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

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 ~ 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.

VT Recipe for InLS (Improved from US Patent (2004): Chandross & Raghavan, Filed 2001) Vacuum Evaporation HMVA (>98%)NH 4 MVA + NH 4 OH Stir Add NH 4 OH Equal vol. Of the org. Liquid-liquid extraction system InLS: In% = 8 S% > 55 L 1/e > 8 m Equal vol. Of the org. Add NH 4 Ac 1. Neutralization 2. On-line purification and solution preparation 3. Solvent extraction and vacuum evaporation InCl 3 Org. Waste TBPO:Tri-n-buylphosphine oxide Fluors: 5g/L PPO (2,5-Diphenyloxazole) and 15 mg/L Bis MSB (p-Bis(o- methylstyryl)benzene) Aq. Waste TBPO-toluene NH 4 MVA Dry Solid of In(MVA) 2 (OH) Dissolve in PC Add fluors Mix together with Hexane InAc 3 + NH 4 Cl In(MVA) 2 (OH) in Hexane

Purification of Methylvaleric Acid (HMVA) 1)Vacuum distillation 2)When transferred into NH 4 MVA solution, equilibrated with 0.5% TBPO-toluene solution twice to remove any organic or inorganic impurities.

Purification of Pseudocumene (PC)  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  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 (Al 2 O 3 ) column. Benzaldehydes (BZH)

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

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 H 2 O coexisting in the indium carboxylates In(Ac) 3 (aq) + MVA - (aq) In(OH)(MVA) 2 in Hexane In(OH)(MVA) 2 Solid InLS in PC Hexane Evaporation PC

Composition Analysis on the InLS (1)  Composition analysis were conducted on the InLS samples. The following chemical groups were measured: In III, MVA, (HMVA) 2, NH 4 +, 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

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  H 2 O/In was found to decrease with the extraction pH by the VT recipe. H 2 O/In = 0.6  The empirical formula for the indium carboxylate at pH 6.88: In(OH) 1 (MVA) 2

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. In MVA OH MVA O HO + + H 2 O Speciation of the Indium Complexes ZVT45 PC #24RT:0.47AV:1NL:6.53E5 T:+ c Q1MS [ ] m/z Relative Abundance zVt45 (pH 6.88, S=54%, d=0.97 g/ml). = ( ), MS Spectrum obtained on ) Mass Spectrometry of the InLS PC Desired Species of indium complex PHYLOWCON2 #45RT:1.27AV:1NL:8.71E6 T:+ p Q1MS [ ] m/z Relative Abundance zVt45 (pH 6.88, S=54%, d=0.97 g/ml). = ( ), MS Spectrum obtained on ) PC Additional species growing with time

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 = % Cs- 137 Sampl e vial PMT High voltag e Signa l Ground Photon yield Scintillation Yield Scintillation Light Yield

Optical Attenuation Length (L 1/e ) 8.6 m after 8 months Transparency of InLS The optical attenuation length (L 1/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. % L 1/e >> 10 m at the prep. L 1/e = 8.6 m after 8 m

SamplepHIn%S%S% at different time Begin1 Mon3 Mon5 Mon8 Mon9 Mon zVt zVt zVt zVt zVt zVt zVt zVt Long Term Stability of L 1/e of InLS  The S values of the samples were found not to change with time.  The L 1/e of the samples synthesized at pH 6.88 were found to stabilize in 3 months, and their L 1/e have stayed > 8 m for 8 months.  Optimum value for the extraction pH ~6.88

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 NH 4 Ac 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 H 2 O and acid and long-term stability; High scintillation yield ( S > 55%); Long attenuation length (L 1/e > 8 m at 430 nm ); L 1/e has been stable for > 8 months. Achievements

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

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.

 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 L scale for MINILENS (needs 200 L) OUTLOOK