and signals from detectors operated from calibrating LEDs.
FIG. 1. A diagram showing one of the setups recording the time-base of the signals from EASparticles in scintillation, Cherenkov and muon detectors.
III. MEASUREMENT OF THE TEMPORAL STRUCTURE OF EAS DISKWITH THE USE OF SCINTILLATION DETECTORS WITH DIFFERENTTHRESHOLDS
The analysis of signals implies treatment of single detectors and their combinations.The point is that detectors have different thresholds, 10 .
5, 1 . . A. Distribution of delayed pulses
On the Fig. 2 a temporal distribution is shown of pulses recorded in showers with theenergy above 10 eV. More than 6000 showers were included in the sample, with zenith anglebetween 0 ◦ and 70 ◦ . It is seen that the main fraction of particles arrives within 2 − µ sand only a small number of showers (mainly inclined with energies above 5 × eV) haspulses delayed by more than 3 − µ s. It is not entirely clear what sort of particles the are.For this work we have performed calculation of lateral distribution functions (LDF) forhadrons, muons, electrons and gamma-photons in showers with E ≥ eV with the useof CORSIKA code (v.6900) and QGSJET01d model [4].It is seen from the Fig. 3 that LDF of neutrons is less steep compared to other components,i.e. neutrons can propagate to larger distances from shower axis. At distances further than2 . c oun t r a t e t , ns FIG. 2. Temporal distribution of delayed EAS particles. -2 -1
100 1000 r , / m r , m e mg n FIG. 3. Lateral distributions of hadrons, muons, electrons and gamma-photons in EAS. below the shower maximum and at large distances from the axis it must be slow neutrons oflower energies and products of their moderation can be low-energy electrons. This hypothesiswas confirmed when electrons were registered in scintillation detectors with the threshold
0 5 10 15 20 25 30 35 40 t , m sec p , lg E = 19.0cos q = 0.975 r = 1000 m g e m n FIG. 4. Arrival time distribution of electromagnetic, muon and neutron components of EAS. ǫ thr. ≥ . E = 10 eV at core distance r ≤ µ s neutrons become a dominant component of EAS. IV. THE NATURE OF DELAYED PULSES RECORDED BY SCINTILLATIONDETECTORS
It is seen from Fig. 5-7 that particles delayed by 5 − µ s and 11 . µ s were recorded inshowers. The pulse with maximal delay was registered by detectors with the area s = 1 m and threshold ǫ thr. ≥ . s = 2 m thesignal is extremely weak or is absent completely. It is also should be mentioned that signalsdelayed by significant time were recorded not in every shower with E ≥ eV. Our taskwas to collect as many such showers as possible and to analyze them and clarify the natureof the delayed particles.After the analysis of the selected showers it was discovered, that pulses delayed by signif-icant time are effectively registered in showers with the energy above 10 eV and with the FIG. 5. The shower registered on 23-05-2008. θ = 45 . ◦ , E = 4 . × eV, r obs.lev. = 464 m, τ = 1 . µ s, τ = 11 . µ s. s = 1 m , ǫ thr. = 1 . A µ /A s = 0 .
26. The division value on horizontal axis is 1 µ s.FIG. 6. The shower registered on 22-03-2013. θ = 45 . ◦ , E = 1 . × eV, r obs.lev. = 444 m, τ = 4 . µ s, τ = 8 µ s. s = 1 m , ǫ thr. = 1 . A µ /A s = 0 .
22. The division value on horizontal axis is 1 µ s. TABLE I. Characteristics of individual showers with E ≥ eV registered by Yakutsk arraycompared to predictions by QGSJET01d model. Here τ / — half-width of a pulse, r — axisdistance, ρ s , ρ µ — the density of charged particles and muons correspondingly, η − — theslope of the charged particles LDF at 300 −
600 m, ρ µ /ρ s — muon fraction with ǫ ≥ r = 600 m from the axis. The last column lists the muon fraction according to QGSJET01d modelfor protons and iron nuclei.date, time lg E τ / , θ ◦ φ ◦ r , ρ s , ρ µ , η − ρ µ /ρ s , ρ µ /ρ s ρ µ /ρ s dd-mm-yy ns m 1/m (exp.) (p) (Fe)11-05-07 06:23:31 19.35 270 9.9 109.3 1298 5.3 0.72 -3.32 0.14 0.15 0.2015-05-07 09:34:33 19.29 188 59.6 282.3 1000 2.6 1.44 -2.35 0.55 – –19-10-07 02:04:13 19.33 190 44.1 29.8 992 4.7 – -2.75 – 0.33 0.4125-01-08 20:34:34 19.00 202 36.0 137.9 768 7.52 1.15 -3.01 0.32 – –01-05-08 19:45:14 19.32 – 45.2 228.1 1298 21.3 6.30 -2.75 0.30 – –01-05-08 07:00:34 19.17 – 11.2 258.2 1600 1.93 0.48 -3.28 0.25 – –02-05-08 13:21:08 19.23 – 19.9 330.2 1150 3.83 1.00 -3.18 0.21 – –02-01-08 08:00:24 19.41 – 30.0 1.00 1400 – – -3.12 0.26 – –08-05-08 20:36:02 19.18 – 19.8 110.1 1325 2.23 0.73 -3.23 0.19 – –16-05-07 04:16:04 19.18 170 23.8 183.6 2310 – – -3.61 – – –23-05-08 00:58:58 19.68 165 45.7 315.5 464 96.5 25.1 -2.79 0.26 – –03-01-09 03:49:59 19.44 213 42.4 14.6 944 6.34 1.63 -2.88 0.26 – –22-01-09 19:51:52 19.64 160 34.5 269.3 1112 10.2 1.77 -2.88 0.17 – –22-01-09 08:40:35 19.56 180 42.7 181.9 1167 0.80 3.47 -2.89 0.23 – –21-01-09 06:15:14 19.10 200 16.9 185.4 993 4.30 0.44 -3.28 0.10 – –03-02-09 22:35:17 19.82 90 41.2 67.5 1169 11.6 1.99 -3.28 0.17 – –22-02-09 14:08:40 19.17 230 28.5 297.4 996 4.40 1.30 -3.13 0.30 – –25-02-09 22:35:34 19.04 150 7.7 122.6 1172 2.12 0.60 -3.33 0.28 – –22-03-09 16:21:47 19.06 150 32.4 84.1 549 16.7 5.30 -3.02 0.32 – –10-05-09 06:14:18 19.03 90 4.4 219.4 1383 1.14 0.52 -3.55 0.46 – –22-03-13 16:36:25 19.25 – 45.8 4.3 444 16.2 3.53 – 0.22 – – FIG. 7. Readings from the detector at r obs.lev. = 105 m in the shower from Fig. 6. s = 0 .
25 m , ǫ thr. = 10 . µ s. zenith angle θ ∼ ◦ . Such showers are listed in the Table I. We suppose that under theseconditions the symmetry of shower development is disrupted, unlike in vertical EAS events.In inclined showers, starting from θ ≥ ◦ a significant separation of particles by their typeoccurs. The particle composition in over-axial and under-axial parts shifts towards muoncomponent with some content of nucleon component which is transferred to far peripheryof particle lateral distribution. As it was pointed in the work [3], these particles could below-energy neutrons which, being neutral, don’t interact with the material of detector or itscontainer but produce low-energy electron and gamma-quanta when are moderated. Theseproducts can be effectively registered by scintillation detectors with 1 . . V. CONCLUSION
From the analysis of showers with energy above 10 eV and different zenith angles, thefollowing conclusions can be summarized:a) in vertical showers at the observation level and at moderate distances from the showeraxis, signals in scintillation detectors are generated by electromagnetic and muon com-ponents, the integration of signals from various sources lasts over 2 − µ s;b) single pulses are registered in strongly inclined showers; they are formed strictly bymuon component;c) when a shower with θ ∼ ◦ is registered, in the time-base recorded at r = ∼
400 mthere are pulses delayed by τ ≥ µ s (see Table I). According to simulation, the delayedpulses are probably associated with neutron component of EAS, which produces low-energy electrons studying moderation in the material of a detector and surroundingmatter. This low energy electrons are registered by scintillation detectors with lowthresholds;d) as a consequence, there is a possibility of registering pulses from wandering neutronsnot associated with EAS by widely spaced detectors (by more than 1000 m) and, thus,the increased probability of registering a false shower. ACKNOWLEDGMENTS