Optimizing Peak Separation for Simultaneous Optimizing Peak Separation for Simultaneous Pu and U MeasurementsPu and U Measurements

Shane KnockemusShane Knockemus

US EPA / NARELUS EPA / NAREL

May 3, 2005May 3, 2005

Why would anyone be interested in Why would anyone be interested in simultaneous determination of Pu and U?simultaneous determination of Pu and U?

Increased laboratory efficiency due to a Increased laboratory efficiency due to a higher degree of sample throughput.higher degree of sample throughput.

Laboratory production would be increased. Laboratory production would be increased. Twice as many alpha spec detectors would Twice as many alpha spec detectors would be available for use.be available for use.

Why do something twice when once is Why do something twice when once is enough?enough?

GoalGoal

I hope to show that a simultaneous I hope to show that a simultaneous determination of Pu and U is possible determination of Pu and U is possible through manipulating important variables. through manipulating important variables. Such variables include:Such variables include:

mass of carrier used during fluoride precipitationmass of carrier used during fluoride precipitation counting the sample at different shelf heights in the counting the sample at different shelf heights in the

alpha spec chamberalpha spec chamber type of filter employed.type of filter employed.

Pu and U measured simultaneously?Pu and U measured simultaneously?

First noticed the possibility after a mistake First noticed the possibility after a mistake during sample separation.during sample separation.

Studies have been concentrated on source Studies have been concentrated on source preparation involving a cerium fluoride preparation involving a cerium fluoride coprecipitation. coprecipitation.

No testing of source preparation with No testing of source preparation with respect to electrodeposition was included in respect to electrodeposition was included in the study.the study.

Experiment parametersExperiment parameters TEVA / TRU separation scheme was used. There are many TEVA / TRU separation scheme was used. There are many

possible scenarios that could enable simultaneous Pu and U possible scenarios that could enable simultaneous Pu and U elution. elution.

Pu and U eluted together with ammonium oxalate from TRU.Pu and U eluted together with ammonium oxalate from TRU. Pu and U precipitated together in the presence of TiClPu and U precipitated together in the presence of TiCl33.. 0.2 um filters used during filtration, later Resolve filters were used.0.2 um filters used during filtration, later Resolve filters were used. Varying amounts of CeVarying amounts of Ce+3+3 used. used. Varying shelf height in alpha spec chambers.Varying shelf height in alpha spec chambers.

Full Width Half Maximum (FWHM)Full Width Half Maximum (FWHM)

FWHM simply refers to peak width. It is defined FWHM simply refers to peak width. It is defined as “the width of the distribution at a level that is as “the width of the distribution at a level that is just half the maximum ordinate (centroid) of the just half the maximum ordinate (centroid) of the peak.” FWHM is measured in terms of energy peak.” FWHM is measured in terms of energy (keV) or channel numbers.(keV) or channel numbers.

Good resolution is a necessity to measurement Good resolution is a necessity to measurement by alpha spec, and is highly dependent on by alpha spec, and is highly dependent on source preparation and deposition.source preparation and deposition.

FWHM DiagramFWHM Diagramcentroid

yy/2Width of peak in keV at y/2

Carrier mass and FWHMCarrier mass and FWHM

FWHM values increase with the FWHM values increase with the amount of carrier mass added amount of carrier mass added during fluoride precipitationduring fluoride precipitation

0

20

40

60

80

100

120

25 50 100 150 200

ug Ce

FW

HM U-238

Pu-238

Finding the optimum mass of carrier to useFinding the optimum mass of carrier to use

How much does the mass of carrier used during How much does the mass of carrier used during precipitation affect the tracer recovery?precipitation affect the tracer recovery?

– Experiments involving 50-100 ug of Ce all showed Experiments involving 50-100 ug of Ce all showed consistently high yields, usually 90+%.consistently high yields, usually 90+%.

– Experiments involving 25 ug had yields that were Experiments involving 25 ug had yields that were consistent, but were less than yields of those precipitated consistent, but were less than yields of those precipitated with 50-100 ug of Ce, usually 75-85%with 50-100 ug of Ce, usually 75-85%

– Some experiments involving 10 ug of Ce were also done. Some experiments involving 10 ug of Ce were also done. These provided yields that were inconsistent and often These provided yields that were inconsistent and often very low.very low.

Spectra comparisonSpectra comparison

200 ug Ce

100 ug Ce

Regions of concernRegions of concern

Listed are nuclides whose Listed are nuclides whose energies could possibly energies could possibly overlap without optimum overlap without optimum peak separationpeak separation

U-234 U-234 4723-4775 keV4723-4775 keV

Pu-242Pu-2424856-4900 keV4856-4900 keV

Pu-239Pu-2395143-5155 keV5143-5155 keV

U-232U-2325263-5320 keV5263-5320 keV

U-232U-2325263-5320 keV5263-5320 keV

Pu-238Pu-2385456-5499 keV5456-5499 keV

Zoom 100 ug Ce high shelfZoom 100 ug Ce high shelf

Spectrum from 25 ug CeSpectrum from 25 ug Ce

25 ug

zoom Peak separation is betterBut still needs improvement

FWHM and peak separationFWHM and peak separation

Good FWHM values do not always translate into adequate peak Good FWHM values do not always translate into adequate peak separation. All FWHM values from this spectrum are less than 80 keV, separation. All FWHM values from this spectrum are less than 80 keV, but peak separation is an obvious problem. Without any peak fitting but peak separation is an obvious problem. Without any peak fitting manipulations, the measured analyte activities would be questionable.manipulations, the measured analyte activities would be questionable.

How to improve peak separation?How to improve peak separation?

All of the previous samples were counted on All of the previous samples were counted on the top shelf in alpha spec chamber. If the the top shelf in alpha spec chamber. If the shelf height of the sample were lowered, the shelf height of the sample were lowered, the resolution and peak separation should resolution and peak separation should improve. Doing this would decrease the improve. Doing this would decrease the efficiency. If you can sacrifice some efficiency. If you can sacrifice some efficiency in exchange for improved peak efficiency in exchange for improved peak separation, this may be advantageous. separation, this may be advantageous.

Bottom shelf spectraBottom shelf spectra

200 ug Ce

100 ug Ce

Spectrum of 25 ug Ce on bottom shelfSpectrum of 25 ug Ce on bottom shelf

Zoom of 25 ug Ce on bottom shelfZoom of 25 ug Ce on bottom shelf

More spectra with 25 ug Ce on low shelfMore spectra with 25 ug Ce on low shelf

Does the type of filter used make a difference Does the type of filter used make a difference in peak separation and resolution?in peak separation and resolution?

Progression of improvementsProgression of improvements

100ugCe-high

100ugCe-low 25ugCe-low

25ugCe-low-Resolve

Experiment resultsExperiment results

The samples used for the experiments were 1 liter The samples used for the experiments were 1 liter tap water samples and were spiked ‘in house’ with tap water samples and were spiked ‘in house’ with known amounts of activity.known amounts of activity.

The recoveries of the spiked analytes were very The recoveries of the spiked analytes were very good and in acceptable ranges when the peak good and in acceptable ranges when the peak separation was adequate.separation was adequate.

Experiment scenarios that produced incomplete Experiment scenarios that produced incomplete peak separation of analyte and tracer peaks were peak separation of analyte and tracer peaks were a problem to quantify accurately. The use of peak a problem to quantify accurately. The use of peak fitting may be a useful tool.fitting may be a useful tool.

Discussion and conclusionDiscussion and conclusion

There are many factors to consider if simultaneous There are many factors to consider if simultaneous measurement of Pu and U were to become a measurement of Pu and U were to become a reality. Experiments showed a noticeable reality. Experiments showed a noticeable improvement in peak separation. The combination improvement in peak separation. The combination of minimizing the mass of carrier used and of minimizing the mass of carrier used and counting the sample on a lower shelf was counting the sample on a lower shelf was effective, but both of those adjustments also have effective, but both of those adjustments also have their drawbacks. Such drawbacks include a their drawbacks. Such drawbacks include a decreased counting efficiency and a slightly lower decreased counting efficiency and a slightly lower yield. yield.