R. Ramachandran is a science writer.
If you were to tell someone that some Indian particle physicists are planning to perform sensitive experiments deep underground in an operational uranium mine, you’ll likelier than not be met with a disbelieving laugh. But this is exactly what is on the anvil: a small cavern, seven by four metres and a little over two metres high, has been converted into an underground laboratory 550 m underground and only about 300 m away from the nearest uranium mining activity in Jaduguda, Jharkhand. It is set to be inaugurated by Shekhar Basu, the chairman of the Atomic Energy Commission (AEC) and secretary of the Department of Atomic Energy (DAE), on September 2.
The reason behind running a particle physics experiment underground – there are many such around the world – is to prevent unwanted particles and radiation, such as those emitted when cosmic rays from outer space interact with particles in Earth’s atmosphere, from contaminating the experiment. So such sites are usually in tunnels under hills, in abandoned mines deep underground, etc. Indeed, it is from this perspective that the proposed India-based Neutrino Observatory (INO), a project sponsored by the DAE, was sought to be located in a cavern inside a mountain, with the rock shielding making for a natural filter. However, it now unfortunately stands stalled by baseless court petitions by activists and politicians, with no immediate end in sight.
What is being proposed at Jaduguda is a particle physics lab close to where natural uranium (mostly U-238) is being continuously mined. From the perspective of keeping emissions from cosmic rays away, the lab’s depth – of 550 m – may be okay, even though it is only less than half the way down compared to the rock overburden at the proposed INO site in Tamil Nadu.
However, the uranium that is being mined is radioactive, as are the isotopes that it is decaying to. They will be constant sources of alpha, beta and gamma radiation that will contaminate the experiment’s readings, producing what is called a noisy background. So what is going on?
A five-year-old proposal
“The idea now basically is to be able to do at least preliminary pilot scale studies on some components of the INO project, like the dark matter experiment and the double beta decay experiment, even as we hope against hope that INO may still see the light of the day,” said Naba Mondal, the spokesperson for the INO project. “But, in some sense, it is also a sign of desperation of scientists who have put in years of hard work to see INO get off the ground, but have been driven to despair by the sheer indifference of the powers that be.”
The hope for INO’s revival may not be entirely misplaced. It was recently reported that sites in Andhra Pradesh are being considered, after the state’s government made some assurances. “But even if that materialises, the original goals of the main neutrino project will have to be revisited in light of other neutrino experiments worldwide, which began later but have overtaken us in the decade that we have lost,” admitted M.V.N. Murthy, a former professor at the Institute of Mathematical Sciences (IMSc), Chennai, and one of the key scientists behind the INO proposal. He had been looking forward to the INO fully characterising the properties of the three kinds of neutrinos (a type of elementary particle) that are known to exist, particularly their mass hierarchy, i.e. which neutrino is lighter and which heavier.
“INO can still provide some interesting insights into neutrino physics as the size of the INO detector will still be unique,” Murthy added.
Both the dark matter experiment, called Darkmatter@INO (DINO), and the neutrinoless double beta decay (NDBD) experiment were part of the original INO proposal. They were to be co-located in the same tunnel that was going to house the core neutrino experiment. But unlike with the core experiment, there have been no breakthroughs in these areas and are still worth pursuing, at least from the Indian perspective.
DINO is meant to look for signatures of an elusive kind of matter called dark matter. It makes up about 27% of all matter and energy in the universe, and is hypothesised to be made of weakly interacting massive particles (WIMPs). So the idea is to see if dark matter particles scatter off of normal matter particles in the detector material and to pick up the signals corresponding to the recoiling nuclei. WIMPs are thought to interact with normal matter only through the force of gravity.
The proposal to use the Jaduguda mine for conducting DINO-related feasibility studies dates back to 2012. The studies were supposed to be conducted together with Rupak Mahapatra and his team from Texas A&M University. They had developed a highly efficient and sensitive cryogenic germanium-based detector called ZIP. The idea was to make a similar 30-kg detector (using silicon instead of germanium) for pilot studies at Jaduguda and scale it up later to a one-tonne detector to be used in the INO tunnel for the full-fledged experiment. The mine had been surveyed then and a possible site for locating the experiment identified.
The Jaduguda mine is the oldest of six uranium mines in and around the town, and is operated by the Uranium Corporation of India Ltd. (UCIL). The mine has two levels – at a depth of 550 m, where mining activity began in 1967, and at 880 m, where mining began much later after resource extraction in the upper levels was no longer economically viable. At present, mining is carried out only at the 880 m level.
A survey team from the Saha Institute of Nuclear Physics (SINP), who were the prime movers of the idea of using the Jaduguda mine, had reckoned that a lab at 550 m with adequate active and passive shielding would be a way to go. They had also then surmised that robust simulations could be used to develop data filters to efficiently separate the ambient particulate noise, which would be recorded by the detector, from the signals of the recoiling nuclei themselves.
But then this proposal was shelved after there were some issues about foreign personnel entering the premises of a high-security mine. Thankfully, with the SINP having developed a detector design and developed it with in-house R&D, the idea was revived at Basu’s urging about 18 months ago. Then, after the DAE failed to break the INO logjam with the Tamil Nadu government, and with no other place having been identified to carry out NDBD-related feasibility studies, it was also tagged with the DINO studies at Jaduguda.
Besides SINP, other institutes associated with the Jaduguda lab are the National Institute of Science Education and Research (NISER), Bhubaneswar; the Bhabha Atomic Research Centre (BARC); and the Tata Institute of Fundamental Research (TIFR), Mumbai.
“DINO studies may be possible, depending on the detector background rejection scheme, as those detection techniques use multiple in-built rejection schemes to be able to identify a nuclear recoil,” said R.G. Pillay, a nuclear physicist from TIFR closely associated with the NDBD experiment. “For NDBD, the background is far too high. But INO seems to be a lost cause and so we are also here.”
So why is the NDBD experiment important?
Beta decay is a kind of radioactive decay in which a neutron of a nucleus decays into a proton by emitting an electron and an antineutrino. Double decay happens when two neutrons in a nucleus decay simultaneously, producing two electrons and two antineutrinos.
Neutrinos (and antineutrinos, the antimatter counterpart) have zero electric charge and were originally believed to have zero mass as well. However, experiments in the 1990s revealed that they have very small masses. This poses a serious problem for the otherwise highly successful clutch of rules called the Standard Model of particle physics. According to the model, neutrinos aren’t supposed to have mass.
One way out of this conflict is to find if neutrinos could be their own antiparticles (i.e. if neutrinos and antineutrinos are the same thing). Because if this is the case, the neutrino would be what is called a Majorana fermion, and will be accommodated within the Standard Model once again.
If a neutrino were a Majorana fermion, the two emitted antineutrinos in a double beta decay event will annihilate themselves, leaving behind only two electrons. This process is called neutrinoless double beta decay, or NDBD. Two electrons emitted simultaneously from a single point in the detector, with no accompanying antineutrinos, will signal an NDBD event. It has not been observed thus far, so the results of the NDBD experiment could tell us how the Standard Model might have to be tweaked.
According to physicists’ calculations, both dark matter and NDBD events are extremely rare, so the sensitivities of the experiments trying to track them down have to be very high as well. They should be very good at picking out genuine signals from all the noise that the detector will be sensitive to. This is why a large amount of ambient background radiation stands to be a deal-breaker.
At Jaduguda, the granite rock samples from 550 m underground have been analysed and found to contain remnant uranium and thorium at 8 parts per million (ppm) and 16 ppm respectively. Naturally occurring uranium and thorium have long half-lives – a few billion years – and are thus barely radioactive. The chief source of the background radiation is the radioactivity from the isotopes that the uranium decays to. These isotopes emit alpha, beta and gamma radiation that then react with the nuclei in the surrounding rock as well as in the detector equipment. Additionally, cosmic rays from outer space that react with particles in Earth’s upper atmosphere produce other particles called muons, and these also have to be dealt with.
The other major concern is the ubiquitous presence of radon, produced by the radioactive decay of radium, itself one of the elements on the uranium decay chain. Apart from its radioactivity, radon is also a gas that is harmful to humans when inhaled. Though it has a very short half-life of about four days, it is constantly replenished by the decay of the long-living uranium and thorium.
And finally, there is the radioactive dust emanating from the constant blasting of rock 300 m below DINO, where mining continues. The dust has to be kept away from the lab area using appropriate provisions for airflow and ventilation.
The purpose of their efforts
From DINO’s perspective, the most important concern is the neutron background. Neutrons could interact with detector nuclei the same way WIMPs would. There’s bound to be a significant ambient neutron background because alpha particles emitted by nuclei in the uranium decay chain will interact with nuclei in the surrounding rocks as well as in the detector material to produce neutrons. The muons from cosmic rays will also interact with rock material to create more neutrons.
Eventually, all of them can mimic a WIMP signal in the detector volume. It is very important therefore to be able to simulate their production and interactions beforehand so that, when they do show up in the data, physicists can identify them for what they are. While the SINP scientists have completed these simulations, they will still have to validate their results using real-world data from within the cavern.
For the NDBD experiment, the chief background annoyance comes from gamma rays produced in the background: their ionising reactions in the detector can mimic the event signal. In this context, the radon background is especially pernicious because radon decays into bismuth, which is a strong gamma emitter. So scientists have to take extra efforts to reduce the radon background.
“The uranium and thorium chains have lifetimes of billions of years, whereas the NDBD lifetime is of the order of 1024 years. Hence, even parts-per-trillion levels of this will give higher count rate than the true NDBD decay events,” Pillay pointed out.
It would seem that there is virtually no end to challenges when a scientific experiment is to be performed in a functional uranium mine. The scientists are at least enthused by the fact that they have been able to secure an underground experimental facility after 25 years (the Kolar Gold Field cosmic ray lab was closed in 1992) and that they may be able to perform meaningful feasibility studies. Nonetheless, one is tempted to ask: With the INO requiring all but an obituary at this point, what can these studies be good for?
“Perhaps to show some non-zero progress towards setting up any underground lab and bring some pressure on the TN government,” one DAE insider said. “The Jaduguda lab is merely a statement of intent rather than a true underground facility designed for low ambient background and rare event studies.”
It has also been suggested that the Jaduguda mine could be taken over entirely once mining stops by 2020 or so – but this does not seem possible for logistical reasons. Currently, there is only one shaft through which people can access both levels. Any large scale lab would require a new shaft to be built, which would be a long-drawn exercise. Then again, as with many of the other considerations discussed thus far, desperation can drive people to do the craziest things.