Muraki, Y

CERN Document Server

29th International Cosmic Ray Conference Pune (2005) 1, 2528Highest Energy Neutrons Detected by a Solar Neutron Telescope inAssociation with the November 28th 1998, Solar FlareY. Muraki , H. Tsuchiya  , P. Evenson  , K. Fujiki , Y. Matsubara , H. Menjyo ,S. Masuda , T. Sako , K. Watanabe , S. Ohnishi, T. Yuda, Y. Katayose, N. Hotta,T. Sakurai 	 , T. Sakai, Y.H. Tan(a) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan(b) Institute of Physical and Chemical Research, Wako 351-0198, Japan(c) Bartol Research Institute, University of Delaware, Newark, DE19716, USA(d) Inst. for Cosmic Ray Research, Univ. of Tokyo, Kashiwa 272-8582, Japan(e) Faculty of Education, Yokohama National univ., Yokohama 240-8501, Japan(f) Dep. of physics, Utsunomiya univ., Utsunomiya 321-8505, Japan(g) Solar Physics div., National Astronomical Observatory, Mitaka 181-8588, Japan(h) College of Industrial Technologies, Nihon University, Narashino 275-0005, Japan(i) Institute of High Energy Physics, Academia Sinica, Beijing 100039, ChinaPresenter: T. Sako (sako@stelab.nagoya-u.ac.jp), jap-sako-T-abs3-sh11-posterA large flare was observed at N17E32 on the solar surface at 5:31 UT with an intensity of X3.3 as measuredby the GOES satellite. In association with this flare, the solar neutron telescope located in Tibet (  )observed an enhancement from the direction of the Sun. In this paper we discuss the possibility that thisenhancement was caused by high energy neutrons. If confirmed, this is the first evidence of solar neutronswith energy beyond ﬁﬀﬃﬂ , and shows that even during winter in the northern hemisphere we can detect solarneutrons under some conditions if we use the telescope function.1. Neutron event on November 28th, 1998In the conference proceeding of 26th ICRC, the authors reported the arrival of solar neutrons in associationwith a large solar flare on November 28th 1998 [1, 2, 3]. In this paper we report further results of the dataanalysis for this event.On November 28th 1998, an X3.3/3N solar flare was observed at N17E32 in active region 8395. Accordingto GOES spacecraft data, the flare onset time and the maximum time were 4:54 UT and 5:52 UT respectively.A complicated loop structure was observed by the soft X-ray telescope on board Yohkoh. The loop startedgrowing at  5:30 UT with thermal emission from the loop reaching a maximum intensity at  5:50 UT. Yohkohdetected hard X-ray emission ( !"$#&%'%)(ﬂ ) between 5:39 UT and 5:43 UT (the peak time was  5:41 UT).CGRO/BATSE also detected hard X-ray emissions ( "*#+',(ﬂ ) with the peak at 5:40:46 UT. Hard X-rayemission started at 5:31:26 UT with a rapid increase beginning at 5:38 UT. The Nobeyama Radio heliographdetected radio emission at a frequency of .-ﬀ0/21 and "34ﬀ0/21 with the maximum emission observed at 5:39:51UT.We have investigated the time profile obtained from the Tibet solar neutron telescope near the time when thesolar flare occurred. There is no noticeable enhancement of the counting rate obtained by the upper scintillators.In this respect, the data from the upper scintillation counters shows the same behavior as the data from theneutron monitor in Tibet [4].Quite surprisingly, as shown in Figure 1, the higher energy counting rate from the south (the direction of theSun) clearly shows an enhancement in comparison with the (off-source) data obtained from the north. Theneutron telescope detects neutrons higher than %-ﬁ56ﬂ and is also sensitive to photons with energy higherthan ,756ﬂ . The difference in the threshold energy comes from the difference in ionization loss between26 Y. Muraki et al.BeijingTime (hour)12:45:00 13:00:00 13:15:00 13:30:00 13:45:00 14:00:00 14:15:00 14:30:00Counts/ 1.0 min2400245025002550260026502700275005:37:00 UTSouth_3chBeijingTime (hour)12:45:00 13:00:00 13:15:00 13:30:00 13:45:00 14:00:00 14:15:00 14:30:00Counts/ 1.0 min23002350240024502500255026002650270005:37:00 UTNorth_3chTibet 1998/11/28Figure 1. Comparison of the ux of neutrons from the north and the south directions. Three directional data for the northand the south were summed up independently. The vertical dotted line indicates the start time of rapid increase of the hardX-rays. An increase of counting rate can be seen for 5 minutes in the data from the south (solar direction), but no excesshas seen in the data from the north.electrons and protons since low energy protons lose more energy in the scintillator and the wood than electrons.In the first stage of the data analysis, we did not considered a possible contamination of high energy photonsin neutron events since photons are much more strongly absorbed in the atmosphere than are neutrons. (Thisis not true if the energy of the photon is over 4ﬀﬃﬂ , at which point a few particles produced by the cascadeshower arrive at high altitude.) In Figure 2, the counting rate of secondary particles produced by verticallyincident neutrons is shown, based on a Monte Carlo calculation made using GEANT4. In the calculation apower law spectrum of incoming solar neutrons is assumed with 8:9;#2%< ' in the energy range =?>@9A toBﬁﬀﬃﬂ . From Figure 2, the number of neutrons over %-756ﬂ at Tibet is estimated as to be C< D' event perhigh energy neutron (over an area equal to that of the detector) at the top of the detector and C< 3D' event foreach photon with energy higher than ,756ﬂ . This implies that the attenuation of electrons and neutrons isthe same order of magnitude and we could expect to see an effect due to photons when high energy neutronsinteract at the top of the atmosphere.Taking into account the zenith angle of the Sun at the time of the event, the atmospheric mass between theSun and the detector was actually -'* . In this case, the number of counts in the detector that simulateneutrons above %-ﬁ56ﬂ is estimated to be more than < , per neutron incident at the top of the atmosphereand more than < B% per photon incident at the top of the atmosphere. We estimate the sensitivity by themultiplying the attenuation of each component by the detection efficiency. The result, obtained from theHighest Energy Neutrons Detected by a Solar Neutron Telescope 27threshold [MeV]1 10 210 310 410rate (>E)[1/events]-310-210-11012gamma@600g/cm0.045threshold [MeV]1 10 210 310 410rate (>E)[1/events]-310-210-11012Neutron@600g/cm0.05Figure 2. Counting rate of secondary particles produced by neutrons for vertical incidence at the Tibet altitude derived froma Monte Carlo calculation using GEANT4. The vertical dotted line of the left panel corresponds to the threshold energyE4FHGJIKLNMPORQfor neutrons and the vertical dotted line of the right panel represents the energy of photons E4S0GUTBLNMPORQwhich can penetrate to the bottom of the detector.Monte Carlo calculation and analytically based on the data is C< %'V and <W%V for neutrons ( X:%-ﬁ56ﬂ ) andphotons ( XY-%756ﬂ ) respectively. Again we can expect the same order of magnitude number of photons andneutrons at the top of the detector.2. A new interpretation for the November 28th 1998, eventOn February 22nd 1999, Japanese solar physicists who observe the Sun at different wave lengths met at theSolar-Terrestrial Environment Laboratory, Nagoya University to discuss this event.One of the difficulties in understanding this event is that the number of events detected by channel 3 ( =[Z or=[\$X]B%756ﬂ ) of the upper scintillators is the same as that in the down side proportional counters ( =2ZJX%D-75^ﬂ ). If the excess of the event were due to neutrons, according to Monte Carlo calculations, the numberof events detected by upper scintillator should be greater than the number of penetrating events. Trigger pulsesare produced by the coincidence between the upper channels (at least X_3756ﬂ deposited energy is necessaryin the scintillator) and lower four layers of the proportional counters. Inside the four layers of the proportionalcounters are two layers of wood with a total thickness of %ﬁ` . According to the Monte Carlo calculation theratio of the upper channel to coincidence signal is expected as to be a factor of four to ten, depending on theenergy of the neutrons in the range %-ﬃ#&Bﬁ56ﬂ [5].Recently we have concluded that high energy (beyond '7ﬀﬃﬂ ) neutrons were involved in this event, requiringthat photons be included in the calculation. Only the most recent version of GEANT4 can treat neutron cas-28 Y. Muraki et al.cades in the atmosphere correctly down to a few MeV (Koi et al., private communication). We have determinedthe best model using a new version of GEANT4 and compared the results to those of Shibata [6]. In this pro-cess, we discovered that the intensity of photons is expected as to be the same order as that of neutrons. Hencewe have arrived at a new interpretation for this event, namely that the flare must have accelerated protons tobeyond Bﬁﬀﬃﬂ and thereby also produced high energy neutrons. In our simulation we have assumed a powerlaw spectrum of neutrons with index 8@9a#2%b<W' . Such high energy neutrons produce not only charged pionsin the atmosphere but also neutral pions, which immediately decay into two photons and initiate an electro-magnetic cascade. Those photons penetrate the solar neutron telescope through the anti-counter which cannotseparate neutrons from photons. The radiation length of upper scintillator is just one radiation length, so mostphotons are converted into electron positron pairs in the scintillator. These are minimum ionizing particleswhich can penetrate both layers of wood and all four layers of the proportional counters.3. Summary and ConclusionsIn the solar flare on November 28th, 1998, ions were accelerated up to Bﬁﬀﬃﬂ with a time profile similar tothat of X-rays observed by the Yohkoh Hard X-ray Telescope. Those high energy ions collided with the atmo-sphere of the Sun to produce high energy neutrons. When the neutrons entered the atmosphere of the Earth theyproduced charged and neutral pions. The neutral pions decayed immediately and started the electromagneticcascades, with the photon component of the cascade shower able to penetrate the atmosphere to the depth ofthe solar neutron telescope.The detector responds to both photons and neutrons and cannot differentiate between them. Electrons andpositrons produced in the scintillator can penetrate two layers of the wood and trigger four layers of the pro-portional counters. This must be the source of the enhancement observed in the solar direction by the neutrontelescope since the adjacent neutron monitor, which is insensitive to photons, showed only a weak enhance-ment in its counting rate. In future versions of the neutron detector will be possible to separate photons fromneutrons by installing a thin lead layer over the anticoincidence shield.We conclude that in this solar flare, particles were accelerated to at least ﬁﬀﬃﬂ , and probably to over ﬁﬀﬃﬂover a time interval of a few minutes. Confirmation of such high energy solar particles will be one of the mostimportant tasks of the next solar cycle.4. AcknowledgmentsThe authors express sincere thanks to the staffs of the international cosmic ray observatory of Acadmica Sinicain Tibet for operating the solar neutron telescope under well condition. The Japanese scientists acknowledgethe Ministry of Education and Science for Grant-in-Aids to make the solar neutron telescope possible.References[1] Hoshida et al., Proceed. 26th ICRC, 6, 38 (1999).[2] Katayose et al., Proceed. 26th ICRC, 6, 58 (1999).[3] Tsuchiya et al., Proceed. 27th ICRC, 8, 3056 (2001).[4] Kohno et al., Proceed. 26th ICRC, 6, 65 (1999).[5] Tsuchiya, PhD thesis to Nagoya University (2001), Proceed. of the cosmic ray research section of NagoyaUniversity, 42-2, 3-104 (2001).[6] Shibata, JGR, 99, 6651 (1994).

Highest Energy Neutrons Detected by a Solar Neutron Telescope in Association with the November 28th 1998, Solar Flare

Abstract

Similar works

Full text

Available Versions

CERN Document Server