A volcanic island with 3 active craters, Stromboli is famous for its frequent eruptions, making it an excellent target for muography. As emulsion plates are in the processes being analyzed from a new Stromboli volcano muography experiment, Valeri Tioukov (INFN) discusses his experience with the first exposure conducted in Stromboli as a collaboration between INFN (Instituo Nazionale di Fisica Nuclere) and INGV (Instituto Nazionale di Geofisica e Volcanologia) e Osservatorio Vesuviano of Napoli and ERI (Earthquake Research Institute of Tokyo University).
Installing this detector in such a harsh and inaccessible environment called for careful planning and challenging work conditions. A helicopter was used to airlift the heaviest components of the detector, however the team also had to hike up to the high-altitude site several times to bring the lighter, more fragile supplies. He also discusses the benefits of using emulsion for this experiment, taken from carefully recycled components of the OPERA project, and analyzed with an international cooperative effort between Italian and Japanese laboratories.
Q: What effect has the volcanic eruptions on Stromboli had on the local community and environment? Why did you choose Stromboli as the site for your study?
Volcanic activity is part of everyday life on Stromboli – everywhere on the island you can hear its roar. In the case of a major eruption which usually occurs once every 10-20 years, the volcano becomes dangerous to the local population and tourists, who would have to quickly evacuate the island. Also, landslides originating from Sciara del Fuoco, which is a collapsed part of the cone, have the potential to provocate tsunamis.
We performed this measurement in collaboration with INFN (Instituo Nazionale di Fisica Nuclere) and INGV (Instituto Nazionale di Geofisica e Volcanologia) e Osservatorio Vesuviano of Napoli. Napolitan volcanologists have a long tradition of studying and monitoring Stromboli. There are several models trying to explain the Strombolian activity. One of them was presented in an article by Giovanni Macedonio and Marcello Martini where the presence of some structures under Sciara del Fuoco was speculated. After some discussions we decided that this is a perfect challenge for the muography technique.
Q: How did you determine the optimal region to place the detector at Stromboli?
As with most volcanoes, Stromboli has a conical shape, so the higher we put the detector, the shorter the path will be inside the rock that has to be crossed by the horizontal muons and the more statistics we can obtain. On the other hand, if we put a detector at a site that is too high, the observable part of the cone above the detector level become too small to be interesting. So our first step was to perform a Monte-Carlo simulation to estimate the optimal range of the altitudes. In this simulation for 1 m2 detector exposed for 4-5 months (winter period), the best altitude was determined to be 600-650 m.
Actually, it’s quite difficult to find a stable, horizontal surface at this altitude on the volcano. The original idea was to perform the detector installation in 2010 but we failed to find a good position during the initial survey in December. On the second day, when we performed an inspection of the cone, we received a storm alert from the civil protection authorities saying that if we do not leave the island in 4 hours there was a risk of being stranded for next 2 weeks (during Christmas) so we rushed down from the mountain to catch the last ferryboat. The installation was postponed to 2011 and in May, Massimo Orazi, Tullio Ricchi (incredible Stromboli expert from INGV who knows every inch of the volcano perfectly) and I performed another survey- we checked the entire island above the altitude of 600 meters, from stem to stern, before finally finding a stable, level surface at 640m with a direct view to Shara del Fuoco. It was the best of what we had seen, but not perfect because the muography observation from this position was affected by the presence of Vancori behind the craters and this had the potential to limit the muography performance starting from some depth below the craters.
Q: It must have been a very complicated procedure to safely transport the muography detector to the observation site. What was that process like and how did you protect the muography equipment during transportation?
We used nuclear emulsion as the sensitive element for the registration of the muons. The emulsion plates are light and shock resistant. It’s possible to transport them with a helicopter but also manually (on foot), even on Stromboli where the access was quite difficult.
In contrast, the overall detector structure should be robust to guarantee protection against bad weather and severe environmental conditions as well as from sunlight heat. The frame should guarantee that the detector stays level, with the requirement that a perfect flatness and stability of the detector plane is maintained at a level of 1 mm over 2 m. We used stainless-steel plates that were 5 mm thick to encapsulate the emulsion foils and partially protect them from environmental radioactivity, which is usually higher on the volcanoes. So, the overall weight of the frame and other supplies was about 400 kg and the only way to bring it to the selected position was by using a helicopter.
The frame was assembled on COA (Centro Operativo Avanzato of Italian Civil Protection) which we used as the base camp. We performed the transportation using three flights: the first flight brought us to to Pizzo where a place suitable for landing was available. During the second flight the frame structure of the detector was hung below the helicopter and brought directly to the installation point, during the last flight a net with all other equipment was transported to the installation point. We installed the structure in position during the first day. Transportation activities of the next day involved hiking up to the installation point with the emulsion and the instruments. After that, we performed the assembling and installation of the modules.
Q: What was the method of assembling the components of the detector for use at Stromboli prior to the helicopter lift? When and how do you put the emulsion film inside the detector?
We brought the emulsions by foot. This was done not because it’s too fragile but just because we had only one day of helicopter availability and we wanted to use that time to bring the structure and all heavy material.
An emulsion detector is like a photographic camera: when you open the camera shutter for a long exposure it must be already positioned and remain still for a full period of exposure, otherwise the image become blurred. The emulsions assembled together inside a module is like the camera shutter in this case. So, this operation has to necessarily be done in place just before fixing the module onto the frame. Within 4 hours inside a small tent, which protects us from wind, dust and a direct sunlight, we inserted 320 plates inside 8 modules and installed them.
Q: What are the advantages of using emulsion films for muography experiments and why was this tool most suitable for this measurement at Stromboli? What are some of the biggest challenges of involved with using emulsion film?
The main advantages of the emulsions:
- They are compact and easy to transport
- The assembled detector is compact and robust – it looks like a single 1-2 cm thick metal plate. Emulsion provides excellent angular resolution (1-3 mrad) with a compact size. On the other hand, electronic detectors are typically larger requiring a base of about one-two meters to obtain a similar angular resolution.
- The emulsion detector is completely autonomous – no electricity or gas supplies are required, nor any registration electronics or disk servers, because emulsion itself is a kind of high capacity data storage device.
Technical challenges relative to the emulsion technique were:
- Environment, and temperature conditions – it was a first time, in my knowledge, that the emulsions were installed outdoors, open to the Sun and rain for so such a long time. It was positioned just 500 m from the craters so when the wind blows from that direction, acids and sulfur products would reach the detector. Each emulsion film was packed inside a light tight and watertight envelope; the envelopes were encapsulated inside stainless steel watertight modules. The South front of the detector was protected against the Sun with heat-isolating materials; we also constructed a small wall from volcanic stones and pieces of rocks to protect it further.
Even with all these precautions, we were not sure to find after the exposure the emulsions would be in good shape. This is the main drawback of the emulsion technique: it is difficult to know in advance if the measurement has gone fine or not before the emulsions have been developed. It has happened before that after several months of effort you develop plates and find them black and completely useless. That’s why no errors can be tolerated in the experiment planning, emulsion handling and installation.
Q: What is the process involved with getting data from the emulsion after the experiment is done? Why did you decide to split the emulsion processing between Napoli, Salerno and Tokyo? Are there differences between the scanning techniques of each lab and how was the task divided?
After the extraction of the emulsions they were transported to Gran Sasso laboratory, developed, and sent for scanning into the laboratories which are equipped with automated microscopes. The Napoli and Salerno labs took part of the OPERA experiment in the LNGS underground laboratory. It was a biggest-ever emulsion project and the system for the analysis of the OPERA emulsions ESS (European Scanning System) was created by us. Later ESS was cloned and installed in several laboratories around the world including in Tokyo ERI. So, all three labs actually used a very similar scanning system that provides compatible data. This emulsion sharing was done mainly to speed up the analysis, which was quite time consuming. We performed also the calibration of our systems to estimate variations of efficiency – this was taken into account when we merge data acquired in different labs for performing the final analysis.
Q: How does the recent emulsion cloud chamber experiment that has been conducted in Stromboli differ from the original experiment?
ECC is the same technique used in OPERA. Sandwiches made of several (10 for example) emulsion layers are interleaved with an absorber (1mm of lead) aid, filtering out the low-momentum particles, the worst kind of background source encountered in muography experiments conducted above ground. Last year we performed a second installation in the same position in front of Sciara del Fuoco using the ECC detector. The plates are under analysis now, emulsion quality is good, the data are pure so we expect to obtain results soon.
I’d like also to acknowledge the great help and support from the Italian Civil Protection personnel as well as from tourist guides. Zaza, a historical tour guide for Stromboli accompanied us in December 2010 when the cone was covered by snow, an extremely rare event.
Massimo Orazi, pictured on the right, from INGV took care of all the logistics and made this mission possible.