NAA Results for KamLAND PPO Samples, activated at MIT 1/27/2001



Zelimir Djurcic and Andreas Piepke
University of Alabama
(Status 2/9/2001)
(Updated 2/21/2001)<


In this note we discuss the results of the analysis of KamLAND PPO samples which have been activated at the research reactor of MIT on 1/27/2001 for one hour each. Analysis of two flux standards at Caltech yielded the following neutron fluxes for these irradiations:
Thermal neutron flux: 4· 1013 / (cm2 s)
Epithermal neutron flux: 1.2· 1012 / (cm2 s)
The Caltech web site gives a summary of their analysis effort.
The following table lists the samples which have been activated and gives the lab which has taken responsibility for the counting.
All samples were prepared in the clean room at UA, shown in the image on the left. The sample containers used were cleaned by double etching with 50% nitric acid followed by rinsing with double de-ionized water. All handling was done with disposable gloves. After filling all vials were packed using virgin zip lock bags.


Sample Summary
Sample code Counted at Sample description Sample mass [g] Irradiation schedule (EST)
CIT 1 CIT Dojindo 2.6477 1/24/2001 13:20 to 14:20
CIT 2 CIT Packard purified (21-1011) 1.2283 1/27/2001 1:09 to 2:09
CIT 3 LBL Packard unpurified (21-1012) 1.8678 1/27/2001 2:12 to 3:12
CIT 4 CIT Large plastic vial 2.6530 1/27/2001 2:12 to 3:12
CIT 5 CIT Small plastic vial 1.3392 1/27/2001 2:12 to 3:12
CIT 6 CIT Packard PPO 9 0.5623 (PPO) 1/27/2001 3:16 to 4:16
in small plastic vial with wool 0.2562 (wool)
CIT 7 LBL Packard PPO (21-0108) 1.4858 1/27/2001 1:09 to 2:09
CIT 8 CIT Flux standard 1/27/2001 2:12 to 3:12
UA 1 LBL Dojindo 2.6314 1/27/2001 4:23 to 5:23
UA 2 UA Packard purified (21-1011) 1.1956 1/27/2001 5:32 to 6:32
UA 3 UA Packard unpurified (21-1012) 1.6442 1/27/2001 6:34 to 7:35
UA 4 UA Small plastic vial with wool 1.3253 (vial) 1/27/2001 7:38 to 8:38
0.4048 (wool)
UA 5 UA Packard (21-0113) 0.3436 (PPO) 1/27/2001 7:38 to 8:38
in small plastic vial with wool 0.3487 (wool)
UA 6 LBL Alfa-Aesar (B8673A) 1.2350 1/27/2001 5:32 to 6:32
UA 7 LBL Aldrich Chemicals (11106C1) 1.1713 1/27/2001 6:34 to 7:35
UA 8 CIT Flux standard 1/27/2001 7:38 to 8:38


Discussion of Results


For all samples we measured the gamma ray spectrum using a shielded Ge detector pictured to the left. Repeated measurements of one hour length each were performed in order to identify the radio isotopes present after the irradiation by both their characteristic gamma peaks and their live time.
In the following we report the counting results obtained at UA. We will first discuss our re-evaluation of Packard PPO lot 21-0113 results (sample UA 5). This is the lot for which we initially reported a high U and Th content. Inspection of the gamma ray spectrum of this sample again shows evidence for the U and Th activation products Np and Pa at the same concentrations as reported before. The time dependence of the Np activity derived from our data is compatible with the known half live. Determination of the half live works well as can be seen in case of a high-statistics 82Br sample. However, in contrast to our previous conclusion removal of the quartz wool from that sample results in a spectrum showing no evidence for either Np or Pa. This is supported by an independent measurement in which an activated sample of quartz wool (which did not come in contact with any PPO) yields the same Np and Pa activities as the PPO sample of interest. We conclude that the U and Th reported previously was contained in the quartz wool used to retain the sample during neutron activation.
In the following we will present the data collected. All activity measurements were made with reference time 1/31/2001 10:04, the time of the first counting run. Activation calculations are then performed referring to the same time. The following table summarizes the activities measured for the various samples. The errors quoted are the sum of the statistical and a 10% systematic error. The systematic error mainly reflects differences in the solid angle between sample changes.




Sample 24Na activity [Bq] 42K activity [Bq] 82Br activity [Bq] 233Pa activity [Bq] 239Np activity [Bq]
UA 5 Packard (21-0113) -3.3± 5.9 1500± 150 -0.3± 0.3 -0.6± 1.6
UA 3 Packard (21-1012; 0.8717 g) 1024± 100 17± 10 124± 12 0.06± 0.2 -0.4± 0.8
UA 2 Packard (21-1011; 1.0002 g) 1163± 116 26600± 2600 -9.6± 2.7
Quartz wool 832± 83 23± 5 16± 3 2.3± 0.4 43.9± 4.5
Small plastic vial 76± 8 0.05± 0.08 1.1± 0.7

For the new Packard samples lots 21-1012 (un-purified) and 21-1011 (purified) we used data sets obtained after externally cleaning the samples vials with nitric acid. Part of the sample had left the vial ("boiled over") during irradiation. Above table gives the re-measured mass after removal of the excess material. For above samples the masses scaled like 0.530 and 0.837 compared to the original mass, respectively. Please note that the measured 82Br activities scaled like 0.53+/-0.08 and 0.87+/-0.12 confirming that the Br is indeed contained in the PPO. The table also shows that Packard's purification introduced a considerable amount of Br which severely limits our sensitivity. However, it is also observed that the 24Na activities scale like 0.37+/-0.05 and 0.56+/-0.08 for samples 21-1012 (un-purified) and 21-1011 (purified), respectively. This over-proportional loss in 24Na activity suggests that some contamination was present on the outside of the sample vial which has been removed by the etching.

The following table summarizes the derived concentrations of the parent elements using NAA. Note that for Packard samples 21-1012 and 21-1011 the vial background has not yet been subtracted. Samples labeled in red were analyzed at UA, green samples at Caltech.



Sample Na [g/g] K [g/g] Br [g/g] Th [g/g] U [g/g]
UA 5 Packard (21-0113) (-0.8± 1.5)· 10-6 (2.7± 0.3)· 10-6 (-900± 900)· 10-12 (-500± 1200)· 10-12
UA 3 Packard (21-1012) (5.2± 0.5)· 10-6 (1.7± 1.0)· 10-6 (92± 9)· 10-9 (80± 300)· 10-12 (-100± 200)· 10-12
UA 2 Packard (21-1011) (5.1± 0.5)· 10-6 (17.1± 1.7)· 10-6 (-10.5± 3)· 10-9
CIT 2 Packard (21-1011) (4.3± 0.4)· 10-6 <1.43 · 10-6 (14.9± 2)· 10-6 <1.2· 10-9 <290· 10-12
CIT 6 Packard lot 9 <5· 10-9 (55± 25)· 10-12 <125· 10-12
CIT 1 Dojindo PPO (0.23± 0.08)· 10-6 <400± 10-9 (15± 2) · 10-9 (170± 70)· 10-12 <100· 10-12
Quartz wool UA analysis (8.6± 0.9)· 10-6 (4.7± 1.0)· 10-6 (140± 20)· 10-9 (6.2± 1.1)· 10-9 (27± 3)· 10-9
Quartz wool CIT analysis (57± 13)· 10-9 (4± 1)· 10-9 (24± 3)· 10-9
Small plastic vial UA analysis (40.0± 4)· 10-9 (40± 60)· 10-12 (200± 130)· 10-12
Small plastic vial CIT analysis (130± 20)· 10-9 (110± 20)· 10-9 (16± 2)· 10-9 (46± 30)· 10-12 <43· 10-12
Large plastic vial CIT analysis (1.1± 0.1)· 10-6 (31± 15)· 10-9 (3.2± 0.3)· 10-9 (30± 23)· 10-12 <40· 10-12
Suprasil quartz vial CIT analysis (37± 4)· 10-9 (180± 17)· 10-9 (16± 1)· 10-12 (20± 1)· 10-12 (32± 3)0· 10-12


Future measurements:
Counting of above samples continues. The dominant side activities 24Na and 82Br are short lived (T1/2=14.96 h and 35.30 h). The long live time of 233Pa (T1/2=26.97 d) will result in a much improved signal to background ratio after the short lived side activities have died out. The Th-sensitivity will hence improve.
We should continue monitoring the PPO purity. However, before performing the next irradiation the problem of sample spill and the Br side activity need to be addressed. We should try to (a) ash the sample as done for our irradiations at Oak Ridge and (b) try to come up with a chemical separation of the relevant elements after activation. If successful this would give us much improved sensitivity.



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