Scintillator

A scintillator is a substance that lights up when a charged particle passes through it. Alongside Cherenkov radiation it is the second great detection principle of neutrino physics — and it is responsible for everything that is too quiet to produce a cone of light.

How the light comes about

A charged particle flying through tears electrons out of their bonds and lifts molecules into excited states. This excitation lasts only nanoseconds. On falling back, the molecule gives off the energy as a flash of light.

In organic scintillators this happens in two steps. The solvent takes up the energy and passes it on to a dissolved fluorescent additive, which emits ultraviolet light. A second admixture, the wavelength shifter, converts this into blue light — for only then are the photomultipliers sensitive to it, and only blue light gets through twenty metres of liquid without being swallowed up.

A modern liquid scintillator therefore consists of three components: the solvent, usually linear alkylbenzene, plus a few grams of PPO per litre and a few milligrams of a shifter.

The decisive advantage

The difference from Cherenkov radiation lies in a single number. A scintillator delivers around ten thousand photons per megaelectronvolt; a water detector manages a few hundred.

Everything else follows from this fiftyfold light yield. The energy can be determined far more precisely, and above all the detection threshold falls drastically: events of a few hundred kiloelectronvolts become visible.

Exactly this was the basis for the success of Borexino, which was the first to count low-energy solar neutrinos in real time — something that would not have been possible with water.

The price for it: the light is radiated uniformly in all directions. The direction of the particle is lost. A water detector such as Super-Kamiokande can say where a neutrino came from; a scintillator detector cannot.

The double signal

The second great merit is the time resolution. A scintillator responds within a few nanoseconds, and that permits a signature which Clyde Cowan and Frederick Reines devised in 1956 and which underpins every reactor measurement to this day.

In inverse beta decay an antineutrino strikes a proton and produces a positron and a neutron. The positron annihilates immediately — first flash. The neutron wanders about for a few microseconds until it is captured — second flash.

Two flashes at the right interval in time are practically impossible to confuse. This is why KamLAND, Daya Bay and JUNO all work with scintillator.

Solid scintillators

Besides liquids there are crystalline and plastic variants.

Crystals such as sodium iodide or caesium iodide, each doped with thallium, deliver still more light and a better energy resolution. They are, however, expensive, sensitive to moisture and not to be had in large quantities.

Plastic scintillators can be cast into any shape and are cheap. They stand in layers between steel plates, for instance at MINOS, where mass and track position matter more than energy resolution.

Purity as the limit

At low energies the natural radioactivity of the detector itself becomes the opponent. Every gram of material contains traces of uranium, thorium and potassium — harmless in everyday life, devastating here.

Borexino therefore achieved a purity of about one uranium atom to 10¹⁸ atoms of the scintillator and was thereby the cleanest place in the world. Developing these purification procedures took longer than building the facility — and it is the reason why JUNO can aim at the same purity in a seventyfold volume at all.

Pulse shape analysis

One final piece of ingenuity: the shape of the flash of light reveals the kind of particle. An alpha particle produces a pulse that rings on somewhat longer than an electron does, because it deposits its energy more densely.

This makes it possible to separate background events from radioactive decays from genuine neutrino events — without additional hardware, purely from the temporal form of the signal.

Cherenkov radiation — the other detection principle
Borexino — the benchmark for purity
JUNO — the same technique on the largest scale
Cowan-Reines experiment — the double signal

Sources

Borexino Collaboration: The Borexino detector at the Laboratori Nazionali del Gran Sasso, Nuclear Instruments and Methods A 600, 568 (2009).
JUNO Collaboration: JUNO Physics and Detector, Progress in Particle and Nuclear Physics 123, 103927 (2022).