Nonlinearity Measurement in Liquid Scintillator Using Compton Scattering Method

1. V. Vorobel, JUNO Euro Meeting, Catania LS nonlinearity measurement in PragueVít Vorobel, Viktor Pěč, Tomáš TmějCharles University in PragueFaculty of Mathematics and Physics

2. Comptonscatteringmethod • Gamma of well known energy scatters in the liquid scintillator and release an electron there. The scattered gamma is registered with HPGe detector. • Coincidence signal amplitudes from both LS and HPGe detectors are recorded event-by-event. Energy of the electron released and deposited in LS is determined from precise measurement of scattered gamma in HPGein each individual event. • Dependence of the LS signal amplitude on the deposited electron energy is measured. precize HPGedetector Compton scattering ELS = Eg – EGe Liquid scintillator ELS EGe Gamma source Eg

3. Measurement setup V. Vorobel, JUNO Euro Meeting, Catania 60Co 241Am LS HPGe gamma detector stay at a fixed position, 50 cm from the LS detector. A stand with the LS detector stay on a support rotating around vertical axis of the detector. Radioisotopes are fixed to the stand, they rotate together with the stand around the detector, so the average scattering angle is adjusted. Radioisotopes, LS vessel and HPGe detector are at the same horizontal plane. Signals from both LS detector and HPGe detector are processed using a digitizer. The signal amplitudes and timestamps are recorded. HPGe

4. Measurement setup – scintillation detector V. Vorobel, JUNO Euro Meeting, Catania • PMT 3” is glued to the scintillator using optical grease. The assembly is installed into a light tight Al tube and equipped with PMT base. The base provides HV supply and signal preamplification. • During the apparatus commissioning, we gained experience using various scintillators and light collection wrappings. The presented data were measured with • Polystyrene cylinderf3”x3” wrapped with teflon tape (side surface) and aluminized mylar (top plain). • Daya Bay LS in Quartz vessel f75 mm, upper part of the vessel white painted, lower part covered with 2 layers of teflon tape and the whole vessel wrapped with aluminized mylar. LS level was just below the vessel neck. • Daya Bay LS in glass vesself55 mm, sides wrapped with 2 layers of teflon tape and with aluminized mylar, LS level covered with 20mm teflon piston.

5. Typical observed single channel spectra V. Vorobel, JUNO Euro Meeting, Catania 60Co – g1173 keV and g1332 keV for scintillator investigation 226Ra – set of gamma lines for HPGe detector calibration 241Am – g59.5 keV for monitoring of the scintillation detector stability. HPGe– calibration 226Ra + 60Co Scintillator 241Am + 60Co HPGe Energy resolution 2.26 keV @ g1332 60Co

6. Gain stability monitoring V. Vorobel, JUNO Euro Meeting, Catania Scintillator peak position 241Am Scintillator Drifts of low energy peak and high energy Compton edge follow the same trends. Drift in a single data set does not exceed statistical fluctuations. HPGe Typical day-night drift ~0.4 keVand week drift ~1.3 keVcan be corrected for with negligible uncertainty. Further stability improvement can be achieved in a room with more stable temperature. 18% Scintillator Compton edge position 60Co 18% HPGepeak position 60Co 0.25%

7. Coincidence of Scintillator with HPGe V. Vorobel, JUNO Euro Meeting, Catania Average scattering angle 40˚ 60Co: g1173 keV, g1332 keV Scintillator calibration Energy resolution FWHM 12-18 % @ 500 keV Time difference between PMT and HPGe signals Time window: 2400 ns DT FWHM ~350 ns

8. V. Vorobel, JUNO Euro Meeting, Catania Nonlinearity observation Average scattering angle 40˚ 60Co: g1173 keV, g1332 keV Sets of nonlinearity points obtained with different average scattering angles follow nearly the same curve in polystyrene case. Deviations from the curve are most probably due to non-uniformity of light collection. Polystyrene Daya Bay LS

9. V. Vorobel, JUNO Euro Meeting, Catania Effect of light collection non-uniformity All scattering angles on a certain circle are equal → energies transfered to electrons are equal. The scatterings which happen close to a wall provide less photons due to worse light collection. Treatment – better light collection, more sophisticated data analysis or collimation with Pb. Scintillator Gamma source HPGe

10. Measurement with LS in glass vessel example of strong non-uniformity effect V. Vorobel, JUNO Euro Meeting, Catania • The glass vessel f55 mm with concave bottom is wrapped with >3 layers of teflon tape and with aluminized mylar, 20 mm teflon piston covers LS top. • We observe significant effect of the light collection non-uniformity, probably due to concave shape of the vessel bottom. • Plan: • To profit from use of this vessel and to learn how to deal with the non-uniformity effect. • To improve the vessel design to minimize the non-uniformity – thick teflon side wall and cover, plain quartz bottom.

11. Nearest plans V. Vorobel, JUNO Euro Meeting, Catania • Non-uniformity effect suppression • Light collection – new vessel is prepared for measurement. • If the improvement of light collection and off line analysis do not help then we will use collimation. • Investigation of readout nonlinearity to be able to separate it from the LS nonlinearity. • Move the setup to a room with more stable temperature in order to stabilize gain in both LS and HPGe detectors. • Use of other isotopes to confirm the present measurement and to move to different energy range.

12. Summary V. Vorobel, JUNO Euro Meeting, Catania First version of the experimental setup is commissioned and its capability to measure scintillator response is proved. Testing measurement data were presented. Nonlinearity was observed. Space for setup improvement – better light collection, more stable temperature, use of collimators. Investigation of readout nonlinearity planed.