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KamLAND Radioassay Progress in the United States Lawrence Berkeley National Laboratory with University of Alabama California Institute of Technology University of Tennessee
Low Background Counting Balloon Film • Goal: Assay radioimpurities in KamLAND • construction materials. • Identify KamLAND background problems. • Method: U/Th daughter and 40K gamma detection • by optimized Germanium detectors. • Only method to verify equilibrium in daughters! • Activity: over 30 samples measured in the past • year. A sample of most KamLAND • construction materials is catalogued • for possible future assay. • Some Results:U (ppb)Th (ppb)40K (ppb) • Balloon Film<3 <3 1.0+/-0.2 • Balloon Ropes<1 <3 0.8+/-0.3 • Cable Guides<4 <9 49+/-3 • Chimney Steel0.6+-0.2 2.0+-0.5 <0.1 • (60Co26+/-2 mBq/kg) Ge Detector KamLAND balloon film inside the Low Background Detector at Caltech
Activation Analysis of KamLAND Scintillator Goal: Reliable, Independent monitor of Purity and Backgrounds • Collection of Scintillator • Ship samples to US • Preconcentration • Remove Reactor-unfriendly organics • Irradiation at Reactor (ORNL/MITR) • Post-chemistry • Separate “Signal” isotopes from “Background” • g-b Counting(HPGe + Scintillator) • Analysis and Results Clean Hot
Neutron Activation Analysis KamLAND Scintillator purity needs are stringent! Reactor ( solar) experiment requirements are K<10-10 (10–14)g/g Th<10-14 (10-16) g/g U<10-14(10-16) g/g Activation Analysis Goals: • Verify scintillator component purity for reactor neutrino phase • Study purification schemes for solar neutrino experiment. Why Activation Analysis? • Direct counting not feasible: <10-14g/g U gives 0.01 decay/day/kg • Mass spectroscopy ultimately limited by chemical blank values. • Neutron capture cross sections and lifetimes are reasonable: 41K + n 1.3b42K(12.4 h) β-42Ca 232Th + n 6.5b 233Th(22.3 m) β-233Pa(27 d) β-233U 238U + n 2.4b239U(23.5 m) β-239Np(2.4 d) β-239Pu
Technical Issues at Irradiation Facilities • Technical Challenges: Pressure build-up through out-gassing of organic samples in high neutron flux environment. • Solutions: • Oak Ridge: Irradiate ashed samples. Utilize pneumatic sample insertion and long-term irradiation at highest-flux. Retention efficiency for U experimentally demonstrated through tracer experiments. • MIT: Irradiate smaller samples e.g. PPO Develop liquidirradiation facility (Engineering Proposal by MIT) Reactor irradiation fees are also significant! Optimize samples for efficiency.
Activation Analysis: Procedures Significant technical progress in the past year towards routine analysis of KamLAND scintillator! • High sensitivity analysis: Slowly evaporate scintillator to PPO residue.Ash residue in synthetic quartz. Irradiate for 10 hours at HFIR ( highest flux in the US) • Faster, lower sensitivity analysis: Irradiate ~2g of PPO in plastic vials in the MIT reactor. • Detection of 233Pa and 239Np by gamma ray spectroscopy with shielded Ge detectors. • Use gamma peak energies and decay times to identify target isotopes. Test irradiations show promising sensitivity and identify challenges! • KamLAND scintillator analyzed : U < 10-13g/g Th~2x10-13g/g • Primary limit on sensitivity comes from side activity interference: e.g. Na Br • Blank values need further control: U/Th ~4-6x10-11g/g from similar run.
Test Data:238U in KamLAND scintillator An example Germanium detector spectrum for 10-10 g U/g in 18.26 g of KamLAND Scintillator. The 239Np peaks at 228 and 277 keV are clearly seen.
Activation Analysis: 232Th in KamLAND Scintillator 232Th + n 233Th 233Pa( 39 days mean life) 233U+b+gs (311 keV) NAA Result: ~2x10-13gTh/g (ICPMS: 4x10-13 gTh/g) Test data showed NAA sensitivity may easily extend below 10-15 g/g through side activity background reduction. 233Pa Ge spectrum from activated scintillator The 233Pa peak at 311 keV can be seen. Background reduction would enhance sensitivity by 2 orders of magnitude.
Radiochemistry Improvements • October 2000 data showed that 239Np/ 233Pa detection is limited • by interfering backgrounds: 24Na, 82Br, 65Zn, 51Cr… • Solve this with chemical separation techniques! • Techniques and Results: • Actinide Absorbing Resin: • Digest sample in nitric acid and pour through a column of Actinide resin. • Np/Pa Efficiency~95% Typical Background rejection: x5-1000. • Tri-butyl Phosphate: • Digest sample in nitric acid and mix with Np/Pa absorbing organic liquid • tri-butyl phosphate. Count the TBP in a β-γ coincidence spectrometer. • Np/Pa Efficiency~90% Typical Background rejection: x10 • Chloroplatinic Acid: • Digest sample in HCl6Pt, evaporate, rinse salt with ethanol, and count. • K Efficiency>40% 24Na rejection >10. • March 2001: U/Th/K Sensitivity gain of ~100
Further Improvements: New Detectors A new ultra low background Germanium detector was added to our capabilities in Time for PPO counting, March `01. The high efficiency detector improves statistics of the counting considerably. Ultra low activity materials were carefully selected for its construction. The shielding hut of the new detector.
PPO Purity Verification • December – April 2001, NAA was employed to monitor • radiopurity of final Packard PPO production. • Sensitivity Goals: At 1.5 g PPO per liter scintillator, • 5ppt PPO impurity yields 1x10-14 g/g impurity in scintillator • January 2001: irradiation of small batches of • PPOs for first pass testing • Result: 5 test lots of PPO proven <500 ppt U/Th • March 2001: focused irradiation of one of final lots • for KamLAND (Lot 21-634) • Radiochemical techniques employed for further sensitivity. • Result: < 2.2 ppt Uranium in PPO • < 7.7 ppt Thorium in PPO • < 8.4 ppt Potassium-40 in PPO • The limits reach the required tolerance of the reactor experiment. • Further study of the final PPO lots continues.
Ge detector spectrum from activated KamLAND PPO:The three spectra compare the spectra before radiochemistry(purple), the spectra after extraction ( actinide ion column)(red), and the ambient detector background(blue).The bottom plot shows a closeup of the 233Pa peak region.
Future Direction: Coincidence Counting Delayed β-γ-e- decay signatures may offer further sensitivity. Spectrometers for this have been built. 239Np Decay Data from a feasibility study with a 239Np source. The TDC measures the delay between a β and conversion e- A Schematic coincidence detection spectrometer
Mass Spectroscopy • We are also investigating mass spectroscopic analysis for • KamLAND. It provides • a faster, but less sensitive, analysis technique than NAA, • an optimal technique for studying water, e.g. from • the KamLAND scintillator purification system. • Progress:In Dec.-Feb. 2001, a technique for preparing PPO • for ICP-MS was developed. • Selected Results: • KamLAND “test” Scintillator 0.2ppt U, 0.4 ppt Th • Dojindo PPO ( a final lot for KamLAND) <100ppt U/Th • Other PPOs compared (Acros, EMScience, Aldrich, • Alfa Aesar). Packard, Dojindo PPO confirmed best.
Summary of Progress • Established long term relationship for irradiation with reactors at Oak Ridge and MIT in the United States • Developed and tested processes to prepare scintillator for irradiation using preconcentration • Developed and tested processes to prepare PPO for irradiation • Scintillator U/Th sensitivity extended to 0.05 ppt. • PPO U/Th/K sensitivity extended to 5 ppt • Feasibility of <10-15 g/g sensitivity demonstrated through side activity background reduction. • Radiochemical tests of this reduction are in progress and have already significantly improved sensitivity.
Current and Future Directions • Further study radiochemical background reduction • Development of chemical actinide separation in progress, with resins or tri-butyl phosphate. • Redigest irradiated organics into scintillator and use delayed β-γ signatures to reject background • Upgrade facilities for fast analysis with a consistent and low sample blank • Dedicated clean rooms in preparation. • Develop pure quartz irradiation vessels • Establish clean Vacuum Ovens for scintillator preconcentration electronically controlled for reproducibility. • Study KamLAND Scintillator and the Purification process