Cryogenics Goes Radioactive in Search for Big Bang Neutrinos

Princeton University’s Plasma Physics Laboratory (PPPL) is readying a facility to detect Big Bang neutrinos by capturing them in tritium, a radioactive isotope of hydrogen. The state-of-the-art project is named PTOLEMY, both as homage to the second-century Greek astronomer and as an acronym for Princeton Tritium Observatory for Light, Early-universe Massive-neutrino Yield.

PTOLEMY consists of a pair of superconducting magnets joined to opposite ends of a five-foot vacuum chamber, with the second magnet connected to a calorimeter that measures electron energy. Its major goal is to demonstrate the ability to measure the mass of Big Bang neutrinos and thus pave the way for a much larger experiment. “We hope to take enough data to measure the neutrino or at least produce the world’s most accurate measurement using calorimeter techniques by the end of 2017,” says physicist Chris Tully.

Big Bang neutrinos are information-rich relics that appeared one second after the Big Bang during the onset of the epoch that fused protons and neutrons to create all the light elements in the universe. Such neutrinos are believed to be everywhere in the universe but have never been seen. The expansion of the universe has stretched them and they are thought to be billions of times colder than neutrinos that stream from the sun. As the oldest known witnesses or “relics” of the early universe, these neutrinos could shed new light on the birth of the cosmos if scientists could pin them down. But that’s a tall order since these ghostly particles can speed through planets as if they were empty space.

The PTOLEMY prototype aims to detect these neutrinos by measuring a tiny boost of energy emitted in tritium decay. The task, which has been compared to detecting a specific heartbeat in a packed sports arena, will require the coldest and darkest conditions achievable to prevent disruption of the exquisitely sensitive instruments.

The hunt for Big Bang neutrinos will begin this summer after several years of preparation. The experiment’s key ingredient is expected to arrive soon: 1/100th of a milligram of tritium loaded onto a postage stamp-sized sheet of graphene, a layer of carbon just a single atom thick. Researchers say this arrangement will produce a clean spectrum of tritium decay when it arrives from Savannah River National Laboratory under a Cooperative Research and Development Agreement approved by the DOE. PPPL will handle this tritium safely in accordance with its DOE-approved Radiation Protection Program. The Laboratory used higher quantities of tritium as a fuel, with the hydrogen isotope deuterium, for fusion experiments conducted on its Tokamak Fusion Test Reactor from 1993 to 1997.

Researchers will position the tritium-loaded graphene inside the first superconducting magnet, the field strength of which is similar to the MRI systems used at hospitals and clinics. This field will guide electrons from the tritium decay into the neighboring vacuum chamber. Low-energy electrons will be filtered as they travel through a series of electrodes placed within the vacuum chamber as the magnetic field first dips in strength and then rises again as the electrons enter the second magnet, leaving only the highest-energy electrons for the calorimeter to analyze.

PTOLEMY's dilution refrigerator. Image: Princeton University Office of Communication, Elle Starkman

PTOLEMY's dilution refrigerator. Image: Princeton University Office of Communication, Elle Starkman

The calorimeter will be the most accurate instrument of its kind in the world. It will be hooked to a dilution refrigerator set at 10-to-50mK, a temperature more than 50 times colder than deep space and a small fraction of a degree above absolute zero. The extreme cold will keep the calorimeter poised between a superconducting state—one in which electrons can flow without resistance—and a non-superconducting state that provides resistance. When an electron with neutrino-supplied extra energy comes along, the calorimeter will signal it by rapidly becoming resistive.

If successful, the PTOLEMY project could lead to a major new experiment at PPPL that explores decay from 100 grams of tritium. The large new experiment would test the theory that predicts that some 330 Big Bang neutrinos per cubic centimeter exist throughout the universe. The enlarged PTOLEMY will count the number of electrons that neutrinos have bumped up in energy to determine whether this prediction is correct. Confirming it would validate current thought about the evolution of the cosmos since the Big Bang, while refuting it could overturn the model and lead to fresh insights.

The expanded experiment could also have other far-reaching effects. It might detect so-called sterile neutrinos, hypothetical Big Bang particles that have no positive or negative charge and could be the source of invisible dark matter.

Support for the initial PTOLEMY comes from grants of $400,000 from the Simons Foundation in New York City and $330,000 from the John Templeton Foundation in Pennsylvania. Tully expects that work on PTOLEMY will attract graduate students and summer interns. “My dream is to prove that measuring neutrino mass can work,” he says, “and to have a beautiful picture of a major new facility that engineers can build.”