Analysis Results for Activated Packard 21-634 PPO

Zelimir Djurcic and Andreas Piepke
University of Alabama
(Status 4/30/2001)
(Updated 5/15/2001)

In this note we discuss our counting results for Packard PPO (batch 21-634) samples UA22, UA23 and UA24. These sample were irradiated together in rabbit C on 3/22/2001 8:39 for 5 hours at the MIT research reactor. Measured activities are reported for a shipping delay of 49.8168 hours. The PPO mass of this sample was 3.4303 g. After receipt of the sample side activities were separated by means of ion exchange. The measured retention efficiency for this procedure, which is described somewhere else in this note, is 0.75. The activities remaining after ion exchange were determined by means of gamma ray counting using two shielded Ge detector set ups at UA. Most of the counting was done with the new low background Ge detector. Compatibility of activities obtained with the two detectors is usually good (within 10%). In the following table we report the sample activities as determined for 4/24/2001 10:28 in [Bq].
Samples UA9, UA10 and UA11 were blank irradiation vials which had been subjected to the same cleaning procedures during filling and to the same ion extraction after irradiation as the PPO samples above. The activities measured for those samples can hence serve as a measure of the background introduced by our sample preparation, handling and extraction. We report the sample activities also for a delay time of 49.8168 hours. The isotope identification was done using the measured energies of the gamma lines and the apparent half life. The activity values are clickable and will show our activity and half life determination. All errors quoted are statistical only. The systematic error is estimated at 10% for each measurement. For activities being significantly larger than zero the graphs for Packard PPO show the temporal development of the activities measured with both Ge detectors to demonstrate their compatibility.

			Activity [Bq]
Sample	Mass [g]	Delay [h]	²⁴Na	⁴⁶Sc	⁵¹Cr	⁵⁹Fe	⁸²Br	¹²²Sb	¹⁹⁸Au	²³³Pa	²³⁹Np
Packard 21-634 UA19-21 No ion exchange	3.4954	54.9840	62230±90	1.30±0.03	34±0.33	3.18±0.05	822±1.2	734±1	14±0.2	0.18±0.08	-1.5±0.6
Packard 21-634 UA22-24 Ion exchange	3.4303	49.8168	137.4±3.6	0.96±0.07	0.17±0.04	2.86±0.014	228.8±0.58	3.37±0.09	3.38±0.053	0.050±0.013	0.032±0.20
Blank UA9-11 Ion exchange		60.0336	-3.6±0.29	0.032±0.004	-0.10±0.055	0.18±0.009	4.94±0.1	8.1±0.11	5.45±0.07	-0.028±0.016	0.062±0.15

From above table we conclude that the ²⁴Na, ⁴⁶Sc, ⁵¹Cr, ⁵⁹Fe, ⁸²Br activities are located in the PPO. The positive values for ¹²²Sb and ¹⁹⁸Au are due to a non-zero sample blank (probably leached out of the sample vial during acid extraction). It is noted that the poly plastic bags used as secondary containment showed very large Sb activities after irradiation. Whenever those bags were molten to the irradition vial during activation the vial was externally contaminated by Sb. The data shows no evidence for the presence of ²³⁹Np. The ²³³Pa signal has a statistical significance of 3.9 sigma. We interpret this result as evidence for a Th contamination of the PPO. The observed counting rate is consistent with being constant in time; supporting the hypothesis of a ²³³Pa signal. However, due to the relatively marginal significance of that signal a clear peak can not be seen when inspecting the energy spectrum.
Using the known neutron flux, irradiation time and Pa/Np retention efficiency we can now convert the measured activities into chemical concentartions. At this point we use the neutron flux as reported by MIT. This conversion is only performed for Th and U for which we have a measured retention efficiency of 0.75 during ion exchange. The other side activities are determined from a PPO sample which was not subjected to ion exchange.

²³²Th: 2.5±0.65^stat ppt or 2.5±0.85 ^total ppt
²³⁸U : 0.15±0.93 ppt or <1.7^stat ppt <2.2 ^total ppt

The sample blank is not the limiting factor at this point. At long delay times the detector background limits our sensitivity. A nitrogen purging and cosmic ray veto is being implemented to reduce detector background. However, a further improvement of our sensitivity (mainly for the short lived Np) will require a reduced blank. We believe that Br is the main source related background as it has a high gama multiplicity. In the next activation run we will try to precipitate the Br after acid digestion using AgNO₃.
The sensitivity of our method is now good enough to probe ppt concentrations. We assume a systematic uncertainty of 10% for the counting efficiency. The determination of the concentration does not yet take into account the scaling of neutron flux along the rabbit. We estimate this from the observed scaling of the measured ²⁴Na activities to be approximately a 20% effect. To be conservative we scale our limits up by a factor 1.3.