Double Chooz

The near detector was completed in September 2014, after construction delays,[1] and started taking data at the beginning of 2015.

Double Chooz used two identical gadolinium-doped liquid scintillator detectors[2] placed in vicinity of two 4.25 GW thermal power reactors to measure antineutrino disappearance.

The far detector was placed inside a hill such that there was a 300 meters of water equivalent of shielding from cosmic muons.

This chamber was filled with 10,000 liters of gadolinium (Gd) loaded (1 gram/liter) liquid scintillator; it was the neutrino target.

It surrounded the neutrino target with a 55 cm thick layer of Gd-free liquid scintillator.

The remainder of the interior space that wasn't occupied by the acrylic double vessel was filled with a non-scintillating mineral oil.

"[3][4] The inner veto surrounded the buffer vessel with a 50 cm thick layer of scintillating mineral oil.

The surrounding 15 cm thick steel casing further served to shield against external γ-rays.

[3][4] Signals from the inner detector and the inner veto were recorded by 8-bit flash ADC electronics with a sampling rate of 500 MHz.

The trigger threshold for the detectors was set to 350 keV, much lower than the 1.02 MeV expected of the electron anti-neutrinos.

[3][4] For several years Double Chooz had operated with only the far detector and had used models such as Bugey4 to calculate the expected flux.

Neutrinos are electrically neutral, extremely light particles that only interact weakly, meaning they can travel vast distances without ever being noticed.

Because one of the neutrino mass-squared differences is much smaller than the other, the Double Chooz experiment only needs to consider a two-flavor oscillation.

[4] In November 2011, first results of the experiment, using 228 days of data, were presented at the LowNu conference in Seoul, hinting at a non-zero value of θ13,[5] followed by an article submitted to arXiv in December 2011.

Shortly after, the Daya Bay experiment provided its confirming measurement with ≥5σ significance.

The central values of both Double Chooz and Daya Bay experiments were in excellent agreement and has remained so (within ≤2σ) so far.

Neutron capture on hydrogen was used to produce independent data, which was analysed to yield a separate measurement in 2013:[8] Using reactor-off data, a background-independent measurement[9] was published July 2014 in Physics Letters B: An improved measurement with reduced background and systematic uncertainties after 467.90 days of data was published in the Journal of High Energy Physics in 2014:[4] Double Chooz was able to identify positronium formation in their detector, which delays positron annihilation and distorts the scintillation signal.