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KamLAND Radioassay Progress in the United States

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.

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KamLAND Radioassay Progress in the United States

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  1. KamLAND Radioassay Progress in the United States Lawrence Berkeley National Laboratory with University of Alabama California Institute of Technology University of Tennessee

  2. 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

  3. 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

  4. 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

  5. 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.

  6. 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. • 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, including 24Na and 82Br

  7. 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.

  8. 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!

  9. 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.

  10. PPO Purity Verification • December – April 2001, NAA was first 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.

  11. 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.

  12. 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

  13. 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. Studies to • improve sensitivity are continuing. • 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.

  14. Accomplishments • We have established the basic counting equipment and reactor agreements necessary to analyze activated samples. • We have developed clean procedures and equipment for preparing samples for analysis. • We have demonstrated feasibility of analyzing organic samples and proven our proficiency in this technique. • We have developed the analysis sensitivity to the level required for the KamLAND reactor experiment. • We have already contributed to the experiment by verifying the purity of test scintillator and the first KamLAND PPO productions.

  15. Directions: I • We will begin detailed study of KamLAND PPO, oil, and scintillator to understand its purity and the purification process in general in time for the start of the reactor experiment. Equipment and procedures for sample transport to the United States are now in place. Operating costs in reactor irradiation fees, hazardous sample shipment from Japan, and clean materials will be substantial!

  16. Directions II • We will continue to upgrade the sensitivity of the analysis techniques pushing towards the requirements of the solar experiment. • Simulations usingthe exterior material radioassay show that the KamLAND solar experiment is still limited by scintillator purity. • Further background reduction through radiochemistry is achievable. • Equipment can be established at the MIT reactor to process samples soon after irradiation, reducing sample cooling delays. • Longer irradiation times (12 hours) have been approved MIT. • Upgrade the basic existing clean room and detector equipment for lower blank and higher sensitivity. • A direct liquid irradiation facility with very low blank is under discussion at the MIT reactor.

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