Chemical Additives in Water Cerenkov Detectors

1. Chemical Additives in Water Cerenkov Detectors R.Svoboda, U.C. Davis /LLNL This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.

2. Improved neutron detection antineutrino tagging (Super-K, SONGS) active neutron shield (LUX) national security (LLNL Portal Monitoring Project) Improved light collection antineutrino spectral measurements reduce required PMT coverage Unwanted additives what makes good water go bad?

3. Pros and Cons • Viable: • GdCl3 relatively inexpensive • Small concentrations of Gd in water improve neutron capture significantly • Gd capture signature (8 MeVγ- cascade) easily detectable • Workable? • What is GdCl3 effect on transparency? • What are the physical effects of GdCl3 caused by extended exposure to SK detector components? • Optimal?

4. Baffled joints Nitrogen purge and relief valve Light transmission arm Injection and Measurement Optics Alignment mirror drain mixing tank and pump Nitrogen purge Storage tank PMT tank Recirculation pump Deionizing Filtering Sterilization

5. LLNL Test Set-Up

6. PMT light integrator Acrylic window Light transmission arm PMT beam splitter reflected beam integrator primary beam integrator Lawrence Livermore National Laboratory LLNL Test Set-Up 337 N2 Laser w/ dye attachement 9.54 meters Not shown: collamators, baffles, filters

7. Typical Waveform for 337nm ns V reflected primary

8. By putting filters of known transmittance (<1% uncertainty) into the injected primary beam, the system is seen to be linear to within 2% over a 40% variation in transmission The system is also stable to variations in PMT gain to better than 1%

9. Stability… pure water fall off in transparency over time (337 nm) Stopped recirc Preliminary +/- 2% 0.9 %/day

10. Test of GdCl3 Addition at 337 nm Injected mixing tank Water and filtered Injected mixing tank Water and filtered Added 0.2% GdCl3 Removed GdCl3 ~13%/day Preliminary

11. Test of GdCl3 Addition at 400 nm Injected Pure Water @13 MOhm Added 0.2% GdCl3 Injected Pure Water @ 8 MOhm Preliminary

12. Test of GdCl3 Addition at 420 nm Injected Pure Water @ 13 MOhm Added 0.2% GdCl3 Injected Pure Water @ 8 MOhm ~8.5% /day Preliminary

13. Results • Pure water in stainless steel slowly looses transparency at 337, 400 and 420 nm. • For 337nm measurements, the water was deoxygenated via nitrogen bubbler to 0.9 ppm (typical air is 8-9 ppm) as measured by dissolved oxygen measurement. • For 400nm & 420nm measurements, initial dissolved oxygen was measured at .15 ppm. • Addition of GdCl3 makes the water transparency drop much faster (factor of 15). • Injection of water from polypro tank shows that water stored there suffered no/little degradation in transparency. • Loss of transparency directly from GdCl3 very small (consistent with 0 at all three wavelengths).

14. GdCl3 effect on transparency

15. Conclusion: GdCl3 is not a suitable additive for detectors with steel walls. May be OK for other materials.

16. Current Work:What makes good water go bad? Super-Kamiokande water must be continuously and cleaned – else transparency drops slowly similar behavior seen in IMB (plastic walls) and SNO (acrylic walls – but much slower degradation) REDUCING THE REQUIREMENT FOR RECIRC WILLLOWER COST OF MEGATON SCALE DETETOR

17. Test with FeCl3 • 10 ppm Fe+3 ion makes water look like ice tea. Clearly very low levels can affect transparency • next week we will test 0.1 ppm • slowly raise concentration to measure molar attenuation coefficient • test Ni, Cr metal ions for similar behavior

18. Future • Change steel pipe for acrylic one • use polypro tank for materials testing of HDPE and other potential plastic liners for LUX and future detectors • Investigate coatings for steel for cryostat treatment • monitor Water SONGS for stability (acrylic sides)

19. Backup slides

20. 15 cm 0.2% wt. GdCl3 50 cm 50 cm 100 cm Water SONGS 1 cm black Delryn lid 8 ea. 8” PMTs (1 cm spacing in both directions) Upper tank (5 sides) 3/8” thick UVT acrylic 4 sides external Tyvek wrap Pure water fill to 10 cm • lower tank (6 sides) • - 3/8” thick UVT acrylic, • Gd-water fill • 5 side external Tyvek wrap - total external dimensions = 100 cm x 50 cm w x 80 cm ht

21. Antineutrino Detection _ 𝜈e + p = n + e+ • The antineutrino interacts with a proton producing… • A 0-7 MeV positron (+ annihilation gammas) • A neutron which thermalizes, captures and creates a delayed 8 MeV gamma cascade • mean time interval ~30 μsec ~ capture time of neutron • Both energy depositions and the time interval are measured • The time since the most recent muon is also measured

22. Background • Antineutrinos are not the only particles that produce our coincident signal • Cosmic ray muons produce fast neutrons, which scatter off protons and can then be captured on Gd • Important to tag muons entering the detector 22