The half-life of /sup 239/Pu was measured by determining the ingrowth of /sup 235/U during a known time by isotope dilution mass spectrometry techniques

Dupzyk, R. J.

Evans, J. E.

Newbury, R. S.

Prindle, A. L.

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Y MASTffl PREPRINT UCRL- 78978 QfrA fr-W&ys^ ̂ 4\ ' • * Lawrence Livennore Laboratory THE HALF-LIFE OF 2 3 9 P U AS DETERMINED BY PULSE-COUNTING MASS SPECTROMETRY AUSTIN L. PRINDLE, JAKES E. EVANS. RAY S. NEWBURY, AND R. JENE DUPZYK 7-1-77 THIS PAPER WAS PREPARED FOR SUBMISSION TO: 25TH ANNUAL CONFERENCE ON MASS SPECTROMETRY AND ALLIED TOPICS AMERICAN SOCIETY FOR MASS SPECTROMETRY MAY 29 - JUNE 3, 1977 WASHINGTON. D. C. 3 This It a praprint of a paper intanded for publication In a ioumal or proceedings. Since changes may '.a made before publication, this preprint is made available with the understanding that It will not be cited or reproduced without the permission of the author. * * • • !U!W' f ».. 0ISTRIBU1 iON QE IHIS QQCUMENI IS UNIOMHEB PULSE-COUNTING MASS SPECTROMETRY Austin L. Prindle, James E. Evans, Ray S. Newbury, and R. Jene Dupzyk Lawrence Livermore Laboratory University of California Livermore, California 94550 239 He have Measured the half-life of Pu by determining the ingrowth of U during a known time by isotope dilution mass spectrometry techniques. 239 235 When Pu decays, it gives off energetic a particles and fonts U. Writing it generally, N-^— M + a + E, in which A is the characteristic decay constant for the process. The relationship between A and the desired half-life is t^/2*s^n2/A. Solving the rate equation for short periods of tine gives: -AN©/NAt - A*P/NAt « Aa9/NAt - ,AE®/NAt - A. The classical radiochemist would use number Qi the calorimetrist, number Oft we mass spec-trometrists chose number©. We received from LASL the master plutonium solution with a cnown number 239 of Pu atoms in it on which to do the half-life study. Some c the master 242 solution was used to calibrate a high-purity Pu tracer mass spectromet-rically. Five portions of the master solution, each containing approximately 237 10 mg of plutonium, were aliquoted for the growth experiment. U tracer which had been calibrated by gamma-ray counting on a Ge(Li) diode was added to two of the five samples and a reagent blank. Each of the snmples underwent purification by ion exchange to completely remove u pric- to 235 237 the U ingrowth period. After purification the samples traced with U 235 were found by gamma counting to contain less than 20 ppm of the U . es<nt in the master solution. Each of the five samples was placed in a volumetric flask, diluted to volume, and weighed. A small aliquot of this solution was weighed and 242 239 diluted for assay with the Pu tracer. This assay established the Pu concentration of the purified solution. The remainder of the solution was 233 transferred to a clean flask and weighed. U tracer, prepared by Allied Chemical Corporation for the Safeguard Analytical Laboratory Evaluation Program (SALE)# was added to each flask and the solution allowed to grow 2 3 5 u . At the end of the growth period, the uranium was separated from pluto-i 233., 235 nium by ion exchange and the time precisely recorded. The ratio of U/ U was measured mass spectroraetfically on two mass spectrometers: MSXL a tandem 60°-sector mass spectrometer of 34 ca radius operating in the surface ionization mode; HSVIII a sequential dc le focusing 60° instrument of 34 cm radius also operating in the surface ionization node. Plutonium or uranium chloride was dried and calcined on the tantalum sample filament and mounted in the source of the spectrometer approximately 1 mm from the rhenium ionizing filament. Xn each system, the electron 01STRIBUT.CN Or TH'S DOCUMENT IS UNLIMITED [ U Multiplier output pulses were fed through a preamplifier into a multisura-ming scaler. A dedicated PDP-8 computer was programmed to abstract the information from the scaler, subtract the background, make multiplier coin­cidence corrections, and output the isotope ratios in a suitable format. Each sample filament was run in both spectrometers. In the case of the Pu samples, mass bias corrections are self-cancelling. NBS uranium standards run in both spectrometers showed no mass bias for uranium. He show here the results for one of the five samples as an example of the 2 3 9 P u half-life determination. Table I shows the data for the assay of 239 242 the dilution of HS-1 for Pu using the Pu tracer. Combining the 14 239 average value of 3.71786 x 10 Pu atoms/gram with the dilution factor (56250.0) we obtain 2.09130 x 1 0 1 9 atoms of 2 3 9 P u in the HS-1 sample. 235 Table II shows the quadruplicate measures of the uranium §4y ratio in HS-1 at the end of the growth period. Combining this ratio with the known amount of tracer added (2.80858 QQADRUP Sample 1 / 2 (3.68430 x 1 0 1 4 atoms) { Table I LICATE PLUTONIUM ASSAY SAMPLES FROM MS-1 ID Pu atoms/q of HS-1 dilution 14 1 4 ( a ) 3.71580 x 10 x" + 0.00026 X 1 0 " 3.71734 X 1 0 1 4 + 0.00020 X 1 0 1 4 3.71S18 x 1 0 1 4 + 0.00039 X 1 0 1 4 3.72311 x 1 0 1 4 + 0.00039 X 1 0 1 4 3.71786 x 1 0 1 4 + 0.00181 X 1 0 1 4 - — - •= 24,liU.l 365.243 days/year) Table II 235 RATIO OF j | | IN MS-1 235 Run # Ratio K » Oranium HS-1-1 MS-1-2 MS-1-3 HS-1-4 AVERAGE XL-1-1 1.31123 + 0.00026 XL-1-2 1.31072 + 0.00020 XL-1-3 1.31292 + 0.00040 HSVIII-1 1.31216 + 0.00059 AVERAGE 1.31176 + 0.00049 (a) standard deviation based on counting statistics. Table III Sample Identification Half-life in Years HS-1 24,110.1 HS-2 24,128.2 HS-3 24,070.6 KS-4 24,071.9 Q -2 24,065.9 AVERAGE 24,089.3 + 12.5 Estimating systematic errors and adding them in quadrature to the 239 random errors calculated gives for the half-life of Pu 24,089 + 22 years. Itotk performed under the auspices of the O.S. Energy Research I Development Administration under contract Ho. W-740S-Eng-48. 

Half-life of /sup 239/Pu as determined by pulse-counting mass spectrometry

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Half-life of /sup 239/Pu as determined by pulse-counting mass spectrometry

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