The Indium-loaded Liquid Scintillator (InLS)

1. The Indium-loaded Liquid Scintillator (InLS) Zheng Chang*, Christian Grieb and Raju S. Raghavan Dept, of Physics, Virginia Tech, Blacksburg, VA 24061 Richard L. Hahn, Minfang Yeh and Alexander Garnov Chemistry Dept., Brookhaven National Lab, Upton, NY 11973 Jay Benziger Dept. of Chemical Engineering, Princeton University, Princeton, NJ 08544

2. Objectives • The synthesis of 125 tons of high quality indium-loaded liquid scintillator (InLS) is a key technology for the success of LENS. Stringent requirements on the InLS must be realized in the development of LENS detector: • Basic robustness and reproducibility of the chemical recipe for metal loading; • High metal loading, typically ~ 8 - 10 wt. %; • Long optical attenuation length, typically > 8 m; • High scintillation yield ~ 55% of the unloaded solvent; • Long term stability on the scale of years; • Choice of solvent vis-a-vis health/safety in an underground environment.

3. 1. Neutralization Add NH4OH HMVA (>98%) NH4MVA + NH4OH 2. On-line purification and solution preparation Equal vol. Of the org. Equal vol. Of the org. TBPO-toluene NH4MVA TBPO-toluene NH4MVA Mix together with Hexane Org. Waste Add NH4Ac InAc3 + NH4Cl InCl3 3. Solvent extraction and vacuum evaporation InLS: In% = 8 S% > 55 L1/e > 8 m Liquid-liquid extraction system Stir In(MVA)2(OH) in Hexane Vacuum Evaporation Dry Solid of In(MVA)2(OH) Dissolve in PC Add fluors Aq. Waste TBPO: Tri-n-buylphosphine oxide Fluors: 5g/L PPO (2,5-Diphenyloxazole) and 15 mg/L Bis MSB (p-Bis(o-methylstyryl)benzene) VT Recipe for InLS (Improved from US Patent (2004): Chandross & Raghavan, Filed 2001)

4. Purification of Methylvaleric Acid (HMVA) • Vacuum distillation • When transferred into NH4MVA solution, equilibrated with 0.5% TBPO-toluene solution twice to remove any organic or inorganic impurities.

5. In the PC from Sigma-Aldrich, three groups of bensaldehydes were observed at the level of 50 ppm. These impurities lower the optical transparency of PC at a wavelength of ~430 nm. GC-MS of Pseudocumene Purification of Pseudocumene (PC) Benzaldehydes (BZH) • BZHs were found as major impurities (<100 ppm) in commercial PC by GC-MS. • BZHs are produced by the oxidation of PC in the air over a long period of time. • BZHs lower the optical transparency of PC. • BZHs can be removed by passing PC through the alumina (Al2O3) column.

6. Hexane In(Ac)3 (aq) + MVA-(aq) In(OH)(MVA)2 in Hexane In(OH)(MVA)2 Solid InLS in PC Evaporation PC Solvent Extraction Org. Phase (Hexane) In(OH)(RCOO)2 InLS • Remove Hexane • Dissolve in PC • Add the fluors In3+ + OH- +2RCOO- In(OH)(RCOO)2 H+ + RCOO- RCOOH NH3 + H2O  NH4+ + OH- In(Ac)3  In3+ + 3Ac- … … Aqueus Phase (Water) RCOOH: 2-methylvaleric acid PC: 1, 2, 4-trimethylbenzene

7. Hexane In(Ac)3 (aq) + MVA-(aq) In(OH)(MVA)2 in Hexane In(OH)(MVA)2 Solid InLS in PC Evaporation PC Solvent Evaporation • Solvent evaporation step provides many advantages, such as • to be able to select a proper solvent for the solvent extraction • to be able to select an ideal chemical as the scintillator solvent • to decrease the H2O coexisting in the indium carboxylates

8. Composition Analysis on the InLS (1) • Composition analysis were conducted on the InLS samples. The following chemical groups were measured: InIII, MVA, (HMVA)2, NH4+, Cl- • Total-MVA/In decreases with the extraction pH. Total-MVA/In = 2.1 • HMVA was found in the form of (HMVA)2 and decreases with the extraction pH. HMVA/In = 0.1

9. Composition Analysis on the InLS (2) • OH- groups was found to maintain constant between pH 3.8 and 5.5, and to increases with the extraction pH. OH-/In = 1.0 • H2O/In was found to decrease with the extraction pH by the VT recipe. H2O/In = 0.6 • The empirical formula for the indium carboxylate at pH 6.88: In(OH)1(MVA)2

10. PHYLOWCON2 # 45 RT: 1.27 AV: 1 NL: 8.71E6 T: + p Q1MS [ 50.00-1500.00] 83.1100 100 95 MVA HO MVA OH In 90 In + MVA MVA 85 MVA O MVA 80 + H2O In In MVA MVA 75 219.2600 Relative Abundance 70 65 121.1200 60 Additional species growing with time 55 PC 50 45 40 35 30 25 20 255.3100 15 739.3600 637.3300 1096.4200 10 550.4300 272.1800 391.3200 5 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 m/z zVt45 (pH 6.88, S=54%, d=0.97 g/ml). Abs@430nm = 0.003 (10-06-05), MS Spectrum obtained on 11-11-05) Mass Spectrometry of the InLS 120.0581 PC Desired Species of indium complex 251.1692 219.1216 zVt45 (pH 6.88, S=54%, d=0.97 g/ml). Abs@430nm = 0.004 (03-22-06), MS Spectrum obtained on 03-29-06) ZVT45 PC # 24 RT: 0.47 AV: 1 NL: 6.53E5 T: + c Q1MS [ 50.00-1200.00] 100 95 90 85 80 75 70 65 60 55 50 Relative Abundance 45 40 35 30 25 20 15 10 5 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 m/z Speciation of the Indium Complexes The InLS samples were studied by electrospray mass spectrometry over time. Indium oligomers were found to grow in amount and types as pH > 7. As pH < 7, less OH- groups are present in the carboxylates, and the oligomerization process becomes insignificant.

11. Cs-137 Sample vial Ground PMT Scintillation Yield High voltage Photon yield Signal Scintillation Light Yield The scintillation yield (S) is mainly affected by the content of the organic acid in the InLS. The chemical speciation of the indium carboxylates does not affect the S% significantly. For Typical Samples At In-loading > 8 wt. % S = 55 - 65%

12. Transparency of InLS 8.6 m after 8 months Optical Attenuation Length (L1/e) The optical attenuation length (L1/e) is very sensitive to the chemical species and trace impurities in the InLS. Both the organic impurities and the oligomers of the indium carboxylates absorb the light at wavelength of ~ 430 nm. For Typical Samples In-loading = 8.2 wt. % L1/e >> 10 m at the prep. L1/e= 8.6 m after 8 m (@430nm)

13. Long Term Stability of L1/e of InLS • The S values of the samples were found not to change with time. • The L1/e of the samples synthesized at pH 6.88 were found to stabilize in 3 months, and their L1/e have stayed > 8 m for 8 months. • Optimum value for the extraction pH ~6.88

14. Achievements A new synthesis procedure, VT recipe, has been developed on the basis of previous experiments. This procedure has two special features: liquid-liquid extraction with high concentration of NH4Ac and vacuum evaporation to get solid indium carboxylates. The final InLS can be made in a desired scintillation solvent (e.g. PC) at the desired indium concentration. With the VT recipe, we have achieved: • Low content of H2O and acid and long-term stability; • High scintillation yield (S > 55%); • Long attenuation length (L1/e> 8 m at 430 nm); • L1/e has been stable for > 8 months.

15. In(III) Species in the Solvent Extraction (Aq.) (Org.) In(Ac)3 In(MVA)2+ In(OH)2+ In(MVA)2+ In(MVA)(OH)+ In(OH)2+ In(MVA)3 In(MVA)2(OH) In(MVA)(OH)2 In(OH)3 In(MVA)4- In(MVA)3(OH)- In(MVA)2(OH)2- In(MVA)(OH)3- In(OH)4- In(MVA)3 In(MVA)2(OH) In(MVA)(OH)2 In(OH)3 (?) In2(MVA)6 In2(MVA)4(OH)2 In2(MVA)2(OH)4 In2(OH)6 (?) Phase Distribution Oligamerization

16. Speciation Study In the Future A speciation study should be conducted in the future to get a comprehensive understanding of the solvent extraction and the oligomerization processes. This knowledge is vital to the design of the large-scale synthesis and to the improvement of the long-term optical stability.

17. OUTLOOK • Study the thermodynamic parameters of the VT process. Monitor long term behavior and optimize recipe • Determine practical quality control parameters for large scale commercial production • Engineering transition from 100 ml production scale to 10-100 L scale for MINILENS (needs 200 L)