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

Zelimir Djurcic and Andreas Piepke
University of Alabama
(Status 2/9/2001)
(Up-dated 2/21/2001)

Sample Description

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: 4.0· 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 in small plastic vial with wool	0.5623 (PPO) 0.2562 (wool)	1/27/2001 3:16 to 4:16
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) 0.4048 (wool)	1/27/2001 7:38 to 8:38
UA 5	UA	Packard (21-0113) in small plastic vial with wool	0.3436 (PPO) 0.3487 (wool)	1/27/2001 7:38 to 8:38
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. The samples were placed on top of the Ge detector end cap, flush with its edge. This placement allows to change the samples and later re-measure them with a reproducebility of ± 10% as far as the detector-source solid angle is concerned. The absolute detection efficiency has been measured using a set of calibrated gamma point sources. Any deviation of the sample geometry from the calibration geometry is absorbed in the ± 10% systematic error applied to each measurement. The measurement error reported below has been calculated adding statistical and systematic errors in quadrature. Whenever multiple gamma lines are detected for any given isotope the individual activities are compared to assure compatibility and get another handle on the isotope assignment. The final activity and its error are then calculated as the weighted sum of the individual activities. Un-physical (negative) activities are reported and not converted into limits. Multiple insignificant activities can then be combined in the same way to arrive at a more stringent limit.
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. The corresponding shipping delays for samples UA2, UA3, UA4 and UA5 are: 100.534 h, 99.482 h, 98.434 h and, 98.434 h, respectively. 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 [Bq]	⁴²K [Bq]	⁵¹Cr [Bq]	⁵⁹Fe [Bq]	⁶⁰Co [Bq]	⁶⁵Zn [Bq]	⁸²Br [Bq]	¹⁹⁸Au [Bq]	²³³Pa [Bq]	²³⁹Np [Bq]
UA 5 Packard (0113)¹		-3.3± 5.9	7.7± 1.7	0.15± 0.44	0.12± 0.02	0.87± 0.1	1440± 150	14± 2.4	0.05± 0.045	-4.1± 1.6
UA 3 Packard (1012)²	389± 120	(17± 10)	100± 11	7.6± 0.8	1.4± 0.3	1.7± 0.23	109± 12	(3.0± 0.34)	0.14± 0.10	-0.94± 0.7
UA 2 Packard (1011)²	530± 130		13± 2.4	0.4± 0.14	<2.1± 0.2	4.5± 0.5	26500± 2600	(0.34± 1.7)	-0.41± 0.18	3.7± 5.0
Quartz wool UA	832± 83	23± 5					16± 3	9.7± 2.0	2.3± 0.4	43.9± 4.5
Small plastic vial UA Blank for UA 5			7.0± 0.7	0.38± 0.44	0.08± 0.01	0.21± 0.03	76± 8	8.1± 0.9	-0.032± 0.026	-0.8± 0.9
Small plastic vial CIT Blank for UA 5		1.8± 0.2	9.1± 0.9	0.75± 0.08	0.31± 0.03	0.28± 0.08	36.5± 3.7	6.0± 0.6	0.0065± 0.0041	<0.4
Large plastic vial CIT Blank for UA 2 & 3	635± 64		9.2± 1	0.61± 0.06	0.21± 0.23	0.53± 0.056	14.9± 1.5	13.7± 1.4	0.089± 0.057	<0.48

(¹)Small vial blank subtracted. Sample mass: 0.3436 g
(²)Large vial blank subtracted. Sample mass: 0.8717 (UA3) and 1.0002 (UA2) g
For results given in brakets background subtraction could not be performed.

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, as determined at Caltech, has been subtracted. In case of ¹⁹⁸Au this subtraction could not be performed as the blank activity exceeds that of the sample (plus blank). For sample Packard 21-0113 the background as determined from sample UA4 at UA has been subtracted. Samples labeled in red were analyzed at UA, green samples at Caltech.

Sample	Na [µ g/g]	K [µ g/g]	Cr [ng/g]	Fe [µ g/g]	Co [ng/g]	Zn [ng/g]	Br [ng/g]	Au [pg/g]	Th [pg/g]	U [pg/g]
UA 5 Packard (21-0113)		-0.8± 1.4	83± 18	0.5± 1.4	1.6± 0.3	167± 19	2650± 280	700± 110	160± 140	-3000± 1200
UA 3 Packard (21-1012)	2.0± 0.6	(1.7± 1.0)	422± 46	9.3± 1.0	7.4± 1.6	130± 17	81± 9	(58± 7)	177± 127	-275± 205
UA 2 Packard (21-1011)	2.5± 0.6		48± 5	0.43± 0.15	<(9.5± 0.9)	298± 33	17400± 1700	(5.9± 2.9)	-453± 200	960± 1300
CIT 2 Packard (21-1011)	4.3± 0.4	<1.43	20± 12	5± 0.6	780± 80	320± 30	14900± 2000	-230± 24	<1200	<92
CIT 6 Packard lot 9									40± 15	400± 150
CIT 1 Dojindo PPO	0.23± 0.08	<0.4	230± 20	1.4± 0.2	36± 4	92± 9	28± 3	-46± 5	51± 47	<100
Small plastic vial UA			19.4± 1.9	0.31± 0.35	0.28± 0.04	10.5± 1.5	36± 4	103± 11	-27± 22	-150± 170
Small plastic vial CIT	0.13± 0.02	0.11± 0.02	20± 2	0.50± 0.05	0.76± 0.10	23± 2.3	17± 2	68± 7	15± 8	<68
Large plastic vial CIT	1.1± 0.1	0.031± 0.015	12± 2	0.31± 0.04	0.65± 0.08	21± 2	3.2± 0.3	81± 9	30± 23	<40
Quartz wool UA	8.6± 0.9	4.7± 1.0					140± 20		6200± 1100	27000± 3000
Quartz wool CIT			80± 30	<0.4	1.5± 1.3	<32	57± 6	-240± 20	4000± 1000	30000± 4000
Suprasil quartz vial CIT	0.037± 0.004	0.18± 0.017					0.016± 0.001		20± 1	32± 3

Future measurements:
Counting of above samples continues. The dominant side activities ²⁴Na and ⁸²Br are short lived (T_1/2=14.96 h and 35.30 h). The long live time of ²³³Pa (T_1/2=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.
A new low background Ge-detector set-up is being prepared at UA.

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Last update March 2, 2001