Liquid Scintillator Tank

Scintillators are transparent materials that exhibit scintillation when charged particles traverse them. A charged particle excites the molecules of the material which, when dis-exciting, produce photons in a number proportional to the energy of the incident radiation. The light is converted into electric current coupling the scintillator with an electronic light sensor such as a photomultiplier tube (PMT), photodiode, or silicon photomultiplier. Reading the output of such a device it is possible to measure the energy of the incident radiation.
Electronic light sensors are characterized are characterized by fast time response, high detection efficiency and a wide choice of material exist, since both inorganic and organic can be used as scintillators.
Inorganic scintillators have excellent energy conversion efficiency, but unfortunately are not easy to handle because of deliquescence, vulnerability to shock and impact.
Organic scintillators include plastic scintillators and liquid scintillators. They display a short decay time and have no deliquescence. Furthermore plastic scintillators are easy to cut and shape, and easy to handle.
Thanks to these properties they have a wide range of applications, such as medical imaging, e.g. PET (Positron Emission Tomography), oil exploration, monitoring of nuclear power stations, and of course particle physics.

A detector set-up, mainly used to detect neutrinos, consists of a tank filled with a liquid scintillator. Indeed the first detection of this particle by Frederick Reines and Clyde Cowan in 1956, was possible using a solution of liquid scintillator and gadolinium in water.
Modern experiments which are using this kind of detector technique are: Kamioka Liquid-scintillator Anti-Neutrino Detector (KamLAND) in Japan, Daya Bay Reactor Neutrino Experiment in China, Double Chooz in France and the Reactor Experiment for Neutrino Oscillation (RENO) in South Korea.

Inside the detector during PMT installation
Inside the detector during PMT installation
© KamLAND - Stanfort University

Experiments of this type detect the anti-neutrino coming from nuclear reactors looking for particles produced through the inverse beta decay (IBD). IBD is a nuclear reaction involving electron anti-neutrino scattering of a proton, creating a positron and a neutron. To also detect the neutron, usually the liquid scintillator is doped with gadolinium which has a high neutron capture cross-section. Their main goal is to study the important phenomenon of the neutrino transition from one flavor to another.

KamLAND is situated in the old Kamiokande cavity in a horizontal mine drift in the Japanese Alps. The experiment consists of an 18 m diameter stainless steel spherical vessel with 1879 Hamamatsu PMTs (type R7250), with 20” diameter and fast time response mounted on the inner surface. Inside the sphere is a 13 m diameter nylon balloon filled with liquid scintillator. On the outside, non-scintillating, highly purified oil provides buoyancy for the balloon and shields it from external radiation. The stainless steel vessel is surrounded by a water Cherenkov detector, which allows it to reject any external background. The anti-neutrinos are produced through the beta decay of the radioactive elements present in the core of 53 Japanese commercial power reactors around the site at distance of about 180 km.

Antineutrino detectors installed in the far hall of the Daya Bay experiment. Credit: Qiang Xiao.

Daya Bay is situated at approximately 52 kilometers northeast of Hong Kong. The experiment consists of eight anti-neutrino detectors, clustered in three locations within 1.9 km of six nuclear reactors. Each detector consists of 20 t of liquid scintillator doped with gadolinium serving as the target for the inverse beta decay reaction. The target is surrounded by the so called “gamma catcher” and is filled with 20 t of undoped liquid scintillator for detecting gamma-rays that escape the target volume. The outer volume contains 37 t of mineral oil which shields the inner volumes against radiation from the detector components. There are 192 Hamamatsu 20 cm R5912 oil-proof PMTs assemblies installed in the outer volume and around the circumference of the stainless steel vessel. The top and the bottom surfaces are not instrumented with PMTs; instead, there are two highly reflective panels. The detectors are submerged in a water pool that provides at least 2.5 m of water to degrade radiation from the rock. The water pool is optically divided into the inner and outer regions, both equipped with Hamamatsu 20 cm R5912 oil-proof PMTs assemblies to serve as water Cherenkov detectors. On the top of the water pool, there are Resistive Plate Chambers serving as another muon detector.

Schematics of the design of the Double-Chooz detectors.
Read more at: Double Chooz Web Page

Double Chooz uses reactors of the Chooz Nuclear Power Plant in France as neutrino source and a set of two detectors situated 400 meters and 1050 meters from the reactors which are filled with liquid scintillator doped with gadolinium. The detectors are composed of a set of concentric cylinders. The innermost volume contains 8.3 tons of liquid scintillator doped with gadolinium inside a transparent acrylic vessel, where the neutrinos interact. This part is surrounded by another acrylic vessel, filled with unloaded scintillator, which is in turn surrounded by a buffer tank filled with mineral oil. The buffer acts as a shield against the radioactivity of the detectors surrounding the most inner part.

The inner volume contains 390 Hamamatsu 25 cm R5912 oil-proof PMTs assemblies and is surrounded again by a volume filled with liquid scintillators, that helps to reject the background coming from outside the detectors equipped with 78 Hamamatsu 20 cm R1408. These last two parts are in turn surrounded by 1 m of water in the nearest detector and 15 cm of steel in the other one. The upper part of the detector is covered by plastic scintillator strips grouped into modules which serves as a veto for muons from cosmic-ray.

RENO is located near the Hanbit nuclear power plant in Yongg-wang, in the southwest coast region of South Korea. The plant consists of six reactors linearly aligned with equal distance of about 260 m. One detector is located at 294 m from the center of the six reactors, while another detector is located at 1383 m. Each detector consists of four layers of nested cylindrical structures and contains different liquids. A main inner detector (ID) is contained in a cylindrical stainless steel vessel of 5.4 m in diameter and 5.8 m in height which houses two nested cylindrical acrylic vessels.

A schematic view of RENO detector (left) and a photo of the detector interior with PMTs installed on the buffer vessel walls (right).
Read more at: ScienceDirect

The 1.5 m thick outer detector (OD) is filled with 350 tons of highly purified water. The OD is intended to identify events coming from the outside. The innermost target vessel, 25 mm thick acrylic vessel of 2.75 m in diameter and 3.15 m in height, holds 16 tons of 0.1% Gd-doped LS as a neutrino target. It is surrounded by a 60 cm thick layer of 29 ton undoped liquid scintillators, useful for recovering γ-rays escaping from the target region.

Outside this layer there is a 70 cm thick buffer region filled with 65 tons of mineral oil which provides shielding against outside background. Light signals emitted from particles interacting in ID are detected by 354 low-background Hamamatsu 25cm R7081 oil-proof assemblies. The OD is equipped with 67 Hamamatsu 25cm R7081 oil-proof assemblies. The two detectors are installed underground to reduce as much as possible all the source of background.

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