- Open Access
Development of a cost-effective plastic scintillator for cosmic-ray muon radiography of a volcano
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
- Received: 28 November 2008
- Accepted: 30 March 2009
- Published: 22 February 2010
Muon radiography has recently made it possible to perform imaging of the internal structure of a volcano. Muon-detecting experiments were carried out at Asama and Usu volcanoes using a newly developed scintillator. The goal was to build a large detector that would improve the quality of the data collected by the muon radiography experimental set-up that is currently being used to study volcanic activities. To this end, a costeffective plastic scintillator was developed by a simple “heating-melting method”. The detector consists of planes of scintillator strips and is currently in the prototyping phase. Using a cosmic-ray muon source, we found that the signal from our scintillator was consistent with the zenith-angle dependence of the cosmic-ray muon spectrum with no significant attenuation effects in the scintillator. We confirmed that our scintillators can be incorporated into a portable assembly-type cosmic-ray muon telescope system.
- Cosmic-ray muon radiography
- plastic scintillator
- detection system
Cosmic-ray muon radiography is similar to X-ray radiography, with the exception that penetrating muons are used instead of X-rays. In terms of determining the internal structure of a volcano, this method measures the absorption of muons along a cross section of the volcano parallel to the plane of the detector, on which the average density along all muon paths is projected. The absorption of the muons by the internal structures of the volcano can then be used to determine the density profile of that volcano. Such measurements would be ideal for studying the time-dependent changes in the crustal structure at sites which cannot be well resolved because of their strong structural heterogeneity and potential difficulty to be accessed and which, therefore, are not amenable to having their structure determined by conventional electromagnetic or seismic techniques. Volcanoes make particularly good study targets for muon radiography because they are axi-symmetric, and it is reasonable to assume that the observed density variations are localized in the vent or crater area. The use of emulsion films (Tanaka et al., 2007a, b, c, 2008; Tanaka and Yokoyama, 2008) has recently made it possible to image the internal structure of a volcano. An emulsion film is a completely power-free particle tracking device that is light enough to be carried up a mountain. However, the image can be obtained only after the emulsion has been developed—at a later time—somewhat like a photographic film. In cosmic-ray muon radiography, a detector with an area on the order of square meters is placed so that it is imaging a volume that is higher in elevation adjacent to the detector. The intensity of the image pixel in the detector is determined by the attenuation of the incident muons caused by absorption in the Earth’s crust. This technique is utterly independent of the geophysical model and directly measures the density length (density × path length). By determining the path lengths from topographic information, the measurement gives provides researchers with the average density 〈ρ〉 along the path lines of cosmic-ray muons through the Earth.
The imaging techniques are aimed at detecting the cosmic-ray muons and are intricately associated with electronic systems and computers. The function of the scintillator in the imaging systems is to convert the energy deposited along the path of the high-energy muons into visible light. The experimental arrangement for a real-time volcano monitoring system requires a relatively large detection area to collect a large number of muon events. With these increasing requirements for volcanic imaging equipment, the demands made on the scintillator as the detection material in imaging systems will be enormous. About 320 kg of scintillator is required for a cosmic-ray muon detection system with a size of 2 × 2 m. In principle, an ideal scintillator for muon radiography should have the following properties: (1) high luminous efficiency; (2) short decay time; (3) no after glow; (4) short attenuation length; (5) low cost. However, in general, the choice of a scintillator for any given project is based on a compromise between light output and cost. Cast plastic scintillators may cost between $40–60 per kilogram. Therefore, the development of new scintillators for cosmic-ray muon radiography will provide a major impulse to research on the diagnosis and monitoring of volcanoes. We report here our development of a new scintillator that is based on a simple “heating-melting method”; this scintillator has a significantly lower price tag than those currently available.
In the field of cosmic-ray muon radiography, the energy of the muons detected is above few GeV. Any improvement of signal-to-noise ratio cannot be obtained by increasing the cosmic-ray source intensity but, rather, is achieved by increasing the sensitivity of detector. Tanaka et al. (2010) recently developed a portable assembly type cosmic-ray muon telescope module. The module comprises a plastic scintillator, acryl light guide, and power-effective photomultiplier tube (PMT) (Hamamatsu H 7724) in a polycarbonate container. At the observation site, 48 modules are arranged so as to construct two segmented scintillation detector planes by which to track muon trails. The length of the module has to be longer than 1 m in order to have a detector plane that is larger than 1 m2. However, the conventional cheap scintillators have a very short light attenuation length—usually shorter than 50 cm. In order to improve the quality of the imaging, it is necessary to reduce the light attenuation effects in the detector.
Physical property of styrene.
Rate of transmission (%/12 cm)
89 at λ = 420 nm; 89.7 at λ = 500 nm
Feature of the present plastic scintillator.
Composition and properties of the plastic scintillator
(1) Base resin (PS) polystyrene [C8H8]
(2) Para-terphenyl (1,4 diphenyl benzene) [C18H14]
(3) POPOP (1,4-Bis[2-5-phenyloxazolyl])-benzene [C24H16N2O2]
B. Specific density (g/cm3)
C. Wavelength of maximum scintillation
D. Decay constant
E. Scintillation efficiency (% Anthracene)
F. Light attenuation length
G. Refractive index
H. Softing point
Our testing of this assembly demonstrates the following important features regarding the cosmic-ray muon radiography of a volcano: (1) for a cosmic-ray muon detector with dimensions of less than 1 × 1 m, the detector can be used without any correction for the light attenuation length of the current scintillator; (2) for a detector with dimensions larger than 1 × 1 m, it is necessary to correct for the light attenuation length of the present scintillator, and the number of events are reduced. A longer attenuation length is needed in order to construct a telescope with a large detection area. Based on the test measurements reported here, we can estimate a realistic enlargement of the detection area by considering the following factors: (1) doping of a small amount of antioxidants or N2-gas purging; (2) use of a vacuum container when casting a scintillator; (3) improving the annealing temperature and time.
It was evident during all steps of our test measurements of our new scintillator that this scintillator can be used for cosmic-ray muon radiography of a volcano. The synthesis method described here is sufficiently simple and inexpensive to be expanded into a much larger scale. This would contribute to time-dependent measurements as well as an extension to three-dimensional tomography.
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