Difference between revisions of "Test bench for measurements of the NOvA scintillator properties"

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'''!It is possible to find current status and all technical features [https://www.overleaf.com/18042451xdfnwgvjmfkk#/68324394/ here]!.'''
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[[User:AlexanderAntoshkin|AlexanderAntoshkin]] ([[User talk:AlexanderAntoshkin|talk]]) 23:53, 21 August 2018 (MSK)

Revision as of 23:53, 21 August 2018

NOvA scientific goals require good knowledge about its scintillator basic properties.

The new test bench was constructed and upgraded at JINR. The main goal of this bench is to measure scintillator properties (for solid and liquid scintillators), namely alpha/beta discrimination and Birk's coefficients for protons and other hadrons (quenching factors). This knowledge will be crucial for recovering the energy of the hadronic part of neutrino interactions with scintillator nuclei.

alpha/beta discrimination was performed on the first version of the bench for LAB-based and NOvA scintillators. It was performed again on the upgraded version of the bench with higher statistic and precision level. Preliminary result of quenching factors for protons was obtained. A technical description of both versions of the bench, analysis and data taking procedures, current results of the measurements and analysis are presented here.

What is a scintillator?

One of the methods of detecting ionizing radiation in experimental physics is to use scintillators. A charged particle passing through a substance deposits its energy within. Part of this energy goes to photon production. For some substances (scintillators) this portion is significant, so that generated light can be detected and measured by photosensors or photodetectors. The spectrum and intensity of the light signal depends on the intensity of the energy release, and the type of passing particle and attributes of the scintillator. Many scintillators, depending on radiation length, are sensitive not only to charged particles, but also to gamma-radiation and neutrons.

These are the main attributes of scintillators: light yield, the spectral composition of radiation, energy resolution, decay time, radiation resistance, radiation length and quenching factors -- Birks Law.

Birks Law

The scintillation response, S, of organic crystals depends on the nature and energy, E, of the incident ionizing particle, of residual range r.

The specific fluorescence, dS/dr, is not in general proportional to the specific energy loss dE/dr. By considering the quenching effect of the molecules damaged by the particle by the "excitons" produced by it, it is possible to show that:

dS/dr = (A*dE/dr)/(1 + kB*dE/dr)

Where A and kB are constants, which have been evaluated for scintillator from observations of S and E, and the range-energy data. The method used for evaluating the relative response is applicable to ionizing particles of any nature or energy, and also to the different organic scintillation crystals or liquids.

The quenching factor is a very important attribute of any scintillator. If we do not know the quenching factor, we cannot say what kind of particle passed through the scintillator and what primary energy it had.

Methodical aspect of the measurement and hardware setup

The first version of the test bench consists of a few components. First is the Black Box with three sections. The top and bottom sections are used like a muon telescope. A muon telescope helps one to divide the light from muons and radioactive sources. Light is measured by PMTs (Photomultiplier tubes) and digitized by ADCs (Analog-to-digital converters). Inside the middle section it is possible to place scintillator samples and all necessary radioactive sources (for alpha/beta ratio measurements). Second is the neutron source (Pu-Be) with the NaI crystal. And the final component is scintillators and different radioactive sources.


Firstly we have to measure the alpha/beta ratio for a scintillator. This procedure is necessary for the calibration of our ADC and testing of our method. After that it is possible to measure the quenching factors for different hadrons.

In the case of NOvA we want to measure Birk's coefficients for protons with energy in the range of 1-14 MeV. But it is not so easy to deliver protons with this energy inside the scintillator. We have to use a neutron source and measure the energy of recoil protons. Recoil protons are produced in the liquid scintillator by neutron-proton scattering events using a neutron generator ING-27 with a monochromatic energy distribution or a Pu-Be source with continuous spectrum. Hence, the proton light output function can be determined from the position of the recoil proton edge in the pulse-height spectra produced by mono-energetic neutrons. In the case of a continuous spectrum it is possible to use the time-of-flight method for speed/energy determination.



Experimental setup of the upgraded test bench

With this hardware configuration we had a problem with precision level. It is mostly connected with the electronics. So we decided to upgrade the test bench.

In addition to the existing part we added several components. The first is a Black metal box with sealing rubber and all necessary connectors. It is used to isolate the inner structure of the box from light. The second is a PMT with divider and teflon cuvette. Additional cuvette (it's inner sizes are 50 x 50 x 50 mm and it is made from optical glass) and PMT (3' Hamamatsu R12772) increases the statistic and allows us to cross-check the measurement process and result. The PMT and cuvette are put inside the box in a vertical orientation. The third is all support electronics like ADC (with higher sampling -- it gives us required precision level), High-Voltage source and laptop with required software.



Tandem of the benches

The final scheme of hardware setup is presented in the picture below. We do not have muon telescope for the addition Black Box.


Data taking

We have 3 active channels -- CH1 is a signal from NaI, CH3 is monitoring LED signal and CH4 is a signal from liquid scintillator.

Logic as follows -- we are looking for coincidence between NaI and Liquid Scintillator (CH1 and CH4) or LED and Liquid Scintillator (CH3 and CH4).


Our ADC is DRS4 manual.

Main parameters of the data taking

  • Flight path -- 1.6 m in our case.
  • 4.43 MeV gamma like the START point.
  • We are interested in time difference between START and STOP (signal from liquid scintillator) points higher than 46 ns and lower than 120 ns (ToF for neutrons). It corresponds to neutrons energy between 1-6 MeV. These energy limits and START point are connected with reaction channel (alpha + 9Be -> 13C* -> 12C* + n -> 4.43 MeV gamma -- first excitation state of 12C). E_{excitation}(13C*) = 10.6 MeV.
  • Connection between time of flight and energy is: T(ns) = (72.3[ns] * L[m])/sqrt(E[MeV])
  • Result of the energy calibration -- we are using several gamma sources (60Co, 137Cs, 228Th). We are looking for Compton edge (half-height). Compton edge energies as follow -- 478 keV for 137Cs, 1041 keV for 60Co, 405 keV and 2381 keV for 228Th.

Energy calibration


Analysis procedure

Our main parameter is time difference between CH1(NaI) and CH4(LS). It is possible to select neutrons with defined energy using this time distribution.

After that one can extract light output on liquid scintillator (charge) on condition that NaI signal corresponds to 4.43 MeV gamma and build Recoil proton energy vs Scintillator response curve. We are looking for half-height of the edge in light yield of liquid scintillator. This point corresponds to single interaction between incident neutron and proton inside the scintillator when neutron transmits all of his energy.


We repeated alpha/beta ratio measurements using the same method but on the new version of the bench with higher statistic and precision level. Also we performed a measurement of the quenching factor for recoil protons using a Pu-Be neutron source and compared it with the German article results for LAB-based scintillator and for the NOvA scintillator.

Results are as follows:


For the Gd+Cm source quenching factors for LAB-based scintillator (\alpha) are equal to 22,2 for Gd (energy 3,183 MeV) and to 16,5 for Cm (energy 5,795 MeV). For the NOvA scintillator they are equal to 23,58 and 17,6.



!It is possible to find current status and all technical features here!.

AlexanderAntoshkin (talk) 23:53, 21 August 2018 (MSK)