FAST NEUTRON DOSIMETRY USING CR-39 PLASTIC
H. Tawara, M. Miyajima, S. Sasaki and K. Hozumi
Radiation and Safety Control Center
National laboratory for High Energy Physics
Ohol-1, Tukuba-shi, Ibaraki-ken, 305, Japan

Abstract
Methods of measuring low level fast neutron dose
have been developed using CR-39 solid state nuclear
track detector. The measured density of etch-pits ofrecoil protons as a function of neutron dose shows
excellent linearity down to 10 J.!SV. 1be sensitivity is
1.34 etch-pits/cm2·J.!Sv in a scanning etch-pits witha sharp tips under an optical microscope with eyes.
In the case of an automatic scanning, the sensitivity
is 0.64 etch-pits/cm2•J.!Sv.
Introduction
CR-39 plastic is highly sensitive martial for thedetection of tracks of recoil protons produced by fastneutrons. Furthermore the aging and fading effects of
CR-39 are small at room temperature for severalmonths. It has pointed out that an accumulated
neutron dose of lOJ.!Sv (= lmrem) or less ismeasurable with CR-39 detector1 .2. Because of these
remarkable properties as a solid state nuclear track
detector, CR-39 plastic has recently being developed
for neutron detection, neutron dosimetry andpersonal monitoring of neutron doses. We havedeveloped new methods of measuring low level
neutron dose (:o;lOOJ.LSv). In this paper, we will
describe the outline of the methods for fast neutron
dosimetry using CR-39 plastic : one is a direct
observation of proton etch-pits under an optical
microscope with eyes3, another is a counting method
using an automatic scanning system4.
Experimental
CR-39 plastic sheets supplied by Sola Optical
Japan, Co. are used in this experiments. The
thickness of the sheets is 1.6 to 1. 7 mm. The stacks
of 3 to 10 layers of CR-39 sheets were irradiated atvarious distances from an Am-Be neutron source. Thedose at etch location was measured by a rem-counterof the Anderson type (Studsvic). Each sheet wasetched in about 6.8N NaOH solution at 70°C for 6 to24 h after irradiation .
In the case of the direct observation of etch-pits
under an optical microscope with eyes, backgroundtracks, which are produced by alpha particles fromairborne alpha-emitters deposited on the surface ofthe plastic, are not completely distinguishable fromthat of recoil protons. We attempted a deep etchingof CR-39 plastic in order to eliminate an incorrect
counting of tracks. If a depth of removed layer inetching is equal to the range of alpha particle withthe energy of around 5 MeV, the residual etch-pits
due to alpha particle show round tips. On the other
hand, many tracks of protons which are recoiled by
fast neutrons inside plastic sheets are observed asetch-pits with sharp tips after such a deep etching.
Although some etch-pits of recoil protons may haveround tips as etch-pits of alpha-particles, the numberof etch-pits with sharp tips does not change, because
the number of etch-pits of recoil proton with sharp

tips lost by deep etching is compensated by newly
formed etch-pits of proton recoiled inside CR-39
plastic in succession. The tips of etch-pits are easily
observable under an optical microscope by changing
its focus point. Therefore, the etch-pits of recoil
protons can be easily distinguished from thebackground etch-pits of alpha-particles. In thescanning of etch-pits under optical microscope witheyes, however, there may be a distinctive feature that
their efficiency tends to be influenced by individual
variation of observers.
An automatic scanning has been tested in order toeliminate differences in scanning efficiency fromperson to person in the above-mentioned humanscanning method. The automatic scanning system iscomposed of a auto-focused optical microscope with avideo camera, a X-Y stage, an image processor and apersonal computer. In this arrangement, features of
etch-pits, labelings, areas, perimeters and diameters,
are automatically analyzed. Etch-pits are counted asbinarised images which are fixed according to a
threshold with a selected grey level. In this case,etch pits of alpha-particle is easily discriminatedbecause they identified by measurement of area anddiameter for their big size. Binarised pictures of
streaks and adhered matters on a surface of CR-39,
however, may becomes backgrounds.
Results and Discussion
The depth dependence in the counted number of
etch-pits of recoil protons is displayed in fig. I. The
front surface of each sheet was scanned. Thedensities of etch-pits were almost constant except
for the first layer in both scanning methods.

...

.0~ 4cnc..

i::.

3()

Cll0 2....
Cll.0E::s1 -ri Q'-t-'Q-e-CR-39(SOJ)

Am-Be neutron source

1• NaOH

6.8N,70°C,20h

Automatic scanning

0 NaOH 6.7N,70°C, 18h
Scanning with eyeszIII

IIIIII

0 o~~--~2~~---4~~--~6~~--~8~~~10Layer of CR-39 sheet

Fig. I Depth dependence of the number of etch-pits
per cm2 for neutrons from an Am-Be source.

-336-

Fig. 2 shows the counted number of etch-pits due
to recoil protons as a function of neutron dose. Th,eresults of scanning with eyes after etching 24 h showthat the density of etch-pits is proportiona l toneutron dose. The sensitivity in the scanning witheyes, 1.34±0.37 etch-pits/cm2 ·!1Sv, was obtained in
the fast neutron dose lower than l 00 11Sv. In theautomatic scanning, it seems that the densities ofetch-pits are biased by the background, which isclose to 24 ± 9 !!Sv/cm2", and the sensitivity was 0.64± 0.08 etch-pits/cm 2·!1Sv. One reason for the low
sensitivity in the automatic scanning is thought that
pictures of small proton etch-pits which wererecognized with visual inspection vanished in thebinarised process of pictures. Another reason is thatthresholds of parameters in the analysis of binarised
pictures set somewhat highly in order to eliminate
counting false pictures due to streaks and adheredmatters on the surface and to count only true tracks.
iSQ _"'E

It is necessary to examine the sensitivity at various

threshold levels in the binarised process and tosearch suitable parameters for feature analysis andtheir thresholds for counting.
Conclusion
For neutron dosimetry using CR-39 detector,
there are many points which must be clarified, forexample , etching condition, the quality of CR-39
plastic itself, and etc. Radiator material of recoil
protons is the CR-39 plastic itself in thisexperiments . However, it is advisable to use ofhydrogenou s front radiator which increase thesensitivity. The developmen t of the automatic
scanning technique is important for personalmonitoring of neutron dose. At present, automatic
scanning method for neutron dosimetry attempted by
us is still at an experimenta l stage. Further
experiments are in progress.
References

CR-39 (SOJ)
Am-Be neutron source
NaOH 6.7N,70"C,24h

1. T. W. Turner, D. L. Henshaw and A. P. Fews,

Nuclear Tracks and Radiation Measuremen ts,

8 (1984) 341.
2. A. P. Fews, T. Portwood, D. L. Henshaw, T. W.
Turner and A. Worley, Fifth sympos~um on

0Ui.

:g_ 100f.s::
0a;
8 -Automatic scanning

0 -- Scanning with eyes

DoseFig.2 Density of etch-pits against neutron dose-337-

Neutron Dosimetry Proceedings, (1985) 479.
3. H. Tawara, H. Takahashi, I. Watanabe, T. Doke,
Y. Kang and M. Miyajima, Nucl. Instr. Meth.,
B24 (1987) 369.
4. H.Tawara, M. Miyajima, S. Sasaki, K. Hozumi,
Ionizing Radiation, 16, 1 (1989) 82 (inJapanese).