Transcript Trace element analysis of K, U and Th in high purity materials by Neutron Activation Analysis P.

Trace element analysis of
K, U and Th in
high purity materials by
Neutron Activation Analysis
P. ILA
Dept. of Earth Atmospheric & Planetary Sciences
Massachusetts Institute of Technology
Cambridge, MA 02139
[email protected]
LRT2004, Sudbury, December 12-13, 2004
What are high purity materials?
Purity level
Common usage
99.9%
High Pure (3 nines purity)
99.95%
Analytical Grade
99.99%
Spectroscopically Pure
99.9999%
High Pure (6 nines purity)
Fe wire, Au foils, Silica powder, HPGe, Cu
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Determination of Th, U and K by
Instrumental Neutron Activation
Analysis (INAA)
(n,g) 233Th --> 233Pa half-life 27 d
g-ray 312 keV
238U (n,g) 239U --> 239Np half-life 2.35 d
g-rays 106, 222, 228 keV
41K (n,g) 42K half life12.36h
g-ray 1524 keV
Usually, we do not do K by INAA,
but can be done if necessary

232Th
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Gamma activity measurement by
Ge detectors – Gamma Spectroscopy
 The
gamma activities from the irradiated
materials are measured by a gamma
spectrometer consisting of HPGe detector
and the related nuclear instrumentation
like the amplifier, analog to digital
converter, multi-channel analyzer and a
computer with data acquisition and
reduction software and other peripherals.
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Interference-free limits
Best limits in clean materials like acrylic
U, Th : 1 ppt; K : 50 ppb
Typical limits
U, Th : 20 ppt, K : 50 ppb
The lower limit of detection is set by the matrix
activities because of the Compton continuum from
higher energy gamma rays than those of interest.
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Interferences from
Cr, Fe, Co, Sc & Na
 Depends on the major element matrix, and
trace elements like Cr, Fe, Co, Sc
These four elements are contaminants arising
from handling of high purity materials ( by way
of molding, milling, cutting etc.)
 Contamination of Na arises from as much as
touching or breathing on the sample.
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Interferences …
Gamma ray Energies keV
106, 222, 278
233Pa 312
42K 1524
239Np
59Fe
142, 192, 1099, 1291
51Cr 321
46Sc 889, 1120
24Na 1368, 2754
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Extending the limits of detection
Limits can be extended below pg/g
by a judicious choice of
 mass of sample,
 irradiation time,
 irradiation flux
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INAA and
Low Background Counting
 If
the activities other than from
Th and U are negligible, LBC
will be useful to go below pg/g
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Radiochemical Neutron Activation
Analysis (RNAA)
In
extreme cases, post
irradiation separation of Pa
and Np activities are
necessary and have been
done as required
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Conclusion

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
INAA is a very useful direct tool but RNAA
coupled with LBC becomes very powerful when
the impurities to be determined are below ppt
level.
The relative ratio of Th or U to total matrix trace
impurities is one issue.
The other issue is the major matrix itself.
We cannot use NAA for determining Th and U in
Fe, or Ge, or Co or Cu, or any other element
where the major element gets activated .
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