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Detectors and sensors

Detectors and sensors

Controversial Italian neutrino experiment gains support online

18 Dec 2017
Photograph of protestors on Gran Sasso
Watershed moment: opposition to the SOX experiment at Gran Sasso

More than 40,000 people have signed an online petition defending a controversial neutrino experiment that is due to start operating next year at the Gran Sasso National Laboratory in Abruzzo, central Italy. The Short Distance Neutrino Oscillations with Borexino Experiment (SOX) is opposed by environmentalists and local residents who fear its source of highly radioactive cerium-144 could contaminate water supplies in the event of an accident. But petitioners believe those fears are overblown and are calling on the president of Abruzzo’s regional government to confirm the experiment’s go-ahead.

That appeal follows a unanimous vote by a regional commission in November to stop the experiment “immediately and definitively”. Meanwhile, a collective of environmental pressure groups known as Forum H2O is to request that the central government revoke the authorization for the experiment that it gave the National Institute for Nuclear Physics (INFN) in February 2016. Forum H2O argues that the INFN failed to tell the relevant ministries that the lab – located within an aquifer inside the Gran Sasso mountain – contains what is effectively a well that feeds one of the region’s main aqueducts.

SOX is designed to provide an intense, laboratory-based source of neutrinos for the Borexino detector, a 17 m-high stainless steel ball containing about 300 tonnes of liquid scintillator that has been used at Gran Sasso over the past 10 years to study neutrinos from the Sun. The idea is that a wave-like distribution of detections throughout the sphere would indicate that ordinary neutrinos from the source are “oscillating” into sterile neutrinos, which are predicted by some extensions of the Standard Model of particle physics but have never been unambiguously observed. SOX’s radioactive source would consist of about 40 g of cerium-144. Currently it is not clear whether the source can actually be built, since the company contracted to do so – Russia’s PA Mayak – has said it is finding it difficult to achieve the desired radioactivity.

Steel and tungsten container

If the cerium-144 can be produced, it would be contained within 10 kg of cerium-oxide powder and sealed inside a double stainless-steel capsule. That would be surrounded by a tungsten cylinder with 19 cm-thick walls and placed in a pit several metres beneath Borexino. The cerium would undergo 5.5 × 1015 beta decays per second (5.5 PBq), and in the process emit intense gamma radiation. According to SOX spokesperson Marco Pallavicini of the University of Genoa, the shielding – which allows the neutrinos from the beta decays to pass through – would mean people handling the source receive a dose about equal to that absorbed on a return transatlantic flight.

Critics fear, however, that the shielding might be opened, either maliciously or accidentally, and that the powder could then enter the water supply. Pallavicini says these concerns are unfounded, explaining that computer simulations show that the tungsten cylinder could not break, even if it fell from a significant height. As to deliberate interference, he says that the cylinder would be extremely difficult to open since it has a cover weighing several hundred kilograms. “For the possibility of a terrorist attack,” he adds, “the Italian prefecture and police have been informed and I assume will take the appropriate measures.”

Augusto De Sanctis, president of the non-profit organization Abruzzo Ornithological Station, argues that SOX would be illegal because it would contravene a 2006 law forbidding the storage of dangerous or radioactive substances within 200 m of a source of drinking water. Pallavicini admits that he does not know how far SOX would be from the well, but insists that the distance is irrelevant. Because SOX would operate for only 18 months, he maintains it would not involve the permanent storage of radioactive material.

Practice run

Controversy over SOX flared up in early October after Forum H2O and a local online newspaper received an anonymous tip-off that an unannounced practice run for the delivery of the radioactive material – involving an empty cylinder but still requiring a police escort – would be held within a few days. Pallavicini says that the reaction to the trial run “got me mad because they accused us of wanting to hide the experiment”, whereas, he maintains, “we didn’t say anything because it wasn’t very interesting”. But De Sanctis says this was the first time that anyone had heard about SOX, arguing that plans for the experiment should have been made public once the INFN had requested authorization in 2014.

The lab came under fire in 2003 over questions of public safety when prosecutors sealed off part of the lab for several months after 50 l of pseudocumene were accidentally released into the lab’s drains. And in August 2016, measurable but not dangerous quantities of dichloromethane were found in the lab’s well.

De Sanctis says that he is “a supporter of scientific research” and that there are dozens of experiments in the lab that he and his fellow campaigners “are proud of and which don’t pose a risk”. But he believes it is a “gamble” to house SOX – as well as Borexino and the Large Volume Detector, which contains 1000 tonnes of white spirit – underground within a huge aquifer in what is a highly seismic area. “Research has its limits,” he says.

Troubled waters

Pallavicini says that the huge response to the petition makes him “reasonably optimistic that in the end we will come through these troubled waters”. He notes that formally the experiment does not need a green light from the regional government since it has already been authorized from Rome. But he says he has no intention of taking any legal action if the regional government is opposed. “I am a physicist,” he says. “I don’t want to do the experiment against the will of the people in Abruzzo.”

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