Study on the etching conditions of
polycarbonate detectors for
particle analysis of safeguards
environmental samples
K. Iguchi, K. T. Esaka, C. G. Lee, J. Inagawa, F. Esaka, T. Onodera, H.
Fukuyama, D. Suzuki, S. Sakurai, K. Watanabe, S. Usuda
Department of Environmental
Sciences, Japan Atomic Energy Research Institute,Tokai, Naka, Ibaraki
319-1195, Japan.
Introduction
Conclusion
The
etching time for detection of particle of interest was changed by the
enrichment of fissile material in the particle. The results obtained suggest the possibility
of
The screening of the uranium particles according to their enrichment by
controlling the etching time.
The improvement of efficiency of the particle
analysis.
A fission
track event from a uranium particle observed by a digital
microscope. The uranium particle located at the center (a) and
fall off from the event (b).
Fission track events
10 mm
(a)
10 mm
(b)
Environmental sample
analysis technique was adopted by the International Atomic Energy Agency
(IAEA) as one of new techniques for the strengthened safeguards system. Its aim is to find undeclared nuclear
activities and materials based on isotope ratio analyses of nuclear materials
taken from the inside and outside of nuclear facilities. One of the particle analysis
techniques for individual particle analysis is based on fission track (FT)
technique combined with thermal ionization mass spectrometry (TIMS). The FT-TIMS method enables us to
analyze sub-micrometer particles containing fissile material. Recently, we simplified FT-TIMS method
; the particles are recovered from the swipe sample onto the polycarbonate
membrane filter by a suction pump.
The filter is dissolved in organic solvent to form a detector of a thin
film, in which particles are confined.
The detector is irradiated by thermal neutrons and then etched with
NaOH solution. The particles containing fissile materials are detected simply
and sensitively with a microscope.
It was found, however, that the particles in the vicinity of the
detector surface were often lost during the etching process. It is expected that the particles in the
environmental samples contain uranium of various 235U enrichment (235U/238U ratio). To detect fissile materials in safeguards environmental samples
with high detection efficiency and less particle loss, basic data for
optimization of the etching condition is required.
The
character and problem of the detector
Proceeding
Purpose
of this study:
To
investigate the relation of etching time and
the observed fission tracks to obtain
appropriate etching condition with high
detection efficiency and less particle
loss.
Thermal neutron
irradiation
The particle in the
vicinity
of the surface falls off.
Fission tracks are created
in the direction of 4π
Simple and high sensitive
detection
Particle loss
FT detector
Etching
Particles containing fissile materials
Enrichment
higher
The number of FT
larger
The damage around the particle
In this work, we focused on the
influence of
the enrichment to etching condition.
Influence of enrichment and particle size in
etching time of detector
Particle size
larger
bigger
Higher etching rate
Behaviors
both the detection of particles and the particle loss are strongly dependent on the enrichment.
(a)
(b)
The changes in the proportions of each events
This result
suggests the possibility that the uranium
particles are screened according to their enrichment, by controlling the etching time.
The
initial time to detect the events both with a particle and without a particle are different to
the etching time and uranium enrichment. And, the hatched region shows the
range in which particle detection without particle loss is possible.
The correlation
between the enrichment and the etching time in detection of events with a
particle and without a particle.
of etching
Results and Discussion
The changes in the number and proportion of events
The number of the events with particles increased steeply after the etching time of 3 min, reached to saturation, and then began to decreases after 14 min.
On the
other hand, the number of the events
without particles monotonically increased
after the etching time of about 8 min.
Fig. 2
Fig. 1
Fig. 3
Fig. 4
Experimental
Thermal neutron
irradiation at JRR-4
Fluence: 8×1014 cm-2
Chemical etching:
6M NaOH at 55℃
Dissolution of a polycarbonate
filter with particles by using a mixed solution of dichloromethane
and dichloromethane
Observation of fission
tracks
by a digital microscope
(VHX100, Keyence Co.)
Formation of the detector of a thin film
Recover of
particles by the filtration
Swipe sample
Filtration
Pump
Polycarbonate
filter
Support
screen
Preparation
of FT detector
Samples
・NBL950a (natural uranium), NBL U050, U100, U350, U850 ( 5, 10, 35, 85% 235U enriched uranium) swiped with cotton cloth (Tex
Wipe®304, 10×10
cm2).
・It was confirmed by a scanning electron microscope (JSM6700F, JEOL Inc.)
that the uranium-oxide samples used in this study had a similar distribution
in particle diameter,
main part of which is in the range
from 1 to 3 mm.
(a)
(Natural uranium)