Final scientific report
Project Title: Ultra-low level detection of the tritium groundwater samples for liquid scintillation spectrometry at underground laboratory Gran Sasso
LNGS Access period: November 2002 - April 2003
Group Leader: M.S. Cuna: National Institute of research-development for isotopic and molecular technologies, Cluj-Napoca (Romania)
Researchers: M.S. Cuna, J. Chereji: National Institute of research-development for isotopic and molecular technologies, Cluj-Napoca (Romania)
C. Baciu : Babes Bolyai University, Geology Department, Cluj-Napoca (Romania)
Delivered access (filled by TARI administration): 86 man-days
Final scientific report (filled by group leader):
Our experiment at LNGS has dealed with measurements of tritium content in water samples by using 1220 Quantulus LS spectrometer. We carried out these measurements in order to check the possibility of using such a high sensitivity counting technique for a lot of future environmental studies connected with tritium abundances in natural waters.
The tritium concentrations in such a samples are usualy very low, so them are measured after the enrichment of the water samples by electrolysis. The water electrolysis is nearly always used for the tritium enrichment. In order to enrich the tritium in the water samples by minimum 10 times we have developed a battery of 5 electrolysis cells. The procedure includes before the enrichment a primary distillation of the samples, and after the enrichment a final distillation.
Three water samples (drinking water, precipitation and atmospheric water) together with theirs duplicates were electrolysed in a single stage to attain en enrichment factor (EF) about 11 for the firs run, respectively of about 22 for the second run. Twenty samples were prepared taking in account a background (BKG) and a standard (STD) sample with a known content of tritium. For the liquid scintillation counting a mixture of 10 ml of water sample and 10 ml of Ultra Gold scintillator was introduced in a glass vial with lower content of kalium.
The EF for tritium was evaluated from EF measured by mass spectrometry for deuterium and by using Bigeleisen relation between separation factor (SF) for electrolysis:
log?=1.4log?, where ? is SF for tritium and ? is SF for deuterium
By using this procedure in order to maintain the overall precision governed by Bigeleisen relation (±5%), a constant value of the initial and final volume ratio was assured with a programmable electronic system that could interrupt the curerent through any cell in which the desired volume of electrolyte was attained.
With a BKG of about 1.47 cpm obtained for a measuring time of 120 hours, the counting statistics of the results revealed an excellent agreement in moisture samples, while a little scater was obtained in precipitation samples. Because the drinking water has no content of tritium, and the enrichment did not affected the count rate in Ch 5-200 (window in SP12 for 3H counting mode), the count rate of the sample 3 to 8 was taken as the background figure. The sample activities are show in the below table:
Sample activities given as fraction of sample STD calculated to the original unenriched activity
Sample notation EF Sample activity/STD activity Sample origin
9 2A0 1 0.001057042 precipitation
10 2B0 1 0.001416467 precipitation
11 2A1 11.4 0.001059844 precipitation
12 2B1 11 0.000945559 precipitation
13 2A2 21.3 0.000843469 precipitation
14 2B2 23 0.001115285 precipitation
15 3A0 1 0.013672498 moisture
16 3B0 1 0.013700565 moisture
17 3A1 11.3 0.013441818 moisture
18 3B1 11 0.013682366 moisture
19 3A2 22.5 0.013237366 moisture
20 3B2 22 0.013261236 moisture
We conclude that the method used for determination of the EF is valuable in the limits of ±5% errors, and has some advantages in comparison with the method that use the "spiked" cells. The most difficult problem that must be surmounted in the measurements of ultra-low concentration of tritium could be the elimination of the contamination during the sample preparation, especially with the tritium content of the moisture from the surrounding atmosphere.
Publications (quote also planned or in preparation) (filled by group leader):
Plastino W., Baciu C., Chereji I., Cuna S. M., De Felice P., Kaihola L.,
"Ultra low level tritium detection by liquid scintillation spectrometry at Gran Sasso National Laboratory", Applied Radiation and Isotopes, 2003, in preparation
Iosif Chereji, Stela Cuna, Wolfango Plastino, Nicolae Lupsa, Gabriela Muresan, Valentin Mirel, Petre Berdea, Calin Baciu, "Tritium enrichment of environmental waters by electrolysis" Studia Universitatis Babes Bolyai, Physica, Special Issue, 2003, in press
Evaluation by USP (October 2003)
The aim of this study was the measurement of tritium content in water samples underground in order to plan future environmental studies. The main objective has been achieved, but a more proper evaluation of the uncertainties of the data set is recommended by the USP. One publication in preparation to be submittedto Applied Radiations and Isotopes, a second in press on Studia Universitatis Babes Bolyai.