A potential space- and power-effective muon sensor module for imaging 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: 30 November 2008
Accepted: 11 June 2009
Published: 22 February 2010
The application of muon radiography will be greatly enhanced by the use of two muon sensor modules that save electric power consumption and are easily transportable. Muon sensor modules used for a volcano observation must have a low electric power consumption requirement and be both waterproof and portable. In this article, we discuss two candidate sensor modules: (1) a portable muon sensor module with wavelength-shifting (WLS) fibers and a multi-anode photomultiplier tube (MAPMT), and (2) a regular scintillator telescope with PMT complemented by a low-power Cockcroft-Walton circuit (CWPMT). A realistic telescope system consisting of a muon sensor module with MAPMT has been tested and found to consume 76 W, most of which (72 W) is used by the redundant electronic circuit required for pulse shaping; this could be modified to drastically improve the power consumption. In comparison, a muon telescope system with a CWPMT was found to consume 7.57 W. We also calculated the muon stopping length in SiO2 by means of a Monte-Carlo simulation. This calculation provided the average density structure along the muon path in rock, where the muon path length was shorter than 1.5 km, with an accuracy of about 5% during a 90-day measurement period by assuming a 1-m2 muon detector with an angular resolution of 25 mrad.
For a number of years, researchers have been attempting to image volcanoes (e.g., Tanaka et al., 2003, 2005, 2007a, b, 2008; Tanaka and Yokoyama, 2008), pyramids (e.g., King et al., 1999), and contrabands (e.g., Borozdin et al., 2003) radiographically using cosmic-ray muons. Tanaka et al. recently succeeded in visualizing the internal density distribution of volcanoes using this technique, with an accuracy in terms spatial resolution that is superior to that possible using conventional geophysical techniques. Observations of volcanoes are challenging because (1) the amount of battery power that can be transported to the system is limited, (2) housing for the telescope system is necessary due to environmental conditions, and (3) the remote and often difficult terrain leading to the observation points require that the unit be easily transportable. These issues were particularly important for the observations performed at Mt. West Iwate and Mt. Asama. Previous observations were performed at a long distance from these volcanoes due to the cumbersome digital sensor system used and the high electrical power requirement of the system. Tanaka et al. overcame these difficulties by using nuclear emulsion photographic films. These films are light and do not require electrical power. Employing this technology, Tanaka et al. obtained radiographic images of the interior density structure of the Mt. Asama (Tanaka et al., 2007a, b, 2008) and Mt. Usu (Tanaka and Yokoyama, 2008) volcanoes with high angular resolution and accuracy. Images with such a high resolution can not be achieved using a conventional geophysical method. The use of nuclear emulsion photographic films for muon observation at volcano was originally impractical due to the numerous particle tracks that had to be read manually. Nakamura et al. (2006) developed an automatic particle track reading machine to facilitate this procedure. The machine had been used in conjunction with the OPERA experiment (Guler et al., 2000) to look at neutrino oscillations and shown to be employable. However, the machine does not enable the interior structure of volcanoes to be observed in real time since the emulsions require analog photographic development. To interpret the dynamics of the volcanic explosion, it is necessary to be able to interpret the variation in the interior density structure of volcanoes over time. For this purpose, we have proposed a new muon sensor system which is lightweight and requires less electrical power. This system was developed and used in 2008 for the first time at Mt. Satsuma-iwojima and Mt. Sakurajima (Tanaka et al., 2010). The use of the MAPMT (multi-anode photomultiplier tube) enables the number of PMTs in the system to be reduced and, in addition, the system can be more compact since the counter does not require to be separated into parts. We have also succeeded in developing a new PMT which runs at a lower voltage. As such, the high voltage (HV) supplier can be removed in the field, enabling the full system to be simplified. For this application, we propose that two different types of muon modules be used.
2.1 Detector design
2.1.1 Muon sensor module with MAPMT
2.1.2 Muon sensor module with CWPMT
One plane contains 12 plastic scintillators arranged in the x and y coordinates and has 144 coincidence elements; its RMS error of point resolution is ΔX = ±2.0 cm, ΔY = ±2.0 cm (Fig. 2). The arrival direction of a muon can be determined by combining two muon sensor planes, and determining the position of a muon in each plane.
2.2 Assembly testing
We observed the analog output signal of each channel of MAPMT and that of CWPMT with the oscilloscope, measured the distribution of pulse height, and optimized the discrimination level.
The zenith angle distribution of the cosmic-ray muons measured by each of the new modules proposed in this paper were compared. The experiment using the following procedures. The analog signal output from each PMT channel was the input to a discriminator (KAIZU WORKS KN1300). As a result, the signal changed into a digital signal with the width of a 20- ns pulse and a NIM level. The coincidence of these two signals was taken (TECHNOLAND CORP N-TM 103). Finally, the output was processed by a visual scaler (TECHNOLAND CORP N-OR 425) and calculated as a 1-D histogram.
The zenith angle distribution of cosmic-ray muons was measured as a coincidence of the two planes with MAPMT. The electrical block diagram used in this experiment is shown in Fig. 3. In this experiment, a multiplicity cut (see Tanaka et al., 2001) is used, which means that when there is more than one hit in any of the four planes of the scintillators, the events are discarded. The result of this measurement was assessed along with the result of the DEIS experiment. By evaluating the results of (2) and (3) with the multiplicity cut, we confirmed that a large number of the soft components were removed using a multiplicity cut.
We measured total electric power consumption of these two muon sensor systems by using power meter (HIOKI 3334 AC/DC POWER HITESTER).
The photographs of the pulse distribution output from the MAPMT, CWPMT, and R7724 systems are shown in Fig. 4.
The muon intensity was measured using the two muon sensor planes proposed in this paper as a function of vertical angles (Fig. 6(b)). The measured muon intensity was adjusted so that the intensity at θ = 90° corresponded to that from the DEIS result.
The muon spectrum measured with MAPMT is shown in Fig. 6(a). Figure 6(b) shows the 2-D histogram resulting from the observation. In this experiment, we used the comparator developed by Uchida et al. (2010); however, the pulse width is so short that the comparator was unable to discriminate the signal. To solve this issue, we placed a discriminator (KAIZU WORKS KN1300) between the modules and the comparator.
We confirmed that the total power consumption of the muon sensor system with the MAPMT was 76 W, with the discriminator consuming 72 W of electrical power. We confirmed that the DC power supply consumed 3.25 W and that the CWPMT consumed 0.09 W when the CWPMT was supplied with 5 V of electrical power. Thus, the total power consumption of the muon sensor system with the CWPMT was 0.09 × 48 + 3.25 = 7.57 W.
The results of these experiments have provided valuable information on important properties of the proposed candidate muon sensor modules which will improve future investigations.
it is possible to make it waterproof due to its mechanical compact arrangement;
the module is easy to assemble due to a decreased number of MAPMT inside the system;
the unit price for each channel is less expensive than in earlier versions of the module. The high consumption of electrical power, 76 W, in this system is due to the use of a discriminator to lengthen the pulse width from the module. The present discriminator consumes 1.5 W/channel. In general, a high-speed amplifier and pulse shaper are not power effective, and their cost per channel is higher. We consider that one way to reduce the power consumption would be to employ a unit consisting of a fast comparator and a FPGA (field-programmable gate array) to modify the pulse height and the pulse width. We expect that the power required for the operation will be less than 100 mW/channel.
This module can potentially be miniaturized, is lightweight, and the unit price per channel is low. In addition, the modules can adapt to the constraints determined by the installation location. For example, we can potentially place the modules at several locations surrounding a volcano and produce 3-D radiographic images of the interior density structure of the volcanoes. One disadvantage of this approach is that if one MAPMT breaks, it is not possible to continue receiving data.
Since the PMTs are separated, even if one or more PMT breaks down, it is possible to continue making observations.
The HV supplier is built into the system, and the system is simplified.
- (1)We fitted the muon spectrum from the DEIS experiment to the following formula (Matsuno et al., 1984).
We generated random numbers, and using these numbers, we generated muons (θ = 80°) adapted to the energy distribution obtained by Eq. (1).
We injected the generated muons into SiO2 (ρ = 2 g/cm3) with a thickness of 500, 1000, and 1500 m, and calculated the physical process in SiO2. Physical processes in the material of the muon were calculated with GEANT4: a toolkit for simulating the passage of particles through matter (Agostinelli et al., 2003).
We calculated the change in the muon angle after they had passed through SiO2. The result of this simulation is shown in Fig. 7 as a 1-D histogram.
3-D information from the topographical map (with a scale of 1 to 25000 provided by Geographical Survey Institute) and the result measured by airborne laser scanner (Fig. 9(b)) (Sasaki et al., 2003) were used to obtain a 2-D map, including the path length in the direction of θ and φ, as seen from the muon sensor.
By generating random numbers, we generated a muon spectrum according to the energy distribution obtained by Eq. (1).
The stopping length of the muons in SiO2 was calculated using Geant4, and a table was constructed to represent the data. For example, the stopping distribution of incident muons at an arrival angle of θ = 80° with energy above 10 GeV, 100 GeV, and 1 TeV is shown in Fig. 8.
By using the result of (2) and (3), we calculated the number of detected muons during a 90-day measurement period by assuming a 1-m2 muon detector with an angular resolution of 25 mrad. The result is shown in Fig. 9(c).
As shown in Fig. 9(c), we can determine the average density structure along the muon path in Mt. Sakurajima, where the muon path length is shorter than 1.5 km (550 m below the summit of Sakurajima), with an accuracy of about 5% during a 90-day measurement period. This value was determined using the simulation result at a confidence level of 1σ. This 90-day measurement period is not long compared with the time scale of volcanic activity. An accuracy of 5% is enough to observe ascending magma in a 100-m conduit in rock with a thickness of 1.6 km.
We confirmed that the total power consumption of the muon sensor system with MAPMT is about 76 W and that of the muon sensor system with CWPMT is about 7.57 W. The Monte-Carlo simulations using GEANT4 revealed that the limit of the angular resolution is about 15 mrad. We also confirmed that we can determine the average density structure along the muon path in Mt. Sakurajima where the muon path length is shorter than 1.5 km (550 m below the summit of Sakurajima) with an accuracy of about 5% during a 90-day trial, assuming a 1-m2 muon detector with an angular resolution of 25 mrad.
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