1. Detection of Solar Neutrons and Protons by the ground level detectors
Y. Muraki, Y. Matsubara, S. Masuda, T. Sako
STE-laboratory, Nagoya University, Nagoya, Japan
E.O. Flueckiger, R. Buetikofer
Physikaliches Institute, University of Bern, Bern, Switzerland
A. Chilingalian and G. Hovsepyan
Yerevan Physics Institute, Yerevan, Armenia
F. Kakimoto, Y. Tsunesada, H. Tokuno
Tokyo Institute of Technology, Tokyo, Japan
A. Veralde
UMSA, Lapaz, Bolivia
T. Sakai
College of Industrial Technology, Nihon University, Narashino, Japan
@ 20th ECRC Sep. 8th 2006 at Lisbon

2. contents
1. Experimental purpose--- Why do we study solar neutrons?
2. The detectors and the Criterion of
the data selection---list of new events
3. An introduction of an interesting event
observed in April 15th, 2001.
4. Tips for the solar cycle 24

3. 1. The experimental Purposes
2. Scientific purposes
1. The experimental Purposes
Sun
Earth
Neutron
Proton, Ion
Magnetic field
Physics aim is to confirm particle acceleration model
at the solar surface. .
When, How?
Positive astronomy
= Experimental Astronomy
However observation of protons does not give us any message about it. Protons are usually coming a few hours later from the flare. This is the reason why we use neutron channel.

4. 2. The detectors and Data Analysis -- SONTEL in European side --
The detector at Gornergrat 　　　　The detector at Aragatz

5. Criterion of the data selection
1. We searched the excesses of events during solar flare
The intensity of flares detected by the GOES X-ray satellite
must be stronger than class >X1.0 .
~100 events were observed during 11years of the solar cycle 23
* at the emission time of hard X-ray ( about 3 minutes) or
* during GOES start and Maximum time ( about 20 minutes)
2. The atmospheric depth must be less than 850 gr/cm2
3. Local time of the detector must be within ±2 hours around the noon
We have found 4 new events by the Gornergrat detector and
1 new event by Aragatz detector since the report by Flueckiger et al
in the 27th ICRC (Hamburg), Vol. 8 (2001) 3044.

6. List of solar neutron events observed by the Gornergrat detector in in the solar cycle 23
Date Begin Max 1/2 Max class position region Gornergrat(g/cm2) σ comments
X S16E )
X N17E ●
X N22W no signal
X N20W ●
X N17W ●
X Y 0M ●
X S19W ●
X S20W )Bolivia event ●
X S16E )Tibet event (Sako)
X S15W )
X S10E )
X Y 0M no signal
X Y 0M )
● mark represents that those events were reported in Proceed. 27th ICRC (Hamburg), 8 (2001) 3044.
0) The telescope function has been introduced in the SONTEL after September 27th, 1999.
1) Only at the highest channel, the excess is seen just before the GLE at 14:00UT. There is also
minor excess at the flare start time 13:19UT in all channels. Only neutron monitor has shown
an excess. A good record of solar activity was observed by Yokoh SXT and HXT imagers.
2) At the south channel and >160 MeV with anti, two peaks are seen at the flare start time (9:32UT)
and at 9:54UT. A clear signal was detected at Tibet at 9:32UT . However no strong HXT signal
was observed. The event was pointed out first by Sako.
3) At the south channel, signal is observed at 12:04UT. The flare start time was 11:51UT.
4) Signals are observed in three channels >40 MeV, >80 MeV and >120 MeV with anti.
5) Clear excess is seen at 10 UT for >120 MeV and > 160 MeV with anti.

7. List of solar neutron events observed by the Mt Aragatz detector in Armenia
Date Begin Max 1/2Max class position region Armenia (g/cm2) σ comments
X S16E
X S11W data handling error
X N33E data handling error
X N16W
X N17E ●
X N22W ●
X N17W ●
X S23W ) ●
X S19W ●
X N19W )
X S16E )
X S10E ? noise?
X N12E (2.6) fluctuation?
X Y 0M
X S12W (2.8) fluctuation?
● mark represents that those data were reported in Proceed. 27th ICRC (Hamburg), 8 (2001) 3044.
1) Signals in association with this event are recorded also in Tibet and Norikura detectors
at 05:06 UT and 5:15 UT.
2) Signal was not observed also in Tibet detector.
3) Neutrons were observed by the neutron monitor located at Tsumeb and Hermanus at 11:06UT.
By the neutron telescopes at Aragatz and Gornergrat, neutrons were observed at 9:51 UT the flare start time.

9. Probability of chance coincidence
Operation : T= 7 years gate width
Coincidence with hard X-ray flare : typical 3 min.
Coincidence with soft X-ray flare : typical 20 min.
Intensity of X-rays > X1.0
Neutron excess frequency probability
(hard X) (soft X)
3.0 σ
3.5 σ
4.0 σ
4.5 σ

10. 3. A detail analysis of an interesting event
Date Begin Max 1/2 Max class position region Gornergrat(g/cm2) σ comments
X S16E )
X N17E ●
X N22W no signal
X N20W ●
X N17W ●
X Y 0M ●
X S19W ●
X S20W )Bolivia event ●
X S16E )Tibet event (Sako)
X S15W )
X S10E )
X Y 0M no signal
X Y 0M )
● mark represents that those events were reported in Proceed. 27th ICRC (Hamburg), 8 (2001) 3044.
0) The telescope function has been introduced in the SONTEL after September 27th, 1999.
1) Only at the highest channel, the excess is seen just before the GLE at 14:00UT. There is also
minor excess at the flare start time 13:19UT in all channels. Only neutron monitor has shown
an excess. There left a good quality of the record of solar activity by Yokoh SXT and HXT.

11. The solar position of 2001.4.15 flare S20W85 GOES X-ray level X14
13:19 start
13:50 maximum
13:55 end
Yokoh HXT

12. Top two panels Gornergrat > 40 MeV >160 MeV GLE is seen from 14:00UT Bottom 3 panels Chacaltaya at 13:48UT Neutron signal is seen

13. Bolivia Chacaltaya 5,250m 5 min value (720g) 13:48 start?

14. Bolivia Chacaltaya 5,250m 1 min value

15. Swiss
Gornergrat
3,150m
1min value
Peak at
13:57UT
(800g)

16. Armenia
Mt. Aragatz
3250m
1 min value
Peak at
13:57UT
(1000g)

17. The April 15th 2005 event　（１）
In comparison with a large solar flare observed in
September 7th 2005 (X17), the signal of neutrons was
15 times weaker than it. However the statistical of April 15th
event, the significance was as large as 8 σ.
The detection efficiency of the solar neutron detector
at Bolivia (made of the plastic scintillator) is calibrated by
the September 7th 2005 event They turn out as
17.8% , 9.5% and 3.2 % for >40MeV, >80MeV and >160MeV
respectively of the total detection efficiency of the neutron monitor.
Therefore, the intensity of neutrons to the new neutron
detector is just one sigma (< 1 σ) : level of the fluctuation.
That is the reason why we could not observe any enhancement
of the signals in the lower energy channel, but…

18. The April 15th 2005 event （2） - - an interpretation --
The enhancement at 13:57 UT must made by the
muons. High energy protons arrived at the top of
the atmosphere. Those Solar Energetic Particles
made nuclear interactions and pion decayed muons
can penetrate the atmosphere and arrived at the two
detectors located at Gornergrat and Aragatz.
The delay time from the flare (13:45UT) is estimated
about 12 minutes. We have observed many such
events until now.

19. Oulu Neutron Monitor and GLE start time is 14:00 UT

20. GOES 8 X-ray data 13:19 start 13:23 C4 13:40 M1 13:43 rapid increase
13:50 X14 maximum

22. Summary of the Event
1) In the solar flare of April 15th 2001 (X14), solar neutrons were detected by the Chacaltaya neutron monitor (8).
2) Those high energy neutrons were produced at the
rapid increase time of the GOES X-ray data when
hard X-rays were simultaneously observed.
3) The enhancements detected by two different detectors
located at Gornergrat and Mt. Aragatz indicate
that high energy protons were coming after 12 minutes
later (13:57UT) and produced pions [They arrived at
the Earth 12minutes later than the highest energy neutrons
(13:45UT)], while low energy GLE starts from 14:00 UT.

23. 4. Tip for the solar cycle 24
1) It is important to make particle identification
more rigidly and strengthen the current detectors
especially on the anti gondola. It is very important
to reject muons and electrons that arrived in the
central scintillator. The power of anti is ~ 85%.
So it should be 99% in the solar cycle 24.
Then we can select weak signals of solar neutrons
by the SONTEL to the level of X1 flare.

25. Bolivia
Chacaltaya
5,250m
3 min value