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Poster Summary: Double Calorimetry System of the JUNO Experiment

March 14, 2017

(by Giuseppe Salamanna) JUNO’s main figure of merit is its resolution to the neutrino energy, crucial to determine the mass hierarchy within an acceptable data taking time (i.e. 3% at 1 MeV to distinguish between normal and inverted at 3 sigma in 6 years). The poster presents and illustrates a novel idea whereby two independent detection systems (PMTs) installed at the JUNO reactor experiment in China should improve the overall energy resolution by reducing the systematic uncertainties.


The figure shows how well the large (left) and small (right) PMT reconstruct the energy as a function of the radius at which a Co60 calibration source was placed in the simulated JUNO detector. The bias at large radii for the 20’’ PMT is clearly visible

Much like a set of two eyes, one larger and one smaller, provide 3D vision, where the larger one collects most of the light but is sometimes blinded while the smaller one just helps in defining the profiles, we suggest that a double calorimetry system based on the baseline 20’’ PMTs for JUNO, plus 3’’ PMTs installed in the interspaces among two large ones, can effectively reduce the non linearity and the non uniformity terms of the energy resolution. This because the small PMTs work almost all the time in the so called “photon counting” regime (i.e. they see 0 or 1 photo-electron -PE- coming from an impinging optical photon generated in the JUNO liquid scintillator).


In this comparison sketch, several neutrino experiments with a similar detection technique and aims are compared. The stress is on how wide the dynamic range of the JUNO baseline 20’’ PMTs is, given the chosen cathode surface, with respect to any predecessor;  and that the 3’’ PMT can complement this information with a punctual photon-counting like illumination regime to redress any saturation or bias effect of the large ones

On the contrary, the large ones offer the advantage of a high geometrical coverage and serve to optimize the stochastic term of the resolution; but risk to “saturate” or to be subject to a pile-up of many PE on the same PMT such that proportionality between the initial neutrino energy and the overall PE count is lost. This is especially true when the typical reactor nubar_e signature in the liquid scintillator (Inverse Beta Decay, or IBD) happens close to the border of the hughe JUNO central detector (20 kt of mass, diameter~36m): at large radii from the centre, in fact, by solid angle many photons hit the same PMTs which is just in face; it can see tens of PE.

By counting how many PE one has seen per energy unit with the smaller PMTs one can calibrate the response of the large ones as a function of E and R. The poster illustrates the problems, shows examples of how the large and small respond differently with a given calibration source (Co60, E~2.5MeV) and as a function of R – therefore the 3’’ ones can serve as a proxy for the “true” energy, breaking the non-linearity and non-uniformity entanglement.

Furthermore one can also use the 3’’ PMT to perform a full analysis of oscillations dominated by the solar terms theta_12: we show an example plot of this, which illustrate that a precise measurement of theta_12 is possible, and can be compared to the one from the 20’’ PMT as an in-situ calibration measurement. Finally, the fact that the tube is short for the 3’’ PMT helps in reducing the TTS and therefore helps in pinning down the times of arrival of photons from e.g. a cosmic muon; making muon tracking and cosmogenic suppression more effective.

The poster has 5 sections:
– “JUNO: an unprecedented Liquid Scintillator Detector”, which presents the context and general problem
– “ A new concept of double calorimetry”, which illustrated the new idea
– “Charge measurement of single channel”, which concentrates on the photon-counting vs many PE issue
– “Implementation for JUNO”, with the technical details of the system and candidate PMT models
– “Other benefits from SPMT” where the Small PMT benefits (TTS, solar nu oscillation parameters) are also shown

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