Performance of the SciBar cosmic ray telescope (SciCRT) toward the detection of high-energy solar neutrons in solar cycle 24
- Yoshinori Sasai1Email author,
- Yuya Nagai1,
- Yoshitaka Itow1,
- Yutaka Matsubara1,
- Takashi Sako1,
- Diego Lopez1,
- Tsukasa Itow1,
- Kazuoki Munakata2,
- Chihiro Kato2,
- Masayoshi Kozai2,
- Takahiro Miyazaki2,
- Shoichi Shibata3,
- Akitoshi Oshima3,
- Hiroshi Kojima4,
- Harufumi Tsuchiya5,
- Kyoko Watanabe6,
- Tatsumi Koi7,
- Jose Francisco Valdés-Galicia8,
- Luis Xavier González8,
- Ernesto Ortiz8,
- Octavio Musalem8,
- Alejandro Hurtado8,
- Rocio Garcia8 and
- Marcos Anzorena8
© Sasai et al.; licensee Springer. 2014
Received: 2 May 2014
Accepted: 17 September 2014
Published: 30 September 2014
We plan to observe solar neutrons at Mt. Sierra Negra (4,600 m above sea level) in Mexico using the SciBar detector. This project is named the SciBar Cosmic Ray Telescope (SciCRT). The main aims of the SciCRT project are to observe solar neutrons to study the mechanism of ion acceleration on the surface of the sun and to monitor the anisotropy of galactic cosmic-ray muons. The SciBar detector, a fully active tracker, is composed of 14,848 scintillator bars, whose dimension is 300 cm × 2.5 cm × 1.3 cm. The structure of the detector enables us to obtain the particle trajectory and its total deposited energy. This information is useful for the energy reconstruction of primary neutrons and particle identification. The total volume of the detector is 3.0 m × 3.0 m × 1.7 m. Since this volume is much larger than the solar neutron telescope (SNT) in Mexico, the detection efficiency of the SciCRT for neutrons is highly enhanced. We performed the calibration of the SciCRT at Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) located at 2,150 m above sea level in Mexico in 2012. We installed the SciCRT at Mt. Sierra Negra in April 2013 and calibrated this detector in May and August 2013. We started continuous observation in March 2014. In this paper, we report the detector performance as a solar neutron telescope and the current status of the SciCRT.
The particle acceleration mechanism on the solar surface has been studied through multi-wavelength observations. High-energy particles are produced related to solar flares. There are two kinds of particles accelerated at the time when a solar flare occurs, i.e., electrons and ions. Information of electron acceleration is obtained by hard X-ray, radio wave, and H α bright line observations. On the other hand, very high-energy protons come to the earth and are sometimes observed. It is, however, difficult to understand the ion acceleration with the information of protons since they are affected by the interplanetary and geomagnetic fields. Gamma rays and neutrons produced by the interaction of accelerated ions and the solar atmosphere are sometimes used to study the ion acceleration as they are not affected by the magnetic field.
Ion acceleration has been mainly studied by gamma ray observation. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was the first gamma ray imaging observatory for solar flares; its results revealed that there are differences between ion and electron acceleration foot point locations for several flares (Hurford et al. 2006). Furthermore, the Fermi Large Area Telescope (LAT) can observe high-energy gamma ray emission (>100 MeV) from the sun. Such high-energy gamma ray emission is generated from pion decay produced by the interaction between accelerated ions and the solar atmosphere. Fermi LAT observed impulsive and long duration gamma ray emission on 7 March 2012 (Ajello et al. 2014).
Solar neutrons have been observed by the near-earth spacecrafts and ground-based detectors. Although solar neutrons are attenuated by the earth’s atmosphere, high-energy neutrons (>100 MeV) may be detected by ground-based detectors. Solar neutron telescopes (SNTs) have been installed near the equator at high-altitude mountains; several solar neutron events were observed (Sako et al. 2006; Watanabe 2005). Since SNTs have a target scintillator for stopping neutron-induced proton and record counting rates when the energy of the recoil protons exceed the energy threshold level, it is not possible to record the energy of neutrons, event by event.
The SciBar Cosmic-Ray Telescope (SciCRT) project has been proposed as a brand new cosmic ray experiment using scintillator bars. The SciCRT is a multi-purpose cosmic ray detector. The main purpose of the SciCRT is 1) solar neutron observation for studying ion acceleration mechanism and 2) muon observation for investigating the anisotropy of Galactic Cosmic Rays (Kato et al. 2014; Nagashima et al. 2012). We performed the calibration of the SciCRT at Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) in Mexico in 2012. The SciCRT was installed at Mt. Sierra Negra (4,600 m) in April 2013 and was calibrated in May and August 2013. In this paper, we will describe the performance of the SciCRT as a solar neutron detector.
Data acquisition system
If more than 1 of the 32 channel signals from one MAPMT exceed the threshold level energy, a hit signal is generated on the FEB and sent to the TRGB via the BEB. 2) Several hit signals are collected on the TRGB. These signals are the test to decide if the neutron trigger condition is satisfied or not. A hold signal is generated if the trigger condition is satisfied. 3) The hold signal is sent to the FEB. The pulse height of each channel signal is kept until the hold signal reaches to the FEB. 4) The pulse heights are digitalized on the BEB. These digitalized values, named ADC data, have the deposited energy information at each channel. 5) ADC data is transferred to a DAQ personal computer (PC) via VME bus.
We need to keep the dead time in mind. Due to the limit of VME bus transfer, the transfer of ADC data is limited up to 1 kHz per one BEB. We are now developing new fast readout electronics. We will describe our new electronics later.
The system of our DAQ hardware trigger is closed in each SB, since the trajectory of high-energy neutron-induced protons seldom penetrate over two SBs. This also mitigate the effect of dead time.
Detection efficiency for neutrons
Sensitivity as a solar neutron telescope
The Mexico SNT is composed of proportional counters (anti counters) at the top, scintillation counters in the middle and proportional (directional) counters underneath. Scintillation counter consists of a 4-m2 array of 30-cm thick plastic scintillator with four energy thresholds (30, 60, 90, 120 MeV). The Mexico SNT records the count rates with anti-coincidence for each threshold. The SciCRT may also obtain the count rates with anti-coincidence with a threshold (7 MeV) determined by a MC calculation.
Observation at Mt. Sierra Negra
We installed the SciCRT as a multi-purpose cosmic ray detector at Mt. Sierra Negra (4,600 m) in eastern Mexico. We especially discussed its capabilities for solar neutron observation. The energy resolution of the SNTs and its sensitivity is not sufficient to elucidate the acceleration mechanism of ions.
The detection efficiency of the SciCRT for neutrons is estimated to be ten times higher than the Mexico SNT, especially at the low-energy region. The sensitivity of the SciCRT will be 1.9 times higher than the current Mexico SNT as shown by a Monte Carlo calculation. We are planning to upgrade electronics to resolve the inconveniently high dead time that cause a limited sensitivity of the SciCRT. The new electronics will improve 3.1 times the sensitivity compared with the Mexico SNT. Since the SciCRT is a fully active scintillator tracker, we can reconstruct the direction and deposited energy of every particle crossing through. We calculated the relationship between the primary neutron energy and neutron-induced proton energy and obtained the function to reconstruct the primary neutron energy. We expect to detect solar neutron events with the SciCRT during remaining solar cycle 24.
We are grateful for the group of the SciBar and SciBooNe experiment for allowing us to use the SciBar as a cosmic ray detector. This work was supported by Grants-in-Aid for Scientific Research (B) 22340054, Scientific Research (C) 23540348 and Fellows 24340054 from JSPS. Grants from CONACyT (181879-I) and PAPIIT-UNAM (IN107911) are also acknowledged. This work is also supported in part by the joint research program of the Solar Terrestrial Environment Laboratory (STEL), Nagoya University.
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